Capsular polysaccharide correlates with immune response to the human gut microbe .

Active inflammatory bowel disease (IBD) often coincides with increases of , a gut microbe found in nearly everyone. It was not known how, or if, this correlation contributed to disease. We investigated clinical isolates of to identify molecular mechanisms that would link to inflammation. Here, we show that only some isolates of produce a capsular polysaccharide that promotes a tolerogenic immune response, whereas isolates lacking functional capsule biosynthetic genes elicit robust proinflammatory responses in vitro. Germ-free mice colonized with an isolate of lacking a capsule show increased measures of gut inflammation compared to those colonized with an encapsulated isolate in vivo. These observations in the context of our earlier identification of an inflammatory cell-wall polysaccharide reveal how some strains of could drive the inflammatory responses that characterize IBD.