Table 3.

The measure of antibiotic synergy by fractional inhibitory concentration index (FICI). Interpretation of the FIC results were as follows: ≤0.5, synergy; 0.5 to 1.0, additivity; >1.0 to 4.0, indifference; and >4, antagonism.

Pse-1 (IMP-13 Producer)	Agar Diffusion (E-Test) MIC	Fractional Inhibitory Concentration Index
Cefiderocol + Imipenem	Cefiderocol alone: 0.064 Cefiderocol combination: 0.016 Imipenem alone: >32 Imipenem combination: 2	0.31
Cefiderocol + Imipenem/relebactam	Cefiderocol alone: 0.064 Cefiderocol combination: 0.016 Imipenem/relebactam alone: >32 Imipenem/relebactam combination: 3	0.34
Cefiderocol + Fosfomycin	Cefiderocol alone: 0.064 Cefiderocol combination: 0.016 Fosfomycin alone: 16 Fosfomycin combination: 6	0.62
Pse-2 (VIM-2 Producer)	Agar Diffusion (E-Test) MIC	Fractional Inhibitory Concentration Index
Cefiderocol + Imipenem	Cefiderocol alone: 0.19 Cefiderocol combination: 0.19 Imipenem alone: >32 Imipenem combination: 32	2
Cefiderocol + Imipenem/relebactam	Cefiderocol alone: 0.19 Cefiderocol combination: 0.19 Imipenem/relebactam alone: >32 Imipenem/relebactam combination: 32	2
Cefiderocol + Fosfomycin	Cefiderocol alone: 0.19 Cefiderocol combination: 0.19 Fosfomycin alone: >256 Fosfomycin combination: 256	2
Pse-3 (IPM-13 Producer)	Agar diffusion (E-Test) MIC	Fractional Inhibitory Concentration Index
Cefiderocol + Imipenem	Cefiderocol alone: 0.047 Cefiderocol combination: 0.016 Imipenem alone: 2 Imipenem combination: 0.5	0.59
Cefiderocol + Imipenem/relebactam	Cefiderocol alone: 0.047 Cefiderocol combination: 0.016 Imipenem/relebactam alone: 1.5 Imipenem/relebactam combination: 0.75	0.84
Cefiderocol + Fosfomycin	Cefiderocol alone: 0.047 Cefiderocol combination: 0.023 Fosfomycin alone: 6 Fosfomycin combination: 2	0.81