Enterococcus faecalis is a commensal and pathogen of humans and insects. In Manduca sexta, E. faecalis is an infrequent member of the commensal gut community, but its translocation to the hemocoel results in a commensal-to-pathogen switch. To investigate E. faecalis factors required for commensalism, we identified E. faecalis genes that are upregulated in the gut of M. sexta using recombinase-based in vivo expression technology (RIVET). The RIVET screen produced 113 clones, from which we identified 50 genes that are more highly expressed in the insect gut than in culture. The most frequently recovered gene was locus OG1RF_11582, which encodes a 6-phosphogluconolactonase that we designated pglA. A pglA deletion mutant was impaired in both pathogenesis and gut persistence in M. sexta and produced enhanced biofilms compared with the wild type in an in vitro polystyrene plate assay. Mutation of four other genes identified by RIVET did not affect persistence in caterpillar guts but led to impaired pathogenesis. This is the first identification of genetic determinants for E. faecalis commensal and pathogenic interactions with M. sexta. Bacterial factors identified in this model system may provide insight into colonization or persistence in other host-associated microbial communities and represent potential targets for interventions to prevent E. faecalis infections.