Antibiotic resistance in microbial keratitis: ten-year experience of corneal scrapes in the United Kingdom

Purpose: To determine the scale of antibiotic resistance in microbial keratitis in East Kent, United Kingdom.

Design: Retrospective, observational case series.

Participants: Corneal scrapes over a 10-year period to December 2008 were identified using the local microbiology database, which provided culture results and antibiotic sensitivity-resistance profiles.

Testing: Isolate sensitivity to chloramphenicol, cefuroxime, gentamicin, and ciprofloxacin was determined by microdilution using the Microscan System (Siemens Diagnostics, Dearfield, IL).

Main outcome measures: Isolates were graded as sensitive, intermediate, or resistant to the tested antibiotics, with minimal inhibitory concentrations interpreted against breakpoints from the Clinical and Laboratory Standards Institute.

Results: There were 476 scrapes from 440 patients (female, 57.6%; mean age, 53.5 years). All samples were cultured. Culture was positive in 163 samples (34.2%), growing 172 organisms. Bacterial keratitis accounted for 162 isolates (94.2%), of which 99 (61.1%) were gram-negative. There was a general increase in the number of gram-negative isolates with time (P=0.003). In vitro testing showed widespread gram-negative resistance to chloramphenicol (74.1%), with reducing sensitivity over the study period (P=0.004). There was 97.3% sensitivity to combination gentamicin and cefuroxime, and 94.4% sensitivity to ciprofloxacin. Ciprofloxacin resistance was found in 8 (17.0%) of 47 gram-positive isolates tested, with no trend toward increasing resistance.

Conclusions: This study has documented the highest levels of gram-negative keratitis in any open retrospective survey to date and highlights a trend of increasing gram-negative infection. We have demonstrated reducing chloramphenicol sensitivity, with high sensitivity to combination gentamicin and cefuroxime, as well as ciprofloxacin. Gram-positive fluoroquinolone resistance was higher than previously reported in the United Kingdom, but showed no evidence of increasing resistance. Second-generation fluoroquinolone monotherapy remains the recommended empirical treatment in microbial keratitis in the United Kingdom, and a change to fourth-generation compounds is not advised. Continued testing is essential to monitor for increasing resistance.

Financial disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article.