DISEASES CAUSED BY BACTERIAL TOXINS (Micro made easy 31)

Staphylococcus aureus: Gram positive cocci, that live in “clusters”, catalase positive, coagulase positive (enzyme that clots blood) and elaborate a golden pigment when cultured in sheep blood agar. (note that Staph epidermidis & saprophyticus are coagulase –ve). Read pp 32 of Micro made easy for summary of Staph aureus. Staph aureus can replicate in foods, and while doing so release enterotoxins (i.e.: exotoxins A-E), and these are eaten by humans when food is eaten. This stimulates peristalsis ≫ therefore causing cardinal diarrhoea symptoms. Takes 2-6 hrs for this to happen.

Bacillus cereus: Spores are deposited in foods, they do not die when foods are cooked≫ spores survive and germinate and produce enterotoxin (like Staph aureus). Two types of enterotoxins produced: 1) Heat labile, 2) Heat stable. Former causes syndromes like E.Coli, latter causes syndromes like Staph aureus.

Clostridium perfringens: Clostridium is anaerobic + spore forming (Bacillus is aerobic + spore forming). Bacteria produces spores that survive cooking ≫ germinate and release powerful exotoxins (alpha most important). Toxins destroy the intestinal epithelium ≫cause diarrhoeal symptoms. Incubation period of 18-35hrs.

Clostridium difficile: Causes pseudomembrane colitis (antibiotic associated). A course of broad spectrum antibiotics destroys the normal colonic flora ≫ allowing over growth of Clostridium difficile. It then releases exotoxins A (causes diarrhea) + B (cytotoxic to colonic mucosal cells). Colonoscopy reveals mucosal ulcers and white fibrinous exudates covering mucosa≫ pseudomembranes. Complications include: toxic megacolon, perforation, peritonitis. Antibiotics that cause this include: penicillin, cephalosporins, and penicillin. Treatment: stop antibiotic ≫ give vancomycin + metronidazole (these are not absorbed in gut – so can kill Clostridium difficile).

Clostridium botulinum: Causes botulism, transmitted by eating home-canned vegetables & smoked fish. Spores are living in such foods BUT are killed when heated (unlike: Bacillus cereus). If not cooked properly ≫ spores germinate (anaerobic environment) ≫ release powerful neurotoxin ≫ binds to pre-synaptic neurons and motor end plates to inhibit release of Ach. Muscle paralysis results. Compare this to Clostridium tetani.

Yersinia spp, Aeromonas spp, and Listeria monocytogenes are psychotrophic bacteria ≫ meaning, they will still grow under refrigerated conditions.

Category: Microbiology Notes