Comparison of in vitro Susceptibilities of Talaromyces marneffei in Mold and Yeast Forms in Malaysia

PURPOSE: This study was aimed to determine minimum inhibitory concentration (MIC) differences between yeast and mold forms of T. marneffei in Malaysia.

PATIENTS AND METHODS: Ninety-seven clinical strains of T. marneffei were received from various Malaysian hospitals from the year 2020 until 2022. Their identities were determined using microscopic, macroscopic and molecular methods. Next, the susceptibility of yeast and mold forms of each isolate against amphotericin B, itraconazole, voriconazole, posaconazole, ketoconazole, isavuconazole, terbinafine, caspofungin and micafungin were tested according to the broth microdilution according to the Clinical and Laboratory Standards Institute (CLSI) M38 and M27 guidelines. The geometric means of minimal inhibitory concentration (GM MIC), MIC50, and MIC90 were determined for each antifungal. Additionally, Wilcoxon signed-rank test was used to compare the significant difference of GM MICs for each antifungal, GM MIC, MIC50 and MIC90 for the combined nine antifungals against different growth forms of T. marneffei. The significance was set at p<0.05.

RESULTS: Micafungin had the highest GM MIC, MIC50 and MIC90 for mold form of T. marneffei. For yeast form, amphotericin B achieved the highest GM MIC and MIC50 while micafungin achieved the highest MIC90. However, the GM MIC, MIC50 and MIC90 of terbinafine and azole antifungals on T. marneffei were similar to each other, namely between 0.03 and 0.60µg/mL. The difference of GM MIC of all tested antifungals except caspofungin and micafungin was insignificant. Overall, GM MIC, MIC50 and MIC90 of the combined nine antifungals against two growth forms were insignificant.

CONCLUSION: The findings suggested either yeast or mold form can be used in the susceptibility testing of T. marneffei against amphotericin B, itraconazole, voriconazole, posaconazole, ketoconazole, isavuconazole and terbinafine.