By Cassandra Willyard
Soon after conception, a human embryo begins to assemble a remarkable organ crucial to its survival. The placenta is both a lifeline and a guardian: it shuttles oxygen, nutrients and immune molecules from the mother’s bloodstream to her developing fetus, but it also serves as a barrier against infections. For more than a century, doctors have assumed that this ephemeral structure — like the fetus and the womb itself — is sterile, unless something goes wrong.
Starting around 2011, Indira Mysorekar began questioning this idea. She and her colleagues had sliced and stained samples from nearly 200 placentas collected from women giving birth at a hospital in St Louis, Missouri. When the researchers examined the samples under a microscope, they found bacteria in nearly one-third of them1. “They were actually inside cells there,” says Mysorekar, a microbiologist at Washington University in St Louis.
Bacteria often signal infection, and infections are a common cause of premature birth. But the microbes that Mysorekar observed didn’t seem to be pathogens. She didn’t see any immune cells near them; nor did she see signs of inflammation. And bacteria weren’t present only in the placentas of women who gave birth early; Mysorekar also found them in samples from women who had normal, healthy pregnancies. “That was our first hint that this may be like a normal microbiome,” she says.
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