Scientists Map the Two Ways Bacterial Vaginosis Comes Back

Bacterial vaginosis is the most common vaginal condition among women of reproductive age, and it has an annoying habit: it comes back. For decades it was treated as a simple imbalance of the vaginal microbiome. Then last year, researchers at Monash University and the Melbourne Sexual Health Centre showed the condition is sexually transmissible, and that treating a male partner alongside the woman cut recurrence by more than 60 per cent.

That discovery won this year's Australian Infectious Diseases Research Centre Eureka Prize and reshaped how doctors think about the condition. But it left an open question. Even when both partners were treated at the same time, bacterial vaginosis still returned in around one in five women.

Now the same team has an answer. In a study published in The Lancet Obstetrics, Gynaecology and Women's Health on September 30, the researchers identified two separate routes that bring bacterial vaginosis back after treatment: reinfection from a partner, and bacteria that simply survive the initial antibiotic course. The finding matters because each route points to a different fix. According to the researchers, the two pathways are distinct but can also overlap in the same patient.

Two ways back to infection

To untangle the routes, the team pooled data from couples in which both partners were treated at the same time. The women took oral antibiotics for a week while their male partners received oral and topical antimicrobials. In total, the analysis covered 188 heterosexual couples, and tracked what happened after the drugs ran their course.

Roughly two-thirds of the women had returned to an optimal vaginal microbiome, rich in protective lactobacilli, the day after finishing treatment. The rest showed microbiome patterns suggesting the infection had not fully cleared before couples resumed sex. That group was nearly three times as likely to develop the infection again within four weeks.

This is why standard treatment fails some patients: their bacterial vaginosis was never fully cleared in the first place. Lead author Dr Lenka Vodstrcil, a senior research fellow at Monash University and Bayside Health's Melbourne Sexual Health Centre, explained that partner treatment improves cure for most women in monogamous relationships, yet some carry BV bacteria that survive the initial therapy before couples resume sex. Spotting that persistence early would let doctors intervene with more intensive treatment before the infection takes hold again.

Testing longer courses and new combinations

That is exactly what the team is doing next. Senior author Professor Catriona Bradshaw, a clinician scientist at Monash University and the Melbourne Sexual Health Centre, said the group is now trialling longer courses of antibiotics and new antibiotic combinations designed to wipe out the bacteria that linger in persistent bacterial vaginosis. The goal is treatment matched to the patient: partner treatment to block reinfection, plus stronger female-focused regimens when the bacteria refuse to leave.

The researchers have a track record of moving findings into practice quickly. After the 2025 study, the Melbourne Sexual Health Centre changed its clinical practice to routinely treat both partners for bacterial vaginosis instead of just the woman, according to the university. Progress on this front sits alongside a broader wave of women's sexual health research, including a topical sildenafil cream that recently cleared a major safety hurdle.

What it means for patients

The new analysis focused on monogamous heterosexual couples, so questions remain about how the findings apply beyond that group. Longer antibiotic courses also come with trade-offs, including side effects and resistance, which is why the team is testing combinations rather than simply extending the standard dose. Still, for the millions of women worldwide who cycle through repeat bacterial vaginosis, the shift is from guesswork to a plan: work out which road the infection is taking, then block it.

Anyone with symptoms of bacterial vaginosis should see a doctor rather than self-treat. Many cases have no noticeable symptoms, and a lab test is needed for a diagnosis. Intimate health also remains one of the hardest topics to bring up in a clinic, which is why researchers say embarrassment keeps patients from getting care.