New research is showing how a daily dose of probiotics may help people cope with mild anxiety and memory problems. At University College, Cork, a study of healthy men who took a probiotic for a month reported less stress and anxiety, and had lower levels of the stress hormone cortisol in the morning, than when they took a placebo instead. They also fared better on memory tests.

For many it is a surprising and staggering truth that we are more bacteria than body – the gut alone holds over 100 trillion bacteria of myriad species, many of which help with breaking down food and play a vital role in immunity. Most of our gut microbiota initially came from our mother’s birth canal as we came into the world, or from skin and the surrounding environment if we were born by caesarean. Once we are out in the open, multiple factors such as diet, antibiotics, genetics and stress will influence the microbiome.

The upshot is a multitude of bugs that weigh about the same as a human brain. And perhaps that makes sense, for while it has long been known that the brain can influence the gut, modern science is showing that communication can go both ways. Recent studies reveal that our gut microbiota could be involved in conditions such as obesity, social behaviour, depression and anxiety.

Scientists in Japan have been using laboratory mice for the last ten years to monitor the effects on bacteria-free animals raised in a sterile environment, and normal animals with microbes. The germ-free mice had greater amounts of stress-related hormones, and they showed differences in a protein that affects the survival, growth and connection of brain cells. In one of the most astonishing studies, a team of researchers transferred gut microbes from an anxious strain of mouse into a germ-free mouse of a more adventurous strain – and vice versa. The result? A behaviour transplant.

As yet there are few studies with humans but more are in the pipeline. Just how big an influence microbes have over our mood has yet to be determined but Professor John Cryan from the University of Cork thinks we might be surprised. ‘It’s worth considering they are the master puppeteers’, he says.