Effects of Treated versus Untreated Polystyrene on Caspofungin In Vitro Activity against Candida Species

Abstract

Significant interlaboratory variability is observed in testing the caspofungin susceptibility of Candida species by both the CLSI and EUCAST broth microdilution methodologies. We evaluated the influence of treated versus untreated polystyrene microtiter trays on caspofungin MICs using 209 isolates of four Candida species, including 16 C. albicans and 11 C. glabrata isolates with defined FKS mutations. Caspofungin MICs were also determined using the commercially available YeastOne and Etest assays and 102 isolates. All C. glabrata isolates had caspofungin MICs of ≥0.5 μg/ml, the clinical breakpoint for caspofungin resistance in this species, measured using trays made of treated polystyrene, regardless of the FKS status. In contrast, susceptible isolates could readily be distinguished from resistant/non-wild-type isolates when caspofungin MICs were measured using untreated polystyrene trays and both the YeastOne and Etest assays. Similar results were also observed for C. krusei isolates, as all isolates had caspofungin MICs above the threshold for resistance measured using treated polystyrene trays. In contrast, C. albicans isolates could be correctly identified as susceptible or resistant when caspofungin MICs were measured with treated or untreated trays and with the YeastOne and Etest assays. MICs falsely elevated above the resistance breakpoint were also not observed for C. tropicalis isolates. These results demonstrated that the use of treated polystyrene may be one factor that leads to falsely elevated caspofungin in vitro susceptibility results and that this may also be a greater issue for some Candida species than for others.

FIG 1

Caspofungin MIC values for Candida glabrata isolates measured by CLSI broth microdilution methods using treated and untreated polystyrene trays and the commercially available YeastOne and Etest assays. A total of 33 clinical C. glabrata isolates were included, including resistant isolates with defined FKS mutations as well as susceptible isolates that had low caspofungin (CAS) MICs by Etest combined with low micafungin MICs. The upper dotted line represents the threshold for caspofungin resistance in C. glabrata per CLSI clinical breakpoints and the lower line the threshold for susceptibility measured using the CLSI broth microdilution methods.

FIG 2

Caspofungin MIC values for Candida albicans isolates measured by CLSI broth microdilution methods using treated and untreated polystyrene trays and the commercially available YeastOne and Etest assays. A total of 29 clinical C. albicans isolates were included, including resistant isolates with defined FKS mutations as well as susceptible isolates that had low caspofungin MICs by Etest and YeastOne assays. The upper dotted line represents the threshold for caspofungin resistance in C. albicans per CLSI clinical breakpoints and the lower line the threshold for susceptibility measured using the CLSI broth microdilution methods.

FIG 3

Caspofungin MIC values for clinical isolates of Candida krusei and C. tropicalis measured by CLSI broth microdilution methods using treated and untreated polystyrene trays and the commercially available YeastOne and Etest assays. A total of 20 isolates per species were included. The upper dotted line represents the threshold for caspofungin resistance in each species per CLSI clinical breakpoints and the lower line the threshold for susceptibility measured using the CLSI broth microdilution methods.