A messy birth could be good for the baby's digestion. So say researchers in Germany, who have found evidence that baby mice squeezing through the birth canal swallow bacterial molecules that help their gut grow healthily. The finding suggests that kids born by caesarean might miss out.

Swarms of friendly bacteria normally live in our guts, and cells lining the intestinal tubes do not attack them. Mathias Hornef at the University Clinic of Freiburg, Germany, and his colleagues, have found that, in mice at least, these intestinal cells 'learn' not to harm the bugs sometime around birth.

The team extracted intestinal cells from mice embryos before birth and exposed them to a component of bacteria. The embryonic cells reacted and produced inflammatory molecules. But the same gut cells from one-day-old newborn mice or adult mice did not. Somehow, the cells in the more developed mice had learned to ignore the bacterial trigger.

The researchers think that bacterial scraps naturally slopping around in the birth canal and mother's faeces are swallowed by the baby mice as they make their entry into the world. These molecules pass down into the gut, where they stimulate the gut cells; a single exposure is enough to teach the cells to tolerate friendly bugs in the future.

Gut reaction

To show this, Hornef's team looked at the responses of gut cells of baby mice born both naturally and by caesarean. Those born through the vagina fired up an inflammatory response in the two hours after birth, a sign that their cells had been stimulated by bacterial molecules. In contrast, babies born by caesarean did not show signs of such activation. But feeding these babies fragments of bacteria after their birth did fire up this response.

This first exposure could teach a newborn infant's gut cells to ignore the harmless bacteria that begin to colonize the intestine in the days and weeks after birth. Hornef's study, reported in the Journal of Experimental Medicine, suggests that the immune systems of babies born naturally have a head start.

In theory, this could mean that the intestines of babies born by caesarean are less welcoming to gut bacteria - perhaps with long-lasting effects for the babies' health. "It's a very interesting speculation," Hornef says.

One study in 2004, for example, showed that human babies born by caesarean seem to be more prone to diarrhoea during their first year of life than babies born naturally. The fact that caesarean babies aren't exposed to bacteria in the birth canal was proposed as a possible cause for this difference, but the idea wasn't tested.

It is possible that children born by caesarean encounter bacteria or other triggers that similarly activate their immune system very soon after birth, perhaps through breastfeeding. But the finding adds to an already vigorous debate about whether caesareans carry greater risks for a mother and baby than vaginal birth.

The results may also have implications for adults with intestinal problems, notes Bruce Vallance from the University of British Columbia in Vancouver, Canada. It will be interesting to examine whether people with inflammatory bowel disease lose the ability to ignore friendly bacteria later in life, he says. "Maybe their intestinal cells go back to this neonatal state."

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