any Gram-positive rod-shaped bacterium of the genus Lactobacillus, which ferments carbohydrates to lactic acid, for example in the souring of milk: family Lactobacillaceae
any of a genus (Lactobacillus) of bacteria that ferment milk, carbohydrates, etc., producing lactic acid and carbon dioxide: found as saprophytes or as parasites in the mouth, intestinal tract, and vagina of humans and other mammals