Lactobacillus is a genus of gram-positive bacteria within the Lactobacillaceae family. Members of the genus are aerotolerant anaerobes or microaerophilic, rod-shaped, and do not form endospores. Until 2020, the genus Lactobacillus comprised 261 phylogenetically, ecologically, and metabolically diverse species; a taxonomic revision of the genus reassigned many former Lactobacillus species to 25 genera (see § Taxonomy below). Lactobacillus species constitute a significant component of the human and animal microbiota at a number of body sites, such as the digestive system and female genital system. Lactobacillus species are normally a major part of the vaginal microbiota. Lactobacillus forms biofilms in the vaginal and gut microbiota, allowing them to persist in harsh environmental conditions and maintain ample populations. Lactobacillus exhibits a mutualistic relationship with the human body, as it protects the host against potential invasions by pathogens, and in turn, the host provides a source of nutrients. Lactobacilli are among the most common probiotics found in food such as yogurt, and the bacteria are diverse in their application in maintaining well-being, by helping to treat diarrhea, vaginal infections, and skin disorders such as eczema.
Metabolism Lactobacilli are homofermentative, i.e., hexoses are metabolized by glycolysis to lactate as the major end product, or heterofermentative, i.e., hexoses are metabolized by the phosphoketolase pathway to lactate, CO2, and acetate or ethanol as major end products. Most lactobacilli are aerotolerant and some species respire if heme and menaquinone are present in the growth medium. Aerotolerance of lactobacilli is manganese-dependent and has been explored (and explained) in Lactiplantibacillus plantarum (previously Lactobacillus plantarum). Lactobacilli generally do not require iron for growth. The Lactobacillaceae are the only family of the lactic acid bacteria (LAB) that includes homofermentative and heterofermentative organisms; in the Lactobacillaceae, homofermentative or heterofermentative metabolism is shared by all strains of a genus. Lactobacillus species are all homofermentative, do not express pyruvate formate lyase, and most species do not ferment pentoses. In L. crispatus, pentose metabolism is strain specific and acquired by lateral gene transfer.
Genomes The genomes of lactobacilli are highly variable, ranging in size from 1.2 to 4.9 Mb (megabases). Accordingly, the number of protein-coding genes ranges from 1,267 to about 4,758 genes (in Fructilactobacillus sanfranciscensis and Lentilactobacillus parakefiri, respectively). Even within a single species, there can be substantial variation. For instance, strains of L. crispatus have genome sizes ranging from 1.83 to 2.7 Mb, or 1,839 to 2,688 open reading frames. Lactobacillus contains a wealth of compound microsatellites in the coding region of the genome, which are imperfect and have variant motifs. Many lactobacilli also contain multiple plasmids. A recent study has revealed that plasmids encode the genes which are required for adaptation of lactobacilli to the given environment.
Species The genus Lactobacillus comprises the following species:
Taxonomy As of 2025, the genus Lactobacillus contains 50 validly published species which are adapted to vertebrate hosts or to insects. In recent years, other members of the genus Lactobacillus (formerly known as the Leuconostoc branch of Lactobacillus) have been reclassified into the genera Atopobium, Carnobacterium, Weissella, Oenococcus, and Leuconostoc. The Pediococcus species P. dextrinicus has been reclassified as a Lapidilactobacillus dextrinicus and most lactobacilli were assigned to Paralactobacillus or one of the 23 novel genera of the Lactobacillaceae. Two websites inform on the assignment of species to the novel genera or species (http://www.lactobacillus.uantwerpen.be/; http://www.lactobacillus.ualberta.ca/).
Phylogeny The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature and the phylogeny is based on whole-genome sequences.
Human health
Vaginal tract Lactobacillus s.s. species are considered "keystone species" in the vaginal microbiota of reproductive-age women. Most, but not all, healthy women have a vaginal microbiota dominated by one of four species of Lactobacillus: L. iners, L. crispatus, L. gasseri, and L. jensenii. Other women have a more diverse mix of anaerobic microorganisms and are still considered to have a healthy microbiome.
Interactions with pathogens Lactobacilli produce lactic acid, which contributes to the vaginal acidity, and this lowered pH is generally accepted to be the main mechanism controlling the composition of the vaginal microbiota. Lactobacilli are also proposed to produce hydrogen peroxide, which inhibits the growth and virulence of the fungal pathogen Candida albicans in vitro, though this is arguably not the main mechanism in vivo. In vitro studies have also shown that lactobacilli reduce the pathogenicity of C. albicans through the production of organic acids and certain metabolites. Both the presence of metabolites, such as sodium butyrate, and decrease in environmental pH caused by the organic acids reduce the growth of hyphae in C. albicans, which reduces its pathogenicity. Lactobacilli also reduce the pathogenicity of C. albicans by reducing C. albicans biofilm formation. On the other hand, following antibiotic therapy, certain Candida species can suppress the regrowth of lactobacilli at body sites where they cohabitate, such as in the gastrointestinal tract. In addition to its effects on C. albicans, Lactobacillus sp. also interact with other pathogens. For example, Limosilactobacillus reuteri (formerly Lactobacillus reuteri) can inhibit the growth of many different bacterial species by using glycerol to produce the antimicrobial substance called reuterin. Another example is Ligilactobacillus salivarius (formerly Lactobacillus salivarius), which interacts with many pathogens through the production of salivaricin B, a bacteriocin.
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