In a surprise find, placentas harbor bacteria

The placenta harbors an unexpected collection of bacteria. Its mix of microbes may promote healthy pregnancies or lead to premature births.

Doctors and scientists have long thought that the womb was sterile. They figured that babies pick up their microbiomes — the collections of bacteria and other microbes that exist in and on them for the rest of their lives — during birth and early childhood.

Last year, however, researchers found that microbes make it to the side of the placenta where it fuses to the mother’s uterus. Many scientists assumed the placenta acts as a barrier preventing bacteria from reaching the fetus, says pediatric infectious disease specialist Anna Bakardjiev of the University of California, San Francisco.

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But research in the May 21 Science Translational Medicine also found bacteria on the baby’s side of the organ, near where the umbilical cord originates, indicating that the placenta has a microbiome of its own. Obstetrician Kjersti Aagaard of Baylor College of Medicine and Texas Children’s Hospital, in Houston, and her colleagues sampled 48 placentas from women who had full-term pregnancies or delivered prematurely.  Women who gave birth to premature babies tended to have a mix of placental microbes that differed from the placental microbiome of women whose pregnancies lasted the full term, the researchers found.  The microbes the researchers found tended to be normal residents of human bodies, not disease-causing invaders.

“It’s exciting that we’re breaking down this dogma that the placenta is sterile,” says Bakardjiev, who was not involved in the work. “The question is, ‘why is it not sterile?’” She wants to know how microbes get to the placenta.

Bacteria’s shortest route to the placenta would be to migrate from the vagina into the uterine cavity, says Aagaard. But she and her colleagues found that the placenta is populated more by bacteria usually found in the mouth than by ones typically found in the vagina. The researchers propose that bacteria travel from a woman’s mouth through her bloodstream to set up shop in the placenta. The finding could help explain why women with periodontal disease or some other infections are at high risk of delivering prematurely.

But the researchers also found that E. coli bacteria are relatively abundant in the placenta. Those bacteria are typically found in the gut and feces. Bakardjiev says the microbes may ascend through the vagina and colonize the uterus even before pregnancy.

One weakness of the study is that the researchers compared the placental microbiomes not with bacteria elsewhere in the mother’s body, but with microbes found in or on various body sites in a major study called the Human Microbiome Project, says Amanda Lewis, a microbiologist at Washington University School of Medicine in St. Louis. She has previously linked preterm birth with an infection called bacterial vaginosis. The bacterial profiles the researchers found in the placentas that nourished premature babies are similar to those in some forms of bacterial vaginosis. Lewis suggests that the microbes could have invaded the placenta not from the mouth but from the vaginas of women with undiagnosed vaginosis. Comparing a mother’s microbes with those in her baby’s placenta might have revealed that connection.

Unfortunately, giving antibiotics to pregnant women who had periodontal disease or urinary tract infections failed to prevent preterm births in other studies, says Antonio Frias, a maternal-fetal medicine researcher at Oregon Health & Science University in Portland. That failure could be an indication that the placenta’s microbiome is difficult to alter, he says. “How to change your microbiome effectively is a whole area of research without a lot of answers yet,” Frias says.

It’s too early to say whether a particular microbial mix promotes healthy pregnancies, researchers say. But the new work “is outstanding,” says George Saade, director of maternal-fetal medicine at the University of Texas Medical Branch at Galveston. “It’s so novel and we really need to pay attention to it.” One difficulty of studying the placental microbiome is that researchers can get access to the organ only after birth. He thinks giving pregnant women blood tests or even collecting their saliva might give clues to which bacteria end up in the placenta. Meanwhile, new techniques for imaging the placenta might show when the microbes arrive.

Women should maintain good oral health in addition to good overall health if they are pregnant or thinking about getting pregnant, researchers say.