PROJECT SUMMARY/ABSTRACT. Pseudomonas aeruginosa is a naturally antibiotic resistant bacterial pathogen that causes severe and deadly acute and chronic infections, particularly in compromised individuals. New therapeutics are needed to treat such infections, as P. aeruginosa is often resistant to commonly used antibiotics. Recently, phage therapy has reemerged as an approach to treat infections caused by P. aeruginosa. The advantage of phages is that they can specifically lyse target bacteria. However, phages replicate and can potentially transfer genetic material including antibiotic resistance or virulence genes from one bacterium to another. Instead of phages, we propose to test the efficacy of the bacteriocins, R-pyocins, as therapeutic agents to treat P. aeruginosa infections. Bacteriocins are antimicrobials produced by a bacterium that are active against the same species. R-pyocins are specifically produced by P. aeruginosa and are related to the contractile tail of P2 bacteriophages, but lack the phage head structure, have no nucleic acids, and thus cannot replicate. R-pyocins can be grouped into three subtypes (R1, R2, and R5) that differ based on their binding to specific sites on the P. aeruginosa lipopolysaccharide structure. Despite the apparent efficacy of R-pyocins against many P. aeruginosa in vitro, there have been relatively few studies validating R-pyocins in mouse models of infection and only a single published study has been reported using a murine model of lung infection. The goal of this new NIH Exploratory/Developmental Research Grant Award (R21 Grant) is to assess P. aeruginosa susceptibility to R- pyocins under in vitro conditions that mimic the lung environment and also in an acute mouse respiratory model of infection. We hypothesize that R-pyocin susceptibility testing in this more infection-relevant growth environment will provide a better indication of true susceptibility and testing additional isolates in vivo will provide the much-needed justification for further consideration of R-pyocins as therapeutics agents.