Diversity of Vaginal Microbiome in Pregnancy: Deciphering the Obscurity

Abstract

Human microbiota plays an indispensable role in physiology, nutrition and most significantly, in imparting immunity. The role of microbiota has remained cryptic for years, until recently meticulous studies revealed the interaction and dynamics of these microbial communities. This diversified state is governed by hormonal, behavioral and physio-chemical changes in the genital tract. Many inclusive studies have revealed "Lactobacillus" to be the most dominant member of vaginal flora in most of the healthy, reproductive age group and pregnant females. A total of five community state types have been described, out of which four are dominated by Lactobacillus while the fifth one by facultative or strict anaerobic species. A variation between species stability and gestational age has also been revealed. Studies have divulged a significant higher stability of vaginal microbiota in early stages of pregnancy and the same increased subsequently. Inter-species and racial variation has shown women belonging to White, Asian, and Caucasian race to harbor more of the anaerobic flora. The vaginal microbiome in pregnancy play a significant role in preterm and spontaneous labor. This Lactobacillus-rich microbiome falls tremendously, becoming more diverse in post-partum period. Apart from these known bacterial communities in human vagina, other microbial communities have also been traced. The major fragment is constituted by vaginal viral virome and very little information exists in relation to vaginal mycobiome. Studies have revealed the abundance of ds DNA viruses in vaginal microbiome, followed by ssDNA, and few unidentified viruses. The eukaryotic viruses detected were very few, with Herpesvirales, and Papillomaviridae being the only pathogenic ones. This flora is transmitted to infants either via maternal gut, vagina or breast milk. Recent studies have given an insight for vaginal microbiome, dissociating the old concept of "healthy" and "diseased." However, more extensive studies are required to study evolution of virome and mycobiome in relation to their association with bacterial communities; to establish and decode full array of vaginal virome under the influence of genotypic and environmental factors, using novel bioinformatic, multi-omic, statistical model, and CRISPR/Cas approaches.

Vaginotypes of the human vaginal microbiome

“…During pregnancy, hormonal changes lead to immune modulation and physico‐chemical changes in the mucosa of the genital tract (Gupta et al ., 2020). These changes can affect the composition and the function of the vaginal microbiota, making it distinctive from non‐pregnant women.…”

“…Analysis of the species richness revealed a statistically significant simplification of the vaginal microbiota of pregnant women (average species richness of 17 ± 14) when compared with non‐pregnant samples (average species richness of 24 ± 18) ( t ‐test p ‐value <0.01) (Fig. S1a), confirming the shift of the vaginal microbiota towards low biodiversity during pregnancy (Romero et al ., 2014; Freitas et al ., 2017; Gupta et al ., 2020). Moreover, HCL and PCoA analysis allowed the identification of associations between vaginotypes and pregnancy (Fig.…”

“…CST IV‐C and CST IV‐D, which are delineated by a predominance of species belonging to the Gardnerella genus or by a heterogeneous group of bacteria, such as Bifidobacterium , Lactobacillus , Alloscardovia , Gardnerella and Atopobium respectively. Furthermore, recent studies revealed that pregnancy induces reduced biodiversity and increases taxonomic composition stability of the vaginal microbiota (Freitas et al ., 2017; Gupta et al ., 2020).…”

Effect of Bacterial Vaginosis (BV)-HIV-1 Co-existence on Maternal and Infant Health: A Secondary Data Analysis

“…The alternative method would have been to culture the bacteria, but at the time, there was limited microbiology culture capacity, and diagnosis of BV was not the primary endpoint of the study. Since then, the development of the field of genomics has expanded on opportunities to describe the microbiota, and in the process, has expanded the spectrum of bacteria identified to be part of the human microbiome ( 10 , 13 ). Studies that have evaluated these newer techniques along with the newer genomic studies have found congruent study findings ( 35 ).…”

“…Although there are no known direct causes of BV, a large body of literature reports an increased probability of infection due to sexual activity. Two studies conducted in Kenya provide evidence for this with the observation that 90% of sexually inactive women had normal vaginal flora (12,13), whereas Bukusi et al, reported a high BV prevalence of 44% among married couples (14). BV, however, does not meet the threshold of being classified as a sexually transmitted infection because the male partner does not experience any illness and there is lack of a single causative agent.…”