According to an article published last week in the research journal mBio, a team of researchers has concluded that a certain form of vaginal bacteria which can be found in some women’s fluids can actually prevent that person’s infection with HIV by essentially “trapping” the HIV virus and preventing it from reaching the body’s immune system.

Cervicovaginal mucus (CVM) can provide a barrier that precludes HIV and other sexually transmitted virions from reaching target cells in the vaginal epithelium, thereby preventing or reducing infections. However, the barrier properties of CVM differ from woman to woman, and the causes of these variations are not yet well understood.

Every year, millions of people around the world rely on condoms to prevent sexually transmitted infections. The thin layer of latex, polyurethane, or other substance provides a barrier that prevents bacteria, viruses, and parasites from infecting a sexual partner.

Still, even without a condom, not all women are infected with an STI after having vaginal sex with a male partner. Some of this is likely a matter of luck: A partner might not be very infectious at the time of sex. But new discoveries about the vaginal microbiome are showing that the issue is far more complex than that. A new study in the open access journal mBio shows that certain bacterial strains living in the vagina can effectively trap virus particles associated with HIV and act as a so-called biological condom.

“We were very, very surprised that subtle differences in microbes could create large differences in their ability to be a barrier,” said Sam Lai, a microbiologist at the University of North Carolina at Chapel Hill, and senior author on the new study.

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