In vivo efficacy of endolysin-deficient phage P954. Survival of mice challenged with clinical MRSA isolate (B911). Groups 1-3 were challenged with MRSA (5 × 107 cells per mouse). Groups 4-6 were not challenged with MRSA and served as controls. The following treatments were administered: groups 1 and 4 (25 mM Tris-HCl, pH 7.5); groups 2 and 5 (two doses of endolysin-deficient phage P954, 200 MOI); groups 3 and 6 (two doses of chloramphenicol, 50 mg/kg).