Comparative efficacy of topical ciprofloxacin for treating Mycobacterium fortuitum and Mycobacterium chelonae keratitis in an animal model.

Mycobacterium fortuitum and M. chelonae are the two most common causes of nontuberculous mycobacterial keratitis, and they may be difficult to differentiate at diagnosis. Mycobacterium fortuitum is generally more sensitive to ciprofloxacin in vitro than is M. chelonae. Using a rabbit model, we compared the efficacy of topical ciprofloxacin (3 mg/ml) against M. chelonae keratitis to its efficacy against M. fortuitum keratitis. After four days of therapy, ciprofloxacin significantly reduced the number of both organisms in treated eyes compared to untreated control eyes (both P values < .001). Mean culture ratios (colony-forming units in treated eye divided by colony-forming units in untreated eye for each rabbit) were used to compare efficacy between groups. When all treated animals were considered, no significant difference was found between groups (P = .13). When outlier values were excluded, ciprofloxacin was more effective against M. fortuitum than M. chelonae (P = .01). When treated and untreated eyes were compared after therapy in the M. fortuitum group, ciprofloxacin treatment was associated with a reduction in mean stromal infiltrate area (P = .03) and in the tendency to form satellite lesions (P = .07). A clinical effect was not observed in the M. chelonae group. Although ciprofloxacin is effective against both organisms, it appears to be less effective against M. chelonae than M. fortuitum in vivo.