Spatial and Functional Characterization of Microbial Residents in the Human Placenta

A healthy placenta is critical to a healthy pregnancy because it is the principal organ that nurtures and protects the fetus until delivery. Among its many functions, it governs fetal nutrition, waste disposal, gas exchange, and fetal tolerance. Notably, the placenta has both maternal and fetal compartments that serve as powerful structural and immunological barriers against pathogens — including bacteria and viruses. The maternal compartment comprises the basal plate, which harbors fetal-derived trophoblasts that are in direct contact with maternal endothelial and immune cells. The fetal compartment harbors the placental villi, which are bathed in maternal blood and serve as site of maternal-fetal exchange. Finally, the fetal membranes encapsulate the amniotic cavity.Historically, the placenta was considered sterile because relatively few pathogens have been shown to breach this robust barrier. Transplacental microbes often incite pathways that lead to local and systemic immune activation and result in adverse outcomes such as loss of fetal viability or preterm birth. Despite advances in maternal and neonatal healthcare, preterm birth remains a global health problem, accounting for the majority of neonatal deaths for children under 5 years old. Contrary to the conventional sterile-womb paradigm, histological, PCR-based, and sequencing analyses point to the presence of “characteristically” non-pathogenic bacteria in normal term placentas. However the localization, impact, and origins of these bacteria on pregnancy and neonatal physiology is not understood.To address spatial localization within the placenta, we profiled the bacteria within the basal plate, placental villi, and fetal membranes using a multi-variable region 16S ribosomal sequencing and described the taxonomic composition, absolute abundance, predominant species, and bacterial diversity within each region. Indeed, we found that placental microbes at term exhibit spatially variable profiles. Furthermore, we found that Ralstonia insidiosa, a waterborne bacterium, dominates the basal plate region in normal term placentas.We tested the cell-specific localization and impact of R. insidiosa within the basal plate using cultured explants derived from term placentas and a cell line that phenotypically resembles fetal trophoblasts within the basal plate. We showed that R. insidiosa resides in the placental BP by direct localization of R. insidiosa within human basal plate biopsies and a trophoblast cell line. Furthermore, R. insidiosa replicates within these models and does not exhibit typical signs of pathogenicity such as overt inflammatory responses or cell death.To address the origins of R. insidiosa within the placenta, we examined potential routes of R. insidiosa entry into the placenta and associated pregnancy outcomes using a pregnant mouse model. We found that R. insidiosa colonizes the placenta via the intrauterine route and does not cause preterm labor. Together, this data supports the hypothesis that R. insidiosa normally seeds the placenta via the intrauterine route. For the first time, we show that R. insidiosa crosses placental barriers without causing adverse responses. Future studies will address the functional relevance of R. insidiosa for placental and neonatal physiology