Vaginal Bacteria Type Can Influence HIV Risk

Researchers have discovered a possible connection between the type of bacteria a woman harbors in her vagina and her risk of contracting HIV during sex.

The Ragon Institute of MGH, MIT and Harvard in Cambridge, Massachusetts, conducted a study involving 236 healthy women between the ages of 19 and 23 from a province in Durban, South Africa, a region notable for its high percentage of HIV-positive indvividuals. They found that women whose vaginal bacteria were highly diverse but lacked sufficient Lactobacillus crispatus, a type of bacteria thought to boost vaginal health, had more genital inflammation and four times the risk of acquiring HIV; these women also had higher numbers of CD4 T-cells in their bacteria mucosa. Although a few previous studies have suggested that vaginal bacteria deficient in Lactobacillus crispatus raised HIV risk, testing had been limited to vaginal smears. Now, with advanced sequencing technologies available, scientists are able to study the vaginal microbiome with greater precision.

The Ragon Institute researchers followed the 236 women for nearly a year, and during that time, all of the test subjects were provided with rigorous HIV prevention counseling. Nevertheless, 31 women contracted HIV during the study period, none of whom had vaginal bacteria dominated by lactobacilli. The women who contracted HIV tended to have a wide array of vaginal bacteria, which included Gardnerella vaginalis, Prevotella, Shuttleworthia, Sneathia, and Megasphaera, among others. To ensure that other factors weren’t influencing the transmission of HIV, the scientists looked at condom use, types of sexual acts, frequency of sex, number of sexual partners, as well as the use of injectable progestin contraceptives, which are a known risk factor for HIV. After accounting for these factors, they concluded that highly diverse anaerobic bacterial communities in the vagina are associated with an increased risk of contracting HIV; a more homogenous bacterial community dominated by Lactobacillus crispatus seems to confer a protective effect and is less inflammatory than a diverse one. Lactobacillus crispatus-dominated communities also have fewer CD4 T-cells, which are prime targets of HIV and increase the risk of transmission.

Distinct differences in risk exist between groups of women based on their ethnic background and geographic location, the researchers noted. “The high-diversity, low Lactobacillus abundance bacterial communities detected in the majority of South African women in our cohort have also been described in women in the US,” said Christina Gosmann, PhD, a research fellow at Ragon and the lead author of the study. “However, there are differences in prevalence between ethnic groups. A study by Ravel [and colleagues] found that 10% of white, 20% of Asian, 40% of Hispanic, and 40% of black asymptomatic women in the US had these more diverse bacterial communities. It is currently unknown whether these differences are due to dietary, environmental, behavioral, genetic, or other reasons. Ongoing research in our lab aims to better understand what determines the makeup of the female genital tract microbiome.”

Women whose vaginal microbiomes likely place them into a “lower-risk” category, such as many white women in the United States and other developed countries, should not be complacent when it comes to HIV and other sexually transmitted diseases, the researchers warned. “All at-risk women should use available methods to protect themselves against HIV, independent of their vaginal bacteria,” Dr. Gosmann said. “Women with Lactobacillus crispatus dominance are at lower risk compared to those with high-diversity bacterial communities, but they are not protected against infection.”

Laurie Saloman, MS, is a health writer with more than 20 years of experience working for both consumer and physician-focused publications. She is a graduate of Brandeis University and the Medill School of Journalism at Northwestern University. She lives in New Jersey with her family.