Gut Bacteria Might Shape Social Behavior

A study of zebrafish reveals the mix of microbes in the intestinal tract could influence brain development

A group of zebrafish swim in a container.
A group of zebrafish Sebastian Gollnow / picture alliance via Getty Images

The gut microbiome—the community of trillions of microorganisms that live in the intestinal tract—is vital to human health. Changes in these microbes can play a role in the development of a number of diseases, including cancer, depression, cardiovascular disease and diabetes.

Now, in a study of zebrafish, researchers have shown that gut bacteria might also contribute to social development.

In a paper published this month in the journal PLOS Biology, researchers write that zebrafish, which are generally known to be social, spent less time interacting with others when they grew up without gut bacteria, Quanta Magazine’s Joanna Thompson reports.

“This study adds further evidence for the role of the microbiome, especially in early life, in shaping social behavior,” John Cryan, a neuroscientist at the University College Cork in Ireland who was not involved in the research, tells Medical News Today’s Deep Shukla.

The findings suggest that zebrafish gut bacteria influence their brains—and that a similar relationship might exist in humans. “Zebrafish brain development is very similar to the brain development of other vertebrates, including humans,” Judith Eisen, a neuroscientist at the University of Oregon and co-author of the new study, tells Medical News Today. “So, it seems likely that similar processes also occur during the development of the brains of other vertebrates.”

A single blue zebrafish in front of some green plants.
Zebrafish are transparent when they are young, so researchers can track their internal development. isoft via Getty Images

Previous research has found links between social behavior and gut microbes in other animals: A study earlier this year revealed honeybees with gut bacteria had more social interactions than those without. Other scientists have examined how microbes influence the social behavior of mice.

For the new study, the researchers bred zebrafish embryos without gut bacteria. When they hatched, some were immediately inoculated with a microbiome, while others were inoculated a week later. The fish that received the microbes later displayed less social behavior two weeks after hatching, per Quanta Magazine.

By studying the brains of the zebrafish, the team pinpointed a possible cause of the antisocial behavior. At two weeks of age, the forebrains of fish that grew without gut bacteria had fewer immune cells called microglia, which trim connections between neurons, per Medical News Today. As a result, these fish had more connections between neurons in the forebrain.

Excessive connections between neurons have been linked with neuropsychiatric disorders such as autism, according to Medical News Today.

“These are big, major changes in the nervous system,” Eisen tells Quanta Magazine.

Still, the researchers aren’t exactly sure how the gut and the brain are communicating. And studying how gut bacteria might shape neurodevelopmental disorders in humans “is very challenging to do,” Kara Margolis, a pediatric gastroenterologist at New York University who was not involved in the research, tells Quanta Magazine.

But scientists hope to better understand the importance of gut bacteria in social development. “Learning more about how gut microbes influence brain development will help us understand how to foster a healthy gut microbiome that supports early nervous system development and function,” Eisen tells Medical News Today.