The vaginal microbiome and preterm birth

As these GEITP pages have continued to emphasize: Multifactorial phenotypes — including complex diseases (e.g. type-2 diabetes, cancer, major depressive disorder), quantitative traits (e.g. height, body mass index), drug efficacy and adverse drug reactions, and responses to environmental toxicants (e.g. dioxin, cigarette smoke, arsenic) — reflect contributions of genetics, epigenetic effects, environmental factors, endogenous influences, and each patient’s microbiome. Some recent breakthroughs in intestinal microbiome research were the topics of yesterday’s GEITP email, whereas today’s GEITP topic concerns the vaginal microbiome (see attached article & editorial).

Microbiomes are located in many areas of the human body (ear, nose, mouth, umbilicus, skin, vagina, as well as our intestine; various regions of skin even have distinctly different microbiota). The vagina houses one of the least diverse microbiomes in the human body. Usually colonized by Lactobacillus species during reproductive years, the acidic metabolites of these species maintain vaginal low pH and low microbiota diversity, impeding colonization by acid-sensitive bacteria — including both gut aerobes (growing in O2) and anaerobes (growing without O2). As vaginal pH becomes more alkaline (e.g. due to infection), the vaginal environment becomes more permissive to colonization by more diverse and undesirable microbial communities; this gives way to bacterial vaginosis (BV). Presence of BV leads to lowered resistance to colonization by pathogens, including HIV. BV is also associated with pelvic inflammation, which during pregnancy can increase risk of premature birth.

There also are individual (genetic) and ethnic variations in the threshold of harmful diversity. It is known, however, that the lack of microbiome diversity is decreased during pregnancy; vaginal changes during pregnancy result in greater Lactobacillus dominance and decreased species diversity. Authors [see attached article] examined single time-points from 1,969 non-pregnant and 613 pregnant women and followed a cohort of 90 pregnant women. They found that early pregnancy differences in the vaginal microbiome — linked to ethnicity — are altered by the gestational dynamics that occur in early pregnancy, with the convergence of all pregnant women toward a Lactobacillus-dominated vaginal microbiome profile, unevenly distributed species with fewer taxa dominating, and a simpler metabolic gene profile.

These data suggest — that in the case of a high-diversity vaginal microbiome during pregnancy — the microbiome composition could be manipulated to reduce adverse risks of pregnancy. Premature-birth-associated taxa were correlated with unwelcome pro-inflammatory cytokines (chemicals secreted by immune cells that evoke responses in other cells) in vaginal fluid. These findings highlight new opportunities for assessment of the risk of preterm birth.


Nat Med June 2019; 25: 1012–1021 & News’n’Views, pp 882-883

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