Towards standardized antifungal susceptibility testing for Sporothrix brasiliensis

Sporotrichosis caused by Sporothrix brasiliensis is increasingly reported in South America. Infections in humans are mostly due to zoonotic transmission by cats. For both hosts, itraconazole is the drug of choice despite long treatment duration. Resistance surveillance is warranted, but protocols for antifungal susceptibility testing (AFST) are not always harmonized between laboratories largely due to Sporothrix species being thermally dimorphic and the absence of well-defined consensus methodologies to address how to deal with yeast/mould phases. To standardize susceptibility testing methodology, we propose to modify the current Clinical Laboratory and Standards Institute (CLSI) M38 broth microdilution methodology with S. brasiliensis culture for 7 days at 30°C (rather than 35°C) on potato dextrose agar (PDA), followed by microscopic confirmation of pure mycelial phase. Incubation temperature would also be modified to 30°C for 72 hours. By following this standardized method for at least itraconazole, terbinafine and amphotericin B, S. brasiliensis non-wild-type isolates and correlations with clinical outcomes can be properly investigated. The proposed method is a consensus-oriented interim approach rather than a fully validated standard.