4.2.5. Evaluation of synergistic interactions using a checkerboard assay

To evaluate the interactions between bisphenylthiazoles and the standard antifungal agents (fluconazole, amphotericin B, 5-fluorocytosine and caspofungin) against 2 C. auris clinical isolates, a standard checkerboard assay was utilized as described in previous studies [51–53]. The fractional inhibitory concentration index (FICI) was calculated for each interaction. Interactions where the FICI was ≤0.5 were categorized as synergistic (SYN). FICI values of >0.5–1.25 were categorized as additive (ADD). FICI values of >1.25–4 were categorized as indifferent, and FICI values of > 4 were categorized as antagonistic [54]. The experiment was repeated three independent times.