Lysostaphin is effective in treating methicillin-resistant Staphylococcus aureus endophthalmitis in the rabbit

Purpose. To determine the effectiveness of lysostaphin treatment of experim ental endophthalmitis caused by methicillin-resistant Staphylococcus aureus (MRSA). Methods. In one experiment, rabbits were injected in the mid-vitreous with 50 or 200 CFU of S. aureus; untreated groups and groups injected intra-vitr eally at 8 or 24 hours postinfection with vehicle or lysostaphin (0.1 mg/ml ) were compared in terms of CFU/ml vitreous at 24 or 48 hours postinfection . Histopathology of untreated and treated eyes was also compared. To quanti fy the potency of lysostaphin, additional rabbits were injected with 50 CFU of S. aureus and untreated eyes and eyes treated at 8 hours with 0.001, 0. 01 or 0.05 mg/ml were compared in terms of CFU/ml vitreous at 24 hours post infection. Results. Vitreous of untreated eyes or vehicle-treated eyes injected with 5 0 or 200 CFU of S. aureus contained 5-10 million CFU/ml at 24 or 48 hours p ostinfection. All eyes treated with lysostaphin at 8 hours postinfection ha d less than 1 log CFU/ml in the vitreous (P less than or equal to 0.0001). Similarly, eyes treated with lysostaphin at 24 hours postinfection had appr oximately 1 log of CFU/ml at 48 hours postinfection. None of the untreated eyes were sterile and 88% or 50% of the eyes treated at 8 or 24 hours posti nfection, respectively, were sterile. Eyes treated with lysostaphin at 8, b ut not 24, hours postinfection had less pronounced pathologic changes than the untreated eyes (P = 0.002). A significant reduction in the CFU/ml vitre ous at 24 hours postinfection was obtained by treating infected eyes at 8 h ours postinfection with lysostaphin at concentrations of greater than or eq ual to0.001 mg/ml (P less than or equal to5 0.0034). Conclusions. Lysostaphin is effective in treating experimental endophthalmi tis mediated by MRSA.