The lactic acid bacteria (LAB) are a large and heterogeneous group of Gram-positive bacteria characterised by a strictly fermentative metabolism with lactic acid as the major end product during sugar fermentation. Typical LAB are non-sporing, catalase-negative, aerotolerant and nutritionally fastidious organisms. They are perfectly adapted to environments rich in nutrients and energy sources and their metabolism is aimed at acid production to outcompete other bacteria sharing the same habitat. Representatives of the LAB can be isolated from nearly all types of food, including fresh and processed meat, fish, cereals, vegetables and dairy products. Some LAB species are normal inhabitants of the mouth and intestine of mammals, whilst others are specially adapted to extreme environments such as alcoholic beverages or foods with a high salt content.