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The Placenta is Sterile, Scientists Find. So Where Do Babies Get Their Microbiomes?

The placenta has no native microbes, scientists say.

By Roni Dengler
Jul 31, 2019 7:59 PMDec 23, 2019 3:13 AM
Pregnant Woman - Shutterstock
(Credit: Blue Planet Studio/Shutterstock)


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The placenta is a unique organ. The temporary body part forms in the uterus during pregnancy and supplies the developing baby with nutrients and oxygen while also carrying away waste products. The organ is also a common source of pregnancy complications.

Recently scientists have wondered whether microbes might be to blame. Some research has suggested that, like the gut, a host of bacteria colonize the placenta. If so, this collection of microorganisms could be what makes the difference between healthy and complicated pregnancies. But now in new research published Wednesday in the journal Nature scientists show the opposite is true: the placenta is sterile. The discovery helps quell a long-standing debate and indicates placental microbes aren’t to blame for pregnancy complications.

“The notion that the placenta is sterile under most conditions would suggest there’s not much point conducting research on the placental microbiome,” said University of Cambridge molecular biologist Stephen Charnock-Jones, who led the new research. 

Microbe Mystery

The thought that placental microbes might have a hand in pregnancy complications intrigued Charnock-Jones. Bacteria are responsible for many other health problems, after all. And more recently researchers are beginning to better understand the microbiome, the host of microorganisms that live on and in us. The gut microbiome, in particular, is implicated in a number of health conditions from autism to irritable bowel syndrome. Microbes might also be involved when things go wrong in pregnancy.

Some previous research had suggested this was the case. But those findings contradicted a long-held belief that the placenta and amniotic fluid are free of microbes. To set the story straight, Charnock-Jones and colleagues set out to discover whether any microbes colonize the placenta and, if so, what effect they might have on pregnancy complications. 

DNA Detective

The researchers collected placentas from more than 500 first-time pregnancies. More than half of the pregnancies endured complications associated with dysfunctional placentas including pre-eclampsia, pre-term birth and babies that are born small. Then, using two different approaches, the scientists sequenced genetic material from the placentas to look for the presence of bacteria. To check for contamination the researchers also sequenced “blank” samples that did not contain placental cells or any other biological material.

Both of the sequencing methods detected a host of bacteria species. But further analyses revealed most of the samples were contaminated. Bacterial DNA showed up in the blank samples, for example. The only true signal came from Streptococcus agalactiae, seen in about five percent of samples. Also known as group B strep, this type of bacteria is fairly common. About 25 percent of healthy women carry the bacteria as part of the vaginal microbiome. In the study, the researchers only detected the microbe in a fraction of carriers’ placentas.

Although group B strep does not typically harm adults, infection in newborns can lead to pneumonia, meningitis and bloodstream infections. But the team did not find any association between group B strep and pre-eclampsia, pre-term birth or babies born small.

Overall, says Charnock-Jones, his team’s findings clearly show the placenta is sterile, though it can harbor disease-causing bacteria such as group B strep. The results should put an end to the debate over whether the placenta has a microbiome or not, and help guide future work into the health of gestating infants.

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