A new study challenges the idea that the placenta has a microbiome

Contrary to earlier reports, the human placenta is largely free of microbes, a study finds. The new result follows years of debate over whether the organ that nourishes and protects a growing fetus also holds bacteria.   

Dueling evidence has been accumulating both for and against the presence of microbes in placentas. Amid the back-and-forth, molecular biologist Stephen Charnock-Jones of the University of Cambridge and colleagues were busy collecting thousands of placenta samples as part of a different study on maternal and fetal health. But the team became interested in the question of a possible collection of bacteria and other microbes, called a microbiome, in the placenta. “We thought, ‘This is an objective thing we can test,’” Charnock-Jones says.

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So Charnock-Jones’ team examined more than 500 of the placenta samples, which were collected after delivery. These samples were from healthy pregnancies and pregnancies that involved complications and both vaginal and cesarean deliveries. The tissue was washed in solutions of salt and frozen. Then, the researchers used two different methods to search for bacterial DNA.

Overall, there were very few signs of bacteria in placentas, the researchers conclude July 31 in Nature. Instead, many of the bacterial DNA signals that were detected came from laboratory tools, the birth canal and, sometimes, the salt solution in which samples were washed, the team says.

Some researchers suspect that microbes in the placenta may have a role in mothers’ or babies’ health. But the findings of the new study question that premise.

To focus on the most reliable bacterial signals, the team tossed out any bacteria that showed up in only one of the tests. Similarly, they ruled out signals that showed up only when researchers used a particular batch of salt solution or other reagents, or a particular lab tool, such as a DNA-sequencer machine. “It doesn’t matter what month, what reagents, which technician,” Charnock-Jones says. “None of that should matter if it’s a real signal.”

As an extra control, the researchers added in a small number of Salmonella bongori bacteria, which are not found in people. The experiments detected these bacteria, but not much else, with one notable exception. In about 5 percent of the placenta samples, researchers found group B Streptococcus bacteria, which can be dangerous for fetuses and newborns. Those samples weren’t tied to poorer health outcomes in the study, though the overall number of those cases was small.

The study’s findings are “very persuasive,” says Roberto Romero, an obstetrician and gynecologist at the Perinatology Research Branch of the National Institute of Child Health and Human Development, which is housed at Wayne State University in Detroit. “More and more evidence is emerging that, probably, the signals we observe represent contamination with reagents and equipment.” 

Not everyone agrees. The analyses were done in a way that may have excluded some important bacteria, says obstetrician and maternal-fetal medicine specialist Kjersti Aagaard of Baylor College of Medicine and Texas Children’s Hospital in Houston.

For instance, the researchers classified several types of vaginal bacteria as “contamination” in the placenta. But it’s possible that those same types of bacteria existed in the placenta before a trip down the birth canal, says Aagaard, who has reported evidence for the placental microbiome (SN: 6/28/14, p. 6). Bacteria known to be in the vagina have also been found in the uterus, which connects to the placenta. “Why would we disregard [those bacteria] and toss them out, when the biology makes perfect sense?” Aagaard says.

Aagaard is convinced there are small amounts of bacteria in the placenta, but remains unsure about what biological role those microbes play, if any.