A probiotic is a food supplement containing living bacteria from species believed to be of benefit to the body. The effects of these micro-organisms are not fully known and very poorly understood. The most widely used probiotics are bacteria belonging to the Lactobacillus and Bifidobacteriumgenera.
These bacteria are added to the very many bacteria in the intestine (the figure usually quoted for human beings is 100,000 billion). The intestinal flora, particularly the flora which develops in the colon, helps to complete digestion and probably to preventing the growth of colonies of pathogenic bacteria. Its exact roles are still poorly understood.