Microbiota in vaginal health and pathogenesis of recurrent vulvo-vaginal infections: a critical review

Abstract
Recurrent vulvovaginal infections (RVVI) have not only become an epidemiological and clinical problem but also include large social and psychological consequences. Understanding the mechanisms of both commensalism and pathogenesis is necessary for the development of efficient diagnosis and treatment strategies for these enigmatic vaginal infections. Through this review, an attempt has been made to analyze vaginal microbiota (VMB) from scratch and to provide an update on its current understanding in relation to health and common RVVI i.e. bacterial vaginosis, vulvovaginal candidiasis and Trichomoniasis, making the present review first of its kind. For this, potentially relevant studies were retrieved from data sources and a critical analysis of the literature was made. Though culture-independent methods have greatly unfolded the mystery regarding vaginal bacterial microbiome, there are only a few studies regarding the composition and diversity of vaginal mycobiome and different Trichomonas vaginalis strains. This scenario suggests a need for further studies based on comparative genomics of RVVI pathogens to improve our perception of RVVI pathogenesis which is still not clear (Fig. 5). Besides this, the review details the rationale for Lactobacilli dominance and changes that occur in healthy VMB throughout a women’s life. Moreover, the list of possible agents continues to expand and new species recognised in both health and VVI are updated in this review. The review concludes with the controversies challenging the widely accepted dogma i.e. “VMB dominated with Lactobacilli is healthier than a diverse VMB”. These controversies, over the past decade, have complicated the definition of vaginal health and vaginal infections with no definite conclusion. Thus, further studies on newly recognised microbial agents may reveal answers to these controversies. Conversely, VMB of women could be an answer, but it is not enough to just look at microbiology. We have to look at the woman itself as VMB which is fine for one woman but may be troublesome for others. These differences in women’s response to the same VMB may be determined by a permutation of behavioural, cultural, genetic and various other anonymous factors, exploration of which may lead to proper definition of vaginal health and disease.

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Keywords: Bacteriobiota, Community state types (CSTs), Mixed infections, Mycobiota, Menopause, Non-albicans Candida (NAC) Species, Vaginal Mycobiome, Vaginal ecosystem and vaginal dysbiosis