Microbes and food 5. Food poisoners
Bacteria Cases
Salmonella 16,987
Campylobacter 62,867
E.coli O157 1,147
Clostridium perfringens 166
Listeria 133
Total 81,280
Table 1: Laboratory reports of the five main foodborne bacteria causing illness in the UK in 2000.
Source: Health Protection Agency.
5.2 Bacteria
Most foodborne illness is caused by bacteria. These single-celled microbes reproduce by splitting in two - often very rapidly. In the right conditions of warmth, acidity and moisture they can produce millions of cells in a few hours.

Some bacteria form spores which are resistant to drying and heating. They can survive cooking and will start to grow again in good conditions.

How do they cause food poisoning?
Food poisoning bacteria cause illness in different ways. For example:
  • They grow in food and produce a toxin. When swallowed this irritates the stomach lining, causing pain, nausea and sickness. The toxins may also enter the intestines and cause diarrhoea. Toxins can survive cooking, even though the bacteria that produced them are killed.
  • They are eaten with food and burrow into the intestine wall where they multiply and may produce toxins. Painful inflammation and diarrhoea follow. More severe symptoms such as kidney and liver damage may occur if the bacteria and toxins enter the blood stream.
  • They enter the body in food and do not cause symptoms in the gut but in other parts of the body.
Which bacteria cause food poisoning?
The table below shows some of the types of bacteria that cause food poisoning.
Photomicrograph of virus particles
Picture 5.2a E.coli bacteria – a common cause of food poisoning.
Salmonella photomicrograph
Picture 5.2b Salmonella with its flagella for moving around.
Bacteria photomicrograph
Picture 5.2c Staphylococcus aureus bacteria.
name of
original source risky foods time to develop symptoms
soil cooked rice and pasta; meat products; vegetables 1–5 hours nausea, vomiting and diarrhoea
This is a soil organism and spore-former. It produces toxins in food that is chilled too slowly.
One toxin causes nausea and vomiting within 1-5 hours, the other causes diarrhoea within 8-16 hours.
Campylobacter jejuni raw meat and poultry undercooked meat and poultry; raw milk and cross-contaminated food 3–5 days of eating infected food fever, severe pain and diarrhoea
A very common cause of food poisoning. It colonises the intestine wall producing enterotoxins.
soil faulty processed canned meat and vegetables; cured meat and raw fish. 1–7 days affects vision, causes paralysis and can be fatal
This is an anaerobic, spore forming soil organism. It produces a powerful neurotoxin in food which is absorbed into the bloodstream through the gut wall.Cases are very rare.
the environment large joints of meat; reheated gravies 8-24 hours nausea, pain and diarrhoea
An anaerobic sporeformer found widely in the environment. A common cause of food poisoning – though illness is not usually serious.
Spores survive cooking and multiply if the food is inadequately chilled. They produce a toxin in the gut. Millions of cells must be eaten to cause infection.
the gut of all humans and animals contaminated water, milk, inadequately cooked meat, cross-contaminated foods 3-4
inflammation, sickness and diarrhoea
Some types cause short attacks of gastro-enteritis, invading the gut wall and causing inflammation, sickness and diarrhoea. E. coli O157:H7 is a very nasty strain. Ingesting a few cells of this bacterium causes fever, bloody diarrhoea and kidney damage. If affects the very old and young worst, and can be fatal.
everywhere soft cheeses, paté, pre-packed salad; cook-chill products varies fever, headache, septicaemia and meningitis
It is carried by most animals and humans without harm if they are healthy. These bacteria are cold-tolerant, although growth is slow below 5°C. When eaten it colonises the gut and then spreads to other parts of the body.
It mostly affects the young, old and immuno-compromised, pregnant women and their babies.
Salmonella gut of animals, birds and humans - spread by faeces into water and food poultry, eggs and raw egg products, vegetables 6-48 hours diarrhoea, sickness and headaches
There are many types, most of which are human pathogens. It produces toxins in the gut wall, leading to illness.
Bacteria can be excreted long after the symptoms have gone. Typhoid fever is a severe Salmonella infection.
the skin and noses of animals and humans cured meat; milk products; unrefrigerated, handled foods 2-6 hours vomiting, pain and sometimes diarrhoea
It is salt tolerant and produces a toxin as it grows in food. The illness is common but short-lived.