In vitro activities of rifampin, colistin, sulbactam and tigecycline tested alone and in combination against extensively drug-resistant Acinetobacter baumannii

The aim of this study was to investigate the in vitro activities of rifampin, colistin, sulbactam and tigecycline alone and in combination against extensively drug-resistant Acinetobacter baumannii (XDR-Ab). Twenty-five XDR-Ab strains were isolated from patients. Broth microdilution assay was used to determine the minimum inhibitory concentration (MIC) for rifampin, colistin, sulbactam and tigecycline against XDR-Ab strains. The checkerboard microdilution method was used to determine the in vitro activities of potential therapeutic combinations of these four antimicrobial agents. Accordingly, the fractional inhibitory concentration (FIC) and FIC index (FICI) were calculated for each of the combinations. According to our results, when tested as single drugs, rifampin, colistin or tigecycline had good bacteriostatic activity against XDR-Ab, whereas sulbactam was not as active against XDR-Ab isolates. On the other hand, when tested in combination, the combinations of colistin/rifampin, rifampin/sulbactam, rifampin/tigecycline and sulbactam/tigecycline showed good in vitro activities against XDR-Ab isolates. More importantly, these combination regimens could exert addictive or partially synergistic effects at the sub-MIC levels against XDR-Ab strains. Compared with single drugs, most of the combinations of these antimicrobial agents could exert partially synergistic and/or addictive effects, which might provide a better alternative when treating XDR-Ab infections.