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Effects of Treated versus Untreated Polystyrene on Caspofungin In Vitro Activity against Candida Species
Annette W Fothergill1, Dora I McCarthy1, Mohammad T Albataineh1
1Fungus Testing Laboratory, Department of Pathology and South Texas Reference Laboratories, University of Texas Health Science Center at San Antonio, San Antonio, Texas, USA.
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.
Insights
Treated polystyrene microtiter trays can lead to falsely elevated caspofungin minimum inhibitory concentrations (MICs) in Candida species testing. Untreated trays or alternative assays improve accurate susceptibility results for antifungal drug resistance.
Area of Science:
- Medical Mycology
- Antimicrobial Susceptibility Testing
- Clinical Microbiology
Background:
- Interlaboratory variability in caspofungin susceptibility testing of Candida species is a significant issue.
- Standard broth microdilution methods (CLSI, EUCAST) show inconsistencies.
- Polystyrene microtiter tray surface treatment is a potential confounding factor.
Purpose of the Study:
- To evaluate the impact of treated versus untreated polystyrene microtiter trays on caspofungin minimum inhibitory concentrations (MICs).
- To assess the influence of tray type on caspofungin susceptibility testing across different Candida species.
- To compare results with commercial assays like YeastOne and Etest.
Main Methods:
- Tested 209 Candida isolates (C. albicans, C. glabrata, C. krusei, C. tropicalis) using broth microdilution with treated and untreated polystyrene trays.
- Included isolates with known FKS mutations, particularly in C. albicans and C. glabrata.
- Determined caspofungin MICs using YeastOne and Etest assays for a subset of 102 isolates.
Main Results:
- Treated polystyrene trays consistently yielded falsely elevated caspofungin MICs for C. glabrata and C. krusei, masking resistance.
- Untreated trays and commercial assays (YeastOne, Etest) allowed accurate differentiation of susceptible and resistant isolates for C. glabrata, C. krusei, and C. albicans.
- C. albicans and C. tropicalis susceptibility results were less affected by tray treatment.
Conclusions:
- Surface treatment of polystyrene microtiter trays is a critical factor contributing to inaccurate in vitro caspofungin susceptibility testing.
- The choice of microtiter tray material significantly impacts caspofungin MICs, with greater effects observed in certain Candida species.
- Utilizing untreated polystyrene or alternative testing platforms like YeastOne and Etest is recommended for reliable caspofungin susceptibility assessment.
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