Disruption of the intestinal mucus barrier has been observed in a broad range of intestinal diseases, including bacterial and parasitic infections and IBD.4,20–23Giardia was previously found to disrupt the mucus barrier by hyperactivating goblet cell secretion and by degrading mucins with its secreted cysteine proteases.11,12 The present study reveals that, in addition to altering the production and secretion of mucins, Giardia alters mucin biochemistry by disrupting O-glycosylation patterns in a manner dependent in part upon a dysbiotic microbiota. Here, CTSB is linked to inflammatory bowel disease.