In vivo development of fluconazole resistance in serial Cryptococcus gattii isolates from a cat

Elevated fluconazole minimum inhibitory concentrations (MICs) are more frequently observed in Cryptococcus gattii compared to C. neoformans isolates; however, the development of in vivo resistance and the molecular mechanisms responsible have not been reported for this species. We report a case of Cryptococcus gattii (molecular type VGIII) that developed reduced susceptibility to fluconazole during therapy and delineate the molecular mechanisms responsible. Multilocus sequence typing and quantitative DNA analysis of the pre- and post-treatment isolates was performed using well-characterized methods. Pre- and post-treatment clinical isolates were confirmed isogenic, and no differences in ERG11 or PDR11 sequences were found. qPCR found an overexpression of ERG11 and the efflux pump PDR11 in the resistant isolate compared to the isolate collected prior to initiation of antifungal therapy. Reversion to wild-type susceptibility was observed when maintained in antifungal-free media confirming the in vivo development of heteroresistance. The in vivo development of heteroresistance to fluconazole in our patient with C. gattii is secondary to overexpression of the efflux pump PDR11 and the drug target ERG11. Additional work in other clinical isolates with elevated fluconazole MICs is warranted to evaluate the frequency of heteroresistance versus point mutations as a cause of resistance.