Aspergillus fumigatus CYP51 sequence: potential basis for fluconazole resistance

Emergence of Triazole Resistance in Aspergillus spp. in Latin America

Purpose of Review

Azole resistance in Aspergillus spp. is becoming a public health problem worldwide. However, data about this subject is lacking in Latin American countries. This review focuses in the epidemiology and molecular mechanisms of azole resistance in Aspergillus spp. emphasizing in Latin America. Data on Aspergillus fumigatus stands out because it is the most prevalent Aspergillus spp. pathogen.

Recent Findings

Azole resistance in Aspergillus spp. emergence was linked with intensive use of these antifungals both in the clinical setting and in the environment (as pesticides). Reports on azole-resistant A. fumigatus strains are being constantly published in different countries. Molecular mechanisms of resistance mainly involve substitution in the azole target (CYP51A) and/or overexpression of this gene. However, several other non-CYP51A-related mechanisms were described. Moreover, intrinsically resistant cryptic Aspergillus species are starting to be reported as human pathogens.

Summary

After a comprehensive literature review, it is clear that azole resistance in Aspergillus spp. is emerging in Latin America and perhaps it is underestimated. All the main molecular mechanisms of azole resistance were described in patients and/or environmental samples. Moreover, one of the molecular mechanisms was described only in South America. Cryptic intrinsic azole-resistant species are also described.

Characterization of Three Pleiotropic Drug Resistance Transporter Genes and Their Participation in the Azole Resistance of Mucor circinelloides

Mucormycosis is a life-threatening opportunistic infection caused by certain members of the fungal order Mucorales. This infection is associated with high mortality rate, which can reach nearly 100% depending on the underlying condition of the patient. Treatment of mucormycosis is challenging because these fungi are intrinsically resistant to most of the routinely used antifungal agents, such as most of the azoles. One possible mechanism of azole resistance is the drug efflux catalyzed by members of the ATP binding cassette (ABC) transporter superfamily. The pleiotropic drug resistance (PDR) transporter subfamily of ABC transporters is the most closely associated to drug resistance. The genome of Mucor circinelloides encodes eight putative PDR-type transporters. In this study, transcription of the eight pdr genes has been analyzed after azole treatment. Only the pdr1 showed increased transcript level in response to all tested azoles. Deletion of this gene caused increased susceptibility to posaconazole, ravuconazole and isavuconazole and altered growth ability of the mutant. In the pdr1 deletion mutant, transcript level of pdr2 and pdr6 significantly increased. Deletion of pdr2 and pdr6 was also done to create single and double knock out mutants for the three genes. After deletion of pdr2 and pdr6, growth ability of the mutant strains decreased, while deletion of pdr2 resulted in increased sensitivity against posaconazole, ravuconazole and isavuconazole. Our result suggests that the regulation of the eight pdr genes is interconnected and pdr1 and pdr2 participates in the resistance of the fungus to posaconazole, ravuconazole and isavuconazole.

Transcriptome analysis of fungicide-responsive gene expression profiles in two Penicillium italicum strains with different response to the sterol demethylation inhibitor (DMI) fungicide prochloraz

Background

Penicillium italicum (blue mold) is one of citrus pathogens causing undesirable citrus fruit decay even at strictly-controlled low temperatures (< 10 °C) during shipping and storage. P. italicum isolates with considerably high resistance to sterol demethylation inhibitor (DMI) fungicides have emerged; however, mechanism(s) underlying such DMI-resistance remains unclear. In contrast to available elucidation on anti-DMI mechanism for P. digitatum (green mold), how P. italicum DMI-resistance develops has not yet been clarified.

Results

The present study prepared RNA-sequencing (RNA-seq) libraries for two P. italicum strains (highly resistant (Pi-R) versus highly sensitive (Pi-S) to DMI fungicides), with and without prochloraz treatment, to identify prochloraz-responsive genes facilitating DMI-resistance. After 6 h prochloraz-treatment, comparative transcriptome profiling showed more differentially expressed genes (DEGs) in Pi-R than Pi-S. Functional enrichments identified 15 DEGs in the prochloraz-induced Pi-R transcriptome, simultaneously up-regulated in P. italicum resistance. These included ATP-binding cassette (ABC) transporter-encoding genes, major facilitator superfamily (MFS) transporter-encoding genes, ergosterol (ERG) anabolism component genes ERG2, ERG6 and EGR11 (CYP51A), mitogen-activated protein kinase (MAPK) signaling-inducer genes Mkk1 and Hog1, and Ca²⁺/calmodulin-dependent kinase (CaMK) signaling-inducer genes CaMK1 and CaMK2. Fragments Per Kilobase per Million mapped reads (FPKM) analysis of Pi-R transcrtiptome showed that prochloraz induced mRNA increase of additional 4 unigenes, including the other two ERG11 isoforms CYP51B and CYP51C and the remaining kinase-encoding genes (i.e., Bck1 and Slt2) required for Slt2-MAPK signaling. The expression patterns of all the 19 prochloraz-responsive genes, obtained in our RNA-seq data sets, have been validated by quantitative real-time PCR (qRT-PCR). These lines of evidence in together draw a general portrait of anti-DMI mechanisms for P. italicum species. Intriguingly, some strategies adopted by the present Pi-R were not observed in the previously documented prochloraz-resistant P. digitatum transcrtiptomes. These included simultaneous induction of all major EGR11 isoforms (CYP51A/B/C), over-expression of ERG2 and ERG6 to modulate ergosterol anabolism, and concurrent mobilization of Slt2-MAPK and CaMK signaling processes to overcome fungicide-induced stresses.

Conclusions

The present findings provided transcriptomic evidence on P. italicum DMI-resistance mechanisms and revealed some diversity in anti-DMI strategies between P. italicum and P. digitatum species, contributing to our knowledge on P. italicum DMI-resistance mechanisms.

Effect of amphotericin B and voriconazole on the outgrowth of conidia of Aspergillus fumigatus followed by time-lapse microscopy

Studies of morphological measurements from the outgrowth of cells to a network of hyphae have been extended from Candida albicans (Nagy et al. in Appl Microbiol Biotechnol 98(11):5185-5194. https://doi.org/10.1007/s00253-014-5696-5 , 2014) to invasive conidiospores of Aspergillus fumigatus upon treatment with antifungal agents. The understanding of mycelial processes is important to optimize industrial processes such as fermentation and contributes to the fight against pathogenic fungi. This brief study combines TLS with digital image analysis. The TLS system was adapted to get information related to the adherence and growth dynamics of filamentous fungi. This approach was used earlier to distinguish among subphases of bacterial and fungal infections of mammal cells by detecting Mycoplasma infection in cell cultures causing serious damages in cell cultures. We describe changes in adherence, germination of spores, and hyphal growth of A. fumigatus, taking place in the absence and presence of amphotericin B (AMB) and voriconazole (VRC). These growth parameters were measured by TLS in CO₂ incubator under physiological Photomicrography by TLS and extended for a longer period of time up to several weeks combined with image analysis represents a comfortable and reliable means to characterize the growth dynamism of A. fumigatus. The most important observation of medical importance related to the pathomechanism of VRC was that it did not adhere to conidiospores, i.e. that it did not contribute to the attachment of spores to the growth surface, and did not prevent germination but delayed hypha protrusion and elongation. In contrast AMB adhered to conidia, inhibited germination, hypha elongation and branching. It was concluded that AMB was efficient against the therapy of growth but not against the prevention of fungal infection.

Intrinsic short-tailed azole resistance in mucormycetes is due to an evolutionary conserved aminoacid substitution of the lanosterol 14α-demethylase

Mucormycoses are emerging and potentially lethal infections. An increase of breakthrough infections has been found in cohorts receiving short-tailed azoles prophylaxis (e.g. voriconazole (VCZ)). Although VCZ is ineffective in vitro and in vivo, long-tailed triazoles such as posaconazole remain active against mucormycetes. Our goal was to validate the molecular mechanism of resistance to short-tailed triazoles in Mucorales. The paralogous cytochrome P450 genes (CYP51 F1 and CYP51 F5) of Rhizopus arrhizus, Rhizopus microsporus, and Mucor circinelloides were amplified and sequenced. Alignment of the protein sequences of the R. arrhizus, R. microsporus, and M. circinelloides CYP51 F1 and F5 with additional Mucorales species (n = 3) and other fungi (n = 16) confirmed the sequences to be lanosterol 14α-demethylases (LDMs). Sequence alignment identified a pan-Mucorales conservation of a phenylalanine129 substitution in all CYP51 F5s analyzed. A high resolution X-ray crystal structure of Saccharomyces cerevisiae LDM in complex with VCZ was used for generating a homology model of R. arrhizus CYP51 F5. Structural and functional knowledge of S. cerevisiae CYP51 shows that the F129 residue in Mucorales CYP51 F5 is responsible for intrinsic resistance of Mucorales against short-tailed triazoles, with a V to A substitution in Helix I also potentially playing a role.

Molecular Tools for the Detection and Deduction of Azole Antifungal Drug Resistance Phenotypes in Aspergillus Species

The incidence of azole resistance in Aspergillus species has increased over the past years, most importantly for Aspergillus fumigatus. This is partially attributable to the global spread of only a few resistance alleles through the environment. Secondary resistance is a significant clinical concern, as invasive aspergillosis with drug-susceptible strains is already difficult to treat, and exclusion of azole-based antifungals from prophylaxis or first-line treatment of invasive aspergillosis in high-risk patients would dramatically limit drug choices, thus increasing mortality rates for immunocompromised patients. Management options for invasive aspergillosis caused by azole-resistant A. fumigatus strains were recently reevaluated by an international expert panel, which concluded that drug resistance testing of cultured isolates is highly indicated when antifungal therapy is intended. In geographical regions with a high environmental prevalence of azole-resistant strains, initial therapy should be guided by such analyses. More environmental and clinical screening studies are therefore needed to generate the local epidemiologic data if such measures are to be implemented on a sound basis. Here we propose a first workflow for evaluating isolates from screening studies, and we compile the MIC values correlating with individual amino acid substitutions in the products of cyp51 genes for interpretation of DNA sequencing data, especially in the absence of cultured isolates.

The Role of In Vitro Susceptibility Testing in the Management of Candida and Aspergillus

Antifungal susceptibility testing has evolved from a research technique to a standardized and well-validated tool for the clinical management of fungal infections and for epidemiological studies. Genetic mutations and phenotypic resistance in vitro have been shown to correlate with clinical outcomes and treatment failures, and this in turn has led to the creation of clinical breakpoints and, more recently, epidemiological cutoff values for clinically relevant fungal pathogens. Resistance mechanisms for Candida and Aspergillus species have been extensively described and their corresponding genetic mutations can now be readily detected. Epidemiological studies have been able to detect the emergence of regional clonal and nonclonal resistance in several countries. The clinical microbiology laboratory is expected to transition from culture and traditional susceptibility testing to molecular methods for detection, identification, and resistance profiling over the next 5-10 years.

Aspergillus fumigatus Intrinsic Fluconazole Resistance Is Due to the Naturally Occurring T301I Substitution in Cyp51Ap

Aspergillus fumigatus intrinsic fluconazole resistance has been demonstrated to be linked to the CYP51A gene, although the precise molecular mechanism has not been elucidated yet. Comparisons between A. fumigatus Cyp51Ap and Candida albicans Erg11p sequences showed differences in amino acid residues already associated with fluconazole resistance in C. albicans The aim of this study was to analyze the role of the natural polymorphism I301 in Aspergillus fumigatus Cyp51Ap in the intrinsic fluconazole resistance phenotype of this pathogen. The I301 residue in A. fumigatus Cyp51Ap was replaced with a threonine (analogue to T315 at Candida albicans fluconazole-susceptible Erg11p) by changing one single nucleotide in the CYP51A gene. Also, a CYP51A knockout strain was obtained using the same parental strain. Both mutants' antifungal susceptibilities were tested. The I301T mutant exhibited a lower level of resistance to fluconazole (MIC, 20 μg/ml) than the parental strain (MIC, 640 μg/ml), while no changes in MIC were observed for other azole- and non-azole-based drugs. These data strongly implicate the A. fumigatus Cyp51Ap I301 residue in the intrinsic resistance to fluconazole.

Structural Insights into Binding of the Antifungal Drug Fluconazole to Saccharomyces cerevisiae Lanosterol 14α-Demethylase

Infections by fungal pathogens such as Candida albicans and Aspergillus fumigatus and their resistance to triazole drugs are major concerns. Fungal lanosterol 14α-demethylase belongs to the CYP51 class in the cytochrome P450 superfamily of enzymes. This monospanning bitopic membrane protein is involved in ergosterol biosynthesis and is the primary target of azole antifungal drugs, including fluconazole. The lack of high-resolution structural information for this drug target from fungal pathogens has been a limiting factor for the design of modified triazole drugs that will overcome resistance. Here we report the X-ray structure of full-length Saccharomyces cerevisiae lanosterol 14α-demethylase in complex with fluconazole at a resolution of 2.05 Å. This structure shows the key interactions involved in fluconazole binding and provides insight into resistance mechanisms by revealing a water-mediated hydrogen bonding network between the drug and tyrosine 140, a residue frequently found mutated to histidine or phenylalanine in resistant clinical isolates.

Azole drug import into the pathogenic fungus Aspergillus fumigatus

The fungal pathogen Aspergillus fumigatus causes serious illness and often death when it invades tissues, especially in immunocompromised individuals. The azole class of drugs is the most commonly prescribed treatment for many fungal infections and acts on the ergosterol biosynthesis pathway. One common mechanism of acquired azole drug resistance in fungi is the prevention of drug accumulation to toxic levels in the cell. While drug efflux is a well-known resistance strategy, reduced azole import would be another strategy to maintain low intracellular azole levels. Recently, azole uptake in Candida albicans and other yeasts was analyzed using [(3)H]fluconazole. Defective drug import was suggested to be a potential mechanism of drug resistance in several pathogenic fungi, including Cryptococcus neoformans, Candida krusei, and Saccharomyces cerevisiae. We have adapted and developed an assay to measure azole accumulation in A. fumigatus using radioactively labeled azole drugs, based on previous work done with C. albicans. We used this assay to study the differences in azole uptake in A. fumigatus isolates under a variety of drug treatment conditions, with different morphologies and with a select mutant strain with deficiencies in the sterol uptake and biosynthesis pathway. We conclude that azole drugs are specifically selected and imported into the fungal cell by a pH- and ATP-independent facilitated diffusion mechanism, not by passive diffusion. This method of drug transport is likely to be conserved across most fungal species.

Assessment of selection and resistance risk for demethylation inhibitor fungicides in Aspergillus fumigatus in agriculture and medicine: a critical review

BACKGROUND

An increasing number of publications have claimed that demethylation inhibitor (DMI) fungicides are confronted with resistance development in the fungus Aspergillus fumigatus and that the origin of resistant isolates may also be outside the medical area. For resistance risk assessment and sourcing the origin of DMI resistance, the primary exposure events ofA. fumigatus with DMI treatments have been analysed case by case, resulting in the pathogen exposure risk (PER).

RESULTS

The calculated maximum exposure concentrations (MEC) are highest during medical treatments (human and veterinary), certain fruit and seed treatments and wood preservation, and are much lower for crop protection applications. Most agricultural DMIs are intrinsically ∼10-100 times less active than medical DMIs for A. fumigatus control and potential resistance selection. However, imazalil is used in agriculture and veterinary medicine (as enilconazole) expressing strong intrinsic activity against A. fumigatus. The majority of mutations in the target gene, cyp51, of DMI-resistant isolates are different in A. fumigatus(e.g. TR34/L98H) in comparison with plant pathogens (e.g. A379G, I381V).

CONCLUSIONS

The assumed selection risk, ASR (MEC × PER) for resistance evolution to DMIs in A. fumigatus is estimated to be highest for human and veterinary applications. However, environmental origin of DMI-resistant spores from certain sites cannot be ruled out.

3D-QSAR and Molecular Mechanics Study for the Differences in the Azole Activity against Yeastlike and Filamentous Fungi and Their Relation to P450DM Inhibition. 1. 3-Substituted-4(3H)-quinazolinones

A combination between 3D-QSAR and molecular mechanics (MM)-docking study was used as a tool to detail and model the mechanism of action of 46 antifungal azoles. Two methods of alignment of the ligands were performed: (i) alignment of the main skeleton without substituents and (ii) alignment of a defined substructure. The best model is characterized by q(2) with the values of 0.70 for yeastlike (yeast), 0.66 for filamentous fungi, and 0.70 for the selectivity against filamentous fungi. 3D-QSAR regression maps derived from six models were used to identify the regions responsible for the differences in the compounds activity against yeast and filamentous fungi. The binding energy of the important substructures (Local Binding Energy-LBE) and its standard deviation were calculated in order to demonstrate quantitatively the contribution of substituents reflecting the diversity of the antifungal activity. The comparisons of these results with the same regions of the contour maps indicated a good correspondence between the 3D-QSAR and MM (LBE) approaches allowing association between the maps and the participating residues in the active sites of P450DM of C. albicans and A. fumigatus. The pi-pi interactions of two or more aromatic groups of the ligands with Phe228 and Tyr132 prove to be most important for the differences in activity against C. albicans. In A. fumigatus there was a better occupation of the inner central I-spiral in the areas around the heme. For the activity against A. fumigatus the pi-pi interactions of aromatic groups of the compounds with Phe509, Phe228, and Tyr132 are significant for the activity.

Mutations in the cyp51A gene and susceptibility to itraconazole in Aspergillus fumigatus serially isolated from a patient with lung aspergilloma

OBJECTIVES

To monitor changes in itraconazole susceptibility of isolates from a patient undergoing treatment for pulmonary Aspergillus infection and relate these changes to genotypic/phenotypic alterations.

METHODS

Six Aspergillus fumigatus isolates were serially recovered from the patient. Itraconazole MICs were determined by Etest and NCCLS methodology. Growth characteristics and phenotype were monitored. Molecular analysis included random amplified polymorphic DNA (RAPD) assay and sequencing of the cyp51A gene.

RESULTS

The MIC of itraconazole against the first isolate before treatment was 0.25 mg/L; the MIC against the second isolate, recovered after 6 months of itraconazole therapy, was >16 mg/L; and that against the third isolate, obtained 2 months after discontinuation of the therapy, was 0.5 mg/L. The MIC against the last three isolates, acquired after restoration of itraconazole therapy for 4-7 months, was >16 mg/L. The six isolates shared identical band patterns of RAPD assay using four primers and the same sequence in intertranscribed spacers (ITS). Therefore, the six isolates were likely to be the same strain of A. fumigatus, and mutations involving itraconazole resistance possibly occurred in these isolates after prolonged itraconazole therapy. Sequencing of the cyp51A gene in the coding region revealed a mutation of M220I in cytochrome P450 sterol 14-alpha-demethylase in the second resistant isolate and a mutation of G54R in the last three resistant isolates. Expression changes of some pump genes, such as MDR3, may also, in part, be related to the resistance to itraconazole.

CONCLUSIONS

We conclude that resistance of A. fumigatus to itraconazole occurred in a patient treated with the drug, and the resistance may result from mutations in the cyp51A gene-the gene encoding the target enzyme for itraconazole.

Cloning and characterization of the lanosterol 14alpha-demethylase (ERG11) gene in Cryptococcus neoformans

The ergosterol pathway in fungal pathogens is an attractive antimicrobial target because it is unique from the major sterol (cholesterol) producing pathway in humans. Lanosterol 14alpha-demethylase is the target for a major class of antifungals, the azoles. In this study we have isolated the gene for this enzyme from Cryptococcus neoformans. The gene, ERG11, was recovered using degenerate PCR with primers designed with a novel algorithm called CODEHOP. Sequence analysis of Erg11p identified a highly conserved region typical of the cytochrome P450 class of mono-oxygenases. The gene was present in single copy in the genome and mapped to one end of the largest chromosome. Comparison of the protein sequence to a number of major human fungal pathogen Erg11p homologs revealed that the C. neoformans protein was highly conserved, and most closely related to the Erg11p homologs from other basidiomycetes. Functional studies demonstrated that the gene could complement a Saccharomyces cerevisiae erg11 mutant, which confirmed the identity of the C. neoformans gene.

Homology Modeling of Lanosterol 14α-Demethylase ofCandida albicansandAspergillus fumigatusand Insights into the Enzyme-Substrate Interactions

The crystal structure of 14alpha-sterol demethylase from Mycobacterium tuberculosis (MT_14DM) provides a good template for modeling the three dimensional structure of lanosterol 14alpha-demethylase, which is the target of azole antifungal agents. Homologous 3D models of lanosterol 14alpha-demethylase from Candida albicans (CA_14DM) and Aspergillus fumigatus (AF_14DM) were built on the basis of the crystal coordinates of MT_14DM in complex with 4-phenylimidazole and fluconazole. The reliability of the two models was assessed by Ramachandran plots, Profile-3D analysis, and by analyzing the consistency of the two models with the experimental data on the P450(14DM). The overall structures of the resulting CA_14DM model and AF_14DM model are similar to those of the template structures. The two models remain the core structure characteristic for cytochrome P450s and most of the insertions and deletions expose the molecular surface. The structurally and functionally important residues such as the heme binding residues, the residues lining the substrate access channel, and residues in active site were identified from the model. To explore the binding mode of the substrate with the two models, 24(28)-methylene-24,25-dihydrolanosterol was docked into the active site of the two models and hydrophobic interaction and hydrogen-bonding were found to play an important role in substrate recognition and orientation. These results provided a basis for experiments to probe structure-function relationships in the P450(14DM). Although CA_14DM and AF_14DM shared similar core structural character, the active site of the two models were quite different, thus allowing the rational design of specific inhibitors to the target enzyme and the discovery of novel antifungal agents with broad spectrum.

Modeling and interactions of Aspergillus fumigatus lanosterol 14-α demethylase `A' with azole antifungals

Recent identification of the sterol 14-alpha demethylase genes (CYP51 A and B) from Aspergillus fumigatus and other species by Mellado et al. (J. Clin. Microbiol. 2001, 39(7), 2431-2438), has opened up possibilities of investigating the interactions of azole antifungals with the enzyme(s) from fungi. This study describes for the first time, a model of the three-dimensional structure of A. fumigatus 14-alpha demethylase (AF-CYP51A), using the crystal structure of Mycobacterium tuberculosis 14-alpha demethylase (PDB code:1EA1) as a template. The paper also describes the various interactions between azole antifungals and the target from A. fumigatus (AF-CYP51A). Quantitative evaluation of these interactions is done using COMBINE analysis to understand contributions of active site residues to ligand activity. It also provides explanation for the activity/inactivity of different ligands for AF-CYP51A.

Characterization of a lanosterol 14 alpha-demethylase from Pneumocystis carinii

Pneumocystis carinii (PC) causes severe pneumonia in immunocompromised patients. PC is intrinsically resistant to treatment with azole antifungal medications. The enzyme lanosterol 14 alpha-demethylase (Erg11) is the target for azole antifungals. We cloned PCERG11 and compared its sequence to Erg11 proteins present in azole-resistant organisms, and performed chromosomal and Northern blot analysis for PCERG11. Of 13 potential sites which could confer resistance to azoles, two were identical to azole-resistant Candida. By site-directed mutagenesis we changed these two sites in PCERG11 to those present in azole-sensitive Candida to generate PCERG11-SDM (E113D, T125K). We tested the susceptibility of ERG11 deletion strains of Saccharomyces cerevisiae (SC) expressing PCERG11, PCERG11-SDM, and wild-type SCERG11 to three azole antifungals: fluconazole, itraconazole, and voriconazole. PCERG11 required a 2.2-fold higher dose of voriconazole and 3.5-fold higher dose of fluconazole than SCERG11 for a 50% reduction in growth. No difference was observed in the sensitivity to itraconazole. PCERG11-SDM has increased sensitivity to fluconazole and voriconazole, but not itraconazole. We believe that the molecular structure of the lanosterol 14 alpha-demethylase encoded by PCERG11 confers inherent resistance to azole antifungals and plays an integral part in the overall resistance of this PC to azole therapy.

A point mutation in the 14alpha-sterol demethylase gene cyp51A contributes to itraconazole resistance in Aspergillus fumigatus

The genes encoding 14alpha-sterol demethylases (cyp51A and cyp51B) were analyzed in 12 itraconazole (ITC)-resistant and three ITC-susceptible clinical isolates of Aspergillus fumigatus. Six ITC-resistant strains exhibited a substitution of another amino acid for glycine at position 54, which is located at a very conserved region of the Cyp51A protein. The cyp51A gene from the A. fumigatus wild-type strain (CM-237) was replaced with the mutated cyp51A gene copy of an ITC-resistant strain (AF-72). Two transformants exhibited resistance to ITC, both of which had incorporated the mutated copy of the cyp51A gene.

Mutations in Aspergillus fumigatus resulting in reduced susceptibility to posaconazole appear to be restricted to a single amino acid in the cytochrome P450 14alpha-demethylase

To better understand the molecular basis of posaconazole (POS) resistance in Aspergillus fumigatus, resistant laboratory isolates were selected. Spontaneous mutants arose at a frequency of 1 in 10(8) and fell into two susceptibility groups, moderately resistant and highly resistant. Azole resistance in A. fumigatus was previously associated with decreased drug accumulation. We therefore analyzed the mutants for changes in levels of transcripts of genes encoding efflux pumps (mdr1 and mdr2) and/or alterations in accumulation of [(14)C]POS. No changes in either pump expression or drug accumulation were detected. Similarly, there was no change in expression of cyp51A or cyp51B, which encode the presumed target site for POS, cytochrome P450 14alpha-demethylase. DNA sequencing revealed that each resistant isolate carried a single point mutation in residue 54 of cyp51A. Mutations at the same locus were identified in three clinical A. fumigatus isolates exhibiting reduced POS susceptibility but not in susceptible clinical strains. To verify that these mutations were responsible for the resistance phenotype, we introduced them into the chromosome of a POS-susceptible A. fumigatus strain under the control of the glyceraldehyde phosphate dehydrogenase promoter. The transformants exhibited reductions in susceptibility to POS comparable to those exhibited by the original mutants, confirming that point mutations in the cyp51A gene in A. fumigatus can confer reduced susceptibility to POS.

Antifungal agents of use in animal health--chemical, biochemical and pharmacological aspects

A limited number of antifungal agents is licensed for use in animals, however, many of those available for the treatment of mycoses in humans are used by veterinary practitioners. This review includes chemical aspects, spectra of activity, mechanisms of action and resistance, adverse reactions and drug interactions of the antifungals in current use.

Resistance of human fungal pathogens to antifungal drugs

Resistance mechanisms can be engaged in clinically relevant fungal pathogens under different conditions when exposed to antifungal drugs. Over past years, active research was undertaken in the understanding of the molecular basis of antifungal drug resistance in these pathogens, and especially against the class of azole antifungals. The isolation of various alleles of the gene encoding the target of azoles has enabled correlation of the appearance of resistance with distinct mutations. Resistance mechanisms to azoles also converge to the upregulation of multidrug transporter genes, whose products have the capacity to extrude from cells several chemically unrelated antifungal agents and toxic compounds. Genome-wide studies of azole-resistant isolates are now permitting a more comprehensive analysis of the impact of resistance on gene expression, and may deliver new clues to their mechanisms. Several laboratories are also exploring, as well as possible alternative resistance pathways, the role of biofilm formation by several fungal species in the development of resistance to various antifungals, including azoles.

Mechanisms of fungal resistance: an overview

The increased use of antifungal agents in recent years has resulted in the development of resistance to these drugs. The significant clinical implication of resistance has led to heightened interest in the study of antifungal resistance from different angles. In this article we discuss antifungal susceptibility testing, the mode of action of antifungals and mechanisms of resistance. Antifungals are grouped into five groups on the basis of their site of action: azoles, which inhibit the synthesis of ergosterol (the main fungal sterol); polyenes, which bind to fungal membrane sterol, resulting in the formation of aqueous pores through which essential cytoplasmic materials leak out; allylamines, which block ergosterol biosynthesis, leading to accumulation of squalene (which is toxic to the cells); candins (inhibitors of the fungal cell wall), which function by inhibiting the synthesis of beta 1,3-glucan (the major structural polymer of the cell wall); and flucytosine, which inhibits macromolecular synthesis. Different mechanisms contribute to the resistance of antifungal agents. These mechanisms include modification of ERG11 gene at the molecular level (gene mutation, conversion and overexpression), over expression of specific drug efflux pumps, alteration in sterol biosynthesis, and reduction in the intracellular concentration of target enzymes. Approaches to prevent and control the emergence of antifungal resistance include prudent use of antifungals, treatment with the appropriate antifungal and conducting surveillance studies to determine the frequency of resistance.