PROJECT SUMMARY/ABSTRACT Understanding drug metabolism by intestinal microbiota is at the forefront of research to explain why some patients do not adequately respond to therapy. However, there remains a lack of research on infectious diseases and the cellular mechanisms by which specific microbes metabolize antibiotic therapies and its impact on treatment outcomes in patients. To advance this research area, we intend to elucidate how Enterococcus sp. gut bacteria metabolize the nitro-containing drug metronidazole. Enterococcus faecalis and Enterococcus faecium are the main colonizers of the human gut, and also cause opportunistic infections. Metronidazole is an important therapy for Clostridioides difficile infection (CDI), a disease that poses an urgent threat to public health. Furthermore, in CDI patients, colonization of with vancomycin-resistant enterococci (VRE) exacerbates disease severity. E. faecalis, but not E. faecium, degrades metronidazole. Our preliminary data suggests this is due to unique interactions between electrochemical components found in E. faecalis and absent in E. faecium. We will now investigate how this mechanism operates in E. faecalis and the extent to which this bacterium promotes treatment failure for metronidazole. In aim 1 we propose to use microbial genetics and metabolomics to elucidate the mechanism(s) of metronidazole degradation and identify the metabolites formed. Aim 2 will co-culture enterococcal and C. difficile species to identify and measure which species protect C. difficile from being killed by metronidazole. Studies will progress to animals, where we measure the effect of enterococci on metronidazole treatment outcomes and severity of CDI disease. This work will advance knowledge of enterococcal physiology relating to its metabolism and aspects that differentiate E. faecalis from E. faecium. These exploratory studies lay the foundation to understand inter-species interactions between enterococci and C. difficile and how this influences disease development and progression. Public health. E. faecalis, E. faecium and C. difficile are leading pathogens causing diseases in hospitals. This study has significant implications for the diagnosis and treatment of CDI, a disease that imposes a major public health and economic burden in the United States.