In vitro activity of colistin plus sulbactam against extensive-drug-resistant Acinetobacter baumannii by checkerboard method.

Objective: To determine in vitro activity of colistin plus sulbactam against extensive-drug-resistant (XDR) Acinetobacter baumannii.
Material and Method: Checkerboard method was used to determine in vitro activity of colistin plus sulbactam against 11 clinical isolates of XDR A. baumannii. The concentrations of colistin and sulbactam used in the study were 0.025 to 128 mg/ 1 and 4 to 256 mg/l, respectively. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of colistin, sulbactam and colistin plus sulbactam at various concentrations were determined. Fractional inhibitory concentration index (FICI) was calculated. The antibiotic combination is considered synergistic if FICI < 0.5, indifferent if FICI 0.5 to 4.0, and antagonistic if FICI > 4.0.
Results: Ten isolates of XDR A. baumannii were susceptible to colistin (MIC < 2 mg/l) and one isolate was resistant to colistin (MIC 8 mg/l). There were no antagonistic effects of colistin plus sulbactam against all study isolates. For 10 isolates of colistin-susceptible XDR A. baumannii, some MIC values of the combinations were lower than those of single antibiotics. However no synergistic effect of colistin and sulbactam was observed in colistin-susceptible XDR A. baumannii isolates. The synergistic effect of colistin and sulbactam was detected in some concentrations of colistin and sulbactam against colistin-resistant XDR A. baumannii isolate.
Conclusion: The combination of colistin and sulbactam showed an indifferent effect against colistin-susceptible XDR A. baumannii. The combination of colistin and sulbactam showed synergistic effect at some concentrations of colistin and sulbactam against a clinical isolate of colistin-resistant XDR A. baumannii. In vitro time-kill method should be performed to confirm the aforementioned observations.