Abstract
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.
Highlights
All of carbapenemresistant Acinetobacter baumannii (CRAB) isolates were susceptible to colistin with minimum inhibitory concentrations (MICs) in a range of 1 to 2 μg/mL whereas MICs of tigecycline against CRAB ranged 0.125 to 3.0 μg/mL
Three out of 12 (25%) CRAB isolates were classified as tigecycline susceptible strains.With synergistic testing, the MICs of both antimicrobials were decreased in most of the CRAB isolates
Neither synergism nor antagonism of the combination of colistin and tigecycline was demonstrated in this study
Summary
To determine in vitro antimicrobial susceptibility and synergistic activity of colistin in combination with tigecycline against clinical strains of carbapenemresistant 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. RESULTS: All CRAB isolates were susceptible to colistin. 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. CONCLUSION: No synergistic effect between colistin and tigecycline against CRAB isolates was detected. Further studies with a larger sample to determine the in vitro synergistic activity of colistin and tigecycline combination are required
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