Evidence for contamination as the origin for bacteria found in human placenta rather than a microbiota

Abstract

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.

Microbial Colonization From the Fetus to Early Childhood—A Comprehensive Review

“…Despite numerous papers published on this field, the concerns of contamination remain unsolved. In the newest study investigating this issue, evidences were put forward for contamination as the origin of bacteria found in human placenta samples (Gschwind et al, 2020).…”

From Short- to Long-Term Effects of C-Section Delivery on Microbiome Establishment and Host Health

“…It is important to highlight that sampling methods have also been assessed by some authors; for example, a study including placenta samples from 20 term and preterm deliveries found bacterial DNA in the samples, but after analyzing the sequences, the differences with the negative controls were not significant [14]. In another study, bacteria were found in fetal membranes and umbilical cords but never inside the placenta, supporting the hypothesis of contamination as the source of bacterial presence in studies that challenge the "sterile womb" hypothesis [15]. As for the amniotic fluid, a recent study in a bovine C-section model revealed some positives (5 out of 24), with low amounts of bacteria in the meconium, but none of the amniotic fluid samples presented different amounts of bacterial DNA compared to the negative controls [16].…”

Presence of distinctive microbiome in the first-pass meconium of newborn infants

“…Recent high-quality studies have questioned the presence of a distinct placental microbiome 15 – 20 and an amniotic fluid microbiome 21 , 22 . Yet, in animal models, maternal bacteria have been suggested to actively colonize the fetal gut before birth 23 , 24 .…”