Microbiology is the study of micro-organisms (microbes). Microorganisms are very small, usually single-celled, organisms which are not individually visible to the naked eye. They can only be seen with the aid of a microscope. They are widely distributed in the environment and are found in foods. Certain of them, if present in food in large enough numbers, can cause food poisoning. Micro-organisms are the main cause of food 'going off', Le. food spoilage. However, not all micro-organisms are undesirable. In fact, they are essential to all forms of life since they break down complex organic matter and return nutrients to the soil. Micro-organisms are used by man in the production of certain foods, e.g. bread and yoghurt.