Although the health benefits of the consumption of dietary fibre have long been implied, the direct effect of fibre polysaccharides on the composition and physiology of the microbiota has remained elusive. Previous studies suggested a link between diet and the status of the colonic mucus layer, which is a complex barrier that is composed of the glycoprotein mucin 2 and provides protection against both commensal and pathogenic microorganisms. Desai et al. assembled a synthetic gut microbiota from human gut bacteria in gnotobiotic mice and showed that a fibre-free diet promotes the enrichment of mucin-degrading bacteria. The fibre-deprived gut microbiota expressed carbohydrate-active enzymes and degraded the colonic mucus barrier, which increased susceptibility to the enteric pathogen Citrobacter rodentium and colitis. Thus, the link between diet, the gut microbiota and intestinal barrier dysfunction could be exploited for dietary therapeutics.