PROJECT SUMMARY Bacteriophages (phages) are gaining traction as antibacterial therapeutics largely due to several high profile uses as emergency-approved experimental biologics. A perceived problem with phage therapy is that the development of bacterial phage resistance will curtail the use of phages as clinically relevant therapies. This conclusion overlooks a fundamental flaw in the development of phage resistance, that is, bacteria often incur reduced fitness as a result of acquired phage resistance. These fitness tradeoffs include enhanced antibiotic susceptibility, reduced virulence, and the inability to stably colonize their host. The work described in this proposal will capitalize on the emergence of phage resistance as a means to successfully treat recalcitrant opportunistic pathogens that reside in the intestine. We will use the Gram-positive intestinal commensals and opportunistic pathogens Enterococcus faecalis and Enterococcus faecium as model organisms to determine how phage resistance phenotypes influence their fitness in the intestine and whether these outcomes can be leveraged as novel therapeutic approaches. Specifically, we will define the mechanisms that drive phage resistance fitness defects of E. faecalis and E. faecium within the intestine by executing three specific aims: 1) to define the mechanism(s) driving the intestinal colonization deficiency of phage resistant strains harboring cell surface exopolysaccharide mutations; 2) to examine how phage-mediated mutations in the peptidoglycan hydrolase gene sagA result in antibiotic sensitivity; and 3) to determine if phage-antibiotic combinations reduce enterococcal intestinal colonization.

PROJECT NARRATIVE Bacteriophage resistance imposes severe fitness defects on bacteria. Using enterococci which are intestinal commensals and established opportunistic pathogens, we will study phage resistance in these bacteria that leads to heightened antibiotic sensitivity and intestinal colonization defects. With this knowledge we can develop phages as precision therapeutics that target drug resistant enterococci that reside in the intestine.