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. 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. 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.