Updated project metadata. Here we have shown that diet-mediated alterations of the gut microbiota composition cause an erosion of the colonic mucus barrier. A compensatory increase in cellular mucus production by the host is not sufficient to re-establish the barrier, possibly due to a lacking increase in mucus secretion. While microbial transplant from mice fed a fiber-rich diet can prevent the mucus defects, the mechanism seems to be independent of general fiber fermentation and rather depend on distinct bacterial species and/or their metabolites.