As researchers look more closely at internal anatomical structures, they reveal more and more how bacteria are happily dwelling in places we once thought impossible under normal, healthy conditions. This week one of the last bastions of human organ understanding, the placenta, surprised researchers when they found bacteria in the organs after healthy, normal pregnancies. However, these “placental microbiomes” may play a role in premature births, scientists speculate.

The report, which was published this week in the American Association for the Advancement of Science’s journal Science Translational Medicine, describes the results from a population-based cohort study in which placental specimens were collected under sterile conditions from 320 subjects.

The researchers used 16S ribosomal DNA-based and whole-genome shotgun DNA sequencing of the placental samples for comparison to the sequencing results from samples collected from oral and nasal cavities, skin, vagina, and gut. The sequence-based metagenomic studies revealed that the placental microbiome is most similar to the oral microbiome. The actual amount of bacteria in the placentas tested was quite low.

“Most people assume that bacteria are crawling along all these surfaces of our body. But, each different bacterial niche actually maintains exquisite specificity,” said Kjersti Aagaard, lead author of the study and researcher at Texas Children’s Hospital in Houston, Texas.

The placenta is the primary organ of exchange between the developing human baby and the mother. It is genetically identical to the baby and contains only the baby’s blood. It has long been assumed to be sterile.

“This idea is really a reflection of both clinical observations and the limits of technology,” said Aagaard.

The authors of the study also found a possible link between placental microbiome and bacterial infection in the mother during pregnancy such as urinary tract infection in the first trimester. They also reported a link to preterm birth defined as birth before 37 weeks.

The next step for Aagaard and colleagues is to examine these findings in a larger study of 500 women at risk for premature births.