Purpose: We compared the resistance patterns of methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible Staphylococcus aureus (MSSA) keratitis isolates with the common topically applied ophthalmic antimicrobials.
Methods: We reviewed the antibiotic susceptibility results of 122 MRSA and 276 MSSA keratitis isolates from January 1993 to November 2012. In vitro susceptibility testing of each Staphylococcus aureus (SA) isolate was performed using Kirby–Bauer disk diffusion based on modified serum interpretations for cefoxitin, bacitracin, cefazolin, ciprofloxacin, gatifloxacin, gentamicin, moxifloxacin, ofloxacin, polymyxin B, sulfamethoxazole, tobramycin, and trimethoprim.
Results: MRSA represented 30.7% (122 of 398) of the total SA isolates. All the SA isolates were susceptible to vancomycin, whereas they were less susceptible to the fluoroquinolones than to the nonfluoroquinolones. In comparison with MSSA, MRSA was significantly more resistant to all the antibiotics tested other than polymyxin B (both equally resistant) and vancomycin (both equally susceptible) (P < 0.001). Besides vancomycin, MRSA demonstrated the best susceptibilities to sulfamethoxazole (94.3%), bacitracin (89.3%), trimethoprim (88.5%), and gentamicin (86.1%). Additionally, MRSA was found to be significantly more resistant to the second-generation fluoroquinolones (ciprofloxacin and ofloxacin) than to the fourth-generation fluoroquinolones (moxifloxacin and gatifloxacin). An increase in resistance to the fourth-generation fluoroquinolones was detected for both MRSA and MSSA over the study period.
Conclusions: The in vitro susceptibilities of commonly used topical antibiotics differ for MRSA and MSSA isolates; thus, successful treatment of bacterial keratitis should be supported with laboratory studies. Vancomycin remains the treatment of choice for MRSA keratitis. The empiric use of second-generation fluoroquinolones seems to be contraindicated in the treatment of MRSA keratitis.
Department of Ophthalmology, University of Pittsburgh Medical Center (UPMC), The Charles T. Campbell Ophthalmic Microbiology Laboratory, Ophthalmology and Visual Sciences Research Center, Pittsburgh, PA.
Reprints: Victoria S. Chang, MD, Department of Ophthalmology, Massachusetts Eye and Ear Infirmary, Harvard Medical School, 243 Charles St, Boston, MA 02114 (e-mail: firstname.lastname@example.org).
Supported by The Eye and Ear Foundation of Pittsburgh, Pittsburgh, PA, a National Institutes of Health core grant for vision research P30 EY008098, and, an unrestricted grant from Research to Prevent Blindness, Inc, New York, NY. Statistical analysis was performed in part by the University of Pittsburgh's Clinical and Translational Science Institute with the support of the National Institutes of Health through grant UL1TR000005.
The authors have no conflicts of interest to disclose.
Presented in part at the Association for Research in Vision and Ophthalmology Annual Meeting, May 2014, Orlando, FL.
Received October 05, 2014
Received in revised form February 15, 2015
Accepted February 16, 2015