Vaginal Microbiota #21

Predicting the risk of preterm birth through vaginal microbiota

^{Liao J, Shenhav L, Urban JA, et al. Microdiversity of the vaginal microbiome is associated with preterm birth. Nat Commun 2023; 14: 4997.}

Respiratory, gastrointestinal and neurodevelopmental complications: preterm birth is the main cause of neonatal morbidity and mortality. The vaginal microbiota seems to be involved, but the underlying mechanisms remain poorly understood. A team of American researchers tracked the genome of the vaginal microbiota of 175 American women throughout their pregnancies (40 of whom subsequently experienced spontaneous preterm delivery, and 135 of whom delivered at full term). The study shows that the two types of pregnancy differ in terms of vaginal microbiota composition: certain bacterial species of the Lactobacillus genus, such as L. helveticus, L. crispatus, L. gasseri and L. jensenii, are associated with fullterm pregnancies, while Megasphaera genomosp, Gardnerella spp. and Atopobium vaginae are linked to preterm births. Another finding is that the genetic diversity of the vaginal microbiota is higher in the first half of pregnancies that end preterm, due to Gardnerella species. More precisely, the nucleotide diversity of Gardnerella spp. increases at the start of pregnancies that end preterm whereas it remains stable in pregnancies that are carried to term. The genetic diversity of Gardnerella spp. could perhaps be used as a biomarker for the early diagnosis of preterm birth. But how can we explain this peak in Gardnerella nucleotide diversity? Compared to other bacteria, Gardnerella shows a 1.5-fold higher growth rate at the start of pregnancy, more frequent genetic recombination and greater selection of mutations that benefit this bacterium (and increased elimination of deleterious mutations). Antibiotics and other xenobiotics are thought to be involved. In fact, the more diversified gene pool of G. swidsinskii seems to correspond to an adaptation to drugs, confirming a previously suggested effect of xenobiotics in the vaginal environment; and vaginal microbiota associated with preterm birth exhibit higher antibiotic resistance potential. Genomic variation in vaginal bacteria is therefore believed to affect the host’s phenotypes (including pregnancy outcomes). However, the authors do not rule out another explanation, even if they consider it unlikely: the associations between microbial genetic diversity and pregnancy outcomes could also result from unmeasured confounding factors (drugs, chemical compounds, etc.) that might act on both variables.