Pathways involved in communication between the gut microbiota and the brain. Effective bacterial molecules can be metabolites released in the gut lumen (e.g. fermentation products or neurotransmitter-like molecules, here in green) or cell structural components (e.g. lipo-polysaccharides of the cell wall, here in purple). They can reach the brain via the systemic circulation (1), signal the immune system and trigger the production of cytokines (2), signal the entero-endocrine cells and trigger the production of neuropeptides (3), or activate the vagal and enteric nervous system afferent fibers (4).