PROJECT SUMMARY The Gram-negative outer membrane is an effective permeability barrier against numerous chemical assaults, including antibiotics. The intrinsic barrier capacity of the outer membrane is at least in part due to its asymmetric nature; the inner leaflet consists of phospholipids while the outer leaflet is composed of lipopolysaccharide (LPS). The ability of the outer membrane to serve as a permeability barrier relies on strong lateral interactions between neighboring LPS molecules, which are disrupted by the presence of phospholipids in the outer leaflet. In addition, the biosynthetic pathways responsible for LPS and phospholipid production utilize a shared pool of biosynthetic precursors, making it critical that the flux through both pathways is balanced to allow equitable access to the precursor pool. Several outer membrane homeostasis factors that confine phospholipids to the inner leaflet or balance LPS and phospholipid biosynthesis have been well characterized in enterobacteria such as Escherichia coli, but little is known about outer membrane homeostasis in the human pathogen Pseudomonas aeruginosa. Further, the increasing rates of multidrug resistant P. aeruginosa highlight the need for a deeper understanding of the factors required to maintain the barrier capacity of its outer membrane. The proposed research aims to characterize outer membrane homeostasis in P. aeruginosa. In Aim 1, we will investigate how flux through the LPS biosynthetic pathway is controlled to fit the needs of the cell. Our preliminary data suggest that LpxC, which catalyzes the second and committed step in the LPS biosynthetic pathway, is regulated by a novel mechanism in P. aeruginosa, which will be genetically and biochemically characterized during the fellowship period. In Aim 2, we will identify and characterize novel factors required for outer membrane homeostasis in P. aeruginosa, including those responsible for the maintenance of outer membrane asymmetry as well as those involved in regulating LPS and phospholipid biosynthesis. Overall, this work will reveal fundamental mechanisms important for drug-resistance in P. aeruginosa with the potential to guide future antibiotic development. The proposed research has been designed to build on the applicant's pre-existing skillset while providing ample training opportunities in new techniques. The work will be completed in a collaborative environment fostered by the sponsor's lab and the Harvard Medical School Department of Microbiology. In addition, the applicant will regularly attend scientific conferences and also participate in career development workshops that cover topics such as scientific communication and lab management. Thus, the applicant will receive comprehensive training in the skills needed for a successful career as an independent researcher.

PROJECT NARRATIVE Pseudomonas aeruginosa is an opportunistic pathogen that poses a serious health threat to individuals that are immunocompromised or those with cystic fibrosis. In addition, a rise in antibiotic resistance among P. aeruginosa infections makes them particularly difficult to treat. This study is focused on understanding how the P. aeruginosa outer membrane, which serves as a barrier for toxic molecules like antibiotics, is both built and maintained.