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Vaginal Bacteria Might Help or Inhibit HIV Prevention

While some vaginal microbes might be assisting in the process of preventing HIV infection, other bacteria might be making drugs less effective by breaking them down, suggest two new studies published in the journal Science.

An antiretroviral vaginal gel used for the prevention of HIV infection has been found to be three times more effective at inhibiting HIV in women with healthy vaginal microbiota than in women with a different microbial distribution. The new findings might explain previous research that shows varying effectiveness of these gels. The gels are medically prescribed to be used before and after sex to prevent HIV infection; results are, however, mixed concerning their effectiveness. While this might be due to incorrect administration of the medication, microbiologist Adam Burgener, author of the first study, wanted to find out whether there was also a biological reason behind.

Therefore, Burgener and his team set out to evaluate the effect of vaginal microbiota on a gel that uses tenofovir as HIV-preventive drug. They examined data from a previous trial conducted on South African women whereby samples of vaginal mucus from participants were taken, and the results showed that the drug lowered HIV infections by 39%. The new study dealt with bacterial proteins in a batch of these samples to identify the vaginal bacteria of the participants to determine whether the effectiveness of the drug was affected by the bacterial species.

The new findings show that over 400 women had Lactobacillus species, the main microbes forming part of healthy vaginal microbiota. These bacteria secrete lactic acid which might be providing a “less hospitable” environment for harmful organisms, says an author of the second study, Khalil Ghanem from Johns Hopkins University School of Medicine. On the other hand, 281 women had more of non-Lactobacillus species, like Gardnerella vaginalis. Among the women of the former group who used the medicated gel, HIV incidence was 61% lower while it was only 18% lower in the non-Lactobacillus dominant group.

“Women with Lactobacillus had three times more protection offered by the gel,” Burgener says. “That’s a pretty remarkable difference in the efficacy of a drug.”

Investigating the samples further, the scientists found that the vaginal gel drug levels were in lower concentrations in the vaginal mucus of the non-Lactobacillus group. When they tested the effect of G. vaginalis on tenofovir in laboratory, they found that the drug had decreased by 50% in the presence of the non-Lactobacillus species. On the other hand, when the drug was tested with Lactobacillus species, its level remained the same: it was not depleted. This suggests that G. vaginalis bacteria used up the drug.

The two studies, thus, show that the bacterial populations present in vaginas constitute another variable that has to be taken into consideration when it comes to preventive treatment for HIV. Women could be using the gel right, but they might not necessarily experience great effectiveness, says Ghanem.

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