FORGET the spring-cleaning. A study has found evidence that bacteria common in soil and dirt could improve people’s spirits.

According to the research, the action of Mycobacterium vaccae (M vaccae) on the brain is similar to that of some commonly used antidepressants.

The bacterium, which is related to the microbe that causes tuberculosis, appears to work by stimulating the body’s immune system. This, in turn, prompts certain cells in the brain to produce more serotonin, a hormone associated with feelings of wellbeing.

“These studies help us to understand how the body communicates with the brain and why a healthy immune system is important for maintaining mental health,” said Dr Chris Lowry, a neuroscientist at Bristol University who carried out the research.

“They also leave us wondering if we shouldn’t all spend more time playing in the dirt.”

The finding follows separate research by other scientists into the impact of bringing children up in “overhygienic” conditions.

They found evidence that exposure to a wide range of common microbes in early life helped to promote healthy development of the immune system.

Without such exposure, the immune system seems more likely to mistake the body’s own cells as invaders and launch attacks on them. This could be one of the mechanisms underlying the surge in conditions such as asthma and eczema.

The research by Lowry and a team of 12 scientists at Bristol and University College London (UCL) takes this “hygiene hypothesis” a step further by linking exposure to the microbes found in dirt with good mental, as well as physical, health.

Interest in the project arose after human cancer patients being treated with M vaccae unexpectedly reported increases in their quality of life.

This could have been caused by the microbe having indirectly activated the brain cells that produce serotonin.

The researchers injected some mice with the bacteria while others were made to inhale it. They then analysed the blood and brains of the infected mice to see what effect the microbes might have had on their immune systems and on serotonin levels.

Details will be published in Neuroscience, an academic journal, this week.

The study is highly unlikely to lead to new therapies for depression in the near future but it does build on the growing body of research showing the importance of the human immune system in regulating even the subtlest aspects of health.

There are a range of studies supporting the hygiene hypothesis and the idea that exposure to microbes is good for long-term health.

In families with several children, the youngest often has the least allergies, most likely because it picks up the elder siblings’ infections so activating the child’s immune system.

Graham Rook, a professor of immunology at UCL who worked with Lowry, has already published research into the link between exposure to microbes and subsequent development of allergies.

Rook and two of his co-researchers are also working with S R Pharma, a company looking into whether M vaccae could become the basis of treatments for conditions such as asthma.

Rook believes that improved cleanliness may be a contributory factor in diseases such as asthma, eczema and hay fever, along with autoimmune diseases such as Type 1 diabetes and inflammatory bowel disorders such as Crohn’s disease. He said: “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.”

The body’s response to such inflammatory diseases is regulated by immune cells which, said Rook, need to encounter harmless bacteria early in life in order to work out how to respond effectively to real threats.

Without these encounters, he said, the regulatory cells can malfunction, leading to health problems.

Mark Pepys, professor of medicine at UCL, said that there was “quite a lot of evidence” to support the hygiene hypothesis but said he would be cautious about extending the theory to mental wellbeing.

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