Vaginal microbes influence whether mucus can trap HIV virus

October 6, 2015
HIV, the AIDS virus (yellow), infecting a human immune cell. Credit: Seth Pincus, Elizabeth Fischer and Austin Athman, National Institute of Allergy and Infectious Diseases, National Institutes of Health.

HIV particles are effectively trapped by the cervicovaginal mucus from women who harbor a particular vaginal bacteria species, Lactobacillus crispatus. The findings, published this week in mBio, an online open-access journal of the American Society for Microbiology, could lead to new ways to reduce or block vaginal transmission of HIV and other sexually transmitted infections (STIs).

"Mucosal surfaces, such as the lung, gastrointestinal tract, or female reproductive tract, are where most infections take place," says Sam Lai, assistant professor of pharmacy and engineering at University of North Carolina at Chapel Hill and senior author on the study. "Our bodies secrete over six liters of everyday as a first line of defense."

Cervicovaginal mucus (CVM) can act as a barrier to prevent pathogens from reaching the underlying vaginal wall cells, but the barrier properties vary greatly from woman to woman and even at different times in the same woman. Lai and his collaborators wanted to know what accounts for those differences.

They collected fresh CVM samples from 31 women of reproductive age, measured various properties of the mucus and used high-resolution, time-lapse microscopy to test whether fluorescent HIV pseudovirus particles became trapped in the mucus or diffused freely.

The researchers observed two distinct populations of CVM samples, one that was very good at trapping HIV and one that was not. Trapping of HIV did not correlate to the mucus' pH, total lactic acid, or Nugent score, a rough measure of vaginal 'health' that reflects how many Lactobacillus bacteria are present compared to other microbes.

One difference between the two groups, higher levels of D-lactic acid in the group that trapped HIV, stood out to the researchers because humans cannot make D-lactic acid. The team suspected that different bacteria living within the were responsible for differences in D-lactic acid. When they sequenced ribosomal genes to identify the bacteria within each sample, the researchers found that the samples fell again into two groups.

L. crispatus bacteria dominated the CVM that trapped HIV. In contrast, CVM that failed to trap HIV either possessed a different Lactobacillus species, L. iners, or had multiple bacterial species present including Gardnerella vaginalis—both conditions that are frequently associated with bacterial vaginosis.

"I was really surprised by how slight differences between Lactobacillus species make a very substantial difference in the barrier properties of mucus," says Lai. In the clinical setting, there is a clear link between bacterial vaginosis and having a higher risk of acquiring and transmitting STI's. Historically, Lai explains, gynecologists considered vaginal microflora to be healthy if it was dominated by any Lactobacillus species. "But our work shows that from a mucus barrier perspective, that is not a good enough distinction," says Lai.

Healthcare workers should be aware that women who harbor L. iners probably have a substantially higher risk of acquiring STIs. And conversely, a L. crispatus dominant microflora may be more protective against HIV and STIs than previously appreciated.

The group also showed that L. crispatus caused CVM to be more 'sticky' against HIV particles—rather than making a tighter mucus mesh. The barrier function was not unique to HIV particles and would likely trap other enveloped viruses as well.

Lai notes that CVM can be thought of as a "biological condom" which could potentially be reinforced by altering a woman's vaginal microbiota. "If we could find a way to tilt the battle in favor of L. crispatus in women, then we would be increasing the barrier properties of their CVM, and improve protection against STIs," he says.

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

Explore further: Why do certain hormonal contraceptives increase the risk of HIV?

More information: The full study can be found online at

Related Stories

Why do certain hormonal contraceptives increase the risk of HIV?

September 1, 2015
In recent years, evidence has been building that injectable contraceptive depot medroxyprogesterone acetate (Depo-Provera or DMPA) is associated with an increased risk of HIV infection. Now a study published in the September ...

Bacterial communities of female genital tract have impact on inflammation, HIV risk

May 19, 2015
A team led by researchers from Massachusetts General Hospital (MGH) and the Ragon Institute of MGH, MIT and Harvard has found that the most common bacterial community in the genital tract among healthy South Africa women ...

Bacteria may contribute to premature births, STDs

April 23, 2013
(Medical Xpress)—New research at Washington University School of Medicine in St. Louis points to a common species of bacteria as an important contributor to bacterial vaginosis, a condition linked to preterm birth and increased ...

Bacterial vaginosis is associated with higher risk of female-to-male transmission of HIV

June 26, 2012
An investigation led by UCSF has found that the risk of female-to-male HIV transmission is increased three fold for women with bacterial vaginosis, a common disorder in which the normal balance of bacteria in the vagina is ...

Researchers identify good bacteria that protects against HIV

March 28, 2014
Researchers at the University of Texas Medical Branch at Galveston by growing vaginal skin cells outside the body and studying the way they interact with "good and bad" bacteria, think they may be able to better identify ...

Recommended for you

Scientists divulge latest in HIV prevention

July 25, 2017
A far cry from the 1990s "ABC" campaign promoting abstinence and monogamy as HIV protection, scientists reported on new approaches Tuesday allowing people to have all the safe sex they want.

Girl's HIV infection seems under control without AIDS drugs

July 24, 2017
A South African girl born with the AIDS virus has kept her infection suppressed for more than eight years after stopping anti-HIV medicines—more evidence that early treatment can occasionally cause a long remission that, ...

Meds by monthly injection might revolutionize HIV care (Update)

July 24, 2017
Getting a shot of medication to control HIV every month or two instead of having to take pills every day could transform the way the virus is kept at bay.

Candidate AIDS vaccine passes early test

July 24, 2017
The three-decade-old quest for an AIDS vaccine received a shot of hope Monday when developers announced that a prototype triggered the immune system in an early phase of human trials.

Paris spotlight on latest in AIDS science

July 21, 2017
Some 6,000 HIV experts gather in Paris from Sunday to report advances in AIDS science as fading hopes of finding a cure push research into new fields.

Scientists elicit broadly neutralizing antibodies to HIV in calves

July 20, 2017
Scientists supported by the National Institutes of Health have achieved a significant step forward, eliciting broadly neutralizing antibodies (bNAbs) to HIV by immunizing calves. The findings offer insights for HIV vaccine ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.