Certain types of vaginal bacteria appear to reduce the efficacy of a common preventive treatment against HIV, tenofovir. This discovery may shed a light on why antiretroviral-based strategies for HIV seem have been more effective in men than in women.

In the last decades, antiretroviral therapy and preventive treatments have stopped thousands of new HIV infections. Being diagnosed as HIV-positive is no longer the death sentence it once was.

Despite these advances, over one million women are still infected with HIV each year, most of them in sub-Saharan Africa, and South Africa in particular.

Recent clinical trials with men who have sex with men have shown that taking the antiretroviral drug tenofovir in prevention markedly reduced their risk of becoming infected with the virus.

However, these promising results have not always been seen in women.

The ability of antiretroviral drugs to prevent HIV infections in women varies greatly, from only 49% in some studies to 75% in others. Until now, researchers had struggle to explain these differences between men and women, but also why some women seemed to respond better to the preventive drugs than others.

The study published in the journal Science now highlights the central role of vaginal bacteria in this process.

Vaginal bacteria communities

The vagina contains many microbial species which can be critical for women's sexual health. In the past few years, new molecular tools have allowed scientists to learn more about vaginal bacteria.

Although there is great heterogeneity between women, scientists have been able to categorise these bacteria communities into two groups: lactobacillus-dominant communities and non- lactobacillus-dominant communities.

In the present study, the scientists worked with 688 women in South Africa, analysing what type of bacteria was present in their vaginas. During a clinical trial, they then tested the efficacy of a tenofovir.

They found that the treatment reduced HIV incidence by 61% in women with Lactobacillus-dominant bacteria, but by only 18% in women with non-Lactobacillus-dominant bacteria (most of these were Gardnerella vaginalis bacteria) - a more than threefold difference in efficacy.

The scientists also showed that high rates of HIV infection were found among those individuals where Gardnerella vaginalis dominated, because the bacteria is able to rapidly metabolise and breakdown the active form of tenofovir.

HIV Israel Truvada
The findings of this study may help change the way clinical trials with women are designed. Getty/KTSDesign/Science Photo Library

These results are valuable for researchers as they can help them identify which women will benefit most from receiving tenofovir. They could also help improve the way clinical trials and public health programs are designed.

"In future trials it will be important to take into account vaginal bacteria and the contributions they may be having on the efficacy of preventive drugs," study author Nichole Klatt from the University of Washington told IBTimes UK.

Looking at the microbiomes of all the women participating in clinical trials is not always possible, but one possibility would be to use pH tests, Klatt said.

"Indeed a high pH is indicative of Gardnerella vaginalis - and for these women we may have to suggest other treatments or to educate them on the risks and urge protected sex or to strictly adhere to the treatment."