In Vitro Activity of Colistin plus Sulbactam against Extensive-Drug-Resistant Acinetobacter baumannii by Checkerboard Method
Visanu Thamlikitkul MD*, Surapee Tiengrim MSc*
Affiliation : * Division of Infectious Diseases and Tropical Medicine, Faculty of Medicine, Siriraj Hospital, Mahidol University, Bangkok, Thailand
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/
l 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 concen-
tration 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.
Keywords : Colistin, Sulbactam, Synergy, A. baumannii
