In Vitro Susceptibility of Sporothrix schenckii to Six Antifungal Agents Determined Using Three Different Methods

Abstract

The in vitro susceptibility of Sporothrix schenckii to antifungal drugs has been determined with three different methods. Nineteen Peruvian clinical isolates of S. schenckii were tested against amphotericin B (AB), flucytosine (FC), fluconazole (FZ), itraconazole (IZ), voriconazole (VZ), and ketoconazole (KZ). Modified NCCLS M38-A, Sensititre YeastOne (SYO), and ATB Fungus 2 (ATBF2) methods were used to determine the MICs. ATCC isolates of Candida parapsilosis, Candida krusei, and Aspergillus flavus were used for quality control. Sporothrix inocula were prepared with the mycelial form growing on potato dextrose agar at 28 +/- 2 degrees C. MICs of AB, FC, FZ, and IZ were determined with all three methods, VZ with M38-A and SYO, and KZ with only SYO. The three methods showed high MICs of FZ and FC (MIC(90) of 0.5 microg/ml), being homogeneously lower than those of IZ and KZ. The M38-A method showed a variable MIC range of VZ (4.0 to 16 microg/ml); the geometric mean (GM) was 9.3 mug/ml. The MIC range of AB was wide (0.06 to 16 microg/ml), but the GM was 1.2 microg/ml, suggesting that the MIC is strain dependent. Agreement (two log(2) dilutions) between commercial techniques and the modified M38-A method was very high with FZ, IZ, and FC. In AB and VZ, the agreement was lower, being related to the antifungal concentrations of each method. The highest activity against S. schenckii was found with IZ and KZ. Lack of activity was observed with FZ, VZ, and FC. When AB is indicated for sporotrichosis, the susceptibility of the strain must be analyzed. Commercial quantitative antifungal methods have a limited usefulness in S. schenckii.