Gut Bacteria May Influence Brain, Behavior

A team of scientists from Sweden and Singapore found that gut bacteria may influence brain development and adult behavior, at least in mice.

"Not only are signal substances like serotonin and dopamine subject to regulation by bacteria, synapse function also appears to be regulated by colonizing bacteria," continues Prof. Sven Pettersson, coordinator of the study. "However, it is important to note that this new knowledge can be applied only to mice, and that it is too early to say anything about the effect of gut bacteria on the human brain."

For the study, published in PNAS, researchers from Karolinska Institutet, Stockholm, and the Genome Institute of Singapore compared behavior and gene expression in two groups of mice: those raised with normal microorganisms, and those raised in the absence of microorganisms (or germ-free). The adult germ-free mice were observed to be more active and engaged in more 'risky' behavior than mice raised with normal microorganisms. When germ-free mice were exposed to normal microorganisms very early in life, as adults they developed the behavioral characteristics of those exposed to microorganisms from birth. In contrast, colonizing adult germ-free mice with bacteria did not influence their behavior.

Subsequent gene profiling in the brain identified genes and signaling pathways involved in learning, memory and motor control that were affected by the absence of gut bacteria, highlighting the profound changes in the mice that developed in the absence of microorganisms. This suggests that, over the course of evolution, colonization of the gut by microorganisms (in total 1.5 kilograms) in early infancy became integrated into early brain development.

"The data suggests that there is a critical period early in life when gut microorganisms affect the brain and change the behavior in later life," says Dr. Rochellys Diaz Heijtz, first author of the study.