Food Poisoning and Foodborne Illness
What this domain tests
This section examines the difference between food poisoning and foodborne illness, the main causative organisms, and how onset time and symptoms point to a likely source. The exam expects named organisms and controls, not vague references to "germs".
Infection versus intoxication
- Infection: live bacteria colonise the gut and cause illness, e.g. Salmonella, Campylobacter, E. coli O157, Listeria monocytogenes.
- Intoxication: pre-formed toxin is eaten, e.g. Staphylococcus aureus, Bacillus cereus (emetic type), Clostridium botulinum.
- Toxin-mediated infection: bacteria grow in the gut and produce toxin, e.g. Clostridium perfringens, B. cereus (diarrhoeal type).
Onset helps: S. aureus 1–6 hours; B. cereus emetic 1–6 hours; C. perfringens 8–18 hours; Salmonella 12–72 hours; Campylobacter 2–5 days; E. coli O157 1–8 days; Listeria can be days to weeks.
Worked example
Cooked rice is left at room temperature overnight, reheated to 60°C, and served. Several customers vomit within 3 hours. The likely organism is Bacillus cereus (emetic toxin), because the toxin forms in rice held in the danger zone and is heat-stable; reheating does not destroy it. The control is to cool rice rapidly (within 90 minutes) and refrigerate, or hold hot above 63°C, and to reheat to 82°C only as a second control - not as a fix for overnight abuse.
Traps
- Blaming the last meal eaten; incubation may span days.
- Saying freezing kills bacteria - it does not; it pauses growth.
- Confusing Staphylococcus aureus toxin with Salmonella infection.
- Forgetting that Listeria grows at refrigeration temperature and affects pregnant, elderly and immunocompromised people.