Bacteria found inside the placentas isolated from cesarean deliveries under controlled conditions do not constitute a microbiota.. No Evidence for a Placenta Microbiota

Until recently the in utero environment of pregnant women was considered sterile. Recent high-sensitivity molecular techniques and high-throughput sequencing lead to reports on evidence for a low-diversity placental microbiome. In parallel, some studies failed to evidence the presence of bacteria using either culture or molecular based techniques. Nevertheless, placental microbiome studies used different protocols and the effect of such methods variability on evidencing the presence of bacteria in utero remains unclear. Thus, the in utero environment sterility deserves evaluation using several methods in the same study to state on the presence or absence of bacteria and to explain such contradictions present in the literature. Here, 38 healthy pregnant women were recruited in 3 maternities. Placenta were collected following 3 protocols (cesarean section with or without Alexis® and vaginal delivery) and 3 areas were sampled (fetal membranes, umbilical cord and chorionic villi). Bacterial presence was analyzed using bacterial culture and qPCR on 34 fetal membranes, umbilical cord and chorionic villi samples. Moreover, shotgun metagenomics was performed on 7 chorionic villi samples. We showed that the isolation of meaningful quantities of viable bacteria or bacterial DNA was possible only outside the placenta (fetal membranes and umbilical cords) highlighting the importance of sampling methods in studying the in utero environment. Bacterial community described by metagenomics analysis was similar in chorionic villi samples and in negative controls and was dependent on database choice for analysis. Thus, our data showed that the placenta does not harbor a microbiota, meaning a resident, specific and functional bacterial community.