Women with a larger diversity of bacteria in their vagina have a greater risk of acquiring HIV compared to those with less microbiota diversity, with some specific groups of vaginal bacteria associated with a substantially increased risk of HIV.

New findings published in the Lancet Infectious Diseases are the first to show significant associations between the concentrations of specific vaginal bacteria and women’s risk of HIV acquisition. This could provide important insight for future HIV prevention research.

In recent years, novel tools and methods to define the vaginal microbiome – the community of bacteria inside the vagina specific to each woman – have led to a better understanding of the potential for the vaginal microbiome to influence HIV risk.

Drawing from five cohorts across six countries in East and Southern Africa, 87 women who had acquired HIV were compared to a control group of 262 women without HIV infection. The women represented three important risk groups for HIV in the region: female sex workers, post-partum women and HIV-negative women in mixed-status relationships.

Using a two-step approach, researchers used RNA gene sequencing to first identify the vaginal bacteria present in a sample of 55 women from each group. Twenty categories (taxa) were then selected for further PCR (polymerase chain reaction) testing to quantify their abundance in the full cohort of women. The researchers then generated odds ratios for increased HIV infection based on the data.

Seven out of the 20 taxa were associated with significantly increased odds of HIV infection, with the odds going up as bacteria concentrations increased. Notably, the Parvimonas species was the strongest predictor of HIV with a 4.6 to one increased odds, followed by the Gemella species with a 3.2 increased odds. These taxa could present either alone, or in combination with others creating a community of bacteria that increase HIV risk.

Interestingly, these taxa are more associated with HIV than a diagnosis of bacterial vaginosis (BV) – already known to increase risk of HIV by 1.5. But BV is very heterogeneous and highly prevalent worldwide, presenting differently regionally and ethnically. Both Parvimonas and Gemella are associated with BV already, thus the research provides better information on the specific bacteria related to BV that increases HIV risk, which could pave the way for more effective treatment-targeting for women in Africa.

While previous studies have shown that women with low-Lactobacillus vaginal bacteria populations (highly prevalent among African women) have an increased risk of HIV, this current study found no significant associations between concentrations of Lactobacillus species and HIV. The authors noted, “low relative abundance of Lactobacillus species might simply reflect the presence of high concentrations of bacterial vaginosis-associated bacteria that affect HIV susceptibility.”

While the mechanisms influencing HIV susceptibility are still under question, the researchers note that both the presence and quantity of these key bacteria could increase HIV risk by way of inflammation, production of HIV inducing factors, and disruption of physical and chemical barriers to infection.

In an accompanying commentary, Jo-Ann S Passmore and Heather B Jaspan praised the importance of this study – saying that it should directly inform future HIV-risk mitigation interventions in Africa. They commented, “Despite our inability to effectively manage bacterial vaginosis in the context of preventing HIV transmission, it is essential that studies direct us towards novel methods to improve the vaginal health of women."