人體消化道的寄生細菌幫助人們更有效地抵抗感染和從食物中攝取能量。此外，腸道細菌還可能影響人的心理健康。

Your digestive tract is home to about a trillion bacteria. And they pay rent. They protect you from infection, they harvest energy for you from what you eat and, if researchers are right, they affect your mental health as well. Because a new study shows that the bacteria that live in your intestines can influence your emotions and behavior. The work appears in the journal Gastroenterology. [Premysl Bercik et al., "The Intestinal Microbiota Affect Central Levels of Brain-Derived Neurotropic Factor and Behavior in Mice"]

Scientists found that treating adult mice with oral antibiotics not only messed with the balance of bugs in their bellies, but it changed their brain chemistry and behavior. The normally timid critters became less cautious and less anxious, perhaps because the gut bacteria influenced a boost in a protein that promotes neural growth. And taking the mice off the drug restored their original intestinal balance and behavior.

To confirm that the bacteria themselves were responsible, the scientists took mice that were raised in germ-free cages, animals that also happened to be naturally passive, and fed them bacteria from mice that were more active and daring. Sure enough, the newly infected mice grew more energetic and bold. So if you react to someone in a hostile way, you might literally be giving them a bellyful.

—Karen Hopkin