Medical researchers in the United Kingdom have found evidence that “friendly” bacteria found in soil may activate the immune system, produce the brain compound serotonin, and help ward off depression.

According to a study published last week in Neuroscience, researchers from Bristol University and University College London found that mice treated with the soil agent Mycobacterium vaccae behaved much like mice treated with anti-depressants. Further research showed that a specific part of the mice brain that produces serotonin — the dorsal raphe nucleus, or DRI — had been energized, and was producing serotonin, which helps govern mood.

Popular anti-depressants such as Prozac and Zoloft work by inhibiting the re-uptake of serotonin in the brain: this bacterium appears to work by indirectly increasing the actual amount produced in the brain.

Medical News Today reported last week that Dr. Chris Lowry of Bristol University became interested in the subject after he heard that cancer patients treated with the soil bacterium reported a better quality of life. Lowry speculated that it might be because the bacteria was activating their “serotonergic system,” as the study appears to confirm.

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“These studies leave us wondering if we shouldn’t all be spending more time playing in the dirt,” Lowry said.

For environmentalists, this is a reminder that dirt itself has gotten something of a bum rap in the last hundred years. Many immunologists now suspect that a little dirt at a young age may help “educate” the immune system and reduce the chance of childhood asthma.

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Another earthy compound considered “dirty” in conventional morality is semen, which — interestingly — also has been shown to have an anti-depressant effect. (The study is controversial, but has not been disproven.)

They say cleaniness is next to Godliness; they don’t mention that it’s also a little depressing.