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International audience; Until recently the in utero environment of pregnant women was considered sterile. Recent high-sensitivity molecular techniques and high-throughput sequencing lead to some evidence for a low-biomass microbiome associated with the healthy placenta. Other studies failed to reveal evidence for a consistent presence of bacteria using either culture or molecular based techniques. Comparing conflicting "placental microbiome" studies is complicated by the use of varied and inconsistent protocols. Given this situation, we undertook an evaluation of the in utero environment sterility using several controlled methods, in the same study, to evaluate the presence or absence of bacteria and to explain contradictions present in the literature. Healthy pregnant women (n = 38) were recruited in three maternity wards. Placenta were collected after cesarean section with or without Alexis ® and vaginal delivery births. For this study we 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. Shotgun metagenomics was performed on seven 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 communities described by metagenomics analysis were similar in chorionic villi samples and in negative controls and were dependent on the database chosen for the analysis. We conclude that the placenta does not harbor a specific, consistent and functional microbiota.

Certaines pathologies semblent avoir une origine développementale et, aujourd’hui, le microbiote apparaît comme un déterminant de l’état de santé de l’homme et son établissement, une étape importante chez le nouveau-né. Des variations dans sa constitution, incluant un déséquilibre (ou dysbiose), ont ainsi été associées à de nombreuses pathologies. De récents travaux suggèrent qu’une colonisation bactérienne de l’utérus, du liquide amniotique ou encore du placenta, des sites auparavant pensés stériles, existerait. Durant les phases de son développement, le foetus pourrait ainsi rencontrer des bactériesin utero. Elles contribueraient à l’établissement de son microbiote avant même l’accouchement et donc avant la rencontre avec les microorganismes des microbiotes vaginal, fécal et cutané, ceux-ci variant selon les modes d’accouchement (voie basse ou césarienne). Les premières études sur l’existence d’un microbiotein utero, qui se caractérise par une faible biomasse, sont cependant controversées.

Some diseases seem to have a developmental origin. Today, the microbiota is recognized as a determinant in health and diseases and one important step is its establishment in the neonate. Some variations in its composition including an imbalance (also called dysbiosis) have been associated to several pathologies. Recent studies suggest a bacterial colonization in the non-pregnant uterus, in the amniotic fluid and in the placenta, which were previously thought sterile. So, during deve-lopmental phases, the fetus could have encounter bacteria in utero. These bacteria could contribute to its microbiota establishment before parturition and therefore before the encounter with all microorganisms from vaginal, fecal and cutaneous microbiotas according to the delivery mode. However, studies stating the existence of such in utero microbiota, characterized by a low biomass, are somewhat disputed.