Abstract
In addition to being a passage for sperm, menstruum, and the baby, the human vagina and its microbiota can influence conception, pregnancy, the mode and timing of delivery, and the risk of acquiring sexually transmitted infections. The physiological status of the vaginal milieu is important for the wellbeing of the host as well as for successful reproduction. High estrogen states, as seen during puberty and pregnancy, promote the preservation of a homeostatic (eubiotic) vaginal microenvironment by stimulating the maturation and proliferation of vaginal epithelial cells and the accumulation of glycogen. A glycogen-rich vaginal milieu is a haven for the proliferation of Lactobacilli facilitated by the production of lactic acid and decreased pH. Lactobacilli and their antimicrobial and anti-inflammatory products along with components of the epithelial mucosal barrier provide an effective first line defense against invading pathogens including bacterial vaginosis, aerobic vaginitis-associated bacteria, viruses, fungi and protozoa. An optimal host-microbial interaction is required for the maintenance of eubiosis and vaginal health. This review explores the composition, function and adaptive mechanisms of the vaginal microbiome in health and those disease states in which there is a breach in the host-microbial relationship. The potential impact of vaginal dysbiosis on reproduction is also outlined.
Highlights
The vaginal mucosal ecosystem is comprised of a stratified squamous non-keratinized epithelium overlaid by a mucosal layer continuously lubricated by cervicovaginal fluid (CVF)
Apart from being an acidic medium containing an assortment of antimicrobial molecules including antibodies (IgA and IgG), mucins, β-defensins, secretory leucocyte protease inhibitor (SLPI), neutrophil gelatinase-associated lipocalin (NGAL), surfactant protein etc., CVF facilitates the confinement of exogenous organisms [1,2,3]
Recent advances in DNA sequencing techniques have revealed that the dominant Lactobacillus species in the vaginal microbiota include L. crispatus, L. gasseri, L. iners, and L. jensenii, while other anaerobes including Gardnerella, Atopobium, Mobiluncus, Prevotella, Streptococcus, Ureaplasma, Megasphaera etc. able to cause infections such as bacterial vaginosis (BV) are kept dormant by the protective action of lactobacilli. These highresolution techniques have enabled the classification of the vaginal microbiota into five community state types (CSTs) with CSTI, II, III and V dominated by L. crispatus, L. gasseri, L. iners, and L. jensenii respectively, while CSTIV is dominated by mixed anaerobes similar to those found in BV [5, 11]
Summary
The vaginal mucosal ecosystem is comprised of a stratified squamous non-keratinized epithelium overlaid by a mucosal layer continuously lubricated by cervicovaginal fluid (CVF). Together, these form a daunting physical and biochemical barrier against extraneous invading organisms. The vagina harbors numerous microorganisms (the “microbiota”), that exist (in conjunction with their genes and products) in a regulated mutualistic relationship with the host (the “microbiome”) [4]. Some of these microorganisms such as Lactobacillus species reinforce the defense against invasion and colonization by opportunistic pathogens. The composition of the vaginal microbiota/microbiome is dynamic and undergoes changes corresponding with hormonal fluctuations throughout the woman’s reproductive life, i.e., from puberty to menopause, and during pregnancy [5]
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