In vitro activities of carbapenem/sulbactam combination, colistin, colistin/rifampicin combination and tigecycline against carbapenem-resistant Acinetobacter baumannii

Objectives: To determine the in vitro activities and interactions of imipenem, colistin and tigecycline with old antibacterial agents against carbapenem-resistant Acinetobacter baumannii Methods: Forty-three carbapenem-resistant A. baurnannii isolates from the intensive care unit of a university hospital were collected and their MICs of imipenem, colistin and tigecycline were determined. With eight randomly selected carbapenem-resistant isolates, an in vitro time-kill study was performed for the evaluation of antibacterial activity of colistin, tigecycline, lmipenem/sulbactam and colistin/ rifampicin. Results: The time-kill study of colistin demonstrated bactericidal activity against A. baumannii at concentrations of 4x MIC and 8x MIC, whereas tigecycline showed bacteriostatic activity at all concentrations. The combination regimens of imlpenem/sulbactam and colistin/rifampicin were synergistic and bactericidal at 1 x MIC. Conclusions: lmipenem/sulbactam combination, colistin and tigecycline showed good in vitro activities against carbapenem-resistant A. baumannii isolates. Even though colistin is bactericidal against carbapenem-resistant A. baumannii, the colistin/rifampicin combination is more warranted in order to be certain.