The brain and the gut are engaged in constant communication. Most of us become aware of such communication when changes in gastrointestinal function are signalled to the brain bringing about sensations such as satiety (fullness), nausea or pain or when stressful experiences lead to an “upset stomach”. The mechanisms underlying gut–brain axis communication involve "hard-wired" neural pathways as well as immune and endocrine mechanisms.
What has emerged more recently that the gut microbiota may have an important role to play in modulating gut-brain communication. Perhaps, most remarkably, evidence suggests a hitherto unrealised dimension to the integration of host and microbiome; that the overall bal-ance in composition of the microbiota, together with the influence of pivotal species that induce specific responses, can influence the CNS leading to the modulation of brain function and consequently mood and behavior.