Project Summary / Abstract Infections caused by intracellular bacteria pose a substantial threat to global health by subverting the immune system to replicate inside host cells. Legionella pneumophila is an excellent system for studying host-pathogen interactions because its ability to infect diverse protozoan species has enabled its emergence as an opportunistic pathogen of humans. Critical early steps in infection are mediated by the Type IV secretion system, including evasion of lysosomal degradation and recruitment of specific host factors to build a protected pathogen-containing vacuole. However, the mechanisms supporting L. pneumophila survival and replication in this niche remain poorly understood. To address this problem, a genetic screen was performed to identify determinants of bacterial fitness during infection of murine macrophages. Among the significant results was a cluster of seven genes that are required for optimal replication of L. pneumophila in macrophages. The predicted products of these genes are a putative, uncharacterized transport system, two nucleotide-modifying enzymes, and a hypothetical protein. The goal of the proposed study is to closely examine the contribution of this genetic locus to fitness during infection by using genetic complementation analyses, animal infection models, and RNA-sequencing to understand its role in the interaction between L. pneumophila and a host cell. Through the training plans devised for this fellowship, the support of my thesis advisor, Dr. Craig Roy, and the collegial atmosphere of the Yale microbiology department, I will be in an ideal position to advance our knowledge of mechanisms supporting L. pneumophila pathogenesis and prepare for the next step in my scientific career.