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

To detect the in vitro activities of sitafloxacin alone and in combination with rifampin, colistin, sulbactam, and tigecycline against extensively drug-resistant Acinetobacter baumannii (XDR-A. baumannii). 24 XDR-A. baumannii strains were isolated from patients' specimens. Broth microdilution assay was used to determine the minimum inhibitory concentration (MIC) for sitafloxacin, rifampin, colistin, sulbactam, and tigecycline against XDR-A. baumannii strains. The checkerboard microdilution method was used to determine the in vitro activities of sitafloxacin combined with the other 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 alone, the rate of susceptibility for sitafloxacin was 91.67% against XDR-A. baumannii, followed by colistin 62.5%, and then tigecycline 54.17%, rifampin 41.67%. Sulbactam, with a 16.67% rate of susceptibility was the least effective one. On the other hand, when tested in combination, all those three combinations except tigecycline/sitafloxacin revealed remarkable synergistic effects. Colistin/sitafloxacin showed the highest indifference rate. These combination regimens could exert addictive or partially-synergistic effects at the sub-MIC levels against XDR-A. baumannii strains. Sitafloxacin has acceptable in vitro activity against XDR-A. baumannii strains as well as tigecycline, rifampin and colistin. Compared with single drugs, most of the combinations of these antimicrobial agents could exert synergistic and/or partially synergistic and/or addictive effects, which might provide a better alternative when treating XDR-A. baumannii infections.