WASHINGTON – The human placenta, the organ that nourishes a developing baby, is not the pristine place some experts had assumed.
Researchers said on Wednesday they have identified a relatively small but thriving group of microbes that inhabit the placenta alongside human cells, a finding that may point to new ways of spotting women at risk for preterm births.
There were clear differences in the mixture of microbes — the microbiome — in the placenta of women who had premature babies compared with those who delivered full-term babies, said Dr. James Versalovic, a professor at Baylor College of Medicine and head of pathology at Texas Children’s Hospital in Houston.
Versalovic said this knowledge could lead to diagnostic tests to forecast which women may be at risk for preterm birth and help obstetricians manage those pregnancies in new ways.
“It allows us to think about the biology of pregnancy in different ways than we have before, that pregnancy and early life aren’t supposed to be these totally sterile events,” said lead researcher Dr. Kjersti Aagaard of Baylor.
Scientists know that microorganisms routinely reside in large numbers in certain parts of the human body such as the gut, which is naturally awash with bacteria.
But would the placenta, which forms within the mother’s womb, provides oxygen and nutrients to the fetus and is pushed out after birth, also support a bunch of bacteria?
“This is a eureka moment where we say, ‘Wow, there are bacteria here in the placenta,’” said Versalovic, who noted that the view among many experts had been that the placenta might be free of such microbes.
“Now we can begin to say: ‘What are they doing? So why are the bacteria there?” Versalovic said in a telephone interview.
Over the eons, such colonies of microbes have been “co-evolving with humans,” Versalovic said. “This intimate relationship has resulted in the maintenance of a microbial community in the placenta during healthy pregnancies that clearly is not impacting the fetus in a negative way and may be helping to nourish the fetus, protect the fetus, enable the fetus to develop.”
Aagaard’s team earlier had studied the microbiome of the vagina, and learned that its composition changes when a woman becomes pregnant. The puzzle: The most common vaginal microbes weren’t the same as the earliest gut bacteria that scientists were finding in newborns.
What else, Aagaard wondered, could be “seeding” the infants’ intestinal tract?
With colleagues from Baylor and Texas Children’s Hospital, Aagaard analyzed 320 donated placentas, using technology that teases out bacterial DNA to evaluate the type and abundance of different microbes.
More than 300 types of bacteria were detected in this placental tissue, with the well-known E. coli the most abundant, Versalovic said. The study found that this placental microbial population more closely resembled that of the mouth than other sites in the women such as the gut or vagina.
The theory: Oral microbes slip into the mother’s bloodstream and make their way to the placenta.
Why does the body allow them to stay? Aagaard said there appears to be a role for different microbes. Some metabolize nutrients. Some are toxic to yeast and parasites. Some act a bit like natural versions of medications used to stop preterm contractions, she said.
Among the 89 placentas that were collected after preterm births, levels of some of the apparently helpful bacteria were markedly lower, she said.
Aagaard is beginning a larger study to explore the link, planning to analyze the oral and placental microbiomes of more than 500 pregnant women at risk of preterm birth.
Aagaard said that while the study found differences in the placental microbiome in women who had preterm babies, there was no such difference in women who, for example, delivered via Caesarean section versus vaginal birth, or who were obese.
The study, part of a project funded by the U.S. National Institutes of Health to understand these microbes and their effect on health, was published in the journal Science Translational Medicine.