Emerging fluoroquinolone resistance in bacterial keratitis: a 5-year review

Abstract

Objective: To identify resistance patterns to the fluoroquinolones for patients with bacterial keratitis.

Design: Retrospective observational case series.

Participants: All cases of bacterial keratitis presenting to the Charles T. Campbell Ophthalmic Microbiology Laboratory at the Eye and Ear Institute of Pittsburgh from January 1993 to December 1997 were reviewed. A total of 1053 ocular isolates from 825 cases of bacterial keratitis were identified.

Main outcome measures: In vitro laboratory susceptibility testing of ocular isolates to ciprofloxacin and ofloxacin was determined by the Kirby-Bauer disk diffusion method and interpreted using the National Committee for Clinical Laboratory Standards serum standards.

Results: The number of cases of bacterial keratitis per year decreased from 284 in 1993 to 75 in 1997. The ratio of gram-positive to gram-negative organisms changed from 81.8%:18.2% in 1993 to 51.4%:48.6% in 1997 (chi-square, 66.00; degrees of freedom, 4; P < 0.000001). Resistance of Staphylococcus aureus to ciprofloxacin significantly increased annually from 5.8% in 1993 to 35.0% in 1997 (chi-square, 19.80; degrees of freedom, 4; P < 0.0001) and for ofloxacin from 4.7% to 35.0% over the same period (chi-square, 21.32; degrees of freedom, 4; P < 0.001). Streptococcus species and coagulase-negative Staphylococcus species showed significant resistance to both fluoroquinolones but no change in resistance over the study period. The gram-negative organisms showed good susceptibility to the fluoroquinolones.

Conclusions: This in vitro study shows a significant increased resistance of S. aureus to the fluoroquinolones from 1993 to 1997. In addition, gaps in fluoroquinolone coverage for Streptococcus and coagulase-negative Staphylococcus species raise concern for the use of monotherapy in treating bacterial keratitis. Contrary to what might be expected, the distribution of gram-positive to gram-negative organisms has shifted, with a decrease in the number of gram-positive organisms identified, while the number of gram-negative isolates has remained stable.