Some women were found to produce a natural protection against HIV and other sexually transmitted infections, or STIs, a new study suggests. Researchers found that the mucus coating the female reproductive tract contains a specific type of naturally occurring bacteria that acts as a “biological condom.”

The study, published in American Society for Microbiology’s online journal mBio, revealed that HIV particles are effectively trapped by the cervicovaginal mucus, or CVM, from a vaginal bacteria species called Lactobacillus crispatus.

“Mucosal surfaces, such as the lung, gastrointestinal tract, or female reproductive tract, are where most infections take place. Our bodies secrete over six liters of mucus every day as a first line of defense,” said Sam Lai, senior author of the study and Assistant Professor of Pharmacy and Engineering at University of North Carolina at Chapel Hill.

CVM, the team claims, can act as a woman’s natural armour to prevent pathogens from reaching the underlying vaginal wall cells. The barrier properties, however, vary greatly in women and even at different times in the same woman.

To find out what accounts for those differences, Lai and his collaborators collected fresh CVM samples from 31 women of reproductive age. For the study, they measured various properties of the mucus and used high-resolution, time-lapse microscopy to test whether fluorescent HIV pseudovirus particles get trapped in the mucus or are diffused freely.

The researchers analysed two distinct populations of CVM samples; one was very good at trapping HIV. According to the team, trapping of HIV was not associated with the mucus' pH, total lactic acid or Nugent score, which is a rough measure of vaginal “health” that reflects how many Lactobacillus bacteria are present compared with other microbes.

The researchers found higher levels of D-lactic acid in the group that trapped HIV. Since humans cannot make this specific acid, they suspect that different bacteria living within the mucus layer were responsible for differences in D-lactic acid.

Analysing further, the team found that women who harbour L. iners bacteria have a substantially higher risk of contracting STIs. Meanwhile, those who harbour L. crispatus bacteria appear to be more protected against HIV.

According to Lai, L. crispatus is also likely to lower the risk of acquiring other STIs that involve enveloped viruses, or those which are wrapped wthin a protective outer coat, such as herpes.

Lai and his colleagues said healthcare workers should be aware of this difference. If there is a way to tilt the battle in favour of L. crispatus in women, they would be able to increase the barrier properties of their CVM and improve protection against STIs, he said.

Richard Cone, a co-author of the study and a biophysicist at Johns Hopkins University in Baltimore, is working on solutions that might deliver a sustained release of lactic acid to the vagina, which would encourage L. crispatus to thrive.

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