Vaginal microbes in mice transfer stress to their pups

Mouse mothers can transmit stress signals to offspring, changing the way the pups’ bodies and brains develop. Some of these stress messages get delivered during birth, scientists suggest July 9 in Nature Neuroscience.

Researchers suspected that vaginal microbes from stressed-out moms could affect male pups in ways that leave them vulnerable to stress later in life (SN: 12/14/2013, p. 13). But earlier studies hadn’t demonstrated whether those microbes, picked up during birth, actually caused some of the changes seen in offspring, or if other aspects of life in utero were to blame.

Tracy Bale of the University of Maryland School of Medicine in Baltimore and colleagues subjected pregnant mice to stressful trials that included smelling the scent of a fox for an hour, listening to unusual sounds overnight and being restrained in a tube for 15 minutes. Other pregnant mice didn’t experience these stressors. Then, researchers delivered pups by cesarean section, so that the pups weren’t exposed to their mothers’ community of vaginal microorganisms, or microbiome. After delivery, researchers dosed the pups with vaginal fluid taken from stressed or unstressed mothers.

For male pups not exposed to stress in the womb, vaginal microbes from a stressed mother changed the amount of certain kinds of gut bacteria. (Just as in earlier studies, female pups didn’t show effects of their mothers’ stress.) When those male pups were older, being restrained led them to release more of the stress hormone corticosteroid than mice dosed with microbiota from unstressed moms. And in the brains of adult mice that had experienced chronic stress, genes involved in metabolism and the development of nerve cells behaved differently depending on whether early microbes came from stressed or unstressed mothers.

But some stress effects didn’t seem to depend on the mothers’ microbiomes, results that suggest those effects came from being stressed in utero. For example, compared with pups that weren’t stressed in utero, pups exposed to stress in the womb had higher levels of certain immune cells, as well as key gut genes behaving differently — both possible signs of inflammation.