Are placental bacteria involved in preterm birth?
The main cause of infant morbidity and mortality in the world is preterm birth; it affects 5% to 18% of pregnancies and represents a real public health issue. A British team has attempted to demonstrate the role of the placental microbiota in different types of premature birth.
Spontaneous preterm birth has many causes and intrauterine infections seem particularly implicated. The hypothesis of an infectious cause–based on data in the literature–would entail an inflammatory response in the mother and/or the fetus, thus triggering premature rupture of the membranes. To confirm this hypothesis, the researchers used sequencing to compare the composition of the placental bacteria of a large cohort of women according to delivery mode: spontaneous preterm, induced, and at term delivery. Another important point: this team also estimated the impact of contamination during sample preparation.
Two genera of bacteria under suspicion
Mycoplasma spp. and Ureaplasma spp., two bacteria previously described as opportunistic intrauterine pathogens strongly associated with premature birth, were overabundant in the women who had had a spontaneous preterm delivery. A third bacterium, Capnocytophaga, which usually colonizes the oral microbiota and rarely the genital tract, also seems associated with spontaneous preterm delivery.
Major methodological pitfalls
Although recent work has suggested that the placental microbiota is specific to delivery mode, this study did not confirm this hypothesis. The sequencing of a biomass as small as that of the placenta raises a methodological and statistical challenge, as the authors remind us: the high risk of contamination of samples during handling or delivery complicates the analysis. Moreover, it does not allow demonstration of bacterial clusters associated with prematurity. Despite all these issues, the study of the placental microbiota is just beginning and its involvement in preterm births still has to be elucidated.
Leon, L. J. et al. Enrichment of clinically relevant organisms in spontaneous preterm delivered placenta and reagent contamination across all clinical groups in a large UK pregnancy cohort. Appl. Environ. Microbiol. (2018). doi:10.1128/AEM.00483-18