Lactobacillus

n. type of bacteria which produces lactic acid

Lactobacillus is a genus of Gram-positive facultative anaerobic bacteria. They are a major part of the lactic acid bacteria group, named as such because most of its members convert lactose and other sugars to lactic acid. They are common and usually benign. In humans they are present in the vagina and the gastrointestinal tract, where they are symbiotic and make up a small portion of the gut flora. Many species are prominent in decaying plant material. The production of lactic acid makes its environment acidic which inhibits the growth of some harmful bacteria. Several members of the genus have had their genome sequenced.
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Lactobacillus /Lac·to·bac·il·lus/ a genus of the family Lactobacillaceae. They are anaerobic or microaerophilic and occur widely in nature and in [more]Lactobacillus - Community and Resources

Noun
1. gram-positive rod-shaped bacteria that produce lactic acid especially in milk
(hypernym) eubacteria, eubacterium, true bacteria
(member-holonym) genus Lactobacillus

Literally milk bacteria, normally found in the mouth, intestinal tract and vagina.