The placenta, once thought sterile, actually harbours a world of bacteria that may influence the course of pregnancy and help shape the bacterial makeup of the infant’s gut, a new study has found.

New research shows that the placenta harbours a unique, low abundant microbiome, said researchers from Baylor College of Medicine and Texas Children’s Hospital.

The findings provide important new insights on the structure of the placental microbial community, the organisms present, and how they might be capable of impacting a pregnancy.

“After we completed our studies of the vaginal microbiome in pregnancy, we noted that the most abundant microbes in the mom’s vagina were not what populated the baby’s intestinal microbiome,” said Dr Kjersti Aagaard, lead and corresponding author on the the research.

“We reasoned that there must be another source ‘seeding’ the infant’s gut at birth, so we sought to examine the placenta,” said Aagaard. In the first and largest study to focus on the placental microbiome, 320 human subjects’ samples were analysed comprehensively following a process called shotgun metagenomic sequencing.

This technology enables microbiologists to uniquely evaluate bacterial diversity and detect the abundance of specific microbes and all their genetic pathways.

The researchers showed that the placenta is not sterile (free from bacteria or other living organisms), but rather harbours a diverse and very unique microbiome.

“The placental microbiome is low in terms of microbial abundance but not as sterile as we previously thought,” said Aagaard.

Of the samples, Escherichia coli (E coli) was the species with the highest abundance in most individuals, a bacteria that lives in the intestines of most healthy individuals.

Prevotella tannerae (gingival crevices) and non-pathogenic Neisseria species (mucosal special surfaces), both species of the oral cavity, were also detected in highest relative abundance.

“Interestingly, when we looked very thoroughly at the placenta in relation to many other sites of the body, we found that the placental microbiome does not bear many similarities to microbiomes closest in terms of anatomic location.

Specifically, it is not much like the vaginal or intestinal microbiome, but rather is most similar to the oral microbiome,” said Aagaard.

The finding has important implications on the likely importance of oral health during pregnancy, she said.

“It reinforces long-standing data relating periodontal disease to risk of preterm birth,” said Aagaard. The finding was published in the journal Science Translational Medicine.