Clarithromycin for experimental Staphylococcus aureus keratitis

Purpose. Clarithromycin, a macrolide antibiotic not previously tested again st the common causes of bacterial keratitis, was analyzed for its effective ness in reducing the number of viable bacteria in a Staphylococcus keratiti s model. An in vivo comparison of the effectiveness of clarithromycin to er ythromycin, minocycline, and tetracycline for three strains of Staphylococc us aureus mras done. Methods. Rabbit eyes were intrastromally injected with 100 colony forming u nits of one of three strains of S. aureus. Two strains were methicillin-sen sitive (ATCC 25923 and MSSA 309) and one strain methicillin-resistant (COL) . Eyes were treated every 30 minutes with 0.3% clarithromycin, erythromycin , tetracycline, or minocycline from 4 to 9 hours postinfection. The number of colony forming units (CFU) per cornea in all eyes was determined at 10 h ours postinfection. Results. Vehicle-treated and untreated eyes (controls) contained over 6 log s of CFU per cornea, a Value significantly higher than any of the antibioti c-treated eyes (P less than or equal to 0.0001). Clarithromycin or erythrom ycin therapy significantly decreased the number of CFU per cornea by approx imately 5 logs in the eyes infected with the methicillin-sensitive strains and by approximately 4 logs in the eyes infected with the methicillin-resis tant strain. Tetracycline and minocycline were also successful in treating these strains, but overall showed less effectiveness than clarithromycin an d erythromycin. Conclusions. Clarithromycin proved to be an effective ocular medication for the therapy of experimental S. aureus keratitis. The effectiveness of clar ithromycin in this model and its known effectiveness for a variety of bacte rial pathogens suggests a role for this drug as a useful ocular antibiotic.