Detection of triazole resistance among Candida species by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS)

Abstract

MALDI-TOF MS can rapidly identify microorganisms to the species level and may be able to detect antimicrobial resistance. We evaluated the ability of this technology to detect triazole resistance in Candida species. 35 C. albicans, 35 C. glabrata, and 37 C. tropicalis strains were exposed to fluconazole, voriconazole, or posaconazole at two different concentrations plus a drug-free control: a midrange concentration (CLSI clinical breakpoint or epidemiologic cut-off value), and a high concentration (fluconazole 64 μg/ml, voriconazole & posaconazole 16 μg/ml). The MALDI-TOF MS spectra at these concentrations were used to create the individual composite correlation index (CCI) matrices for each isolate. When the CCI of the midrange/highest concentration was lower than that of the midrange/null concentration, the strain was classified as resistant. These results were then compared to the classifications for susceptible or resistant obtained by measuring the MICs according to the CLSI M27-A3 antifungal susceptibility testing (AFST) method. The MALDI-TOF MS assay was able to classify triazole susceptibility against all strains. Overall, essential agreement between MALDI-TOF MS and AFST varied between 54% and 97%, and was highest for posaconazole against C. glabrata. The reproducibility of the MALDI-TOF MS assay varied between 54.3 and 82.9% and was best for fluconazole against C. albicans and posaconazole against C. glabrata. Reproducibility was also higher for C. glabrata isolates compared to C. albicans and C. tropicalis. These results demonstrate that MALDI-TOF MS may be used to simultaneously determine the Candida species and classification as susceptible or resistant to triazole antifungals. Further studies are needed to refine the methodology and improve the reproducibility of this assay.