Many Bacteria Use Toxins as Virulence Factors

Chapter 9 This chapter discusses the relationship between humans and microorganisms, focusing on the role of microorganisms in causing infections. It explores different types of infections caused by bacteria, viruses, fungi, and parasites, as well as the challenges of antibiotic resistance. The chapter highlights the importance of vaccines and immunotherapy in protecting against infections. Many bacteria use toxins as virulence factors Exotoxins are secreted molecules, mostly protein enzymes Exotoxins directly injure cells by damaging cell membranes or entering cells and changing their function Exotoxins are immunogenic and elicit the production of antibodies known as antitoxins Endotoxins cause a profound immune and inflammatory response Comparison of Exotoxin and Endotoxin Exotoxin: Gram-negative and gram-positive microorganisms Endotoxin: Gram-negative microorganisms Exotoxin is a secreted protein, while endotoxin is an integral part of the cell wall Exotoxin is highly antigenic, while endotoxin is poorly antigenic Exotoxins can be converted to a toxoid and used for vaccine development, while endotoxins cannot be converted to a toxoid Staphylococcus aureus Infections Different strains of Staphylococcus aureus cause various infections in different organ systems Infections may depend on the toxin produced Gram-negative bacteria and endotoxin Gram-negative microbes produce endotoxin (lipopolysaccharide [LPS]) Endotoxin is released during growth, lysis, or destruction of the bacteria Endotoxin activates the inflammatory process and produces fever Antibiotics cannot prevent the toxic effects of endotoxins Sepsis Bacteremia occurs when bacteria are present in the blood Sepsis or septicemia occurs when bacteria are growing in the blood and release toxins Clinical symptoms and signs are caused by large amounts of toxins released by bacteria Overproduction of proinflammatory cytokines and reactive oxygen species can lead to fever, platelet activation, coagulation abnormalities, hypotension, and potentially septic shock and multiple organ failure Viral infection Viruses are obligatory intracellular microbes with a nucleic acid genome protected by a protein shell The viral genome can be dsDNA, ssDNA, dsRNA, or ssRNA Viral replication depends on infecting a permissive host cell that cannot resist viral invasion and replication Examples of viral diseases Adenoviruses: Acute febrile pharyngitis Herpes simplex viruses 1 and 2: Lesions in mouth, pharynx, genital area, conjunctivitis, meningitis Epstein-Barr virus: Mononucleosis, Burkitt lymphoma Cytomegalovirus: Mononucleosis, congenital infection Varicella-zoster virus: Chickenpox, shingles Papillomavirus: Warts, cervical carcinoma Rotavirus: Severe diarrhea Picornaviruses: Nonspecific febrile illness, conjunctivitis, meningitis Hepatitis A virus: Acute hepatitis Poliovirus: Poliomyelitis Viral Pathogens AIDS, Acquired immunodeficiency syndrome; DNA, deoxyribonucleic acid; ds, doublestranded; HIV, human immunodeficiency virus; RNA, ribonucleic acid; RT, reverse transcriptase; SARS, severe acute respiratory syndrome; SS, single-stranded. Transmission of Viral Pathogens Transmission is usually from one infected individual to an uninfected individual Transmission can occur through aerosols of respiratory tract fluids, contact with infected blood, sexual contact, transmission from an animal reservoir (zoonotic infection), or through a vector, such as mosquitoes Stages of Viral Infection Attachment to a specific receptor on the target cell Penetration (entrance into the cell by endocytosis or membrane fusion) Uncoating is the release of viral nucleic acid from the viral capsid by viral or host enzymes Replication is the synthesis of viral proteins and messenger RNA (mRNA) Assembly is the formation of new virions Release or shedding of new virions is by lysis or budding from the cell membrane Effects of Viral Infection on Cells The effects of a virus on an infected cell vary greatly and often result in destruction of the cell Destructive processes can include cessation of DNA, RNA, and protein synthesis Effects of viral infection Viral infections can cause damage to cell membranes resulting in the release of digestive lysosomal enzymes that can kill the cell (e.g., herpesvirus) Some viruses can cause fusion of host cells, producing multinucleated giant cells (e.g., respiratory syncytial virus [RSV]) Viruses can alter the antigenic properties of infected cells, causing the individual's immune system to attack the cell as if it were foreign (e.g., hepatitis B virus) Certain viruses can transform host cells into cancerous cells, resulting in uninhibited and unregulated growth (e.g., human papillomavirus [HPV]) Viruses can promote secondary bacterial infections in tissues damaged by viruses (e.g., RSV and influenza viruses) Some viruses can hide within cells and evade normal inflammatory or immune responses in a process called viral latency (e.g., varicella zoster virus, HPV, and HIV) Antigenic changes and immune evasion Viruses can hide from the immune system by making small changes to the genes that produce viral surface antigens, a process known as antigenic variation Influenza A virus undergoes regular antigenic changes known as drift, where the HA and NA antigens change slightly, resulting in seasonal infections Occasionally, a major antigenic change known as antigenic shift occurs in influenza viruses when two or more different viruses combine to form a new strain with a mixture of surface antigens. This can lead to severe disease and pandemics Monitoring and prevention Infections involving major antigenic shifts are closely monitored by organizations like the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) Prevention measures, including the development and administration of vaccines, are recommended to control the spread of viral infections Types of Infections: Viral and Fungal Viruses express new antigens, making them less susceptible to residual immunity Viral infections rarely produce toxins but can cause tissue damage through cytokines produced by infected cells Symptoms of viral infection are generally mild and caused by the individual's own inflammatory response Fungal infections are caused by eukaryotic microorganisms with thick walls Fungi can exist as yeasts or molds and reproduce through simple division or budding Fungal cell walls are rigid and composed of different polysaccharides than bacteria Molds are aerobic, while yeasts are facultative anaerobes