Fluconazole exposure rather than clonal spreading is correlated with the emergence of Candida glabrata with cross-resistance to triazole antifungal agents

Abstract

The emergence of antifungal resistance in Candida species has raised concern in recent years, especially resistance toward triazole. Several newer triazole antifungal agents have been introduced which have a broader spectrum for fungal infections, such as voriconazole. However, cross-resistance among triazoles is a major concern with regard to their clinical application. Antifungal susceptibility was performed using E-test for 166 clinical isolates (29 blood and 137 nonblood isolates) in 2003 and 2004. We applied pulsed-field gel electrophoresis for genotyping. Ninety isolates of C. albicans, 47 isolates of C. tropicalis, 27 isolates of C. glabrata, and two isolates of C. krusei were included. All isolates were susceptible to amphotericin B. Eleven (40.7%) of the 27 C. glabrata had intermediate resistance to caspofungin. Forty-seven (28.3%) of the 166 isolates were not susceptible to fluconazole, including two C. albicans, 16 C. tropicalis, 27 C. glabrata, and two C. krusei isolates. All except seven of the C. glabrata isolates were susceptible to voriconazole. All the triazole drugs had a positive correlation among their minimum inhibitory concentrations (MICs). Fluconazole MIC was a good predictor for susceptibility to voriconazole, as determined using a receiver operating characteristic curve. Furthermore, a high diversity of pulsotypes for the 27 clinical isolates of C. glabrata was observed. Previous fluconazole exposure within 3 months was associated with reduced triazole susceptibility for C. glabrata. We demonstrated a significant positive correlation of MIC values among the four tested triazole drugs. No amphotericin B and caspofungin resistant isolates were found in this study. The cross-resistance to triazole among C. glabrata isolates was associated with previous fluconazole exposure as opposed to clonal spreading. Selection pressure due to fluconazole use may play a major role in triazole cross-resistance.

Figure 1

Scatterplot matrix and pairwise correlation of log₂ values for the minimum inhibitory concentrations of the six tested antifungal agents.

Figure 2

(A) Using the ROC curve method to determine cut‐off MIC values of surrogate markers—fluconazole, itraconazole, or ketoconazole—for voriconazole nonsusceptibility of C. glabrata; (B) using the ROC curve method to determine cut‐off MIC values of surrogate markers—fluconazole, itraconazole, or ketoconazole—for voriconazole nonsusceptibility of the Candida species. MIC = minimum inhibitory concentration; ROC = receiver operating characteristic.

Figure 3

Dendrogram of pulsed field gel electrophoresis results for 27 C. glabrata isolates.