Activity of gatifloxacin against Haemophilus influenzae and Moraxella catarrhalis, including susceptibility test development, E-test comparisons, andquality control guidelines for H-influenzae

In vitro antimicrobial activity and susceptibility testing interpretation c riteria and quality control were studied for gatifloxacin, a new 8-methoxy fluoroquinolone, tested against Haemophilus influenzae. Moraxella catarrhal is (600 strains) and H. influenzae (1,400 strains) from the SENTRY Antimicr obial Surveillance Program in North America (Canada and the United States) were also tested against gatifloxacin and 12 other antimicrobial agents. Ga tifloxacin (MIC at which 90% of the isolates are inhibited [MIC90], less th an or equal to 0.03 mu g/ml; 100.0% of strains inhibited at less than or eq ual to 2 mu g/ml) was the most active agent tested against H. influenzae an d was similar to four comparison fluoroquinolones (MICs, less than or equal to 0.03 to 2 mu g/ml) against M. catarrhalis. A subset of 300 recent clini cal isolates of H. influenzae were tested by using media (Haemophilus Test Medium agar and broth) and procedures recommended by the National Committee for Clinical Laboratory Standards (NCCLS) and with the E-test (AB BIODISK, Solna, Sweden). Gatifloxacin (MIC50, 0.008 mu g/ml) was slightly more acti ve than levo-floxacin, and E-test results were generally elevated by 0.5 lo g, dilution step compared to reference MICs. The gatifloxacin 5-mu g disk t est produced zone diameters that were routinely above 30 mm for H. influenz ae strains, corresponding to gatifloxacin MICs of 0.008 or 0.016 mu g/ml. T he gatifloxacin susceptibility breakpoint proposed for nonfastidious specie s (less than or equal to 2 mu g/ml; greater than or equal to 18 mm) was als o suggested for H. influenzae testing. No interpretive errors were observed . Quality control guidelines for H. influenzae ATCC 49247 were determined b y using the NCCLS M23-T3 (1998) study design. The results from the nine-lab oratory protocol suggested the following control ranges: for broth microdil ution tests, 0.004 to 0.03 mu g/ml; for disk diffusion testing, 33 to 41 mm . Gatifloxacin appears to be a potent anti-Haemophilus fluoroquinolone comp ound with in vitro testing interpretive criteria that will produce accurate results (disk diffusion, broth microdilution, and E-test).