The vaginal microbiome explained: lactobacilli, pH and your health
In 30 seconds
- The vagina has its own community of bacteria, the vaginal microbiome. In most women it is dominated by lactobacilli1.
- Lactobacilli help keep the vagina slightly acidic, which protects against infection. When they drop, conditions like BV can develop1,3.
- Probiotics are popular, but current evidence is not strong enough to recommend them to treat BV or thrush4,2.
Your gut isn’t the only place with its own bacteria. The vagina has a microbiome too, and it plays a big part in vaginal health. Here’s what the science says.
A landmark study
In 2011, researchers mapped the vaginal bacteria of 396 women without symptoms, from four ethnic groups in North America1. They found five main types of bacterial “community”:
| Community type | Main bacterium | What it means |
|---|---|---|
| I | Lactobacillus crispatus | Lactobacillus-dominated |
| II | Lactobacillus gasseri | Lactobacillus-dominated |
| III | Lactobacillus iners | Lactobacillus-dominated |
| IV | Mixed, fewer lactobacilli | More anaerobic bacteria; higher pH |
| V | Lactobacillus jensenii | Lactobacillus-dominated |
Four of the five types were dominated by lactobacilli. The fifth had fewer lactobacilli and more of other bacteria. Vaginal pH and community type also varied between ethnic groups1.
Why lactobacilli and pH matter
Lactobacilli make lactic acid, which keeps the vagina slightly acidic1. A normal vaginal pH is below 4.52. This acidity helps keep unwanted bacteria in check.
When lactobacilli drop and other bacteria take over, the vagina becomes less acidic. This is what happens in bacterial vaginosis (BV), and it lowers the vagina’s natural defences against infection3. Around a quarter of women of reproductive age have BV at any one time5.
Good to know
A “mixed” microbiome isn’t always a problem. In the 2011 study, all the women had no symptoms, including those with fewer lactobacilli1. What’s normal can differ between women.
What upsets the balance
The NHS lists things that make BV more likely, including a new sexual partner, an IUD and perfumed products used in or around the vagina3. Douching is also linked with BV6. The vagina cleans itself, and normal discharge is part of that7.
What about probiotics?
Probiotics are widely sold for vaginal health. But the evidence is limited:
- A Cochrane review found there isn’t enough evidence to recommend probiotics to treat BV4.
- US guidelines say there isn’t enough evidence to support probiotics for treating thrush2, and no studies support them for BV6.
That may change as research continues. For now, if you have symptoms, the proven treatments are antibiotics for BV and antifungals for thrush3,8.
Quick check: What is a normal vaginal pH?
Below 4.5, which is slightly acidic2.
References
- Ravel J, Gajer P, Abdo Z, et al. Vaginal microbiome of reproductive-age women. Proc Natl Acad Sci USA. 2011;108(Suppl 1):4680-4687. doi.org/10.1073/pnas.1002611107
- Workowski KA, et al. Sexually Transmitted Infections Treatment Guidelines, 2021: Vulvovaginal candidiasis. Centers for Disease Control and Prevention. www.cdc.gov/std/treatment-guidelines/candidiasis.htm
- NHS. Bacterial vaginosis. www.nhs.uk/conditions/bacterial-vaginosis/
- Senok AC, Verstraelen H, Temmerman M, Botta GA. Probiotics for the treatment of bacterial vaginosis. Cochrane Database Syst Rev. 2009;(4):CD006289. doi.org/10.1002/14651858.CD006289.pub2
- Peebles K, Velloza J, Balkus JE, McClelland RS, Barnabas RV. High global burden and costs of bacterial vaginosis: a systematic review and meta-analysis. Sex Transm Dis. 2019;46(5):304-311. doi.org/10.1097/OLQ.0000000000000972
- Workowski KA, Bachmann LH, Chan PA, et al. Sexually Transmitted Infections Treatment Guidelines, 2021: Bacterial vaginosis. Centers for Disease Control and Prevention. www.cdc.gov/std/treatment-guidelines/bv.htm
- NHS. Vaginal discharge. www.nhs.uk/symptoms/vaginal-discharge/
- NHS. Thrush in men and women. www.nhs.uk/conditions/thrush-in-men-and-women/
How we rate evidence. Strong: systematic reviews, large studies or national and international guidelines. Moderate: a single good-quality study, expert reviews or official guidance on a narrower question. Limited: one observational study or little direct research. Early: small or pilot studies that need confirming.
This article is for general information and does not replace medical advice. If you are worried about your symptoms, contact your GP, a sexual health clinic, NHS 111 or, in pregnancy, your midwife.
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