- any long, slender, rod-shaped, anaerobic bacterium of the genus Lactobacillus, that produces large amounts of lactic acid in the fermentation of carbohydrates, especially in milk.
- 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
