Diffusion-Mediated Quantification of Dose-Dependent Antifungal Drug Tolerance

Antimicrobial resistance is a global health problem, with drug-resistant fungi posing major challenges for the treatment of immunocompromised patients. Antifungal tolerance is a recently discovered phenomenon, distinct from antifungal resistance, whereby pathogenic fungi grow slowly above minimum inhibitory drug concentrations. We combine physics-based spatiotemporal models with microbiology experiments to quantitatively investigate the emergence of tolerance to all three major classes of antifungal drugs used to treat invasive infections in the multidrug-resistant pathogenic yeast Candidozyma auris (formerly Candida auris). Specifically, we combine Fick's second law, solved using a finite difference method to simulate drug diffusion, with experimental data from disk diffusion assays to determine the concentration ranges associated with the emergence of antifungal-tolerant colonies. Tolerant colonies emerged experimentally within distinct drug concentration ranges for itraconazole, amphotericin B, and caspofungin in patient-derived C. auris isolates. This study provides a method to quantify dose-dependent antifungal tolerance and establishes that tolerance is a dose-dependent phenomenon in C. auris.