Checkerboard array synergy testing.

The DDM method described above was used to prepare checkerboard arrays in which two drugs were each dispensed in 7 to 9 2-fold dilutions. Each two-drug combination was tested against every C. auris strain, with growth determinations made after 24 h of incubation. Growth inhibition was determined as described above. For combinations in which both drugs use 90% inhibition for MIC determination (AMB plus MIN, AMB plus RIF, and MIN plus RIF), combination wells were considered inhibitory when growth was inhibited by 90% relative to growth control wells; for all other combinations, combination wells were considered inhibitory when growth was inhibited by at least 50% relative to growth control wells. The fractional inhibitory concentration index (FIC_I) for each drug was calculated as FIC_I = [MIC drug A (in combination)/MIC drug A (alone)] + [MIC drug B (in combination)/MIC drug B (alone)]; if the MIC of a drug was off-scale, the highest concentration tested was assigned an FIC of 0.5. A minimum FIC_I (FIC_I-MIN) of ≤0.5 was considered synergistic, an FIC_I-MIN of >4.0 antagonistic, and an intermediate FIC_I-MIN indifferent (38).