Identifying Factors that Protect the Host from Clostridium difficile Infection

Abstract

Clostridium difficile is the most prevalent single cause of nosocomial infection in the United States. A major risk factor for Clostridium difficile infection (CDI) is exposure to antibiotics. Antibiotics increase susceptibility to CDI by altering the gut microbial community, enabling increased germination of spores and growth of vegetative cells. Despite being a major risk factor, antibiotics are currently the principal treatment for the disease. The cycle of antibiotic usage associated with CDI is likely why twenty-five percent of patients with CDI fail antibiotic therapy and experience recurrent disease. An alternative treatment, such as limiting colonization with toxigenic C. difficile by prior colonization of susceptible patients with non-toxigenic strains of C. difficile has been utilized with some success in clinical trials. However the mechanisms underlying how prior colonization with C. difficile limits disease are unknown. The work described in this dissertation sought to determine the relative contribution of host and bacterial factors in mediating protection from CDI. Using a murine model of C. difficile, I present evidence in support of a novel paradigm of colonization resistance. In this model depletion of the amino acid glycine by prior colonization with one strain of C. difficile protects from lethal CDI by limiting germination of the incoming strain. Additionally, I show that unlike other gastrointestinal infections, adaptive immunity is not required for clearance of C. difficile, rather clearance is associated the presence of two members of the Lachnospiraceae family in the indigenous untreated microbial community. Finally, I describe the use of human intestinal organoids to study CDI in the context of a complex human epithelium. Together these results underscore the importance of bacterial interactions in providing protection from colonization while attributes of the host and pathogen may alter the pathogenesis of the infection. These results are important because they provide new insights into this important nosocomial infection. Furthermore the role of glycine in providing colonization resistance provides a novel target for the development of next-generation probiotic therapies for CDI.