Can a Woman Get Bacterial Vaginosis From a Man?

Evidence accumulated over the past decade strongly supports the idea that a woman can acquire bacterial vaginosis from a male sexual partner. For years, BV was framed as a “disruption in vaginal flora” rather than a sexually transmitted condition, but microbiome studies now show that the bacteria responsible for BV live on the penis and are exchanged during intercourse. A landmark 2024 clinical trial found that treating male partners with antibiotics nearly halved BV recurrence in women, which is hard to explain unless men are a meaningful source of reinfection.

BV-Associated Bacteria Live on the Penis

The shift in how researchers think about BV and male partners came largely from studies that compared the microbial communities of couples. When scientists sequenced the bacteria on penile skin and in the male urethra, they found many of the same organisms implicated in BV, including Gardnerella vaginalis, Sneathia, Prevotella, and Dialister. In one study, the penile and urethral microbiome of a man whose female partner had BV looked significantly more similar to her vaginal microbiome than to the vaginal microbiome of unrelated women with BV. That level of microbial matching between specific couples suggests direct sharing, not coincidence.¹

A predictive modeling study went further. Researchers found that the composition of a man’s penile microbiome could predict whether his female partner would develop BV with high accuracy. The bacteria most predictive of her future BV included several species long recognized as BV hallmarks, like Gardnerella vaginalis and Sneathia sanguinegens.² A separate study of couples in Uganda confirmed the pattern: men carrying certain penile bacterial communities were significantly more likely to have a female partner with BV.³

These bacteria tend to concentrate in the area under the foreskin and in the distal urethra.⁴ Men typically experience no symptoms from these organisms, which is one reason the role of male partners went underappreciated for so long. The bacteria are benign residents on the penis but can displace protective lactobacilli once introduced to the vaginal environment.⁵

How Semen Itself Can Trigger BV

Sexual transmission of specific bacteria is only part of the story. The physical act of unprotected intercourse introduces semen, which is alkaline. A healthy vagina is acidic, and that acidity is maintained largely by lactobacilli. Semen temporarily raises vaginal pH, and that pH shift alone can favor the growth of BV-associated anaerobes over protective lactobacilli.⁶

A study that tested women for prostate-specific antigen (a marker of recent semen exposure) found that women with detectable semen had roughly two and a half times the risk of prevalent BV compared with women without recent exposure, after adjusting for age, contraceptive use, and condom habits.⁷ This suggests that even apart from whatever bacteria a man carries, unprotected sex can create conditions in the vagina where BV-causing organisms thrive.

Researchers have also documented what appears to be a Gardnerella biofilm that forms on vaginal tissue and can be found on the genitals of male partners. One study traced a specific cohesive Gardnerella strain between partners retrospectively over 15 years, and found that the biofilm form was present in all patients with proven BV and their partners.⁸ Biofilms are notoriously difficult to eradicate with antibiotics, which helps explain why BV so often comes back after treatment.

The Trial That Showed Partner Treatment Works

The strongest piece of evidence that men contribute to BV came from a randomized controlled trial published in the New England Journal of Medicine in 2024. In this trial, called StepUp, male partners of women with BV were either given a combination antibiotic regimen or a placebo. Among women whose partners received the real antibiotics, BV recurred in about 35%. Among women whose partners received placebo, the recurrence rate was 63%.⁹

That difference is large and difficult to attribute to anything other than the man’s bacterial load. If BV were purely a matter of a woman’s own vaginal ecosystem going haywire, treating her partner should not have made a difference. The fact that it cut recurrence by almost half strongly implies that male partners serve as a reservoir, harboring the bacteria and reintroducing them during sex.

This trial was a long time coming. For decades, clinical guidelines in most countries did not recommend treating male partners, and many still don’t. The prevailing view had been that BV was not sexually transmitted in the traditional sense. But the recurrence problem was always hard to explain otherwise: more than half of women who clear BV with antibiotics will have it come back within a year.¹⁰ Treating the woman alone while leaving an untreated male partner in the picture now looks like a recipe for that frustrating cycle.

What Condoms and Circumcision Reveal

Two other lines of evidence reinforce the sexual transmission picture. First, condom use. A study of women without BV at enrollment found that consistent condom use reduced the risk of acquiring BV by roughly 40% compared with inconsistent use.¹¹ A separate analysis found a similar protective effect for women who were BV-free at baseline, though consistent condom use did not significantly reduce BV prevalence in women who already had it.¹² Condoms block both bacterial transfer and semen exposure, so it makes sense that they would be more effective at preventing new BV than curing existing infections.

Second, male circumcision. A large randomized trial in Uganda found that BV prevalence was about 40% among wives of circumcised men compared with about 51% among wives of uncircumcised men. Severe BV showed an even bigger reduction.¹³ A review of that same trial data emphasized that circumcised men in the study had lower counts of the anaerobic bacteria associated with BV on their penile tissue, which is likely why their partners benefited.¹⁴ The sub-preputial space (the area under the foreskin) is warm, moist, and oxygen-poor, which creates an ideal environment for the anaerobic bacteria implicated in BV. Removing that niche reduces the bacterial load a man can carry.

Neither condoms nor circumcision eliminates BV risk entirely, which points to the fact that sexual transmission from a male partner is an important but not the only pathway. Other factors matter too.

BV Is Not Exclusively About Male Partners

One reason scientists were slow to call BV sexually transmitted is that it occurs in women who have never had sex with men. BV is well-documented in women who have sex with women, and the concordance rates within female couples are striking. In one community-based study, 87% of lesbian couples had matching vaginal flora, a degree of agreement far higher than chance would predict.¹⁵ A systematic review of BV in women who have sex with women found that having more female partners and having a partner with confirmed BV were both associated with increased risk.¹⁶

BV also occurs in women who are not currently sexually active at all, though less commonly. Vaginal douching is one non-sexual risk factor with consistent evidence behind it. Women who douche at least once a month have a higher frequency of BV, and those who douched within the past seven days are at roughly double the risk.¹⁷ A longitudinal study confirmed that regular douching, compared with no douching, increased BV risk by about 21%.¹⁸ Douching disrupts the vaginal ecosystem in much the same way that semen does: it raises pH and washes away lactobacilli.

So the honest framing is that BV can be sexually transmitted from men, from women, or can arise from non-sexual disruptions to vaginal flora. Sexual contact with a male partner is one of the most common pathways, but it is not the only one.

Why BV Keeps Coming Back

BV has one of the highest recurrence rates of any vaginal condition. A review of the problem identified three broad explanations: the BV-associated biofilm may persist on vaginal tissue even after antibiotics clear the free-floating bacteria; BV-associated bacteria may re-emerge from other body sites; and reinfection from a sexual partner may restart the process.¹⁰ In most women, recurrence is probably driven by more than one of these mechanisms simultaneously.

The StepUp trial results strongly suggest that the partner-reinfection pathway accounts for a significant share of recurrences. Treating the male partner cut the recurrence rate from about 4.2 episodes per person-year to 1.6, but it didn’t reduce it to zero.⁹ That leftover recurrence likely reflects the biofilm persistence and other non-partner-related factors. Achieving sustained cure will probably require attacking BV from multiple angles: antibiotics for both partners, strategies to restore a healthy vaginal microbiome, and possibly treatments that target the biofilm directly.

How BV-Associated Bacteria Affect Men

Men who carry BV-associated bacteria on their genitals generally have no idea. They don’t develop symptoms the way women do. In qualitative research, men reported that BV had little impact on them personally beyond worry about their partner’s well-being and self-esteem.⁵ But the absence of symptoms does not mean these bacteria are harmless to men in every respect.

Research on the semen microbiome has started to connect BV-associated organisms with male fertility problems. One study found that when BV-associated bacteria were detected in semen samples, those samples showed a tendency toward abnormal sperm shape, and specific organisms like Gardnerella vaginalis and Ureaplasma urealyticum were linked to altered semen characteristics.¹⁹ A systematic review found that semen dominated by Prevotella, a BV-associated genus, appeared to have worse sperm quality, while semen dominated by Lactobacillus showed better parameters.²⁰

Lab experiments have probed the mechanism more directly. Toxins produced by BV-associated bacteria initially caused sperm to become hyperactivated when added to samples, but after several hours the sperm showed significantly reduced motility and progressive movement compared to untreated controls.²¹ The practical implication is still being worked out, but it raises the possibility that in couples struggling with fertility, the man’s genital microbiome deserves attention alongside the woman’s.

The Emotional Toll and the Stigma Problem

BV’s ambiguous status between “imbalance” and “infection” has created a stigma problem that falls almost entirely on women. Because it has not traditionally been classified as a sexually transmitted infection, many women feel blindsided when it recurs and blame themselves for hygiene failures. Qualitative research has documented the shame and embarrassment women experience around BV, much of it tied to broader societal stigma about female sexuality and the assumption that genital symptoms reflect promiscuity.²²

The growing evidence that male partners play a role in transmission and recurrence may actually help with this. If BV is understood as something that can be passed between partners rather than a personal failing of the woman’s body, it becomes easier to talk about openly. It also distributes responsibility: if a man is part of the problem, it follows that he should be part of the solution, whether through condom use, accepting antibiotic treatment, or simply understanding what BV is. The men interviewed in one study were generally willing to be treated and viewed it as a shared problem worth addressing together.⁵

Clinical guidelines in most countries have not yet caught up with the partner-treatment evidence. If you are dealing with recurrent BV and your provider has not discussed your male partner’s potential role, the StepUp trial gives you a concrete basis for that conversation. The evidence is strong enough that the question is no longer whether men contribute to BV, but how quickly clinical practice will adapt to reflect what the research now shows.