Lactobacilli are non-spore-forming, gram-positive rods that are an important part of the normal human bacterial flora commonly found in the mouth, gastrointestinal (GI) tract, and female genitourinary tract. They can also appear as coryneform with a bent morphology or tend to grow in chains. Most species of lactobacilli are facultative anaerobes growing in either the presence or absence of an anaerobic environment.
The GI tracts of various mammals are commonly colonized with Lactobacillus spp. The most common species of lactobacilli isolated from GI tracts are Lactobacillus brevis, L. casei, L. acidophilus, L. plantarum, L. fermentum, and L. salivarius. Lactobacilli have also been isolated from tooth plaque, saliva, and the vaginal tracts of humans and other mammals.
Lactobacilli are required to maintain a healthy GI tract and are not usually considered to be pathogens in the healthy host except when associated with dental caries. They are considered protective organisms and are thought to inhibit the growth of pathogenic organisms via the production of lactic acid and other metabolites.