Neuro-active compounds produced by probiotics. Towards a microbiota-(gut-) brain axis control ?

The gut microbiota provides the host with multiple functions (e.g. by contributing to food digestion, vitamin supplementation, defense against pathogenic strains) and interact with the host organism through both direct contact (e.g. through surface antigens) and soluble molecules which are produced by the microbial metabolism. The existence of the so called gut-brain axis of bi-directional communication between the gastrointestinal tract and the central nervous system actually also supports a communication pathway between the gut microbiota and the host neural circuits, including the central nervous system. An increasing body of evidences has shown that gut microbiota is able to modulate gut and brain functions, including mood, cognitive functions and behavior. Nonetheless, given the extreme complexity of this communication network, its understanding is just at its beginning. The present chapter will attempt to provide a state-of-the art description of the gut-brain axis structure and functions and the multiple cellular and molecular (i.e. neural, immune and humoral) mechanisms by which the gut microbiota can affect this communication circuit. In this context, special attention will be given to microbial strains with ascertained or potential probiotic activity which produce neuroactive molecules (e.g. glutamate, GABA, serotonin and trace amines, opioid peptides). As the knowledge in this domain is increasing, new unexpected and exciting perspectives for both basic and applied science have emerged. From the basic science viewpoint, the birth of the “microbial endocrinology” represents those theories which consider neurochemicals produced by both multicellular organisms and prokaryotes (e.g. serotonin, GABA, glutamate) as a common shared language enabling interkingdom communication. With regards to application, researches in this area open the possibility to a future use of neuroactive molecule-producing probiotics as therapeutic agents for the treatment of neurogastroenteric and/or psychiatric disorders