Pathogenic Gram-Positive Bacteria

Pathogenic Gram-Positive Bacteria Outline Facultatively anaerobic Gram-positive cocci Staphylococcus spp. Streptococcus spp. Enterococcus spp. Facultatively anaerobic Gram-positive bacilli Bacillus spp. Listeria spp. Corynebacterium spp. Nocardia and Actinomyces spp. Lecture Summary Staphylococcus Bacteria in the genus Staphylococcus live and reproduce on of human skin as normal members of the microbiota. However, they can be opportunistic pathogens, causing minor to lifethreatening disease. Structure and Physiology Staphylococcus is a genus of Gram-positive, facultatively anaerobic prokaryotes whose spherical cells are typically clustered in grapelike arrangements. Staphylococcal cells are salt tolerant; thus, they tolerate the salt deposited on human skin by sweat glands. They also tolerate desiccation, radiation, and heat. Further, they synthesize catalase. The most virulent species is Staphylococcus aureus which can colonize the skin and anterior nares of 10-30% of healthy individuals. Staphylococcus aureus is the only human staphylococci that produces coagulase enzyme. Therefore, staphylococci other than S. aureus is referred to as coagulase negative staphylococci (CNS). Staphylococcus epidermidis and other coagulase negative staphylococci are normal microbiota members of the skin and are important opportunistic pathogens. Pathogenicity Staph infections result when staphylococci breach the body’s physical barriers. The virulence factors aid in disease formation is given below. Structural Defenses Against Phagocytosis S. aureus cells are coated with protein A that interferes with antibody immune responses and the complement cascade by binding to the stem of IgG. The outer surfaces of most pathogenic strains of S. aureus contain bound coagulase (clumping factor: CF), an enzyme that converts fibrinogen into fibrin, producing clots that hide the bacteria from phagocytes. Both S. aureus and S. epidermidis (and certain other coagulase negative staph) also synthesize polysaccharide slime layers that inhibit chemotaxis of, and phagocytosis by, leukocytes and facilitate attachment to biotic and abiotic surfaces. Cells of S. aureus can be covered by a polysaccharide capsule which inhibits phagocytosis. Enzymes Staphylococci produce a number of enzymes that contribute to their pathogenicity. S. aureus produces cell-free coagulase in addition to the bound form. Other enzymes produced include hyaluronidase and staphylokinase, which enable the bacteria to spread within the body; lipases, which allow growth in sebaceous glands; and a -lactamase, which allows S. aureus to break down beta-lactam antimicrobial drugs. Toxins S. aureus, and sometimes S. epidermidis, produce cytolytic toxins that lyse cells and exfoliative toxins that cause sloughing of the skin. Toxic shock syndrome toxin and enterotoxins cause toxic shock syndrome and food poisoning, respectively. Epidemiology As noted, staphylococci are found on human skin, as well as in the upper respiratory, gastrointestinal, and urogenital tracts. They are transmitted through direct contact and indirect contact via fomites, and proper handwashing and aseptic techniques are essential in preventing their transfer in health care settings. Staphylococcal Diseases Staphylococcus aureus cause a variety of medical problems that can be categorized as toxin mediated diseases, suppurative and systemic infections. Staphylococcus epidermidis and other CNS are important opportunists in healthcare settings. They can also be isolated as contaminants in blood culture. Staphylococcus saprophyticus is a common pathogen causing acute non complicated urinary tract infections in young women. Toxin mediated diseases Food poisoning is one noninvasive disease caused by Staphylococcus. The food must remain at room temperature or warmer for several hours for the bacteria to grow and secrete enterotoxin, which produces the signs and symptoms of the disease. Signs and symptoms include vomit- ing, diarrhea, headache, and abdominal pain, and they usually appear within four (1-5) hours of ingestion. The disease is self-limiting, usually resolving within about 24 hours. Staphylococcal scalded skin syndrome is caused by strains that produce exfoliative toxins and produces reddening of the skin that typically begins near the mouth, spreads over the entire body. Within two days, the affected skin peels off in sheets. It is mostly seen in infants and young children. In older children and adults, a localized form called bullous impetigo which is characterized by large blisters that contain clear fluid lacking bacteria or white blood cells can be seen. Toxic shock syndrome, non-streptococcal (TSS), which is characterized by fever, vomiting, red rash, low blood pressure, and loss of sheets of skin in response to production of TSS toxin. The toxin can pass mucosal barriers, therefore systemic signs and symptoms occur even though bacteria does not invade the bloodstream. Suppurative and systemic infections Staphylococcus aureus causes localized pyogenic lesions. Impetigo is a skin disorder in children in which small, flattened, red patches appear on the face and limbs, and develop into pus-filled vesicles that eventually crust over. Folliculitis is an infection of a hair follicle, and when it occurs at the base of an eyelid, it is called a sty. A furuncle, or boil, is a large, painful, raised nodular extension of folliculitis into surrounding tissue. When several furuncles fuse, they form a carbuncle, which extends deeper into the tissues. S. aureus is a common cause of bacteremia, the presence of bacteria in the blood. It may attack the lining of the heart, producing a condition called endocarditis, or it may invade the lungs, causing pneumonia. In 10% of patients with staphylococcal pneumonia, the fluid filling the alveoli is pus; this condition is called empyema. When Staphylococcus invades bone, it causes osteomyelitis, inflammation of the bone marrow and surrounding bone. S. aureus is the primary cause of septic arthritis in young children and in adults who are receiving intraarticular injections or who have mechanically abnormal joints. Staphylococcus epidermidis can cause a variety of opportunistic infections (endocarditis, catheter and shunt infections, prosthetic joint infections) and is the most common coagulase negative staphylococci isolated from clinical specimens. S. saprophyticus has a predilection for causing non complicated urinary tract infections in young, sexually active women. Diagnosis, Treatment, and Prevention Culture is the gold standard. In direct microscopic examination done from pus, blood, or other fluids Gram-positive cocci in grapelike arrangements is detected. Staphylococci can be distinguished from streptococci and enterococci by catalase test. Coagulase-positive S. aureus can be easily differentiated from other staphylococci.