bClostridium difficile infection (CDI) is an urgent public health concern causing considerable clinical and economic burdens. CDI can be treated with antibiotics, but recurrence of the disease following successful treatment of the initial episode often occurs. Surotomycin is a rapidly bactericidal cyclic lipopeptide antibiotic that is in clinical trials for CDI treatment and that has demonstrated superiority over vancomycin in preventing CDI relapse. Surotomycin is a structural analogue of the membraneactive antibiotic daptomycin. Previously, we utilized in vitro serial passage experiments to derive C. difficile strains with reduced surotomycin susceptibilities. The parent strains used included ATCC 700057 and clinical isolates from the restriction endonuclease analysis (REA) groups BI and K. Serial passage experiments were also performed with vancomycin-resistant and vancomycin-susceptible Enterococcus faecium and Enterococcus faecalis. The goal of this study is to identify mutations associated with reduced surotomycin susceptibility in C. difficile and enterococci. Illumina sequence data generated for the parent strains and serial passage isolates were compared. We identified nonsynonymous mutations in genes coding for cardiolipin synthase in C. difficile ATCC 700057, enoyl-(acyl carrier protein) reductase II (FabK) and cell division protein FtsH2 in C. difficile REA type BI, and a PadR family transcriptional regulator in C. difficile REA type K. Among the 4 enterococcal strain pairs, 20 mutations were identified, and those mutations overlap those associated with daptomycin resistance. These data give insight into the mechanism of action of surotomycin against C. difficile, possible mechanisms for resistance emergence during clinical use, and the potential impacts of surotomycin therapy on intestinal enterococci. C lostridium difficile is a Gram-positive anaerobic spore-forming bacterium and a causative agent of health care-associated infections (HAIs). C. difficile infection (CDI; alternatively called C. difficile-associated diarrhea) can occur after disruption of the normal microbiota of the gastrointestinal (GI) tract, thereby decreasing colonization resistance and allowing for C. difficile outgrowth (1). Toxin production by C. difficile can lead to severe disease. Over the last two decades, the incidence of CDI has increased dramatically, especially among elderly health care patients (1-3). A recent Centers for Disease Control and Prevention report on antibiotic resistance threats in the United States classifies C. difficile as an urgent public health threat, as it causes an estimated 250,000 infections per year, leading to an estimated 14,000 deaths and at least $1 billion in medical costs (2).CDI can be treated with antibiotics, but recurrence can occur after a successful therapy and is a persistent concern (3). Vancomycin, metronidazole, and the recently FDA-approved drug fidaxomicin are used to treat CDI (1, 3, 4). Surotomycin (CB-183,315) is a cyclic lipopeptide antibiotic and daptomycin analogue that...