Gut microbes may also communicate with the brain in other ways, scientists say, by modulating the immune system or by producing their own versions of neurotransmitters.

"I'm actually seeing new neurochemicals that have not been described before being produced by certain bacteria," says Mark Lyte of the Texas Tech University Health Sciences Center in Abilene, who studies how microbes affect the endocrine system. "These bacteria are, in effect, mind-altering microorganisms."