Bacteria may transfer mom’s stress to fetus

SAN DIEGO — When stress during pregnancy disrupts a growing baby’s brain, blame bacteria. Microbes take part in an elaborate chain reaction, a new study finds: First, stress changes the populations of bacteria dwelling in a pregnant mouse’s vagina; those changes then affect which bacteria colonize a newborn pup’s gut; and the altered gut bacteria change the newborn’s brain.

The research, presented at the annual Society for Neuroscience meeting, may help explain how a stressful environment early in life can make a person more susceptible to disorders such as autism or schizophrenia. The finding also highlights the important and still mysterious ways that the bacteria living in bodies can influence the brain.

“This is really fascinating and promising work,” said neuroscientist Cory Burghy of the University of Wisconsin–Madison. “I am excited to take a look at how these systems interact in humans,” she said.

Stress during pregnancy dramatically shifts the mix of bacteria that dwell in the vagina, Christopher Howerton of the University of Pennsylvania reported November 11. The alarming odor of foxes, loud noise, physical restraints and other stressful situations during a mouse’s pregnancy changed the composition of its vaginal bacteria, he and his colleagues found.

The population of helpful Lactobacillus bacteria, for instance, decreased after stress. And because newborn mouse pups populate their guts with bacteria dwelling in their mother’s birth canal, microbes from mom colonize the baby’s gut. Mice born to moms with lower levels of Lactobacillus in the vagina had lower levels of Lactobacillus in their guts soon after they were born, the team reported.

Lower levels of Lactobacillus in the newborn mice seem to influence the brain, Howerton reported. Genes in a brain region called the hypothalamus behaved differently in mice with low levels of Lactobacillus in their guts.

Those results make sense, said neuroscientist Jane Foster of McMaster University in Hamilton, Ontario. Previous studies have shown that stress can influence the bacteria in human bodies, she said, and in turn these bacteria also influence how a person’s body and brain respond to stress. “It is exciting that neuroscientists are starting to pay attention to this important area of research,” Foster said.

It’s not clear how the bacteria in newborn mice’s guts influence their brains, but the researchers have some hints: Levels of some key chemicals important for brain development were different in mice born to stressed mothers, the researchers found, an effect that could come from altered nutrient absorption in the gut. Another possibility is that stress changes the mix of bacteria in the vagina by shifting the levels of immune cells, allowing more dangerous bacteria to slip in and ultimately make it into the baby, edging out the friendly Lactobacillus, Howerton said.

The complex chain of events outlined in the new study — from stress to mom’s vaginal bacteria to baby’s gut bacteria to baby’s brain — might help explain how stress early in life, perhaps even during gestation, can make a person more vulnerable to psychiatric disorders later, said study coauthor Tracy Bale of the University of Pennsylvania. “Every neuropsychiatric disorder, without exception, is influenced by stress,” she said. Figuring out how stress becomes dangerous might ultimately help scientists identify who is at risk, and how to prevent the ill effects of stress.

So far, the work has been restricted to mice, but Bale and her colleagues plan to study the effects of stress on bacteria in pregnant women and their newborn babies.