Extreme changes of the gut microbiota affect brain function and behaviour, as measured in animal models. A question arises as to the potential clinical relevance of these findings, especially when one considers the influence of dietary habits on the gut flora diversity. The issue attracts public awareness and media attention, causing perhaps diffusion of more interpretations than solid data. A growing body of knowledge, however, supports a role for different gut bacteria, or dietary antigens, in affecting the immune system function, causing potential changes in clinically relevant susceptibility to noxious stimuli.