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Antimicrobial Agents and Chemotherapy, September 2008, p. 3092-3098, Vol. 52, No. 9
0066-4804/08/$08.00+0     doi:10.1128/AAC.00088-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Mutations in the fks1 Gene in Candida albicans, C. tropicalis, and C. krusei Correlate with Elevated Caspofungin MICs Uncovered in AM3 Medium Using the Method of the European Committee on Antibiotic Susceptibility Testing

Institut Pasteur, Centre National de Référence Mycologie et Antifongiques, Unité de Mycologie Moléculaire, CNRS URA3012, 75724 Paris Cedex 15,¹ Université Paris Descartes, Faculté de Médecine, AP-HP, Hôpital Européen Georges Pompidou, Unité de Parasitologie-Mycologie, 75015 Paris, France²

Received 21 January 2008/ Returned for modification 1 April 2008/ Accepted 18 June 2008

Mutations in two specific regions of the Fks1 subunit of 1,3-β-D-glucan synthase are known to confer decreased caspofungin susceptibility on Candida spp. Clinical isolates of Candida spp. (404 Candida albicans, 62 C. tropicalis, and 21 C. krusei isolates) sent to the French National Reference Center were prospectively screened for susceptibility to caspofungin in vitro by the broth microdilution reference method of the Antifungal Susceptibility Testing Subcommittee of the European Committee on Antibiotic Susceptibility Testing (AFST-EUCAST). Twenty-eight isolates (25 C. albicans, 2 C. tropicalis, and 1 C. krusei isolate) for which the caspofungin MIC was above the MIC that inhibited 90% of the isolates of the corresponding species (MIC₉₀) were subjected to molecular analysis in order to identify mutations in the fks1 gene. Substitutions in the deduced protein sequence of Fks1 were found for 8 isolates, and 20 isolates had the wild-type sequence. Among the six C. albicans isolates harboring mutations, six patterns were observed involving amino acid changes at positions 641, 645, 649, and 1358. For C. tropicalis, one isolate showed an L644W mutation, and for one C. krusei isolate, two mutations, L658W and L701M, were found. Two media, RPMI medium and AM3, were tested for their abilities to distinguish between isolates with wild-type Fks1 and those with mutant Fks1. In RPMI medium, caspofungin MICs ranged from 0.25 to 2 µg/ml for wild-type isolates and from 1 to 8 µg/ml for mutant isolates. A sharper difference was observed in AM3: all wild-type isolates were inhibited by 0.25 µg/ml of caspofungin, while caspofungin MICs for all mutant isolates were 0.5 µg/ml. These data demonstrate that clinical isolates of C. albicans, C. tropicalis, and C. krusei with decreased susceptibility to caspofungin in vitro have diverse mutations in the fks1 gene and that AM3 is potentially a better medium than RPMI for distinguishing between mutant and wild-type isolates using the AFST-EUCAST method.

Published ahead of print on 30 June 2008.