Loss of sterol demethylase activity reduces both azole and echinocandin antifungal susceptibility in a clinical isolate of C. parapsilosis

The present study describes a novel mechanism of antifungal resistance affecting the susceptibility of both the azole and echinocandin antifungals in an azole-resistant isolate from a matched pair of Candida parapsilosis isolates obtained from a patient with prosthetic valve endocarditis. Whole genome sequencing revealed a mutation in the ERG3 gene leading to a G111R amino acid substitution in the resistant isolate. Subsequent introduction of this allele in the native ERG3 locus in the susceptible isolate resulted in a fluconazole MIC of >64 ug/ml and a caspofungin MIC of 8 ug/ml. Corresponding allelic replacement of the wildtype allele for the mutant allele in the resistant isolate resulted in a drop in MIC to 1 ug/ml for both fluconazole and caspofungin. Sterol profiles indicated a loss of sterol demethylase activity as a result of this mutation. This work demonstrate that this G111R mutation is wholly responsible for the resistant phenotype in the C. parapsilosis resistant isolate and is the first report of this multidrug resistance mechanism.