Glossary · Dysbiosis and disease
Bacterial vaginosis
In one sentence
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.
Also known as: BV
What is bacterial vaginosis?
Bacterial vaginosis (BV) is a form of dysbiosis in the vaginal microbiota. Lactobacilli, which normally dominate and keep pH low, are depleted and replaced by a diverse anaerobic community: Gardnerella, Prevotella, Fannyhessea (formerly Atopobium) vaginae, Mobiluncus and Megasphaera, among others.
Unlike vaginitis, there is usually little visible inflammation, hence “vaginosis”. Many women have no symptoms. When present, the classic picture is thin, grey-white, homogeneous discharge with a fishy odour that is often worse after a period or sex. BV has been associated with a higher risk of acquiring sexually transmitted infections and with obstetric complications, although causality is not established in every case.
How it is diagnosed
Two classic methods remain the reference standard:
- Amsel criteria (Amsel and colleagues, American Journal of Medicine, 1983). BV is diagnosed when at least three of four findings are present: thin homogeneous discharge, vaginal pH above 4.5, clue cells on wet mount, and a positive whiff test (fishy amine odour after adding 10% potassium hydroxide).
- Nugent score (Nugent, Krohn and Hillier, Journal of Clinical Microbiology, 1991). A Gram-stained smear is scored from 0 to 10 according to lactobacillus, Gardnerella/Bacteroides and curved-rod morphotypes. A score of 7–10 is consistent with BV; 4–6 is intermediate.
Commercial molecular assays that quantify several BV-associated bacteria are also available. Microbiota sequencing describes the whole community but does not replace these diagnostic criteria.
Example
A woman attends her GP for the fourth time in a year with malodorous discharge. Vaginal pH is 5.0 and the whiff test is positive, and the episode is confirmed using Amsel criteria. Given the recurrences, she is referred to gynaecology, where a vaginal microbiota analysis is requested. The report shows predominant Gardnerella and Fannyhessea with very few lactobacilli. The result does not change the diagnosis, but it documents the community and gives a baseline for comparison after treatment.
What the evidence says
The cause of BV is not fully understood. Gardnerella biofilms are thought to be central, yet Gardnerella is also found in healthy women. Recurrence is common. A randomised trial published in the New England Journal of Medicine in 2025 found fewer recurrences when male partners were treated as well, strengthening the case for a sexually transmitted component. The role of probiotics remains uncertain.
Bacterial vaginosis at Xenogene
Xenogene’s AMC Vaginal profiles the vaginal ecosystem by NGS. According to the product sheet, it reports the dominant lactobacilli, BV-associated species such as Gardnerella and Atopobium, diversity indices and a microbiota-based pH estimate, and recurrent bacterial vaginosis is among its listed indications. It is a complementary test: it is ordered by a healthcare professional, who interprets it alongside the clinical picture and diagnostic tests.
Frequently asked questions
Is bacterial vaginosis a sexually transmitted infection?
It is not classed as a classic STI, but it is linked to sexual activity and can also occur in women who have never had sex. A 2025 trial found fewer recurrences when male partners were treated too. Treatment decisions belong to the clinician.
Why does bacterial vaginosis keep coming back?
Recurrence after treatment is common. Proposed reasons include bacterial biofilms that survive treatment, reinfection from a partner and lactobacilli failing to recover. Repeated episodes warrant a gynaecological assessment.
How can you tell BV from thrush?
BV is a bacterial imbalance, typically with thin discharge, a fishy odour and raised pH. Thrush is caused by Candida yeasts and usually brings itching and thick discharge with normal pH. They need different treatments, so getting the diagnosis right matters.
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.
- Gardnerella vaginalis
Gardnerella vaginalis is a facultatively anaerobic vaginal bacterium associated with bacterial vaginosis. It is also found in women without symptoms, so its presence alone is not enough for a diagnosis.
- Lactobacillus
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.
- Candidiasis
Candidiasis is an infection caused by Candida yeasts, most often Candida albicans. It commonly affects the vulva and vagina or the mouth, and occurs when the yeast overgrows beyond normal colonisation.
- Dysbiosis
Dysbiosis is a change in the composition or function of the microbiota away from what is considered compatible with health: loss of diversity, loss of beneficial microbes or overgrowth of others.
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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