Read more: 鈥淚nfectious moods: How bugs control your mind鈥�
Down in the dumps? A shot of friendly bacteria could give you the boost you need
It was meant to be a new way to fight cancer. The idea was that injecting a certain bacterium into people would stimulate their immune systems to destroy tumours. Unfortunately, the treatment had little effect on the survival of the terminally ill lung cancer patients in the first trial. It did have one unexpected effect, though: those injected with the bacterium experienced a radical improvement in their mood and quality of life.
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鈥淚t was supposed to be a double-blind study, but the investigators could nearly always tell who was on the genuine treatment because their whole attitude, their demeanour, changed,鈥� says Charles Akle, chair of in London, UK. 鈥淭hey just looked better.鈥�
鈥淭he investigators could tell who had been injected with the bacteria because their whole attitude changed鈥�
How can injecting a bacterium brighten someone鈥檚 mood? We don鈥檛 yet know all the details, but animal studies suggest that the immune response triggered by Mycobacterium vaccae causes neurons in the prefrontal cortex to release large amounts of serotonin, boosting mood and well-being (). This might seem odd, given that immune stimulation can also lead to depression (see opposite), but our relationship with M. vaccae goes back a long way. Such 鈥渙ld friends鈥� are thought to prime the immune system in interesting ways. 鈥淲e think M. vaccae is inducing regulatory cells which will dampen down and terminate unwanted inflammatory responses,鈥� says Graham Rook of the Royal Free and University College Medical School in London.
Whatever the mechanism, the discovery adds to the growing evidence that bacteria affect our mind as well as our body. 鈥淏acteria and bacterial products can clearly have an effect on the brain and pathways leading to the brain,鈥� says Bienenstock. 鈥淭here are quite a few papers now suggesting that you can influence behaviour, and that the microbiome has something to do with cortisol production, which are pretty hard-core, basic human reactions to stress and things like that.鈥�
In fact, Rook recently proposed that one reason depression is so prevalent in western countries is because people are no longer routinely exposed to organisms like M. vaccae during early life. The 鈥渉ygiene hypothesis鈥� was originally proposed to explain soaring rates of asthma and allergies, but Rook believes it could also be implicated in psychiatric disorders ().
So could M. vaccae be used to make people happy? It is much harder to get approval to inject live bacteria into people with depression than those with terminal cancer, so Immodulon鈥檚 next trial will be in people with prostate cancer. If there is a strong mood-boosting effect again, the company may focus more on its potential for treating depression. Ultimately, if the precise mechanism can be uncovered, it might be possible to develop drugs that mimic the bacterium鈥檚 effect.
