A recent study, "Identification of an immune-responsive mesolimbocortical serotonergic system: potential role in regulation of emotional behavior," suggests that treatment with a specific soil bacterium, Mycobacterium vaccae, may be able to alleviate the symptoms of depression. The bacterium, when injected into mice, activate a set of serotonin-releasing neurons in the brain—the same nerves targeted by Prozac.
According to the paper's authors as reported in Discover Magazine's Raw Data blog, "the results so far suggest that simply inhaling M. vaccae 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," said Christopher Lowry, co-author and a neuroscientist at the University of Bristol in England.
"Graham Rook, an immunologist at University College London and another coauthor of the paper, adds that depression itself may be in part an inflammatory disorder. By triggering the production of immune cells that curb the inflammatory reaction typical of allergies, M. vaccae may ease that inflammation and hence depression."
In addition, the Raw Data blog also points out several other research projects that inadvertently noted the psychological effects of Mycobacterium vaccae on humans. "For example, lung cancer patients who were injected with killed M. vaccae reported better quality of life and less nausea and pain." And, "other reports show that it can improve mood."