Food Poisoning and Food-Borne Illness
This domain tests your knowledge of the main food-borne pathogens: where they live, which foods are associated with them, how long they take to cause symptoms, and which control measure breaks the chain. Questions typically ask you to match a pathogen to a food vehicle, or to explain why a particular practice - cooling rice quickly, for example - matters.
Bacteria and their signatures
- Salmonella - poultry, eggs, raw meat, unwashed produce. Incubation 12–72 hours. Diarrhoea, fever, abdominal pain.
- Campylobacter - the most common UK cause of bacterial gastroenteritis; raw poultry, unpasteurised milk, contaminated water. Incubation 2–5 days.
- Clostridium perfringens - spores survive cooking; grows in slowly cooled stews, gravies and large meat joints. Incubation 8–22 hours. Control: cool fast and reheat thoroughly.
- Bacillus cereus - cooked rice left at room temperature (emetic type, 1–6 hours) and starchy foods (diarrhoeal type, 8–16 hours). Reheating does not destroy the emetic toxin.
- Staphylococcus aureus - hands, skin, nose, wounds; transferred to cooked ham, cream cakes and milk. Toxin is heat-stable.
- E. coli O157 - raw minced beef, unpasteurised dairy, animal contact. Very low infectious dose; can cause haemolytic uraemic syndrome.
- Listeria monocytogenes - chilled ready-to-eat foods, soft cheese, pâté, smoked fish. Grows at refrigeration temperature; dangerous in pregnancy and for immunocompromised people.
- Clostridium botulinum - anaerobic, low-acid canned and vacuum-packed foods; heat-labile toxin but heat-resistant spores.
Viruses and other agents
Norovirus spreads person-to-person and via shellfish and ready-to-eat food handled by an infected person; it is not killed by cooking reliably and survives on surfaces. Hepatitis A is linked to shellfish and unwashed produce. Scombroid (histamine) poisoning follows poor temperature control of tuna and mackerel; shellfish toxins are not destroyed by cooking.
Worked example
A party where guests ate cooked rice at 7 pm and several vomited by 11 pm. Reasoning: extremely short incubation plus vomiting points to Bacillus cereus emetic toxin, formed while the rice sat warm. Reheating will not help, because the toxin is heat-stable. The control is to cool rice within 90 minutes and refrigerate, or keep it above 63 °C.
Traps that cost marks
- Assuming reheating makes any previously mishandled food safe - true for vegetative bacteria, false for pre-formed toxins.
- Mixing up B. cereus emetic (fast, vomiting) and diarrhoeal (slower) forms.
- Ignoring that Listeria multiplies in the fridge.
- Forgetting viruses need hand hygiene, not just cooking.