Getting dirty might help lift our spirits, according to a new study which reveals that common soil bacteria could act like antidepressant drugs.
Mycobacterium vaccae, a harmless bacteria normally found in dirt, has been found to stimulate the immune system of mice and boost the production of serotonin, a mood-regulating brain chemical.
The bacterium has already been successfully used in people as a vaccine against tuberculosis. It is also being tested as a treatment for cancer patients and in asthma sufferers, as a way to control the allergic reaction and help 'rebalance' the immune system.
Now, studies on mice led by neuroscientist Christopher Lowry at the University of Bristol in England, suggests that the bacteria may have other applications as a treatment for mood disorders like depression.
"These studies help us understand how the body communicates with the brain and why a healthy immune system is important for maintaining mental health. They also leave us wondering if we shouldn't all spend more time playing in the dirt," said Lowry.
Interest in the unusual antidepressant properties of M.vaccae arose by accident following an experimental treatment for human lung cancer led by cancer researcher Mary O'Brien at the Royal Marsden Hospital in London, England. Under that treatment, patients received heat-killed inoculations of the bacteria.
Following the tests, O'Brien's team observed not only fewer symptoms of cancer, but also improvements in their patients' vitality, emotional health and mental abilities.
Lowry and his colleagues speculated that the bacteria in these earlier experiments might have activated brain cells to release mood-lifting chemicals. To investigate the idea further, they injected heat-killed bacteria into a group of mice and found that they initiated an immune response, which activated serotonin-producing neurons in the brain.
Low levels of serotonin cause depression – an illness which afflicts around 1.3 million Australians. The most commonly prescribed antidepressant medications help treat depression by delaying the re-uptake of serotonin, thus raising levels in the brain.
According to Lowry, the strange effect of the bacteria may work by prompting the body's immune cells to release cytokines, chemicals known to activate sensory nerves that stimulate the brain. The findings are published in the journal Neuroscience.
"We believe that the brain then responds by activating serotonin neurons," he said. "These studies help us understand how the body communicates with the brain and why a healthy immune system is important for maintaining mental health."
They also raise the question of whether exposure to common bacteria from a young age, could make us less vulnerable to disease. "We believe that prolonged exposure to [M.vaccae] from childhood could have a beneficial effect," said Lowry.
Further studies are required to confirm the effect in people and to see if other types of bacteria might have a similar effect, he said.
The study lends further support the 'hygiene hypothesis' whereby exposure to bacteria and pathogens from an early age helps balance the immune system. The idea is that some experience of disease early in life prevents our immune systems from attacking our own bodies - leading to allergies, asthma and other so-called auto-immune diseases.
Gardening benefits in combating depression are not limited exclusively to the bacteria related weaponry. Dr Cosmo Hallstrom, a psychiatrist in Chelsea and member of the Royal College of Psychiatrists, said gardening provides also a distraction therapy, vital in helping deal with depression.
"If I was seeing you in cognitive behavior therapy (CBT) I might say, 'Let's look at three things you enjoy doing,' and let's say you say one of them is gardening, I would then say, 'OK let's do one hour's gardening,' he said. "CBT is a modern form of psychological therapy dealing with the here and now as opposed to your past experiences looking at thinking and behavior and can include all manner of techniques.
"When you get depressed you stop doing things and get isolated which makes you more depressed. The theory is that if you do pleasurable things you will in time get better
"Gardening is a pleasurable activity and it focuses you away from thinking about your health problems.
"Why gardening and not running? Well I think at first it is a bit much doing things that are too physical. It is important to find something you enjoy."
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