Combined antimicrobial susceptibility test against pan-drug-resistant Acinetobacter baumannii with E-test and microdilution checkerboard assay
Objective To evaluate the activity of antibiotics against pan-drug-resistant (PDR) Acinetobacter baumannii by combination antimicrobial susceptibility test in viro with epsilometric methods (Etest method) and microdilution checkerboard (CB method),and to detect a good correlation between timekill curve with the above mentioned two assays.Methods Thirty-one clinical isolates of PDR Acinetobacter baumannii were selected for mono and combination antimicrobial susceptibility test in vitro by E-test and CB method,then a comparison was conducted between the test results and the time-kill curve.Mono drugs involved tigecycline,colistin,imipenem and amikacin,and combinations involved two of drugs above,and three drugs involved imipenem/tigecycline,plus amikacin combination.Results Synergistic effect was detected in imipenem plus colistin and tigecycline plus imipenem combination.A high comparability was revealed between the E-test method with antimicrobial drugs added into the culture medium and the time-kill curves.Synergy in the combination of imipenem/tigecycline,plus amikacin was detected by the CB method and time-kill curves.Conclusion The results showed that the effect of specific combination of antibiotics against PDR Acinetobacter baumannii could be predicted by testing their synergistic effect with combination antimicrobial susceptibility test. 
 
Key words: 
E-test method;  Broth microdilution checkerboard method;  Time-kill curve;  Pan-drug resistant Acinetobacter baumannii