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Each of us harbors a unique collection of bacteria, on our outsides and our insides. Now, scientists are finding that the bacteria you get at birth may depend on how you got here. Because babies born vaginally have a different set of microbes than those that arrive by Caesarean-section. The work appears in the Proceedings¹ of the National Academy of Sciences.

The uterus is a sterile² environment. So, in the womb, babies don’t have any bacteria to call their own. It’s only once they enter the world that they begin to collect the microbes that will colonize³ their bodies and help shape their immunity⁴. But where do these bugs⁵ come from?

Scientists sampled the bacterial⁶ flora⁷ from nine newborns and their mothers. And they found that babies who passed through the birth canal harbored bacteria that matched their moms’ vaginal microbiota. Whereas babies delivered by C-section had microbes typically found on the skin.

The scientists say the results could help explain why babies that arrive by C-section are more susceptible⁸ to certain infections. Vaginal microbes might be quicker than skin bugs at snagging all the best bodily real estate, a land grab that could keep harmful infections from getting a foot in the door.

Thanks for the minute for Scientific American’s 60-Second Science. I’m Karen Hopkin.