Evaluation of in vitro susceptibility testing criteria for gemifloxacin when tested against Haemophilus influenzae strains with reduced susceptibility to ciprofloxacin and ofloxacin.

Although Hemophilus influenzae resistance to fluoroquinolones is low, it is important to monitor the development of populations with decreased susceptibility to this class of antimicrobials. The activity of gemifloxacin, a new fluoroquinolone, was measured against a collection of susceptible wild-type strains (MIC(90,) 0.008 microg/ml for both beta-lactamase-negative and -positive strains) and strains less susceptible to quinolones (MIC(90,) 1 microg/ml). Using a 5microg disk, the gemifloxacin-susceptible strains had MIC results ranging from </= 0.004 to 0.25 microg/ml (the proposed breakpoint), corresponding to zone diameters of 20 to 39 mm. However, some H. influenzae strains having multiple QRDR mutations were less susceptible to gemifloxacin (MIC range, 0.016-2 microg/ml), although gemifloxacin remained active against 57.1% of strains. In vitro testing, using these selected resistant strains, defined the categorical interpretive criteria for routine clinical laboratory use.