The wide use of antifungal prophylaxis might trigger the emergence of rare yeast species that are resistant against commonly used yeast-active agents, such as fluconazole and echinocandins. Among this rare, but emerging yeasts are Candida ciferrii, C. inconspicua, and C. rugosa. Although studies suggest a very low incidence of these species, they are of medical great interest as infections with them are associated with unfavorable patient outcome. Individual case reports suggest that C. ciferii isolates have a reduced susceptibility against fluconazole, amphotericin B, caspofungin, and anidudalfungin, while against C. rugosa and C. inconspicua voriconazole and fluconazole had limited activity. Due to the rareness of C. ciferii, C. inconspicua, and C. rugosa comprehensive studies on antifungal susceptibility are missing and epidemiological cut-off values (ECOFFs) and clinical breakpoints (CBP) are missing for both reference methods, the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the Clinical & Laboratory Standards Institute (CLSI).
Therefore, the aim of our study is to increase the knowledge on the best method used for antifungal susceptibility testing (AST), minimal inhibitory concentrations (MIC) distributions, and underlying molecular mechanism of resistance.
To this aim, we will perform AST from a comprehensive test set and study the molecular mechanism involved in antifungal resistance. Currently AST are performed for C. ciferrii, C. inconspicua and C. rugosa using EUCAST broth micro dilution and the commercial assay E-test. Data from the first hundred tested isolates show that all species have a normal distribution for all antimycotics with some outliers (isolates with high MIC values) for echinocandins. Moreover, our data confirm that all three species are resistant against fluconazole.
After finalizing the AST, and categorizing isolates into wild type and mutant, we will study the molecular mechanisms of resistance. The common causes of resistance in yeast are modification in the target molecule lanosterol 14-α-demethylase (ERG11; primary target of azoles) and 1,3-beta-glucan synthase (FKS1 and FKS2; primary targets of echinocandins) and up regulation of membrane transporter proteins (CDR1,CDR2, SNQ2). Whole genome sequencing of the three species will be performed using next generation sequencing, we will search for homologue genes and search for SNPs (single nucleotide polymorphism) that could explain the resistance of these species on a molecular level.