Comparative Minimal Inhibitory and Mutant Prevention Drug Concentrations of Four Fluoroquinolones Against Ocular Isolates of Haemophilus influenzae

Abstract

Haemophilus influenzae is a known ocular pathogen, particularly in the pediatric population. The purpose of this study was to determine the minimal inhibitory concentration (MIC) and mutant prevention concentration (MPC) values of third-generation (i.e., ciprofloxacin and ofloxacin) and fourth-generation (i.e., gatifloxacin and moxifloxacin) fluoroquinolone antibiotics against clinical ocular isolates of H. influenzae. Twenty-six clinical isolates of H. influenzae were evaluated. For MIC testing, approximately 10(5) colony-forming units/mL of test organism was added to an H. influenzae test medium containing twofold dilution increments of each fluoroquinolone. Cultures were incubated for 18 to 24 hours at 35 degrees C to 37 degrees C in 5% CO2. The lowest drug concentration that prevented growth was recorded as the MIC. The lowest drug concentration that inhibited 90% of strains tested was recorded as the MIC90. To determine MPC values, more than 10(10) bacteria were inoculated to agar plates containing varying drug concentrations and incubated for 24 to 48 hours at 35 degrees C to 37 degrees C in 5% CO2. To confirm MPC values, bacteria isolated from drug-containing plates were subcultured to drug-free agar, incubated, and then subcultured to a plate with the same drug concentration. The lowest drug concentration that allowed no growth was recorded as the MPC. All tested fluoroquinolones were highly active against H. influenzae. The MIC90 values were 0.016 microg/mL for gatifloxacin and ciprofloxacin and 0.031 microg/mL for moxifloxacin and ofloxacin. The MPC90 values were 0.125 microg/mL for gatifloxacin and 0.5 microg/mL for moxifloxacin, ofloxacin, and ciprofloxacin. All evaluated fluoroquinolones were effective against H. influenzae. The MIC90 and MPC90 values for gatifloxacin were consistently lower than those for moxifloxacin.