Digestive diseases, such as inflammatory bowel disease (IBD) and colorectal cancer (CRC), pose increasing problems to health worldwide. These diseases are now thought to emerge from a confluence of host genetics, dietary and environmental triggers and the community of microbes that lives in the human colon. The goal of this project will be to explore the role of a prominent and controversial dietary additive, the algal polysaccharide carrageenan, in triggering IBD and eventually CRC. Carrageenan has been shown to cause both IBD and CRC in animal models. We have identified, for the first time, intestinal bacteria that are present in 20-30% of healthy US adults that are able to degrade carrageenan. Based on these observations, we hypothesize that the presence of carrageenan-degrading bacteria is beneficial in preventing carrageenan-mediated intestinal inflammation. We will test this using a variety of approaches that may ultimately lead to new ways to treat and prevent IBD and CRC.