Clostridium difficile infection (CDI) is recognized as a major clinical complication in patients with ulcerative colitis (UC). However, the mechanism associated with increased susceptibility to C. difficile in UC patients remains poorly understood. Given the evidence that the gut microbiota plays a critical role in the prevention of CDI and that gut microbiota is perturbed in UC patients, we hypothesized that UC-associated gut dysbiosis contributes to the increased susceptibility of patients to CDI.