In studies of mice they identified a small group of neurons containing the neurotransmitter serotonin that was activated by the ‘friendly’ bacterium Mycobacterium vaccae, normally found in the soil. Treatment of mice with M. vaccae led to increases in serotonin metabolism in the prefrontal cortex, a region of the brain involved in cognitive function and regulation of mood. The lack of serotonin in the brain is thought to cause depression in people, thus M. vaccae’s effects on the behaviour of mice may be due to increasing the release of serotonin in parts of the brain that regulate mood. Indeed, treatment with M. vaccae led to antidepressant-like behavioural responses.

The authors hypothesise that this small group of serotonin neurons normally increases the ability to cope with stressful life events, but when the system is over-activated, for example by prolonged immune activation or chronic stress, it can desensitise, leading to an inability to effectively cope with everyday stress (a common feature of major depression). The work raises interesting questions relating to how the body communicates with the brain to regulate our behaviour and our emotions. It also help us understand how the body communicates with the brain and why a healthy immune system is important for maintaining mental health.

This research makes us wonder if we shouldn't spend more time playing in the dirt

The interest in M.vaccae arose following its discovery in soil samples from the shores of Lake Kyoga in Uganda when colleagues were looking for a bacterium that could help the human immune system respond to virulent bacteria like M. tuberculosis, which causes tuberculosis. Subsequently, they found that M. vaccae was also an effective vaccine for leprosy and could improve the autoimmune symptoms of leprosy patients free from bacterial infection. Since then, M. vaccae-based products have been evaluated in clinical trials for the treatment of asthma, cancer and tuberculosis.

Interest in the antidepressant qualities of M.vaccae emerged after human cancer patients treated with M. vaccae unexpectedly reported increases in their quality of life. Lowry and his colleagues reasoned that this effect could be mediated by activation of serotonin-containing neurons in the brain. The new research supports this hypothesis but future studies will be designed to determine if M. vaccae or other bacteria have antidepressant properties, through activation of this group of serotonin neurons.

Dr Christopher Lowry / Laboratories for Integrative Neuroscience and Endocrinology