In Vitro Activities of Colistin and Sitafloxacin Combinations against Multidrug-, Carbapenem-, and Colistin-ResistantAcinetobacter baumanniiUsing the Broth Microdilution Checkerboard and Time-Kill Methods

Drug-resistantAcinetobacter baumannii (A. baumannii) infections are a critical global problem, with limited treatment choices. This study aims to determine the in vitro activities of colistin-sitafloxacin combinations against multidrug-, carbapenem- and colistin-resistantA. baumannii(MDR-AB, CRAB, CoR-AB, respectively) clinical isolates from tertiary care hospitals. We used the broth microdilution checkerboard and time-kill methods in this study. Synergy was found using both methods. The colistin-sitafloxacin combination showed synergy in MDR-AB, CRAB, and CoR-AB isolates (3.4%, 3.1%, and 20.9%, respectively). No antagonism was found in any type of drug-resistant isolate. The majority of CoR-AB isolates became susceptible to colistin (95.4%). The time-kill method also showed that this combination could suppress regrowth back to the initial inocula of all representative isolates. Our results demonstrated that the colistin-sitafloxacin combination might be an interesting option for the treatment of drug-resistantA. baumannii. However, further in vivo and clinical studies are required.