Synergistic Activities of Colistin with Tigecycline Combination Against Clinical Isolates of Carbapenem-resistant Acinetobacter baumannii

OBJECTIVES: To determine in vitro antimicrobial susceptibility and synergistic activity of colistin in combination with tigecycline against clinical strains of carbapenem- resistant Acinetobacter baumannii (CRAB).MATERIAL AND METHODS: Colistin and tigecycline minimum inhibitory concentrations (MICs) of 12-clinical CRAB isolates were determined by broth microdilution. Checkerboard testing was performed to assess the interaction of the colistin-tigecycline combination. Fractional inhibitory concentration indexes (ΣFIC) in the range of 0.5 to 1.0, > 1.0 to < 4.0 and ≥ 4.0 are considered as additive, indifferent, and antagonistic effects, respectively.RESULTS: All CRAB isolates were susceptible to colistin. Three out of 12 CRABs were susceptible to tigecycline based on a pharmacokinetic-pharmacodynamics (PK-PD) breakpoint (MICs ≤ 0.25 μg/mL). The MIC of both antimicrobials was decreased in most of the CRAB isolates in the checkerboard synergy testing. The interaction of colistin and tigecycline combination revealed both additive and indifferent effects in five and seven of the 12-CRAB isolates, respectively. Neither synergism nor antagonism of colistin and tigecycline combination was demonstrated.CONCLUSION: No synergistic effect between colistin and tigecycline against CRAB isolates was detected. However, the combination of these two drugs is likely to result in a decrease in the MIC of both drugs. Further studies with a larger sample to determine the in vitro synergistic activity of colistin and tigecycline combination are required.