The Usual Suspects.
Salmonella lives in raw poultry and eggs; Listeria grows in the fridge; Bacillus cereus waits in cooked rice. Each rule of food safety is aimed at someone.
Salmonella lives in the guts of animals and reaches raw poultry, eggs and meat; cooking kills it. E. coli O157 comes from cattle; a very small dose can cause severe illness, which is why minced beef is cooked through (XVIII.8). Listeria monocytogenes lives in soil and in food factories, grows at fridge temperatures and threatens pregnancy (XIII.2). Clostridium perfringens and Bacillus cereus form spores that survive cooking and germinate as the food cools. Staphylococcus aureus comes from people's skin and noses and makes a toxin that cooking does not destroy (XII.9). Norovirus, not a bacterium, does not grow in food at all but passes from sick hands.
Each habit sets a rule: cook poultry thoroughly; cook minced beef through; keep the fridge at 5 °C or below and respect use-by dates; cool large dishes quickly; do not leave rice warm; wash hands and stay out of the kitchen when ill. The workshop compares the organisms.
Below: how fast each organism doubles at the temperature you choose and how many minutes at 60 °C give a tenfold kill (microbe bench), with each one's habits in a table.
- Name the main foodborne pathogens and their sources.
- Explain spores and heat-stable toxins.
- Match each pathogen to the rule that controls it.
- Know which grow in the fridge.
Know the enemy
At 4 °C only Listeria of these six can grow at all, doubling in this model about every 27 hours — slow, but over a week or two in the fridge that is enough to matter, which is why ready-to-eat foods carry use-by dates and why pregnant women are advised to avoid deli meats, unpasteurised soft cheeses and cold-smoked fish unless heated until steaming. The others need warmth: their minimum temperatures range from about 5 °C to 15 °C.
Spores and toxins are the reason some rules exist at all. Cooking kills the growing cells of C. perfringens and B. cereus but not their spores, which wake as the food cools through the danger zone; hence the rules on cooling large dishes and rice quickly (XII.7). S. aureus's toxin and B. cereus's emetic toxin survive boiling; once made, they cannot be cooked away (XII.9). Norovirus needs no warmth at all, only a sick person's hands: anyone with vomiting or diarrhoea should not cook for others until 48 hours after symptoms end.
Look through your fridge for ready-to-eat foods — sliced meats, smoked fish, soft cheese, prepared salads — and check their use-by dates and how long they have been open.
Terms
- Spore
- A dormant, heat-resistant form of certain bacteria.
- Toxin
- A poison made by a microbe; some survive cooking.
- Ready-to-eat food
- Food eaten without further cooking, such as deli meat.
- Infective dose
- The number of organisms needed to cause illness.
Sources
- ICMSF, Microorganisms in Foods 5: Characteristics of Microbial Pathogens, Blackie, 1996 — growth limits (minimum temperature, pH and water activity) and typical heat resistance; the course rounds them.
- Heat resistance of Listeria monocytogenes in meat: D(60 °C) of a few minutes, z about 6–7 °C (summarised in ICMSF 1996 and Food Code annex 3). Teaching values within the range.
- Clostridium perfringens grows between about 15 and 50 °C with an optimum near 43–45 °C, where it can double in under ten minutes; its spores survive cooking (ICMSF 1996; FDA Bad Bug Book, 2nd ed., 2012).
- Bacillus cereus forms heat-resistant spores and the emetic toxin cereulide, which survives reheating; cooked rice held warm is the classic vehicle (FDA Bad Bug Book, 2nd ed., 2012).