A patient’s gut may not be the most obvious place to look for the origins of depression. But that was the hunch of George Porter Phillips in the early 20th Century.

As he walked the wards of London’s notorious Bethlem Royal Hospital, Phillips had observed that his patients with melancholia often suffered from severe constipation, along with other signs of a “general clogging of the metabolic processes” – including brittle nails, lustreless hair and a sallow complexion.

The natural assumption might have been that the depression had led to those physiological problems, but Phillips wondered if the arrow of causation instead pointed in the other direction. By targeting the gut, could you ease the melancholia?

To find out, he fed the patients a reduced diet devoid of all meats, except fish. He also offered them a fermented milk drink known as kefir, which contains the lactobacillus bacteria, a “friendly” microbe that was already known to ease digestion.

Amazingly, it worked. Of the 18 patients Phillips tested, 11 were cured completely, with two others showing significant improvement – offering some of the first evidence that our gut bacteria can have a profound influence over our mental wellbeing.

If you like this, you may also enjoy:

BBC Future’s Microbes and Me series has now examined various claims about the power of our gut microbiota to harm or heal – but the notion that they could be responsible for our mental health is perhaps the most difficult to behold. How could these microscopic scavengers, feeding on the debris of our digestion, possibly affect the brain?

As we have seen with the other articles in this series, some of these findings have been overhyped. But more than a century after Phillips’ initial experiment, the fundamental idea of a gut-brain axis is now remarkably solid. “There is no debate, in my mind, that microbes influence mental health,” says Jane Allyson Foster, whose lab at McMaster University in Canada is leading research in this area. And that means we may be able to heal the brain through the belly. “There is the potential both for the development of novel therapeutics and for precision medicine.”

Foster emphasises that an unhealthy gut is just one of many possible causes of mental illness, meaning that only a subset of patients will respond well to the new “psychobiotic” treatments. But for those patients who are suffering from an imbalance in their gut bacteria, the new therapies might bring much-needed relief.