AAC Accepts, published online ahead of print on 2 November 2009

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Antimicrob. Agents Chemother. doi:10.1128/AAC.01256-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Susceptibility testing of Candida species to echinocandins: comparison of EUCAST EDef 7.1, CLSI M27-A3, Etest, disk diffusion and agar-dilution using RPMI and IsoSensitest medium.

Maiken Cavling Arendrup^*, Guillermo Garcia-Effron, Cornelia Lass-Flörl, Alicia Gomez Lopez, Juan-Luis Rodriguez-Tudela, Manuel Cuenca-Estrella, and David S. Perlin

Unit of Mycology and Parasitology, Statens Serum Institut, Copenhagen, Denmark; Public Health Research Institute, UMDNJ-New Jersey Medical School, Newark, USA; Dept. of Hygiene and Medical Microbiology, Innsbruck Medical University, Innsbruck, Austria; and Servicio de Micología, Centro Nacional de Microbiología, Instituto de Salud Carlos III, Majadahonda, Spain

^* To whom correspondence should be addressed. Email: mad{at}ssi.dk.

This study compared nine susceptibility testing methods and 12 endpoints for anidulafungin, caspofungin and micafungin on the same collection of blinded FKS hot spot mutant (no. 29) and wild type isolates (no. 94). Susceptibility test: EUCAST Edef 7.1, agar dilution, Etest and disk diffusion with RPMI-1640+2% glucose (2G) and IsoSensitest-2G media and CLSI M27A-3. Microdilution plates were read after 24 and 48 hours. The following test parameters were evaluated: fks hot spot mutants overlapping the wild type distribution; distance between the two populations, number of very major errors (VMEs, fks mutants mis-classified as susceptible) and major errors (MEs, wild type isolates classified as resistant) using a wild-type-upper-limit-value (WT-UL) (two 2-fold dilutions higher than MIC₅₀) as susceptibility breakpoint.

The methods with the lowest number of errors (given as VMEs/MEs) across the three echinocandins were CLSI (12%/1%)), agar dilution with RPMI-2G medium (14%/0%) and Etest with RPMI-2G medium (8%/3%). Fewest errors were overall observed for anidulafungin (4%/1% for EUCAST, 4%/3% for CLSI and 3%/9% for Etest with RPMI-2G). For micafungin, 10-71% VMEs were observed. For caspofungin, agar dilution with either medium was superior (VMEs/MEs: 0%/1%) while CLSI, EUCAST with IsoSensitest-2G medium and Etest were less optimal (7%, 10% and 10% VMEs, respectively). Applying the CLSI breakpoint (S: 2 μg/ml) for CLSI results, 89.2% fks hot spot mutants were classified as anidulafungin susceptible, 60.7% as caspofungin susceptible and 92.9% as micafungin susceptible.

In conclusion, no test was perfect but anidulafungin susceptibility testing using WT-UL to define susceptibility reliably identified fks hot spot mutants.