This from Discovery Magazine – microbes in dirt act as antidepressents and elevate mood.

For the greatest mood elevating effect, get dirty with a partner.

Is Dirt the New Prozac?

Injections of soil bacteria produce serotonin—and happiness—in mice. by Josie Glausiusz

THE STUDY “Identification of an Immune-Responsive Mesolimbocortical Serotonergic System: Potential Role in Regulation of Emotional Behavior,” by Christopher Lowry et al., published online on March 28 in Neuroscience.

THE MOTIVE Some researchers have proposed that the sharp rise in asthma and allergy cases over the past century stems, unexpectedly, from living too clean. The idea is that routine exposure to harmless microorganisms in the environment—soil bacteria, for instance—trains our immune systems to ignore benign molecules like pollen or the dandruff on a neighbor’s dog. Taking this “hygiene hypothesis” in an even more surprising direction, recent studies indicate that treatment with a specific soil bacterium, Mycobacterium vaccae, may be able to alleviate depression. For example, lung cancer patients who were injected with killed M. vaccae reported better quality of life and less nausea and pain. Now a team of neuroscientists and immunologists may have figured out why this works. The bacteria, when injected into mice, activate a set of serotonin-releasing neurons in the brain—the same nerves targeted by Prozac.

THE METHODS Some studies have found that treatment with M. vaccae, the inoffensive soil bacterium, eases skin allergies, and other reports—such as the cancer study—show that it can improve mood. Christopher Lowry, a neuroscientist at the University of Bristol in England, had a hunch about how this process might work. “What we think happens is that the bacteria activate immune cells, which release chemicals called cytokines that then act on receptors on the sensory nerves to increase their activity,” he says.

…THE MEANING The results so far suggest that simply inhaling M. vaccae—you get a dose just by taking a walk in the wild or rooting around in the garden—could help elicit a jolly state of mind. “You can also ingest mycobacteria either through water sources or through eating plants—lettuce that you pick from the garden, or carrots,” Lowry says.