Fig. 3

3D scatterplot of the cases based on the Minimum Inhibitory Concentration (MIC) of the antibiotics in the combination setting. Labeled values indicate the overlap of a case in the additive category with a case in the synergy (A), antagonism (B), or additive (C) categories. Cases farther along the axes are displayed with reduced opacity for better visualization. The axes show the absolute MICs of each drug in the combination setting, whereas the color-coding reflects the interaction category based on the fractional inhibitory concentration index (FICI), which normalizes each combination MIC to the isolate’s corresponding single-agent MIC. Therefore, cases can share very similar combination MIC triplets yet fall into different categories if their single-agent MICs differ (e.g., lower baseline MICs yield larger fractional values at the same MIC in combination). Antagonism points correspond to cases in which the required concentrations in combination are close to or higher than the single-agent MIC(s), resulting in FICI ≥ 4.