Food Contamination by Micro-organisms

Food contamination is usually caused by bacteria, but it can also sometimes be caused by viruses or parasites. 
Some of the main sources of contamination and incubation periods (time between eating contaminated food and start of symptoms) are:


Campylobacter bacteria are usually found in raw or undercooked meat (particularly poultry) unpasteurised milk and untreated water.
Incubation period: 2 – 5 days, symptoms last less than a week 


Salmonella bacteria are usually found in raw or undercooked meat, raw eggs, milk and other dairy products.
Incubation period: 12 – 72 hours, symptoms last 4 – 7 days


Listeria bacteria maybe found in a range of chilled, ready to eat foods including pre-packed sandwiches, cooked slice meats and pate and soft cheeses (such as Brie or Camembert). These foods should be eaten within their “use-by” dates. This is particularly important for pregnant women, because a listeria infection in (known as listeriosis) in pregnancy can cause pregnancy and birth complications and can result in miscarriage. 
Incubation period: Few days to several weeks, symptoms normally pass within 3 days


Escherichia coli, often known as E. coli, are bacteria found in the digestive systems of many animals, including humans. Most strains are harmless but some can cause serious illness. Most cases of E. coli food poisoning occur after eating undercooked beef (particularly mince, burgers and meatballs) or drinking unpasteurised milk.
Incubation period: one to eight days. The symptoms usually last for a few days or weeks.


Shigella bacteria can contaminate any food that has been washed in contaminated water. An infection caused by Shigella bacteria is known as bacillary dysentery or shigellosis. 
Incubation period: 7 days, last a week 


Norovirus is the most common virus that cause diarrhoea and vomiting. It's easily spread from person to person, through contaminated food or water. Raw shellfish, particularly oysters, can also be a source of infection.
Incubation period: 24-48 hours, symptoms usually last a few days.
Rotavirus is a common cause of infection in young children and is caused by consumption of contaminated food. 
Incubation period: 7 days, last 5 – 7 days.


Food poisoning by parasites is rare in the UK and is more common in developing countries. The following are parasitic infections that can be spread in contaminated food:

  • giardiasis – an infection caused by a parasite called Giardia intestinalis
  • cryptosporidiosis – an infection caused by a parasite called Cryptosporidium
  • ameobiasis – a type of dysentery caused by a single-cell parasite (ameoba) called Entamoeba histolytica (this is very rare in the UK)

Incubation period: 10 days – several weeks, duration of symptoms depends on treatment. If left untreated, the symptoms can last several weeks or even a few months.

Food contamination

Food can become contaminated at any stage during its production, processing or cooking and but can be avoided by:

  • not consuming food that has been touched by someone who is ill or has been in contact with someone experiencing vomiting and diarrhoea 
  • storing foods that need to be chilled correctly at or below 8o c 
  • cooking foods thoroughly (particularly meat)
  • keeping cooked foods in refrigeration
  • cooling foods quickly and putting into refrigeration
  • preventing cross contamination between cooked/ready to eat foods and raw foods.

Cross-contamination can occur, for example when you prepare raw chicken on a chopping board and don’t wash the board before preparing food that won’t be cooked, such as salad as the harmful bacteria can be spread from the chopping board to the salad. 
It can also occur if raw meat is stored above ready to eat foods and the juices of the meat drip on to the food below. Read more about Food poisoning. 

Food businesses

Owners of all food premises have a responsibility to comply with the Food Safety Act 1990 and other regulations. It is also the owner’s responsibility to ensure that their staff act upon that information and comply with current legislation regarding food safety.


There is no requirement to wear gloves when handling food. Gloves maybe worn as a form of protective clothing but it is important to ensure that gloves are kept clean and are regularly changed after handling raw products and before handling ready to eat products to prevent the risk of cross contamination 
If gloves are not worn, hands must be kept clean, wounds must be covered and jewellery (sleepers and a plain wedding band are acceptable) and false nails should not be worn. 

Fitness to work 

If you handle food, you have a responsibility to do all you can to avoid spreading germs. Read more about fitness to work 

Food sampling

Under the Food Safety Act 1990, Environmental Health Officers from the local authority have powers to carry out bacterial sampling of food, food equipment and food handlers. The local authorities will send samples off to a laboratory for microbiological analysis. 

Last updated: 29 November 2019