In VitroAntibiotic Synergy in Extensively Drug-Resistant Acinetobacter baumannii: the Effect of Testing by Time-Kill, Checkerboard, and Etest Methods

ABSTRACTThis study examined the in vitroeffects of polymyxin B, tigecycline, and rifampin combinations on 16 isolates of extensively drug-resistant Acinetobacter baumannii, including four polymyxin-resistant strains. In vitrosynergy was demonstrated in 19 (40%) of a possible 48 isolate-antibiotic combinations by time-kill methods, 8 (17%) by checkerboard methods, and only 1 (2%) by Etest methods. There was only slight agreement between Etest and checkerboard methods and no agreement between results obtained by other methods.