Lactobacilli are non-spore-forming, gram-positive rods that play a crucial role in the normal human bacterial flora. They are commonly found in the mouth, gastrointestinal (GI) tract, and female genitourinary tract. These bacteria can exhibit a coryneform appearance with a bent morphology and often grow in chains. Most Lactobacillus species are facultative anaerobes, thriving in both aerobic and anaerobic environments.

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The GI tracts of various mammals are frequently colonized by Lactobacillus species, with the most commonly isolated being Lactobacillus brevis, L. casei, L. acidophilus, L. plantarum, L. fermentum, and L. salivarius. Additionally, lactobacilli have been identified in tooth plaque, saliva, and the vaginal tracts of both humans and other mammals.

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These bacteria are essential for maintaining a healthy GI tract and are typically not considered pathogens in healthy individuals, except in cases related to dental caries. Lactobacilli are regarded as protective organisms, believed to inhibit the growth of pathogenic bacteria through the production of lactic acid and other metabolites.