The frequency of antifungal resistance, particularly to the azole class of ergosterol biosynthetic inhibitors, is a growing global health problem. Survival rates for those infected with resistant isolates are exceptionally low. Beyond modification of the drug target, our understanding of the molecular basis of azole resistance in the fungal pathogen Aspergillus fumigatus is limited. We reasoned that clinically relevant antifungal resistance could derive from transcriptional rewiring, promoting drug resistance without concomitant reductions in pathogenicity. Here we report a genome-wide annotation of transcriptional regulators in A. fumigatus and construction of a library of 484 transcription factor null mutants. We identify 12 regulators that have a demonstrable role in itraconazole susceptibility and show that loss of the negative cofactor 2 complex leads to resistance, not only to the azoles but also the salvage therapeutics amphotericin B and terbinafine without significantly affecting pathogenicity.

SummaryCYP51 encodes the cytochrome P450 sterol 14a-demethylase, an enzyme essential for sterol biosynthesis and the target of azole fungicides. In Fusarium species, including pathogens of humans and plants, three CYP51 paralogues have been identified with one unique to the genus. Currently, the functions of these three genes and the rationale for their conservation within the genus Fusarium are unknown.Three Fusarium graminearum CYP51s (FgCYP51s) were heterologously expressed in Saccharomyces cerevisiae. Single and double FgCYP51 deletion mutants were generated and the functions of the FgCYP51s were characterized in vitro and in planta.FgCYP51A and FgCYP51B can complement yeast CYP51 function, whereas FgCYP51C cannot. FgCYP51A deletion increases the sensitivity of F. graminearum to the tested azoles. In DFgCYP51B and DFgCYP51BC mutants, ascospore formation is blocked, and eburicol and two additional 14-methylated sterols accumulate. FgCYP51C deletion reduces virulence on host wheat ears.FgCYP51B encodes the enzyme primarily responsible for sterol 14a-demethylation, and plays an essential role in ascospore formation. FgCYP51A encodes an additional sterol 14a-demethylase, induced on ergosterol depletion and responsible for the intrinsic variation in azole sensitivity. FgCYP51C does not encode a sterol 14a-demethylase, but is required for full virulence on host wheat ears. This is the first example of the functional diversification of a fungal CYP51.

Plant fungal pathogens can have devastating effects on a wide range of crops, including cereals and fruit (such as wheat and grapes), causing losses in crop yield, which are costly to the agricultural economy and threaten food security. Azole antifungals are the treatment of choice; however, resistance has arisen against these compounds, which could lead to devastating consequences. Therefore, it is important to understand how these fungicides are used and how the resistance arises in order to tackle the problem fully. Here, we give an overview of the problem and discuss the mechanisms that mediate azole resistance in agriculture (point mutations in the CYP51 amino acid sequence, overexpression of the CYP51 enzyme and overexpression of genes encoding efflux pump proteins). © 2015 Society of Chemical Industry.

Candida auris has emerged as a multidrug-resistant pathogen of great clinical concern. Approximately 90% of clinical C. auris isolates are resistant to fluconazole, the most commonly prescribed antifungal agent, and yet it remains unknown what mechanisms underpin this fluconazole resistance. To identify novel mechanisms contributing to fluconazole resistance in C. auris, fluconazole-susceptible C. auris clinical isolate AR0387 was passaged in media supplemented with fluconazole to generate derivative strains which had acquired increased fluconazole resistance in vitro. Comparative analyses of comprehensive sterol profiles, [3H]fluconazole uptake, sequencing of C. auris genes homologous to genes known to contribute to fluconazole resistance in other species of Candida, and relative expression levels of C. auris ERG11, CDR1, and MDR1 were performed. All fluconazole-evolved derivative strains were found to have acquired mutations in the zinc-cluster transcription factor-encoding gene TAC1B and to show a corresponding increase in CDR1 expression relative to the parental clinical isolate, AR0387. Mutations in TAC1B were also identified in a set of 304 globally distributed C. auris clinical isolates representing each of the four major clades. Introduction of the most common mutation found among fluconazole-resistant clinical isolates of C. auris into fluconazole-susceptible isolate AR0387 was confirmed to increase fluconazole resistance by 8-fold, and the correction of the same mutation in a fluconazole-resistant isolate, AR0390, decreased fluconazole MIC by 16-fold. Taken together, these data demonstrate that C. auris can rapidly acquire resistance to fluconazole in vitro and that mutations in TAC1B significantly contribute to clinical fluconazole resistance. IMPORTANCE Candida auris is an emerging multidrug-resistant pathogen of global concern, known to be responsible for outbreaks on six continents and to be commonly resistant to antifungals. While the vast majority of clinical C. auris isolates are highly resistant to fluconazole, an essential part of the available antifungal arsenal, very little is known about the mechanisms contributing to resistance. In this work, we show that mutations in the transcription factor TAC1B significantly contribute to clinical fluconazole resistance. These studies demonstrated that mutations in TAC1B can arise rapidly in vitro upon exposure to fluconazole and that a multitude of resistance-associated TAC1B mutations are present among the majority of fluconazole-resistant C. auris isolates from a global collection and appear specific to a subset of lineages or clades. Thus, identification of this novel genetic determinant of resistance significantly adds to the understanding of clinical antifungal resistance in C. auris.

Sterol analysis identified four Candida albicans erg3 mutants in which ergosta 7,22-dienol, indicative of perturbations in sterol ⌬ 5,6 -desaturase (Erg3p) activity, comprised >5% of the total sterol fraction. The erg3 mutants (CA12, CA488, CA490, and CA1008) were all resistant to fluconazole, voriconazole, itraconazole, ketoconazole, and clotrimazole under standard CLSI assay conditions (MIC values, >256, 16, 16, 8, and 1 g ml ؊1 , respectively). Importantly, CA12 and CA1008 retained an azole-resistant phenotype even when assayed in the presence of FK506, a multidrug efflux inhibitor. Conversely, CA488, CA490, and three comparator isolates (CA6, CA14, and CA177, in which ergosterol comprised >80% of the total sterol fraction and ergosta 7,22-dienol was undetectable) all displayed azole-sensitive phenotypes under efflux-inhibited assay conditions. Owing to their ergosterol content, CA6, CA14, and CA177 were highly sensitive to amphotericin B (MIC values, <0.25 g ml ؊1 ); CA1008, in which ergosterol comprised <2% of the total sterol fraction, was less sensitive (MIC, 1 g ml ؊1 ). CA1008 harbored multiple amino acid substitutions in Erg3p but only a single conserved polymorphism (E266D) in sterol 14␣-demethylase (Erg11p). CA12 harbored one substitution (W332R) in Erg3p and no residue changes in Erg11p. CA488 and CA490 were found to harbor multiple residue changes in both Erg3p and Erg11p. The results suggest that missense mutations in ERG3 might arise in C. albicans more frequently than currently supposed and that the clinical significance of erg3 mutants, including those in which additional mechanisms also contribute to resistance, should not be discounted.