9 The Vaginal Microbiome
The Vaginal Microbiome
The vaginal microbiome is a reproductive organ-specific niche that harbors a unique collection of microorganisms that are important in a variety of aspects to human health. This microbial community has significance in maintaining vaginal homeostasis, protecting against urogenital infections, host immunity, and reproductive capacities. In recent years, next generation sequencing (NGS) techniques have been employed to classify the vaginal microbiome into community state types (CSTs), or vaginotypes, based upon composition, which can enhance epidemiological studies to make better associations between the microbiome and host-vaginal health (France et al., 2020, Mancabelli et al., 2021).
Composition and Role
The vaginal microbiome is composed of over 200 different bacterial species, though it is primarily dominated by members from Lactobacillus genus (Ma et al., 2012, Auriemma et al., 2021). The Lactobacilli protect the vagina from potential pathogen invasion through their fermentation of vaginal epithelial cell-produced glycogen into lactic acid, which lowers vaginal pH, as well as their production of various antimicrobial compounds, and resource/space competitive inhibition (Boskey et al., 2001, Amabebe and Anumba, 2018, Chee et al., 2020, Jang et al., 2019). Though, the bacteriome is usually the emphasized feature of the vaginal microbiome, it also harbors protists, fungi, archaea and viruses, with each occupying their own niche in the normal healthy network (Belay et al., 1990, Bradford and Ravel, 2017, Happel et al., 2020, Chacra and Fenollar, 2021). The composition isn’t always fixed however, as factors like the menstrual cycle, hormone fluctuation, sexual partners, hygiene, genetics, age, the environment, drug use, and other aspects of lifestyle can affect the microbial makeup (Aagaard et al., 2012, Fettweis et al., 2014, Hyman et al., 2014, Zapata and Quagliarello, 2015, Martin and Marrazzo, 2016, Diop et al., 2019, Chacra and Fenollar, 2021).
As mentioned earlier, using 16S rRNA sequencing, the vaginal microbiome has been categorized into CSTs and allowed for deeper analysis, where comparisons of the consortia within these classifications can be used to make associations with host and vaginal health. There are 5 main CSTs each predominated by a particular species of Lactobacilli, except for group IV, which has been further dissected into additional subgroups: CST I—L. crispatus dominated, CST II—L. gasseri dominated, CST III—L. iners dominated, and CST V—L. jensenii dominated (Ravel et al., 2011, France et al., 2020, Chacra and Fenollar, 2021, Mancabelli et al., 2021). CST IV is not dominated by any particular species, and contains a mixture of both strict and facultative anaerobes including Gardnerella, Atopobium, Lactobacillus, Bifidobacterium, etc., where certain controversial subgroups (e.g. CST IV-A, CST IV-B, CST IV-C, CST IV-D, CST IV-G, etc.) have various combinations depending on the st