Dr Northrup loves it when family wisdom proves true! She remembers what her Dad, a dentist, always said when someone dropped a bit of food on the floor or in the dirt: “Don’t worry about it. A little dirt won’t hurt you. Let the earth pass through you. Then you’ll be immune to everything.” “It turns out he was right!” Dr. Northrup proclaims with pride and delight.

Researchers from Great Britain’s University of Bristol and University College of London have discovered that a bacterium found in soil—common, garden-variety dirt—has an unexpected health benefit. The bacterium, Mycobacterium vaccae, may be effective in fighting depression. This bacterium was already known to help stimulate the immune system and has been used in clinical trials to treat patients with cancer.

“Children who aren’t exposed to enough dirt are at risk for asthma and allergies,” she says. “Now, it looks like some of the bacteria in dirt is even helpful for mood improvement. Fascinating and right up my alley!” As a champion of alternative medicine and a believer in the unity of all life, Dr. Northrup enjoys finding evidence that validates the healing power available in nature. Dad was right—dirt is good for you!  

The discovery that M. vaccae was beneficial for depression came when cancer patients reported an improved quality of life during treatment with the bacterium. Dr. Christopher Lowry, leader of the research team, theorized that exposure to M. vaccae might cause an increase in serotonin, a mood-enhancing neurotransmitter. To investigate further, Dr. Lowry and his team inoculated a group of mice with the friendly bacterium and looked to see what effect it had on their brains. M. vaccae induces the brain to produce serotonin, a neurotransmitter level that is often low in people with depression.

According to Dr. Lowry, we are exposed to friendly bacteria every day and absorb many through the food we eat and the air we breathe. M. vaccae is one of about 90 species of environmental mycobacteria that are abundant in water as well as in dirt in most climates. While it isn’t possible right now to buy a “dirt” pill to take for depression, Dr. Lowry and his team will continue working to discover how this unusual remedy for mood regulation might work to the benefit of humans.

Graham Rook, a professor of immunology who worked with Dr. Lowry, has researched the link between exposure to microbes and the development of allergies. Rook believes that the modern world’s improved cleanliness may contribute to increases in such conditions as asthma, eczema, and hay fever, as well as autoimmune diseases such as type 1 Diabetes and Crohn’s disease (an inflammatory bowel disorder). “We’ve known for a couple of decades now that a whole group of chronic inflammatory disorders are becoming much commoner in the rich developed world,” he says.

Christopher Lowry joked that while doctors are not yet ready to prescribe dirt for improving your mental health, it couldn’t hurt to make mud pies or roll around in the dirt on a down day.  Could it be that part of the great appeal of gardening is our instinctive need to keep ourselves connected to the soil? Remember Dad’s words, “Let the earth pass through you.” A little dirt can’t hurt and might be a great big help.