Antifungal drug resistance

Using C. elegans for antimicrobial drug discovery

INTRODUCTION: The number of microorganism strains with resistance to known antimicrobials is increasing. Therefore, there is a high demand for new, non-toxic and efficient antimicrobial agents. Research with the microscopic nematode Caenorhabditis elegans can address this high demand for the discovery of new antimicrobial compounds. In particular, C. elegans can be used as a model host for in vivo drug discovery through high-throughput screens of chemical libraries. AREAS COVERED: This review introduces the use of substitute model hosts and especially C. elegans in the study of microbial pathogenesis. The authors also highlight recently published literature on the role of C. elegans in drug discovery and outline its use as a promising host with unique advantages in the discovery of new antimicrobial drugs. EXPERT OPINION: C. elegans can be used, as a model host, to research many diseases, including fungal infections and Alzheimer's disease. In addition, high-throughput techniques, for screening chemical libraries, can also be facilitated. Nevertheless, C. elegans and mammals have significant differences that both limit the use of the nematode in research and the degree by which results can be interpreted. That being said, the use of C. elegans in drug discovery still holds promise and the field continues to grow, with attempts to improve the methodology already underway.

Gene Expression and Identification Related to Fluconazole Resistance of Candida glabrata Strains

Objectives

Candida glabrata has become one of the most common causes of Candida bloodstream infections worldwide. Some strains of C. glabrata may be intermediately resistant to all azoles. The several possible mechanisms of azole resistance have been reported previously, but the exact resistant mechanism is not clear. In this study, we identified differentially expressed genes (DEGs) of fluconazole-resistant C. glabrata and compared the gene expression of fluconazole-resistant strains with that of fluconazole-susceptible strains to identify gene corresponding to fluconazole resistance.

Methods

Using antifungal susceptibility test, several C. glabrata strains were selected and used for further study. The expression of CgCDR1 and CgCDR2 genes was investigated by slot hybridization against fluconazole-susceptible, -resistant, and resistant-induced strains. In addition, ERG3 and ERG11 genes were sequenced to analyze DNA base substitution. DEGs were identified by reverse transcription-polymerase chain reaction using DEG kit composed of 120 random primers.

Results

In slot hybridization, CgCDR1 gene was expressed more than CgCDR2 gene in resistant strains. Though base substitution of ERG11 and ERG3 genes was observed in several base sequences, just one amino acid change was identified in resistant strain. In the results of reverse transcription-polymerase chain reaction, 44 genes were upregulated and 34 genes were downregulated. Among them, adenosine triphosphate-binding cassette transporter-related genes, fatty acid desaturase, lyase, and hypothetical protein genes were upregulated and aldehyde dehydrogenase, oxidoreductase, and prohibitin-like protein genes were downregulated. Other DEGs were also identified.

Conclusion

This study showed that CgCDR1 gene was more closely related to fluconazole resistance of C. glabrata than CgCDR2 gene. In addition, several other genes related with fluconazole resistance of C. glabrata were identified.

Antifungal drug resistance of oral fungi

Fungi comprise a minor component of the oral microbiota but give rise to oral disease in a significant proportion of the population. The most common form of oral fungal disease is oral candidiasis, which has a number of presentations. The mainstay for the treatment of oral candidiasis is the use of polyenes, such as nystatin and amphotericin B, and azoles including miconazole, fluconazole, and itraconazole. Resistance of fungi to polyenes is rare, but some Candida species, such as Candida glabrata and C. krusei, are innately less susceptible to azoles, and C. albicans can acquire azole resistance. The main mechanism of high-level fungal azole resistance, measured in vitro, is energy-dependent drug efflux. Most fungi in the oral cavity, however, are present in multispecies biofilms that typically demonstrate an antifungal resistance phenotype. This resistance is the result of multiple factors including the expression of efflux pumps in the fungal cell membrane, biofilm matrix permeability, and a stress response in the fungal cell. Removal of dental biofilms, or treatments to prevent biofilm development in combination with antifungal drugs, may enable better treatment and prevention of oral fungal disease.

The current role of amphotericin B lipid complex in managing systemic fungal infections

BACKGROUND

An increase in the number of immunocompromised patients has led to a rising burden of systemic fungal infections. Historically, conventional amphotericin B has been used to treat these infections due to its broad spectrum of activity. The development of lipid-based amphotericin B agents, such as Abelcet * (ABLC), has allowed clinicians to take advantage of the broad spectrum of activity of amphotericin B while reducing adverse events. As well as this, a number of new antifungal agents have been developed in recent years which have significantly added to the treating physician's antifungal armamentarium. * Abelcet is a registered trade name of Cephalon Ltd, Herts, UK.

OBJECTIVES

Review the clinical data that support the use of ABLC and discuss the evidence for its continuing role in the treatment of invasive fungal infections in light of the introduction of newer antifungal agents.

METHODS

Published studies were identified by searching the MEDLINE database and the Cochrane Centre for Reviews up to August 2009. The search was conducted using the following key words: Amphotericin, Lipid, Abelcet, AmBisome, Efficacy, Nephrotoxicity, Renal, Toxicity.

FINDINGS

ABLC is effective and well-tolerated in the treatment of systemic fungal infections and remains a valuable therapeutic option in a variety of immunocompromised patients due to its broad antifungal spectrum and rarity of resistance.

LIMITATIONS

Data from randomised controlled trials of lipid-based amphotericin B formulations, as well as head-to-head comparison studies between ABLC and other antifungal agents are limited. In addition, the review uses a narrative approach and relies to a great extent on the authors' personal views and experiences.

Candida albicans hyphal formation and virulence assessed using a Caenorhabditis elegans infection model

Candida albicans colonizes the human gastrointestinal tract and can cause life-threatening systemic infection in susceptible hosts. We study here C. albicans virulence determinants using the nematode Caenorhabditis elegans in a pathogenesis system that models candidiasis. The yeast form of C. albicans is ingested into the C. elegans digestive tract. In liquid media, the yeast cells then undergo morphological change to form hyphae, which results in aggressive tissue destruction and death of the nematode. Several lines of evidence demonstrate that hyphal formation is critical for C. albicans pathogenesis in C. elegans. First, two yeast species unable to form hyphae (Debaryomyces hansenii and Candida lusitaniae) were less virulent than C. albicans in the C. elegans assay. Second, three C. albicans mutant strains compromised in their ability to form hyphae (efg1Delta/efg1Delta, flo8Delta/flo8Delta, and cph1Delta/cph1Delta efg1Delta/efg1Delta) were dramatically attenuated for virulence. Third, the conditional tet-NRG1 strain, which enables the external manipulation of morphogenesis in vivo, was more virulent toward C. elegans when the assay was conducted under conditions that permit hyphal growth. Finally, we demonstrate the utility of the C. elegans assay in a screen for C. albicans virulence determinants, which identified several genes important for both hyphal formation in vivo and the killing of C. elegans, including the recently described CAS5 and ADA2 genes. These studies in a C. elegans-C. albicans infection model provide insights into the virulence mechanisms of an important human pathogen.

Fluconazole dosing for the prevention or treatment of invasive candidiasis in young infants

BACKGROUND

Young infants are susceptible to developmental factors influencing the pharmacokinetics of drugs. Fluconazole is increasingly used to prevent and treat invasive candidiasis in infants. Dosing guidance remains empiric and variable because limited pharmacokinetic data exist.

METHODS

Our population pharmacokinetic model derived from 357 fluconazole plasma concentrations from 55 infants (23-40 week gestation) illustrates expected changes in fluconazole clearance based upon gestational age, postnatal age, weight, and creatinine. We used a Monte Carlo simulation approach based on parametric description of a patient population's pharmacokinetic response to fluconazole to predict fluconazole exposure (median: 10th and 90th percentile population variability range) after 3, 6, and 12 mg/kg dosing.

RESULTS

For the treatment of invasive candidiasis, a dose of at least 12 mg/kg/d in the first 90 days after birth is needed to achieve an area under the concentration curve (AUC) of >400 mg*h/L and an AUC/minimum inhibitory concentration (MIC) >50 for Candida species with MIC <8 microg/mL in > or =90% of <30 week gestation infants and 80% of 30 to 40 week gestation infants. The more preterm infants achieve a higher median AUC (682 mg*hr/L) compared with more mature infants (520 mg*hr/L). For early prevention of candidiasis in 23 to 29 week infants, a dose of 3 or 6 mg/kg twice weekly during the first 42 days of life is equivalent to an AUC of 50 and 100 mg*hr/L, respectively, and maintains fluconazole concentrations > or =2 or 4 microg/mL, respectively, for half of the dosing interval. For late prevention, the 6 mg/kg dose every 72 hours provides similar exposure to 3 mg/kg daily dose. Infants with serum creatinine > or =1.3 mg/dL have delayed drug clearance and dose adjustment is indicated if creatinine does not improve within 96 hours.

CONCLUSIONS

A therapeutic concentration of fluconazole in premature infants with invasive candidiasis requires dosing substantially greater than commonly recommended in most reference texts. To prevent invasive candidiasis, twice weekly prophylaxis regimens can provide adequate exposure when unit specific MICs are taken into account.

Remodeling of global transcription patterns of Cryptococcus neoformans genes mediated by the stress-activated HOG signaling pathways

The ability to sense and adapt to a hostile host environment is a crucial element for virulence of pathogenic fungi, including Cryptococcus neoformans. These cellular responses are evoked by diverse signaling cascades, including the stress-activated HOG pathway. Despite previous analysis of central components of the HOG pathway, its downstream signaling network is poorly characterized in C. neoformans. Here we performed comparative transcriptome analysis with HOG signaling mutants to explore stress-regulated genes and their correlation with the HOG pathway in C. neoformans. In this study, we not only provide important insights into remodeling patterns of global gene expression for counteracting external stresses but also elucidate novel characteristics of the HOG pathway in C. neoformans. First, inhibition of the HOG pathway increases expression of ergosterol biosynthesis genes and cellular ergosterol content, conferring a striking synergistic antifungal activity with amphotericin B and providing an excellent opportunity to develop a novel therapeutic method for treatment of cryptococcosis. Second, a number of cadmium-sensitive genes are differentially regulated by the HOG pathway, and their mutation causes resistance to cadmium. Finally, we have discovered novel stress defense and HOG-dependent genes, which encode a sodium/potassium efflux pump, protein kinase, multidrug transporter system, and elements of the ubiquitin-dependent system.

The effect of plagiochin E alone and in combination with fluconazole on the ergosterol biosynthesis of Candida albicans

Plagiochin E (PLE), a macrocyclic bis(bibenzyl) isolated from the liverwort Marchantia polymorpha, has been reported to have antifungal activity and resistance reversal effects on Candida albicans. In order to understand the underlying mechanisms, we studied the effects of PLE alone and in combination with fluconazole (FLC) on the ergosterol biosynthetic pathway against both FLC-sensitive and FLC-resistant strains by analyzing the sterol content and the ergosterol pathway gene (ERG) expression. Relative quantitative analysis of different ergosterol precursors was carried out by employing the hyphenated technique of gas chromatography-high resolution mass spectrometry (GC-HR-MS). We observed that for FLC-resistant strain PLE itself can cause the accumulation of lanosterol and the decrease of 14alpha-methylfecosterol. When it combined with FLC, a significant decrease was observed in ergosterol formation and corresponding accumulation of 14alpha-methylated sterols was also found. Real-time reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed that the transcription level of ERG11 was decreased in FLC-resistant strain when exposed to PLE alone or PLE plus FLC. These results suggest that PLE potentiates FLC antifungal activity by interfering with the FLC-targeted ergosterol biosynthesis pathway.

Efflux-mediated antifungal drug resistance

Fungi cause serious infections in the immunocompromised and debilitated, and the incidence of invasive mycoses has increased significantly over the last 3 decades. Slow diagnosis and the relatively few classes of antifungal drugs result in high attributable mortality for systemic fungal infections. Azole antifungals are commonly used for fungal infections, but azole resistance can be a problem for some patient groups. High-level, clinically significant azole resistance usually involves overexpression of plasma membrane efflux pumps belonging to the ATP-binding cassette (ABC) or the major facilitator superfamily class of transporters. The heterologous expression of efflux pumps in model systems, such Saccharomyces cerevisiae, has enabled the functional analysis of efflux pumps from a variety of fungi. Phylogenetic analysis of the ABC pleiotropic drug resistance family has provided a new view of the evolution of this important class of efflux pumps. There are several ways in which the clinical significance of efflux-mediated antifungal drug resistance can be mitigated. Alternative antifungal drugs, such as the echinocandins, that are not efflux pump substrates provide one option. Potential therapeutic approaches that could overcome azole resistance include targeting efflux pump transcriptional regulators and fungal stress response pathways, blockade of energy supply, and direct inhibition of efflux pumps.

In vitro activities of retigeric acid B alone and in combination with azole antifungal agents against Candida albicans

The vitro antifungal activity of retigeric acid B (RAB), a pentacyclic triterpenoid from the lichen species Lobaria kurokawae, was evaluated alone and in combination with fluconazole, ketoconazole, and itraconazole against Candida albicans using checkerboard microdilution and time-killing tests. The MICs for RAB against 10 different C. albicans isolates ranged from 8 to 16 microg/ml. A synergistic action of RAB and azole was observed in azole-resistant strains, whereas synergistic or indifferent effects were observed in azole-sensitive strains when interpreted by a separate approach of the fractional inhibitory concentration index and DeltaE model (the difference between the predicted and measured fungal growth percentages). In time-killing tests, we used both colony counts and a colorimetric assay to evaluate the combinational antifungal effects of RAB and azoles, which further confirmed their synergistic interactions. These findings suggest that the natural product RAB may play a certain role in increasing the susceptibilities of azole-resistant C. albicans strains.

Enhanced susceptibility to antifungal oligopeptides in yeast strains overexpressing ABC multidrug efflux pumps

The susceptibility to several oligopeptide and amino acid antifungals of a Saccharomyces cerevisiae strain carrying multiple deletions in yeast multidrug resistance genes was compared to transformants containing the CDR1, CDR2, or MDR1 genes that encode the major Candida albicans drug efflux pumps. Recombinant yeast strains overexpressing Cdr1p and Cdr2p showed enhanced susceptibilities to all tested oligopeptide antifungals. The enhanced susceptibilities of multidrug-resistant yeast strains to oligopeptide antifungals corresponded to higher rates of oligopeptide uptake. Yeast cells overexpressing Cdr1p or Cdr2p effluxed protons at higher rates than the reference cells lacking these ABC transporters. An increased plasma membrane electrochemical gradient caused by the functional overexpression of Cdr1p or Cdr2p appeared to increase cellular susceptibility to oligopeptide antifungals by stimulating their uptake via oligopeptide permeases.

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.

Antifungal and antioxidant activities of the phytomedicine pipsissewa, Chimaphila umbellata

Bioassay-guided fractionation of Chimaphila umbellata (L.) W. Bart (Pyrolaceae) ethanol extracts led to the identification of 2,7-dimethyl-1,4-naphthoquinone (chimaphilin) as the principal antifungal component. The structure of chimaphilin was confirmed by 1H and 13C NMR spectroscopy. The antifungal activity of chimaphilin was evaluated using the microdilution method with Saccharomyces cerevisiae (0.05mg/mL) and the dandruff-associated fungi Malassezia globosa (0.39mg/mL) and Malassezia restricta (0.55mg/mL). Pronounced antioxidant activity of C. umbellata crude extract was also identified using the DPPH (2,2-diphenyl-1-picrylhydrazyl) assay, suggesting this phytomedicine has an antioxidant function in wound healing. A chemical-genetic profile was completed with chimaphilin using approximately 4700 S. cerevisiae gene deletion mutants. Cellular roles of deleted genes in the most susceptible mutants and secondary assays indicate that the targets for chimaphilin include pathways involved in cell wall biogenesis and transcription.

Reavaliação da suscetibilidade de Candida à anfotericina B: estudo comparativo com isolados de três hospitais do estado do Rio Grande do Sul

Comparou-se a suscetibilidade à anfotericina B de Candida spp isoladas de candidemias, sendo: 41 do Hospital Universitário de Santa Maria, 56 do Hospital de Clínicas de Porto Alegre e 47 da Santa Casa, Complexo Hospitalar de Porto Alegre. Os testes foram baseados no documento M27-A2 do Clinical Laboratory Standards Institute. Todavia, foram empregadas 20 concentrações de anfotericina B, variáveis entre 0,1 e 2µg/ml. Os testes foram realizados nos meios RPMI 1640 com glicose, antibiotic medium 3 e yeast nitrogen base dextrosado. O caldo antibiotic medium 3 gerou amplas faixas de concentrações inibitórias mínimas e concentrações fungicidas mínimas quando comparado aos demais. As variações de suscetibilidade entre os hospitais foram melhor detectadas no antibiotic médium 3; os isolados do Hospital Universitário de Santa Maria evidenciaram menor sensibilidade do que os da Santa Casa, Complexo Hospitalar de Porto Alegre (p < 0,05). As causas das variações de suscetibilidade não foram avaliadas mas apontam para a necessidade de vigilância da suscetibilidade a anfotericina B.

A first Portuguese epidemiological survey of fungaemia in a university hospital

A prospective, observational study was conducted at the biggest Portuguese hospital, aiming to evaluate the epidemiology of bloodstream fungal infection. During a period of 12 months (2004), all yeasts isolated from the blood cultures of patients with fungaemia admitted at a university hospital of Porto were collected. Demographic and clinical data, as well as haematological and biochemical profiles, were registered. Antifungal susceptibility was evaluated. The incidence of fungaemia and nosocomial fungaemia were 2.7 and 2 per 1,000 hospital admissions, respectively. Blood strains from 117 patients were identified. Thirty-five percent of yeast isolates were Candida albicans, followed by C. parapsilosis (25.6%). The mortality rate associated with fungaemia was 39.3%; the highest values were found in patients with C. glabrata and C. tropicalis infection. Seventy-five percent of the fungaemia episodes were nosocomial, with 48% mortality; the main predisposing factors were parenteral nutrition, gastric protection with omeprazole, surgical drainage and the presence of central venous catheters (CVCs). Thrombocytopaenia, urinary catheter, gastrointestinal pathology and nosocomial fungaemia were independently associated with a poor outcome. Antifungal susceptibility testing showed high fluconazole resistance (15%), mostly in C. tropicalis. We observed a high incidence of nosocomial fungaemia with high mortality rates. Important predisposing factors were identified, deserving further investigation. Local surveillance is warranted to monitor the incidence of in vitro antifungal resistance.

Enantiomeric separation of several antimycotic azole drugs using supercritical fluid chromatography

The chiral resolution of four antifungal compounds, three imidazoles (miconazole, econazole and sulconazole) and one triazole (itraconazole) using supercritical fluid chromatography on the amylose-based chiral stationary phase Chiralpak AD, is presented in this work. The influence of pressure, type and percentage of organic modifier and temperature on retention times and resolution was studied. The enantiomeric separation of the three imidazoles was achieved with resolutions higher than two and analysis times lower than 10 min, obtaining the best results using methanol as modifier. However, the analysis time of the triazole was higher than 80 min due to the existence of a high number of functional groups that were able to interact with the chiral stationary phase. In this case, the resolution of the four stereoisomers was achieved only partially with mixtures of ethanol and 2-propanol as modifier. The isoenantioselective temperatures were obtained from the study of the influence of the temperature, they were above the range of temperatures assayed, except for sulconazole using 2-propanol.

Antifungal effect of voriconazole on intracellular Candida glabrata, Candida krusei and Candida parapsilosis in human monocyte-derived macrophages

Infections caused by Candida species other than Candida albicans are increasingly common, and decreased susceptibility to azoles has made them more difficult to treat. Since phagocytic killing is important in elimination of Candida infections, intracellular killing of fluconazole-resistant Candida glabrata, Candida krusei and Candida parapsilosis (four strains each) by voriconazole was investigated in human monocyte-derived macrophages (MDMs). MDMs were infected with Candida, and voriconazole was then added. MDMs were lysed at 0, 24 or 48 h after infection, and viable Candida in the lysates enumerated. Compared to the starting inoculum, the number of viable intracellular C. parapsilosis and C. glabrata in untreated MDMs increased to 28,121 and 351 %, respectively, in 48 h. In contrast, the number of C. krusei decreased to 42 %. In MDMs treated with voriconazole, the decrease in viable count was dependent upon drug concentration. At 48 h, C. glabrata was killed only at 5x MIC (P < 0.05), C. krusei was killed at all voriconazole concentrations, while C. parapsilosis was inhibited at 0.5 and 1x MIC and killed at > or = 2.5x MIC (P < 0.05). The data show that intracellular growth and survival of these Candida species in the absence or presence of voriconazole vary markedly. The activity of voriconazole depends on the concentration of the drug and the time of exposure. For the 12 Candida strains studied, regression curves show that the maximum intracellular anticandidal activity of voriconazole was reached at 3.5-5x MIC.

Modification of the fluorescein diacetate assay for screening of antifungal agents against Candida albicans: Comparison with the NCCLS methods

A modified fluorescein diacetate (FDA) assay has been compared with standard NCCLS broth macrodilution and broth microdilution methods for the detection of antifungal activity. The FDA assay was performed in a medium containing bacteriological peptone, NaCl, yeast extract and glucose (0.2%, 0.1%, 0.1% and 1% w/v, respectively) and buffered with 10 mM BES buffer. The MICs of amphotericin B, fluconazole, miconazole and flucytosine (representing three major classes of antifungal agents) obtained by the three methods were compared. The results obtained with the FDA assays correlated well with the NCCLS macrodilution method for MICs of amphotericin B, miconazole and fluconazole, but not for flucytosine. However, the MIC values of flucytosine obtained with the FDA assay were well within the quality control range for the two reference strains recommended by the NCCLS. The FDA assay described is an attractive alternative to the NCCLS methods for screening for antifungal agents, with the added advantage of objectivity of fluorescence measurement.

Biochemical characterization of recombinant dihydroorotate dehydrogenase from the opportunistic pathogenic yeast Candida albicans

Candida albicans is the most prevalent yeast pathogen in humans, and recently it has become increasingly resistant to the current antifungal agents. In this study we investigated C. albicans dihydroorotate dehydrogenase (DHODH, EC 1.3.99.11), which catalyzes the fourth step of de novo pyrimidine synthesis, as a new target for controlling infection. We propose that the enzyme is a member of the DHODH family 2, which comprises mitochondrially bound enzymes, with quinone as the direct electron acceptor and oxygen as the final electron acceptor. Full-length DHODH and N-terminally truncated DHODH, which lacks the targeting sequence and the transmembrane domain, were subcloned from C. albicans, recombinantly expressed in Escherichia coli, purified, and characterized for their kinetics and substrate specificity. An inhibitor screening with 28 selected compounds was performed. Only the dianisidine derivative, redoxal, and the biphenyl quinoline-carboxylic acid derivative, brequinar sodium, which are known to be potent inhibitors of mammalian DHODH, markedly reduced C. albicans DHODH activity. This study provides a background for the development of antipyrimidines with high efficacy for decreasing in situ pyrimidine nucleotide pools in C. albicans.

Mechanisms of azole resistance in a clinical isolate of Candida tropicalis

Azole resistance has been insufficiently investigated in the yeast Candida tropicalis. Here we determined the molecular mechanisms responsible for azole resistance in a clinical isolate of this pathogenic yeast. Antifungal susceptibility testing performed by a disk diffusion method showed resistance or markedly decreased susceptibility to azoles, which was confirmed by determination of MICs. Considering the relationship between azole susceptibility and the respiration reported for other yeast species, the respiratory activity of this isolate was investigated. Flow cytometry using rhodamine 123 and oxygraphy demonstrated an increased respiratory activity, which was not linked to an overexpression or increased number of copies of the mitochondrial genome. Among previously described resistance mechanisms, an increased activity of efflux pumps was investigated by flow cytometry using rhodamine 6G. However, the efflux of rhodamine 6G was lower in the resistant isolate than in susceptible ones. Likewise, real-time reverse transcription-PCR quantification of the expression of C. tropicalis MDR1 (CtMDR1), which encodes an efflux protein belonging to the major facilitator superfamily, did not show overexpression of this gene. In contrast, the resistant isolate overexpressed the CtERG11 gene coding for lanosterol 14alpha-demethylase. This was in agreement with the larger amount of ergosterol found in this isolate. Moreover, sequencing of CtERG11 showed a point mutation leading to a tyrosine substitution in the protein sequence, which might lead to decreased binding affinity for azoles. In conclusion, overexpression of CtERG11 associated with a missense mutation in this gene seemed to be responsible for the acquired azole resistance of this clinical isolate.

Therapeutic potential of yeast killer toxin-like antibodies and mimotopes

This review focuses on the potential of yeast killer toxin (KT)-like antibodies (KTAbs), that mimic a wide-spectrum KT through interaction with specific cell wall receptors (KTR) and their molecular derivatives (killer mimotopes), as putative new tools for transdisease anti-infective therapy. KTAbs are produced during the course of experimental and natural infections caused by KTR-bearing micro-organisms. They have been produced by idiotypic vaccination with a KT-neutralizing mAb, also in their monoclonal and recombinant formats. KTAbs and KTAbs-derived mimotopes may exert a strong therapeutic activity against mucosal and systemic infections caused by eukaryotic and prokaryotic pathogenic agents, thus representing new potential wide-spectrum antibiotics.

Candida glabrata Fungemia in Transplant Patients Receiving Voriconazole after Fluconazole

The clinical impact of voriconazole resistance in Candida glabrata is not well described. Five hematopoietic stem cell transplant recipients that developed breakthrough Candida glabrata bloodstream infections while receiving voriconazole are described and the clinical management and susceptibility profiles of their isolates are reported. All patients were markedly immunosuppressed, and in all cases, voriconazole use was preceded by prolonged fluconazole exposure (median 60 days); median voriconazole exposure prior to candidemia was 48 days. Isolates from 4 patients were shown to be resistant to fluconazole and itraconazole when tested in vitro; these same isolates had MICs to voriconazole and posaconazole > or = 2 microg/ml. Clinical failure to voriconazole may result from deficits in host defense, retained infected foci, and adaptation of the organism to environmental pressures, the specific sequence and mechanisms of which warrant further study. Clinicians must maintain a high index of suspicion for these infections in highly susceptible hosts despite voriconazole therapy, particularly when voriconazole use is preceded by prolonged fluconazole exposure.

Establishing Surrogate Markers for Fluconazole Resistance inCandida albicans

Azole-resistant Candida can be a confounding factor for clinical management of opportunistic infections in immunocompromised patients, but rapid identification of such resistant organisms can improve patient outcome. New target-based molecular diagnostic strategies have the potential to identify resistant organisms faster than current culture-based assays. It was the objective of this study to determine whether target site mutations and/or drug pump over-expression are suitable surrogate markers of drug resistance that could aid new molecular-based diagnostic assays. A collection of 59 clinical isolates displaying a range of azole susceptibilities were assayed for mutations within the target gene Erg 11 and for over-expression of drug-efflux pumps Cdr 1, Cdr 2, Flu 1, and Mdr 1, as well as drug target gene Erg 11 by quantitative real-time PCR with molecular beacons. A fluconazole-resistant (MIC>or=64 microg/ml) phenotype was closely associated with over-expression of Cdr 1 (p=0.005), Cdr 2 (p=0.01), and Mdr 1 (p=0.03) along with four mutations in Erg 11 (T 229 A, Y 132 F, S 405 F, G 464 S). Changes in expression levels for Erg 11 and Flu 1 were not statistically correlated with resistance (p=0.27 and p=0.86, respectively). Overall, these findings provide a statistical basis to establish Erg 11 mutations and drug pump over-expression as surrogate markers for phenotypic fluconazole resistance.

Effects of voriconazole, granulocyte-macrophage colony-stimulating factor, and interferon γ on intracellular fluconazole-resistant Candida glabrata and Candida krusei in human monocyte-derived macrophages

Infections caused by fluconazole-resistant Candida glabrata and Candida krusei are increasingly common causes of morbidity and mortality. We investigated the intracellular killing of fluconazole-resistant C. glabrata and C. krusei by cytokine-activated human monocyte-derived macrophages (MDM) in the presence and absence of voriconazole. For C. glabrata, MDM were activated with either granulocyte-macrophage colony-stimulating factor (GM-CSF) or interferon gamma (IFN-gamma) before infection, after infection, or both before and after infection, whereas for C. krusei MDM were activated with cytokines both before and after infection. Activated MDM were infected, treated with voriconazole, and then lysed, and viable yeast in the lysates enumerated at 0, 24, or 48 h after infection. In the presence of voriconazole (2.5 x MIC), the best activity against C. glabrata occurred when MDM were activated with GM-CSF for 24 h before infection as well as after infection or when they were activated for 24 h before infection alone. A lesser effect was observed when MDM were activated for at least 1 h before infection or when they were treated with cytokines only after infection. IFN-gamma activation had a significant but lesser effect than GM-CSF. Activity against C. krusei in the presence of voriconazole was greatest when MDM were activated with IFN-gamma rather than GM-CSF. Our results suggest that cytokines increase the intracellular anticandidal effect of voriconazole and may be useful as therapeutic adjuvants to voriconazole for treatment of infections caused by fluconazole-resistant C. glabrata and C. krusei.

Rapid, high-throughput, multiplex, real-time PCR for identification of mutations in the cyp51A gene of Aspergillus fumigatus that confer resistance to itraconazole

Aspergillus fumigatus is an important cause of life-threatening invasive fungal disease in patients with compromised immune systems. Resistance to itraconazole in A. fumigatus is closely linked to amino acid substitutions in Cyp51A that replace Gly54. In an effort to develop a new class of molecular diagnostic assay that can rapidly assess drug resistance, a multiplexed assay was established. This assay uses molecular beacons corresponding to the wild-type cyp51A gene and seven mutant alleles encoding either Arg54, Lys54, Val54, Trp54, or Glu54. Molecular beacon structure design and real-time PCR conditions were optimized to increase the assay specificity. The multiplex assay was applied to the analysis of chromosomal DNA samples from a collection of 48 A. fumigatus clinical and laboratory-derived isolates, most with reduced susceptibility to itraconazole. The cyp51A allelic identities for codon 54 were established for all of the strains tested, and mutations altering Gly54 in 23 strains were revealed. These mutations included G(54)W (n = 1), G(54)E (n = 12), G(54)K (n = 3), G(54)R (n = 3), and G(54)V (n = 4). Molecular beacon assay results were confirmed by DNA sequencing. Multiplex real-time PCR with molecular beacons is a powerful technique for allele differentiation and analysis of resistance mutations that is dynamic and suitable for rapid high-throughput assessment of drug resistance.

Incidence and management of proven and probable fungal infections in patients with acute leukemia: a single center experience

The incidence of fungal infections and the role of liposomal amphotericin B (Ambisome) in proven and probable infections were evaluated in acute leukemic patients, intolerant to conventional amphotericin B. During 1999-2002, 307 febrile episodes occurred in 231 patients. Fungi were responsible for 3% of bloodstream infections. Ambisome was employed in 5 fungal sepsis (1 Candida albicans, 1 C. famata, 1 C. tropicalis, 1 C. krusei, 1 Geotrichum capitatum) 2 Aspergillosis, 2 probable fungal pneumonia cases. A favorable response was achieved in 78% of patients (4 fungemia, 2 aspergillosis, 1 probable), an unfavorable response in 1 C. krusei fungemia and in 1 probable pneumonia. Our antimicrobial pattern documented a high resistance rate to azoles. We concluded that Ambisome is an effective and well tolerated agent and its introduction has changed the outcome for many patients, although in some refractory diseases other strategies must be considered.

Phenotypic and genotypic evaluation of fluconazole resistance in vaginalCandidastrains isolated from HIV-infected women from Brazil

We conducted a study to determine the antifungal susceptibility of vaginal Candida isolates from HIV-infected Brazilian women. Among 127 women enrolled, positive cultures for yeast were obtained from 31 of 38 (81%) women with symptomatic vulvovaginitis, and from 41 of 89 (46%) asymptomatic women. Susceptibility testing demonstrated 11 of the 72 isolates had either resistance or dose-dependent susceptibility to azole drugs, including four Candida albicans strains. Expression of the MDR1, CDR1, CDR2 and ERG11 genes was evaluated in all of the C. albicans isolates, and all four of the strains with reduced susceptibility to fluconazole had increased expression of CDR1 as compared to the fluconazole-sensitive strains. No increased expression of the other genes was identified. This large survey of Candida isolates from HIV-infected women from Brazil demonstrates that reduced susceptibility to azoles occurs at a low frequency among vaginal yeast isolates, and when present in C. albicans, azole resistance is associated with increased expression of CDR1.

Impact of real-time fungal susceptibility on clinical practices

PURPOSE OF REVIEW

In the past two decades standardized in-vitro antifungal susceptibility tests have been developed in response to increasing invasive fungal infections. Our purpose is to review the utility of real-time Candida antifungal susceptibility testing and its effects on clinical outcomes in the context of the new antifungal agents, existing antifungal susceptibility testing methods, and the changing epidemiology and susceptibilities of Candida spp.

RECENT FINDINGS

New antifungal agents active against Candida spp. are now available. Caspofungin is approved for the treatment of invasive candidiasis but standardized antifungal susceptibility testing has not been developed. The newer triazoles, such as voriconazole, posaconazole and ravuconazole, are not yet approved for invasive candidiasis. As infections caused by non-albicans Candida spp. with growing resistance to fluconazole increase, antifungal susceptibility testing may be important to guide therapy. Unfortunately, few studies exist describing the impact of real-time antifungal susceptibility testing on the treatment of invasive fungal infections.

SUMMARY

Newer antifungal agents with broader anti-Candida activity, fewer adverse events and minimal resistance are currently available. The challenge clinicians face is choosing when to use these agents appropriately, while considering the changing Candida epidemiology and susceptibility trends without over-treating fluconazole-sensitive pathogens. Unfortunately, the correlation of antifungal susceptibility testing results by any method and clinical outcome is mostly lacking. We suggest that a concerted examination of the utility of real-time antifungal susceptibility testing and its effect on clinical outcomes by guiding appropriate antifungal therapy for high-risk patients with invasive candidiasis is warranted.

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.

Terbinafine resistance mediated by salicylate 1-monooxygenase in Aspergillus nidulans

Resistance to antifungal agents is a recurring and growing problem among patients with systemic fungal infections. UV-induced Aspergillus nidulans mutants resistant to terbinafine have been identified, and we report here the characterization of one such gene. A sib-selected, 6.6-kb genomic DNA fragment encodes a salicylate 1-monooxygenase (salA), and a fatty acid synthase subunit (fasC) confers terbinafine resistance upon transformation of a sensitive strain. Subfragments carrying salA but not fasC confer terbinafine resistance. salA is present as a single-copy gene on chromosome VI and encodes a protein of 473 amino acids that is homologous to salicylate 1-monooxygenase, a well-characterized naphthalene-degrading enzyme in bacteria. salA transcript accumulation analysis showed terbinafine-dependent induction in the wild type and the UV-induced mutant Terb7, as well as overexpression in a strain containing the salA subgenomic DNA fragment, probably due to the multicopy effect caused by the transformation event. Additional naphthalene degradation enzyme-coding genes are present in fungal genomes, suggesting that resistance could follow degradation of the naphthalene ring contained in terbinafine.

Evaluation of fluconazole resistance mechanisms in candida albicans clinical isolates from HIV-infected patients in Brazil

In this study, we describe resistance mechanisms in fluconazole-resistant isolates of C. albicans isolated from AIDS patients from nine Brazilian hospitals. These mechanisms include the presence of point mutations in the ERG11 gene and overexpression of ERG11, and several genes encoding efflux pumps, as measured by quantitative real-time reverse transcriptase polymerase chain reaction. Several fluconazole-resistant strains had multiple mechanisms of resistance. Four mutations previously described, Y132F, K143R, E266D, and V437I, were identified among the strains, whereas some isolates contained more than one mutation. Fourteen novel mutations were identified. Interestingly, all Brazilian fluconazole-resistant isolates showed homozygosity at mating-type loci (MTL) associated with fluconazole resistance. This is the first comprehensive assessment at molecular level of mechanisms of fluconazole resistance in C. albicans isolates from South America.

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.

Molecular cloning and biochemical characterization of Candida albicans acyl-CoA:sterol acyltransferase, a potential target of antifungal agents

To determine whether Candida albicans acyl CoA:sterol acyltransferase (ASAT) can be a potential target enzyme for the protoberberine derivative (HWY-289), we have isolated a gene encoding Ca-ASAT and examined inhibitory effects of HWY-289 on the overexpressed Ca-ASAT. HWY-289 specifically inhibits Ca-ASAT in a non-competitive manner in vitro (IC(50) [9.2microM], K(i) [5.15microM]). The cloned CaARE2 gene (1830 nucleotides [nt]) encodes active Ca-ASAT protein that exhibits a calculated molecular mass of 71.3kDa. The amino acid sequence of CaAre2p is 33.4% and 35.1% identical to those of Saccharomyces cerevisiae ScAre1p and ScAre2p homologues, respectively. Recombinant and endogenous Ca-ASAT displayed identical patterns of inhibition upon exposure to HWY-289 and a preference for cholesterol and oleoyl-CoA as substrates. Northern blot analysis showed that CaARE2 was activated by HWY-289, but not by CI-976 (a human acyl-coenzyme A:cholesterol acyltransferase inhibitor), in a dose-dependent manner (up to 5mg/L), suggesting different selectivities of action between HWY-289 and CI-976 on Ca-ASAT activity.

Enzyme-assisted suicide: molecular basis for the antifungal activity of 5-hydroxy-4-oxonorvaline by potent inhibition of homoserine dehydrogenase

The structure of the antifungal drug 5-hydroxy-4-oxonorvaline (HON) in complex with its target homoserine dehydrogenase (HSD) has been determined by X-ray diffraction to 2.6 A resolution. HON shows potent in vitro and in vivo activity against various fungal pathogens despite its weak (2 mM) affinity for HSD in the steady state. The structure together with structure-activity relationship studies, mass spectrometry experiments, and spectroscopic data reveals that the molecular mechanism of antifungal action conferred by HON involves enzyme-dependent formation of a covalent adduct between C4 of the nicotinamide ring of NAD(+) and C5 of HON. Furthermore, novel interactions are involved in stabilizing the (HON*NAD)-adduct, which are not observed in the enzyme's ternary complex structure. These findings clarify the apparent paradox of the potent antifungal actions of HON given its weak steady-state inhibition characteristics.

Aspergillus fumigatus variant with decreased susceptibility to multiple antifungals

Isolates of Aspergillus fumigatus that demonstrate resistance to itraconazole (ITZ) have been described previously; however, the prevalence and clinical significance of ITZ resistance are not completely understood. In this study we assessed the ITZ susceptibilities of 128 A. fumigatus isolates that caused invasive infection in 82 stem cell transplant patients before and after the use of ITZ in our institution (study period, 1991 to 2000). The MICs for 10 isolates obtained from seven patients were high, > or 1 microg/ml. The average ITZ MIC increased after institutional use of the drug began in 1995. The majority of the isolates for which MICs were high (6 of 10) and one isolate for which the MIC was low (0.06 microg/ml) demonstrated an unusual phenotype, appearing as predominantly white colonies. For all seven atypical isolates, voriconazole MICs were high (> or = 2 microg/ml), and minimal effective concentrations of caspofungin were high (> or 4 microg/ml). For two of the seven atypical isolates, amphotericin B MICs were high (> or 2 microg/ml). The isolates appeared white due to slow sporulation; however, after prolonged incubations, the isolates sporulated with no difference in conidial color or conidiophore morphology compared with typical isolates. Randomly amplified polymorphic DNA-PCR patterns of these isolates were distinct compared with those of other A. fumigatus isolates. Sequencing of 18S rRNA genes confirmed that all were A. fumigatus; however, the mitochondrial cytochrome b gene sequences of all the atypical isolates were unique. These data suggest the potential presence of a genetically unique, poorly sporulating variant of A. fumigatus that demonstrates decreased susceptibilities to several antifungals.

Fungal cell wall septation and cytokinesis are inhibited by bleomycins

When the essential and distinctive cell walls of either pathogenic or nonpathogenic fungi break, cytoplasmic membranes rupture and fungi die. This fungicidal activity was discovered previously on nonproliferating Saccharomyces cerevisiae cells treated briefly with the oxidative tool and anticancer drug family of bleomycins. The present studies investigated effects of bleomycin on growing fungal organisms. These included the medically important Aspergillus fumigatus and Cryptococcus neoformans, as well as the emerging human pathogen and fungal model, S. cerevisiae. Bleomycin had its highest potency against A. fumigatus. Scanning electron microscopy and thin-section transmission electron microscopy were used to study morphological growth characteristics. Killing and growth inhibition were also measured. Long, thin, and segmented hyphae were observed when A. fumigatus was grown without bleomycin but were never observed when the mold was grown with the drug. Bleomycin arrested conidial germination, hyphal development, and the progression and completion of cell wall septation. Similarly, the drug inhibited the construction of yeast cell wall septa, preventing cytokinesis and progression in the cell division cycle of S. cerevisiae. Even when cytoplasms of mother and daughter cells separated, septation and cell division did not necessarily occur. Bizarre cell configurations, abnormally thickened cell walls at mother-daughter necks, abnormal polarized growth, large undivided cells, fragmented cells, and empty cell ghosts were also produced. This is the first report of a fungicidal agent that arrests fungal growth and development, septum formation, and cytokinesis and that also preferentially localizes to cell walls and alters isolated cell walls as well as intact cell walls on nongrowing cells.

Antifungal prophylaxis with azole derivatives

In recent years, several reports have underlined the increasing role of fungal infections as a cause of morbidity and mortality in hospitalised patients. For this reason, and also in light of the high mortality rate associated with these infections, chemoprophylaxis has been advocated by several authors. The available evidence suggests that both fluconazole and itraconazole are able to decrease candida colonisation and infection, when compared with placebo or with nonabsorbable antifungals. Data seem also to suggest that a decrease in fungus-related mortality can be achieved with prophylaxis, although with little effect on overall mortality, probably because of the importance of severe underlying diseases. Itraconazole proved to be effective in the prevention of fungal infections, including invasive aspergillosis, although with increased incidence of side-effects, often leading to treatment discontinuation. The other side of the coin is that antifungal prophylaxis might have untoward effects, such as the selection of triazole-resistant Candida strains or the induction of resistance. In addition, some authors have suggested that the use of triazoles might modulate the pattern of infecting organisms in cancer patients, increasing the risk of both aspergillosis and bacteremia. In conclusion, antifungal prophylaxis with triazole antifungals should be used with caution, only in patients at high risk for invasive fungal infections. These include allogeneic bone marrow transplant patients (especially those with mismatched or unrelated donors), acute myeloid leukaemia patients treated with high-dose cytarabine (C-ara), very-low-birth-weight infants, patients with chronic granulomatous disease, and high-risk surgical and intensive-care unit patients.