Lactobacillus is the name given to a group of Gram-negative bacteria that do not form spores but derive energy from the conversion of the sugar glucose into another sugar known as lactose. The name of the genus derives from the distinctive sugar use. Lactobacillus has a number of commercial uses, especially in aspects of dairy production, including the manufacture of yogurt. As well, Lactobacillus is part of the normal microbial population of the human adult vagina, where it exerts a protective effect.

Prominent examples of the genus include Lactobacillus acidophilus, Lactobacillus GG, Bifidobacterium bifidum, and Bifidobacterium longum.

A distinctive feature of the members of the genus Lactobacillus is the formation of lactic acid from glucose. This is the property that confers the sour taste to natural, Lactobacillus-containing yogurt. As well, the lactic acid lowers the pH of the environment that the bacteria dwell in. In the case...