Genetic prerequisites for additive or synergistic actions of 5-fluorocytosine and fluconazole in baker's yeast

Toxic eburicol accumulation drives the antifungal activity of azoles against Aspergillus fumigatus

Azole antifungals inhibit the sterol C14-demethylase (CYP51/Erg11) of the ergosterol biosynthesis pathway. Here we show that the azole-induced synthesis of fungicidal cell wall carbohydrate patches in the pathogenic mold Aspergillus fumigatus strictly correlates with the accumulation of the CYP51 substrate eburicol. A lack of other essential ergosterol biosynthesis enzymes, such as sterol C24-methyltransferase (Erg6A), squalene synthase (Erg9) or squalene epoxidase (Erg1) does not trigger comparable cell wall alterations. Partial repression of Erg6A, which converts lanosterol into eburicol, increases azole resistance. The sterol C5-desaturase (ERG3)-dependent conversion of eburicol into 14-methylergosta-8,24(28)-dien-3β,6α-diol, the "toxic diol" responsible for the fungistatic activity against yeasts, is not required for the fungicidal effects in A. fumigatus. While ERG3-lacking yeasts are azole resistant, ERG3-lacking A. fumigatus becomes more susceptible. Mutants lacking mitochondrial complex III functionality, which are much less effectively killed, but strongly inhibited in growth by azoles, convert eburicol more efficiently into the supposedly "toxic diol". We propose that the mode of action of azoles against A. fumigatus relies on accumulation of eburicol which exerts fungicidal effects by triggering cell wall carbohydrate patch formation.

Factors enforcing the species boundary between the human pathogens Cryptococcus neoformans and Cryptococcus deneoformans

Hybridization has resulted in the origin and variation in extant species, and hybrids continue to arise despite pre- and post-zygotic barriers that limit their formation and evolutionary success. One important system that maintains species boundaries in prokaryotes and eukaryotes is the mismatch repair pathway, which blocks recombination between divergent DNA sequences. Previous studies illuminated the role of the mismatch repair component Msh2 in blocking genetic recombination between divergent DNA during meiosis. Loss of Msh2 results in increased interspecific genetic recombination in bacterial and yeast models, and increased viability of progeny derived from yeast hybrid crosses. Hybrid isolates of two pathogenic fungal Cryptococcus species, Cryptococcus neoformans and Cryptococcus deneoformans, are isolated regularly from both clinical and environmental sources. In the present study, we sought to determine if loss of Msh2 would relax the species boundary between C. neoformans and C. deneoformans. We found that crosses between these two species in which both parents lack Msh2 produced hybrid progeny with increased viability and high levels of aneuploidy. Whole-genome sequencing revealed few instances of recombination among hybrid progeny and did not identify increased levels of recombination in progeny derived from parents lacking Msh2. Several hybrid progeny produced structures associated with sexual reproduction when incubated alone on nutrient-rich medium in light, a novel phenotype in Cryptococcus. These findings represent a unique, unexpected case where rendering the mismatch repair system defective did not result in increased meiotic recombination across a species boundary. This suggests that alternative pathways or other mismatch repair components limit meiotic recombination between homeologous DNA and enforce species boundaries in the basidiomycete Cryptococcus species.

Several mechanisms enforce species boundaries by either preventing the formation of hybrid zygotes, known as pre-zygotic barriers, or preventing the viability and fecundity of hybrids, known as post-zygotic barriers. Despite these barriers, interspecific hybrids form at an appreciable frequency, such as hybrid isolates of the human fungal pathogenic species, Cryptococcus neoformans and Cryptococcus deneoformans, which are regularly isolated from both clinical and environmental sources. C. neoformans x C. deneoformans hybrids are typically highly aneuploid, sterile, and display phenotypes intermediate to those of either parent, although self-fertile isolates and transgressive phenotypes have been observed. One important mechanism known to enforce species boundaries or lead to incipient speciation is the DNA mismatch repair system, which blocks recombination between divergent DNA sequences during meiosis. The aim of this study was to determine if genetically deleting the DNA mismatch repair component Msh2 would relax the species boundary between C. neoformans and C. deneoformans. Progeny derived from C. neoformans x C. deneoformans crosses in which both parental strains lacked Msh2 had higher viability, and unlike previous studies in Saccharomyces, these Cryptococcus hybrid progeny had higher levels of aneuploidy and no observable increase in meiotic recombination at the whole-genome level.

Gene Duplication Associated with Increased Fluconazole Tolerance in Candida auris cells of Advanced Generational Age

Candida auris is an emerging multi-drug resistant yeast that causes systemic infections. Here we show that C. auris undergoes replicative aging (RA) that results from asymmetric cell division and causes phenotypic differences between mother and daughter cells similar to other pathogenic yeasts. Importantly, older C. auris cells (10 generations) exhibited higher tolerance to fluconazole (FLC), micafungin, 5- flucytosine and amphotericin B compared to younger (0–3 generation) cells. Increased FLC tolerance was associated with increased Rhodamine 6G (R6G) efflux and therapeutic failure of FLC in a Galleria infection model. The higher efflux in the older cells correlated with overexpression of the efflux pump encoding gene CDR1 (4-fold). In addition, 8-fold upregulation of the azole target encoding gene ERG11 was noted in the older cells. Analysis of genomic DNA from older cells by qPCR indicates that transient gene duplication of CDR1 and ERG11 causes the observed age-dependent enhanced FLC tolerance in C. auris strains. Furthermore, older cells exhibited a thickened cell wall, decreased neutrophil killing (24% vs 50%), increased epithelial cell adhesion (31.6% vs 17.8%) and upregulation of adhesin protein Als5p. Thus, this study demonstrates that transient gene duplication can occur during RA, causing increased FLC tolerance in old C. auris cells.

A Ser29Leu substitution in the cytosine deaminase Fca1p is responsible for clade-specific flucytosine resistance in Candida dubliniensis

The population structure of the opportunistic yeast pathogen Candida dubliniensis is composed of three main multilocus sequence typing clades (clades C1 to C3), and clade C3 predominantly consists of isolates from the Middle East that exhibit high-level resistance (MIC(50) > or = 128 microg/ml) to the fungicidal agent flucytosine (5FC). The close relative of C. dubliniensis, C. albicans, also exhibits clade-specific resistance to 5FC, and resistance is most commonly mediated by an Arg101Cys substitution in the FUR1 gene encoding uracil phosphoribosyltransferase. Broth microdilution assays with fluorouracil (5FU), the toxic deaminated form of 5FC, showed that both 5FC-resistant and 5FC-susceptible C. dubliniensis isolates exhibited similar 5FU MICs, suggesting that the C. dubliniensis cytosine deaminase (Fca1p) encoded by C. dubliniensis FCA1 (CdFCA1) may play a role in mediating C. dubliniensis clade-specific 5FC resistance. Amino acid sequence analysis of the CdFCA1 open reading frame (ORF) identified a homozygous Ser29Leu substitution in all 12 5FC-resistant isolates investigated which was not present in any of the 9 5FC-susceptible isolates examined. The tetracycline-inducible expression of the CdFCA1 ORF from a 5FC-susceptible C. dubliniensis isolate in two separate 5FC-resistant clade C3 isolates restored susceptibility to 5FC, demonstrating that the Ser29Leu substitution was responsible for the clade-specific 5FC resistance and that the 5FC resistance encoded by FCA1 genes with the Ser29Leu transition is recessive. Quantitative real-time PCR analysis showed no significant difference in CdFCA1 expression between 5FC-susceptible and 5FC-resistant isolates in either the presence or the absence of subinhibitory concentrations of 5FC, suggesting that the Ser29Leu substitution in the CdFCA1 ORF is the sole cause of 5FC resistance in clade C3 C. dubliniensis isolates.