Dirt has a microbiome, and it may double as an antidepressant

May 30, 2017, 05:00 PM

No one would blame you for not wanting your body to be infested with creatures from your garden. But maybe you should rethink your position. Your garden has its own microbiome, and research suggests it’s good for you. Our health depends on the flourishing microbiome in our guts—and on how much of the natural world’s microbiome we let infiltrate. Lately, thanks to modern life, we don’t let in a lot. But in a string of pioneering studies, scientists are beginning to look at what would happen if we literally inject microbes from the soil into our bodies, reintroducing us to the ancient relationship between bacteria and human. So far, the results have been uplifting—to both the scientists and the subjects they study. In 2004, Mary O’Brien, an oncologist at the Royal Marsden Hospital in London, published a paper with unexpected results: She injected lung cancer patients with a common, harmless soil bacteria, Mycobacterium vaccae, to see if it could prolong their life. M. vaccae had some success in earlier trials where it was tested for its abilities to fight drug-resistant pulmonary tuberculosis and boost immune system response. O’Brien thought maybe the bacteria could help her patients’ immune systems beat back the cancer in their lungs. It failed. Only, it succeeded elsewhere: the bacteria injection “significantly improved patient quality of life,” O’Brien wrote in the paper detailing the findings. Her patients were happier, expressed more vitality, and better cognitive functioning—in short, it reduced the emotional toll of advanced cancer.

A few years later, Christopher Lowry, a neuroscientist at the University of Bristol, injected M. vaccae into mice and subjected them to a series of stress tests. The ones inoculated with the bacteria showed far less stressed behavior than their untreated counterparts—in fact, they acted as if they were on antidepressants. In a 2007 paper published in the journal Neuroscience, Lowry and his team wrote that the bacteria activated groups of neurons in the mouse brains responsible for producing serotonin—a neurotransmitter that, when impaired, can cause depression. Even more intriguingly, the neurons that lit up were also known to be related to immune response, suggesting an intimate connection between the immune system and emotional health. The world of biomedical research has already fallen in love with the promising realm of the human gut microbiome. A body of emerging evidence tells us the millions of microbes in our digestive tract influence our immune systems, our smells, our mood, and possibly even our attractiveness to mosquitoes—and to other people. But M. vaccae expands this thinking to the microbiome of the pile of mulch in your backyard. There’s now pretty good evidence to draw at least an outline of a conclusion: Breathing in, playing in, and digging in dirt may be good for your health. Our modern, sterilized life in sealed-off office buildings and homes are likely not. Researchers have already found clear evidence that childhood exposure to outdoor microbes is linked to a more robust immune system; for example, Bavarian farm children who spent time in family animal stables and drank farm milk had drastically lower rates of asthma and allergies throughout their lives than their neighbors who did not. But the rest of us, not raised on farms, may be missing out on that sort of protection. Some counterbalance, like spending time in a garden, might change that.

One reason dirt is so good for us might be M. vaccae, which, after Lowry’s 2007 paper, emerged on the scene as a sort of celebrity bacterium. Papers published since have described feeding mice M. vaccae-laced peanut butter sandwiches, and watching them race through challenging mazes far faster than their counterparts, suggesting the bacteria gave them a significant brain boost, in addition to apparently elevating their mood. That paper also demonstrated that eating the bacteria, instead of injecting it, could still give the mi...