Australian scientists identify why some bacterial vaginosis cases resist treatment

A team of Melbourne researchers has spent the past year chasing a stubborn puzzle: why do some women keep getting bacterial vaginosis even after they and their partners receive antibiotics together? The answer, published this week in The Lancet Obstetrics, Gynaecology & Women's Health, points to two separate mechanisms at work—and it could reshape how doctors treat a condition that affects roughly one in four women worldwide.

Bacterial vaginosis, or BV, is fundamentally a microbial imbalance. The vagina normally hosts protective bacteria called Lactobacilli, but in BV, these beneficial organisms decline while other bacteria flourish unchecked. The condition can cause itching, burning, and discomfort, though some women notice nothing at all. What makes it medically urgent is what it enables: increased vulnerability to other infections, and in pregnancy, a higher risk of complications during delivery.

Last year, the same Melbourne team made headlines with a discovery that shifted how the field understood transmission. They found that BV could pass from male partners to women during sex—a finding that led to a major insight. When couples received concurrent antibiotic treatment, recurrence dropped by more than 60%. But that success came with a catch: roughly one in five women treated alongside their partners still developed BV again. The researchers wanted to know why.

For their new study, they enrolled 188 couples. Women received a week of oral antibiotics; men got both oral and topical antimicrobials. The team then tracked what happened to the women's vaginal microbiomes and whether infection returned. What emerged was a picture of two distinct pathways. About two-thirds of the women, the day after finishing treatment, had restored their optimal microbiome—abundant in protective Lactobacilli. The remaining third told a different story. Their microbiomes suggested the original infection had never fully cleared. These women were nearly three times as likely to develop BV again within four weeks.

Dr. Lenka Vodstrcil, the study's lead author and a senior research fellow at Monash University's Melbourne Sexual Health Centre, described the divergence plainly: some women's bodies simply did not eliminate the infection despite a full course of antibiotics. Others, presumably, were reinfected by partners who carried the bacteria. The distinction matters because it points toward different solutions. Persistence requires more aggressive or prolonged antibiotic strategies. Reinfection requires ensuring partners are fully treated—a lesson the concurrent-treatment data already suggested.

One unexpected finding emerged around intrauterine devices. Thirty percent of the female participants used an IUD, and the data hinted that these devices might make BV persistence more likely after treatment, though the researchers cautioned that more work was needed to confirm the link. The study also acknowledged its own limits: participants were drawn from urban Australia, and the findings may not apply to women in low-income countries or other settings where BV is especially common.

Vodstrcil and her colleagues are now planning trials of longer antibiotic courses and combinations of multiple drugs. The goal is direct and human: to stop the cycle in which women find themselves taking antibiotics two, three, or four times a year. Prof. Catriona Bradshaw, the study's senior author and head of research translation at Monash, framed the next phase as a move toward tailored treatment—using partner therapy to prevent reinfection while deploying more intensive strategies against bacterial persistence. The hope is that understanding these two pathways will finally allow doctors to cure more women, not just temporarily suppress their symptoms.