It’s in the dirt! Bacteria in soil may make us happier, smarter

This post was written by Naomi Sachs, a landscape designer and Founding Director of the Therapeutic Landscapes Network, a non-profit organization that provides information, education and advocacy about landscapes that promote health and well-being.

Boy munching on trowel
Photo by Guy Ambrosino

We have so many reasons to get outside and play: Fresh air, Vitamin D, exercise, nurturing a sense of wonder, fascination, and connection with the natural world, and, of course, FUN.

And now we have one more excuse, not just to go outside, but to really get into nature: Dirt can be good for us! Mycobacterium vaccae, a bacterium in soil, has been found to trigger the release of seratonin, which in turn improves mood and possibly even brain function.

Which means that contact with soil, through play and gardening and other means (see photo on right), is beneficial. Furthermore, new research has found that exposure to bacteria in soil may reduce the risk of asthma.

Dirty, Happy Mice

When Mary O’Brien, an oncologist at Royal Marsden Hospital in London, was inoculating cancer patients with a strain of M. vaccae (pronounced “emm vah-kay”), she noticed that in addition to fewer cancer symptoms, patients also showed improved emotional health, vitality, and even cognitive function.

Dr. Chris Lowry, at Bristol University, decided to put O’Brien’s findings to the test, this time with mice. He hypothesized that the body’s immune response to the bacterium causes the brain to produce seratonin. Lack of seratonin is one symptom, or perhaps even cause, of depression. Lowry injected mice with the M. vaccae bacterium and found that the M. vaccae mice – both physiologically and behaviorally – showed lower stress levels.

Dorothy Matthews and Susan Jenks, at the Sage Colleges in Troy, NY tested the findings, again on mice, but this time instead of injecting the M. vaccae, they fed the mice tiny peanut butter sandwiches with a little of the bacterium smeared on. Yummm. Then they ran the mice through a difficult maze. Compared to those that did not eat the bacterium, the M. vaccae mice “navigated the maze twice as fast and exhibited half of the anxiety behaviors.”

Muddy Boys
© Lenard Sanders/

Seratonin is also thought to play a role in learning, so it may have helped the mice not just by making them less anxious, but by helping them to concentrate. Once the bacterium was removed from their diet, the mice continued to perform better than the control group for about three weeks. As the bacterium left their system, the superhero effects tapered off and by the third week, the difference was no longer statistically significant.

Matthews and Jenks’ research is important because it indicates that the bacterium could potentially affect us through normal everyday contact and not just injection. Just how does M. vaccae affect people (as opposed to mice), and how much would be needed to influence us? We don’t know that just yet. Nevertheless, says Matthews,

“Gardeners inhale these bacteria while digging in the soil, but they also encounter M. vaccae in their vegetables or when soil enters a cut in their skin…From our study we can say that it is definitely good to be outdoors–it’s good to have contact with these organisms. It is interesting to speculate that creating learning environments in schools that include time in the outdoors where M. vaccae is present may decrease anxiety and improve the ability to learn new tasks.”

Fewer asthma cases on farms?

Even more recently, a study published in the New England Journal of Medicine reported that children who grow up on traditional farms are 30% – 50% less likely than other children to develop asthma. Researchers correlated the high diversity of bacteria and fungi in household dust – from soil and farm animals – with the low likelihood of asthma.

So, are you making the connection yet between these research findings and what it means for kids to be outdoors? Even parents and teachers who believe in the joys of connecting with nature may hesitate to let their kids fully engage (in other words, get dirty). But perhaps a little extra soap and elbow grease at home is a fair trade for healthier, happier, and smarter kids.

This article was first published on the Therapeutic Landscapes Network Blog. Please visit the blog for full citations and other links.