Protection and Risk: Male and Female Genital Microbiota and Sexually Transmitted Infections

Abstract

Unique compositional and functional features of the cervicovaginal microbiota have been associated with protection against and risk for sexually transmitted infections (STI). In men, our knowledge of the interaction between the penile microbiota and STI is less developed. The current state of our understanding of these microbiota and their role in select STIs is briefly reviewed, along with strategies that leverage existing findings to manipulate genital microbiota and optimize protection against STIs. Finally, we focus on major research gaps and present a framework for future studies.

Keywords: Lactobacillus; STI; anaerobes; microbiome; penile; vagina.

Figure 1.

A, Studies have shown that reproductive-aged women can be grouped into several distinct vaginal microbial community state types (CSTs) based on diversity and relative abundance of bacteria [44, 50]. There are 4 species of Lactobacillus that are consistently observed in surveys of vaginal microbiota and each is capable of dominating a vaginal community state type. CST I is dominated by Lactobacillus crispatus, CST II by L. gasseri, CST III by L. iners, and CST V by L. jensenii. Low-Lactobacillus communities (CST IV-A, CST IV-B, and CST IV-C) are dominated by a variety of bacterial vaginosis-associated bacteria (BVAB; eg, Atopobium, Prevotella, Dialister, Gardnerella, Megasphaera, Peptoniphilus, Sneathia, Eggerthella, Aerococcus, Finegoldia, and Mobiluncus) [44]. These low-Lactobacillus communities (defined by tools such as 16S rRNA gene amplicon sequencing, quantitative PCR, and metagenomic sequencing) are collectively termed molecular-BV [26]. B, While the vaginal microbiota in some women may remain stable over time dominated by Lactobacillus spp. or in the absence of Lactobacillus, in others it can fluctuate dramatically, over hours to days [43, 49].

Figure 2.

A, Although Lactobacillus spp. may sometimes be found, the vaginal microbiota of premenarchal girls is generally characterized by low quantities of these bacteria. They are often present after birth, in part likely due to maternal estrogen stimulating the temporary maturation of the vaginal epithelium that supports the growth of Lactobacillus spp. As girls progress through puberty and eventually menarche (and estrogen levels increase [54]) the vaginal microbiota can transition to become Lactobacillus dominated [55, 56]. In menopause, as estrogen levels decline, Lactobacillus spp. decrease [57–61]. Pregnancy, a high-estrogen and -progesterone state, is associated with increased stability and Lactobacillus dominance of the vaginal microbiota [62, 63]. B, Increased estrogen at certain times of the menstrual cycle may correlate with increased Lactobacilli and increased stability [43]. Finally, exogenous sex hormones in the form of hormonal contraceptives, or topical estrogens used to treat atrophic vaginitis in postmenopausal women, have also been shown in some [64, 65], although not all studies [34] to increase vaginal Lactobacillus. The effects of environmental mimetic estrogens or xenoestrogens on the vaginal microbiota are currently unknown [66].