Fig. 2.

Vaginal communities and risk of sexually transmitted diseases (STDs). Risk of STD acquisition and transmission increases with increasing diversity of vaginal flora and is lowest with Lactobacillus crispatus–dominant communities. Higher levels of lactic acid have been strongly associated with vaginal health, and production of lactic acid is conserved across healthy vaginal communities. L- and D-lactic acid isomers may have different functions within the vaginal microenvironment, and their ratio may influence expression of host genes and immune response. CST, community-state types; HIV, human immunodeficiency virus; HSV, herpes simplex virus; HPV, human papillomavirus. *Data from references 10 and 19. ^†Data from reference 10 and the following: Witkin SS, Mendes-Soares H, Linhares IM, Jayaram A, Ledger WJ, Forney LJ. Influence of vaginal bacteria and D- and L-lactic acid isomers on vaginal extracellular matrix metalloproteinase inducer: implications for protection against upper genital tract infections. MBio 2013 Aug 6;4. pii: e00460–13. DOI: 10.1128/mBio.00460-13.