Sulbactam enhances the in vitro activity of sitafloxacin against extensively-drug resistant Acinetobacter baumannii

The present study aimed to determine the in vitro activities of sulbactam and sitafloxacin against extensively-drug resistant Acinetobacter baumannii (XDR-A. baumannii). A total of 50 strains of XDR-A. baumannii were isolated from clinical specimens. Broth microdilution assay was applied to determine the minimum inhibitory concentration (MIC) for sulbactam and sitafloxacin. Microdilution checkerboard method was used to determine the in vitro activity of this antimicrobial combination. Accordingly, the fractional inhibitory concentration (FIC) and FIC index (FICI) were calculated. Time-kill study was also carried out for four strains with different susceptibilities to determine the bactericidal activities of individual or combined use of sitafloxacin and sulbactam. Isolates with MICs of sitafloxacin ≤2 mg/l were considered to be susceptible to sitafloxacin. The susceptibility rate for sitafloxacin was 92% originally. When combined with sulbactam, this rate increased to 96%. Microdilution checkerboard results indicated that, when tested in combination, sulbactam/sitafloxacin exhibited marked synergistic and partial synergistic effects on 16 and 50% of the 50 strains, respectively. Time-kill assay suggested that sulbactam enhanced the bactericidal activity of sitafloxacin and the combination induced a synergistic effect. For strains that were not susceptible to sitafloxacin, the bactericidal activities of the combination of sitafloxacin and sulbactam at a sub-MIC concentration were impaired. However, this impairment could be overcome with the increase of the concentration to 1X MIC. The present study demonstrated that sulbactam enhanced the in vitro antimicrobial activity of sitafloxacin against XDR-A. baumannii.