In vitro antifungal susceptibility of Sporothrix globosa isolates from Jilin Province, northeastern China: comparison of yeast and mycelial phases

The dimorphic fungus Sporothrix globosa is the predominant etiologic agent causing sporotrichosis in China, particularly in the northeast. It has been demonstrated that the incubation temperature and growth phase can influence in vitro antifungal susceptibility profiles of S. schenckii sensu stricto and S. brasiliensis (sibling species of S. globosa). Few studies have reported on the antifungal susceptibility of S. globosa, especially using large numbers of isolates. In this study, we assessed the susceptibility of 80 isolates of S. globosa originating from Jilin Province, northeastern China, to six antifungal agents (itraconazole, terbinafine, voriconazole, posaconazole, fluconazole, and amphotericin B), at varying incubation temperatures and in different fungal growth phases. The isolates were most sensitive to terbinafine (geometric mean [GM] of the minimum inhibitory concentration [MIC]: 0.0356 μg/ml for the mycelial phase at 30 °C, 0.0332 μg/ml for the mycelial phase at 35 °C, and 0.031 μg/ml for the yeast phase, respectively), followed by posaconazole (GM of the MIC: 4.2501 μg/ml for the mycelial phase at 30 °C, 1.4142 μg/ml for the mycelial phase at 35 °C, and 0.7195 μg/ml for the yeast phase, respectively) and itraconazole (GM of the MIC: 6.8448 μg/ml for the mycelial phase at 30 °C, 3.1383 μg/ml for the mycelial phase at 35 °C, and 1.0263 μg/ml for the yeast phase, respectively). The isolates were relatively resistant to fluconazole (GM of the MIC: 76.7716 μg/ml for the mycelial phase at 30 °C, 66.2570 μg/ml for the mycelial phase at 35 °C, and 24.4625 μg/ml for the yeast phase, respectively) and voriconazole (GM of the MIC: 26.2183 μg/ml for the mycelial phase at 30 °C, 13.6895 μg/ml for the mycelial phase at 35 °C, and 1.3899 μg/ml for the yeast phase, respectively). For all the tested azole drugs, the MICs at 30 °C were significantly higher than those at 35 °C (P < 0.001); for all agents except terbinafine, the MICs of S. globosa in the yeast phase were significantly lower than those of the strains in the mycelial phase (P < 0.001). These results show that the sensitivities of S. globosa to antifungal compounds are dependent on incubation temperature and growth phase. To the best of our knowledge, this is the largest study of antifungal susceptibility of S. globosa isolates reported to date. To establish epidemiological cutoff values for S. globosa, further antifungal susceptibility testing studies by independent laboratories located in different regions and using uniform conditions are required.