Glossary · Microbiota
Lactobacillus
In one sentence
Lactobacillus is a genus of Gram-positive, lactic acid-producing bacteria. Species such as L. crispatus, L. gasseri, L. iners and L. jensenii commonly dominate the vaginal microbiota of women of reproductive age.
Also known as: lactobacilli · Lactobacillus crispatus · Lactobacillus iners · Lactobacillus gasseri · Lactobacillus jensenii
What is Lactobacillus?
Lactobacillus is a genus of rod-shaped, Gram-positive bacteria that tolerate little or no oxygen and ferment sugars into lactic acid. Members of the group live in the gut, mouth and vagina, and are used widely in fermented foods.
The genus was split up in 2020. Zheng and colleagues (International Journal of Systematic and Evolutionary Microbiology, 2020) reclassified the old genus, then more than 260 species, into 25 genera. Many food and probiotic species moved: Lactobacillus rhamnosus became Lacticaseibacillus rhamnosus, and L. reuteri became Limosilactobacillus reuteri. The main vaginal species, L. crispatus, L. iners, L. gasseri and L. jensenii, stayed in Lactobacillus. “Lactobacilli” remains a useful umbrella term for the whole group.
What it does in the vagina
In the vaginal microbiota, lactobacilli are linked to a stable environment through several mechanisms described in the literature:
- Acidification: lactic acid keeps vaginal pH low.
- Competition: they occupy niches and nutrients other organisms could use.
- Other antimicrobial compounds, such as bacteriocins, whose real-world importance is still being studied.
Species differ. In Ravel and colleagues’ 2011 PNAS study, four of the five vaginal community state types were each dominated by a different Lactobacillus species. Communities dominated by L. crispatus have been associated with a lower likelihood of bacterial vaginosis. L. iners behaves differently: it is found both in stable communities and in transitional ones, and its role is debated.
Example
A vaginal microbiota report shows L. iners at 85% relative abundance with a small fraction of Gardnerella, in a woman without symptoms. The requesting clinician reads this as a Lactobacillus-dominated community with no finding that, on its own, points to disease, and considers whether her history, such as previous episodes of vaginosis, warrants follow-up. The figures here are illustrative; reference ranges depend on the method and the laboratory.
Lactobacillus and probiotics
Many probiotics contain lactobacilli or species reclassified from them. Effects are strain-specific, and the evidence for vaginal indications is mixed. Finding, or not finding, lactobacilli in a test is not by itself a reason to start treatment.
Lactobacillus at Xenogene
Xenogene’s AMC Vaginal uses NGS to profile the vaginal microbiota. According to the product sheet, it reports the dominant lactobacilli (L. crispatus, L. gasseri, L. iners) alongside species associated with dysbiosis, diversity indices and a microbiota-based pH estimate. The test is ordered by a healthcare professional, who is responsible for interpreting the result in clinical context.
Frequently asked questions
What does it mean if a test shows low vaginal lactobacilli?
A low proportion of lactobacilli is associated with profiles such as bacterial vaginosis, but it is also seen in women with no symptoms. On its own it is not a diagnosis; the clinician interprets it together with symptoms and other tests.
Is Lactobacillus iners a good or bad bacterium?
Neither label fits well. L. iners dominates the vaginal community of many healthy women, but it is also common during transitions towards less stable profiles. Its role is still debated, and the overall community matters more than one species.
Do probiotic lactobacilli raise vaginal Lactobacillus levels?
The evidence is mixed and depends on the strain, route and indication. Some trials have tested specific strains, for example of L. crispatus, with variable results. It is worth discussing with a healthcare professional before using them.
Related terms
- Vaginal microbiota
The vaginal microbiota is the community of microorganisms living in the vagina. In many women of reproductive age it is dominated by Lactobacillus species, which produce lactic acid and help keep vaginal pH low.
- Bacterial vaginosis
Bacterial vaginosis is a shift in the vaginal microbiota in which lactobacilli decline and anaerobic bacteria such as Gardnerella overgrow. It can cause discharge and odour, but is often symptom-free.
- Probiotic
A probiotic is a live microorganism that, when administered in adequate amounts, confers a health benefit on the host. This is the FAO/WHO definition, retained by the ISAPP consensus panel in 2014.
- Eubiosis
Eubiosis is the state in which the microbiota is stable, diverse and compatible with the host's health. It is the opposite of dysbiosis, although neither term has an agreed operational definition.
Educational content for healthcare professionals and the general public. It does not replace assessment by a healthcare professional: ordering and interpreting any test is the responsibility of the requesting clinician.
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