There's been a whole lot of research over the past year highlighting a link between mental health and gut bacteria. And now a small study has strengthened that link, after showing that a group of men who took probiotic capsules daily for a month felt less stressed and performed better on memory tests.

The study only involved a small group, so the results definitely need to be replicated before we all rush off to the shops and buy probiotics, but they're a promising sign that we might one day be able to help manage our mental health through our guts.

Our colonies of gut bacteria - or microbiome - make up a unique ecosystem of thousands of bacteria that live inside our intestines and help us to break down our food and regulate our immune system.

In the new research, 22 men were given a capsule containing more than a billion Bifidobacterium longum 1714 bacteria for a month, before unknowingly being switched to a placebo for the second month.

After that first month, the researchers found that not only did the participants report less stress than both before the trial and during the second month, but they also had lower levels of the stress hormone cortisol in their blood stream.

When they were given memory tests that required them to pair objects together, such as a colour and a number, the participants also performed better after taking a month of probiotics, as opposed to a month of placebos.

"When they were given these bacteria they were less anxious and their capacity to memorise material seemed to be enhanced," lead researcher Ted Dinan from the University College Cork in Ireland, told The Guardian"The effect was large enough for them to perceive less stress."

The results of the trial were presented at the Society for Neuroscience meeting in Chicago over the weekend, where Dinan admitted that the study was too small to show a conclusive link between the probiotics and anxiety and memory.

But the findings mirror the results of Dinan's previous research on mice, which found that probiotics were able to improve their memories and had an antidepressant effect.

"This study is one of the few examples where someone has taken a probiotic from preclinical studies and put it into humans and found pretty much the same thing as in rodents," he said.

The challenge now will be not only to replicate this same effect in more humans, but to explain exactly how probiotics could have an impact on anxiety levels and memory performance. 

Dinan suggests that it's possible the B. longum bacteria are releasing substances that activate the vagus nerve, which connects the brain and the gut, but further research is needed to show what's really going on.

Still, this isn't the first time that boosting gut bacteria has been shown to help with anxiety - back in June, researchers found that students who ate foods rich in probiotics, such as yoghurt, pickles, and sauerkraut, reported feeling less social anxiety and neuroticism.

If the link can be verified between the two, it'll be a pretty exciting development for mental health, particularly given the fact that probiotics have so few side effects.

"By their very nature, probiotics are safe bacteria, and if we can delineate those that have benefits for mental health, that would be a major advance for neuroscience and psychiatry," said Dinan. "My hope is that within the next five years we'll have a probiotic or two on the market that will be effective for treating mild forms of anxiety and depression."