Medical News
Placenta experiments may have been corrupted by contaminationSteveAllenPhoto/Getty
By Jessica HamzelouIs the placenta home to communities of bacteria? Recent evidence that the organ has its own microbiome promised to rewrite our understanding of it, but now research suggests that the earlier experiments may have been contaminated.
While the gut is known to host a rich, diverse community of bacteria, as well as viruses and fungi, the fetus and placenta have long been assumed to be sterile. “Over decades of old-fashioned microbiology, no one had been able to isolate bacteria from the placenta,” says Julian Parkhill at the Wellcome Sanger Institute in Cambridge.
That picture started to change seven years ago, when evidence emerged that babies are born with a gut microbiome, which seems to start taking shape before birth. Then in 2014, a team led by Kjersti Aagaard at Baylor College of Medicine in Houston, Texas, found low levels of bacteria in the placenta. The bacteria found in the placenta most closely resembled those found in the adult mouth, and the team reasoned that the bacteria might work its way to the placenta from the mother’s mouth. Other teams have found more evidence for a placental microbiome since.
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Parkhill and his colleagues wanted to find out if the make up of the placenta’s microbiome might influence the outcome of a woman’s pregnancy. But as his team’s research progressed, they came to a different conclusion: that the placenta doesn’t seem to have a microbiome after all.
Parkhill’s team performed tests on samples of placenta taken from 578 women. In one experiment, the team spiked some of their placenta samples with a very small amount of DNA from bacteria that would not normally be found in people because it does not infect warm-blooded animals. When they looked for bacterial DNA, they did find other types, but because these were at levels even lower than those of the bacteria they introduced, they are not likely to represent an active community of bacteria, says Parkhill.
And the bacteria they found is likely to come from the experiments themselves, Parkhill adds. He found traces of E. Coli in the placenta samples, for example. But when he compared across the samples, the same strain of E. Coli was present in all of them. “That’s impossible from a cohort of hundreds of pregnant women,” he says. The bacteria must have come from the experiment kit, he says.
Contamination controversy
Parkhill’s team also found DNA from bacteria typically found in the vagina. Aagaard argues that these bacteria can also make its home in the placenta. But Parkhill’s team found more of these bacteria in placentas delivered vaginally compared to those delivered by Caesarean section, and reason that the placenta is contaminated during birth.
“We are not the first to say this, but all the signals [of a microbiome] that you find in the placenta are due to contamination,” says Parkhill.
Frederic Bushman at the University of Pennsylvania in Philadelphia agrees with Parkhill. Bushman’s own research supports the idea that evidence of a placenta microbiome is the result of contamination. “The methods [used to identify bacterial DNA in the placenta] are so sensitive that even a handful of molecules can be amplified… and the world is full of bacterial DNA.,” he says. “It really looks like it’s sterile in there.”
But what Bushman and Parkhill see as contamination, Aagaard views as evidence of bacteria that make a home in the placenta, even if the total level of bacteria is much lower than that found in other body organs. She points to many other studies that support her theory.
“Reasonable minds looking at their data through a different lens might actually conclude that they have defined a placental microbiome,” says Aagaard. While the microbiome of the placenta might not be as rich and diverse as that of the gut, it still might have an important role to play in birth outcomes and a baby’s metabolic and brain development, she says.
The new research findings “won’t put the issue to rest”, says Bushman. There, Aagaard agrees with him. “Believe you me I am as exhausted by the seemingly endless back and forth as anyone is,” she says.
Journal reference: Nature, DOI: 10.1038/s41586-019-1451-5
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