The gut microbiota, which contains approximately one kg of bacteria, can be modulated by diet and many other factors. It is not static and can change from day to day, starting at birth.

Early life stress, such as maternal separation, is known to induce long-term changes in the microbiome. Dinan and his colleagues review one study that assessed the potential benefits of a specific probiotic, B infantis, in rats displaying depressive behaviour due to maternal separation.

The probiotic treatment normalised both their behavior and their previously abnormal immune response.

This preclinical study and others like it strongly support the hypothesis that probiotics have the potential to exert behavioral and immunological effects.

Some psychobiotics have been shown to have anti-inflammatory effects. This is important because depression and stress are both associated with inflammation in the body.

"The intestinal microbial balance may alter the regulation of inflammatory responses and in so doing, may be involved in the modulation of mood and behavior," the authors said.

In one human study, healthy volunteers received either a probiotic combination (L helveticus R0052 and B longum) or placebo for 30 days.

Those who received the probiotics reported lower stress levels. In a separate study, volunteers who consumed a yogurt containing probiotics reported improved mood.

"What is clear at this point is that, of the large number of putative probiotics, only a small percentage have an impact on behaviour and may qualify as psychobiotics," said Dinan.

The review article appeared in the journal Biological Psychiatry published by Elsevier.