Researchers have discovered a small community of bacteria living in a most unlikely place: the placenta, the organ that nourishes a developing fetus through the umbilical cord. The finding overturns the conventional wisdom that the placenta is sterile. The study also suggests that these microbes may come from the mouth, affirming that good oral hygiene may be important for a healthy pregnancy.

The placenta is a pancake-shaped mass of tissue on the side of the uterus that provides oxygen, food, and waste removal to a fetus. Medical experts have long assumed that any bacteria found in the organ must have been picked up when it passed through the vagina after delivery. But more recently, researchers have realized that a baby has a community of bacteria in its gut when it is born. And these bacteria don’t match those in the vagina, suggesting some other source, such as the placenta, says fetal medicine specialist Kjersti Aagaard of Baylor College of Medicine in Houston, Texas.

Aagaard and co-workers are collaborators on the U.S. Human Microbiome Project, which is studying microbiomes—communities of bacteria, fungi, and viruses—that live in various places on and in our bodies. They looked for a placental microbiome by analyzing carefully collected placentas from 320 pregnancies. The researchers extracted DNA from the placentas and sequenced it for snippets and entire bacterial genomes in order to identify and quantify microbial species and the genes they carried. This analysis revealed low levels of a diverse set of bacteria, mostly nondisease causing strains of Escherichia coli, which dominate our intestinal tracts, but also others from five broad groups, or phyla. Most were benign species known to provide services such as metabolizing vitamins.

Surprisingly, the mix of bacteria in the placenta looked more like the microbiome in an adult human’s mouth than the vaginal, skin, gut, or other body microbiomes, Aagaard’s team reports today in Science Translational Medicine. The researchers think the microbes may get to the placenta from the mother’s mouth through her bloodstream, perhaps when she brushes her teeth and dislodges them into the blood. That possibility is intriguing, because there’s a well-known correlation between gum disease and preterm birth. Indeed, the array of bacteria in the placenta differed in women who gave birth early, before 37 weeks.

“This reemphasizes the importance of oral health” during pregnancy, Aagaard says. In fact, women may need to pay attention to their teeth even before they may become pregnant, because the placenta develops early in pregnancy, she says. That may be a challenge for low-income women who can’t afford dental care, Aagaard adds. The team also found a correlation between the composition of the placental microbiome and urinary tract infections, which suggests that such illnesses or antibiotics taken to treat them could alter the microbiome in unhealthy ways.

“This study is the first to suggest that all placentas contain a small amount of bacteria,” says perinatal researcher Roberto Romero of the National Institute of Child Health and Human Development campus in Detroit, Michigan. “These bacteria may live there and have a specific purpose,” such as seeding the fetus’s intestinal microbiome or building its immune system, adds biologist Indira Mysorekar of Washington University in St. Louis, who has reported finding bacteria inside certain placental cells.

However, Romero and others caution that it’s too soon to say exactly how the placental microbiome got there and what it’s doing. The bacteria could have been in the uterus before pregnancy and evolved to resemble those in the mouth, Mysorekar says. Despite these unknowns, says microbiologist Seth Bordenstein of Vanderbilt University in Nashville, the discovery of a placental microbiome “continues to build the snowball that no tissue in the human body is sterile.”