Antibiotic resistant bacteria, such as Vancomycin-resistant Enterococcus (VRE) or carbapenem-resistant Klebsiella pneumoniae (KPC), are an increasing problem in hospitalized patients and commonly cause infections following antibiotic therapy. Infections with these opportunistic pathogens generally begin by colonization of the intestinal epithelium. Our intestinal tract is inhabited by hundreds of commensal bacterial species that suppress intestinal colonization by antibiotic resistant pathogens. Commensal microbes might prevent infection by releasing inhibitory molecules or by competing for nutrients. In addition, we have shown that the microbiota confers protection against intestinal pathogens by inducing the immune response. Disruption of the microbiota after antibiotic administration enhances intestinal colonization by antibiotic resistant bacteria. However, not much is known about how antibiotic-induced changes in the microbiota increase the risk of infection. Using high-throughput 16S rDNA sequencing, We have begun to study the effect of antibiotic treatment on intestinal microbial populations and VRE/KPC colonization. We have shown that ampicillin treatment induces changes on the commensal flora that extend beyond the course of antibiotic administration. These long-lasting changes allowed establishment and persistence of VRE and KPC in the intestine. We hypothesize that some of the specific populations, lost after antibiotic administration, are key in suppressing VRE and KPC colonization. The goal of this proposal is to identify those specific microbial populations that confer protection against VRE or KPC. The mechanisms by which those bacterial species confer protection will also be studied. We will use a combination of microbiology, immunology and high-throughput sequencing techniques to pursue this goal. The results obtained in this proposal may lead to new therapeutic approaches to treat and prevent infections with antibiotic resistant pathogens.