Many people find gardening relaxing, and now research has found out why exposure to microorganisms in soil has a stress-busting effect

Thirty years after scientists coined the term ‘hygiene hypothesis’ to suggest that increased exposure to microorganisms could benefit human health, University of Colorado at Boulder researchers have identified an anti-inflammatory fat in a soil-dwelling bacterium that may be responsible for it.

The discovery may partly explain how the bacterium, Mycobacterium vaccae, quells stress-related disorders.

We think there is a special sauce driving the protective effects in this bacterium, and this fat is one of the main ingredients in that special sauce,” says Professor Christopher Lowry, of the Department of Integrative Physiology.

British scientist David Strachan first proposed the controversial “hygiene hypothesis” in 1989, suggesting that in our modern, sterile world, lack of exposure to microorganisms in childhood was leading to impaired immune systems and higher rates of allergies and asthma.

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Researchers have since refined that theory, suggesting that it is not lack of exposure to disease-causing germs at play, but rather to “old friends” – beneficial microbes in the soil and the environment that we have long lived alongside – and that mental health is also impacted.

The wonders of rural life

“The idea is that as humans have moved away from farms and an agricultural or hunter-gatherer existence into cities, we have lost contact with organisms that served to regulate our immune system and suppress inappropriate inflammation,” says Prof Lowry. “That has put us at higher risk for inflammatory disease and stress-related psychiatric disorders.”

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Linking healthy bacteria to good mental health

Prof Lowry has published numerous studies demonstrating a link between exposure to healthy bacteria and mental health.

One study showed that children raised in a rural environment, surrounded by animals and bacteria-laden dust, grow up to have more stress-resilient immune system and may be at lower risk of mental illness than pet-free city dwellers are.

Another study found that when a particular soil-dwelling bacterium, Mycobacterium vaccae, is injected into rodents, it alters the animals’ behaviour in a way similar to that of antidepressants and has long-lasting anti-inflammatory effects on the brain. (Studies suggest exaggerated inflammation boosts the risk of trauma and stressor related disorders, such as posttraumatic stress disorder.)

A 2016 study showed that injections of M. vaccae prior to a stressful event could prevent a “PTSD-like” syndrome in mice, fending off stress-induced colitis and making the animals act less anxious when stressed again later.

In awe of the natural power of soil

In the new study, Lowry and his team identified discovered that inside immune cells, Mycobacterium vaccae acted like a key in a lock, binding to a specific receptor, peroxisome proliferator-activated receptor (PPAR), and inhibiting a host of key pathways which drive inflammation. They also found that when cells were pre-treated with the lipid they were more resistant to inflammation when stimulated.

“This is just one strain of one species of one type of bacterium that is found in the soil but there are millions of other strains in soils,” Lowry says. “We are just beginning to see the tip of the iceberg in terms of identifying the mechanisms through which they have evolved to keep us healthy. It should inspire awe in all of us.”

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Source: The University of Colorado at Boulder via

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