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<i>Science Translational Medicine</i>: Bacteria Live in Healthy Placentas

The placenta, long thought to be sterile, has a bacterial community similar to the one found in the mouth. | AAAS/Carla Schaffer

A small but diverse bacterial community calls the placenta home, scientists have discovered. The findings, from a new study of hundreds of placentas taken from healthy women, uproot the long-held notion that the placenta is a sterile environment.

The research in the 21 May issue of Science Translational Medicine also hints at an association between the bacterial makeup of the placenta and preterm births.

The placenta, an organ that develops during pregnancy, is one of the last mysteries of the human body. While it's understood that the placenta's main tasks are to remove waste and supply a growing fetus with oxygen and nutrients, the long-term effects of the organ on maternal and fetal health remain unclear. Research like the Human Placenta Project aims to dig deeper and make connections between placental health and adverse pregnancy outcomes such as preterm birth.

The placenta has long been thought to be sterile. "This idea is really a reflection of both clinical observations and the limits of technology," says Kjersti Aagaard, lead author of the study and researcher at Texas Children's Hospital in Houston, Texas. For instance, most observations of placental bacteria came from women who had inflammation in those tissues, Aagaard said, so healthy women were assumed to have no placental bacteria.

These days, genomic techniques allow researchers to recover and sequence bacterial DNA directly from placental tissues. In their examination of more than 300 placentas, Aagaard and her colleagues found that while sparse, the bacterial makeup of the placenta is most similar to the microbial community found in the mouth.

"Most people assume that bacteria are crawling along all these surfaces of our body. But, each different bacterial niche actually maintains exquisite specificity," said Aagaard.

The authors suspect that oral microbes from the mother might slip into the bloodstream and end up in placenta. The mostly non-pathogenic bacteria living in the placenta have important day-to-day functions, like metabolizing vitamins and cofactors (for example, biotin and folic acid) at healthy levels for a developing fetus.

Taking a closer look at bacterial diversity in placental specimens of women who had experienced preterm births, the researchers also saw that certain kinds of bacteria were more prevalent in these placentas compared to those from full-term pregnancies.

Aagaard and her team plan to further explore this connection between placenta bacterial composition and preterm births in a future study looking at the oral and placental microbiomes of over 500 women at risk for preterm birth.

Author

Nadia Ramlagan