Copyright © 1997, American Society for Microbiology Activities of Levofloxacin, Ofloxacin, and Ciprofloxacin, Alone and in Combination with Amikacin, against Acinetobacters as Determined by Checkerboard and Time-Kill Studies

A total of 101 Acinetobacter genospecies (77 Acinetobacter baumannii strains and 24 non-A. baumannii strains) were tested for their susceptibilities to levofloxacin, ofloxacin, and ciprofloxacin and for synergy between the quinolones and amikacin by checkerboard titration and time-kill analyses. The MICs at which 50 % of the isolates are inhibited (MIC50)/MIC90s for the 101 strains were as follows (in micrograms per milliliter): levofloxacin, 0.25/16.0; ofloxacin, 0.5/32.0; ciprofloxacin, 0.25/>64.0; and amikacin, 1.0/>32.0. At empiric breakpoints of <2.0 mg/ml, 61 % of the strains were susceptible to all three quinolones. At a breakpoint of <16.0 mg/ml, 84 % of the strains were susceptible to amikacin. Checkerboard titrations yielded synergistic fractional inhibitory concentration (FIC) indices (<0.5) for one strain with levofloxacin and amikacin and for two strains with ofloxacin and amikacin. Indices of>0.5 to 1.0 were seen for 57, 54, and 55 strains with levofloxacin plus amikacin, ofloxacin plus amikacin, and ciprofloxacin plus amikacin, respectively, and indices of>1.0 in 43, 45, and 46 strains, respectively, were found with the above three combinations. No strains yielded antagonistic FIC indices (>4.0). Most FIC results of>1.0 occurred in strains for which the quinolone MICs were>2.0 mg/ml and for which the amikacin MICs were>32.0 mg/ml. By contrast, synergy (defined as>2 log10 decrease compared to the more active compound alone by time-kill analysis) was found in all seven strains tested for which the quinolone MICs were <2.0 mg/ml. For eight other strains for which the quinolone MICs were>2.