Unraveling the Dynamics of the Human Vaginal Microbiome

Abstract

Four Lactobacillus species, namely L. crispatus , L. iners, L. gasseri, and L. jensenii, commonly dominate the vaginal communities of most reproductive-age women. It is unclear why these particular species, and not others, are so prevalent. Historically, estrogen-induced glycogen production by the vaginal epithelium has been proffered as being key to supporting the proliferation of vaginal lactobacilli. However, the 'fly in the ointment' (that has been largely ignored) is that the species of Lactobacillus commonly found in the human vagina cannot directly metabolize glycogen. It would appear that this riddle has been solved as studies have demonstrated that vaginal lactobacilli can metabolize the products of glycogen depolymerization by α-amylase, and fortunately, amylase activity is found in vaginal secretions. These amylases are presumed to be host-derived, but we suggest that other bacterial populations in vaginal communities could also be sources of amylase in addition to (or instead of) the host. Here we briefly review what is known about human vaginal bacterial communities and discuss how glycogen-derived resources and resource competition might shape the composition and structure of these communities.

Keywords: alpha-amylase; glycogen; microbial community; microbiome; vagina; vaginal microbiome.

Figure 1

Composition and structure of vaginal bacterial communities found in 396 reproductive age women. The bacterial populations in each sample were classified based on partial 16S rRNA gene sequences and the communities were clustered based on the relative abundances of these bacterial populations. Major groups of communities were used to define community state types I to V. This heatmap shows the relative abundance (see color key) of bacterial taxa (listed on the left) in each community. (From [3]. Reprinted with permission from the Proceedings of the National Academy of Sciences USA.)

Figure 2

Temporal dynamics of vaginal bacterial communities. Heat maps (top) and interpolated bar plots (bottom) depict changes in the relative abundances of bacterial taxa in the vaginal communities of four women over 16 weeks. The color code for each taxon is shown below the figure. (From [8]. Reprinted with permission from AAAS.)

Figure 3

Schemes for the production and use of glycogen-derived resources by vaginal bacterial communities. Panel A depicts our current understanding of how glycogen-derived resources are produced and used by vaginal bacterial communities. Host estrogen stimulates the production and accumulation of glycogen, which is degraded by human α-amylase to produce simpler sugars that are consumed by vaginal lactobacilli and fermented to produce lactic acid. In Panel B we propose that bacterial populations in vaginal communities could also be sources of α-amylase in addition to (or instead of) the host. The resulting simpler sugars could serve as a ‘common good’ that is available to the entire bacterial community, thus setting the stage for interspecies competition for these resources. Both panels illustrate ecological networks that include species of Lactobacillus, various other bacterial populations and the host.