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.
Lai’s work started not with microbes, but with mucus. Although skin might be more famous as the body’s protective external layer, mucus also helps defend against pathogens. Mucus coats the tissues that are most likely to be infected by pathogens, including the nose, lungs, stomach, intestines, and vagina. It’s a sticky, gel-like substance that can range from thin and runny (like you experience when you have a cold) to thick and dense. Mucin, one of the key proteins in mucus, has long, branched projections that enable it to stick to viruses, bacteria, and other particles like Velcro. If these pathogens are stuck in mucus, they can’t infect the underlying tissue.
The body, however, doesn’t rely on mucus alone. A protective barrier of microbes beneath the mucus provides another layer of protection against invading pathogens. The large number of beneficial bacteria in the microbiome prevent harmful microbes from gaining a foothold in the body. These microbes are tremendously dynamic, varying dramatically from person to person, and even within the same person over time. This is as true for the microbes in the GI tract as it is for the vaginal microbiome.
“We like to think of mucus as the first line of defense. One of the reasons we don’t get sick that often is that the mucus is already doing a really good job,” Lai said.
Historically, researchers believed that the microbiome of a healthy vagina was colonized by a bacterium called Lactobacillus. As its name suggests, these bacteria produce lactic acid, which lowers the pH of the vagina, making it even more inhospitable to invading microbes. However, more recent surveys have shown that not all Lactobacilli are the same, and that a substantial minority of women have an otherwise healthy vaginal microbiome that isn’t dominated by Lactobacillus at all.
“This system is protecting women against pathogens. That most women are healthy most of the time is really quite stunning and shows that this system is working pretty well,” said Larry Forney, a microbiologist at the University of Idaho.
In all of this confusion, Lai and colleagues began to wonder whether the type of microbes dominating the vaginal microbiome could affect the likelihood that a woman could become infected with HIV after vaginal sex. A 2008 study in the Journal of Acquired Immune Deficiency Syndromes revealed that women with bacterial vaginosis whose vaginal microbiomes were dominated by Gardnerella vaginalis and other anaerobic bacteria were more likely to contract HIV. An independent study in the Journal of Virology showed that acidic mucus effectively trapped HIV, but it didn’t characterize the contribution of the individual microbes. Other work was showing subtle but noticeable differences in the different species of Lactobacillus commonly found in the vagina.
In the current work, published last week in mBio, the researchers found that the ability of vaginal mucus to trap HIV was related to the presence of a particular strain, Lactobacillus crispatus. To determine how different bacterial strains might alter HIV infection, Liu and colleagues obtained samples of cervicovaginal mucus from 31 racially diverse women between the ages of 19 and 33, and divided them into three groups: those whose vaginal microbiomes were dominated by L. crispatus, those dominated by L. iners, and those dominated by G. vaginalis. Confirming their previous results, the mucus with more lactic acid, and therefore a lower pH, did a better job of trapping non-infectious HIV particles. In particular, an L. crispatus-dominant microbiome produced more lactic acid and thus did a much better job of trapping HIV. The women with lots of L. crispatus were able to trap more than 90 percent of the virus. Those with vaginal microbiomes dominated by L. iners or G. vaginalis trapped less than 90 percent.
“If HIV can’t get to the cells they want to infect, you won’t have transmission. These microbes act as a barrier, which is what a condom does. So I like to think of them as a biological condom,” Lai said.
To reproductive epidemiologist Deborah Nelson of Temple University, this work adds an important piece of knowledge to what we know about how the vaginal microbiome contributes to a woman’s risk of sexually transmitted infections.
“I was really excited to see this study. It opens the door to the idea that women could alter the microbes in their genital tract to prevent HIV infection either before or after they might have been exposed,” said Nelson, who was not involved in the research.
Forey points out that while this study is of both high quality and importance, it’s a little premature to start calling the protective effect a “condom.” The barrier provided by a condom provides a much greater level of protection than those currently provided by L. crispatus.
“There’s still a lot of work to be done. It’s not a failsafe, but it’s also one of the only papers that has tried to tackle the issue head-on,” said Forey, who was not involved in the research.
Even so, the work opens the door to a new type of condom that can potentially be created using a probiotic supplement or suppository and used by a woman without requiring the consent or approval of her partner. Although the current study only tested the ability of vaginal mucus to trap HIV, researchers are investigating how it affects transmission of other STIs. It might not be a revolution, but the results are pointing to yet another way that your microbiome just might save your life.