Lactobacillus is a genus of Gram-positive facultative bacteria, named as such because most of its members convert lactose and other simple sugars to lactic acid. They are common and usually benign -- indeed, necessary -- inhabitants of the bodies of humans and other animals - for example, they are present in the gastrointestinal tract and the vagina. Many species are prominent in decaying plant material. The production of lactic acid makes their environment acidic which inhibits the growth of some harmful bacteria. Some Lactobacillus species are used industrially for the production of yoghurt, sauerkraut, pickles, and other fermented foods, such as silage. Sourdough bread is made using a "starter" (also known as a "sponge") which is a symbiotic culture of yeast and lactobacteria growing in a water and flour medium. Some yoghurt drinks contain Lactobacillus bacteria as a dietary supplement. Many lactobacilli are unique among living things in requiring no iron and having extremely high hydrogen peroxide tolerance. Several members of the genus have had their genome sequenced.

Many lactobacilli are unusual in that they operate using homofermentative metabolism (that is, they produce only lactic acid from sugars) and are aerotolerant despite the complete absence of a respiratory chain. This aerotolerance is manganese-dependent and has been explored (and explained) in Lactobacillus plantarum.