Controlling Microbial Growth in the Body Antimicrobial Drugs

Controlling Microbial Growth inthe Body: Antimicrobial Drugs Chemicals that affect physiology in any manner, such as caffeine, alcohol, and tobacco, are called drugs. Drugs that act against diseases are called chemotherapeutic agents. Examples include insulin, anticancer drugs, and drugs for treating infections—called antimicrobial agents or antimicrobials, the subject of this chapter. In the pages that follow, we’ll examine the mechanisms by which antimicrobial agents act, the factors that must be considered in the use of antimicrobials, and several issues surrounding resistance to antimicrobial agents among microorganisms. First, however, we begin with a brief history of antimicrobial chemotherapy. ­ ­ ­ Staphylococcus aureus (bacterium) ­ Penicillium chrysogenum (fungus) Zone where bacterial growth is inhibited The History of Antimicrobial Agents ▲ FIGURE10.1 Antibiotic effect of the mold Penicillium ­ ­ ­ The little girl lay struggling to breathe as her parents stood mutely by, willing the doctor to do something—anything—to relieve the symptoms that had so quickly consumed their fouryear-old daughter’s vitality. Sadly, there was little the doctor could do. The thick “pseudomembrane” of diphtheria, composed of bacteria, mucus, blood-clotting factors, and white blood cells, adhered tenaciously to her pharynx, tonsils, and vocal cords. He knew that trying to remove it could rip open the underlying mucous membrane, resulting in bleeding, possibly additional infections, and death. In 1902, there was little medical science could offer for the treatment of diphtheria; all physicians could do was wait and hope. At the beginning of the 20th century, much of medicine involved diagnosing illness, describing its expected course, and telling family members either how long a patient might be sick or when they might expect her to die. Even though physicians and scientists had recently accepted the germ theory of disease and knew the causes of many diseases, very little could be done to inhibit pathogens, including Corynebacterium diphtheriae (kŏ-rī’nē-bak-tēr’ēŭm dif-thi’rē-ī), and alter the course of infections. In fact, one-third of children born in the early 1900s died from infectious diseases before the age of five. It was at this time that Paul Ehrlich (1854–1915), a visionary German scientist, proposed the term chemotherapy to describe the use of chemicals that would selectively kill pathogens while having little or no effect on a patient. He wrote of “magic bullets” that would bind to receptors on germs to bring about their death while ignoring host cells, which lacked the receptor molecules. Ehrlich’s search for antimicrobial agents resulted in the discovery of arsenic compounds, one of which worked against M10_BAUM7206_05_SE_C10.indd 287 the bacterial agent of syphilis. A few years later, in 1928, the British bacteriologist Alexander Fleming (1881–1955) reported the antibacterial action of penicillin released from Penicillium (pen-i-sil’ē-ŭm) mold, which creates a zone where bacteria don’t grow (FIGURE 10.1). Though arsenic compounds and penicillin were discovered first, they were not the first antimicrobials in widespread use: Ehrlich’s arsenic compounds are toxic to humans, and penicillin was not available in large enough quantities to be useful until the late 1940s. Instead, sulfanilamide, discovered in 1932 by the German chemist Gerhard Domagk (1895–1964), was the first practical antimicrobial agent efficacious in treating a wide array of bacterial infections. Selman Waksman (1888–1973) discovered that other microorganisms are sources of useful antimicrobials, most notably species of soil-dwelling bacteria in the genus Streptomyces (strep-tō-mī’sēz). Waksman coined the term antibiotics to describe antimicrobial agents that are produced naturally by an organism. In common usage today, antibiotic denotes an antibacterial agent, including synthetic compounds and excluding agents with antiviral and antifungal activity. Other scientists produced semisynthetic antimicrobials— chemically altered antibiotics—that are more effective, longer lasting, or easier to administer than naturally occurring antibiotics. Antimicrobials that are completely synthesized in a laboratory are called synthetic drugs. Most commonly used antimicrobials are either natural or semisynthetic. TABLE 10.1 on p. 288 provides a list of some common antibiotics and semisynthetics and their sources. ­ Fleming, and Gerhard Domagk in the development of antimicrobials. 10.2 Explain how semisynthetic and synthetic antimicrobials differ from antibiotics. ­ 10.1 Describe the contributions of Paul Ehrlich, Alexander chrysogenum. Alexander Fleming observed that this mold secretes penicillin, which inhibits the growth of bacteria, as is apparent with Staphylococcus aureus growing on this blood agar plate. ­ LE A R N I N G O U TC O M E S TELL ME WHY Why aren’t antibiotics effective against the common cold? 11/23/16 3:03 AM p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t ha       C Although they can have a variety of effects on pathogens, antimicrobial drugs can be categorized into several general groups according to their mechanisms of action (FIGURE 10.2): ­ TABLE 10.1 Sources of Some Common Antibiotics and Semisynthetics Microorganism Antimicrobial Penicillium chrysogenum P Fungi Penicillium griseofulvum Griseofulvin Acremoniuma Cephalothin spp.b • Drugs that inhibit cell wall synthesis. These drugs are selectively toxic to certain fungal or bacterial cells, which have cell walls, but not to animals, which lack cell walls. • Drugs that inhibit protein synthesis (translation) by targeting the differences between prokaryotic and eukaryotic ribosomes. • Drugs that disrupt unique components of a cytoplasmic membrane. • Drugs that inhibit general metabolic pathways not used by humans. • Drugs that inhibit nucleic acid synthesis. • Drugs that block a pathogen’s recognition of or attachment to its host. enicillin G Bacteria Bacillus polymyxa ifampin Bacitracin olymyxin Gentamicin Pseudomonas fluorescens Mupirocin Saccharopolyspora erythraea E Micromonospora purpurea rythromycin Streptomyces griseus Streptomycin Streptomyces fradiae Neomycin Streptomyces aureofaciens Tetracycline Streptomyces venezuelae Chloramphenicol Streptomyces nodosus Amphotericin B Streptomyces avermitilis Ivermectin aThis bspp. In the following sections, we examine these mechanisms in turn. ▶ ANIMATION: Chemotherapeutic Agents: Modes of Action Inhibition of Cell Wall Synthesis genus was formerly called Cephalosporium. is the abbreviation for multiple species of a genus. LE A R N I N G O UTC O M E 10.5 Describe the actions and give examples of drugs that Mechanisms of Antimicrobial Action affect the cell walls of bacteria or fungi. LE A R N I N G O U TC O M E S 10.3 Explain the principle of selective toxicity. 10.4 List six mechanisms by which antimicrobial drugs affect pathogens. ­ ­ As Ehrlich foresaw, the key to successful chemotherapy against microbes is selective toxicity; that is, an effective antimicrobial agent must be more toxic to a pathogen than to the pathogen’s host. Selective toxicity is possible because of differences in structure or metabolism between the pathogen and its host. Typically, the more differences, the easier it is to discover or create an effective antimicrobial agent. Because there are many differences between the structure and metabolism of pathogenic bacteria and their eukaryotic hosts, antibacterial drugs constitute the greatest number and diversity of antimicrobial agents. Fewer antifungal, antiprotozoan, and anthelmintic drugs are available because fungi, protozoa, and helminths—like their animal and human hosts— are eukaryotic chemoheterotrophs (see Chapter 6) and thus share many common features. The number of effective antiviral drugs is also limited, despite major differences in structure, because viruses utilize their host cells’ enzymes and ribosomes to metabolize and replicate. Therefore, drugs that are effective against viral replication are likely toxic to the host as well. M10_BAUM7206_05_SE_C10.indd 288 A cell wall protects a cell from the effects of osmotic pressure. Both pathogenic bacteria and fungi have cell walls, which animals and humans lack. First, we examine drugs that act against bacterial cell walls. Inhibition of Synthesis of Bacterial Walls The major structural component of a bacterial cell wall is its peptidoglycan layer. Peptidoglycan is a huge macromolecule composed of polysaccharide chains of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) molecules that are cross-linked by short peptide chains extending between NAM subunits (see Chapter 3, Figure 3.14). To enlarge or divide, a cell synthesizes more peptidoglycan by adding new NAG and NAM subunits to existing NAG-NAM chains, and the new NAM subunits must then be bonded to other new, nearby NAM subunits (FIGURE 10.3a, b). Many common antibacterial agents act by preventing the cross-linkage of NAM subunits. Most prominent among these drugs are beta-lactams (such as penicillins, cephalosporins, and carbapenems), which are antimicrobials whose functional portions are called beta­lactam (ß­lactam) rings (FIGURE 10.3c). Beta-lactams inhibit peptidoglycan formation by irreversibly binding to the enzymes that cross-link NAM subunits (FIGURE 10.3d). In the absence of correctly formed peptidoglycan, growing bacterial cells have weakened cell walls that are less resistant to the effects of osmotic pressure. ­ Bacillus licheniformis R Amycolatopsis rifamycinica Vancomycin P Amycolatopsis orientalis 11/23/16 3:03 AM       Mechanisms of Antimicrobial Action Inhibition of pathogen’s attachment or entry into host cell Arildone Pleconaril Enfuvirtide Inhibition of cell wall synthesis Penicillins Carbapenems Cephalosporins Lipoglycopeptides Bacitracin Isoniazid Ethambutol Echinocandins (antifungal) Inhibition of protein synthesis Aminoglycosides Tetracyclines Chloramphenicol Macrolides Antisense nucleic acids Human cell membrane Inhibition of DNA or RNA synthesis Actinomycin Nucleotide analogs Quinolones Rifampin Disruption of cytoplasmic membrane Polymyxins Polyenes (antifungal) Inhibition of general metabolic pathway Sulfonamides Trimethoprim Dapsone ▲ FIGURE10.2 Mechanisms of action of microbial drugs. Also listed are representative drugs for each type of action. M10_BAUM7206_05_SE_C10.indd 289 ­ bacitracin (bas-i-trā’sin), blocks the transport of NAG and NAM across the cytoplasmic membrane to the wall. Like betalactams, lipoglycopeptides, cycloserine, and bacitracin result in cell lysis due to the effects of osmotic pressure. Since all these drugs prevent bacteria from increasing the amount of cell wall material but have no effect on existing peptidoglycan, they are effective only on bacterial cells that are growing or reproducing; dormant cells are unaffected. Bacteria of the genus Mycobacterium (mī’kō-bak-tēr’ē-ŭm), notably the agents of leprosy and tuberculosis, are characterized by unique, complex cell walls that contain waxy lipids called mycolic acids in addition to the usual peptidoglycan of bacterial cells. Isoniazid (ī-sō-nī’ă-zid), or INH,1 and ethambutol (eth-am’boo-tol) disrupt the formation of mycolic acid. ­ The underlying cytoplasmic membrane bulges through the weakened portions of cell wall as water moves into the cell, and eventually the cell lyses (FIGURE 10.3e). Chemists have made alterations to natural beta-lactams, such as penicillin G, to create semisynthetic derivatives such as methicillin and imipenem (see Figure 10.3c), which are more stable in the acidic environment of the stomach, more readily absorbed in the intestinal tract, less susceptible to deactivation by bacterial enzymes, or more active against more types of bacteria. Other antimicrobials, such as lipoglycopeptides—for example, vancomycin (van-kō-mī’sin), oritavancin (or-it-avan’sin)—and cycloserine, a semisynthetic, disrupt cell wall formation in a different manner. They directly interfere with particular alanine-alanine bridges that link the NAM subunits in many Gram-positive bacteria. Those bacteria that lack alanine-alanine crossbridges are naturally resistant to these drugs. Still another drug that prevents cell wall formation, 1From isonicotinic acid hydrazide, the correct chemical name for isoniazid. 11/23/16 3:03 AM (a) p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t ha       C (b) NAM Crossbridge between NAM and NAM NAG NAG-NAM chain New NAG and NAM subunits Growth A bacterial cell wall is made of peptidoglycan, which is made of NAG-NAM chains that are cross-linked by peptide bridges between the NAM subunits. d-lactam ring (c) CH2 O H C N S N O O CH3 HO H H H N H3C NH S S N O Penicillin G (natural) O New NAG and NAM subunits are inserted into the wall by enzymes, allowing the cell to grow. Other enzymes link new NAM subunits to old NAM subunits with peptide cross-links. H C N CH3 O H C N O O N CH2 O C CH3 COOH COOH Imipenem (semisynthetic) Cephalothin (natural) Carbapenems O CH2 O Cephalosporins S N Methicillin (semisynthetic) Penicillins (d) (e) New cross-links inhibited by beta-lactam Previously formed crossbridge Growth Beta-lactam interferes with the linking enzymes, and NAM subunits remain unattached to their neighbors. However, the cell continues to grow as it adds more NAG and NAM subunits. The cell bursts from osmotic pressure because the integrity of peptidoglycan is not maintained. ▲ FIGURE10.3 Bacterial cell wall synthesis and the inhibitory effects of ­ beta-lactams on it. (a) A schematic depiction of a normal peptidoglycan cell wall showing NAG-NAM chains and cross-linked NAM subunits. (b) A bacterium grows by adding new NAG and NAM subunits and linking new NAM subunits to older ones. (c) Structural formulas of some beta-lactam drugs. Their functional portion is the beta-lactam ring. (d) A schematic depiction of the effect of penicillin on peptidoglycan in preventing NAM-NAM cross-links. (e) Bacterial lysis due to the effects of a beta-lactam drug. After a beta-lactam weakens a peptidoglycan molecule by preventing NAM-NAM cross-linkages, what force actually kills an affected bacterial cell? Figure 10.3 Osmotic movement of water into the cell causes cells to lyse. Mycobacteria typically reproduce only every 12 to 24 hours, in part because of the complexity of their cell walls, so antimicrobial agents that act against mycobacteria must be administered for months or even years to be effective. It is often difficult to ensure that patients continue such a long regimen of treatment. M10_BAUM7206_05_SE_C10.indd 290 Inhibition of Synthesis of Fungal Walls Fungal cell walls are composed of various polysaccharides containing a sugar—1,3-d-glucan—that is not found in mammalian cells. A new class of antifungal drugs called echinocandins, among them caspofungin, inhibit the enzyme that synthesizes 11/23/16 3:03 AM       Mechanisms of Antimicrobial Action LE A R N I N G O U TC O M E S 10.6 Describe the actions and give examples of six antimicro- bial drugs that interfere with ribosomes. 10.7 Describe how mupirocin interferes with protein synthesis. ­ ­ Cells use proteins for structure and regulation, as enzymes in metabolism, and as channels and pumps to move materials across cell membranes. Thus, a consistent supply of proteins is vital for the active life of a cell. Given that all cells, including human cells, use ribosomes to translate proteins using information from messenger RNA templates, it is not immediately obvious that drugs could selectively target differences related to protein synthesis. However, there are differences between prokaryotic and eukaryotic ribosomes as well as differences in the enzymes that “load” amino acids onto tRNA molecules. ­ Inhibition of Protein Synthesis the enzymatic site of the 50S subunit (FIGURE 10.4c), which prevents translation. Lincosamides, streptogramins, and macrolides (mak’rō-līds, such as erythromycin) bind to a different portion of the 50S subunit, preventing movement of the ribosome from one codon to the next (FIGURE 10.4d); as a result, translation is frozen, and protein synthesis is halted. Other drugs that block protein synthesis are antisense nucleic acids (FIGURE 10.4e). These RNA or single-stranded DNA molecules are designed to be complementary to specific mRNA molecules of pathogens. They block ribosomal subunits from attaching to that mRNA with no effect on human mRNA. Fomivirsen is the first of this class of drugs to be approved. It inactivates cytomegalovirus and is used to treat eye infections. Oxazolidinones (oks-ā’zō-lĭ-dĭ-nōnz) are antimicrobial drugs that work to stop protein synthesis by blocking initiation of translation (FIGURE 10.4f). Oxazolidinones are used as a last resort in treating infections of Gram-positive bacteria resistant to other antimicrobials, including vancomycin- and methicillin-resistant Staphylococcus aureus (staf’i-lō-kok’ŭs o’rē-ŭs). ­ glucan; without glucan, fungal cells cannot make cell walls, leading to osmotic rupture. M10_BAUM7206_05_SE_C10.indd 291 ­ ­ Prokaryotic ribosomes differ from eukaryotic ribosomes in structure and size: Prokaryotic ribosomes are 70S and composed of 30S and 50S subunits, whereas eukaryotic ribosomes are 80S with 60S and 40S subunits (see Figure 7.15). Many antimicrobial agents take advantage of the differences between ribosomes to selectively target bacterial protein translation without significantly affecting eukaryotes. Note, however, that because some of these drugs affect eukaryotic mitochondria, which also contain 70S ribosomes like those of prokaryotes, such drugs may be harmful to animals and humans, especially very active cells in the liver and bone marrow. Understanding the actions of antimicrobials that inhibit protein synthesis requires an understanding of the process of translation because various parts of ribosomes are the targets of antimicrobial drugs. Both the 30S and 50S subunits of a prokaryotic ribosome play a role in the initiation of protein synthesis, in codon recognition, and in the docking of tRNA– amino acid complexes. Additionally, the 50S subunit contains the enzymatic portion that actually forms peptide bonds (see Figure 7.18). Among the antimicrobials that target the 30S ribosomal subunit are aminoglycosides and tetracyclines. Aminoglycosides (am’i-nō-glī’kō-sīds), such as streptomycin and gentamicin, change the shape of the 30S subunit, making it impossible for the ribosome to read the codons of mRNA correctly (FIGURE 10.4a). Other aminoglycosides and tetracyclines (tet-ră-sī’klēns) block the tRNA docking site (A site), which then prevents the incorporation of additional amino acids into a growing polypeptide (FIGURE 10.4b). Some antimicrobials interfere with the function of the 50S subunit. Chloramphenicol and similar drugs block At the base of each transfer RNA molecule there is an anticodon composed of three nucleotides complementary to the nucleotides of a codon that codes for a specific amino acid (see Figure 7.14). Enzymes called aminoacyl­tRNA synthetases “charge” or “load” amino acids onto tRNA molecules. There is one type of aminoacyl-tRNA synthetase for each amino acid. Specificity of the enzymes ensures that the amino acid loaded at the top of a tRNA molecule (on the acceptor stem) is appropriate and correct for the anticodon at the base of each particular tRNA. A topical drug, mupirocin, selectively binds to isoleucyltRNA synthetase, the enzyme that loads isoleucine in Grampositive bacteria, particularly Staphylococcus and Streptococcus. The drug is selectively toxic because it does not bind to any eukaryotic tRNA molecules. In the presence of the drug, Grampositive cells cannot load isoleucine, so protein synthesis cannot proceed past isoleucine codons; thus, mupirocin cripples a bacterium’s protein production. The World Health Organization (WHO) lists mupirocin as an essential medicine for bacterial skin infections needed in a basic health system. ­ Interference with Prokaryotic Ribosomes Interference with Charging of Transfer RNA Molecules Disruption of Cytoplasmic Membranes LE A R N I N G O UTC O M E 10.8 Describe the action of antimicrobial drugs that interfere with cytoplasmic membranes. Some antibacterial drugs, such as the short polypeptide gram­ icidin, disrupt the cytoplasmic membrane of a targeted cell, often by forming a channel through the membrane, damaging its integrity. This is also the mechanism of action of a group of antifungal and antiprotozoan drugs called polyenes (pol-ēēns’). The polyenes nystatin and amphotericin B (FIGURE 10.5a) 11/23/16 3:03 AM p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t ha       C ◀ FIGURE10.4 Some mechanisms by which antimicrobials target prokaryotic ribosomes to inhibit protein synthesis. (a) Aminoglycosides change Incorrect amino acids the shape of the 30S subunit, causing incorrect pairing of t NA anticodons with m NA codons. (b) Tetracyclines block the V e t NA docking site (A site) on the 30S subunit, preventing protein Tu r elongation. (c) Chloramphenicol blocks enzymatic activity of the 50S subunit, preventing the formation of peptide bonds between amino acids. (d) Lincosamides or macrolides bind to the 50S subunit, preventing movement of the ribosome along the m NA. (e) Antisense nucleic acids bind to m NA, blocking ribosomal subunits. (f) Oxazolidinones inhibit initiation of translation. Which tRNA anticodon should align with the codon CUG? mRNA (a) R Some aminoglycosides, for example streptomycin, cause change in 30S shape, so mRNA is misread. 30S o R to R id R R Ribosome Figure 10.4 Anticodon GAC is the correct complement for the codon CUG. Tetracycline and some aminoglycosides block docking site of tRNA. 50S mRNA (b) Amino acids ­ Chloramphenicol blocks peptide bond formation. are fungicidal because they attach to ergosterol, a lipid constituent of fungal membranes (FIGURE 10.5b), in the process disrupting the membrane and causing lysis of the cell. The cytoplasmic membranes of humans are somewhat susceptible to amphotericin B because they contain cholesterol, which is similar to ergosterol, though cholesterol does not bind amphotericin B as well as does ergosterol. Azoles, such as f luconazole, and allylamines, such as terbinafine, are two other classes of antifungal and antiprotozoan drugs that disrupt cytoplasmic membranes. They act by inhibiting the synthesis of ergosterol; without ergosterol, a fungal cell’s cytoplasmic membrane does not remain intact, and the cell dies. Azoles and allylamines are generally 50S mRNA (c) Lincosamides or macrolides bind to 50S subunit, blocking proper mRNA movement through ribosome. Synthesis stops. H 3C OH O 50S mRNA (d) CH3 CH3 O HO HO OH HO 50S Antisense nucleic acid HO Amphotericin B O mRNA 30S Phospholipid HO O HOOC Ergosterol Pore OH OH O (e) NH2 CH3 HO Amphotericin B fMet (a) 50S (f) M10_BAUM7206_05_SE_C10.indd 292 30S Oxazolidinone mRNA (b) ▲ FIGURE10.5 Disruption of the cytoplasmic membrane by the antifungal amphotericin B. (a) The structure of amphotericin B. (b) The proposed action of amphotericin B. The drug binds to molecules of ergosterol, which then congregate, forming a pore. 11/23/16 3:03 AM       Mechanisms of Antimicrobial Action Inhibition of Metabolic Pathways LE A R N I N G O U TC O M E S 10.9 Explain the action of antimicrobials that disrupt synthe- sis of folic acid. 10.10 Define the term analog as it relates to antimicrobial drugs. 10.11 Describe the action of antiviral drugs that interfere with metabolism. Metabolism can be defined simply as the sum of all chemical reactions that take place within an organism. Whereas most living things share certain metabolic reactions—for example, glycolysis—other chemical reactions are unique to certain organisms. Whenever differences exist between the metabolic processes of a pathogen and its host, antimetabolic agents can be effective. Various kinds of antimetabolic agents are available, including: • Atovaquone, which interferes with electron transport in protozoa and fungi • Heavy metals (such as arsenic, mercury, and antimony), which inactivate enzymes • Agents that disrupt tubulin polymerization and glucose uptake by many protozoa and parasitic worms • Drugs that block the activation of viruses • Metabolic antagonists, such as sulfanilamide, the first commercially available antimicrobial agent Sulfanilamide and similar compounds, collectively called sulfonamides or sulfa drugs, act as antimetabolic drugs because they are structural analogs of—that is, are chemically very similar to—para-aminobenzoic acid (PABA) (FIGURE 10.6a). PABA is crucial in the synthesis of nucleotides required for M10_BAUM7206_05_SE_C10.indd 293 ­ ­ ­ ­ DNA and RNA synthesis. Many organisms, including some pathogens, enzymatically convert PABA into dihydrofolic acid and then dihydrofolic acid into tetrahydrofolic acid (THF), a form of folic acid that is used as a coenzyme in the synthesis of purine and pyrimidine nucleotides (FIGURE 10.6b). As analogs of PABA, sulfonamides compete with PABA molecules for the active site of the enzyme involved in the production of dihydrofolic acid (FIGURE 10.6c). This competition leads to a decrease in the production of THF and thus of DNA and RNA. The end result of sulfonamide competition with PABA is the cessation of cell metabolism, which leads to cell death. Note that humans do not synthesize THF from PABA; instead, we take simple folic acids found in our diets and convert them into THF. As a result, human metabolism is unaffected by sulfonamides. Another antimetabolic agent, trimethoprim, also interferes with nucleic acid synthesis. However, instead of binding to the enzyme that converts PABA to dihydrofolic acid, trimethoprim binds to the enzyme involved in the conversion of dihydrofolic acid to THF, the second step in this metabolic pathway. Some antiviral agents target the unique aspects of the metabolism of viruses. After attachment to a host cell, viruses must penetrate the cell’s membrane and be uncoated to release viral genetic instructions and assume control of the cell’s metabolic machinery. Some viruses of eukaryotes are uncoated as a result of the acidic environment within phagolysosomes (vesicles derived after a lysosome fuses with a phagosome in phagocytosis). Amantadine, rimantadine, and weak organic bases can neutralize the acid of phagolysosomes and thereby prevent viral uncoating; thus, these are antiviral drugs. The Centers for Disease Control and Prevention (CDC) showed in 2008 that amantadine is no longer effective against influenza type A virus, so physicians currently do not prescribe this drug for use against type A influenzaviruses. Protease inhibitors interfere with the action of protease—an enzyme that HIV needs near the end of its replication cycle. These drugs, when used as part of a “cocktail” of drugs including reverse transcriptase inhibitors (discussed shortly), have revolutionized treatment of AIDS patients in industrialized countries. Researchers have reduced the number of pills in the daily cocktail to just a few that contain all the drugs formerly found in 35 or more daily pills. ­ ­ harmless to humans because human cells do not manufacture ergosterol. Most bacterial membranes lack sterols, so these bacteria are naturally resistant to polyenes, azoles, and allylamines; however, there are other agents that disrupt bacterial membranes. An example of these antibacterial agents is polymyxin, produced by Bacillus polymyxa (ba-sil’ŭs po-lē-miks’ă). Polymyxin is effective against Gram-negative bacteria, particularly Pseudomonas (soo-dō-mō’nas), but because it is toxic to human kidneys, it is usually reserved for use against external pathogens that are resistant to other antibacterial drugs. Pyrazinamide disrupts transport across the cytoplasmic membrane of Mycobacterium tuberculosis (too-ber-kyū-lō’sis). The pathogen uniquely activates and accumulates the drug. Unlike many other antimicrobials, pyrazinamide is most effective against intracellular, nonreplicating bacterial cells. Some antiparasitic drugs also act against cytoplasmic membranes. For example, praziquantel and ivermectin change the permeability of cell membranes of several types of parasitic worms. Inhibition of Nucleic Acid Synthesis LE A R N I N G O UTC O M E 10.12 Describe the antimicrobial action of nucleotide and nucleoside analogs, quinolones, drugs that bind to RNA or DNA, and reverse transcriptase inhibitors. The nucleic acids DNA and RNA are built from purine and pyrimidine nucleotides and are critical to the survival of cells. Several drugs function by blocking either the replication of DNA or its transcription into RNA. 11/23/16 3:03 AM t p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs ha       C O H2N O C H 2N OH S O NH2 H2N S O PABA N O Sulfanilamide O H H2N N H S N O O H3C CH3 Sulfamethoxazole O N CH3 Sulfisoxazole (a) Para-aminobenzoic acid (PABA) and some of its structural analogs, the sulfonamides PABA Enzyme Dihydrofolic acid Enzyme Tetrahydrofolic acid (THF) Enzymes PABA O C NH2 Purine and pyrimidine nucleotides OH Dihydrofolic acid Enzymes DNA and RNA NH2 O S O Dihydrofolic acid not produced NH2 Active site Enzyme Enzyme (b) Role of PABA in folic acid synthesis in bacteria and protozoa (c) Inhibition of folic acid synthesis by sulfonamide P P P ▲ FIGURE10.6 The antimetabolic action of sulfonamides in inhibiting nucleic acid synthesis. (a) ara-aminobenzoic acid ( ABA) and representative members of its structural analogs, the sulfonamides. The analogous portions of the compounds are shaded. (b) The metabolic pathway in bacteria and protozoa by which folic acid is synthesized from ABA. (c) The inhibition of folic acid synthesis by the presence of a sulfonamide, which deactivates the enzyme by binding irreversibly to the enzyme’s active site. 2Nucleotides are composed of a pentose sugar, phosphate, and a nitrogenous base; nucleosides lack the phosphate. M10_BAUM7206_05_SE_C10.indd 294 ­ analogs or nucleoside analogs, which are molecules with structural similarities to the normal nucleotide building blocks of nucleic acids (FIGURE 10.7 ). The structures of certain nucleotide or nucleoside analogs, such as the anti-AIDS drug AZT, enable them to be incorporated into the DNA or RNA of pathogens, where they distort the shapes of the nucleic acid molecules and prevent further replication, transcription, or translation. Nucleotide and nucleoside analogs are most often used against viruses because viral DNA polymerases are tens to hundreds of times more likely to incorporate these nonfunctional nucleotides into nucleic acids than is human DNA polymerase. Additionally, complete viral nucleic acid synthesis is more rapid than cellular nucleic acid synthesis. These characteristics make viruses more susceptible to nucleotide analogs than their hosts are, though nucleotide analogs can also be effective against rapidly dividing cancer cells. Other antimicrobial agents function by binding to and inhibiting the action of RNA polymerases during the synthesis of RNA from a DNA template. Several drugs, including rifampin (rif’am-pin), bind more readily to prokaryotic RNA polymerase than to eukaryotic RNA polymerase; as a result, rifampin is more toxic to prokaryotes than to eukaryotes. Clofazimine binds ­ Because only slight differences exist between the DNA of prokaryotes and eukaryotes, drugs that affect DNA replication often act against both types of cells. For example, actinomycin binds to DNA and effectively blocks DNA synthesis and RNA transcription not only in bacterial pathogens but in their hosts as well. Generally, drugs of this kind are not used to treat infections, though they are used in research of DNA replication and may be used carefully to slow replication of cancer cells. One exception is the synthetic drugs called quinolones, commonly fluoroquinolones, which contain fluorine. Quinolones inhibit DNA gyrase, the bacterial enzyme necessary for correct coiling and uncoiling of replicating bacterial DNA; they typically have little effect on eukaryotes or viruses, though they may act against replication of mitochondrial DNA in some eukaryotes. Recall that mitochondrial DNA is similar to bacterial DNA. Other compounds that can act as antimicrobials by interfering with the function of nucleic acids are nucleotide 2 11/23/16 3:03 AM       Mechanisms of Antimicrobial Action NH2 N N O O O O O O O O O N N N N O HO N N O HO Valaciclovir O N N N N N O HO N HO N OH O HO O N O O CH3 O NUCLEOSIDES N N O HO N O N O OH Thymidine Ganciclovir N N O HO O N O HO SH Cytidine Dideoxycytidine (ddC) Lamivudine O I CF3 N ▲ FIGURE10.7 Nucleosides and some of their antimicrobial analogs. The arrows indicate synthesis pathways. ­ ­ N N O OH Azidothymidine (AZT) Acyclovir (ACV) N N O HO N3 Stavudine (d4T) Guanosine CH3 N N O HO OH Adenosine O N O OH Adenosine arabinoside CH3 N N HO O O N N HO Adefovir O O N Penciclovir N N N O N N N O HO N N O OH OH N N N N O N N N O N Ribavirin NH2 NH2 N P N HO Dideoxyinosine (ddI) NH2 N HO HO Tenofovir O O N N CH3 O N N N O HO O O C N N O H2N N N N N P O O O N O HO (For simplicity, the nucleotides and analogs are shown without phosphate groups; that is, they are depicted as nucleosides.) How do nucleoside analogs interfere with DNA replication and RNA transcription? ­ HO O N ­ O OH Trifluridine to bacterial DNA and evidently prevents normal transcription (and DNA replication). Rifampin and clofazimine are used primarily against species of mycobacteria such as M. tuberculosis and M. leprae (lep’rī), the causative agents of tuberculosis and leprosy, respectively. Pentamidine and propamidine isethionate bind to protozoan DNA, inhibiting DNA replication and RNA transcription. The result is that the protozoa cannot reproduce or metabolize. Reverse transcriptase inhibitors, such as emtricitabine—a part of AIDS cocktails—act against reverse transcriptase, which is an enzyme HIV uses early in its replication cycle to make DNA copies of its RNA genome (reverse transcription). Since people lack reverse transcriptase, the inhibitor does not harm patients. M10_BAUM7206_05_SE_C10.indd 295 Figure 10.7 Because of the distortions they cause in nucleic acids, nucleoside (and nucleotide) analogs do not form proper base pairs with normal nucleotides. This increases the number of mismatches in the transcription of RNA and replication of DNA. OH Iododeoxyuridine Prevention of Virus Attachment, Entry, or Uncoating LE A R N I N G O UTC O M E 10.13 Describe the action of antimicrobial attachment antago- nists and of drugs that inhibit viral entry. Many pathogens, particularly viruses, attach to their host’s cells via the chemical interaction between attachment proteins on the pathogen and complementary receptor proteins on a host cell. Attachment of viruses can be blocked by peptide and sugar analogs of either attachment or receptor proteins. 11/23/16 3:03 AM p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t ha       C ­ When analogs block these sites, viruses can neither attach to nor enter their host’s cells. The use of such substances, called attach­ ment antagonists, is an exciting new area of antimicrobial drug development. Pleconaril is a synthetic antagonist of the receptors of picornaviruses such as some cold viruses, polioviruses, and coxsackievirus. The drug blocks viral attachment and deters infections. Arildone is a synthetic antiviral that prevents removal of poliovirus capsids (uncoating), thereby interrupting the viral replication cycle. TELL ME WHY No agent has all of these qualities, so doctors and medical laboratory technicians must evaluate antimicrobials with respect to several characteristics: the types of pathogens against which they are effective; their effectiveness, including the dosages required to be effective; the routes by which they can be administered; their overall safety; and any side effects they produce. We consider each of these characteristics of antimicrobials in the following sections. Spectrum of Action LE A R N I N G O UTC O M E Some antimicrobial drugs are harmful to humans. Why can physicians safely use such drugs despite the potential danger? 10.14 Distinguish narrow-spectrum drugs from broad-spec- ­ trum drugs in terms of their targets and side effects. • Readily available • Inexpensive • Chemically stable (so that it can be transported easily and stored for long periods of time) • Easily administered • Nontoxic and nonallergenic • Selectively toxic against a wide range of pathogens ­ ­ Even though some fungi and bacteria commonly produce antibiotics, most of these chemicals are not effective for treating diseases because they are toxic to humans and animals, are too expensive, are produced in minute quantities, or lack adequate potency. The ideal antimicrobial agent to treat an infection or disease would be one that has all these characteristics: ­ Clinical Considerations in Prescribing Antimicrobial Drugs The number of different kinds of pathogens a drug acts against is known as its spectrum of action (FIGURE 10.8); drugs that work against only a few kinds of pathogens are narrow-spectrum drugs, whereas those that are effective against many different kinds of pathogens are broad-spectrum drugs. For instance, because tetracycline acts against many different kinds of bacteria, including Gram-negatives, Gram-positives, chlamydias, and rickettsias, it is considered a broad-spectrum antibiotic. In contrast, penicillin cannot easily penetrate the outer membrane of a Gram-negative bacterium to reach and prevent the formation of peptidoglycan, so its efficacy is limited largely to Gram-positive bacteria. Thus, penicillin has a narrower spectrum of action than tetracycline. The use of broad-spectrum antimicrobials is not always as desirable as it might seem. Broad-spectrum antimicrobials can also open the door to serious secondary infections by transient pathogens or superinfections by members of the normal microbiota unaffected by the antimicrobial. This results because the killing of normal microbiota reduces microbial antagonism, the competition between normal microbes and pathogens The Spectrum of Activity of Selected Antimicrobial Drugs Prokaryotes Mycobacteria Gram-negative bacteria Eukaryotes Gram-positive bacteria Chlamydias, rickettsias Protozoa Fungi Isoniazid Viruses Helminths Niclosamide Polymyxin Azoles Penicillin Arildone Ribavirin Praziquantel Acyclovir Streptomycin Erythromycin Tetracycline Sulfonamides ▲ FIGURE10.8 Spectrum of action for selected antimicrobial agents. The more kinds of pathogens a drug affects, the broader its spectrum of action. M10_BAUM7206_05_SE_C10.indd 296 11/23/16 3:03 AM Antibiotic Overkill for nutrients and space. Microbial antagonism reinforces the body’s defense by limiting the ability of pathogens to colonize the skin and mucous membranes. Thus, a woman using erythromycin to treat strep throat (a bacterial disease) could develop vaginitis resulting from the excessive growth of Candida albicans (kan’did-ă al’bi-kanz), a yeast that is unaffected by erythromycin and is freed from microbial antagonism when the antibiotic kills normal bacteria in the vagina. Effectiveness R applications, she was unable to completely remove the film. Furthermore, she had excessive vaginal discharges consisting of a cheeselike white substance. When she began to have vaginal itching, she finally decided it was time to seek help. eluctantly, she revisited her personal physician and described the symptoms. Her doctor explained the symptoms and provided additional prescriptions to alleviate her distress. 1. What happened to the young woman in this situation? 2. How had her body’s defenses been violated? 3. How can she avoid a repeat of this situation? If all antimicrobials diffused at the same rate, then a larger zone of inhibition would indicate a more effective drug. However, drugs with low molecular weights generally diffuse more quickly and so might have a larger zone of inhibition than a more effective but larger molecule. The size of a zone of inhibition must be compared to a standard table for that particular drug before accurate comparisons can be made. Diffusion susceptibility tests enable scientists to classify pathogens as susceptible, intermediate, or resistant to each drug. ­ ­ ­ ­ ­ ­ A young woman was taking antibiotic pills for a urinary infection. Several days into her course of medication, she began to experience peculiar symptoms. At first, they were hardly noticeable. Very quickly, however, they worsened and became embarrassing and unbearable. She noticed a white coating on her tongue, bad breath, and an awful taste in her mouth. Despite persistent brushing and mouthwash ­ CLINICAL CASE STUDY       P Clinical Considerations in rescribing Antimicrobial Drugs Bacterial lawn Zone of inhibition LE A R N I N G O U TC O M E 10.15 Compare and contrast diffusion susceptibility tests, Etest, MIC test, and MBC test. To effectively treat infectious diseases, physicians must know which antimicrobial agent is most effective against a particular pathogen. To ascertain the efficacy of antimicrobials, microbiologists conduct a variety of tests, including diffusion susceptibility tests, the minimum inhibitory concentration test, and the minimum bactericidal concentration test. Diffusion Susceptibility Test M10_BAUM7206_05_SE_C10.indd 297 ▲ FIGURE10.9 Zones of inhibition in a diffusion susceptibility (Kirby-Bauer) test. Disks are impregnated with an antimicrobial agent. In general, the larger the zone of inhibition around the disks, the more effective that antimicrobial is against the organism growing on the plate. The organism is classified as either susceptible, intermediate, or resistant to the antimicrobials tested based on the sizes of the zones of inhibition. If all of these antimicrobial agents diffuse at the same rate and are equally safe and easily administered, which one would be the drug of choice for killing this pathogen? Figure 10.9 The drug in the uppermost disk. Diffusion susceptibility tests, also known as Kirby-Bauer tests, involve uniformly inoculating a Petri plate with a standardized amount of the pathogen in question. Then small disks of paper containing standard concentrations of the drugs to be tested are firmly arranged on the surface of the plate. The plate is incubated, and the bacteria grow and reproduce to form a “lawn” everywhere except the areas where effective antimicrobial drugs diffuse through the agar. After incubation, the plates are examined for the presence of a zone of inhibition—that is, a clear area where bacteria do not grow (FIGURE 10.9). A zone of inhibition is measured as the diameter (to the closest millimeter) of the clear region. 11/23/16 3:03 AM p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t ha       C Turbid wells Clear wells Increasing concentration of drug test in wells. Minimum Inhibitory Concentration (MIC) Test gradient of the antimicrobial agent of interest. The MIC is estimated to be the concentration printed on the strip where the zone of inhibition intersects the strip. In this example, the MIC is 0.75 mg/ml. samples taken from clear MIC tubes (or, alternatively, from zones of inhibition from a series of diffusion susceptibility tests) are transferred to plates containing a drug-free growth medium (FIGURE 10.12). The appearance of bacterial growth in these subcultures after appropriate incubation indicates that at least some bacterial cells survived that concentration of the Concentration of antibacterial drug (μg/ml) ­ Once scientists identify an effective antimicrobial agent, they quantitatively express its potency as a minimum inhibitory concentration (MIC), often using the unit mg/ml. As the name suggests, the MIC is the smallest amount of the drug that will inhibit growth and reproduction of the pathogen. The MIC can be determined via a broth dilution test, in which a standardized amount of bacteria is added to serial dilutions of antimicrobial agents in tubes or wells containing broth. After incubation, turbidity (cloudiness) indicates bacterial growth; lack of turbidity indicates that the bacteria were either inhibited or killed by the antimicrobial agent (FIGURE 10.10). Dilution tests can be conducted simultaneously in wells, and the entire process can be automated, with turbidity measured by special scanners connected to computers. Another test that determines minimum inhibitory concentration combines aspects of an MIC test and a diffusion susceptibility test. This test, called an Etest,3 involves placing a plastic strip containing a gradient of the antimicrobial agent being tested on a plate uniformly inoculated with the organism of interest (FIGURE 10.11). After incubation, an elliptical zone of inhibition indicates antimicrobial activity, and the minimum inhibitory concentration can be noted where the zone of inhibition intersects a scale printed on the strip. ­ ▲ FIGURE10.10 Minimum inhibitory concentration (MIC) ▲ FIGURE10.11 An Etest, which combines aspects of Kirby-Bauer and MIC tests. The plastic strip contains a ­ Clear MIC tube Minimum Bactericidal Concentration (MBC) Test Similar to the MIC test is a minimum bactericidal concentration (MBC) test, though an MBC test determines the amount of drug required to kill the microbe rather than just the amount to inhibit it, as the MIC does. In an MBC test, 3The name Etest has no specific origin. M10_BAUM7206_05_SE_C10.indd 298 8 μg/ml Bacterial colonies 16 μg/ml No colonies 25 μg/ml No colonies Drug-free media ▲ FIGURE10.12 A minimum bactericidal concentration (MBC) test. In this test, plates containing a drug-free growth medium are inoculated with samples taken from zones of inhibition or from clear MIC tubes. After incubation, growth of bacterial colonies on a plate indicates that the concentration of antimicrobial drug (in this case, 8 mg/ml) is bacteriostatic. The lowest concentration for which no bacterial growth occurs on the plate is the minimum bactericidal concentration; in this case, the MBC is 16 mg/ml. 11/23/16 3:03 AM Battling the Enemy Like his father and grandfather before him, Ben is a Marine 1. Lactobacillus reuteri. sergeant and proud to serve his country. He was nearing the end of his second tour of duty in Afghanistan and was looking forward to returning home to his wife and two sons. However, two weeks before he was scheduled to leave, Ben was seriously wounded in a suicide bomber’s attack and almost lost his leg. He initially responded well to treatment, but three days into his recovery, his wound was obviously ­  ­ ­ ­ antimicrobial drug and were able to grow and multiply once placed in a drug-free medium. Any drug concentration at which growth occurs in subculture is bacteriostatic, not bactericidal, for that bacterium. The lowest concentration of drug for which no growth occurs in the subcultures is the minimum bactericidal concentration (MBC). Routes of Administration infected and getting worse. His doctors needed to act fast. They prescribed cephalexin, which is a semisynthetic cephalosporin. The nurse swabbed the wound and sent a sample to the lab for a diffusion susceptibility test. 1. What does the term semisynthetic mean in reference to an antimicrobial drug? 2. What is the active site of the drug cephalexin called? 3. If the bacterium proves to be resistant to cephalexin, resistance is likely due to what enzyme? 4. What is another name for a diffusion susceptibility test? ­ CLINICAL CASE STUDY       P Clinical Considerations in rescribing Antimicrobial Drugs for example, cilastatin inhibits a kidney enzyme that would destroy imipenem—a type of beta-lactam. In addition to considering the route of administration, physicians must consider how the blood distributes the antimicrobial agents to infected tissues. For example, an agent removed rapidly from the blood by the kidneys might be the drug of choice for a bladder infection but would not be chosen to treat an infection of the heart. Finally, given that blood vessels in the brain, spinal cord, and eye are almost impermeable to many LE A R N I N G O U TC O M E 10.16 Discuss the advantages and disadvantages of the differ- Administration method Oral ­ For an antimicrobial agent to be effective, an adequate amount of it must reach a site of infection. For external infections such as athlete’s foot, drugs can be applied directly. This is known as topical or local administration. For internal infections, drugs can be administered orally, intramuscularly (IM), or intravenously (IV). Each route has advantages and disadvantages. Even though the oral route is simplest (it requires no needles and is self-administered), the drug concentrations achieved in the body are lower than occur via other routes of administration (FIGURE 10.13). Further, because patients administer the drug themselves, they do not always follow prescribed timetables. IM administration via a hypodermic needle allows a drug to diffuse slowly into the many blood vessels within muscle tissue, but the concentration of the drug in the blood is never as high as that achieved by IV administration, which delivers the drug directly into the bloodstream through either a needle or a catheter (a plastic or rubber tube). The amount of a drug in the blood is initially very high for the IV route, but unless the drug is continuously administered, the concentration can rapidly diminish as the liver and kidneys remove the drug from the circulation. Physicians can administer nonantimicrobial chemicals that prolong an antimicrobial’s life span in the body; Relative concentration of drug in blood ent routes of administration of antimicrobial drugs. M10_BAUM7206_05_SE_C10.indd 299 Intramuscular and one-time intravenous Continuous intravenous 1 Time (hours) 2 ▲ FIGURE10.13 The effect of route of administration on blood levels of a chemotherapeutic agent. Although ­ intravenous (IV) and intramuscular (IM) administration achieve higher drug concentrations in the blood, oral administration has the advantage of simplicity. 11/23/16 3:03 AM A       Chapter 10    Controlling Microbial Growth in the Body: ntimicrobial Drugs antimicrobial agents (because the tight structure of capillary walls in these structures creates the blood-brain barrier), infections there are often difficult to treat. drug’s therapeutic window, which is the range of concentrations of the drug that are effective without being excessively toxic. Safety and Side Effects Allergies LE A R N I N G O U TC O M E S 10.17 Identify three main categories of side effects of antimi- crobial therapy. 10.18 Define therapeutic index and therapeutic range. Another aspect of chemotherapy that physicians must consider is the possibility of adverse side effects. These fall into three main categories—toxicity, allergies, and disruption of normal microbiota. Toxicity Though antimicrobial drugs are ideally selectively toxic against microbes and harmless to humans, many antimicrobials in fact have toxic side effects. The exact cause of many adverse reactions is poorly understood, but drugs may be toxic to the kidneys, the liver, or nerves. For example, polymyxin and aminoglycosides can have fatally toxic effects on kidneys. Not all toxic side effects are so serious. Metronidazole (Flagyl), a drug effective against a variety of anaerobic protozoa and bacteria, may cause a harmless temporary condition called “black hairy tongue,” which results when the breakdown products of hemoglobin accumulate in the papillae of the tongue (FIGURE 10.14a). Doctors must be especially careful when prescribing drugs for pregnant women because many drugs that are safe for adults can have adverse affects when absorbed by a fetus. For instance, tetracyclines form complexes with calcium that can become incorporated into bones and developing teeth, causing malformation of the skull and stained, weakened tooth enamel (FIGURE 10.14b). Researchers are able to estimate the safety of an antimicrobial drug by calculating the drug’s therapeutic index (TI), which is essentially a ratio comparing the largest dose of the drug that is not toxic to the drug’s smallest effective dose. The higher the TI, the safer the drug. Clinicians refer to a In addition to toxicity, some drugs trigger allergic immune responses in sensitive patients. Although relatively rare, such reactions may be life threatening, especially in an immediate, violent reaction called anaphylactic shock. For example, about 0.1% of Americans have an anaphylactic reaction to penicillin, resulting in approximately 300 deaths per year. However, not every allergy to an antimicrobial agent is so serious. Recent studies indicate that patients with mild allergies to penicillin frequently lose their sensitivity to it over time. Thus, an initial mild reaction to penicillin need not preclude its use in treating future infections. (Chapter 18 discusses allergies in more detail.) Disruption of Normal Microbiota Drugs that disrupt the normal microbiota and their microbial antagonism may allow opportunistic pathogens to proliferate as secondary infections. In some cases, when a member of the normal microbiota is not affected by a drug, it can become an opportunistic pathogen and can overgrow, causing a disease. For example, long-term use of broad-spectrum antibacterials often results in explosions in the growth rate of Candida albicans in the vagina (vaginitis) or mouth (thrush) and the multiplication of Clostridium difficile (klos-trid’ē-ŭm di’fe-sēl) in the colon, causing a potentially fatal condition called pseudomembranous colitis. Such opportunistic pathogens are of great concern for hospitalized patients, who are often not only debilitated and susceptible to opportunistic infections but are also more likely to be exposed to other pathogens with resistance to antimicrobial drugs—the topic of the next section. Beneficial Microbes: Probiotics: Using Live Microorganisms to Treat or Prevent Disease focuses on how normal microbiota may help keep pathogens in check. TELL ME WHY Why don’t physicians invariably prescribe the antimicrobial with the largest zone of inhibition? ­ ◀ FIGURE10.14 Some side effects resulting from toxicity of antimicrobial agents. (a) “Black hairy ­ tongue,” caused by the antiprotozoan drug metronidazole (Flagyl). (b) Discoloration and damage to tooth enamel caused by tetracycline. Who should avoid taking tetracycline? Figure 10.14 Pregnant women and children should not use tetracycline. (a) M10_BAUM7206_05_SE_C10.indd 300 (b) 06/03/2018 10:02       R esistance to Antimicrobial Drugs Resistance to Antimicrobial Drugs The process by which a bacterial strain develops resistance is depicted in FIGURE 10.15. In the absence of an antimicrobial drug, resistant cells are usually less efficient than their normal neighbors because they must expend extra energy to maintain resistance genes and proteins. Under these circumstances, resistant individuals remain the minority in a population because they reproduce more slowly. However, when an antimicrobial agent is present, the majority of microbes, which are sensitive to the antimicrobial, are inhibited or die while the resistant microbes continue to grow and multiply, often more rapidly because they then face less competition. The result is that resistant organisms soon replace the sensitive ones as the majority in the population. The microbe has evolved resistance. It should be noted that the presence of an antimicrobial agent does not produce resistance but instead selects for the replication of resistant microbes that were already present in the population. ▶ ANIMATION: Antibiotic Resistance: Origins of ­ ­ ­ Among the major challenges facing microbiologists today are the problems presented by pathogens that are resistant to antimicrobial agents (see Emerging Disease Case Study: Community-Associated MRSA on p. 302). In the sections that follow, we examine the development of resistant populations of pathogens, the mechanisms by which pathogens are resistant to antimicrobials, and some ways that resistance can be retarded. The Development of Resistance in Populations LE A R N I N G O U TC O M E S 10.19 Describe how populations of resistant microbes can arise. Resistance 10.20 Describe the relationship between R plasmids and resis- tant cells. Mechanisms of Resistance Not all pathogens are equally sensitive to a given therapeutic agent; a population may contain a few individuals that are naturally either partially or completely resistant. Among bacteria, individual cells can acquire such resistance in two ways: through new mutations of chromosomal genes or by acquiring resistance genes on extrachromosomal pieces of DNA called R plasmids (or R factors) via the processes of horizontal gene transfer—transformation, transduction, or conjugation. We focus here on resistance in populations of bacteria, but resistance is known to occur among other microbes as well, including protozoa, fungi, and viruses. LE A R N I N G O UTC O M E S 10.21 List seven ways by which microorganisms can be resis- tant to antimicrobial drugs. 10.22 List two ways that genes for drug resistance are spread between bacteria. The problem of resistance to antimicrobial drugs is a major health threat to our world. An “alphabet soup” of resistant pathogens and diseases plague health care professionals: E R E E B N FICIAL MIC OB S Probiotics: Using Live Microorganisms to Treat or Prevent Disease Antibiotics are powerful tools in modern medicine that kill or inhibit the growth of microbes. The general concept behind antibiotics is to remove pathogenic microbes from a patient’s body, or at least inhibit them, so that the immune system can do its job. As scientists have learned more about the microbial ecology of the human body, they have discovered that some diseases arise from an imbalance between “good” microbes and “bad” ones. Probiotics is the concept that it is sometimes better to introduce beneficial microbes to resolve or prevent a disease rather than kill pathogens after they attack. One advantage of this proactive approach is that it mitigates the development of antibiotic resistance. Lactobacillus is a well-studied bacterium that can act as a probiotic. Lactobacilli are the predominant organisms in yogurt and also live naturally in the human gut and vagina. esearchers have known for a while that Lactobacillus can help reduce the frequency and duration of diarrhea, relieve milk allergies, and possibly treat 1 µm R SEM M10_BAUM7206_05_SE_C10.indd 301 inflammatory bowel diseases. Now, they are beginning to understand how this probiotic bacterium might accomplish its beneficial tasks. Some strains of Lactobacillus convert the amino acid histidine into histamine, which can then bind to histamine receptors on the surface epithelial cells that line the human gut. This dampens inflammation and may be one of the major ways that our normal bacteria protect themselves from a strong immune response so as to persist in the gut. Further, some lactobacilli can synthesize and secrete an antimicrobial compound called reuterin, which kills many pathogens, including E. coli O157:H7 and Listeria monocytogenes, but which doesn’t harm most bacteria that normally live in humans, including Lactobacillus. Thus, reuterin allows lactobacilli to “protect their own turf,” which is our gut. We benefit from this effect. Continuing research may result in wider use of probiotics to promote human health. 11/23/16 3:03 AM U E e CAS ST DY as GING D e is M ER t p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs E ha       C Community-Associated MRSA R Drug-sensitive cells R R R ­ Jason’s pain was so severe that he wondered only when he might die to be free of the agony. For years, health care workers have battled healthcareassociated M SA (HA-M SA), which commonly afflicts patients in hospitals. Now, researchers are concerned that victims who have never been in a hospital are succumbing; M SA has escaped hospitals and travels communities worldwide, including among athletes, students in middle schools, and customers of unsafe tattooists. The bacterium can be spread between individuals who share fomites—towels, razors, clothing, or sheets. hysicians drained Julie’s lesions and prescribed oral antimicrobials, including trimethoprim-sulfamethoxazole, doxycycline, and clindamycin. Jason received intravenous vancomycin. Both eventually recovered, but her tattoo is now a reminder of a terrible experience and not a happy birthday. (For more about staphylococcal infections, see Chapter 19.) P R ­ ­ Julie was proud and excited about her new tattoo; it was cool! Jason, her boyfriend, had bought it as a gift for her 18th birthday, and she loved the way it covered her arm. A week later something else covered her arm . . . and his leg. Julie and Jason had fallen victim to an emerging disease— community-associated, methicillin-resistant Staphylococcus aureus (CA-M SA) infection. Julie’s arm was covered with red, swollen, painful, pus-containing lesions, and she had a fever. Jason thought he had a spider bite on Wednesday, but by Thursday morning a bright red line extended from his ankle to his groin. He could barely walk, and then his condition worsened. By the weekend, he was hospitalized, and his fever hit 107°F during his 10 days there. M SA had entered his blood (bacteremia) and infected his bones (osteomyelitis). 1. How were Julie and then Jason likely infected? 2. Why is CA-MRSA considered an “emerging disease,” given that MRSA has been around for years? 3. Why doesn’t the map show more countries in Africa and South America with cases of CA-MRSA? Drug-resistant mutant Exposure to drug (a) Population of microbial cells Remaining population grows over time (b) Sensitive cells inhibited by exposure to drug (c) Most cells now resistant E E ▲ FIGURE10.15 The development of a resistant strain of bacteria. (a) A bacterial population contains both drug-sensitive and drug-resistant cells, although sensitive cells constitute the vast majority of the population. (b) xposure to an antimicrobial drug inhibits the sensitive cells; as long as the drug is present, reduced competition from sensitive cells facilitates the multiplication of resistant cells. (c) ventually, resistant cells constitute the majority of the population. Why do resistant strains of bacteria more often develop in hospitals and nursing homes than in college dormitories? Figure 10.15 Resistant strains are more likely to develop in hospitals and other health care facilities because the extensive use of antimicrobial agents in those places inhibits the growth of sensitive strains and selects for the growth of resistant strains. M10_BAUM7206_05_SE_C10.indd 302 11/23/16 3:03 AM       R esistance to Antimicrobial Drugs Lactam ring H O H C C N H O Penicillin S CH3 CH3 N COOH d-lactamase (penicillinase) breaks this bond H O H C C N C C O C N H S OH H Inactive penicillin CH3 CH3 COOH ▲ FIGURE10.16 How beta-lactamase (penicillinase) renders penicillin inactive. The enzyme acts by breaking a bond in the lactam ring, the functional portion of the drug. ­ MRSA,4 VRSA,5 VISA,6 VRE,7 MDR-TB,8 and XDR-TB9 are just some of these. How do microbes gain resistance to antimicrobial drugs? Consider the path a typical antimicrobial drug must take to affect a microbe: The drug must cross the cell’s wall, then cross the cytoplasmic membrane to enter the cell; there the antimicrobial binds to its target (receptor) molecule. Only then can it inhibit or kill the microbe. Microbes gain resistance by blocking some point in this pathway. There are at least seven mechanisms of resistance: ­ ­ • Resistant cells may produce an enzyme that destroys or deactivates the drug. This common mode of resistance is exemplified by beta (ß)-lactamases (penicillinases), which are enzymes that break the beta-lactam rings of penicillin and similar molecules, rendering them inactive (FIGURE 10.16). Many MRSA strains have evolved resistance to penicillin-derived antimicrobials in this manner. Over 200 different lactamases have been identified. Frequently their genes are located on R plasmids. • Resistant microbes may slow or prevent the entry of the drug into the cell. This mechanism typically involves changes in the structure or electrical charge of the cytoplasmic membrane proteins that constitute channels or pores. Such proteins in the outer membranes of Gramnegative bacteria are called porins (see Figure 3.15b). Altered pore proteins result from mutations in chromosomal genes. Resistance against tetracycline and penicillin are known to occur via this mechanism. • Resistant cells may alter the target of the drug so that the drug either cannot attach to it or binds it less effectively. This form of resistance is often seen against antimetabolites (such as sulfonamides) and against drugs that thwart protein translation (such as erythromycin). • Resistant cells may alter their metabolic chemistry, or they may abandon the sensitive metabolic step altogether. For example, a cell may become resistant to a drug by S. aureus. S. aureus. 6S. aureus with intermediate level of resistance to vancomycin. 7 Vancomycin-resistant enterococci. 8Multi-drug-resistant tuberculosis. 9Extensively drug-resistant tuberculosis. 4Methicillin-resistant 5Vancomycin-resistant M10_BAUM7206_05_SE_C10.indd 303 producing more enzyme molecules for the affected metabolic pathway, effectively reducing the power of the drug. Alternatively, cells become resistant to sulfonamides by abandoning the synthesis of folic acid, absorbing it from the environment instead. • Resistant cells may pump the antimicrobial out of the cell before the drug can act. So-called efflux pumps, which are typically powered by ATP, are often able to pump more than one type of antimicrobial from a cell. Some microbes become multi-drug resistant (perhaps to as many as 10 or more drugs) by utilizing resistance pumps. • Bacteria within biofilms resist antimicrobials more effectively than free-living cells. Biofilms retard diffusion of the drugs and often slow metabolic rates of species making up the biofilm. Lower metabolic rates reduce the effectiveness of antimetabolic drugs. • Some resistant strains of the bacterium Mycobacterium tuberculosis have a novel method of resistance against fluoroquinolone drugs that bind to DNA gyrase. These strains synthesize an unusual protein that forms a negatively charged, rodlike helix about the width of a DNA molecule. This protein, called MfpA, binds to DNA gyrase in place of DNA, depriving fluoroquinolone of its target site. DNA competes with and replaces MfpA in binding to DNA gyrase, allowing gyrase to function normally. This is the first known method of antibiotic resistance that involves a decoy to protect the target of an antimicrobial drug rather than, say, changing the target or deactivating the drug. MfpA protein probably slows down cellular division of M. tuberculosis, but that is better for the bacterium than being killed. ▶ ANIMATION: Antibiotic Resistance: Forms of Resistance Despite decades of research, scientists and health care workers still do not fully comprehend what makes certain species, such as Staphylococcus aureus, more prone to developing resistance; how and with what frequency resistance to antimicrobials spreads through a population; or whether there are effective ways to limit proliferation of resistance. It has been shown that horizontal gene transfer, most often by conjugation and less often by transformation, accounts for the spread of antimicrobial resistance among densely growing cells of biofilms. Often, several resistance genes travel together between cells. 11/23/16 3:03 AM p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t ha       C Multiple Resistance and Cross Resistance Disk with semisynthetic amoxicillin-clavulanic acid Disk with semisynthetic aztreonam LE A R N I N G O U TC O M E 10.23 Define cross resistance, and distinguish it from multiple resistance. LE A R N I N G O U TC O M E 10.24 Describe four ways to retard development of resistance. ­ ­ The development of resistant populations of pathogens can be averted in at least four ways. The first way is to maintain sufficiently high concentrations of the drug in a patient’s body for a long enough time to inhibit the pathogens, allowing the body’s defenses to defeat them. Discontinuing a drug too early promotes the development of resistant strains. For this reason, it is important that patients finish their entire antimicrobial prescription and resist the temptation to “save some for another day.” A second way to avert resistance is to use antimicrobial agents in combination so that pathogens resistant to one drug will be killed by other drugs, and vice versa. Additionally, one drug sometimes enhances the effect of a second drug in a process called synergism (sin’er-jizm) (FIGURE 10.17 ). For example, the inhibition of cell wall formation by penicillin makes it easier for streptomycin molecules to enter bacteria and interfere with protein synthesis. Synergism can also result from combining an antimicrobial drug and a nonantimicrobial chemical, as occurs when clavulanic acid enhances the effect of penicillin by deactivating b@lactamase. (Not all M10_BAUM7206_05_SE_C10.indd 304 green represents the synergistic enhancement of antimicrobial activity beyond the activities of the individual drugs (outlined in white). What does clavulanic acid do? ­ drugs act synergistically; some combinations of drugs can be antagonistic—interfering with each other. For example, drugs that slow bacterial growth are antagonistic to the action of penicillin, which acts only against growing and dividing cells.) A third way to reduce the development of resistance is to limit the use of antimicrobials to necessary cases. Unfortunately, many antimicrobial agents are used indiscriminately, both in developed countries and in less developed regions, where many agents are available without a physician’s prescription. For example, antibacterial drugs have no effect on cold and flu viruses; therefore, 100% of antibacterial prescriptions for treating these diseases are superfluous. The use of antimicrobial agents encourages the reproduction of resistant bacteria by limiting the growth of sensitive cells; therefore, inappropriate use of such drugs increases the likelihood that resistant strains of bacteria will multiply. Finally, scientists can combat resistant strains by developing semisynthetic drugs, perhaps by adding different side chains to the original molecule. In this way, scientists develop second-generation drugs. If resistance develops to these drugs, third-generation drugs may be developed to replace them. Alternatively, scientists search for new antimicrobials. Researchers explore diverse habitats, such as peat bogs, ocean sediments, garden soil, and people’s mouths, for organisms that produce novel antibiotics. Researchers see potential in bacteriocins—antibacterial proteins coded by bacterial plasmids. Bacteria use bacteriocins to inhibit other bacterial strains; now we can do the same. Tinkering with these and other antibiotics is yielding promising new semisynthetics, though the cost of bringing a new antimicrobial into production can be ­ Retarding Resistance ▲ FIGURE10.17 An example of synergism between two antimicrobial agents. The portion of the zone of inhibition outlined in Figure 10.17 Clavulanic acid deactivates b@lactamase, allowing penicillins to work. A given pathogen can acquire resistance to more than one drug at a time, especially when resistance is conferred by R plasmids, which are exchanged readily among bacterial cells. Such multiresistant strains of bacteria frequently develop in hospitals and nursing homes, where the constant use of many kinds of antimicrobial agents eliminates sensitive cells and encourages the development of resistant strains. Multiple-drug-resistant pathogens (erroneously called superbugs in the popular press) are resistant to three or more different types of antimicrobial agents. Multiple-drug-resistant strains of Staphylococcus, Streptococcus (strep-tō-kok’ŭs), Enterococcus (en’ter-ō-kok’ŭs), Pseudomonas, Mycobacterium tuber­ culosis, and Plasmodium (plaz-mō’dē-ŭm) pose unique problems. Caregivers must treat infected patients without effective antimicrobials while taking care to protect themselves and others from infection. Resistance to one antimicrobial agent may confer resistance to similar drugs, a phenomenon called cross resistance. Cross resistance typically occurs when drugs are similar in structure. For example, resistance to one aminoglycoside drug, such as streptomycin, may confer resistance to similar aminoglycoside drugs. 11/23/16 3:03 AM       Resistance to Antimicrobial Drugs Tough Decision staggering—the cost to prove a drug’s effectiveness and safety can be almost $2 billion. With the advent of genome sequencing and enhanced understanding of protein folding, some scientists predict that we are moving into a new era of antimicrobial drug discovery. They point out that researchers who know the exact shapes of microbial proteins should be able to design drugs complementary to those shapes—drugs that will inhibit microbial proteins without affecting humans. Currently, scientists are examining ways to inhibit microbes by inhibiting the following: secretion systems, which allow bacteria to attach to and inject proteins into a cell; attachment molecules and their receptors; biofilm signaling molecules; and bacterial RNA polymerase. Despite researchers’ best efforts, other scientists wonder whether drug developers can stay ahead of the development of resistance by pathogens. M10_BAUM7206_05_SE_C10.indd 305 drug-susceptible strain, but his father carried a strain of TB that was resistant to several standard anti-tuberculosis drugs. It would take many weeks to determine which of the two strains was affecting the baby. Doctors had to make a tough decision: Should they immediately begin treating the infant for the multi-drug-resistant strain (MDR-TB), which involves using five to seven different drugs with multiple and painful side effects for many months, or should they treat him for the more normal form of TB, using a more typical and less stressful drug regimen for an equal length of time? 1. What issues must be considered to determine the drug therapy for the baby? 2. What treatment would you guess was used for the infant? Reference: Adapted from MMWR 40:373–375, 1991. TELL ME WHY Why is it incorrect to say that an individual bacterium develops resistance in response to an antibiotic? Selected antimicrobial agents, their modes of action, clinical considerations, and other feat ures are summarized in TABLES 10.2 through 10.6. Particular use of antimicrobial drugs against specific pathogens is covered in the relevant chapters of this book. ­ A young mother brought her frail infant son to a southern Texas emergency room. While she waited, her baby began to have seizures. The staff stabilized the child while the mother explained that he had been ill for many days. Feverish at times, her son had lost weight because he was too short of breath to nurse. Other family members were ill with bloody coughs. The boy was diagnosed with a form of tuberculosis (TB) that affected his brain. He was hospitalized and isolated. Health department officials discovered that two individuals had exposed the baby to TB: an uncle and his father. The strain infecting the uncle was a ­ CLINICAL CASE STUDY 16. 06/03/2018 08:47 TABLE 10.2 p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t ha       C Antibacterial Drugs Drug Description and Mode of Action Clinical Considerations Method of Resistance Spectrum of action: (G + ) bacteria esistance most often involves changes in bacterial cell membranes that prevent bacitracin from entering the cell Bacitracin Isolated from Bacillus licheniformis growing on a patient named Tracy; appears to have three modes of action: Interference with the movement of peptidoglycan precursors through the bacterial cell membrane to the cell wall Gram-positive Route of administration: Adverse effects: Topical Toxic to kidneys R Antibacterial Drugs That Inhibit Cell Wall Synthesis R Inhibition of NA transcription Damage to the bacterial cytoplasmic membrane The latter two modes of action have not been proven definitely Spectrum of action: Natural drugs have limited action against most Gramnegative (G - ) bacteria because they do not readily cross the outer membrane; semisynthetics have broader spectra of action Large number of natural and semisynthetic derivatives from the fungi Penicillium (penicillins) and Acremonium or Streptomyces (cephalosporins); bind to and deactivate the enzyme that cross-links the NAM subunits of peptidoglycan Monobactams have only a single ring instead of the two rings seen in other beta-lactams Cycloserine Analog of alanine that interferes with the formation of alaninealanine bridges between NAM subunits Spectrum of action: bacteria, mycobacteria revents the formation of mycolic acid; used in combination with other antimycobacterial drugs Spectrum of action: Mycobacteria, including M. tuberculosis and M. leprae Develops in three ways in G bacteria: Change their outer membrane structure to prevent entrance of the drug Modify the enzyme so that the drug no longer binds Monobactams have a limited spectrum of action, affecting only aerobic, G - bacteria Synthesize beta-lactamases that cleave the functional lactam ring of the drug; genes for lactamases are often carried on plasmids R P Route of administration: enicillin V, a few cephalosporins (e.g., cephalexin), and monobactams: oral; penicillin G and many semisynthetics (e.g., methicillin, ampicillin, carbenicillin, cephalothin): IM or IV Adverse effects: Allergic reactions against beta-lactams in some adults; monobactams are least allergenic Route of administration: Some G + bacteria enzymatically deactivate the drug Oral Adverse effects: Toxic to nervous system, producing depression, aggression, confusion, and headache Route of administration: Adverse effects: M10_BAUM7206_05_SE_C10.indd 306 Some G + Oral esistance is due to random mutations of bacterial chromosomes that result in alteration of target site R Ethambutol P R R R R P R P P R Beta-Lactams epresentative natural penicillins: enicillin G enicillin V epresentative semisynthetic penicillins: Amoxicillin Ampicillin Cloxacillin Dicloxacillin Methicillin Nafcillin Oxacillin iperacillin epresentative semisynthetic carbapenems: Imipenem Meropenem epresentative natural cephalosporin: Cephalothin epresentative semisynthetic cephalosporins: Cefotaxime Ceftriaxone Cefuroxime Cephalexin Cephamycin epresentative semisynthetic monobactam: Aztreonam None 11/23/16 3:03 AM       R esistance to Antimicrobial Drugs TABLE 10.2 Antibacterial Drugs—Continued Drug Description and Mode of Action Clinical Considerations Method of Resistance Spectrum of action: Mycobacteria, including M. tuberculosis and M. leprae esistance is due to random mutations of bacterial chromosomes that result in alteration of target site or overproduction of target molecules Isoniazid (isonicotinic acid hydrazide, INH) Analog of the vitamins nicotinamide and pyridoxine; blocks the gene for an enzyme that forms mycolic acid Route of administration: Adverse effects: Vancomycin Route of administration: Dalbavancin R epresentatives: Directly interfere with the formation of certain alanine-alanine bridges between NAM subunits E Oritavancin Spectrum of action: ffective against most G + bacteria but often reserved for use against strains resistant to other drugs, such as methicillin-resistant Staphylococcus aureus (M SA) Lipoglycopeptide R Oral May be toxic to liver IV Adverse effects: Damage to ears and kidneys, allergic reactions, may cause depression R Antibacterial Drugs That Inhibit Cell Wall Synthesis—Continued G - bacteria are naturally resistant because the drug is too large to pass through the outer membrane; some G + bacteria (e.g., Lactobacillus) are naturally resistant because they do not form alanine-alanine bonds between NAM subunits Antibacterial Drugs That Inhibit Protein Synthesis Kanamycin P Neomycin aromomycin Spiramycin Streptomycin Tobramycin R epresentative semisynthetic: roduced by Streptomyces lincolnensis; binds to 50S ribosomal subunit and stops protein elongation Spectrum of action: ffective against G + and anaerobic G - bacteria and some protozoa Clindamycin P Lincosamides U Spectrum of action: Broad but rarely used except in treatment of typhoid fever P arely used drug produced by Streptomyces venezuelae that binds to the 50S subunits of prokaryotic ribosomes, preventing them from moving along m NA R Chloramphenicol Adverse effects: Often toxic to kidneys and to organs of inner ear or auditory nerves, causing deafness ptake of these drugs is energy dependent, so anaerobes (with less AT available) are less susceptible; aerobic bacteria alter membrane pores to prevent uptake or synthesize enzymes that alter or degrade the drug once it enters; rarely, bacteria alter the binding site on the ribosome; some bacteria make biofilms when exposed to the drugs Route of administration: traverses blood-brain barrier Oral; Develops via gene carried on an plasmid that codes for an enzyme that deactivates drug R Gentamicin Route of administration: Oral, IV, or IM, depending on drug; do not traverse blood-brain barrier Adverse effects: In 1 of 24,000 patients, causes aplastic anemia, a potentially fatal condition in which blood cells fail to form; can also cause neurological damage E Capreomycin Spectrum of action: Broad: effective against most G - bacteria and some protozoa R Amikacin (semisynthetic) Compounds in which two or more amino sugars are linked with glycosidic bonds; were originally isolated from species of the bacterial genera Streptomyces and Micromonospora. Inhibit protein synthesis by irreversibly binding to the 30S subunit of prokaryotic ribosomes, which either causes the ribosome to mistranslate m NA, producing aberrant proteins, or causes premature release of the ribosome from m NA, stopping synthesis. Also bactericidal by destroying outer membranes of G bacteria R epresentatives: R R Aminoglycosides Route of administration: Oral or IV; does not traverse blood-brain barrier Develops via changes in ribosomal structure that prevent drug from binding; resistance genes are same as those of aminoglycosides and macrolides Adverse effects: Gastrointestinal distress, including nausea, diarrhea, vomiting, and pain Azithromycin E Clarithromycin rythromycin Natamycin Telithromycin Spectrum of action: ffective against G + and a few G - bacteria and fungi (natamycin) E Group of antimicrobials typified by a macrocyclic lactone ring; the most prescribed is erythromycin, which is produced by Saccharopolyspora erythraea; act by binding to the 50S subunit of prokaryotic ribosomes and preventing the elongation of the nascent protein Route of administration: Oral; do not traverse blood-brain barrier Adverse effects: Nausea, mild gastrointestinal pain, vomiting; associated with narrowing of the stomach in children under six months old; erythromycin increases risk of cardiac arrest Develops via changes in ribosomal NA that prevent drugs from binding, or via plasmid genes coding for the production of macrolide-digesting enzymes; resistance genes are same as those of lincosamides and aminoglycosides R epresentatives: R R Macrolides Continued— M10_BAUM7206_05_SE_C10.indd 307 11/23/16 3:03 AM TABLE 10.2 p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t ha       C Antibacterial Drugs—Continued Drug Description and Mode of Action Clinical Considerations Method of Resistance roduced by Pseudomonas fluorescens; binds to bacterial t NAIle synthetase, which prevents charging of isoleucine onto t NA, blocking polypeptide synthesis at isoleucine codons Spectrum of action: ffective primarily against G + bacteria esistance develops from mutations that change the shape of t NAIle Synthetic; inhibits initiation of polypeptide synthesis Spectrum of action: R R epresentatives: Dalfopristin R Tetracyclines epresentatives: Minocycline Tetracycline Tigecycline R R Method of resistance not known Oral, IV roduced by species of Streptomyces, composed of four hexagonal rings with various side groups; prevent t NA molecules, which carry amino acids, from binding to ribosomes at the 30S subunit’s docking site Spectrum of action: Most are broad: effective against many G + and G bacteria as well as against bacteria that lack cell walls, such as Mycoplasma P Spectrum of action: Broad but reserved for use against multipledrug-resistant strains R Doxycycline G + bacteria Route of administration: roduced by species of Streptomyces; binds to 50S ribosomal subunit, stops protein synthesis; the synergistic drugs quinupristin and dalfopristin are taken together P R Quinupristin None reported R Tedizolid epresentatives: Adverse effects: Topical Adverse effects: ash, diarrhea, loss of appetite, constipation, fever Linezolid Streptogramins Route of administration: cream Route of administration: Adverse effects: pain Not known IV Muscle and joint Route of administration: crosses poorly into brain Develops in three ways; bacteria may: Alter gene for pores in outer membrane such that new pore prevents drug from entering cell Oral; Alter binding site on the ribosome to allow t NA to bind even in presence of drug R R Oxazolidinones E Mupirocin P Antibacterial Drugs That Inhibit Protein Synthesis—Continued Adverse effects: Nausea, diarrhea, sensitivity to light; forms complexes with calcium, which stains developing teeth and adversely affects the strength and shape of bones Actively pump drug from cell Antibacterial Drugs That Alter Cytoplasmic Membranes Spectrum of action: A bacteriocin naturally produced by Lactococcus lactis that disrupts a bacterial cytoplasmic membrane Spectrum of action: bacteria in food roduced by Bacillus polymyxa; destroys cytoplasmic membranes of susceptible cells Spectrum of action: ffective against G - bacteria, particularly Pseudomonas (and some amoebas) Route of administration: Route of administration: preservative in food None reported M10_BAUM7206_05_SE_C10.indd 308 R esults from changes in cell membrane that prohibit entrance of the drug Topical Toxic to kidneys Spectrum of action: tuberculosis Mycobacterium Route of administration: Adverse effects: diarrhea Not known sed as a E P Adverse effect: Disrupts membrane transport and prevents Mycobacterium from repairing damaged proteins Topical G + and G - Route of administration: Pyrazinamide Not known Adverse effects: In allergic individuals, itching, swelling, and local irritation Adverse effect: Polymyxin G + bacteria Oral esults from point mutations in bacterial gene for enzyme necessary to activate drug R Nisin Short polypeptide produced by Bacillus brevis that forms pore across cytoplasmic membrane, allowing single-charged cations to cross freely U Gramicidin Malaise, nausea, 11/23/16 3:03 AM       R esistance to Antimicrobial Drugs TABLE 10.2 Antibacterial Drugs—Continued Drug Description and Mode of Action Clinical Considerations Method of Resistance Spectrum of action: Mycobacteria; reserved for use for multi-drug-resistant tuberculosis Not known Antibacterial Drugs That Are Antimetabolites Synthetic drug that inhibits an AT synthesis enzyme unique to mycobacteria P Bedaquiline Route of administration: Oral Adverse effects: Disturbance in heart’s electrical activity, nausea, headache, joint pain Dapsone Synthetic analog of paraaminobenzoic acid; interferes with synthesis of folic acid Spectrum of action: M. tuberculosis M. leprae, Route of administration: Not known Oral Adverse effects: Insomnia, headache, nausea, vomiting, increased heart rate R epresentatives: Sulfadiazine Sulfadoxine Synthetic drugs; first produced as a dye; analogs of ABA that bind irreversibly to enzyme that produces dihydrofolic acid; synergistic with trimethoprim P Sulfonamides Sulfamethoxazole Spectrum of action: Broad: effective against G + and G - bacteria and some protozoa and fungi; however, resistance is widespread Route of administration: Oral R Adverse effects: are: allergic reactions, anemia, jaundice, mental retardation of fetus if administered in last trimester of pregnancy Pseudomonas is naturally resistant because of permeability barriers; organisms that use folic acid from the diet are also naturally resistant; chromosomal mutations result in lowered affinity for the drugs Sulfanilamide Blocks second metabolic step in the formation of folic acid from ABA; synergistic with sulfonamides, such as sulfamethoxazole P Trimethoprim Spectrum of action: Broad: effective against G + and G - bacteria and some protozoa and fungi; however, resistance is widespread Route of administration: Oral Adverse effects: Allergic reactions or liver damage in some patients Pseudomonas is naturally resistant because of permeability barriers; organisms that use folic acid from the diet are also naturally resistant; chromosomal mutations result in lowered affinity for the drug Antibacterial Drugs That Inhibit Nucleic Acid Synthesis Clofazimine Binds to DNA, preventing replication and transcription Spectrum of action: Mycobacteria, especially M. tuberculosis, M. leprae, and M. ulcerans Route of administration: Not known Oral Adverse effects: Diarrhea, discoloration of skin and eyes R epresentatives: Ciprofloxacin Levofloxacin Synthetic agents that inhibit DNA gyrase, which is needed to correctly replicate bacterial DNA; penetrate cytoplasm of cells Spectrum of action: Broad: G + and G - bacteria are affected Anaerobic conditions reduce these synthetic molecules, which then damage DNA, preventing correct DNA replication Spectrum of action: Obligate anaerobic bacteria and protozoa R Not known Natural and semisynthetic derivatives from Amycolatopsis rifamycinica that bind to bacterial NA polymerase, preventing transcription of NA; used with other antimicrobial bacterial drugs Spectrum of action: Bacteriostatic against aerobic G + bacteria; bactericidal against mycobacteria esults from chromosomal mutation that alters binding site on enzyme; G - bacteria are naturally resistant because of poor uptake Moxifloxacin Route of administration: Adverse effects: rupture Oral esults from chromosomal mutations that lower affinity for drug, reduce its uptake, or protect gyrase from drug R Quinolones Tendonitis, tendon Nalidixic acid Ofloxacin R Nitroimidazoles epresentatives: Benznidazole Metronidazole ifampin ifaximin M10_BAUM7206_05_SE_C10.indd 309 R R R R R epresentatives: Route of administration: Adverse effects: significance Oral R Tinidazole Rifamycins None of major 11/23/16 3:03 AM p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t ha       C TABLE 10.3 Antiviral Drugs Drug Description and Mode of Action Clinical Considerations Method of Resistance Synthetic molecule that blocks attachment molecule on host cell or pathogen Spectrum of action: icornaviruses (e.g., poliovirus, some cold viruses, and, coxsackieviruses) Not known Pleconaril P Attachment Antagonists Route of administration: Adverse effects: R Neuraminidase Inhibitors epresentatives: Oseltamivir Synthetic drugs that prevent influenzaviruses from attaching to or exiting from cells Zanamivir Oral None Spectrum of action: Influenzavirus Not known Route of administration: Oral (oseltamivir) or aerosol (zanamivir) Adverse effects: None Antiviral Drugs That Inhibit Viral Entry Enfuvirtide Synthetic drug that binds viral protein gp41, preventing fusion of viral envelope with host cell membrane, thereby blocking viral entry Spectrum of action: HIV-1 Route of administration: Not known IV Adverse effects: Tingling, allergic reaction at injection site Antiviral Drugs That Inhibit Viral Uncoating Amantadine Synthetic drug that neutralizes acid environment within phagolysosomes that is necessary for viral uncoating Spectrum of action: A virus Influenza Route of administration: Oral Mutation resulting in a single amino acid change in a membrane ion channel leads to viral resistance Arildone Synthetic antiviral that prevents removal of poliovirus capsids (uncoating) Spectrum of action: Synthetic drug that neutralizes phagolysosomal acid, preventing viral uncoating Spectrum of action: A virus olioviruses Not known Route of administration: Topical Adverse effects: Rimantadine P Adverse effects: Toxic to central nervous system; results in nervousness, irritability, insomnia, and blurred vision None reported Influenza Route of administration: adults only Oral, Mutation resulting in a single amino acid change in a membrane ion channel leads to viral resistance Adverse effects: Toxic to central nervous system; results in nervousness, irritability, insomnia, and blurred vision M10_BAUM7206_05_SE_C10.indd 310 11/23/16 3:03 AM       R esistance to Antimicrobial Drugs TABLE 10.3 Antiviral Drugs—Continued Drug Description and Mode of Action Clinical Considerations Method of Resistance Spectrum of action: Viruses that code for kinase enzymes: human herpesvirus, pstein-Barr virus, cytomegalovirus, and varicella virus Mutations in genes for kinase enzymes may render the drugs ineffective at drug activation hosphorylation by virally coded kinase enzyme activates these synthetic nucleotide analogs; inhibit DNA and NA synthesis R P Acyclovir (ACV) Ganciclovir Penciclovir E Antiviral Drugs That Inhibit Nucleic Acid Synthesis Route of administration: Sofosbuvir hosphorylation by cell-coded kinase enzyme activates this synthetic drug; inhibits DNA synthesis; viral DNA polymerase more likely to incorporate the drugs than human DNA polymerase Spectrum of action: Herpesvirus Cell-coded kinase phosphorylates this nucleotide analog, which inhibits DNA replication Spectrum of action: C virus P Adenosine Arabinoside Oral None esults from mutation of viral DNA polymerase R Adverse effects: Route of administration: IV Adverse effects: Fatal to host cells that incorporate the drug into cellular DNA; anemia Hepatitis Route of administration: Not known Oral Adverse effects: Fatigue, headache, nausea, insomnia, rash, diarrhea R Adefovir E Azidothymidine (AZT) ntecavir R Lamivudine Synthetic molecules phosphorylated by cell-coded kinase enzyme that activates these drugs; inhibits DNA synthesis; HIV and hepatitis B virus reverse transcriptases more likely to incorporate some of these drugs; used in conjunction with protease inhibitor to treat HIV ibavarin Tenofovir Spectrum of action: Human immunodeficiency, hepatitis B and C, respiratory syncytial, influenza A, measles, and some hemorrhagic fever viruses esults from mutation of HIV reverse transcriptase R Other Nucleotide Analogs epresentatives (see also Figure 10.7): Route of administration: Oral, aerosol, or IV, depending on drug Adverse effects: Vary with drug but include nausea, bone marrow toxicity, and fetal harm Valaciclovir Antiviral Drugs That Inhibit Protein Synthesis Fomivirsen Synthetic nucleic acid complementary to specific m NA; binding prevents protein synthesis by blocking ribosomes R epresentative: Spectrum of action: Specific to species with complementary m NA; fomivirsen specific against cytomegalovirus Not known R R Antisense Nucleic Acids Adverse effects: glaucoma P Route of administration: Fomivirsen injected weekly into eyes ossible Antiviral Drugs That Inhibit Viral Proteins R epresentatives: Boceprevir Darunavir Fosamprenavir Ledipasvir Computer-assisted modeling of protease enzymes, which are unique to HIV and hepatitis C virus, allowed the creation of drugs that block the enzymes’ active sites, often used in conjunction with drugs active against nucleic acid synthesis Spectrum of action: hepatitis C HIV and Route of administration: Adverse effects: esults from mutation in protease gene R Protease Inhibitors Oral None Telaprevir M10_BAUM7206_05_SE_C10.indd 311 11/23/16 3:03 AM p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t ha       C TABLE 10.4 Antimicrobials Against Eukaryotes: Antifungal Drugs Drug Description and Mode of Action Clinical Considerations Method of Resistance Antifungal action due to inhibition of ergosterol synthesis, which is essential for fungal cytoplasmic membranes Spectrum of action: Not known Synthetic antifungal and antiprotozoan drug that inhibits synthesis of membrane sterols Spectrum of action: protozoa Antifungal Drugs That Inhibit Cell Membranes epresentative: Terbinafine R Azoles epresentatives: Fluconazole Ketoconazole Oral, IV Adverse effects: Headache, nausea, vomiting, diarrhea, liver damage, rash Fungi and Route of administration: Adverse effects: cancer in humans Itraconazole Fungi Route of administration: P R Allylamines Mutation in gene for target enzyme Topical, IV ossibly causes Voriconazole R epresentatives: Amphotericin B Nystatin Associate with molecules of ergosterol, forming a pore through the fungal membrane, which leads to leakage of essential ions from the cell; amphotericin B is produced by Streptomyces nodosus Spectrum of action: amoebas Synthetic analog of coenzyme Q of Pneumocystis and of several protozoa; interrupts electron transport Spectrum of action: Pneumocystis, protozoa (especially Plasmodium) Fungi, some are; decrease in amount or change in chemistry of ergosterol R Polyenes Route of administration: Amphotericin B: IV; nystatin: topical Adverse effects: fever Chills, vomiting, Other Antifungal Drugs Route of administration: Adverse effects: diarrhea, headache Cells modify the structure of their electron transport chain proteins Oral ossible rash, Synthetic antimetabolite that appears to disrupt DNA repair, inhibit mitosis, and interfere with some aspects of intracellular transport Spectrum of action: especially Trichophyton Natural product of Aspergillus and semisynthetic derivatives that inhibit synthesis of glucan subunits in some fungal cell walls Spectrum of action: Aspergillus Fungi, but not mammals, have an enzyme that converts this synthetic drug into 5-fluorouracil, an analog of uracil that inhibits NA function Spectrum of action: Cryptococcus, Aspergillus Isolated from Penicillium griseofulvum; deactivates tubulin, preventing cytokinesis and segregation of chromosomes during mitosis (see Chapter 12) Spectrum of action: infections Fungi, Route of administration: Not known Topical Adverse effects: ossible burning sensation at application site P Ciclopirox P Atovaquone R epresentative: Route of administration: esults from mutation in glucan synthase gene IV Adverse effects: ash, facial swelling, respiratory spasms, gastrointestinal distress, toxicity to human embryos R Caspofungin Candida, R Echinocandins R 5-Fluorocytosine Griseofulvin M10_BAUM7206_05_SE_C10.indd 312 Candida, Route of administration: Adverse effects: None Molds of skin Route of administration: oral Adverse effects: Oral Develops from mutations in the genes for enzymes necessary for utilization of uracil Not known Topical, None 11/23/16 3:03 AM       R esistance to Antimicrobial Drugs TABLE 10.5 Antimicrobials Against Eukaryotes: Anthelmintic Drugs Drug Description and Mode of Action Clinical Considerations Method of Resistance Interferes with neuromuscular physiology of helminths; also possibly uncouples electron transport from phosphorylation (AT production) Spectrum of action: Not known Inhibit microtubule formation and glucose uptake Spectrum of action: protozoa Anthelmintic Drugs That Are Antimetabolites P Bithionol epresentatives: Route of administration: Oral Adverse effects: Diarrhea, abdominal cramping, nausea Helminths, Albendazole Route of administration: Mebendazole Adverse effects: P R Benzimidazole Derivatives Helminths Not known Oral ossible diarrhea Thiabendazole Triclabendazole Ivermectin Metrifonate Spectrum of action: Inhibits oxidative phosphorylation of AT by mitochondria Spectrum of action: P Niclosamide Semisynthetic drugs modified from a natural product of Streptomyces; produce flaccid paralysis by blocking neurotransmitters and changing membrane permeability Route of administration: Pyrantel Pamoate Diethylcarbamazine Not known Oral Adverse effects: Allergic reactions may result from antigens of dead helminths Cestodes Route of administration: Adverse effects: nausea, diarrhea Praziquantel Helminths Abdominal pain, Synthetic drug that changes membrane permeability to calcium ions, which are required for muscular contraction; induces complete muscular contraction in helminths Spectrum of action: trematodes Synthetic drugs that bind to neurotransmitter receptors, causing complete muscular contraction of helminths Spectrum of action: Cestodes, Route of administration: Adverse effects: Not known Oral None Nematodes Route of administration: Adverse effects: Not known Oral Not known Oral None Anthelmintic Drugs That Inhibit Nucleic Acid Synthesis Niridazole Oltipraz Oxamniquine When partially catabolized by schistosome enzymes, binds to DNA, preventing replication Spectrum of action: Synthetic drug that possibly acts by reducing the supply of deoxyribonucleotides Spectrum of action: Schistosome enzyme activates semisynthetic drug, which then inhibits DNA synthesis Spectrum of action: mansoni Schistosoma Route of administration: Not known Oral Adverse effects: Central nervous system toxicity, hallucinations Schistosoma Route of administration: Not known Oral Adverse effects: Central nervous system and gastrointestinal toxicity Schistosoma Route of administration: Not known Oral Adverse effects: Central nervous system toxicity, hallucinations M10_BAUM7206_05_SE_C10.indd 313 11/23/16 3:03 AM TABLE 10.6 t p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs ha       C Antimicrobials Against Eukaryotes: Antiprotozoan Drugs Drug Description and Mode of Action Clinical Considerations Method of Resistance Spectrum of action: ffective against some protozoa and G + and anaerobic G - bacteria Develops via changes in ribosomal structure that prevent drug from binding; resistance genes are same as those for resistance to aminoglycosides and macrolides R epresentative: Clindamycin roduced by Streptomyces lincolnensis; binds to 50S ribosomal subunit and stops protein elongation P Lincosamides E Antiprotozoan Drugs That Inhibit Protein Synthesis Route of administration: Oral or IV; does not traverse blood-brain barrier Adverse effects: Gastrointestinal distress, including nausea, diarrhea, vomiting, and pain Aminoglycoside produced by Streptomyces krestomuceticus that interferes with bacterial 30S ribosomal subunits. Its method of action against protozoa is not known Spectrum of action: ffective against some protozoa, e.g., Leishmania and Entamoeba, and most G - and G + bacteria E Paromomycin Route of administration: Decreased drug uptake due to altered cytoplasmic membrane Oral Adverse effects: Allergic reactions in some patients; intestinal blockage Antiprotozoan Drugs That Are Antimetabolites Atovaquone R Azoles epresentatives: Albendazole Ketokonazole Synthetic analog of coenzyme Q of several protozoa and of Pneumocystis; interrupts electron transport Spectrum of action: rotozoa (especially Plasmodium), fungus—Pneumocystis Synthetic antiprotozoan and anthelmintic drugs that inhibit microtubule formation and glucose uptake Spectrum of action: Deactivate enzymes by breaking hydrogen bonds necessary for effective tertiary structure; drugs containing arsenic were the first recognized selectively toxic chemotherapeutic agents Spectrum of action: cells Mode of action unknown Spectrum of action: Plasmodium Route of administration: Adverse effects: dizziness Route of administration: Adverse effects: headache Oral Cells modify the structure of their electron transport chain proteins Oral ossible rash, diarrhea, Helminths, protozoa Route of administration: Adverse effects: Mutation in Ca2+ transporter gene Nausea, vomiting, itching, P Artesunate Spectrum of action: P epresentative: Derived from Chinese wormwood shrub; interferes with heme detoxification and with Ca2+ transport P R Artemisinins Not known Oral ossible diarrhea Mebendazole R Heavy Metals (e.g., Hg, As, Cr, Sb) epresentatives: Meglumine antimonate Melarsoprol (contains As) Salvarsan (contains As) Metabolically active Route of administration: Not known Topical, oral Adverse effects: Toxic to active cells, such as those of the brain, kidney, liver, and bone marrow Sodium stibogluconate (contains Sb) Iodoquinol Intestinal amoebas Route of administration: Adverse effects: Lumefantrine Nifurtimox Spectrum of action: Synthetic drug that interferes with electron transport Spectrum of action: Plasmodium Route of administration: Not known Oral Headache, dizziness, Trypanosoma cruzi Route of administration: Adverse effects: M10_BAUM7206_05_SE_C10.indd 314 Fever, chills, rash Synthetic drug that prevents detoxification of heme released from hemoglobin of damaged red blood cells Adverse effects: weakness Not known Oral Not known Oral Abdominal pain, nausea 11/23/16 3:03 AM       R esistance to Antimicrobial Drugs TABLE 10.6 Antimicrobials Against Eukaryotes: Antiprotozoan Drugs—Continued Drug Description and Mode of Action Clinical Considerations Method of Resistance Antiprotozoan Drugs That Are Antimetabolites—Continued Spectrum of action: Synthetic drugs that block second metabolic step in the formation of folic acid from ABA; synergistic with sulfonamides Spectrum of action: Broad: effective against G + and G - bacteria and some protozoa and fungi; however, resistance is widespread P Proguanil Pyrimethamine Synthetic drug believed to interfere with anaerobic electron transport chain P Nitazoxanide rotozoa Route of administration: Adverse effects: Gastrointestinal distress Route of administration: Adverse effects: patients Oral Spectrum of action: Broad: effective against G + and G - bacteria and some protozoa and fungi; however, resistance is widespread Suramin Synthetic drug that inhibits specific enzymes in some protozoa Spectrum of action: ( ast African variant) Route of administration: Oral Organisms that do not synthesize folic acid are naturally resistant; chromosomal mutations result in lowered affinity for the drugs Adverse effects: are: Allergic reactions, anemia, jaundice, mental retardation of fetus if administered in last trimester of pregnancy R P Synthetic drugs; first produced as a dye; analogs of ABA that bind irreversibly to enzyme that produces dihydrofolic acid; synergistic with trimethoprim Trypanosoma brucei Route of administration: Adverse effects: P Synthetic drug that blocks second metabolic step in the formation of folic acid from ABA; synergistic with sulfonamides Not known E R Cells that require folic acid as a vitamin are also naturally resistant; chromosomal mutations result in lowered affinity for the drugs Allergic reactions in some Sulfonamides epresentatives: Sulfadiazine Sulfadoxine Sulfanilamide Trimethoprim Not known Oral Oral None Spectrum of action: Broad: effective against some protozoa, fungi, and some G + and G - bacteria; however, resistance is widespread Route of administration: Oral Organisms that cannot synthesize folic acid are naturally resistant; chromosomal mutations result in lowered affinity for the drugs Adverse effects: Allergic reactions or liver damage in some patients Antiprotozoan Drugs That Inhibit DNA Synthesis Eflornithine Synthetic drug that inhibits synthesis of precursors of nucleic acids Spectrum of action: Trypanosoma brucei gambiense (West African variant) Route of administration: Not known Oral Adverse effects: Anemia, inhibition of blood clotting, nausea, vomiting Anaerobic conditions reduce these synthetic drugs, which then appear to damage DNA, preventing correct replication and transcription Spectrum of action: protozoa and bacteria Pentamidine Synthetic drug that binds to nucleic acids, inhibiting replication, transcription, and translation Spectrum of action: rotozoa, including Trypanosoma brucei (West African variant), and fungi (e.g., Pneumocystis) Obligate anaerobic Route of administration: Not known Oral Adverse effects: Metronidazole and tinidazole cause cancer in laboratory rodents P R Nitroimidazoles epresentatives: Benznidazole Metronidazole Tinidazole Route of administration: Not known IM, IV R Adverse effects: ash, low blood pressure, irregular heartbeat, kidney and liver failure Spectrum of action: Plasmodium Route of administration: Adverse effects: disturbances Oral Allergic reactions, visual esults from the presence of quinoline pumps that remove the drugs from parasite’s cells R Natural and semisynthetic drugs derived from the bark of cinchona tree; though considered antibacterial, they inhibit metabolism of malaria parasites by one or more unknown methods P P R Quinolones epresentatives: Natural quinine Semisynthetic quinines: Chloroquine Mefloquine iperaquine rimaquine M10_BAUM7206_05_SE_C10.indd 315 11/23/16 3:03 AM p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t Dr. Warner tells Thomas that his symptoms indicate he has antibiotic-associated diarrhea (AAD), a side effect of the antibiotic Thomas was taking for his bronchitis. The doctor assures Thomas that this is quite common and easy to treat. Dr. Warner collects a stool sample for analysis and tells Thomas to stop taking the antibiotic for his bronchitis. He also instructs Thomas to drink more water and to eat saltine crackers and bananas to help restore electrolytes. Two days later, Thomas calls the doctor’s office because he is still not feeling well. Dr. Warner asks Thomas to come in as soon as possible. The results of the stool analysis indicated that Thomas has Clostridium difficile infection; this, in combination with Thomas’s lack of improvement, leads Dr. Warner to order an endoscopy so he can examine the inside of Thomas’s intestinal tract. The endoscopy results confirm the severity of infection; the intestinal walls are inflamed, and psuedomembranes (yellowish plaques resulting from the inflammatory response) are present. Dr. Warner prescribes metronidazole (another antimicrobial that works a different way) to treat the C. dif f icile infection. He also advises Thomas to keep drinking fluids and eating the saltines and bananas. Within a few days, Thomas’s cramping and diarrhea are gone. He finishes his course of metronidazole, and at his follow-up visit with Dr. Warner, he is told that he has made a full recovery. 1. Thomas’s bronchitis was treated with levofloxacin, a fluoroquinolone. What is the mechanism of action for this class of antibiotics? 2. How could antibiotic treatment, which is meant to cure an infection, lead to an infection with another organism? Check your answers to Micro in the Clinic Follow- p questions in the MasteringMicrobiology Study Area. U A Cure Leading to Disease? ­ ha       C CHAPTER SUMMARY ­ 4. Antimicrobial agents that inhibit protein synthesis by interfering with 70S ribosomes include aminoglycosides and tetracyclines, which inhibit functions of the 30S ribosomal subunit, and chloramphenicol, lincosamides, streptogramV e ins, and macrolides, which inhibit 50S subunits. Tu r Oxazolidinones block initiation of translation. Antisense nucleic acid molecules inhibit protein synthesis by preventing ribosomes from assembling on mRNA. 2. Antimicrobial drugs affect pathogens by inhibiting cell wall synthesis, inhibiting the translation of proteins, disrupting cytoplasmic membranes, inhibiting general metabolic pathways, inhibiting nucleic acid synthesis, blocking the attachment of viruses to their hosts, or blocking a pathogen’s recognition of its host. ▶ ANIMATION: Chemotherapeutic Agents: Modes of Action ­ 3. Beta-lactams—penicillins, carbapenems, cephalosporins—have a functional lactam ring. They prevent bacteria from cross-linking NAM subunits of peptidoglycan in the bacterial cell wall during growth. Lipoglycopeptides (for example, vancomycin) and cycloserine also disrupt cell wall formation in many Grampositive bacteria. Bacitracin blocks NAG and NAM transport from the cytoplasm. Isoniazid (INH) and ethambutol block M10_BAUM7206_05_SE_C10.indd 316 o to ­ 1. Successful chemotherapy against microbes is based on selective toxicity, that is, using antimicrobial agents that are more toxic to pathogens than to the patient. ­ Mechanisms of Antimicrobial Action (pp. 288–296) id ­ 1. Chemotherapeutic agents are chemicals used to treat diseases. Among them are antimicrobial agents, or antimicrobials, which include antibiotics (biologically produced agents), semisynthetic antimicrobials (chemically modified antibiotics), and synthetic drugs. mycolic acid synthesis in the walls of mycobacteria. Echinocandins block synthesis of fungal cell walls. ­ The History of Antimicrobial Agents (pp. 287–288) 5. Mupirocin binds to isoleucine aminoacyl-tRNA synthetase—the enzyme that loads isoleucine onto the tRNA. 6. Polyenes, azoles, and allylamines disrupt the cytoplasmic membranes of fungi. Polymyxin acts against the membranes of Gram-negative bacteria. 7. Sulfonamides are structural analogs of para-aminobenzoic acid (PABA), a chemical needed by some microorganisms but not by humans. The substitution of sulfonamides in the metabolic pathway leading to nucleic acid synthesis kills those organisms. Trimethoprim also blocks this pathway. 8. Drugs that inhibit nucleic acid replication in pathogens include actinomycin, nucleotide analogs and nucleoside analogs, quinolones, and rifampin. 11/23/16 3:03 AM Clinical Considerations in Prescribing Antimicrobial Drugs (pp. 296–300) 7. Clinicians use the term therapeutic window to indicate the range of concentrations of a drug that are effective without being excessively toxic. ­ ­ 1. Chemotherapeutic agents have a spectrum of action and may be classed as either narrow-spectrum drugs or broad-spectrum drugs, depending on how many kinds of pathogens they affect. Resistance to Antimicrobial Drugs (pp. 301–305) 2. Diffusion susceptibility tests, such as the Kirby-Bauer test, reveal which drug is most effective against a particular pathogen; in general, the larger the zone of inhibition around a drugsoaked disk on a Petri plate, the more effective the drug. 3. The minimum inhibitory concentration (MIC), usually determined by either a broth dilution test or an Etest, is the smallest amount of a drug that will inhibit a pathogen. 4. A minimum bactericidal concentration (MBC) test ascertains whether a drug is bacteriostatic and the lowest concentration of a drug that is bactericidal.       R Questions for eview 1. Some members of a pathogenic population may develop resistance to a drug because of extra DNA pieces called R plasmids or the mutation of genes. Microorganisms may resist a drug by producing enzymes such as beta (ß)-lactamases that deactivate the drug, by inducing changes in the cell membrane that prevent entry of the drug, by altering the drug’s target to prevent its binding, by altering the cell’s metabolic pathways, by removing the drug from the cell with efflux pumps, or by protecting the drug’s target by binding another molecule to it. ▶ ANIMATIONS: Antibiotic Resistance: Origins of Resistance, Forms of Resistance 3. Synergism describes the interplay between drugs that results in efficacy that exceeds the efficacy of either drug alone. Some drug combinations are antagonistic. STIONS FO VI W E Q RE 6. A drug’s therapeutic index (TI) is essentially a ratio of the drug’s tolerated dose to its effective dose. The higher the TI, the safer the drug. R 2. Cross resistance occurs when resistance to one chemotherapeutic agent confers resistance to similar drugs. Multiple-drug-resistant pathogens are resistant to three or more types of antimicrobial drugs. UE 5. In choosing antimicrobials, physicians must consider effectiveness, how a drug is best administered—orally, intramuscularly, or intravenously—and possible side effects, including toxicity and allergic responses. Answers to the Questions for Review (except Short Answer questions) begin on p. A-1.                                3. The key to successful chemotherapy is a. selective toxicity b. a diffusion test c. the minimum inhibitory concentration test d. the spectrum of action . 4. Which of the following statements is relevant in explaining why sulfonamides are effective? a. Sulfonamides attach to sterol lipids in the pathogen, disrupt the membranes, and lyse the cells. b. Sulfonamides prevent the incorporation of amino acids into polypeptide chains. M10_BAUM7206_05_SE_C10.indd 317 6. Multiple-drug-resistant microbes . a. are resistant to all antimicrobial agents b. respond to new antimicrobials by developing resistance c. frequently develop in hospitals d. all of the above 7. Which of the following is most closely associated with a beta-lactam ring? a. penicillin c. bacitracin b. vancomycin d. isoniazid 8. Drugs that act against protein synthesis include a. beta-lactams c. polymyxin b. trimethoprim d. aminoglycosides                                                               2. In a Kirby-Bauer susceptibility test, the presence of a zone of inhibition around disks containing antimicrobial agents indicates . a. that the microbe does not grow in the presence of the agents b. that the microbe grows well in the presence of the agents c. the smallest amount of the agent that will inhibit the growth of the microbe d. the minimum amount of an agent that kills the microbe in question 5. Cross resistance is . a. the deactivation of an antimicrobial agent by a bacterial enzyme b. alteration of the resistant cells so that an antimicrobial agent cannot attach c. the mutation of genes that affect the cytoplasmic membrane channels so that antimicrobial agents cannot cross into the cell’s interior d. resistance to one antimicrobial agent because of its similarity to another antimicrobial agent                                                               1. Diffusion and dilution tests that expose pathogens to antimicrobials are designed to determine . a. the spectrum of action of a drug b. which drug is most effective against a particular pathogen c. the amount of a drug to use against a particular pathogen d. both b and c c. Humans and microbes use PABA differently in their metabolism. d. Sulfonamides inhibit DNA replication in both pathogens and human cells.                                Multiple Choice . 11/23/16 3:03 AM p er 10    Controlling Microbial Growth in the Body: Antimicrobial Drugs t ha       C 9. Which of the following statements is false concerning antiviral drugs? a. Macrolide drugs block attachment sites on the host cell wall and prevent viruses from entering. b. Drugs that neutralize the acidity of phagolysosomes prevent viral uncoating. c. Nucleotide analogs can be used to stop microbial replication. d. Drugs containing protease inhibitors retard viral growth by blocking the production of essential viral proteins. 2. What specific test for antimicrobial efficacy is shown? What does this test measure? Draw an oval to predict the size and shape of the zone of inhibition if the drug concentration on the strip were increased twofold.                                10. PABA is . a. a substrate used in the production of penicillin b. a type of (ß)-lactamases c. molecularly similar to cephalosporins d. used to synthesize folic acid VISUALIZE IT! 1. Label each figure below to indicate the class of drug that is stopping polypeptide translation. Incorrect amino acids fMet a. ___________________________ block initiation. 3. Why is the fact that drug Z destroys the NAM portions of a cell’s wall structure an important factor in considering the drug for chemotherapy? 30S b. _____________________________ change 30S subunit. Amino acids Peptide bonding 50S 50S 4. Given that both human cells and pathogens synthesize proteins at ribosomal sites, how can antimicrobial agents that target this process be safe to use in humans? 5. Support or refute the following statement: Antimicrobial agents make cells become resistant. 6. Given that resistant strains of pathogens are a concern to the general health of a population, what can be done to prevent their development? 7. Why are antiviral drugs difficult to develop? 30S c. ___________________________ block ribosome attachment. 1. What characteristics would an ideal chemotherapeutic agent have? Which drug has these qualities? 2. Contrast narrow-spectrum and broad-spectrum drugs. Which are more effective? 50S 30S Short Answer d. _____________________________ inhibits peptide bonding. 8. A man has been given a broad-spectrum antibiotic for his stomach ulcer. What unintended consequences could arise from this therapy? 9. Compare and contrast the actions of polyenes, azoles, allylamines, and polymyxin. 10. What is the difference in drug action of synergists contrasted with that of antagonists? 50S 50S e. ____________________________ f. ____________________________ block ribosome movement. M10_BAUM7206_05_SE_C10.indd 318 g. _____________________________ block tRNA docking. 11/23/16 3:03 AM       R Questions for eview R C ITICAL THINKING 1. AIDS is treated with a “cocktail” of several antiviral agents at once. Why is the cocktail more effective than a single agent? What is a physician trying to prevent by prescribing several drugs at once? 2. How does Penicillium escape the effects of the penicillin it secretes? 9. Why might amphotericin B affect the kidneys more than other human organs? 10. Antiparasitic drugs in the benzimidazole family inhibit the polymerization of tubulin. What effect might these drugs have on mitosis and flagella? 4. Fewer than 1% of known antibiotics have any practical value in treatment of disease. Why is this so? 11. Your cousin reads in a blog that the U.S. Food and Drug Administration (FDA) has approved an antimicrobial called tigecycline. The blog says that the drug is an analog of tetracycline. She asks you what that means and how the drug works. What should you tell your cousin? 5. In an issue of News of the Lepidopterists’ Society, a recommendation was made to moth and butterfly collectors to use antimicrobials to combat disease in the young of these insects. What are the possible ramifications for human health of such usage of antimicrobials? 12. Scientists have cultured bacteria isolated from within frozen mammoths, which are thousands of years old. Why would it not be surprising if these microbes were to show some resistance to modern antimicrobials that didn’t exist when the mammoths died? 6. Even though aminoglycosides such as gentamicin can cause deafness, there are still times when they are the best choice for treating some infections. What laboratory test would a clinical scientist use to show that gentamicin is the best choice to treat a particular Pseudomonas infection? 13. It would be impractical and expensive for every American to take amantadine during the entire flu season to prevent influenza infections. For what group of people might amantadine prophylaxis be cost effective? 3. How might a colony of Bacillus licheniformis escape the effects of its own bacitracin? 7. Your pregnant neighbor has a sore throat and tells you that she is taking some tetracycline she had left over from a previous infection. Give two reasons why her decision is a poor one. 14. Sometimes it is not possible to conduct a susceptibility test because of either a lack of time or an inability to access the bacteria (for example, from an inner-ear infection). How could a physician select an appropriate therapeutic agent in such cases? 8. Acyclovir has replaced adenosine arabinoside as treatment for herpes infections. Compare the ways these drugs are activated (see Table 10.3 on pp. 310–311). Why is acyclovir a better choice? 15. Enterococcus faecium is frequently resistant to vancomycin. Why might this be of concern in a hospital setting in terms of developing resistant strains of other genera of bacteria? M10_BAUM7206_05_SE_C10.indd 319 11/23/16 3:03 AM A M APP CONC EPT       Chapter 10    Controlling Microbial Growth in the Body: ntimicrobial Drugs ING Using the following terms, fill in the following concept map that describes antimicrobial resistance. Penicillin Transduction Transformation Efflux pumps Entry of antimicrobials into cell Mutation Pathogen’s enzymes Altered targets Beta-lactamase Cell division Conjugation Resistance to antimicrobials originates by 1 spreads vertically by 2 3 mechanisms include spreads horizontally by 4 5 6 inactivate Altered pore proteins prevent 10 prevent 9 Antimicrobials Binding of antimicrobials Rapid removal of antibiotic due to 11 an example is 7 inactivates 8 M10_BAUM7206_05_SE_C10.indd 320 06/03/18 4:51 PM