From Science Daily

Treatment of mice with a 'friendly'  soil bacteria altered their behavior in a way similar to effects from antidepressant drugs, reports research published in the latest issue of Neuroscience.

This leaves us wondering if we shouldn't all be spending more time playing in the dirt.

When the team looked closely at the brains of mice, they found that treatment with M. vaccae activated a group of neurons that produce the brain chemical serotonin. The lack of serotonin in the brain is thought to cause depression in people, thus M. vaccae's effects on the behavior of mice may be due to increasing the release of serotonin in parts of the brain that regulate mood.

Go ahead, play in the dirt, your mom won't mind!