Methods: Five clinically unique mcr-1-positive E. coli were obtained from the CDC Antimicrobial Resistance Bank. MICs for polymyxin B and rifampin were determined in triplicate by Etest and broth microdilution. Synergy testing was performed in triplicate by a MIC:MIC Etest method. Results were read at 24h and summation fractional inhibitory concentration (∑FIC) calculated: synergy ≤0.5. Synergy testing by time-kill assay was performed using ½MIC of polymyxin B and rifampin. Synergy was defined as ≥2 log10 decrease in CFU/ml after 24h by the combination compared to the most potent agent alone.
Results: MICs (µg/ml) were: polymyxin B, 3-6 (Etest) and 2-4 (broth microdilution) (>2 is considered resistant by Eucast 2018); rifampin, >32 (Etest) and 6 to >48 (broth microdilution) (no rifampin breakpoints established). The MIC:MIC Etest method showed synergy with polymyxin B plus rifampin against all 5 mcr-1-positive E. coli isolates (∑FICs, 0.2-0.3). Using the ½MIC combination of polymyxin B and rifampin, time-kill assay also showed synergy against all isolates.
Conclusion: Synergy was seen against all mcr-1-positive E. coli isolates using polymyxin B plus rifampin by both the MIC:MIC Etest method and time-kill assay at ½MIC. In vitro synergy may or may not correlate clinically.
D. Ashcraft, None
G. Pankey, None