aureus (MSSA) and MRSA strains, from our collection. Cell viability was reduced by ≥ 90% with both Selleckchem ATM Kinase Inhibitor phages (Figure 4). Similarly, the host range of each phage was the same on a panel of 20 phage-sensitive and phage-resistant clinical isolates (data provided as Additional file 2 A-1210477 Table S1). Figure 4 Bactericidal activity of parent and lysis-deficient phage P954. Bactericidal activity of parent and lysis-deficient

phage P954 (10 MOI equivalent) on eight clinical isolates of MRSA (B910, B954, B9053, B9194, B9195) and MSSA (B911, B9007, B9030). Phage resistant isolate indicated with asterix (*). The error bars represent standard deviation (n = 3, single experiment). In vivo efficacy of endolysin-deficient phage P954 An IP injection of the MRSA isolate B911 (5 × 107 MCC-950 cells/mouse) resulted in the onset of disease in 80% of mice (group 1), indicated by dullness, ruffled fur, and death within 48 hr (Figure 5). However, IP administration of endolysin-deficient phage P954 as two doses (immediately and after 2 hr) post B911 challenge fully protected the mice against lethality (group 2). Similarly, chloramphenicol (dose regimen similar to phage) protected mice against lethality (group 3); however, one

animal died in each of the chloramphenicol treatment groups of unknown causes (groups 3 and 6). Endolysin-deficient phage alone was not toxic or lethal to neutropenic mice, demonstrating its safety (group 5). Endolysin-deficient phage demonstrated significant efficacy against MRSA B911in the tested animal model (P value = 0.0001). Figure 5 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). Discussion Bacteriophage endolysins are peptidoglycan Inositol monophosphatase 1 hydrolases that

function at the end of the phage multiplication cycle, lysing the bacterial cell and releasing new phages to infect other bacteria. Many efforts to develop therapeutic phages have focused on the lytic endpoint of phage infection to destroy the bacterium. However, cell lysis by phage may present the problem of endotoxin release and serious consequences as known in the case of antibiotics [33]. Antibiotic-induced release of Lipotiechoic acids and peptidoglycan (PG) in case of gram positive bacteria has been shown to enhance systemic inflammatory responses [34]. An endolysin-deficient phage does not degrade the bacterial cell wall, thus progeny are not released until the cell disintegrates or is lysed by other means.