Micro organisms are very small life forms that can be seen only with a microscope. They include bacteria, viruses, moulds and yeasts.

Certain micro organisms can cause food spoilage and some of these (pathogens) also cause food borne illness. There are also pathogens that do not cause obvious spoilage. Other micro organisms are beneficial and can be used to change food properties as part of food processing (for example fermentation as part of the preparation of sausages, yoghurt and cheese) and to extend the shelf life of certain products.