Figure 1.

The structure of the mucus layer is affected by the gut microbiota. Gut bacteria are separated from the host epithelium by the intestinal mucus layer, which is fortified with host defense molecules, such as defensins, Ly6/PLAUR domain containing protein 8 (LYPD8), zymogen granulae protein 16 (ZG16), Regenerating islet-derived proteins 3 (REG3α/γ) and others. The presence of gut microbiota induces expression of the genes encoding mucin 2 (Muc2) and galactoside 2-alpha-L-fucosyltransferase 2 (Fut2), thereby affecting mucus strength and mucin glycan structure. This can in return affect gut-microbiota composition, as specific gut bacteria, such as Bacteroides thetaiotaomicron, can bind and metabolize mucin glycans as an energy source. Degradation of glycans leads to production of microbial metabolites, which not only affect goblet cell differentiation, but also via cross-feeding the abundance of other microbial species, for example Faecalibacterium prausnitzii. Accordingly, the composition of the gut microbiota affects mucus function through the availability of microbial metabolites.