Baby’s first bacteria arrive sooner than we thought

For more than a century, scientists have assumed that babies come into the world germ-free after nine months in a sterile womb. Not so, new studies find: Bacteria lurk in the placenta, amniotic fluid and umbilical cord blood, making the womb a germy place.


We are not alone. Bacteria in and on our bodies outnumber our own cells 10 to 1. As the hunt for bacteria intensifies, researchers are starting to turn up microbes in locales that were once thought to be sacred ground: The brain is teeming with bacteria. Urine is most definitely not sterile, our resident pee expert Erika Engelhaupt reports. And now, a new study, described by our inimitable Tina Hesman Saey, reveals that the placenta, a pancake-shaped organ that nourishes a developing fetus, is awash with microbes.

The results shatter the idea that the womb, and the fetus nestled inside, is sterile. The old thinking, proposed by French pediatrician Henry Tissier over a century ago, was that the fetus, nestled in the inner sanctum of the uterus, grows germ-free. Born sterile, babies pick up bacteria only as they pass through the birth canal, are held by adoring adults and are breastfed.

Not so, the new study and others say. Bacteria breaks into the fetus long before the big squeeze into life on the outside. Scientists have spotted bacteria in amniotic fluid, blood in the umbilical cord, the membrane that surrounds the fetus and even babies’ first poop. All of those tissues should be sterile if everything else in there is. Babies in these studies were all healthy, suggesting that these bugs aren’t harmful. Instead, they’re just a part of normal human development.

Bacteria-sharing from mom to youngster isn’t limited to people. In cockroaches, bacteria squeeze into eggs just prior to ovulation. Stinkbugs smear their newly laid eggs with bacteria-rich feces. And mouse pups harbor particular kinds of bacteria present in their mom, even when they’re born by sterile C-section. Those are just a few examples of the wide range of bacterial transfer described in a 2013 PLOS Biology article by Lisa Funkhouser and Seth Bordenstein of Vanderbilt University in Nashville.

“We can no longer ignore the fact that exposure to microbes in the womb is likely and may even be a universal part of human pregnancy, serving as the first inoculation of beneficial microbes before birth,” Funkhouser and Bordenstein write.

Because scientists are just starting to tally up these microbes, it’s not yet clear what role they play in a newborn. A tantalizing hint comes from the new placental bacteria study: Certain mixes of bacteria were linked with a higher risk of premature birth. Of course, correlation isn’t causation. Those bacteria could just be along for the ride, or a side effect of whatever caused the premature birth in the first place. But it’s possible that the microbes aren’t just bystanders. They could be influencing how a baby grows, in ways both good and bad.

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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