Read more: “Infectious moods: How bugs control your mind”
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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“It 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,” says Charles Akle, chair of in London, UK. “They just looked better.”
“The investigators could tell who had been injected with the bacteria because their whole attitude changed”
How can injecting a bacterium brighten someone’s mood? We don’t 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 “old friends” are thought to prime the immune system in interesting ways. “We think M. vaccae is inducing regulatory cells which will dampen down and terminate unwanted inflammatory responses,” 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. “Bacteria and bacterial products can clearly have an effect on the brain and pathways leading to the brain,” says Bienenstock. “There 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.”
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 “hygiene hypothesis” 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’s 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’s effect.
