Mechanisms of action of the intestinal microbiome on the gastrointestinal barrier. Commensal bacteria support the digestion of fibres and other nutrients, thereby contributing to energy and substrate supply. They regulate epithelial functions such as mucus production in goblet cells, defensin release from Paneth cells and tight junction protein synthesis in normal epithelial cells. They prevent colonisation of pathogens in the gut and regulate the mucosal immune system, for example, by inducing and maintaining gut-associated lymphoid tissue and by stimulating mucosal immunoglobulin A production. For details and references, see text 'Underlying mechanisms'.