It is increasingly well understood that the countless microbes in our guts help us to digest our food , to absorb and produce essential nutrients , and to prevent harmful organisms from settling in . Less intuitive — perhaps even outlandish — is the idea that those microbes may also affect our mood , our mental health and how we perform on cognitive tests . But there is mounting evidence that they do .

For nearly two decades , neuroscientist John Cryan of University College Cork in Ireland has been uncovering ways in which intestinal microbes affect the brain and behavior of humans and other animals . To his surprise , many of the effects he ’ s seen in rodents appear to be mirrored in our own species . Most remarkably , research by Cryan and others has shown that transplanting microbes from the guts of people with psychiatric disorders

like depression to the guts of rodents can cause comparable symptoms in the animals .

These effects may occur in several ways — through the vagus nerve connecting the gut to the brain , through the influence of gut bacteria on our immune systems , or by microbes synthesizing molecules that our nerve cells use to communicate . Cryan and coauthors summarize the science in a set of

articles including “ Man and the Microbiome : A New Theory of Everything ?,” published in the Annual Review of Clinical Psychology . Cryan told Knowable Magazine that even though it will take much more research to pin down the mechanisms and figure out how to apply the insights , there are some things we can do already .

This conversation has been edited for length and clarity .