Food Poisoning, Bacteria & Cross-Contamination

What Causes Food Poisoning?

Food poisoning is an illness caused by consuming food contaminated with harmful bacteria or the toxins those bacteria produce. In the UK, the Food Standards Agency (FSA) estimates that around 2.4 million cases of food-borne illness occur each year — including infections caused by bacteria, viruses and other pathogens. Understanding which bacteria cause most cases, and what conditions allow them to thrive, is essential for anyone preparing food.

Conditions Bacteria Need to Multiply: Temperature, pH, Moisture and Time

Bacteria multiply by dividing in two — one cell becomes two, two become four, and so on. Under ideal conditions, harmful bacteria such as Salmonella can double in number every 20 minutes. That means a single bacterium can become over one million in just under seven hours if left at the ideal temperature. Four factors control this multiplication rate:

Temperature is the most important control factor in a kitchen. Bacteria multiply fastest between 5°C and 63°C — this range is called the danger zone. Below 5°C (refrigerator temperature) multiplication slows significantly. Cooking needs a safe temperature-and-time combination at the centre; for example, 75°C for 30 seconds. Freezing stops reproduction altogether, though it does not kill bacteria.

pH refers to the acidity or alkalinity of the food. Most harmful bacteria prefer a near-neutral pH of 6.5–7.5. Very acidic foods (vinegar-pickled foods, citrus-based marinades) and highly alkaline foods inhibit bacterial growth. This is why pickling is an effective preservation method, and why acidic marinades slow bacterial growth on raw meat held in the fridge.

Moisture (water activity) is essential for bacterial growth. High-protein, high-moisture foods — cooked rice, chicken, soft cheese, cream-based sauces — provide the ideal substrate. Dry foods such as crackers, dried pasta and breakfast cereals have very low water activity and do not support rapid bacterial growth.

Time links directly to temperature. Bacteria left in the danger zone for extended periods reach dangerous levels. The general food-hygiene principle of “the two-hour rule” — not leaving prepared food at room temperature for more than two hours — reflects this. After two hours in warm conditions, bacterial counts in high-risk foods can reach levels that cause illness.

The Four Key Food-Poisoning Bacteria

The specification names four bacteria that can cause food poisoning. Each has a distinct source, a typical incubation period, and characteristic symptoms.

Salmonella is found in raw poultry, raw eggs, raw meat and unpasteurised dairy products. Contamination of a chicken fillet left to defrost on a kitchen counter at room temperature is a classic scenario. Symptoms include nausea, vomiting, abdominal cramps, diarrhoea and fever, typically appearing 12–72 hours after eating the contaminated food. Salmonella is destroyed by thorough cooking to a safe core temperature.

Campylobacter is the most common cause of food-borne illness in the UK according to the FSA. It lives in the gut of poultry and is found on raw chicken, unpasteurised milk and contaminated water. Even a small number of cells can cause illness. Symptoms appear 2–5 days after exposure and include severe diarrhoea (sometimes bloody), stomach cramps and fever. Prevention relies on: not washing raw chicken (which spreads droplets of contaminated water), thorough cooking to 75°C at the core, and rigorous hand-washing and surface cleaning.

E. coli (specifically the strain E. coli O157:H7) is associated with undercooked minced beef (burgers, meatballs), unpasteurised milk and contaminated salad vegetables. The symptoms can be severe — bloody diarrhoea, kidney complications — and very young children and elderly people are particularly at risk. Cooking minced beef thoroughly throughout (not just on the surface, as the grinding process moves surface bacteria into the centre of the meat) is critical.

Staphylococcus aureus is carried on human skin, in the nose and throat, and in spots and cuts. It can be transferred directly to food by handling and then multiply to produce a heat-stable toxin that causes rapid-onset illness — often within 1–6 hours of eating. Unlike most other pathogens, the Staphylococcus toxin is not destroyed by reheating, even to 75°C. High-risk foods include cooked meats, cream-filled pastries and ready-to-eat foods handled without adequate hygiene. Prevention relies on strict personal hygiene: washing hands thoroughly, covering cuts and sores, and not handling ready-to-eat food when unwell.

Types of Cross-Contamination and Their Prevention

Cross-contamination is one of the leading causes of food-borne illness in both homes and commercial kitchens. It occurs when bacteria spread from a contaminated source to food that will be eaten, usually without any further cooking to destroy them. There are three types:

Direct cross-contamination occurs when a contaminated food touches another food directly. A raw chicken breast resting against a cooked chicken breast on the same plate is a direct route for Campylobacter from the raw meat to reach the cooked meat.

Indirect cross-contamination occurs via a surface, utensil, cloth or person’s hands. A chopping board used to prepare raw mince and then used without washing to slice tomatoes for a salad is a classic indirect route. The same applies to a cook who handles raw poultry, then opens a fridge door without washing their hands — bacteria are now on the door handle.

Drip contamination occurs when raw meat or fish juices drip onto other foods in storage. Raw chicken stored above cooked leftovers in a fridge will drip contaminated juices onto the lower shelf.

Consequences of Poor Food Hygiene

Inadequate food hygiene causes food wastage as well as illness. Food that has been contaminated or allowed to spoil must be discarded, representing a financial loss to households and businesses. On a wider scale, food waste contributes to greenhouse gas emissions — particularly methane from organic matter decomposing in landfill. WRAP estimates that UK households wasted food and drink worth about £1,000 per household of four in 2022. In a commercial kitchen, a food-poisoning outbreak can result in closure by environmental health officers, significant fines, reputational damage and civil claims from affected customers — consequences that far exceed the small cost of correct food handling.