In vitro synergy of colistin combinations against extensively drug-resistant Acinetobacter baumannii producing OXA-23 carbapenemase

Abstract

Fifty extensively drug-resistant Acinetobacter baumannii (XDRAB) were isolated from patients. The chequerboard microdilution method was used to determine the in vitro activities of five colistin (COL)-based combinations including COL+fosfomycin (FOS), COL+rifampicin (RIF), COL+imipenem (IMP), COL+sulbactam (SUP) and COL+levofloxacin (LVX). The synergistic activity was evaluated by the fractional inhibitory concentration index (FICI). According to our results, the combination of COL was synergistic with FOS, RIF, IMP, SUP and LVX with the ratios of 50, 72, 88, 92 and 64%, respectively. When combined with COL, the other five agents showed increased antimicrobial activities. In addition, two of the combinations, COL+RIF and COL+IMP, were more active than the combinations of COL+FOS, COL+SUP and COL+LVX. More importantly, these combination regimens could exert synergistic effects at the sub-minimum inhibitory concentration (MIC) levels against XDRAB strains.

Keywords:

• Antimicrobial resistance
• Acinetobacter
• Synergy
• Susceptibility
• Combination therapy

Acknowledgement

This work was supported by grants from the National Natural Science Foundation of China [grant number 81172737 and 81373072].