Examining abiotic and biotic factors influencing bacterial and host interactions in the female reproductive tract

Abstract

Currently, the leading route of new HIV-1 infection is via heterosexual transmission, in which women are disproportionately burdened. One key factor associated with a fourfold increased risk of HIV-1 acquisition is a dysbiotic vaginal microbiota (VMB). A dysbiotic VMB is characterized by a diverse mix of anaerobic species without any appreciable amounts of beneficial Lactobacillus species. Our understanding of the species-specific manner by which vaginal bacteria interact with one another and with the host to induce susceptibility remains incompletely understood. With this, this study was designed to elucidate the interactions between common vaginal bacteria and host vaginal epithelial cells. The phenotypic and metabolic characteristics of these bacteria were also examined to provide a deeper understanding about the conditions in which each species may be able to survive and thrive. Common vaginal bacteria analyzed included dysbiosis associated species Gardnerella vaginalis and Prevotella bivia, as well as Lactobacillus species L. crispatus and L. iners. The presence of P. bivia, G. vaginalis and L. iners cocultured with vaginal epithelial cells in vitro resulted in reduced viability of vaginal epithelial cells, reduced barrier integrity and the production of pro-inflammatory cytokines. Conversely, the presence of L. crispatus did not, and was able to negate these adverse effects when placed in a dual species coculture with either of the other species. Additionally, we found that L. crispatus was the only one of these four species to produce hydrogen peroxide, and its supernatant was capable of inhibiting the growth of G. vaginalis and P. bivia. While we found that all four vaginal species could use glycogen for their growth, L. crispatus was able to use the widest range of carbohydrates tested. This translated to L. crispatus significantly outcompeting the other three bacterial species when cocultured in bacterial broth media with various carbohydrates tested. Our data provides insight into the species-specific nature by which common vaginal bacteria may interact with vaginal epithelial cells to increase host susceptibility to infection through cytotoxicity, decreased barrier function, and inflammation. We importantly observed the ability of L. crispatus to largely mitigate these effects and our phenotypic characterization place L. crispatus as the species most adept to provide protection in the FRT. Together, this work contributes to a better understanding of the interactions that govern the dynamics of the VMB and can be built upon to develop more rationale therapeutic or prophylactic interventions to improve the reproductive health of many vulnerable women.