Influence of CO(2) incubation on quinolone activity against Streptococcus pneumoniae and Haemophilus influenzae

Quinolone activity can be influenced by the pH change that occurs during CO(2) incubation when testing capnophilic organisms such as Streptococcus pneumoniae and Haemophilus influenzae. This study compares the activity of ciprofloxacin, clinafloxacin, enrofloxacin, fleroxacin, grepafloxacin, levofloxacin, moxifloxacin, norfloxacin, ofloxacin, pefloxacin, sparfloxacin, trovafloxacin, and the glycopeptide vancomycin, under ambient air and supplemental CO(2) incubation conditions. Etest (AB BIODISK, Solna, Sweden) was used to determine the MIC values of 30 S. pneumoniae strains including S. pneumoniae ATCC 49619 and 6030; and 29 H. influenzae strains including H. influenzae ATCC 49247 and 49766, incubated with and without 5% CO(2.) Reference broth microdilution and agar dilution tests were performed under similar conditions. Results determined that MIC values of all quinolones agreed (> or = 95%) within +/- one log(2) dilution for the three test methods tested under identical incubation conditions. Quinolone MICs for S. pneumoniae were minimally influenced by CO(2,) while MIC values for H. influenzae ranged from 0.5 to two log(2) dilutions higher when incubated in the CO(2) environment. The differences in quinolone activity against H. influenzae may be due to a combination of pH effects and enhanced growth in the CO(2) test conditions. Quality control and interpretative criteria, especially for H. influenzae and quinolones published by the National Committee for Clinical Laboratory Standards, may not be fully applicable to results obtained under CO(2) incubation. This may generate potential therapeutic interpretive dilemmas in the susceptibility testing of H. influenzae, where clinical isolates require CO(2) to sustain acceptable growth.