For centuries people have been finding solace and joy futzing around in their gardens. Now, in the 21st Century, science has figured out why.
Researchers have long theorized that the sharp rise in autoimmune conditions, like asthma and allergies, could stem from living too clean (the “hygiene hypothesis”). The idea is that routine exposure to harmless microorganisms in the environment, like soil bacteria, strengthens our immune system and trains it to ignore benign molecules like pollen or hairs on a neighbor’s cat. A series of studies reported in Neuroscience took this hypothesis a step further by treating depression with a specific soil bacterium, Mycobacterium vaccae. They found, for example, that lung cancer patients injected with killed M. vaccae reported better quality of life and less nausea and pain. The injections also eased skin allergies in other patients. Additional studies have shown promise with M. vaccae in improving cognitive function, Crohn’s disease and even rheumatoid arthritis.
It appears that the bacteria activates a set of serotonin-releasing neurons in the brain – the same nerves targeted by Prozac – with no side effects. Lack of serotonin has been linked to depression, anxiety, obsessive compulsive disorder and bipolar problems. According to Christopher Lowry, a neuroscientist at the University of Bristol, England, “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.”
When researchers looked at mouse brains to see which neurons, if any, were activated after the bacterial injections, they discovered that serotonin-producing neurons in the area of the brain that regulates mood, were more active in treated mice. When another set of mice were subjected to a stress-response test – dropping each mouse in water to see how long it would take the animal to switch from active swimming to passive floating, the M. vaccae-injected mice swam for nearly twice as long as the control group. Past research had shown that antidepressants increase active swimming and decrease immobility. “The bacteria,” Lowry explains, “had the exact same effect as antidepressant drugs.”
But it’s not necessary to inject M. vaccae to get these mood enhancing effects. Studies suggest that simply inhaling M. vaccae – just walking in the wild or rooting around in the garden – can elicit a happy state of mind. Lowry notes, “You can also ingest M. vaccae through water sources or through eating plants – lettuce that you pick from the garden, or carrots.”
It’s nice that science has reaffirmed the feelings of well-being we experience in the great outdoors, but let’s not depend too much on those guys in labs with reams of data to tell us whether or not we’re delusional. Mankind has survived for thousands of years by careful observation of Nature and the trial and error practice of living. You don’t have to wait for the studies; listen to your gut instincts and learn.
“Identification of an Immune-Responsive Mesolimbocortical Serotonergic System: Potential Role in Regulation of Emotional Behavior,” by Christopher Lowry et al, published in Neuroscience
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