The inhibitory effect of vancomycin ointment on the manifestation of MRSA keratitis in rabbits

The purpose of this study was to investigate an experimental model of methicillin-resistant Staphylococcus aureus (MRSA) keratitis, and to evaluate the inhibitory effect of vancomycin ointment on the manifestation of keratitis in this model. For the development of the MRSA keratitis model, 16 rabbits received 0.3-ml intrastromal injections of 1 × 107 colony forming units/ml MRSA inoculated to each of their corneas. After the MRSA inoculation, the rabbits were examined for a period of 14 days. Vehicle only was applied to each left eye for 2 days as an experimental control. Prototype vancomycin ointments, at concentrations of 0.03%, 0.1%, 0.3%, and 1.0%, were applied to each right eye for 2 days. The treatment commenced immediately after the MRSA inoculation and was administered five times a day at 2-h intervals. Four rabbits were used for each vancomycin ointment concentration. Anterior segments were examined daily, and the minimum inhibitory concentration of vancomycin (percentage in the ointment) for treating MRSA keratitis was determined on day 2. On day 14, no recurrences of infection were observed in the eyes that had not exhibited keratitis on day 2. Forty-eight hours post-inoculation, all left eyes demonstrated severe conjunctivitis, stromal infiltrates at injection sites, and fibroid reactions in the anterior chamber. MRSA keratitis was completely inhibited by 0.3% and 1.0% vancomycin ointments. Although the 0.1% ointment generally could prevent keratitis, punctate infiltrates were found in some eyes. Numerous infiltrates were found in eyes treated with 0.03% ointment. In conclusion, the present study substantiates the view that vancomycin ointment has a potent inhibitory effect upon the manifestations of MRSA keratitis, and indicates that a 0.3% concentration is necessary for effective treatment.