Detection of Caspofungin Resistance in Candida spp. by Etest

Abstract

The caspofungin susceptibilities of 28 Candida sp. clinical isolates, including 8 caspofungin-resistant isolates characterized by mutations in the Fks1 protein, were determined by the Etest in RPMI and AM3 media. Good discrimination between wild-type and mutant isolates was obtained. These results suggest that the Etest is valuable for the detection of caspofungin resistance in Candida spp.Most of the large surveillance studies on the in vitro susceptibility of Candida species isolates to caspofungin have been performed by using the reference broth microdilution techniques of the CLSI (25, 27) and 7,9). Nevertheless, in routine clinical microbiology laboratories, alternative methods such as the commercially available agar diffusion technique Etest are commonly used. The Etest is a reproducible and reliable technique for the determination of in vitro caspofungin activity, and good agreement with reference techniques is obtained (1,5,13,21,28).Caspofungin resistance in Candida spp. remains uncommon (27). However, several clinical case reports have been published on infections with Candida species isolates exhibiting high caspofungin MICs, some of which were associated with treatment failure (2, 8, 11, 14, 15, 18-20, 22, 23, 26, 30). In several cases, resistance was confirmed in animal models (15,19,26), and mutations in the fks1 gene were associated with the resistance phenotype (2,17,18,20,22,26). It has been shown that mutations in Fks1 are sufficient to confer reduced susceptibility to caspofungin on Candida spp. (26). Even though high MICs have been recorded using the Etest on a few Candida isolates (2, 19), this technique has never been evaluated for its ability to detect caspofungin resistance. We recently characterized a collection of Candida albicans, C. tropicalis, and C. krusei isolates with decreased susceptibility to caspofungin, associated with various Fks1 mutations (10). The aim of the present study was to evaluate the Etest for its ability to discriminate between wild-type (caspofungin-susceptible) and Fks1 mutant (caspofungin-resistant) isolates of Candida spp.The clinical isolates were collected at the French National Reference Center for Mycoses and Antifungals as part of our surveillance activities. All isolates were identified to the species level by standard mycological procedures including the assimilation patterns obtained with the commercialized ID32C strips (bioMérieux, Marcy-l'Etoile, France). For all C. albicans isolates, a specific PCR amplification (12) was performed to exclude C. dubliniensis. In a recent study (10), we sequenced and translated (to obtain the deduced protein sequence) the two hot spot (HS) regions of the fks1 gene, HS1 and HS2, for 28 clinical isolates, including Candida albicans (n ϭ 25), C. tropicalis (n ϭ 2), and C. krusei (n ϭ 1), for which caspofungin MICs were determined by the AFST-EUCAST method (29). MICs of Ն0.5 g/ml recorded in AM3 medium were associated with mutations in the deduced Fks1 sequence (6 C. albicans, 1 C. tropicalis, and 1 C. krusei isolate), w...

Echinocandin Susceptibility Testing ofCandidaSpecies: Comparison of EUCAST EDef 7.1, CLSI M27-A3, Etest, Disk Diffusion, and Agar Dilution Methods with RPMI and IsoSensitest Media

“…These WT-UL values are considerably lower than the currently recommended CLSI breakpoint for susceptibility of an S value of Յ2 g/ml (9, 38). However, the vast majority of isolates with FKS hot spot mutations were classified as susceptible by CLSI and Etest using the CLSI breakpoint, in agreement with the increasing concern that the breakpoint may be too high (5,7,13,14,18,22,25). This was particularly true for anidulafungin and micafungin, in agreement with the fact that the MICs of these compounds are in general lower than that of caspofungin and much lower than the breakpoint (12,29,30).…”

“…As no significant differences in clinical response were noted among the various species, results for all species were merged, and a susceptibility breakpoint of 2 g/ml was found to encompass the vast majority of isolates, while not bisecting the population of Candida parapsilosis. The crucial issue is whether current susceptibility testing methods and breakpoints clearly and reliably identify isolates with resistance mechanisms associated with treatment failures (5,7,8,13,14,16,18,22,25,26,33,40). Not only have cases involving isolates classified as susceptible using the reference methods been shown to contain resistance mutations (5,7,13,14,22,25), but also recent studies suggest that a breakpoint of an S value of Յ2 g/ml may be too high for anidulafungin and micafungin, considering the 1,3-ß-D-glucan synthase kinetic inhibition data of wild-type and mutant enzymes from resistant strains (17,18).…”

Comparison of European Committee on Antimicrobial Susceptibility Testing (EUCAST) and Etest Methods with the CLSI Broth Microdilution Method for Echinocandin Susceptibility Testing of Candida Species

“…In contrast to both the CLSI and EUCAST BMD methods, the agar-based Etest method has been proposed as a more sensitive means of discriminating strains of Candida species with fks mutations from wild-type (WT) strains by virtue of much higher MIC results observed with the mutant strains (1,3,12,13). As a result of these observations, we have used a large global collection of Candida species bloodstream infection (BSI) isolates to define the WT MIC distributions and to establish epidemiological cutoff values (ECVs) for each echinocandin and species of Candida using the CLSI BMD method (31).…”

Quick Detection of FKS1 Mutations Responsible for Clinical Echinocandin Resistance in Candida albicans

“…This expanding use of echinocandins has brought about the emergence of drug resistance (9)(10)(11)(12)(13)(14)(15).…”