The existence of bidirectional communication between the gut and the brain, known as the gut-brain axis, has long been recognized. On the one hand, the brain modulates the gastrointestinal tract by regulating motility, secretion, absorption, and blood flow. On the other hand, the gut can affect the brain functions through neurological, immunological, and endocrine pathways. Microorganisms present in the gut are responsible for the production of different immunological and neurological signaling molecules, such as short-chain fatty acids (SCFAs) and neurotransmitters.
In recent years, the primary role of the gut microbiota in this interaction has become increasingly evident. Microbial metabolism byproducts are able to reach the brain through the central, enteric, and autonomic nervous system as well as the hypothalamic-pituitary-adrenal axis, affecting not only brain physiology but also its adaptation to psychological conditions like anxiety and stress. For this reason, the gut microbiome has recently been recognized as a new hallmark linked to mental health, with the latest studies focusing their attention on the cause-effect relationship between gut microbiota dysbiosis and acute and/or chronic stress, as well as on cognitive dysfunctions. Finally, recent advances in metagenomics sequencing have revealed that dysregulation in the composition of gut microbiota (dysbiosis) is present in a variety of neurological diseases.