Micro-organisms are very small life-forms which include

  • bacteria
  • viruses
  • moulds and yeasts.


Size, Shape and Sources

A bacterium is a simple structure consisting of one cell that is not visible to the naked eye and can only be seen through a powerful microscope at a magnification of 100 times. One million bacteria, clumped together, would cover a pinhead. They vary in shape but those found in food are usually rod-shaped (bacilli), spherical (cocci) or comma-shaped (vibrios).

Bacteria are found everywhere - in air, water, soil, animals, people and food.

Most are not harmful and many bacteria serve a useful or even essential purpose. They are used, for example, in:

  • the manufacture of foods such as cheese and yoghurt;
  • the production of certain medicines;
  • the body's digestive process;
    the decaying process which is essential for sewage treatment or the breakdown of vegetable matter for compost.

A few types of bacteria, however are undesirable in food and fall into two categories:

Spoilage bacteria: certain types of bacteria cause food to rot and decay, but do not necessarily make people ill. They hasten deterioration of food and, although they are not visible, they produce changes in the food which may be detectable such as altered smell, taste, colour and texture. These effects are usually sufficient to alert people to the fact that the food is no longer fit to eat.

Pathogens: a few types of bacteria in food are responsible for causing illness and are referred to as pathogens. They may be present in food in large numbers but are not visible and may not cause obvious changes to the food so that it still looks, tastes and smells perfectly wholesome.

Spore Formation

A spore is a body containing the essential constituents of the bacteria but does not itself cause illness and is unable to multiply. Certain bacteria, Bacillus and Clostridium, are able to form spores which are capable of surviving in conditions which the active bacteria find unacceptable, e.g. high temperatures, lack of moisture, etc. As soon as conditions return to those in which bacteria may grow, the spore returns to the vegetative state (i.e. becomes an active bacterial cell), and will be able to grow and multiply once more.

Toxin Production

Some bacteria release poisons known as toxins which give rise to food-poisoning. Bacteria such as Staphylococcus aureus, Clostridium botulinum and Bacillus cereus produce toxins, known as exotoxins, whilst they multiply in food. The toxins of Staphylococcus and Bacillus are not easily destroyed by cooking and may remain in food once they have developed. Other bacteria such as Clostridium perfringens produce toxins inside the human body only after the food has been eaten. These are referred to as endotoxins.

Detection of Bacteria

Bacteria can normally only be seen through a powerful microscope and their detection and identification requires specialist work within a laboratory. Since individual cells are so small, most laboratory work normally starts by growing them to high numbers. Clusters of bacteria will develop, known as colonies. These are often of a distinctive shape, size and colour which will help with initial identification. Further examination and more complex testing may then need to be undertaken to analyse the specific type of organism.

Laboratory analysis is not only able to confirm the type of organism (e.g. Salmonella) but also the particular species (e.g. Salmonella enteritidis) and may, if necessary, subdivide further (e.g. Salmonella entertidis PT2 or PT4 [phage type 2 or 4]). This is very useful in food-poisoning outbreak investigation since analysis may help to demonstrate that exactly the same pathogen is present in food and faeces and thus clearly establish the source of the outbreak.


Viruses are even smaller than bacteria and can only be seen through an electron microscope. Most viruses which can cause food poisoning only multiply in human cells therefore it is difficult to detect them in the laboratory.

Viral food-poisoning can be caused by contamination of food by infected food handlers either from faeces or vomit. If the food is cooked these viruses are destroyed, but contaminated ready-to-eat foods or undercooked foods can cause illness.

The consumption of shellfish, such as cockles, mussels, oysters and clams, is commonly associated with viral food-poisoning. This is because shellfish are filter feeders and if they feed in water contaminated by sewage they can concentrate the viruses within their bodies. Shellfish are often eaten raw or lightly cooked which does not kill the viruses and therefore illness can occur.

Reports of viruses associated with food-borne illness have risen from approximately 320 in 1986 to 2050 in 1997.

Moulds and Yeasts

Moulds are a type of fungi which will grow on most foods and over a wide range of temperatures. Some are used in food production such as cheese manufacture but those which are undesirable are generally considered to be significant in terms of food spoilage rather than food-poisoning.

A very few species of mould produce toxins in food such as nuts, cereals and rice, which can be harmful to man and animals, but incidents of illness from these mycotoxins are extremely rare in the UK.

Yeasts are another type of fungi which will grow in food. They are used in the manufacture of foods such as bread and beer but also cause spoilage in many foods including jam, fruit juice, yoghurts and meats.