Abstract: Clostridioides difficile, the etiology of pseudomembranous colitis, causes substantive morbidity, mortality and close to $5 billion/year in US healthcare costs. Commensals provide primary protection against C. difficile infections though the underlying mechanisms of action remain ill-defined. We have identified individual bacterial species that provide long-term survival against virulent C. difficile strains, and other species that can make the infection worse. Our proposed aims will define specific commensal activities and commensal genes mediating these effects on the pathogen, and test their functions in vivo, in mice carrying mouse vs human complex microbiota, for the purposes of developing defined bacteriotherapeutics and biomarkers to predict successful therapy.