In vitro activity of three fluoroquinolones against enterococci and Pseudomonas aeruginosa: modified categorization...

The fluoroquinolones ciprofloxacin, norfloxacin, and ofloxacin were examined in vitro against 103 enterococcal isolates and 138 isolates of Pseudomonas aeruginosa, in an attempt to determine which respective proposed interpretive criteria (minimal inhibitory concentration (MIC) versus diameter of agar disk diffusion inhibition zones) correlated best, i.e., resulted in fewer discrepant results. A previously proposed system for grouping discrepant results (very major, major, and minor discrepancies) of Barry and co-workers was modified to comprise 6 categories (very major, major, minor, slight, minimal, and negligible); this expanded system rendered the encountered discrepant in vitro test results more transparent. Overall, the currently employed interpretive criteria for norfloxacin (MICs versus 10-micrograms disks) resulted in fewer discrepancies with the above two groups of bacterial isolates than those proposed for ciprofloxacin and ofloxacin (MICs versus 5-micrograms disks, respectively).