Understanding Echinocandin Resistance in the Emerging Pathogen Candida auris.

Candida auris has simultaneously emerged on five continents as a fungal pathogen causing nosocomial outbreaks. The challenges in the treatment of C. auris infections are the variable antifungal susceptibility profiles among clinical isolates and the development of resistance to single or multiple classes of available antifungal drugs. Here, the in vitro susceptibility to echinocandin antifungal drugs was determined and FKS1 sequencing was performed on 106 C. auris clinical isolates. Four isolates were identified to be resistant to all tested echinocandins (MIC ≥ 4 mg/liter) and harbored an S639F mutation in FKS1 hot spot region 1. All remaining isolates were FKS1 wild type (WT) and echinocandin susceptible, with micafungin being the most potent echinocandin (MIC50 = 0.125 mg/liter). Antifungal susceptibility testing with caspofungin was challenging due to the fact that all FKS1 WT isolates exhibited an Eagle effect (also known as the paradoxical growth effect), which occurred at various intensities. To assess whether the Eagle effect resulted in pharmacodynamic resistance, 8 representative isolates were evaluated for their in vivo drug response in a murine model of invasive candidiasis. All isolates were susceptible to caspofungin at a human therapeutic dose, except for those harboring the S639F mutation. The data suggest that only isolates carrying mutations in FKS1 are echinocandin resistant and that routine in vitro testing of C. auris isolates for susceptibility to caspofungin by the broth microdilution method should be viewed cautiously or avoided.