PROJECT SUMMARY Infection of gut epithelia by attenuated rotavirus (RV) vaccines serves to protect millions of lives annually. It is increasingly appreciated that gut microbiota, which shape the milieu in which rotaviruses encounter the epithelium is a pivotal determinant of rotavirus vaccine efficacy. Indeed, RV vaccines have proven to be much less effective in developing countries which are known to have stark differences in gut microbiota composition. Our published work demonstrates that microbiota can both directly impact RV and, moreover, impact gut epithelia to prevent against its entry and replication. Furthermore, our unpublished work compellingly argues for a central role for gut microbiota composition in RV vaccine inefficacy. Specifically, we find that transplant of microbiota from non-RV vaccine responding children confers a microbiota-dependent non-responder phenotype. In addition, culturing infant fecal samples in bioreactor continuous culture systems yields supernatants that differentially inhibit RV vaccine replication in vitro. Hence, the central goals of this proposal are to identify and isolate microbiota constituents that drive RV vaccine inefficacy in humans and determine mechanisms by which they act. Findings from this work will advance our fundamental knowledge on bacterial-viral interactions in the gut, mechanistically enlighten how microbiota can impact replication and response to live, attenuated oral vaccines and potentially aid in the development of novel strategies to reduce the burden of enteric viral infections.