Abstract
Enterococcus faecalis is a Gram-positive opportunistic pathogen that inhabits the gastrointestinal tract. Because of the high frequency of antibiotic resistance among Enterococcus clinical isolates, interest in using phage to treat enterococcal infections and to decolonize high-risk patients for antibiotic-resistant Enterococcus is rising. Bacteria can evolve phage resistance, but there is little published information on these mechanisms in E. faecalis. In this report, we identified genetic determinants of resistance to ϕNPV1 and their effects on E. faecalis susceptibilities to daptomycin and sodium chloride-induced osmotic stress. We found that loss-of-function mutations in epaR confer ϕNPV1 resistance by blocking phage adsorption. We attribute the inability of the phage to adsorb to the loss of an extracellular polymer in strains with inactivated epaR. Phage-resistant epaR mutants exhibited increased daptomycin and osmotic stress susceptibility. Our results demonstrate that spontaneous resistance to ϕNPV1 comes at a cost in E. faecalis OG1RF, resulting in concomitant increased susceptibilities to daptomycin and sodium chloride-induced osmotic stress.