Antifungal drug resistance of pathogenic fungi

Evaluation of the new Micronaut-Candida system compared to the API ID32C method for yeast identification

A new system, Micronaut-Candida, was compared to API ID32C to identify 264 yeast (Candida albicans, C. parapsilosis, C. tropicalis, C. krusei, C. inconspicua, C. norvegensis, C. lusitaniae, C. guilliermondii, C. dubliniensis, C. pulcherrima, C. famata, C. rugosa, C. glabrata, C. kefyr, C. lipolytica, C. catenulata, C. neoformans, Geotrichum and Trichosporon species, Rhodotorula glutinis, and Saccharomyces cerevisiae) clinical isolates. Results were in concordance in 244 cases. Eighteen out of the 20 of discordant results were correctly identified by Micronaut-Candida but not by API ID32C, as confirmed by PCR ribotyping.

Current status of antifungal susceptibility testing methods

Antifungal susceptibility testing is a very dynamic field of medical mycology. Standardization of in vitro susceptibility tests by the Clinical and Laboratory Standards Institute (CLSI) and the European Committee for Antimicrobial Susceptibility Testing (EUCAST), and current availability of reference methods constituted the major remarkable steps in the field. Based on the established minimum inhibitory concentration (MIC) breakpoints, it is now possible to determine the susceptibilities of Candida strains to fluconazole, itraconazole, voriconazole, and flucytosine. Moreover, utility of fluconazole antifungal susceptibility tests as an adjunct in optimizing treatment of candidiasis has now been validated. While the MIC breakpoints and clinical significance of susceptibility testing for the remaining fungi and antifungal drugs remain yet unclear, modifications of the available methods as well as other methodologies are being intensively studied to overcome the present drawbacks and limitations. Among the other methods under investigation are Etest, colorimetric microdilution, agar dilution, determination of fungicidal activity, flow cytometry, and ergosterol quantitation. Etest offers the advantage of practical application and favorable agreement rates with the reference methods that are frequently above acceptable limits. However, MIC breakpoints for Etest remain to be evaluated and established. Development of commercially available, standardized colorimetric panels that are based on CLSI method parameters has added more to the antifungal susceptibility testing armamentarium. Flow cytometry, on the other hand, appears to offer rapid susceptibility testing but requires specified equipment and further evaluation for reproducibility and standardization. Ergosterol quantitation is another novel approach, which appears potentially beneficial particularly in discrimination of azole-resistant isolates from heavy trailers. The method is yet investigational and requires to be further studied. Developments in methodology and applications of antifungal susceptibility testing will hopefully provide enhanced utility in clinical guidance of antifungal therapy. However, and particularly in immunosuppressed host, in vitro susceptibility is and will remain only one of several factors that influence clinical outcome.

Plagiochin E, a botanic-derived phenolic compound, reverses fungal resistance to fluconazole relating to the efflux pump

AIM

In this study, we investigated the effect of plagiochin E (PLE), a botanic-derived phenolic natural product, on reversal of fungal resistance to fluconazole (FLC) in vitro and the related mechanism.

METHODS AND RESULTS

A synergistic action of PLE and FLC was observed in the FLC-resistant Candida albicans strains and was evaluated using the fractional inhibited concentration index. The effect of PLE on FLC intracellular uptake was investigated in FLC-resistant C. albicans cells by liquid chromatography-tandem mass spectrometry, and the effect on efflux drug pump was assessed by measuring the efflux of Rhodamine 123 (Rh123). PLE significantly inhibited the efflux, but not the absorption, of Rh123 in FLC-resistant strains in phosphate-buffered saline with 5% glucose. Overexpression of the multidrug-resistance gene CDR1 in FLC-resistant C. albicans isolates was detected, and the introduction of PLE to the cells showed a significant reduction of the CDR1 expression in those FLC-resistant isolates.

CONCLUSIONS

These findings indicate that PLE could reverse the fungal resistant to FLC by inhibiting the efflux of FLC from C. albicans, and this effect may be related to the efflux pump.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results indicate that the combination of PLE and FLC may provide an approach for the clinical therapy of fungus infection induced by FLC-resistant strains.

Mechanism-based pharmacokinetic-pharmacodynamic models of in vitro fungistatic and fungicidal effects against Candida albicans

Mechanism-based pharmacokinetic-pharmacodynamic (PK-PD) models describing the fungistatic activity of fluconazole and the fungicidal activity of caspofungin were developed using dynamic in vitro models. Antifungal-drug pharmacokinetics was simulated in vitro, assuming a one-compartment model with an elimination half-life of 3 h and using a wide (1 to 10,000) range of initial concentrations. The number of CFUs over time was determined for up to 31 h and used for PK-PD modeling. A model incorporating first-order natural growth and natural death, plus a maximum number of viable Candida cells, was used to characterize Candida growth in the absence of a drug. Fluconazole was considered to inhibit Candida growth and caspofungin to stimulate Candida death according to an Emax pharmacodynamic model. The data were analyzed with Nonmem, using a population approach. A good fit of the data was obtained with satisfactory estimates of PK-PD parameters, especially with drug concentrations producing 50% of the maximal effect: 50% inhibitory concentrations for fluconazole growth inhibition and 50% effective concentrations for caspofungin death stimulation. In conclusion, mechanistic PK-PD models were successfully developed to describe, respectively, the fungistatic and fungicidal activities of fluconazole and caspofungin in vitro. These models provide much better information on the drug effects over time than the traditional PK-PD index based on MICs. However, they need to be further characterized.

Candida albicans Sun41p, a putative glycosidase, is involved in morphogenesis, cell wall biogenesis, and biofilm formation

The SUN gene family has been defined in Saccharomyces cerevisiae and comprises a fungus-specific family of proteins which show high similarity in their C-terminal domains. Genes of this family are involved in different cellular processes, like DNA replication, aging, mitochondrial biogenesis, and cytokinesis. In Candida albicans the SUN family comprises two genes, SUN41 and SIM1. We demonstrate that C. albicans mutants lacking SUN41 show similar defects as found for S. cerevisiae, including defects in cytokinesis. In addition, the SUN41 mutant showed a higher sensitivity towards the cell wall-disturbing agent Congo red, whereas no difference was observed in the presence of calcofluor white. Compared to the wild type, SUN41 deletion strains exhibited a defect in biofilm formation, a reduced adherence on a Caco-2 cell monolayer, and were unable to form hyphae on solid medium under the conditions tested. Interestingly, Sun41p was found to be secreted in the medium of cells growing as blastospores as well as those forming hyphae. Our results support a function of SUN41p as a glycosidase involved in cytokinesis, cell wall biogenesis, adhesion to host tissue, and biofilm formation, indicating an important role in the host-pathogen interaction.

The zinc cluster transcription factor Tac1p regulates PDR16 expression in Candida albicans

The Candida albicans PDR16 gene, encoding a putative phosphatidylinositol transfer protein, is co-induced with the multidrug transporter genes CDR1 and CDR2 in azole-resistant (A(R)) clinical isolates and upon fluphenazine exposure of azole-susceptible (A(S)) cells, suggesting that it is regulated by Tac1p, the transcriptional activator of CDR genes. Deleting TAC1 in an A(R) isolate (5674) overexpressing PDR16, CDR1 and CDR2 decreased the expression of the three genes and fluconazole resistance to levels similar to those detected in the matched A(S) isolate (5457), demonstrating that Tac1p is responsible for PDR16 upregulation in that strain. Deleting TAC1 in the A(S) strain SC5314 abolished CDR2 induction by fluphenazine and decreased that of PDR16 and CDR1, uncovering the participation of an additional factor in the regulation of PDR16 and CDR1 expression. Sequencing of the TAC1 alleles identified one homozygous mutation in strain 5674, an Asn to Asp substitution at position 972 in the C-terminus of Tac1p. Introduction of the Asp(972) allele in a tac1Delta/Delta mutant caused high levels of fluconazole resistance and TAC1, PDR16, CDR1 and CDR2 constitutive induction. These results demonstrate that: (i) Tac1p controls PDR16 expression; (ii) Asn(972) to Asp(972) is a gain-of-function mutation; and (iii) Tac1p is positively autoregulated, directly or indirectly.

Antifungal potential of disulfiram

Disulfiram, an alcohol antagonistic drug has been on the market since 1949 with 80% bioavailability and an established safety profile. Recently it has been reported as a P-glycoprotein efflux pump modulator. Herein we report its antifungal potential. The MIC50 and MIC90 of disulfiram for yeast isolates is 4 and 8 microg/ml, respectively, and the MIC range is 1-16 micro g/ml for both fluconazole sensitive and resistant strains. Interestingly, disulfiram also showed fungicidal activity on Aspergillus spp. with MIC50 and MIC90 of 2 and 8 microg/ml, respectively.

The Mechanism of Overcoming Multidrug Resistance (MDR) of Fungi by Amphotericin B and Its Derivatives

Comparative studies were performed to determine the activity and cytotoxicity of amphotericin B (AmB) and its derivatives on standard strain of Saccharomyces cerevisiae and its transformants with cloned genes from Candida albicans encoding multidrug resistance (MDR) pumps of ATP-binding cassette and major facilitator superfamilies. The AmB derivatives: amphotericin B 3-dimethylaminopropyl amide and N-methyl-N-D-fructopyranosylamphotericin B methyl ester were shown to be fungistatic and fungicidal towards MDR strains, by membrane permeabilization mechanism. Antibiotic-cell interaction monitored by energy transfer method indicates similar membrane affinity in parent strain and its MDR transformants. Experiments with fungal cells loaded with rhodamine 6G point to lack of competition between this dye and AmB and its derivatives for efflux driven by CDR2p. It can be thus assumed that AmB and its derivatives overcome fungal MDR by not being substrates of the multidrug exporting pumps, presumably due to their large molecular volumes.

Synthesis and in vitro activity of 2-thiazolylhydrazone derivatives compared with the activity of clotrimazole against clinical isolates of Candida spp

In this paper, we report on the synthesis of a novel series of 2-thiazolylhydrazone derivatives and the influence of the substituents on the thiazole ring on antifungal activity. All synthesized compounds were screened for their in vitro activities against 22 clinical isolates of Candida spp., representing six different species, compared to clotrimazole as a reference compound. Some of the tested compounds were found to possess significant antifungal activity when compared to clotrimazole, in particular compound 14 which exhibited higher potency against most of the Candida spp. considered. The compounds that were most active as anti-Candida agents were also submitted to cytotoxic screening by the Trypan Blue dye exclusion assay and in general they were shown to induce low cytotoxic effects.

Synthesis, characterisation and antimicrobial activity of copper(II) and manganese(II) complexes of coumarin-6,7-dioxyacetic acid (cdoaH2) and 4-methylcoumarin-6,7-dioxyacetic acid (4-MecdoaH2): X-ray crystal structures of [Cu(cdoa)(phen)2].8.8H(2)O and [Cu(4-Mecdoa)(phen)2].13H2O (phen=1,10-phenanthroline)

Two novel coumarin-based ligands, coumarin-6,7-dioxyacetic acid (1) (cdoaH(2)) and 4-methylcoumarin-6,7-dioxyacetic acid (2) (4-MecdoaH(2)), were reacted with copper(II) and manganese(II) salts to give [Cu(cdoa)(H(2)O)(2)].1.5H(2)O (3), [Cu(4-Mecdoa)(H(2)O)(2)] (4), [Mn(cdoa)(H(2)O)(2)] (5) and [Mn(4-Mecdoa)(H(2)O)(2)].0.5H(2)O (6). The metal complexes, 3-6, were characterised by elemental analysis, IR and UV-Vis spectroscopy, and magnetic susceptibility measurements and were assigned a polymeric structure. 1 and 2 react with Cu(II) in the presence of excess 1,10-phenanthroline (phen) giving [Cu(cdoa)(phen)(2)].8.8H(2)O (7) and [Cu(4-Mecdoa)(phen)(2)].13H(2)O (8), respectively. The X-ray crystal structures of 7 and 8 confirmed trigonal bipyramidal geometries, with the metals bonded to the four nitrogen atoms of the two chelating phen molecules and to a single carboxylate oxygen of the dicarboxylate ligand. The complexes were screened for their antimicrobial activity against a number of microbial species, including methicillin-resistant Staphylococcus aureus (MRSA), Escherichia coli and Candida albicans. The metal-free ligands 1 and 2 were active against all of the microbes. Complexes 3-6 demonstrated no significant activity whilst the phen adducts 7 and 8 were active against MRSA (MIC(80)=12.1microM), E. coli (MIC(80)=14.9microM) and Patonea agglumerans (MIC(80)=12.6microM). Complex 7 also demonstrated anti-Candida activity (MIC(80)=22microM) comparable to that of the commercially available antifungal agent ketoconazole (MIC(80)=25microM).

Integrated Proteomics and Genomics Strategies Bring New Insight into Candida albicans Response upon Macrophage Interaction

The interaction of Candida albicans with macrophages is considered a crucial step in the development of an adequate immune response in systemic candidiasis. An in vitro model of phagocytosis that includes a differential staining procedure to discriminate between internalized and non-internalized yeast was developed. Upon optimization of a protocol to obtain an enriched population of ingested yeasts, a thorough genomics and proteomics analysis was carried out on these cells. Both proteins and mRNA were obtained from the same sample and analyzed in parallel. The combination of two-dimensional PAGE with MS revealed a total of 132 differentially expressed yeast protein species upon macrophage interaction. Among these species, 67 unique proteins were identified. This is the first time that a proteomics approach has been used to study C. albicans-macrophage interaction. We provide evidence of a rapid protein response of the fungus to adapt to the new environment inside the phagosome by changing the expression of proteins belonging to different pathways. The clear down-regulation of the carbon-compound metabolism, plus the up-regulation of lipid, fatty acid, glyoxylate, and tricarboxylic acid cycles, indicates that yeast shifts to a starvation mode. There is an important activation of the degradation and detoxification protein machinery. The complementary genomics approach led to the detection of specific pathways related to the virulence of Candida. Network analyses allowed us to generate a hypothetical model of Candida cell death after macrophage interaction, highlighting the interconnection between actin cytoskeleton, mitochondria, and autophagy in the regulation of apoptosis. In conclusion, the combination of genomics, proteomics, and network analyses is a powerful strategy to better understand the complex host-pathogen interactions.

Mechanism of action of coumarin and silver(I)-coumarin complexes against the pathogenic yeast Candida albicans

The anti-fungal activity and mode of action of a range of silver(I)-coumarin complexes was examined. The most potent silver(I)-coumarin complexes, namely 7-hydroxycoumarin-3-carboxylatosilver(I), 6-hydroxycoumarin-3-carboxylatosilver(I) and 4-oxy-3-nitrocoumarinbis(1,10-phenanthroline)silver(I), had MIC80 values of between 69.1 and 4.6 microM against the pathogenic yeast Candida albicans. These compounds also reduced respiration, lowered the ergosterol content of cells and increased the trans-membrane leakage of amino acids. A number of the complexes disrupted cytochrome synthesis in the cell and induced the appearance of morphological features consistent with cell death by apoptosis. These compounds appear to act by disrupting the synthesis of cytochromes which directly affects the cell's ability to respire. A reduction in respiration leads to a depletion in ergosterol biosynthesis and a consequent disruption of the integrity of the cell membrane. Disruption of cytochrome biosynthesis may induce the onset of apoptosis which has been shown previously to be triggered by alteration in the location of cytochrome c. Silver(I)-coumarin complexes demonstrate good anti-fungal activity and manifest a mode of action distinct to that of the conventional azole and polyene drugs thus raising the possibility of their use when resistance to conventional drug has emerged or in combination with such drugs.

Risk of resistance associated with fluconazole prophylaxis: systematic review

BACKGROUND

Several studies have shown that fluconazole prophylaxis reduces the risk of fungal colonization and of invasive fungal infection in high-risk patients with minimal toxicity. This systematic review was designed to estimate the risk of emergence of colonization and infection either with azole susceptible-dose dependent or with resistant strains.

METHODS

We searched Medline, EMBASE, the Cochrane Collaboration database and our own files for randomized controlled trials assessing the effect of fluconazole on the incidence of fluconazole-resistant strains.

RESULTS

This systematic review of randomized clinical trials shows that fluconazole prophylaxis increases the risk for colonization with fluconazole susceptible-dose dependent or with resistant yeasts, the percentage of non-albicans Candida isolates and the percentage of fluconazole susceptible-dose dependent or resistant fungal isolates. Fluconazole prophylaxis did not significantly affect the risk of invasive disease with fluconazole susceptible-dose dependent or resistant fungi. The sample size was too small to assess the effect of fluconazole prophylaxis on the risk for breakthrough infections with non-albicans Candida.

CONCLUSION

Evidence from randomized trials suggests that fluconazole prophylaxis increases the risk for colonization with fluconazole susceptible-dose dependent or with resistant fungi, but does not significantly affect the risk for invasive infections with fluconazole susceptible-dose dependent or with resistant fungi. The risk for breakthrough infections remains a concern and needs to be addressed in large prospective studies.

Filamentous fungi in a tertiary care hospital: environmental surveillance and susceptibility to antifungal drugs

OBJECTIVE

To evaluate filamentous fungi with respect to environmental load and potential drug resistance in a tertiary care teaching hospital.

DESIGN

Monthly survey in 2 buildings of the hospital during a 12-month period.

SETTING

Hippokration Hospital in Thessaloniki, Greece.

METHODS

Air, surface, and tap water sampling was performed in 4 departments with high-risk patients. As sampling sites, the solid-organ transplantation department and the hematology department (in the older building) and the pediatric oncology department and the pediatric intensive care unit (in the newer building) were selected.

RESULTS

From January to May of 2000, the fungal load in air (FLA) was low, ranging from 0 to 12 colony-forming units (cfu) per m(3) in both buildings. During the summer months, when high temperature and humidity predominate, the FLA increased to 4-56 cfu/m(3). The fungi commonly recovered from culture of air specimens were Aspergillus niger (25.9%), Aspergillus flavus (17.7%), and Aspergillus fumigatus (12.4%). Non-Aspergillus filamentous fungi, such as Zygomycetes and Dematiaceous species, were also recovered. The pediatric intensive care unit had the lowest mean FLA (7.7 cfu/m(3)), compared with the pediatric oncology department (8.7 cfu/m(3)), the solid-organ transplantation department (16.1 cfu/m(3)), and the hematology department (22.6 cfu/m(3)). Environmental surfaces were swabbed, and 62.7% of the swab samples cultured yielded filamentous fungi similar to the fungi recovered from air but with low numbers of colony-forming units. Despite vigorous sampling, culture of tap water yielded no fungi. The increase in FLA observed during the summer coincided with renovation in the building that housed the solid-organ transplantation and hematology departments. All 54 Aspergillus air isolates randomly selected exhibited relatively low minimum inhibitory or effective concentrations for amphotericin B, itraconazole, voriconazole, posaconazole, micafungin, and anidulafungin.

CONCLUSION

Air and surface fungal loads may vary in different departments of the same hospital, especially during months when the temperature and humidity are high. Environmental Aspergillus isolates are characterized by lack of resistance to clinically important antifungal agents.

Human pharmacogenomic variations and their implications for antifungal efficacy

Pharmacogenomics is defined as the study of the impacts of heritable traits on pharmacology and toxicology. Candidate genes with potential pharmacogenomic importance include drug transporters involved in absorption and excretion, phase I enzymes (e.g., cytochrome P450-dependent mixed-function oxidases) and phase II enzymes (e.g., glucuronosyltransferases) contributing to metabolism, and those molecules (e.g., albumin, A1-acid glycoprotein, and lipoproteins) involved in the distribution of antifungal compounds. By using the tools of population genetics to define interindividual differences in drug absorption, distribution, metabolism, and excretion, pharmacogenomic models for genetic variations in antifungal pharmacokinetics can be derived. Pharmacogenomic factors may become especially important in the treatment of immunocompromised patients or those with persistent or refractory mycoses that cannot be explained by elevated MICs and where rational dosage optimization of the antifungal agent may be particularly critical. Pharmacogenomics has the potential to shift the paradigm of therapy and to improve the selection of antifungal compounds and adjustment of dosage based upon individual variations in drug absorption, metabolism, and excretion.

Fluconazole susceptibility testing of Candida species by flow cytometry

OBJECTIVE

Currently, antifungal drug susceptibility testing is labor-intensive, limited by delays in obtaining results and high costs. The purpose of this study was to determine the usefulness of flow cytometry (FCM) antifungal drug susceptibility testing as a routine laboratory procedure.

METHODS

A total of 24 clinical isolates of Candida spp. and reference strains were tested for susceptibility to fluconazole by FCM using propidium iodide (PI) as an indicator of viability. The minimum inhibitory concentration (MIC) was defined as the lowest concentration of fluconazole that resulted in an increase of 30% in mean channel fluorescence (MCF), compared to the growth control. FCM results were compared with MIC results as determined by the Clinical and Laboratory Standards Institute (CLSI) method.

RESULTS

An 8h incubation was sufficient for determination of the MICs. The results by FCM at 8h and the NCCLS methods at 24h showed 87.5% agreement to within two drug dilutions. However, the FCM method is labor-intensive in proportion to the larger number of samples. For Candida lusitaniae, MICs by the FCM method showed poor correlation with the CLSI method.

CONCLUSIONS

Further evaluation is necessary to assess the usefulness of FCM as a technique for routine antifungal MIC testing in the clinical laboratory.

Impact of changes in the target P450 CYP51 enzyme associated with altered triazole-sensitivity in fungal pathogens of cereal crops

Control of diseases caused by fungi in both medicine and agriculture is heavily dependent on the use of triazoles. As a consequence, resistance to triazoles is a threat to both human health and the sustainability of agricultural production systems. In human pathogens, particularly Candida albicans, mutations encoding alterations in the target cytochrome P450 sterol 14α-demethylase (CYP51; where CYP is cytochrome P450) enzyme are the primary determinants of triazole resistance. In fungal pathogens of cereals, CYP51A1 modifications, some at positions known to contribute to a resistant phenotype in human pathogens, have also been identified in isolates with altered triazole-sensitivity. However, unlike medicine where resistance to triazoles is a major clinical problem, failures of triazoles to control crop diseases in the field are rare with mean population sensitivities generally remaining low, perhaps due to differences in the selection pressures imposed on human and cereal pathogen populations. Nonetheless, the biological potential for resistance exists, and the question remains as to whether widespread triazole resistance can develop in an important cereal pathogen.

Identification of novel cell-wall active antifungal compounds

Fungal infections of humans and other animals are serious and often life threatening, especially in immunocompromised patients. One difficulty in treating fungal disease is the limited arsenal of antifungal compounds. The recent regulatory approval of three cell-wall active antifungal compounds encourages the search for additional clinical candidates that inhibit fungal cell wall formation. We have screened a small portion of a unique chemical library and have found eight compounds that appear to be inhibitors of fungal cell wall assembly.

Resistance mechanisms in fluconazole-resistant Candida albicans isolates from vaginal candidiasis

Candida albicans is the most frequently identified yeast species causing mycotic vaginitis. A significant number of vaginal yeast isolates are resistant to azole antifungal agents in vitro. Here we investigated the molecular mechanisms of resistance in 22 randomly selected fluconazole-resistant vaginal C. albicans isolates. Twelve isolates in this collection were found to be cross-resistant to itraconazole and 15 to voriconazole. Most of them also displayed decreased susceptibility to terbinafine. Northern blot analyses revealed overexpression of the MDR1 gene in all isolates, which in some isolates was accompanied by elevated levels of CDR1/CDR2 and ERG11 expression. Sequence analysis of the polymerase chain reaction-amplified ERG11 gene of selected azole-resistant isolates identified D116E and V488I amino acid alterations in Erg11p that are known to be conserved in fluconazole-resistant strains. The results demonstrate that decreased susceptibilities of vaginal yeast isolates to clinically used azole derivatives are the result of a combination of several molecular mechanisms involving drug efflux and alterations in the structure or cellular amount of 14-alpha-lanosterol demethylase.

Aspergillus fumigatus C-5 sterol desaturases Erg3A and Erg3B: role in sterol biosynthesis and antifungal drug susceptibility

Two erg3 genes encoding C-5 sterol desaturase enzymes (Erg3A and Erg3B) in Aspergillus fumigatus were characterized with respect to their nucleotide sequences and null mutant phenotypes. Targeted disruption of the erg3A and erg3B genes and a double gene knockout, erg3A- erg3B-, showed that they are not essential for A. fumigatus viability. Mutant phenotypes clearly showed that Erg3B is a C-5 sterol desaturase, but no apparent role for Erg3A in A. fumigatus ergosterol biosynthesis was found. Susceptibility to amphotericin B, itraconazole, fluconazole, voriconazole, and ketoconazole was not altered in isolates in which erg3A and erg3B were knocked out alone and in combination.

The use of new probes and stains for improved assessment of cell viability and extracellular polymeric substances in Candida albicans biofilms

Phenotypic and genotypic cell differentiation is considered an important feature that confers enhanced antifungal resistance in candidal biofilms. Particular emphasis has been placed in this context on the viability of biofilm subpopulations, and their heterogeneity with regard to the production of extracellular polymeric substances (EPS). We therefore assessed the utility of two different labeled lectins Erythrina cristagalli (ECA) and Canavalia ensiformis (ConA), for EPS visualization. To evaluate the viability of candidal biofilms, we further studied combination stains, SYTO9 and propidium iodide (PI). The latter combination has been successfully used to assess bacterial, but not fungal, viability although PI alone has been previously used to stain nuclei in fungal cells. Candida albicans biofilms were developed in a rotating disc biofilm reactor and observed in situ using confocal scanning laser microscopy (CSLM). Our data indicate that SYTO9 and PI are reliable vital stains that may be used to investigate C. albicans biofilms. When used together with ConA, the lectin ECA optimized EPS visualization and revealed differential production of this material in mature candidal biofilms. The foregoing probes and stains and the methodology described should help better characterize C. albicans biofilms in terms of cell their viability, and EPS production.

Chitinase fromParacoccidioides brasiliensis: molecular cloning, structural, phylogenetic, expression and activity analysis

A full-length cDNA encoding a chitinase (Pbcts1) was cloned by screening a cDNA library from the yeast cells of Paracoccidioides brasiliensis. The cDNA consists of 1888 bp and encodes an ORF of 1218 bp corresponding to a protein of 45 kDa with 406 amino acid residues. The deduced PbCTS1 is composed of two signature family 18 catalytic domains and seems to belong to fungal/bacterial class. Phylogenetic analysis of PbCTS1 and other chitinases suggests the existence of paralogs of several chitinases to be grouped based on specialized functions, which may reflect the multiple and diverse roles played by fungi chitinases. Glycosyl hydrolase activity assays demonstrated that P. brasiliensis is able to produce and secrete these enzymes mainly during transition from yeast to mycelium. The fungus should be able to use chitin as a carbon source. The presence of an endocytic signal in the deduced protein suggests that it could be secreted by a vesicular nonclassical export pathway. The Pbcts1 expression in mycelium, yeast, during differentiation from mycelium to yeast and in yeast cells obtained from infected mice suggests the relevance of this molecule in P. brasiliensis electing PbCTS1 as an attractive drug target.

Update on antifungal drug resistance mechanisms of Aspergillus fumigatus

Although the arsenal of agents with anti-Aspergillus activity has expanded over the last decade, mortality due to invasive aspergillosis (IA) remains unacceptably high. Aspergillus fumigatus still accounts for the majority of cases of IA; however less susceptible to antifungals non-fumigatus aspergilli began to emerge. Antifungal drug resistance of Aspergillus might partially account for treatment failures. Recent advances in our understanding of mechanisms of antifungal drug action in Aspergillus, along with the standardization of in vitro susceptibility testing methods, has brought resistance testing to the forefront of clinical mycology. In addition, molecular biology has started to shed light on the mechanisms of resistance of A. fumigatus to azoles and the echinocandins, while genome-based assays show promise for high-throughput screening for genotypic antifungal resistance. Several problems remain, however, in the study of this complex area. Large multicenter clinical studies--point prevalence or longitudinal--to capture the incidence and prevalence of antifungal resistance in A. fumigatus isolates are lacking. Correlation of in vitro susceptibility with clinical outcome and susceptibility breakpoints has not been established. In addition, the issue of cross-resistance between the newer triazoles is of concern. Furthermore, in vitro resistance testing for polyenes and echinocandins is difficult, and their mechanisms of resistance are largely unknown. This review examines challenges in the diagnosis, epidemiology, and mechanisms of antifungal drug resistance in A. fumigatus.

Posaconazole: a broad-spectrum triazole antifungal

Posaconazale is a new triazole drug being investigated in phase III clinical trials for the treatment and prevention of invasive fungal infections. In-vitro and in-vivo studies showed that posaconazole has broad-spectrum activity against most Candida species, Cryptococcus neoformans, Aspergillus species, Fusarium species, zygomycetes, and endemic fungi. Posaconazole is given orally two to four times daily. This triazole is widely distributed in the body, metabolised mainly by the liver, and is well tolerated, even in long-term courses. Adverse events are generally mild and include headache and gastrointestinal complaints. Posaconazole has shown promising clinical efficacy against life-threatening fungal infections that are often refractory to the currently available antifungal therapies-eg, invasive aspergillosis, fusariosis, and the emerging zygomycosis.

The power of the pump: mechanisms of action of P-glycoprotein (ABCB1)

Members of the superfamily of ATP-binding cassette (ABC) transporters mediate the movement of a variety of substrates including simple ions, complex lipids and xenobiotics. At least 18 ABC transport proteins are associated with disease conditions. P-glycoprotein (Pgp, ABCB1) is the archetypical mammalian ABC transport protein and its mechanism of action has received considerable attention. There is strong biochemical evidence that Pgp moves molecular cargo against a concentration gradient using the energy of ATP hydrolysis. However, the molecular details of how the energy of ATP hydrolysis is coupled to transport remain in dispute and it has not been possible to reconcile the data from various laboratories into a single model. The functional unit of Pgp consists of two nucleotide binding domains (NBDs) and two trans-membrane domains which are involved in the transport of drug substrates. Considerable progress has been made in recent years in characterizing these functionally and spatially distinct domains of Pgp. In addition, our understanding of the domains has been augmented by the resolution of structures of several non-mammalian ABC proteins. This review considers: (i) the role of specific conserved amino acids in ATP hydrolysis mediated by Pgp; (ii) emerging insights into the dimensions of the drug binding pocket and the interactions between Pgp and the transport substrates and (iii) our current understanding of the mechanisms of coupling between energy derived from ATP binding and/or hydrolysis and efflux of drug substrates.

Novel endoperoxides: synthesis and activity against Candida species

Fifteen new endoperoxides have been synthesised and tested for activity against pathogenic Candida species. These endoperoxides can be prepared in high yields, in one to three steps, from inexpensive starting materials. Despite chemical and structural similarities, their inhibitory activity against Candida growth varied greatly from one endoperoxide to another, and one species to another. This study of susceptibility to endoperoxide compounds presented here may lead to the development of potent new antifungal agents.

Therapy of Systemic Fungal Infections: A Pharmacologic Perspective

An increase in the incidence of severe, invasive, systemic fungal infections has been noted over the last decade in human and veterinary medicine. Reports of drug resistance and therapeutic failure to currently available antifungal agents have also been on the rise. Many factors are likely to be involved in these trends, including immune suppression and the use of broad-spectrum antibiotics. The use of fungistatic drugs, suboptimal doses, compounded drugs, poorly absorbed drug formulations, and inadequate tissue penetrations of antifungals also contribute to the development of acquired resistance. Because of the unique chemical complexities of the antifungal agents, drug/drug and drug/food interactions may also play a significant role in poor therapeutic outcome. This review summarizes the pharmacology and toxicology of the antifungal agents in current use for systemic mycosis and introduces some of the newer antifungal agents that anecdotally show very promising results.

Voriconazole-resistant disseminated Paecilomyces variotii infection in a neutropenic patient with leukaemia on voriconazole prophylaxis

Paecilomyces variotii, an emerging hyalohyphomycetes, has been reported to be susceptible in vitro to voriconazole. We describe a case of disseminated P. variotii infection in a neutropenic child with relapsed leukaemia who was on voriconazole prophylaxis. The P. variotii isolate was resistant to voriconazole in vitro. The patient responded to liposomal amphotericin B.

Two mutants selectively resistant to polyenes reveal distinct mechanisms of antifungal activity by nystatin and amphotericin B

Polyene macrolides nystatin and amphotericin B are widely used in the treatment of fungal infections. In order to characterize factors affecting polyene activity, we have isolated Saccharomyces cerevisiae mutants showing selective resistance to nystatin and amphotericin B. Characterization of two of these mutants (nystatin-resistant mutant X1/16 and amphotericin B-resistant mutant X3/33) is presented. Genetic analysis revealed that resistance in each of these mutants is caused by a mutation in one gene with a different mode of inheritance. Nystatin resistance in mutant X1/16 is caused by changes in sterol spectrum while amphotericin B resistance in mutant X3/33 is probably related to modification of the cell wall. Our results suggest that, in spite of their structural similarity, nystatin and amphotericin B differ significantly in mechanisms of their antifungal activity.

Tin(IV) complexes of pyrrolidinedithiocarbamate: synthesis, characterisation and antifungal activity

The reaction of ammonium pyrrolidinedithiocarbamate, [NH4{S2CN(CH2)4}], with SnCl2, [Sn(C6H5)2Cl2], [Sn(C6H5)3Cl], [Sn(C4H9)2Cl2] and [Sn(C6H11)3Cl] produced in good yield the compounds [Sn{S2CN(CH2)4}2Cl2] (1), [Sn{S2CN(CH2)4}2Ph2] (2), [Sn{S2CN(CH2)4}Ph3] (3), [Sn{S2CN(CH2)4}2 n-Bu2] (4) and [Sn{S2CN(CH2)4}Cy3] (5). The complexes were characterised by infrared, multinuclear NMR (1H, 13C{1H} and 119Sn{1H}) and 119Sn Mössbauer spectroscopies. In addition, the crystal structure of 4 was determined by X-ray crystallography. The in vitro antifungal activity of the tin(IV) complexes as well of the ligand was performed on human pathogenic fungi, Candida albicans, in concentrations of 0.025; 0.050; 0.100; 0.200; 0.400; 0.800; 1.600 and 3.200 mM. The microorganism presented resistance to the dithiocarbamate ligand and all tin(IV) complexes tested were actives. The highest activity was found for compounds 1 and 4.

Inactivation of the FCY2 gene encoding purine-cytosine permease promotes cross-resistance to flucytosine and fluconazole in Candida lusitaniae

In a previous work, we described the possible relationship between a defect of purine-cytosine permease and the acquisition of a cross-resistance to the antifungal combination flucytosine (5FC) and fluconazole (FLC) in Candida lusitaniae (T. Noël, F. François, P. Paumard, C. Chastin, D. Brethes, and J. Villard, Antimicrob. Agents Chemother. 47:1275-1284, 2003). Using degenerate PCR and chromosome walking, we cloned two FCY2-like genes in C. lusitaniae. Northern blot analysis revealed that only one gene was expressed; it was named FCY2. The other one behaved as a pseudogene and was named FCY21. In order to better characterize the possible role of FCY2 in cross-resistance to 5FC-FLC, disruption experiments with auxotrophic strain 6936 ura3(D95V) FCY2 with an integrative vector carrying the URA3 gene and a partial sequence of the C. lusitaniae FCY2 gene were undertaken. Southern blot analysis revealed that homologous recombination events occurred in all transformants analyzed at rates of 50% at resident locus FCY2 and 50% at resident locus URA3, resulting in the genotypes ura3 fcy2::URA3 and ura3::URA3 FCY2, respectively. It was then demonstrated that only transformants harboring a disrupted fcy2 gene were resistant to 5FC, susceptible to FLC, and resistant to the 5FC-FLC combination. Finally, complementation experiments with a functional FCY2 gene restored 5FC and FLC susceptibilities to the wild-type levels. The results of this study provide molecular evidence that inactivation of the sole FCY2 gene promotes cross-resistance to the antifungal association 5FC-FLC in C. lusitaniae.

Disulfiram, an old drug with new potential therapeutic uses for human cancers and fungal infections

Disulfiram, a drug used to treat alcoholism, has recently been indicated to play a primary as well as an adjuvant role in the treatment of many cancers and in the reversal of fungal drug-resistance. This review discusses the molecular mechanism of action of disulfiram and its potential use in the treatment of human cancers and fungal infections.

Synthesis and X-ray crystal structure of [Ag(phendio)2]ClO4 (phendio = 1,10-phenanthroline-5,6-dione) and its effects on fungal and mammalian cells

The Cu(II) and Ag(I) complexes, [Cu(phendio)3](ClO4)2 x 4H2O and [Ag(phendio)2]ClO4 (phendio = 1,10-phenanthroline-5,6-dione), are prepared in good yield by reacting phendio with the appropriate metal perchlorate salt. The X-ray crystal structure of the Ag(I) complex shows it to have a pseudo tetrahedral structure. 'Metal-free' phendio and the Cu(II) and Ag(I) phendio complexes strongly inhibit the growth of the fungal pathogen Candida albicans, and are more active than their 1,10-phenanthroline analogues. The simple Ag(I) salts, AgCH3CO2, AgNO3 and AgClO4 x H2O display superior anti-fungal properties compared to analogous simple Cu(II) and Mn(II) salts, suggesting that the nature of the metal ion strongly influences activity. Exposing C. albicans to 'metal-free' phendio, simple Ag(I) salts and [Ag(phendio)2]ClO4 causes extensive, non-specific DNA cleavage. 'Metal-free' phendio and [Ag(phendio)2]ClO4 induce gross distortions in fungal cell morphology and there is evidence for disruption of cell division. Both drugs also exhibit high anti-cancer activity when tested against cultured mammalian cells.

Increased frequency of non-fumigatus Aspergillus species in amphotericin B– or triazole–pre-exposed cancer patients with positive cultures for aspergilli

Invasive aspergillosis (IA) can occur despite prior prophylactic or empiric use of triazoles or amphotericin B (AMB). Although profound immunosuppression may account for breakthrough IA, resistance of Aspergillus to antifungals may also play a role. To examine this question, we measured the minimal inhibitory concentration of 105 Aspergillus isolates recovered from 105 cancer patients (64 with IA, 41 with Aspergillus colonization) to AMB, itraconazole (ITC), and voriconazole (VRC) using the National Committee for Clinical Laboratory Standards (NCCLS) M38-A microdilution and E-test methods. We also determined the minimal fungicidal concentration (MFC) of these agents and the minimal effective concentration (MEC) of caspofungin (CAS) using standardized methods. We then collected information regarding pre-exposure to AMB or triazoles (fluconazole, ITC, VRC) within 3 months before Aspergillus isolation. Pre-exposure of cancer patients to AMB or triazoles was associated with increased frequency of non-fumigatus Aspergillus species. Aspergillus isolates recovered from patients who previously received AMB exhibited higher E-test AMB MICs compared with isolates from patients without prior AMB exposure (P = 0.01). In addition, the AMB MICs by E-test were higher in triazole-pre-exposed patients compared with those not exposed to triazoles (P = 0.001). The ITC and VRC MICs by E-test were not affected by prior AMB or triazole exposure. Finally, neither the AMB, ITC, and VRC MICs and MFCs by NCCLS method nor CAS MECs showed such changes. In conclusion, cancer patients with positive Aspergillus cultures who are pre-exposed to AMB or triazoles have high frequency of non-fumigatus Aspergillus species. These Aspergillus isolates were found to be AMB-resistant by the more sensitive E-test method.

Perspectives for the management of febrile neutropenic patients with cancer in the 21st century

Over the past several decades, there has been substantial progress in the management of patients with febrile neutropenia. However, the ever-changing patterns of infection, ecology, and antibiotic-resistance trends do not allow the development of treatment guidelines that could be applied universally. Hence, the institution's predominant pathogens and resistance patterns should guide the empirical choice of antimicrobials. Prompt initiation of antimicrobial therapy remains the gold standard. Monotherapy with the newer broad-spectrum antimicrobials has tended to replace the classic combination therapy. Empirical administration of glycopeptides, such as vancomycin, without documentation of a gram-positive infection is not favored. The development of risk-stratification models has allowed for identification of low-risk patients with additional treatment options, such as early discharge and exclusively outpatient treatment with oral antimicrobials. The initiation of empirical antifungal therapy in persistently febrile neutropenic patients has become common practice, especially recently, since the introduction of new, effective, less toxic antifungal drugs. It is hoped that the development of new nonculture-based diagnostic methods will allow for the early detection of invasive fungal infections and, thus, the replacement of empirical antifungal therapy with pathogen-specific, preemptive therapy.

Transcriptional response of Candida albicans upon internalization by macrophages

The opportunistic fungal pathogen Candida albicans is both a benign gut commensal and a frequently fatal systemic pathogen. The interaction of C. albicans with the host's innate immune system is the primary factor in this balance; defects in innate immunity predispose the patient to disseminated candidiasis. Because of the central importance of phagocytic cells in defense against fungal infections, we have investigated the response of C. albicans to phagocytosis by mammalian macrophages using genomic transcript profiling. This analysis reveals a dramatic reprogramming of transcription in C. albicans that occurs in two successive steps. In the early phase cells shift to a starvation mode, including gluconeogenic growth, activation of fatty acid degradation, and downregulation of translation. In a later phase, as hyphal growth enables C. albicans to escape from the macrophage, cells quickly resume glycolytic growth. In addition, there is a substantial nonmetabolic response imbedded in the early phase, including machinery for DNA damage repair, oxidative stress responses, peptide uptake systems, and arginine biosynthesis. Further, a surprising percentage of the genes that respond specifically to macrophage contact have no known homologs, suggesting that the organism has undergone substantial evolutionary adaptations to the commensal or pathogen lifestyle. This transcriptional reprogramming is almost wholly absent in the related, but nonpathogenic, yeast Saccharomyces cerevisiae, suggesting that these large-scale and coordinated changes contribute significantly to the ability of this organism to survive and cause disease in vivo.

Genetically recombinant antibodies: new therapeutics against candidiasis

Historically, the therapy of serious fungal infection has been dominated by monotherapy with the polyene antibiotic amphotericin B. Clinical failures, side effects, the lack of alternatives and the toxicity of this drug have heightened the need to produce alternative therapies, which have included fluconazole, voriconazole and caspofungin. The observation that recovery from disseminated candidiasis was associated with an antibody response to the 47 kDa Candida heat-shock protein (HSP)90 homologue, coupled with the ability to sequence all the antibodies from patients who have recovered from the infection and to re-express the dominant ones as fragments in Escherichia coli, has opened the possibility of immunotherapy. The first recombinant antibody fragment, Mycograb (Neu Tec Pharma plc), against Candida HSP90 is now in clinical trials in patients with disseminated candidiasis in Europe and the US. Laboratory and early clinical data support the concept of synergy between Mycograb and amphotericin B. This should improve outcome and diminish the risk of resistance occurring to either drug, without an increase in toxicity, as this should be minimal in a human antibody fragment representing the natural antibody that a patient produces on recovery.

Induction of apoptosis in yeast and mammalian cells by exposure to 1,10-phenanthroline metal complexes

1,10-Phenanthroline (phen) and metal-phen complexes display fungicidal and fungiststic activity, disrupt mitochondrial function and induce oxidative stress. We have examined the effect of these drugs on the structure of yeast and mammalian cell organelles and the integrity of cellular DNA. Exposure of Candida albicans to [Mn(phen)2(mal)].2H2O or [Ag2(phen)3(mal)].2H2O (mal H2 = malonic acid) resulted in DNA degradation whereas exposure to phen or [Cu(phen)2(mal)].2H2O did not. All drugs induced extensive changes to the internal structure of yeast cells including retraction of the cytoplasm, nuclear fragmentation and disruption of the mitochondrion. In the case of cultured mammalian cells [Cu(phen)2(mal)].2H2O induced apoptosis as evidenced by the ladder pattern of DNA fragments following gel electrophoresis and also the blebbing of the cell membrane. The other drugs produced non-specific DNA degradation in mammalian cells. In conclusion, phen and metal-phen complexes have the potential to induce apoptosis in fungal and mammalian cells. Given their distinct mode of action compared to conventional anti-fungal drugs, phen and metal-phen complexes may represent a novel group of anti-fungal agents for use either in combination with existing drugs or in cases where resistance to conventional drugs has emerged.

Aspergillus terreus: an emerging amphotericin B-resistant opportunistic mold in patients with hematologic malignancies

BACKGROUND

Invasive aspergillosis (IA) has emerged as a common cause of morbidity and mortality among immunocompromised patients. At The University of Texas M. D. Anderson Cancer Center (Houston, TX), Aspergillus terreus is second to A. fumigatus as the most common cause of IA. In the current study, the authors compared the risk factors and outcomes associated with IA caused by A. terreus and IA caused by A. fumigatus.

METHODS

The authors retrospectively reviewed the medical records of 300 patients who received care at our institution between 1995 and 2001 and who had cultures that were positive for Aspergillus infection, including 90 patients whose cultures were positive for A. fumigatus and 70 patients whose cultures were positive for A. terreus.

RESULTS

Thirty-two patients with IA caused by A. terreus and 33 patients with IA caused by A. fumigatus were evaluated. The two groups were comparable in terms of age, gender, and underlying disease. Leukemia was the most common underlying malignancy (84%). More than 40% of patients in each group had undergone bone marrow transplantation. There was a trend toward a higher frequency of neutropenia among patients with IA caused by A. terreus (P = 0.12). IA caused by A. terreus was considered to be nosocomial in origin significantly more frequently compared with IA caused by A. fumigatus (P = 0.03). In vitro, A. terreus was found to be more resistant to amphotericin B (minimal inhibitory concentration [MIC90], 4.0 microg/mL) than to antifungal therapy (MIC90, 1.0 Hg/mL) in the isolates that were tested (< 50% of all isolates). The overall rate of response to antifungal therapy was 39% for patients with A. fumigatus infection, compared with 28% for patients with A. terreus infection (P = 0.43).

CONCLUSIONS

Despite the decreased in vitro susceptibility of A. terreus (relative to A. fumigatus) to amphotericin B, the two groups within the current patient population had comparably poor responses to amphotericin B preparation and somewhat improved responses to posaconazole.

A new era of antifungal therapy

Invasive fungal infections pose major management problems for clinicians caring for hematopoietic cell transplant patients. Two major fungal genera, Candida and Aspergillus, account for most fungal infections. Rates of systemic Candida infection range from 15% to 25%, mostly in the pre-engraftment period. Prophylaxis by fluconazole has dramatically reduced the frequency of early Candida infections. Caspofungin has recently been shown to offer an excellent alternative to amphotericin B (with less toxicity) or fluconazole (with a broader spectrum) for therapy of systemic Candida infections. Aspergillus infections occur in 15% to 20% of allogeneic hematopoietic cell transplant patients, most frequently in the post-engraftment period; they are associated with a severe diminution of cell-mediated immune responses by graft-versus-host disease and prolonged corticosteroid use. Voriconazole, a recently introduced broad-spectrum azole, has excellent activity against Aspergillus and is generally well tolerated. Voriconazole currently offers the best prospect for success and tolerance as a first-line treatment for aspergillosis. Second-line therapies include lipid formulations of amphotericin B, caspofungin, or intravenous itraconazole. Unfortunately, early initiation of therapy for aspergillosis is frequently not possible because of inaccurate diagnostics. One new diagnostic, the galactomannan assay, has recently been approved, and others are in development; these offer promise for earlier diagnosis without the need for invasive procedures. It is hoped that these new therapies and new diagnostics will usher in a new era of antifungal therapy.

Emerging fungal resistance

There has been an increase in systemic fungal infections over the past several decades, partially because of an increasing number of critically ill patients, surgical procedures, and immunosuppressive therapies, as well as the use of more invasive diagnostic and therapeutic medical procedures. Concomitant with this increase in infections has been the increase in azole-resistant Candida species and opportunistic molds with intrinsic resistance to many of the currently available antifungal agents. This review focuses on antifungal resistance, with emphasis on emerging resistance patterns and emerging fungi that are intrinsically resistant to antifungal agents.