Fig. 1.

Bidirectional lines of communication between the gut microbiota and the brain. Neural, endocrine, and immune pathways mediate signals between the central nervous system (CNS) and the intestinal microbiota. The microbiota can produce a variety of neuro- and immunomodulatory substances that can either act locally on immune populations and enteroendocrine cells in the gut, or modulate distant functions in the CNS. This can alter mood, behavior, and neuroinflammatory responses. Stress signals from the brain are conducted via the efferent nerves, as well as the hypothalamic–pituitary–adrenal (HPA) axis, which can alter gastrointestinal function and microbiota composition. PYY = peptide YY; 5-HT, 5-hydroxytryptamine; SCFA = short-chain fatty acid; LPS = lipopolysaccharide