Our war with HIV is far from won, and determined scientists keep adding to our ever-growing antiviral armamentarium in hope of eventual defeat. But how about fighting microbe with microbe? That’s the idea of one group of researchers, who have found that a particular species of bacteria helps create a protective net in the vagina that catches HIV and prevents sexual transmission.
“Mucosal surfaces, such as the lung, gastrointestinal tract, or female reproductive tract, are where most infections take place,” senior author Sam Lai from the University of North Carolina said in a statement. “Our bodies secrete over 6 liters [10 pints] of mucus everyday as a first line of defense.”
But it’s not an impenetrable barrier: Sexually transmitted infections, colds and stomach bugs are evidence of that. They also slightly differ from person to person, which plays a part in the variation in disease susceptibility that we observe. Take the vagina, for example. Alongside hosting a complex, delicate array of beneficial microbes that help fend off intruders, the cervicovaginal mucus (CVM) also makes it tough for invaders to get to the underlying susceptible cells, but its properties vary significantly not just between individuals, but also at different times in the same person.
So what’s behind this? A team of researchers decided to find out. As described in mBio, they began by gathering CVM samples from 31 women which they scrutinized for differences in an array of properties. Next, they tested the CVMs capabilities of trapping HIV particles, which the team tagged with fluorescent markers, by tracking and filming the movement of the viruses through the mucus using time-lapse microscopy. This revealed that the women fell broadly into two categories: those who had CVM samples that were adept at catching the virus, and those who showed a lesser ability to do so.
Further testing found that neither pH levels, which are kept low to avoid infections, nor total lactic acid – produced by vaginal microbes – seemed to be a predictor of HIV trapping ability. Interestingly, neither was one measure clinicians use to assess overall vaginal health, called the Nugent score, which reflects ratio of friendly Lactobacillus bacteria compared with other species.
But what they did notice was that levels of a different chemical, called D-lactic acid, did seem to correlate with the ability to trap HIV. And that piqued the researchers’ interest, because the body can’t actually make it, so they hypothesized that there must be a microbial element to their observations.
They tested this idea out by sequencing the bacterial genomes in the samples to identify the species present, which revealed that those with the highest D-lactic acid levels, and thus those that trapped HIV most efficiently, were dominated by a species called Lactobacillus crispatus. Conversely, those less efficient at catching the virus, with the lowest D-lactic acids, demonstrated an abundance of different microbes: L. iners or G. vaginalis, the latter of which is actually associated with bacterial vaginosis.
It turns out that L. crispatus imparts a stickiness to the CVM that helps entrap HIV particles, rather than making this “net” tighter, so to speak. And they don’t think that this is unique to HIV either, as the team predicts this sticky nature likely also helps prevent other microbes from reaching the underlying cells.
The researchers envisage that this data could ultimately be used to reduce new infections, not just of HIV but other sexually transmitted agents. If a simple way to adjust the ratio of bacteria in favor of L. crispatus can be developed, then women may be better protected against infection.