Understanding Echinocandin Resistance in the Emerging Pathogen Candida auris

Abstract

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.

Keywords: Candida; Candida auris; anidulafungin; antifungal resistance; antifungal susceptibility testing; caspofungin; echinocandins; micafungin; susceptibility.

FIG 1

Growth intensity-drug concentration curves for Candida auris isolates which presented different patterns of response to anidulafungin (A), caspofungin (B), and micafungin (C). VPCI 265/P/14 was resistant to all echinocandins, VPCI 513/P/14 was susceptible to all echinocandins with a high CAS Eagle effect, C57942 was susceptible to all echinocandins with a medium CAS Eagle effect, and VPCI 107/P/14 was susceptible to all echinocandins with a low CAS Eagle effect. Growth intensity was calculated as follows: (i) the absorbance of the no-growth control was subtracted from the absorbance of all the wells, and (ii) the values obtained were compared to the value for the growth control (which was not treated with an antifungal drug), which was set at 100%.

FIG 2

Comparison of the kidney burdens among the different treatment groups of mice infected with C. auris FKS1 WT isolates which exhibited Eagle effects of different intensities in vitro (isolates VPCI 463/P/14, VPCI 107/P/14, and B11800 exhibited a low Eagle effects; isolate C57942 exhibited a medium Eagle effect; and isolate VPCI 471/P/13 exhibited a high Eagle effect) and C. auris fks1 mutant isolates with the S639F substitution (isolates VPCI 1133/P/13 - R, VPCI 462/P/14, and VPCI 471a/P/14 - R, which were resistant to all echinocandins tested) at 24 h postinfection.