The pathogenesis of ulcerative colitis (UC) involves genetic susceptibility, immune-mediated tissue injury and environmental factors including disturbances of the gut microbiota. Nearly all current approved therapies modify host immunity, rather than directly targeting the microbiota. Fecal microbiota transplantation provides encouraging evidence for the therapeutic potential of gut microbiome modulation. Bacteria in the GI tract are ecologically differentiated by their ability to use specific glycans as growth substrates, making glycans a promising and safe alternative to target the microbiome.