A research team from the University of Cambridge has finally answered a heavily disputed question about the presence or absence of a microbiome in the human placenta. Scientists have traditionally viewed the placenta as sterile, with the assumption that the presence of microbes would endanger a pregnancy. However, a 2014 study suggested the presence of a unique microbiota in the placenta similar to bacteria found in human mouths. This most recent study, published in Nature this week, provided evidence that in healthy pregnancies, the human placenta does not contain microorganisms. Additionally, it’s unlikely that infants develop their microbiota in utero.
The research team analyzed 537 placenta samples, representing the largest sample size for a study of this kind. They used Salmonella bongori, which does not occur in humans, as a positive control. They used two DNA sequencing methods; the first of which targeting genes common to all bacteria. The second method used all DNA present in the sample and mapped it to specific bacterial and animal species. They discovered that any present bacteria were acquired during labor and delivery and that certain microbes occur in higher concentrations if a woman had a vaginal delivery versus cesarean section.
After accounting for contaminants, the one bacterium that researchers found in 5% of the samples was Streptococcus agalactiae. According to a news article in Nature regarding the study, S. agalactiae can be transmitted from mother to infant during the birthing process and may cause pneumonia, septicemia, and meningitis.
Kjersti Aagaard, obstetrician and maternal-fetal medicine specialist at Baylor College of Medicine and Texas Children’s Hospital, disagrees with these results. Aagaard was also the lead author on the 2014 study that discovered the unique placental microbiome. Mainly, Aagaard disagrees with the researchers’ choice to disregard placental microbes that overlapped with those found in the vagina. As she stated to Science News, “why would we disregard and toss them out, when the biology makes perfect sense?”
Nature concludes that this “carefully controlled, large-scale study was needed to provide strong evidence for the absence of bacteria in the placenta.” While the author recommends further investigations, they state that we should feel confident that “the placenta is not a microbial reservoir and therefore is not a major direct stream of diverse microbes to the fetus under healthy conditions."