Checkerboard Test

Six different combinations of antimicrobials were selected and tested by the checkerboard test: MEM plus AK, MEM plus COL, MEM plus TGC, AK plus COL, AK plus TGC and COL plus TGC. The information on the six isolates is shown in Table 1. The MICs of drug combinations were determined after incubation at 35°C for 18–24 h in ambient air.

Isolates Used in the Time–Kill Assay and G. mellonella Survival Assay

Abbreviations: MEM, meropenem; TGC, tigecycline; COL, colistin; AK, amikacin; R, resistant; S, susceptible.

The interaction between antimicrobial agents was determined based on the calculated fractional inhibitory concentration index (FICI). The FIC of each drug was calculated as a ratio of the MIC of drug A (or B) when used in the combination (combo) and the MIC of drug A (or B) when tested alone, according to the following formula: FICI=FIC_A+FIC_B=MIC_(A–combo)/MIC_(A–alone)+MIC_(B–combo)/MIC_(B–alone).¹⁹ The results were interpreted as follows: synergism, FICI ≤0.5; partial synergism, 0.5<FICI<1; additivity, FICI=1; indifference, 1<FICI<4; and antagonism, FICI ≥4.²⁰