The vaginal microbiota has long been considered healthy if it was dominated by any species of lactobacillus, The research team found that a specific species of lactobacillus -- lactobacillus crispatus -- appears to play a key role in sustaining the mucus barrier against HIV and other sexually transmitted infections. The findings could lead to the development of new strategies to protect women against HIV.

The research team examined mucus from 31 women of reproductive age using high-resolution, time-lapse microscopy to test whether fluorescent HIV pseudovirus particles became trapped in the mucus or spread freely. The team's findings were published in the journal mBio, 

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

However, one difference between the two groups did stand out: higher levels of D-lactic acid in the group that trapped HIV. Humans cannot make D-lactic acid, so the team suspected that bacteria living within the mucus layer were responsible for the difference. The researchers found that mucus that trapped HIV had predominantly lactobacillus crispatus. Samples that did not trap HIV were either dominated by lactobacillus iners or had multiple bacterial species present including gardnerella vaginalis, both conditions that are frequently associated with bacterial vaginosis.

http://uncnews.unc.edu/2015/10/08/unc-chapel-hill-researchers-reveal-type-of-vaginal-bacteria-that-protects-women-from-hiv-infection/