In Vitro Antifungal Susceptibility Of Isolates Of Sporothrix Schenckii By Microdilution Broth Method

Objective Evaluate the convertion rate of mycelial phase to yeast phase of Sporothrix shenckii isolates after successive passages. Test the in vitro susceptibility of potassium iodide, fluconazole, itraconazole, terbinafine to Sporothrix shenckii, and investigate the in vitro effect of potassium iodide to itraconazole and terbinafine. Methods We incubated ninety five clinical isolates of Sporothrix shenckii at 37℃on BHIA, subcultivation was done every ten days, and recorded the successive passages conversion rate. Then according to a modified CLSI microdilution method M38-A and M27-A2, we tested the in vitro susceptibility of fifty four isolates of Sporothrix shenckii against potassium iodide, fluconazole, itraconazole, terbinafine. Mycelial phase was incubated at 30℃, and yeast phase at 35℃. We added potassium iodide in RPMI-1640, compared the in vitro susceptibility results of itraconazole and terbinafine with results of the former. The ATCC 6258 were used for quality control. Result The conversion rate is 24.2%, 38.9%, 49.5%, 55.8% for one to four successive passages. Potassium iodide does not have the in vitro antifungal function. The MIC of terbinafine is the best, with GM of 0.30μg/ml and 0.17μg/ml for mycelial phase and yeast phase, the MIC ranges are both 0.03～1μg/ml for two phases. The MIC of itraconazole is 0.06～8μg/ml(1.33μg/ml) for mycelial phase and 0.03～8μg/ml(0.98μg/ml) for yeast phase. Fluconazole has a high MIC of >64μg/ml. With potassium iodide in RPMI-1640, the MICs of terbinafine are significant different from the normal results (p<0.05). There is no significant differences between MICs of isolates from cutaneous sporotrichosis and lymphocutaneous sporotrichosis (p>0.05). Conclusion It needs several successive passages for Sporothrix schenckii mycelial phase convert to yeast phase at 37℃, and continuous subcultivation can improve the conversion rate. Potassium iodide can improve the in vitro antifungal susceptibility of terbinafine. Further studies are expected to continue analyze the mechanism of potassium iodide. M27-A2 is an alternative method for testing yeast phase of Sporothrix schenckii.