Characterization of echinocandin-resistant mutants of Candida albicans: genetic, biochemical, and virulence studies

Molecular characterisation of Candida auris isolates from immunocompromised patients in a tertiary-care hospital in Kuwait reveals a novel mutation in FKS1 conferring reduced susceptibility to echinocandins

BACKGROUND

Candida auris is an emerging, potentially multidrug-resistant pathogen that exhibits clade-specific resistance to fluconazole and also develops resistance to echinocandins and amphotericin B easily. This study analysed 49 C auris isolates for alterations in hotspot-1 and hotspot-2 of FKS1 for the detection of mutations conferring reduced susceptibility to echinocandins.

METHODS

C auris isolates (n = 49) obtained from 18 immunocompromised patients during June 2016-December 2018 were analysed. Antifungal susceptibility testing was performed by Etest and broth microdilution-based MICRONAUT-AM assay. Mutations in hotspot-1 and hotspot-2 regions of FKS1 were detected by PCR sequencing and fingerprinting of the isolates was done by short tandem repeat typing.

RESULTS

The patients had multiple comorbidities/risk factors for Candida spp. infection including cancer/leukaemia/lymphoma/myeloma (n = 16), arterial/central line (n = 17), urinary catheter (n = 17), mechanical ventilation (n = 14) and major surgery (n = 9) and received antifungal drugs as prophylaxis and/or empiric treatment. Seven patients developed C auris candidemia/breakthrough candidemia, nine patients had candiduria with/without candidemia and four patients developed surgical site/respiratory infection. Resistance to fluconazole and amphotericin B was detected in 44 and four isolates, respectively. Twelve C auris isolates from eight patients showed reduced susceptibility to echinocandins. Seven isolates contained hostspot-1 mutations and three isolates from a candidemia patient contained R1354H mutation in hotspot-2 of FKS1. Ten patients died, five were cured, two were lost to follow-up and treatment details for one patient were not available.

CONCLUSIONS

Our findings describe development of a novel mutation in FKS1 conferring reduced susceptibility to echinocandins in one patient during treatment and unfavourable clinical outcome for many C auris-infected patients.

Resistance to Antifungal Drugs

Pathogenic fungi have several mechanisms of resistance to antifungal drugs, driven by the genetic plasticity and versatility of their homeostatic responses to stressful environmental cues. We critically review the molecular mechanisms of resistance and cellular adaptations of pathogenic fungi in response to antifungals and discuss the factors contributing to such resistance. We offer suggestions for the translational and clinical research agenda of this rapidly evolving and medically important field. A better understanding of antifungal resistance should assist in developing better detection tools and inform optimal strategies for preventing and treating refractory mycoses in the future.

Site-Directed Mutagenesis of the 1,3-β-Glucan Synthase Catalytic Subunit of Pneumocystis jirovecii and Susceptibility Assays Suggest Its Sensitivity to Caspofungin

The echinocandin caspofungin inhibits the catalytic subunit Gsc1 of the enzymatic complex synthesizing 1,3-β-glucan, an essential compound of the fungal wall. Studies with rodents showed that caspofungin is effective against Pneumocystis asci. However, its efficacy against asci of Pneumocystis jirovecii, the species infecting exclusively humans, remains controversial. The aim of this study was to assess the sensitivity to caspofungin of the P. jirovecii Gsc1 subunit, as well as of those of Pneumocystis carinii and Pneumocystis murina infecting, respectively, rats and mice. In the absence of an established in vitro culture method for Pneumocystis species, we used functional complementation of the Saccharomyces cerevisiae gsc1 deletant. In the fungal pathogen Candida albicans, mutations leading to amino acid substitutions in Gsc1 confer resistance to caspofungin. We introduced the corresponding mutations into the Pneumocystis gsc1 genes using site-directed mutagenesis. In spot dilution tests, the sensitivity to caspofungin of the complemented strains decreased with the number of mutations introduced, suggesting that the wild-type enzymes are sensitive. The MICs of caspofungin determined by Etest and YeastOne for strains complemented with Pneumocystis enzymes (respectively, 0.125 and 0.12 μg/ml) were identical to those upon complementation with the enzyme of C. albicans, for which caspofungin presents low MICs. However, they were lower than the MICs upon complementation with the enzyme of the resistant species Candida parapsilosis (0.19 and 0.25 μg/ml). Sensitivity levels of Gsc1 enzymes of the three Pneumocystis species were similar. Our results suggest that P. jirovecii is sensitive to caspofungin during infections, as are P. carinii and P. murina.

Yeast species-specific, differential inhibition of β-1,3-glucan synthesis by poacic acid and caspofungin

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Poacic acid antifungal activity is both strains and species dependent for a range of Candida species.

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The calcineurin pathway regulates poacic acid sensitivity in C. albicans.

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Point mutations in β-1,3-glucan synthase Fks1 differentially affect poacic acid and echinocandin sensitivity.

The rise of widespread antifungal resistance fuels the need to explore new classes of inhibitory molecules as potential novel inhibitors. Recently a plant natural product poacic acid (PA) was shown to inhibit β-1,3-glucan synthesis, and to have antifungal activity against a range of plant pathogens and against Saccharomyces cerevisiae. As with the echinocandins, such as caspofungin, PA targets the synthesis of cell wall β-1,3-glucan and has potential utility in the treatment of medically important fungi. However, the antifungal activity of PA against human pathogenic Candida species has not been explored and the precise mode of action of this compound is not understood. Here, we show that PA sensitivity is regulated by the calcineurin pathway and that susceptibility to PA varied significantly between Candida species, but did not correlate with in vitro β-glucan synthase activity, cell wall β-glucan content or the sensitivity of the species to caspofungin. Strains with point mutations (S645Y or S645P) in the hotspot1 region of the β-1,3-glucan synthase subunit Fks1, had decreased sensitivity to caspofungin but increased sensitivity to PA. C. guilliermondii, C. orthopsilosis, and C. parapsilosis were more sensitive to PA than C. albicans, C. dubliniensis, C. tropicalis, and C. glabrata. These observations suggest that there are significant differences in the mode of action of PA and caspofungin and that PA or PA analogues are not likely to have broad spectrum activity in the treatment of Candida infections.

Role of FKS Gene in the Susceptibility of Pathogenic Fungi to Echinocandins

Echinocandins are antifungal agents that specifically inhibit the biosynthesis of 1,3-β-D-glucan, a major structural component of fungal cell walls. Echinocandins are recommended as first-line or alternative/salvage therapy for candidiasis and aspergillosis in antifungal guidelines of various countries. Resistance to echinocandins has been reported in recent years. The mechanism of echinocandin resistance involves amino acid substitutions in hot spot regions of the FKS gene product, the catalytic subunit of 1,3-β-D-glucan synthase. This resistance mechanism contributes to not only acquired resistance in Candida spp., but also inherent resistance in some pathogenic fungi. An understanding of the echinocandin resistance mechanism is important to develop both effective diagnosis and treatment options for echinocandin-resistant fungal diseases.

Targeting Candida spp. to develop antifungal agents

Invasive fungal infections are a complex challenge throughout the world because of their high incidence, mainly in critically ill patients, and high mortality rates. The antifungal agents currently available are limited; thus, there is a need for the rapid development of new drugs. In silico methods are a modern strategy to explore interactions between new compounds and specific fungal targets, but they depend on precise genetic information. Here, we discuss the main Candida spp. target genes, including information about null mutants, virulence, cytolocalization, co-regulatory genes, and compounds that are related to protein expression. These data will provide a basis for the future in silico development of antifungal drugs.

Comparative study of Candida spp. isolates: Identification and echinocandin susceptibility in isolates obtained from blood cultures in 15 hospitals in Medellín, Colombia

OBJECTIVES

Invasive candidiasis has a high impact on morbidity and mortality in hospitalised patients. Accurate and timely methods for identification of Candida spp. and determination of echinocandin susceptibility have become a priority for clinical microbiology laboratories.

METHODS

This study was performed to compare matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF/MS) identification with sequencing of the D1/D2 region of the rRNA gene complex 28 subunit in 147 Candida spp. isolates obtained from patients with candidaemia. Antimicrobial susceptibility testing was performed by broth microdilution (BMD) and Etest. Sequencing of the FKS1 and FKS2 genes was performed.

RESULTS

The most common species isolated were Candida albicans (40.8%), followed by Candida parapsilosis (23.1%) and Candida tropicalis (17.0%). Overall agreement between the results of identification by MALDI-TOF/MS and molecular identification was 99.3%. Anidulafungin and caspofungin susceptibility by the BMD method was 98.0% and 88.4%, respectively. Susceptibility to anidulafungin and caspofungin by Etest was 93.9% and 98.6%, respectively. Categorical agreement between Etest and BMD was 91.8% for anidulafungin and 89.8% for caspofungin, with lower agreements in C. parapsilosis for anidulafungin (76.5%) and C. glabrata for caspofungin (40.0%). No mutations related to resistance were found in the FKS genes, although 54 isolates presented synonymous polymorphisms in the hotspots sequenced.

CONCLUSIONS

MALDI-TOF/MS is a good alternative for routine identification of Candida spp. isolates. DNA sequencing of the FKS genes suggested that the isolates analysed were susceptible to echinocandins; alternatively, unknown resistance mechanisms or limitations related to antifungal susceptibility tests may explain the resistance found in a few isolates.

Characterization of In Vitro Resistance Development to the Novel Echinocandin CD101 in Candida Species

CD101 is a novel echinocandin with a long half-life undergoing clinical development for treatment of candidemia/invasive candidiasis and vulvovaginal candidiasis. The potential for and mechanisms underlying the development of resistance to CD101 in Candida species were investigated by using spontaneous resistance and serial passage selection methodologies. Four Candida spp. (C. albicans, C. glabrata, C. parapsilosis, and C. krusei) were chosen for resistance characterization with CD101, anidulafungin, and caspofungin. The frequency of spontaneous, single-step mutations conferring reduced susceptibility to CD101 at 1× the agar growth inhibition concentration was low across all species, with median frequencies ranging from 1.35 × 10(-8) to 3.86 × 10(-9), similar to ranges generated for anidulafungin and caspofungin. Serial passage of Candida spp. on agar plates containing drug gradients demonstrated a low potential for resistance development, with passage 20 CD101-selected strains possessing increases in MICs equivalent to or lower than those for the majority of strains generated under selection with anidulafungin and caspofungin. A total of 12 fks "hot spot" mutations were identified, typically in strains with the highest MIC shifts. Cross-resistance was broadly observed among the 3 echinocandins evaluated, with no CD101-selected mutants (with or without fks hot spot mutations) exhibiting reduced susceptibility to CD101 but not also to anidulafungin and/or caspofungin. Consistent with currently approved echinocandins, CD101 demonstrates a low potential for resistance development, which could be further enhanced in vivo by the high maximum concentration of drug in serum (Cmax)/area under the concentration-time curve (AUC) plasma drug exposure achieved with once-weekly dosing of CD101.

Discontinuation of echinocandin and azole treatments led to the disappearance of an FKS alteration but not azole resistance during clonal Candida glabrata persistent candidaemia

To give an indication of a fitness cost conferred by FKS mutation-associated echinocandin resistance in Candida glabrata during human infection. Six C. glabrata clinical strains sequentially isolated from blood and a hepatic abscess in a solid organ transplant recipient were analysed. The patient had received long-term azole and echinocandin therapy for invasive aspergillosis and persistent candidaemia. Minimal inhibitory concentrations were determined by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) broth microdilution method. Molecular mechanisms of antifungal resistance were determined by sequencing hot spots of the FKS. Strain relatedness was determined using a microsatellite-based typing method. Typing analysis revealed an identical microsatellite pattern for all isolates, supporting a close relation. The first C. glabrata isolate showed wild-type phenotype (i.e. susceptibility to echinocandins and low level of azole resistance). After voriconazole therapy, the C. glabrata quickly acquired pan-azole resistance. Later, echinocandin treatment led to the emergence of a FKS2 S663P alteration and echinocandin resistance. After disruption of both azole and echinocandin therapy in favour of liposomal amphotericin B, C. glabrata isolates regained full susceptibility to echinocandin and lost the FKS2 S663P alteration while nonetheless maintaining their pan-azole resistance. Our clinical report supports the potential existence of a fitness cost conferred by FKS mutation in C. glabrata, as disruption of treatment led to a rapid disappearance of the resistant clone. This suggests that a more restricted use and/or a discontinuous administration of echinocandins may limit the spread of clinical resistance to this class.

Tipping the balance both ways: drug resistance and virulence in Candida glabrata

Among existing fungal pathogens, Candida glabrata is outstanding in its capacity to rapidly develop resistance to currently used antifungal agents. Resistance to the class of azoles, which are still widely used agents, varies in proportion (from 5 to 20%) depending on geographical area. Moreover, resistance to the class of echinocandins, which was introduced in the late 1990s, is rising in several institutions. The recent emergence of isolates with acquired resistance to both classes of agents is a major concern since alternative therapeutic options are scarce. Although considered less pathogenic than C. albicans, C. glabrata has still evolved specific virulence traits enabling its survival and propagation in colonized and infected hosts. Development of drug resistance is usually associated with fitness costs, and this notion is documented across several microbial species. Interestingly, azole resistance in C. glabrata has revealed the opposite. Experimental models of infection showed enhanced virulence of azole-resistant isolates. Moreover, azole resistance could be associated with specific changes in adherence properties to epithelial cells or innate immunity cells (macrophages), both of which contribute to virulence changes. Here we will summarize the current knowledge on C. glabrata drug resistance and also discuss the consequences of drug resistance acquisition on the balance between C. glabrata and its hosts.

Emergence of echinocandin-resistant Candida spp. in a hospital setting: a consequence of 10 years of increasing use of antifungal therapy?

Since their introduction in the 2000s, echinocandin drugs have become widely used for the treatment and prophylaxis of invasive fungal infections and, notably, invasive candidiasis. Although cases of breakthrough candidiasis in patients receiving echinocandins have been reported, clinical failure during echinocandin treatment due to the acquisition of resistance by a normally susceptible Candida spp. isolate is considered rare. To date, no publications have been published correlating the use of echinocandins and the emergence of echinocandin resistance among Candida species. So, our goal is to report an initial analysis of echinocandin use in relation to the emergence of resistant Candida isolates. We report here a single-centre experience of the emergence of eight resistant isolates belonging to normally susceptible Candida species in six patients receiving echinocandins. We describe the context and analyse the use of echinocandins over the previous decade. For seven of these isolates, we identified FKS gene mutations involved in decreased susceptibility. Seven isolates were obtained in 2011, on the heels of a ten-fold increase in caspofungin use over the preceding decade. In contrast, in 2012, the use of echinocandins decreased in our institution by 19.5 % and, in that year, only one Candida-resistant isolate was detected, despite the stable global epidemiology of invasive candidaemia. This work underlines the necessity of improving the prescription of antifungal drugs. Improvement in the monitoring of strain susceptibility should also be considered in order to better detect the emergence of resistant or non-susceptible yeast strains.

Chromosome 5 monosomy of Candida albicans controls susceptibility to various toxic agents, including major antifungals

Candida albicans is a prevailing fungal pathogen with a diploid genome that can adapt to environmental stresses by losing or gaining an entire chromosome or a large portion of a chromosome. We have previously found that the loss of one copy of chromosome 5 (Ch5) allows for adaptation to the toxic sugar l-sorbose. l-Sorbose is similar to caspofungin and other antifungals from the echinocandins class, in that it represses synthesis of cell wall glucan in fungi. Here, we extended the study of the phenotypes controlled by Ch5 copy number. We examined 57 strains, either disomic or monosomic for Ch5 and representing five different genetic backgrounds, and found that the monosomy of Ch5 causes elevated levels of chitin and repressed levels of 1,3-β-glucan components of the cell wall, as well as diminished cellular ergosterol. Increased deposition of chitin in the cell wall could be explained, at least partially, by a 2-fold downregulation of CHT2 on the monosomic Ch5 that encodes chitinase and a 1.5-fold upregulation of CHS7 on Ch1 that encodes the protein required for wild-type chitin synthase III activity. Other important outcomes of Ch5 monosomy consist of susceptibility changes to agents representing four major classes of antifungals. Susceptibility to caspofungin increased or decreased and susceptibility to 5-fluorocytosine decreased, whereas susceptibility to fluconazole and amphotericin B increased. Our results suggest that Ch5 monosomy represents an unrecognized C. albicans regulatory strategy that impinges on multiple stress response pathways.

Comparison of three statistical methods for establishing tentative wild-type population and epidemiological cutoff values for echinocandins, amphotericin B, flucytosine, and six Candida species as determined by the colorimetric Sensititre YeastOne method

The Sensititre YeastOne (SYO) method is a widely used method to determine the susceptibility of Candida spp. to antifungal agents. CLSI clinical breakpoints (CBP) have been reported for antifungals, but not using this method. In the absence of CBP, epidemiological cutoff values (ECVs) are useful to separate wild-type (WT) isolates (those without mechanisms of resistance) from non-WT isolates (those that can harbor some resistance mechanisms), which is the goal of any susceptibility test. The ECVs for five agents, obtained using the MIC distributions determined by the SYO test, were calculated and contrasted with those for three statistical methods and the MIC(50) and modal MIC, both plus 2-fold dilutions. The median ECVs (in mg/liter) (% of isolates inhibited by MICs equal to or less than the ECV; number of isolates tested) of the five methods for anidulafungin, micafungin, caspofungin, amphotericin B, and flucytosine, respectively, were as follows: 0.25 (98.5%; 656), 0.06 (95.1%; 659), 0.25 (98.7%; 747), 2 (100%; 923), and 1 (98.5%; 915) for Candida albicans; 8 (100%; 352), 4 (99.2%; 392), 2 (99.2%; 480), 1 (99.8%; 603), and 0.5 (97.9%; 635) for C. parapsilosis; 1 (99.2%; 123), 0.12 (99.2%; 121), 0.25 (99.2%; 138), 2 (100%; 171), and 0.5 (97.2%; 175) for C. tropicalis; 0.12 (96.6%; 174), 0.06 (96%; 176), 0.25 (98.4%; 188), 2 (100%; 209), and 0.25 (97.6%; 208) for C. glabrata; 0.25 (97%; 33), 0.5 (93.9%; 33), 1 (91.9%; 37), 4 (100%; 51), and 32 (100%; 53) for C. krusei; and 4 (100%; 33), 2 (100%; 33), 2 (100%; 54), 1 (100%; 90), and 0.25 (93.4%; 91) for C. orthopsilosis. The three statistical methods gave similar ECVs (within one dilution) and included ≥ 95% of isolates. These tentative ECVs would be useful for monitoring the emergence of isolates with reduced susceptibility by use of the SYO method.

Echinocandin Resistance in Candida Species: Mechanisms of Reduced Susceptibility and Therapeutic Approaches

OBJECTIVE:

To summarize published data regarding mechanisms of reduced echinocandin susceptibility in Candida spp., the impact of echinocandin resistance on the fitness and virulence of Candida isolates, and current and future treatment approaches.

DATA SOURCES:

A search of MEDLINE databases (1966-September 2011) was conducted.

STUDY SELECTION AND DATA EXTRACTION:

Databases were searched using the terms echinocandin, resistance, and Candida. Citations from publications were reviewed for additional references.

DATA SYNTHESIS:

Echinocandins have in vitro activity against most Candida spp. and are first-line agents in the treatment of candidemia. However, case reports describing echinocandin treatment failure due to resistant isolates have been published. Reduced echinocandin susceptibility has been shown to occur via 3 main mechanisms: (1) adaptive stress responses, which result in elevated cell wall chitin content and paradoxical growth in vitro at supra minimum inhibitory concentrations (MICs); (2) acquired FKS mutations, which confer reduced glucan synthase sensitivity, elevated MICs, and are associated with clinical failure; and (3) intrinsic FKS mutations, which are naturally occurring mutations in C. parapsilosis and C. guilliermondii, which confer elevated MIC levels but a lower level of reduced glucan synthase sensitivity compared with acquired FKS mutations. Some FKS mutants have been shown to have significantly reduced fitness and virulence versus wild type isolates and may contribute to the low incidence of echinocandin resistance reported in large surveillance studies. Treatment strategies evaluated for FKS mutants include echinocandin dose escalation and combination with agents such as calcineurin inhibitors, HSP90 inhibitors, and chitin synthase inhibitors.

CONCLUSIONS:

While the incidence of echinocandin resistance in Candida spp. is low, it can present a significant therapeutic challenge, especially in multidrug-resistant Candida isolates. Dose escalation is unlikely to be effective in treating FKS mutant isolates, and significant adverse effects limit the clinical use of agents evaluated as combination therapy. Patients with infections failing to respond to echinocandin therapy should undergo susceptibility testing and be treated with an alternative antifungal agent if possible.

The potential impact of antifungal drug resistance mechanisms on the host immune response to Candida

A large number of studies have been published over the last two decades examining molecular mechanisms of antifungal resistance in Candida species. However, few of these studies have explored how such mechanisms influence the host immune response to this opportunistic pathogen. With recent advances in our understanding of host immunity to Candida, a body of emerging literature has begun to explore how intrinsic and adaptive resistance mechanisms in Candida alter host immune system evasion and detection, which could have important implications for understanding (1) why certain resistance mechanisms and Candida species predominate in certain patient populations, (2) the biological context for understanding why high in vitro levels of resistance in may not necessarily correlate with risk of drug failure in vivo and (3) insight into effective immunotherapeutic strategies for combatting Candida resistance. Although this area of research is still in its infancy, two themes are emerging: First, the immunoevasion and intracellular persistence of C. glabrata may be a key factor in the capability of this species to persist in the course of multiple antifungal treatments and develop multidrug resistance. Second, changes in the cell wall associated with antifungal resistance often favor evasion for the host immune response.

Chitinases in Pneumocystis carinii pneumonia

Pneumocystis pneumonia remains an important complication of immune suppression. The cell wall of Pneumocystis has been demonstrated to potently stimulate host inflammatory responses, with most studies focusing on β-glucan components of the Pneumocystis cell wall. In the current study, we have elaborated the potential role of chitins and chitinases in Pneumocystis pneumonia. We demonstrated differential host mammalian chitinase expression during Pneumocystis pneumonia. We further characterized a chitin synthase gene in Pneumocystis carinii termed Pcchs5, a gene with considerable homolog to the fungal chitin biosynthesis protein Chs5. We also observed the impact of chitinase digestion on Pneumocystis-induced host inflammatory responses by measuring TNFα release and mammalian chitinase expression by cultured lung epithelial and macrophage cells stimulated with Pneumocystis cell wall isolates in the presence and absence of exogenous chitinase digestion. These findings provide evidence supporting a chitin biosynthetic pathway in Pneumocystis organisms and that chitinases modulate inflammatory responses in lung cells. We further demonstrate lung expression of chitinase molecules during Pneumocystis pneumonia.

Reconstitution of high-level micafungin resistance detected in a clinical isolate of Candida glabrata identifies functional homozygosity in glucan synthase gene expression

OBJECTIVES

A mechanism for the acquisition of high-level echinocandin resistance in Candida glabrata was investigated. FKS mutants were constructed to: determine whether clinically significant micafungin resistance requires a hot-spot mutation in FKS1 and a premature stop codon in FKS2, as was observed in a clinical isolate; select for variants with reduced susceptibility and locate mutations in FKS genes; and assess the roles of FKS1 and FKS2.

METHODS

A panel of FKS mutants was constructed using micafungin-susceptible parents by site-directed mutagenesis. Drug susceptibility, gene expression and glucan synthase activities were compared between mutants. Mutations acquired by selection were identified by DNA sequence analysis of FKS genes from selected variants. Single FKS deletants were constructed and their phenotypes examined.

RESULTS

Introduction of the hot-spot mutation in FKS1 alone conferred an intermediate reduction in susceptibility, and the premature stop codon in FKS2 alone had no effect on susceptibility, while severely reduced susceptibility equivalent to that of the clinical isolate required both mutations. Exposure of susceptible strains to micafungin yielded variants with an intermediate reduction in susceptibility that possessed a hot-spot mutation in FKS1. Further exposure to micafungin yielded variants with severely reduced susceptibility that acquired various single mutations in FKS2. The phenotypes of Δfks1 and Δfks2 mutants indicate that the two FKS genes are functionally redundant, while deletion of both FKS1 and FKS2 conferred synthetic lethality.

CONCLUSIONS

In the laboratory mutants of C. glabrata, clinically significant reduced susceptibility to micafungin required single nucleotide changes in both FKS1 and FKS2, and both genes encoded β-1,3-glucan synthase catalytic subunits.

Fungal infections: their diagnosis and treatment in transplant recipients

Systemic fungal infections typically occur in individuals who are seriously ill with recognized risk factors such as those frequently found in transplant recipients. Unfortunately, they are often diagnosed late, when the efficacy of the available treatments is low, often less than 50%, and the cost in terms of lives lost, hospital length of stay, and total hospital costs is substantially increased. The application of antifungal therapies associated with reported efficacy rates greater than 50% are those used prophylactically. When used prophylactically, these infections are reduced in greater than 95% of the expected cases. The choice of a prophylactic agent should be based upon its ease of administration, lack of adverse effects, reduced likelihood of potential drug interactions, and its efficacy in patients with established risk factors and comorbid disease processes that include renal, hepatic, and chronic pulmonary disease. The indications for the use of currently available antifungal agents, their adverse effects, drug interactions, ease of dosing, and applicability in patients with preexisting disease states, and especially in liver transplant recipients, are presented in this paper.

Fitness and virulence costs of Candida albicans FKS1 hot spot mutations associated with echinocandin resistance

The identification of FKS1 mutations in Candida albicans associated with echinocandin resistance has raised concerns over the spread of drug-resistant strains. We studied the impact of fks1 mutations on C. albicans virulence and fitness. Compared with wild-type strains for FKS1, echinocandin-resistant C. albicans strains with homozygous fks1 hot-spot mutations had reduced maximum catalytic capacity of their glucan synthase complexes and thicker cell walls attributable to increased cell wall chitin content. The fks1 mutants with the highest chitin contents had reduced growth rates and impaired filamentation capacities. Fks1 mutants were hypovirulent in fly and mouse models of candidiasis, and this phenotype correlated with the cell wall chitin content. In addition, we observed reduced fitness of echinocandin-resistant C. albicans in competitive mixed infection models. We conclude that fks1 mutations that confer echinocandin resistance come at fitness and virulence costs, which may limit their epidemiological and clinical impact.

Treatment of candiduria with micafungin: A case series

There has been a gradual increase in the incidence of non-Candida albicans-related nosocomial infections. Candida glabrata urinary tract infections have increased in frequency, and treating these infections can be difficult because the organism may be resistant to fluconazole. A newer antifungal agent, micafungin, which belongs in the class of echinocandins, provides an alternative and effective therapy against C glabrata. The present report describes three cases of C glabrata-associated urinary tract infections successfully treated with micafungin. To the authors' knowledge, this is the first report of successful treatment of C glabrata and azole-resistant C albicans-associated urinary tract infection with an echinocandin.

Comparison of echinocandin antifungals

The incidence of invasive fungal infections, especially those due to Aspergillus spp. and Candida spp., continues to increase. Despite advances in medical practice, the associated mortality from these infections continues to be substantial. The echinocandin antifungals provide clinicians with another treatment option for serious fungal infections. These agents possess a completely novel mechanism of action, are relatively well-tolerated, and have a low potential for serious drug-drug interactions. At the present time, the echinocandins are an option for the treatment of infections due Candida spp (such as esophageal candidiasis, invasive candidiasis, and candidemia). In addition, caspofungin is a viable option for the treatment of refractory aspergillosis. Although micafungin is not Food and Drug Administration-approved for this indication, recent data suggests that it may also be effective. Finally, caspofungin- or micafungin-containing combination therapy should be a consideration for the treatment of severe infections due to Aspergillus spp. Although the echinocandins share many common properties, data regarding their differences are emerging at a rapid pace. Anidulafungin exhibits a unique pharmacokinetic profile, and limited cases have shown a potential far activity in isolates with increased minimum inhibitory concentrations to caspofungin and micafungin. Caspofungin appears to have a slightly higher incidence of side effects and potential for drug-drug interactions. This, combined with some evidence of decreasing susceptibility among some strains of Candida, may lessen its future utility. However, one must take these findings in the context of substantially more data and use with caspofungin compared with the other agents. Micafungin appears to be very similar to caspofungin, with very few obvious differences between the two agents.

Anidulafungin in the treatment of invasive fungal infections

More antifungal agents have reached clinical use in the past two decades than at any other time. The echinocandins have been a welcome addition to this group, with the latest being anidulafungin. There are several lines of evidence to support anidulafungin’s role as primary therapy for the treatment of invasive candidiasis in non-neutropenic patients, and as alternative therapy to fluconazole in patients with esophageal candidiasis with azole intolerance or triazole-resistant Candida. Pharmacokinetic–pharmacodynamic studies in animals have demonstrated superior efficacy, defined as maximal microbial kill, when compared to fluconazole, regardless of the fluconazole susceptibility of the Candida species. These studies, as well as dose-effect studies in patients, also support the currently recommended dose of anidulafungin. A well designed randomized controlled trial has demonstrated anidulafungin’s efficacy in patients with invasive candidiasis. In this paper, we argue that anidulafungin may be preferable to fluconazole for the treatment of candidemia. However, as of yet, the difference between anidulafungin and the other two licensed echinocandins as first-line therapy for invasive candidiasis is unclear. On the other hand, there is insufficient evidence as of yet to support first-line use of anidulafungin in patients with neutropenia or aspergillosis.

Caspofungin: when and how? The microbiologist's view

The echinocandins are antifungal agents, which act by inhibiting the synthesis of β-(1,3)-D-glucan, an integral component of fungal cell walls. Caspofungin, the first approved echinocandin, demonstrates good in vitro and in vivo activity against a range of Candida species and is an alternative therapy for Aspergillus infections. Caspofungin provides an excellent safety profile and is therefore favoured in patients with moderately severe to severe illness, recent azole exposure and in those who are at high risk of infections due to Candida glabrata or Candida krusei. In vivo/in vitro resistance to caspofungin and breakthrough infections in patients receiving this agent have been reported for Candida and Aspergillus species. The types of pathogens and the frequency causing breakthrough mycoses are not well delineated. Caspofungin resistance resulting in clinical failure has been linked to mutations in the Fksp subunit of glucan synthase complex. European Committee for Antimicrobial Susceptibility Testing and Clinical and Laboratory Standards Institute need to improve the in vitro susceptibility testing methods to detect fks hot spot mutants. Caspofungin represents a significant advance in the care of patients with serious fungal infections.

Regulatory circuitry governing fungal development, drug resistance, and disease

Pathogenic fungi have become a leading cause of human mortality due to the increasing frequency of fungal infections in immunocompromised populations and the limited armamentarium of clinically useful antifungal drugs. Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are the leading causes of opportunistic fungal infections. In these diverse pathogenic fungi, complex signal transduction cascades are critical for sensing environmental changes and mediating appropriate cellular responses. For C. albicans, several environmental cues regulate a morphogenetic switch from yeast to filamentous growth, a reversible transition important for virulence. Many of the signaling cascades regulating morphogenesis are also required for cells to adapt and survive the cellular stresses imposed by antifungal drugs. Many of these signaling networks are conserved in C. neoformans and A. fumigatus, which undergo distinct morphogenetic programs during specific phases of their life cycles. Furthermore, the key mechanisms of fungal drug resistance, including alterations of the drug target, overexpression of drug efflux transporters, and alteration of cellular stress responses, are conserved between these species. This review focuses on the circuitry regulating fungal morphogenesis and drug resistance and the impact of these pathways on virulence. Although the three human-pathogenic fungi highlighted in this review are those most frequently encountered in the clinic, they represent a minute fraction of fungal diversity. Exploration of the conservation and divergence of core signal transduction pathways across C. albicans, C. neoformans, and A. fumigatus provides a foundation for the study of a broader diversity of pathogenic fungi and a platform for the development of new therapeutic strategies for fungal disease.

Comparative in vivo dose-dependent activity of caspofungin and anidulafungin against echinocandin-susceptible and -resistant Aspergillus fumigatus

BACKGROUND

Echinocandin resistance in Aspergillus species is rare. We examined if mutations in FKS1 would result in a complete loss of echinocandin activity in vivo in an experimental model of aspergillosis.

METHODS

Neutropenic mice were infected with either an echinocandin-susceptible Aspergillus fumigatus (AF 293) or an echinocandin-resistant A. fumigatus laboratory strain harbouring 'hot-spot' substitution in Fks1p (AF Ser678Pro). Mice then received daily treatment with either anidulafungin or caspofungin at varying dosages (0.25-16 mg/kg/day) for 5 days and Aspergillus lung fungal burden was assessed by quantitative real-time PCR.

RESULTS

Both strains produced histological evidence of progressive invasive pulmonary aspergillosis, but AF Ser678Pro was less virulent than AF 293, as evidenced by lower lung fungal burden and longer median survival time. At > 0.5 mg/kg/day, both anidulafungin and caspofungin reduced the lung fungal burden in neutropenic animals infected with AF 293, but had mixed efficacy against the resistant AF Ser678Pro strain. For caspofungin, the fungal burden was reduced only at doses <1 mg/kg/day. Anidulafungin also modestly reduced AF Ser678Pro lung fungal burden, but only at > 4 mg/kg/day.

CONCLUSIONS

Despite a lack of appreciable antifungal activity in vitro, both anidulafungin and caspofungin were still modestly effective in vivo against a laboratory-generated A. fumigatus mutant harbouring the Ser678Pro mutation in Fks1p. This persistent activity, combined with impaired fitness of the isolate in vivo, could partially explain why microbiologically documented echinocandin-resistance in Aspergillus species remains a rare clinical occurrence.

Differential activities of three families of specific beta(1,3)glucan synthase inhibitors in wild-type and resistant strains of fission yeast

Three specific β(1,3)glucan synthase (GS) inhibitor families, papulacandins, acidic terpenoids, and echinocandins, have been analyzed in Schizosaccharomyces pombe wild-type and papulacandin-resistant cells and GS activities. Papulacandin and enfumafungin produced similar in vivo effects, different from that of echinocandins. Also, papulacandin was the strongest in vitro GS inhibitor (IC(50) 10(3)-10(4)-fold lower than with enfumafungin or pneumocandin), but caspofungin was by far the most efficient antifungal because of the following. 1) It was the only drug that affected resistant cells (minimal inhibitory concentration close to that of the wild type). 2) It was a strong inhibitor of wild-type GS (IC(50) close to that of papulacandin). 3) It was the best inhibitor of mutant GS. Moreover, caspofungin showed a special effect for two GS inhibition activities, of high and low affinity, separated by 2 log orders, with no increase in inhibition. pbr1-8 and pbr1-6 resistances are due to single substitutions in the essential Bgs4 GS, located close to the resistance hot spot 1 region described in Saccharomyces and Candida Fks mutants. Bgs4(pbr)(1-8) contains the E700V change, four residues N-terminal from hot spot 1 defining a larger resistance hot spot 1-1 of 13 amino acids. Bgs4(pbr)(1-6) contains the W760S substitution, defining a new resistance hot spot 1-2. We observed spontaneous revertants of the spherical pbr1-6 phenotype and found that an additional A914V change is involved in the recovery of the wild-type cell shape, but it maintains the resistance phenotype. A better understanding of the mechanism of action of the antifungals available should help to improve their activity and to identify new antifungal targets.

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.

Oropharyngeal candidiasis in the era of antiretroviral therapy

Oropharyngeal candidiasis (OPC) remains a common problem in the HIV-infected population despite the availability of antiretroviral therapy (ART). Although Candida albicans is the most frequently implicated pathogen, other Candida species also may cause infection. The emergence of antifungal resistance within these causative yeasts, especially in patients with recurrent oropharyngeal infection or with long-term use of antifungal therapies, requires a working knowledge of alternative antifungal agents. Identification of the infecting organism and antifungal susceptibility testing enhances the ability of clinicians to prescribe appropriate antifungal therapy. Characterization of the responsible mechanisms has improved our understanding of the development of antifungal resistance and could enhance the management of these infections. Immune reconstitution has been shown to reduce rates of OPC, but few studies have evaluated the current impact of ART on the epidemiology of OPC and antifungal resistance in these patients. Preliminary results from an ongoing clinical study showed that in patients with advanced AIDS, oral yeast colonization was extensive, occurring in 81.1% of the 122 patients studied, and symptomatic infection occurred in one-third. In addition, resistant yeasts were still common, occurring in 25.3% of patients colonized with yeasts or with symptomatic infection. Thus, OPC remains a significant infection in advanced AIDS, even with ART. Current knowledge of the epidemiology, pathogenesis, clinical presentation, treatment, and mechanisms of antifungal resistance observed in OPC are important in managing patients with this infection and are the focus of this review.

Determination of Echinocandin MICs for Candida species in less than 8 hours: comparison of the rapid susceptibility assay with the Clinical and Laboratory Standards Institute's broth microdilution assay

The echinocandins prevent fungal cell wall synthesis by inhibiting beta-1,3-glucan synthesis, a significant glucose-consuming process. Previous studies suggested that echinocandin inhibitory activity is evident within 1 h of exposure. We hypothesized that a susceptibility assay based on glucose consumption may provide clinically useful MICs rapidly. The rapid susceptibility assay (RSA), which provides MICs in less than 8 h, was compared with the standard broth microdilution susceptibility assay (Clinical and Laboratory Standards Institute, document M27-A3, 2008) for 56 Candida species strains. Variables which are known to influence MICs determined by the M27-A3 method were also assessed for their effects on the RSA results. Excellent agreement (>90%) between the results of the RSA and M27-A3 methods was achieved for all three FDA-approved echinocandins (micafungin, caspofungin, and anidulafungin). Candida lusitaniae strains were responsible for most of the discordant results. Assay variables such as the test medium, the age of the inoculum culture, and the presence of human serum affected MIC results from the RSA and the M27-A3 method similarly. The RSA is equivalent to the standard M27-A3 method for determining echinocandin MICs for Candida species. The RSA provides MIC results in less than 8 h and can be applied to old and young yeast colonies. The assay could potentially provide clinically useful MICs on the same day that yeast growth from a specimen is first detected on solid medium.

[The mechanisms of resistance to echinocandin class of antifungal drugs]

The echinocandin (candin) class of antifungal drugs inhibit beta-1,3-glucan synthase and block synthesis of beta-1,3-glucan , an important polysaccharide in fungal cell walls. Candins are used widely for treatment of systemic infections caused by Candida and Aspergillus because of their high potency and low toxicity to humans. The incidence of candin resistance has been rare compared to that of azole resistance, although candin-resistant clinical isolates of C. albicans, C. glabrata, C. krusei and C. tropicalis have been reported in the USA and Europe in recent years. These isolates possess hundred-fold higher MIC values for candins than sensitive strains, as well as candin-resistant beta-1,3-glucan synthase activities. Their candin resistance is associated with amino acid substitutions in the echinocandin resistant region (Ech) of the FKS gene that encodes a catalytic subunit of the beta-1,3-glucan synthase. However, the effect of these amino acid substitutions on the drug-protein interaction and the molecular basis for the resistance is unknown. The exposure of fungi to candin drugs induces stress responses that activate networks involving transcriptional regulators and components controlling signal transduction of the pathways responsible for maintenance of fungal cell wall integrity. The fungal cell wall is still an attractive drug target and further investigation into the mechanisms of candin resistance and structural analysis of the beta-1,3-glucan synthase protein complex will facilitate the development of broad spectrum inhibitors of fungal cell wall synthesis.

Herpes simplex virus type 1 dysregulates anti-fungal defenses preventing monocyte activation and downregulating toll-like receptor-2

We investigated the interplay occurring between pathogens in the course of dual infections, using an in vitro model in which the THP-1 monocytic cell line is first infected with HSV-1 and then exposed to Ca or Cn. These three pathogens share some pathogenic features: they cause opportunistic infections, target macrophages and are neurotropic. Here, we show that HSV-1-infected THP-1 cells exhibited augmented phagocytosis against the two opportunistic fungi but reduced capability to counteract fungal infection: the better ingestion by monocytes was followed by facilitated fungal survival and replication. Reduced IL-12 production was also observed. Cytofluorimetric analysis showed that HSV-1-infected monocytes exhibit: (i) downregulated TLR-2 and TLR-4, critical structures in fungal recognition; (ii) reduced expression of CD38 and CD69, known to be important markers of monocyte activation; and (iii) enhanced expression of apoptosis and necrosis markers, in the absence of altered cell proliferation. Overall, these findings imply that HSV-1 infection prevents monocyte activation, thus leading to a significant dysfunction of the monocyte-mediated anti-Candida response; HSV-1 induced apoptosis and necrosis of monocytes further contribute to this impairment.

[The fungal cell wall and the mechanism of action of anidulafungin]

The fungal cell wall is a structure with a high plasticity that protects the cell from different types of environmental stresses including changes in osmotic pressure. In addition to that, the cell wall allows the fungal cell to interact with its environment, since some of its proteins are adhesins and receptors. Some of its components are highly immunogenic. The structure of the fungal cell wall is unique to the fungi, and it is composed of glucan, chitin and glycoproteins. Since humans lack the components present in the cell walls of fungi, this structure is an excellent target for the development of antifungal drugs. Anidulafungin, like the rest of echinocandins acts on beta-1,3-D-glucan synthase inhibiting the formation of beta-1,3-D-glucan and causing, depending on the type of fungus, a fungicidal or either a fungistatic effect.

Development of caspofungin resistance following prolonged therapy for invasive candidiasis secondary to Candida glabrata infection

We report a case of Candida glabrata invasive candidiasis that developed reduced susceptibility to caspofungin during prolonged therapy. Pre- and posttreatment isolates were confirmed to be isogenic, and sequencing of hot spots known to confer echinocandin resistance revealed an F659V substitution within the FKS2 region of the glucan synthase complex.

Characterization of caspofungin susceptibilities by broth and agar in Candida albicans clinical isolates with characterized mechanisms of azole resistance

Caspofungin (CSP) susceptibilities of Candida albicans, as determined by broth microdilution methods, have not been found to be related to azole susceptibilities or resistance. In contrast, it has been observed that azole-resistant clinical isolates that overexpress the efflux pump gene CDR2 are less susceptible to CSP when tested using an agar dilution method commonly employed with Saccharomyces cerevisiae. The goal of this study was to further understand the effects of azole resistance mechanisms on CSP susceptibility testing. A collection of 69 isolates exhibiting known mechanisms of azole resistance and resistance-associated phenotypes were analyzed by broth microdilution methods to determine standard minimum inhibitory concentrations (MICs) for CSP. The same isolates were then analyzed as to their MIC to CSP by Etest strips, an agar-based method that has been shown generally to be comparable to broth methods. The MICs found with both methods were not significantly different. However, a collection of strains overexpressing the efflux pump CDR2 did exhibit a spectrum of CSP susceptibilities when examined by agar dilution susceptibility tests, ranging from standard to reduced susceptibilities. This work demonstrated that a change in CSP susceptibility with CDR2 overexpressing cells in agar dilution studies is a variable phenotype and it is not the result of growth conditions (i.e., broth versus agar).

Increase of virulence and its phenotypic traits in drug-resistant strains of Candida albicans

There is concern about the rise of antifungal drug resistance, but little is known about comparative biological properties and pathogenicity of drug-resistant strains. We generated fluconazole (FLC; CO23 RFLC)- or micafungin (FK; CO23 RFK)-resistant strains of Candida albicans by treating a FLC- and FK-susceptible strain of this fungus (CO23 S) with stepwise-increasing concentrations of either drug. Molecular analyses showed that CO23 RFLC had acquired markedly increased expression of the drug-resistance efflux pump encoded by the MDR1 gene, whereas CO23 RFK had a homozygous mutation in the FSK1 gene. These genetic modifications did not alter to any extent the growth capacity of the drug-resistant strains in vitro, either at 28 degrees C or at 37 degrees C, but markedly increased their experimental pathogenicity in a systemic mouse infection model, as assessed by the overall mortality and target organ invasion. Interestingly, no apparent increase in the vaginopathic potential of the strains was observed with an estrogen-dependent rat vaginal infection. The increased pathogenicity of drug-resistant strains for systemic infection was associated with a number of biochemical and physiological changes, including (i) marked cellular alterations associated with a different expression and content of major cell wall polysaccharides, (ii) more rapid and extensive hypha formation in both liquid and solid media, and (iii) increased adherence to plastic and a propensity for biofilm formation. Overall, our data demonstrate that experimentally induced resistance to antifungal drugs, irrespective of drug family, can substantially divert C. albicans biology, affecting in particular biological properties of potential relevance for deep-seated candidiasis.

Regulation of sugar transport and metabolism by the Candida albicans Rgt1 transcriptional repressor

The ability of the fungal pathogen Candida albicans to cause systemic infections depends in part on the function of Hgt4, a cell surface sugar sensor. The orthologues of Hgt4 in Saccharomyces cerevisiae, Snf3 and Rgt2, initiate a signalling cascade that inactivates Rgt1, a transcriptional repressor of genes encoding hexose transporters. To determine whether Hgt4 functions similarly through the C. albicans orthologue of Rgt1, we analysed Cargt1 deletion mutants. We found that Cargt1 mutants are sensitive to the glucose analogue 2-deoxyglucose, a phenotype probably due to uncontrolled expression of genes encoding glucose transporters. Indeed, transcriptional profiling revealed that expression of about two dozen genes, including multiple HGT genes encoding hexose transporters, is increased in the Cargt1 mutant in the absence of sugars, suggesting that CaRgt1 represses expression of several HGT genes under this condition. Some of the HGT genes (probably encoding high-affinity transporters) are also repressed by high levels of glucose, and we show that this repression is mediated by CaMig1, the orthologue of the major glucose-activated repressor in S. cerevisiae, but not by its paralogue CaMig2. Therefore, CaRgt1 and CaMig1 collaborate to control expression of C. albicans hexose transporters in response to different levels of sugars. We were surprised to find that CaRgt1 also regulates expression of GAL1, suggesting that regulation of galactose metabolism in C. albicans is unconventional. Finally, Cargt1 mutations cause cells to hyperfilament, and suppress the hypofilamented phenotype of an hgt4 mutant, indicating that the Hgt4 glucose sensor may affect filamentation by modulating sugar import and metabolism via CaRgt1.

In vitro susceptibility of invasive isolates of Candida spp. to anidulafungin, caspofungin, and micafungin: six years of global surveillance

The echinocandins are being used increasingly as therapy for invasive candidiasis. Prospective sentinel surveillance for the emergence of in vitro resistance to the echinocandins among invasive Candida sp. isolates is indicated. We determined the in vitro activities of anidulafungin, caspofungin, and micafungin against 5,346 invasive (bloodstream or sterile-site) isolates of Candida spp. collected from over 90 medical centers worldwide from 1 January 2001 to 31 December 2006. We performed susceptibility testing according to the CLSI M27-A2 method and used RPMI 1640 broth, 24-h incubation, and a prominent inhibition endpoint for determination of the MICs. Of 5,346 invasive Candida sp. isolates, species distribution was 54% C. albicans, 14% C. parapsilosis, 14% C. glabrata, 12% C. tropicalis, 3% C. krusei, 1% C. guilliermondii, and 2% other Candida spp. Overall, all three echinocandins were very active against Candida: anidulafungin (MIC50, 0.06 microg/ml; MIC90, 2 microg/ml), caspofungin (MIC50, 0.03 microg/ml; MIC90, 0.25 microg/ml), micafungin (MIC50, 0.015 microg/ml; MIC90, 1 microg/ml). More than 99% of isolates were inhibited by < or = 2 microg/ml of all three agents. Results by species (expressed as the percentages of isolates inhibited by < or = 2 microg/ml of anidulafungin, caspofungin, and micafungin, respectively) were as follows: for C. albicans, 99.6%, 100%, and 100%; for C. parapsilosis, 92.5%, 99.9%, and 100%; for C. glabrata, 99.9%, 99.9%, and 100%; for C. tropicalis, 100%, 99.8%, and 100%; for C. krusei, 100%, 100%, and 100%; and for C. guilliermondii, 90.2%, 95.1%, and 100%. There was no significant change in the activities of the three echinocandins over the 6-year study period and no difference in activity by geographic region. All three echinocandins have excellent in vitro activities against invasive strains of Candida isolated from centers worldwide. Our prospective sentinel surveillance reveals no evidence of emerging echinocandin resistance among invasive clinical isolates of Candida spp.

A Ser678Pro substitution in Fks1p confers resistance to echinocandin drugs in Aspergillus fumigatus

An S678P substitution in Fks1p, the major subunit of glucan synthase, was sufficient to confer echinocandin resistance in Aspergillus fumigatus. The equivalent mutation in Candida spp. has been implicated in echinocandin resistance. This work demonstrates that modification of Fks1p is a conserved mechanism for echinocandin resistance in pathogenic fungi.

Multiple resistance mechanisms to azole antifungals in yeast clinical isolates

The use of antifungal agents, especially the azole class, has increased in parallel with a higher incidence of fungal infections, particularly in immunocompromised patients. This situation has favored the appearance of Candida species, prominent among them C. albicans and C. globrata, with acquired resistance to these agents. This review focuses on the latest developments in investigations of molecular mechanisms contributing to azole resistance. Multiple resistance mechanisms have been described that can coexist in resistant clinical isolates. Understanding resistance mechanisms is of value not only for the design of new antifungal agents but also the development of strategies of overcome or delay the emergence of resistance.

Pneumocystis carinii: a fungus resistant to antifungal therapies - mechanisms of action of antipneumocystis drugs

Pneumocystis carinii is a pathogen that causes a potentially lethal pneumonia in patients with AIDS and other immunodeficiency states. This review discusses the mechanisms of action of four classes of antipneumocystis agents: inhibitors of ergosterol synthesis and function, 1,3-beta-glucan synthase inhibitors, antifolates and DNA binding agents. Investigations of P. carinii's biologic pathways affected by the antipneumocystis actions of each of these classes of agents has generated important insights into the organism's basic biology and supports the organism's classification as a fungus. In addition, this review discusses some recent P. carinii research and its potential impact on drug development.

Resistance to echinocandin-class antifungal drugs

Invasive fungal infections cause morbidity and mortality in severely ill patients, and limited drug classes restrict treatment choices. The echinocandin drugs are the first new class of antifungal compounds that target the fungal cell wall by blocking beta-1,3-d-glucan synthase. Elevated MIC values with occasional treatment failure have been reported for strains of Candida. Yet, an uncertain correlation exists between clinical failure and elevated MIC values for the echinocandin drugs. Fungi display several adaptive physiological mechanisms that result in elevated MIC values. However, resistance to echinocandin drugs among clinical isolates is associated with amino acid substitutions in two "hot-spot" regions of Fks1, the major subunit of glucan synthase. The mutations, yielding highly elevated MIC values, are genetically dominant and confer cross-resistance to all echinocandin drugs. Prominent Fks1 mutations decrease the sensitivity of glucan synthase for drug by 1000-fold or more, and strains harboring such mutations may require a concomitant increase in drug to reduce fungal organ burdens in animal infection models. The Fks1-mediated resistance mechanism is conserved in a wide variety of Candida spp. and can account for intrinsic reduced susceptibility of certain species. Fks1 mutations confer resistance in both yeasts and moulds suggesting that this mechanism is pervasive in the fungal kingdom.

A novel role for catalase B in the maintenance of fungal cell-wall integrity during host invasion in the rice blast fungus Magnaporthe grisea

Asexual spores of the rice blast fungus germinate to produce a specialized and melanized infection structure, the appressorium, which is pivotal to successful plant penetration. To investigate whether Magnaporthe grisea counteracts the toxic burst of H2O2 localized beneath the site of attempted invasion, we examined the temporal expression of five candidate antioxidant genes. Of these, the putatively secreted large subunit catalase CATB gene was 600-fold up-regulated in vivo, coincident with penetration, and moderately up-regulated in vitro, in response to exogenous H2O2. Targeted gene replacement of CATB led to compromised pathogen fitness; the catB mutant displayed paler pigmentation and accelerated hyphal growth but lower biomass, poorer sporulation, fragile conidia and appressoria, and impaired melanization. The catB mutant was severely less pathogenic than Guy 11 on barley and rice, and its infectivity was further reduced on exposure to H2O2. The wild-type phenotype was restored by the reintroduction of CATB into the catB mutant We found no evidence to support a role for CATB in detoxification of the host-derived H2O2 at the site of penetration. Instead, we demonstrated that CATB plays a part in strengthening the fungal wall, a role of particular importance during forceful entry into the host.