Up to 80 per cent of people with irritable bowel syndrome experience increased anxiety and depression. And those with autism — a syndrome associated with problems interacting with others — are more likely to have abnormal levels of gut bacteria. […]

In a study published last year, they changed the behaviour of mice by giving them fecal transplants of intestinal bacteria.

It involved giving adventurous mice bacteria from timid ones, thereby inducing timid behaviour. Before the transplant, adventurous mice placed in a dark, protected enclosure spent much of their time exploring an attached bright, wide-open area. After the transplant, they rarely ventured beyond their enclosure.

This effect has also been demonstrated in humans to some degree:

Mayer scanned both groups' brains before and after treatment, while showing them photos of people with emotional facial expressions such as anger or fear.

The women who drank probiotics showed a reduced brain response to the faces, compared with the women who weren’t given probiotics.