*4.5. The Microbiota-Gut-Brain Axis*

The microbiota-gut-brain axis (microbiome) is a dynamic matrix of tissues and organs including the brain, glands, gut, immune cells, and gastrointestinal microbiota that communicate in a complex multidirectional manner to maintain homeostasis. It is, thus, regarded as a modulator of various central processes affecting changes in neuroinflammation, as well as neurotransmission and behavior, including stress adaption and immune response. Therefore, gut microbiome dysbiosis might be detrimental, contributing to the development of a number of CNS disorders such as aberrant anxiety and fear responses, despair and anhedonia via mechanisms not yet unraveled. This triggered largely preclinical animal studies investigating the influence of the microbiome, searching for mechanisms by which the microbiome may affect mental health. Some of the studies demonstrate encouraging results in the treatment of depression (for review see [89]), while studies in clinical cohorts have, mostly, been diagnostic in nature, and warrant further ones with pre- and pro-biotic interventions (for review see [90]).
