Bacterial Contamination and Food Poisoning
How Bacteria Get into Food
Bacterial contamination rarely arrives in one dramatic moment — it usually creeps in through a series of small mistakes. A chopping board used for raw chicken that is then used for salad leaves, a food handler who forgets to wash their hands after visiting the toilet, a pet walking across a kitchen surface — these are the real routes by which harmful bacteria reach the food we eat. Understanding the sources of bacterial contamination is the first step in preventing it.
The main sources of contamination are:
- Other contaminated foods — particularly raw foods: Raw meat, poultry, eggs, seafood and unwashed vegetables all carry bacteria naturally. Raw chicken is one of the most significant sources of campylobacter in the UK. Even raw vegetables from the soil can carry bacteria such as e-coli if they have been in contact with animal manure during growing or harvesting.
- Work surfaces and equipment: Bacteria survive on worktops, chopping boards, knives and mixing bowls for hours. A surface that was used for raw fish and wiped down with a damp cloth (rather than washed with hot soapy water) remains a contamination risk.
- The people cooking: Hands are the main vector from person to food. Poor personal hygiene — sneezing over open food, touching the face, failing to wash hands after handling raw meat — is a leading cause of staphylococcus aureus contamination specifically.
- Pests: Mice, rats, cockroaches and flies carry bacteria on their bodies and in their droppings. A fly landing on cooked food and then on raw meat transfers bacteria in both directions.
- Waste food and rubbish: Decomposing food in bins creates a reservoir of bacteria that can spread if bins are stored near food preparation areas or if hands are not washed after handling waste.
The five sources of bacterial contamination are: other contaminated foods (especially raw meat, poultry, eggs, seafood and vegetables); work surfaces and equipment; the people cooking; pests; and waste food and rubbish.
The Five Food-Poisoning Bacteria
You need to know five specific bacteria by name — their typical sources, the conditions that favour them and the key control methods. A short-answer question asking you to name two and give their sources is very common. Memorise all five: campylobacter, e-coli, salmonella, listeria and staphylococcus aureus. Students who know only salmonella consistently lose marks on this topic.
Campylobacter is found primarily in raw and undercooked poultry, particularly chicken. It is also found in unpasteurised milk, contaminated water and the intestines of farm animals. It is responsible for the majority of bacterial food poisoning cases in the UK each year. The key control is thorough cooking — campylobacter is destroyed when poultry reaches an internal temperature of 75°C throughout. Do not wash raw chicken: washing spreads campylobacter-laden droplets up to a metre around the sink. Campylobacter does not multiply particularly fast in food at room temperature; its main risk is survival from undercooking.
E-coli (Escherichia coli): Most strains of e-coli are harmless gut bacteria, but certain strains (particularly E. coli O157) produce toxins that cause severe illness. Sources include undercooked ground beef (minced meat), contaminated water, and raw vegetables grown in soil fertilised with animal manure. Control methods: thorough cooking of minced meat to 75°C throughout; washing raw vegetables; good hand hygiene; pasteurising milk.
Salmonella: Found in raw poultry, raw eggs, raw meat and some unpasteurised dairy products. Contamination can come from the inside of an egg if the laying hen was infected, which is why runny fried eggs carry a risk if the eggs are not from a Lion Code flock. Control: thorough cooking to 75°C; good personal hygiene when handling raw eggs; refrigerating raw poultry separately from cooked foods.
Listeria (Listeria monocytogenes) is unusual because it can multiply at refrigerator temperatures — between 0°C and 5°C — making it a particular threat in chilled ready-to-eat foods. Sources include soft mould-ripened cheeses (such as Brie and Camembert), pâté, cold deli meats and smoked fish. Listeria is especially dangerous for pregnant women, where it can cause miscarriage or serious illness in the newborn. Control: keep chilled food below 5°C and consume within use-by date; vulnerable groups should avoid high-risk products.
Staphylococcus aureus is carried naturally on human skin, in the nose, throat and hair. It reaches food when food handlers sneeze, cough, touch their face or have skin infections such as cuts or boils. Once in the food, it multiplies and produces toxins that are heat-resistant — meaning cooking the food again will not destroy the toxin even if the bacteria are killed. High-risk situations include cooked rice, egg mayonnaise, and cream-filled pastries left at room temperature. Control: strict personal hygiene; keeping food below 5°C or above 63°C; covering cuts with blue waterproof plasters.
The five food-poisoning bacteria you must know: campylobacter (raw poultry), e-coli (undercooked minced beef, contaminated water), salmonella (raw poultry, raw eggs), listeria (soft cheeses, chilled ready-to-eat meats — can grow at fridge temperature), staphylococcus aureus (human skin/nose/throat, cooked foods at room temperature).
Symptoms of Food Poisoning
Food poisoning is caused either by the bacteria themselves infecting the gut, or by toxins they have already produced in the food before it is eaten. The general symptoms are consistent across most types, although their severity and timing vary:
- Nausea (feeling sick)
- Vomiting
- Diarrhoea
- Stomach cramps and abdominal pain
- Fever (raised body temperature)
- Headache and general malaise
Symptoms typically develop between one hour and several days after eating contaminated food, depending on the bacterium. Staphylococcus aureus toxin can cause symptoms within 30 minutes to 8 hours (often 1 to 6 hours) because the toxin is already present in the food. Campylobacter infections typically take two to five days to develop because the bacteria must first establish in the gut. Most healthy adults recover within a few days, but young children, elderly people, pregnant women and those with weakened immune systems can develop serious complications, including dehydration and, in rare cases, kidney failure.