Antifungal susceptibility profiles of Candida species to triazole: application of new CLSI species-specific clinical breakpoints and epidemiological cutoff values for characterization of antifungal resistance

Abstract

The Clinical and Laboratory Standards Institute (CLSI) Subcommittee on Antifungal Susceptibility Testing has newly introduced species-specific clinical breakpoints (CBPs) for fluconazole and voriconazole. When CBPs can not be determined, wild-type minimal inhibitory concentration (MIC) distributions are detected and epidemiological cutoff values (ECVs) provide valuable means for the detection of emerging resistance. The aim of this study is to determine triazole resistance patterns in Candida species by the recently revised CLSI CBPs. A total of 140 Candida strains isolated from blood cultures of patients with invasive candidiasis hospitalized in various intensive care units in Turkey and sent to our reference laboratory between 2011-2012, were included in the study. The isolates were identified by conventional methods, and susceptibility testing was performed against fluconazole, itraconazole and voriconazole, by the 24-h CLSI broth microdilution (BMD) method. Azole resistance rates for all Candida species were determined using the new species-specific CLSI CBPs and ECVs criteria, when appropriate. The species distribution of the isolates were as follows; C.parapsilosis (n= 31 ), C.tropicalis (n= 26 ), C.glabrata (n= 21), C.albicans (n= 18), C.lusitaniae (n= 16), C.krusei (n= 16), C.kefyr (n= 9), C.guilliermondii (n= 2), and C.dubliniensis (n= 1). According to the newly determined CLSI CBPs for fluconazole and C.albicans, C.parapsilosis, C.tropicalis [susceptible (S), ≤ 2 µg/ml; dose-dependent susceptible (SDD), 4 µg/ml; resistant (R), ≥ 8 µg/ml], and C.glabrata (SDD, ≤ 32 µg/ml; R≥ 64 µg/ml) and for voriconazole and C.albicans, C.parapsilosis, C.tropicalis (S, ≤ 0.12 µg/ml; SDD, 0.25-0.5 µg/ml; R, ≥ 1 µg/ml), and C.krusei (S, ≤ 0.5 µg/ml; SDD, 1 µg/ml; R, ≥ 2 µg/ml), it was found that three of C.albicans, one of C.parapsilosis and one of C.glabrata isolates were resistant to fluconazole, while two of C.albicans and two of C.tropicalis were resistant to voriconazole. The ECVs of 0.5 µg/ml for voriconazole and C.glabrata were used to differentiate wild-type (MIC ≤ ECV) from non-wild-type (MIC > ECV) strains. Five of C.glabrata were non-WT for voriconazole. Due to the lack of CBPs for the less common species, the ECVs for fluconazole, itraconazole and voriconazole, respectively, were used for C.lusitaniae (2 µg/ml, 0.5 µg/ml, 0.03 µg/ml), C.guilliermondii (8 µg/ml, 1 µg/ml, 0.25 µg/ml), C.dubliniensis (0.5 µg/ml, 0.25 µg/ml, 0.03 µg/ml), and C.kefyr (1 µg/ml, 0.015 µg/ml) to categorize isolates of these species as wild- and non-wild-type. When the ECVs were used for fluconazole, one each of C.lusitaniae, C.dubliniensis and C.kefyr; for voriconazole, three of C.lusitaniae and one of C.kefyr were detected as non-wild-type. Overall, a total of five Candida species were resistant to fluconazole and four to voriconazole and among these species one each of C.parapsilosis, C.tropicalis, C.glabrata, C.lusitaniae, C.kefyr and three of C.albicans exhibited cross-resistance at least against two azoles. It was concluded that, the strains identified as resistant and non-wild-type in this in vitro study should be supported by molecular and in vivo studies for the determination of their clinical validity.