Bactericidal and bacteriostatic activity of gemifloxacin against Acinetobacter spp. in vitro

This study compared the in vitro bacteriostatic activity of gemifloxacin (S B-265805) and a panel of test antimicrobial agents against 100 clinical iso lates of Acinetobacter spp. (47 Acinetobacter baumannii, 18 Acinetobacter a nitratus, 18 Acinetobacter Iwoffii, 13 Acinetobacter calcoaceticus and four other Acinetobacter spp.). Gemifloxacin (MIC50/90 0.06/16 mg/L) was more t han eight-fold more potent than ciprofloxacin (0.5/>128 mg/L), two- to eigh t-fold more potent than grepafloxacin, moxifloxacin, levofloxacin, ofloxaci n and gatifloxacin, and of similar potency to trovafloxacin and sparfloxaci n. Cross-resistance was seen only within the quinolone group and did not ex tend to non-quinolone antimicrobials. The bactericidal activities of gemifl oxacin and the six comparator quinolones were investigated by dose-response and time-kill studies against A. baumannii ATCC 19606 at their optimum bac tericidal concentration (OBC) and at 4 x MIC. At the OBC there was no signi ficant difference between the quinolones, but at 4 x MIC gemifloxacin showe d superior activity, reducing the viable count by almost 2 log(10) in 30 mi n compared with a 1 log(10) reduction seen with the other drugs. This enhan ced killing extended over 24 h, reducing cell numbers by >4 log(10). These data suggest that gemifloxacin has the potential to be of therapeutic value in the treatment of infection by Acinetobacter spp.