Fungal beta(1,3)-D-glucan synthesis

New triazoles and echinocandins: mode of action, in vitro activity and mechanisms of resistance

Different types of mycoses, especially invasive mycoses caused by yeasts and molds, are a growing problem in healthcare. The most notable explanation for this increase is a rise in the number of immunocompromised patients owing to advances in transplantation, the emergence of AIDS and a rise in the number of invasive surgical procedures. Despite advances in medical practice, some therapeutic problems remain. In addition, intrinsic or acquired antifungal resistance may pose a serious problem to antifungal therapy. A new generation of triazole agents (voriconazole, posaconazole, isavuconazole, ravuconazole and albaconazole) and the recent class of the echinocandins (caspofungin, micafungin and anidulafungin) have become available, and represent an alternative to conventional antifungals for serious fungal infection management. Currently, only two of the recent triazole generation (voriconazole and posaconazole) and all three echinocandins are available for clinical use. More precisely, voriconazole and posaconazole are indicated for the treatment of invasive fungal infections and the echinocandins for the treatment of specific candidiasis. Voriconazole and posaconazole have a very broad spectrum of antifungal activity that includes Candida species, and filamentous and dimorphic fungi. Their activity extends to both fluconazole- and itraconazole-resistant strains of Candida. A major difference between posaconazole and voriconazole is that posaconazole has activity against Zygomycetes including Mucor spp., Rhizopus spp. and Cunninghamella spp., and voriconazole has no activity against this class of fungi. Ravuconazole, isavuconazole and albaconazole have shown very potent in vitro activity against species of Candida, Cryptococcus and Aspergillus, and they are currently in various stages of development. All three echinocandin agents, caspofungin, micafungin and anidulafungin, are similar in their spectrum of activity. Echinocandins do not possess in vitro activity against important basidiomycetes, including Cryptococcus, Rhodotorula and Trichosporon. This review attempts to deliver the most up-to-date knowledge on the mode of action and mechanisms of resistance to triazoles and echinocandins in fungal pathogens. In addition, the in vitro activity data available on triazoles and echinocandins are reported.

Occidiofungin, a unique antifungal glycopeptide produced by a strain of Burkholderia contaminans

Bacterial strain Burkholderia contaminans MS14 was isolated from soil that suppressed brown patch disease of lawn grass. An antifungal compound was purified from the liquid culture of this bacterium. In this study, complete covalent structures of two purified closely related antifungal compounds were determined by the experiments of TOCSY, NOESY, ROESY, 13C HSQC 2D NMR, and ESI-MS and GC. The analysis of monoisotopic masses of the purified preparation indicated the presence of two related compounds with masses determined to be 1199.543 and 1215.518 Da; the difference corresponds to the mass of an oxygen atom. GC analysis identified a xylose sugar attached to the antifungal compound. NMR experiments revealed that the compound is cyclic and composed of eight amino acids, two of which are beta-hydroxy derivatives of Tyr and Asn, and one being a novel amino acid. The novel amino acid serves as the scaffold for the attachment of the xylose and a short acyl chain. The spectrum and concentration of antifungal activity were determined using a microtiter plate assay. The antifungal compound demonstrated potent antifungal activities against a broad panel of fungal plant and animal pathogens, as well as two Pythium spp. Microscopic observations showed that the antifungal compound disrupts normal membrane morphology. The cells fill with large inclusion bodies and the membrane becomes irregularly shaped and swollen following the exposure to subinhibitory concentrations of the antifungal compound. Our data support the identification of a novel fungicide and the compound has been named occidiofungin, meaning fungal killer.

Effect of Candida glabrata FKS1 and FKS2 mutations on echinocandin sensitivity and kinetics of 1,3-beta-D-glucan synthase: implication for the existing susceptibility breakpoint

Thirteen Candida glabrata strains harboring a range of mutations in hot spot regions of FKS1 and FKS2 were studied. The mutations were linked to an echinocandin reduced susceptibility phenotype. Sequence alignments showed that 11 out of the 13 mutants harbored a mutation in FKS1 or FKS2 not previously implicated in echinocandin reduced susceptibility in C. glabrata. A detailed kinetic characterization demonstrated that amino acid substitutions in Fks1p and Fks2p reduced drug sensitivity in mutant 1,3-beta-D-glucan synthase by 2 to 3 log orders relative to that in wild-type enzyme. These mutations were also found to reduce the catalytic efficiency of the enzyme (Vmax) and to influence the relative expression of FKS genes. In view of the association of FKS mutations and reduced susceptibility of 1,3-beta-D-glucan synthase, an evaluation of the new CLSI echinocandin susceptibility breakpoint was conducted. Only 3 of 13 resistant fks mutants (23%) were considered anidulafungin or micafungin nonsusceptible (MIC > 2 microg/ml) by this criterion. In contrast, most fks mutants (92%) exceeded a MIC of >2 microg/ml with caspofungin. However, when MIC determinations were performed in the presence of 50% serum, all C. glabrata fks mutants showed MICs of > or = 2 microg/ml for the three echinocandin drugs. As has been observed with Candida albicans, the kinetic inhibition parameter 50% inhibitory concentration may be a better predictor of FKS-mediated resistance. Finally, the close association between FKS1/FKS2 hot spot mutations provides a basis for understanding echinocandin resistance in C. glabrata.

Recent approaches to antifungal therapy for invasive mycoses

Invasive fungal infections with primary and opportunistic mycoses have become increasingly common in recent years and pose a major diagnostic and therapeutic challenge. They represent a major area of concern in today's medical fraternity. The occurrence of invasive fungal diseases, particularly in AIDS and other immunocompromised patients, is life-threatening and increases the economic burden. Apart from the previously known polyenes and imidazole-based azoles, newly discovered triazoles and echinocandins are more effective in terms of specificity, yet some immunosuppressed hosts are difficult to treat. The main reasons for this include antifungal resistance, toxicity, lack of rapid and microbe-specific diagnoses, poor penetration of drugs into sanctuary sites, and lack of oral or intravenous preparations. In addition to combination antifungal therapy, other novel antimycotic treatments such as calcineurin signaling pathway blockers and vaccines have recently emerged. This review briefly summarizes recent developments in the pharmacotherapeutic treatment of invasive fungal infections.

Surfactant protein D increases phagocytosis of hypocapsular Cryptococcus neoformans by murine macrophages and enhances fungal survival

Cryptococcus neoformans is a facultative intracellular opportunistic pathogen and the leading cause of fungal meningitis in humans. In the absence of a protective cellular immune response, the inhalation of C. neoformans cells or spores results in pulmonary infection. C. neoformans cells produce a polysaccharide capsule composed predominantly of glucuronoxylomannan, which constitutes approximately 90% of the capsular material. In the lungs, surfactant protein A (SP-A) and SP-D contribute to immune defense by facilitating the aggregation, uptake, and killing of many microorganisms by phagocytic cells. We hypothesized that SP-D plays a role in C. neoformans pathogenesis by binding to and enhancing the phagocytosis of the yeast. Here, the abilities of SP-D to bind to and facilitate the phagocytosis and survival of the wild-type encapsulated strain H99 and the cap59Delta mutant hypocapsular strain are assessed. SP-D binding to cap59Delta mutant cells was approximately sixfold greater than binding to wild-type cells. SP-D enhanced the phagocytosis of cap59Delta cells by approximately fourfold in vitro. To investigate SP-D binding in vivo, SP-D(-/-) mice were intranasally inoculated with Alexa Fluor 488-labeled cap59Delta or H99 cells. By confocal microscopy, a greater number of phagocytosed C. neoformans cells in wild-type mice than in SP-D(-/-) mice was observed, consistent with in vitro data. Interestingly, SP-D protected C. neoformans cells against macrophage-mediated defense mechanisms in vitro, as demonstrated by an analysis of fungal viability using a CFU assay. These findings provide evidence that C. neoformans subverts host defense mechanisms involving surfactant, establishing a novel virulence paradigm that may be targeted for therapy.

Peptide antibiotics: an alternative and effective antimicrobial strategy to circumvent fungal infections

Mycosis, caused by both filamentous fungi and pathogenic yeasts is a major concern nowadays especially in the immunocompromised patient population. The emergence of pathogenic fungi resistant to current therapies in the last few decades has intensified the search for new antifungals like cationic peptides, which are the key components of innate defense mechanism. The review provides an inventory of different peptides from a diverse array of organisms from bacteria to mammals with proven antifungal activity, their therapeutic options and also about those which are in various stages of preclinical development. Literature, on the total and semi-synthetic variants of the parent peptides that exhibit an improved antifungal activity is also reviewed.

Serum markers in interstitial pneumonia with and without Pneumocystis jirovecii colonization: a prospective study

Background

In patients with chronic respiratory disease, Pneumocystis jirovecii (P. jirovecii) colonization is observed, and may influence disease progression and systemic inflammation. Pneumocystis pneumonia causes interstitial changes, so making a diagnosis of PCP in patients who have interstitial pneumonia (IP) with P. jirovecii colonization is sometimes difficult based on radiography.

Methods

This study investigated the prevalence of P. jirovecii colonization in IP patients and assessed pulmonary injury due to P. jirovecii colonization by measurement of serum markers (KL-6, SP-A, SP-D, and (1→3) β-D-glucan (β-D-glucan)) and the peripheral lymphocyte counts, prospectively. A total of 75 patients with idiopathic pulmonary fibrosis (n = 29), collagen vascular-related interstitial pneumonia (n = 19), chronic bronchitis or pneumonia (n = 20), and Pneumocystis pneumonia (n = 7) were enrolled in this prospective study. P. jirovecii DNA was detected in sputum samples, while serum markers and the lymphocyte count were measured in the peripheral blood.

Results

IP patients (idiopathic pulmonary fibrosis and collagen vascular-related IP) who received oral corticosteroids had a high prevalence of P. jirovecii colonization (23.3%). In IP patients, oral corticosteroid therapy was a significant risk factor for P. jirovecii colonization (P < 0.05). Serum markers did not show differences between IP patients with and without P. jirovecii colonization. The β-D-glucan level and lymphocyte count differed between patients with Pneumocystis pneumonia or P. jirovecii colonization.

Conclusion

Serum levels of KL-6, SP-A, SP-D, and β-D-glucan were not useful for detecting P. jirovecii colonization in IP patients. However, the serum β-D-glucan level and lymphocyte count were useful for distinguishing P. jirovecii colonization from pneumocystis pneumonia in IP patients.

Role for Fks1 in the intrinsic echinocandin resistance of Fusarium solani as evidenced by hybrid expression in Saccharomyces cerevisiae

The opportunistic mold Fusarium solani is intrinsically resistant to cell wall synthesis-inhibiting echinocandins (ECs), including caspofungin and micafungin. Mutations that confer acquired EC resistance in Saccharomyces cerevisiae and other normally susceptible yeast species have been mapped to the Fks1 gene; among these is the mutation of residue 639 from Phe to Tyr (F639Y) within a region designated hot spot 1. Fks1 sequence analysis identified the equivalent of Y639 in F. solani as well as in Scedosporium prolificans, another intrinsically EC-resistant mold. To test its role in intrinsic EC resistance, we constructed Fks1 hybrids in S. cerevisiae that incorporate F. solani hot spot 1 and flanking residues. Hybrid construction was accomplished by a PCR-based method that was validated by studies with Fks1 sequences from EC-susceptible Aspergillus fumigatus and paired EC-susceptible and -resistant Candida glabrata isolates. In support of our hypothesis, hybrid Fks1 incorporating F. solani hot spot 1 conferred significantly reduced EC susceptibility, 4- to 8-fold less than that of wild-type S. cerevisiae and 8- to 32-fold less than that of the same hybrid with an F639 mutation. We propose that Fks1 sequences represent determinants of intrinsic EC resistance in Fusarium and Scedosporium species and, potentially, other fungi.

Biochemical characterization of potential virulence markers in the human fungal pathogen Pseudallescheria boydii

The ubiquitous Pseudallescheria boydii (anamorph Scedosporium apiospermum) is a saprophytic filamentous fungus recognized as a potent etiologic agent of a wide variety of infections in immunocompromised as well as in immunocompetent patients. Very little is known about the virulence factors expressed by this fungal pathogen. The present review provides an overview of recent discoveries related to the identification and biochemical characterization of potential virulence attributes produced by P. boydii, with special emphasis on surface and released molecules. These structures include polysaccharides (glucans), glycopeptides (peptidorhamnomannans), glycolipids (glucosylceramides) and hydrolytic enzymes (proteases, phosphatases and superoxide dismutase), which have been implicated in some fundamental cellular processes in P. boydii including growth, differentiation and interaction with host molecules. Elucidation of the structure of cell surface components as well as the secreted molecules, especially those that function as virulence determinants, is of great relevance to understand the pathogenic mechanisms of P. boydii.

Correlating echinocandin MIC and kinetic inhibition of fks1 mutant glucan synthases for Candida albicans: implications for interpretive breakpoints

A detailed kinetic characterization of echinocandin inhibition was performed for mutant 1,3-beta-d-glucan synthase enzymes from clinical isolates of Candida albicans with nine different FKS1 mutations resulting in high MICs. Among 14 mutant Fks1p enzymes studied, the kinetic parameters 50% inhibitory concentration and K(i) increased 50-fold to several thousandfold relative to those for the wild type. Enzymes with mutations at Ser645 (S645P, S645Y, and S645F) within hot spot 1 showed the most prominent decrease in sensitivity, while those with mutations at the N- and C-terminal ends of hot spot 1 generally retained greater sensitivity to all three drugs. Kinetic inhibitions by caspofungin, micafungin, and anidulafungin were comparable among the fks1 mutant enzymes, although absolute values did vary with specific mutations. Amino acid substitutions in Fks1p did not alter K(m) values, although some mutations decreased the V(max). Given the association of FKS1 mutations with clinical resistance, an evaluation of the kinetic parameters for the inhibition of mutant 1,3-beta-D-glucan synthase as a function of the MIC enabled an independent evaluation of the recently adopted susceptibility breakpoint for echinocandin drugs. Overall, a breakpoint MIC of >or=2 microg/ml for caspofungin captured nearly 100% of fks1 C. albicans strains when a kinetic inhibition rise threshold of <or=50-fold for the K(i) was used as a measure of susceptibility. A similar MIC breakpoint for micafungin and anidulafungin was less inclusive, and a projected MIC of >or=0.5 microg/ml was required for >95% coverage of clinical isolates. However, when MIC determinations were performed in the presence of 50% serum, all fks1 mutants showed MIC values of >or=2 microg/ml for the three echinocandin drugs. The 1,3-beta-D-glucan synthase kinetic inhibition data support the proposed susceptibility breakpoint for caspofungin in C. albicans, but a lower susceptibility breakpoint (<or=0.5 microg/ml) may be more appropriate for anidulafungin and micafungin. Overall, the data indicate that MIC testing with caspofungin may serve as a surrogate marker for resistance among the class of echinocandin drugs.

[Anidulafungin: a new therapeutic approach in antifungal therapy. Pharmacology of anidulafungin]

Anidulafungin is a new echinocandin antifungal agent which inhibits beta-1,3-D-glucan synthase and disrupts fungal cell-wall synthesis. It has marked antifungal activity against Candida spp. and Aspergillus spp., including amphotericin B and triazole resistant strains. Due to the limited oral availability, anidulafungin in clinical use is available for parenteral administration only. Elimination of anidulafungin takes place via slow non-enzymatic degradation to inactive metabolites. Less than 10% and 1% of the initially administered drug is excreted unchanged into feces and urine, respectively. It does not require dosage adjustment in subjects with hepatic or renal impairment established. Anidulafungin is generally well tolerated. Adverse events appear not to be dose or infusion related. The most common treatment related adverse events are phlebitis, headache, nausea, vomiting and pyrexia. The lack of interactions with tacrolimus, cyclosporine and corticosteroids and its limited toxicity profile places anidulafungin as an attractive new option for the treatment of invasive fungal infections especially in transplant patients.

Stimulation of chitin synthesis rescues Candida albicans from echinocandins

Echinocandins are a new generation of novel antifungal agent that inhibit cell wall β(1,3)-glucan synthesis and are normally cidal for the human pathogen Candida albicans. Treatment of C. albicans with low levels of echinocandins stimulated chitin synthase (CHS) gene expression, increased Chs activity, elevated chitin content and reduced efficacy of these drugs. Elevation of chitin synthesis was mediated via the PKC, HOG, and Ca²⁺-calcineurin signalling pathways. Stimulation of Chs2p and Chs8p by activators of these pathways enabled cells to survive otherwise lethal concentrations of echinocandins, even in the absence of Chs3p and the normally essential Chs1p, which synthesize the chitinous septal ring and primary septum of the fungus. Under such conditions, a novel proximally offset septum was synthesized that restored the capacity for cell division, sustained the viability of the cell, and abrogated morphological and growth defects associated with echinocandin treatment and the chs mutations. These findings anticipate potential resistance mechanisms to echinocandins. However, echinocandins and chitin synthase inhibitors synergized strongly, highlighting the potential for combination therapies with greatly enhanced cidal activity.

Fungal pathogens are increasingly important agents of human disease and are also difficult to treat since few antifungal agents kill the invading organism. The cell wall of a fungus is essential for its viability and this can be attacked by a new generation of antifungal antibiotics called echinocandins. Echinocandins such as caspofungin are normally cidal for the human pathogen Candida albicans. These inhibit the synthesis of β(1,3)-glucan, a major strength-imparting polysaccharide in the cell wall. Treatment of C. albicans with echinocandins in vitro stimulated the formation of a second cell wall polysaccharide—chitin, which rescued the cells. Treatments that increased the chitin content of the C. albicans cell wall reduced the efficacy of echinocandins and could even induce the formation of novel structures such as a salvage septum that enabled the cells to continue to undergo cell division under otherwise lethal conditions. Combined treatments with echinocandins and chitin synthase inhibitors synergized strongly, highlighting the potential for potent combination therapies with enhanced fungicidal activity.

The cell wall: a carbohydrate armour for the fungal cell

The cell wall is composed of a polysaccharide-based three-dimensional network. Considered for a long time as an inert exoskeleton, the cell wall is now seen as a dynamic structure that is continuously changing as a result of the modification of culture conditions and environmental stresses. Although the cell wall composition varies among fungal species, chemogenomic comparative analysis have led to a better understanding of the genes and mechanisms involved in the construction of the common central core composed of branched beta1,3 glucan-chitin. Because of its essential biological role, unique biochemistry and structural organization and the absence in mammalian cells of most of its constitutive components, the cell wall is an attractive target for the development of new antifungal agents. Genomic as well as drug studies have shown that the death of the fungus can result from inhibition of cell wall polysaccharide synthases. To date, only beta1,3 glucan synthase inhibitors have been launched clinically and many more targets remain to be explored.

The pharmacology and clinical use of caspofungin

Caspofungin was the first echinocandin to be licensed for the treatment of invasive fungal infections. Caspofungin has in vitro and in vivo activity against Candida spp. and Aspergillus spp., which constitute the majority of medically important opportunistic fungal pathogens. Caspofungin inhibits the synthesis of the 1,3-beta-glucan, with resultant osmotic instability and lysis. The pharmacology of caspofungin is relatively complex. Trafficking of drug into tissues is an important determinant of the shape of the concentration-time relationship. Caspofungin has demonstrated efficacy in experimental models of invasive candidiasis and aspergillosis, which reflect its activity in the treatment of oropharyngeal, esophageal and disseminated candidiasis, as well as salvage therapy for patients with invasive aspergillosis.

An anti-beta-glucan monoclonal antibody inhibits growth and capsule formation of Cryptococcus neoformans in vitro and exerts therapeutic, anticryptococcal activity in vivo

In this study we tested the in vitro and in vivo anti-Cryptococcus neoformans activity of an antilaminarin (anti-beta-glucan) monoclonal antibody (MAb 2G8) (immunoglobulin G2b) which was previously shown to inhibit the growth of beta-glucan-exposing Candida albicans cells. Here we show that MAb 2G8 binds to the cell wall of C. neoformans and inhibits its growth to an extent comparable to that observed for C. albicans. Binding and growth inhibition were detected almost equally for encapsulated and acapsular C. neoformans strains. In addition, at subinhibitory concentrations, MAb 2G8 reduced the capsule thickness without affecting protease or phospholipase production. Acapsular fungal cells, but not encapsulated fungal cells, were opsonized by the antibody and more efficiently phagocytosed and killed by human monocytes and by murine peritoneal macrophages. A single administration of MAb 2G8 resulted in a reduction in the fungal burden in the brains and livers of mice systemically infected with a highly virulent, encapsulated C. neoformans strain. This protective effect was also detected in neutropenic mice. Overall, these findings demonstrate that cell wall beta-glucan of encapsulated C. neoformans is accessible to antibodies which can exert remarkable anticryptococcal activities in vitro and in vivo.

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.

Current insights into the biology and pathogenesis of Pneumocystis pneumonia

The fungal infection Pneumocystis pneumonia is the most prevalent opportunistic infection in patients with AIDS. Although the analysis of this opportunistic fungal pathogen has been hindered by the inability to isolate it in pure culture, the use of molecular techniques and genomic analysis have brought insights into its complex cell biology. Analysis of the intricate relationship between Pneumocystis and the host lung during infection has revealed that the attachment of Pneumocystis to the alveolar epithelium promotes the transition of the organism from the trophic to the cyst form. It also revealed that Pneumocystis infection elicits the production of inflammatory mediators, culminating in lung injury and impaired gas exchange. Here we discuss these and other recent findings relating to the biology and pathogenesis of this intractable fungus.

The immunology of parasite infections in immunocompromised hosts

Immune compromise can modify the severity and manifestation of some parasitic infections. More widespread use of newer immnosuppressive therapies, the growing population of individuals with immunocompromised states as well as the prolonged survival of these patients have altered the pattern of parasitic infection. This review article discusses the burden and immunology of parasitic infections in patients who are immunocompromised secondary to congenital immunodeficiency, malnutrition, malignancy, and immunosuppressive medications. This review does not address the literature on parasitic infections in the setting of HIV-1 infection.

Dectin-2 is a pattern recognition receptor for fungi that couples with the Fc receptor gamma chain to induce innate immune responses

Antigen presenting cells recognize pathogens via pattern recognition receptors (PRR), which upon ligation transduce intracellular signals that can induce innate immune responses. Because some C-type lectin-like receptors (e.g. dectin-1 and DCSIGN) were shown to act as PRR for particular microbes, we considered a similar role for dectin-2. Binding assays using soluble dectin-2 receptors showed the extracellular domain to bind preferentially to hyphal (rather than yeast/conidial) components of Candida albicans, Microsporum audouinii, and Trichophyton rubrum. Selective binding for hyphae was also observed using RAW macrophages expressing dectin-2, the ligation of which by hyphae or cross-linking with dectin-2-specific antibody led to protein tyrosine phosphorylation. Because dectin-2 lacks an intracellular signaling motif, we searched for a signal adaptor that permits it to transduce intracellular signals. First, we found that the Fc receptor gamma (FcRgamma) chain can bind to dectin-2. Second, ligation of dectin-2 on RAW cells induced tyrosine phosphorylation of FcRgamma, activation of NF-kappaB, internalization of a surrogate ligand, and up-regulated secretion of tumor necrosis factor alpha and interleukin-1 receptor antagonist. Finally, these dectin-2-induced events were blocked by PP2, an inhibitor of Src kinases that are mediators for FcRgamma chain-dependent signaling. We conclude that dectin-2 is a PRR for fungi that employs signaling through FcRgamma to induce innate immune responses.

The echinocandins: comparison of their pharmacokinetics, pharmacodynamics and clinical applications

Caspofungin, micafungin and anidulafungin are three drugs of the echinocandin class of antifungals available for intravenous treatment of invasive candidiasis and aspergillosis. They exhibit high in vitro and in vivo activities against Candida spp. and Aspergillus spp. In various clinical studies investigating candidemia and invasive candidiasis, Candida esophagitis, and fever in neutropenia, the clinical efficacy of the echinocandin tested was similar to that of established antifungals. Antifungal activity against strains no longer susceptible to conventional antifungal agents, such as fluconazole and amphotericin B suggests that echinocandins can be used as salvage therapy in life-threatening fungal infections. There is no cross-resistance to other antifungals. Excellent safety and tolerability of treatment with caspofungin has been documented over a total of 4.3 million patient days. Echinocandins are poor substrates of the cytochrome P450 enzyme family and can be safely co-administered with most drugs without the need for dosage adaptation. No dose reduction is required in renal impairment. A reduction in the daily maintenance dose has been recommended for caspofungin, but not for micafungin and anidulafungin in patients presenting with mild to moderate hepatic failure.

The structure and synthesis of the fungal cell wall

The fungal cell wall is a dynamic structure that protects the cell from changes in osmotic pressure and other environmental stresses, while allowing the fungal cell to interact with its environment. The structure and biosynthesis of a fungal cell wall is unique to the fungi, and is therefore an excellent target for the development of anti-fungal drugs. The structure of the fungal cell wall and the drugs that target its biosynthesis are reviewed. Based on studies in a number of fungi, the cell wall has been shown to be primarily composed of chitin, glucans, mannans and glycoproteins. The biosynthesis of the various components of the fungal cell wall and the importance of the components in the formation of a functional cell wall, as revealed through mutational analyses, are discussed. There is strong evidence that the chitin, glucans and glycoproteins are covalently cross-linked together and that the cross-linking is a dynamic process that occurs extracellularly.

An alpha-(1,4)-amylase is essential for alpha-(1,3)-glucan production and virulence in Histoplasma capsulatum

Histoplasma capsulatum is a dimorphic fungus that causes respiratory and systemic disease and is capable of surviving and replicating within macrophages. The virulence of Histoplasma has been linked to cell wall alpha-(1,3)-glucan; however, the role of this polysaccharide during infection, its organization within the cell wall, and its synthesis and regulation remain poorly understood. To identify genes involved in the biosynthesis of alpha-(1,3)-glucan, we employed a forward genetics strategy to isolate physically marked mutants with reduced alpha-(1,3)-glucan. Insertional mutants were generated in a virulent strain of H. capsulatum by optimization of Agrobacterium tumefaciens-mediated transformation. Approximately 90% of these mutants possessed single insertions with no chromosomal rearrangements or deletions in the host genome. To confirm the role and specificity of identified candidate genes, we phenocopied the disrupted locus by either RNA interference or targeted gene deletion. Our findings indicate alpha-(1,3)-glucan production requires the function of the AMY1 gene product, a novel protein with homology to the alpha-amylase family of glycosyl hydrolases, and UGP1, a UTP-glucose-1-phosphate uridylyltransferase which synthesizes UDP-glucose monomers. Loss of AMY1 function attenuated the ability of Histoplasma to kill macrophages and to colonize murine lungs.

Efficacy of caspofungin against Aspergillus flavus, Aspergillus terreus, and Aspergillus nidulans

The echinocandin caspofungin is a potent inhibitor of the activity of 1,3-beta-D-glucan synthase from Aspergillus flavus, Aspergillus terreus, and Aspergillus nidulans. In murine models of disseminated infection, caspofungin prolonged survival and reduced the kidney fungal burden. Caspofungin was at least as effective as amphotericin B against these filamentous fungi in vivo.

FsFKS1, the 1,3-beta-glucan synthase from the caspofungin-resistant fungus Fusarium solani

The cell wall, a mesh of carbohydrates and proteins, shapes and protects the fungal cell. The enzyme responsible for the synthesis of one of the main components of the fungal wall, 1,3-beta-glucan synthase, is targeted by the antifungal caspofungin acetate (CFA). Clinical isolates of Candida albicans and Aspergillus fumigatus are much more sensitive to CFA than clinical isolates of Fusarium species. To better understand CFA resistance in Fusarium species, we cloned and sequenced FsFKS1, which encodes the Fusarium solani f. sp. pisi beta(1,3)-D-glucan synthase, used RNA interference to reduce its expression and complemented deletion of the essential fks gene of the CFA-sensitive fungus A. fumigatus with FsFKS1. Reduction of the FsFKS1 message in F. solani f. sp. pisi reduced spore viability and caused lysis of spores and hyphae, consistent with cell wall defects. Compensating for the loss of A. fumigatus fks1 with FsFKS1 caused only a modest increase in the tolerance of A. fumigatus for CFA. Our results suggest that FsFKS1 is required for the proper construction of F. solani cell walls and that the resistance of F. solani to CFA is at best only partially due to resistance of the FsFKS1 enzyme to this antifungal agent.

The plasma membrane proteome of Saccharomyces cerevisiae and its response to the antifungal calcofluor

Calcofluor is an antifungal compound known to induce structural perturbations of the cell wall by interfering with the synthesis of chitin microfibril. Proteins from a stripped plasma membrane fraction were solubilized with the neutral and non-denaturing detergent, the n-dodecyl beta-D-maltoside. Proteins were then resolved using a recently described ion-exchange chromatography (IEC)/lithium dodecyl sulfate (LDS)-PAGE procedure. Nearly 90 proteins were identified and clustered, based on their pI, molecular weight, abundance and/or hydrophobicity. This method was then applied to profile the plasma membrane response to calcofluor. The LDS-PAGE patterns obtained from whole plasma membrane proteins were similar for the non-treated and calcofluor-treated samples. However, IEC/LDS-PAGE analysis revealed subtle changes in the expression of several proteins of low abundance, in response to calcofluor. These proteins include Pil1p and Lsp1p, two sphingolipid long-chain base-responsive inhibitors of protein kinases involved in signaling pathways for cell wall integrity and Rho1p, a small GTPase. It was recently hypothesized that Pil1p and Lsp1p could associate with, and regulate, the plasma membrane beta-1-3-glucan synthase, responsible for the synthesis of another major microfibril for yeast cell wall. Results are discussed with respect to both calcofluor effects on the plasma membrane proteins and the power of the IEC/LDS-PAGE procedure in the search for new potential therapeutics targets.

Candida albicans and Candida glabrata clinical isolates exhibiting reduced echinocandin susceptibility

A recognized hotspot for mutations conferring reduced echinocandin susceptibility (RES) is residue S645 of Candida albicans Gsc1(Fks1). We report that the mutation F641Y is associated with RES in a C. albicans isolate. The analogous Fks2 residue is mutated F to V in a Candida glabrata RES isolate; the introduction of this mutation into susceptible C. glabrata confirmed its role in RES. Y641-equivalent Fks residues were identified in intrinsically RES Fusarium species and Candida guilliermondii.

Cell wall integrity is dependent on the PKC1 signal transduction pathway in Cryptococcus neoformans

Cell wall biogenesis and integrity are crucial for fungal growth, pathogenesis and survival, and are attractive targets for antifungal therapy. In this study, we identify, delete and analyse mutant strains for 10 genes involved in the PKC1 signal transduction pathway and its regulation in Cryptococcus neoformans. The kinases Bck1 and Mkk2 are critical for maintaining integrity, and deletion of each of these causes severe phenotypes different from each other. In stark contrast to results seen in Saccharomyces cerevisiae, a deletion in LRG1 has severe repercussions for the cell, and one in ROM2 has little effect. Also surprisingly, the phosphatase Ppg1 is crucial for cell integrity. These data indicate that the mechanisms of maintaining cell integrity differ between the two fungi. Deletions in SSD1 and PUF4, potential alternative regulators of cell integrity, also exhibit phenotypes. This is the first comprehensive analysis examining genes involved the maintenance of cell integrity in C. neoformans and sets the foundation for future biochemical and virulence studies.

Current state of three-dimensional characterisation of antifungal targets and its use for molecular modelling in drug design

The alarming rise in life-threatening systemic fungal infections due to the emergence of drug-resistant fungal strains had produced an increased demand for new antimycotics, especially those targeting novel antifungal structures. Drug discovery has developed from screening natural products and chemical synthesis to a modern approach, namely structure-based drug design. Whilst many antifungal agents currently in use were discovered more than 30 years ago, characterisation of various drug targets has only been achieved recently, contributing immensely to understanding the structure-activity relationships of antifungals and their targets. Three-dimensional characterisation has become a well established tool for modern antifungal drug research and should play an important role in investigations for new antifungal agents.

Specific substitutions in the echinocandin target Fks1p account for reduced susceptibility of rare laboratory and clinical Candida sp. isolates

An association between reduced susceptibility to echinocandins and changes in the 1,3-beta-d-glucan synthase (GS) subunit Fks1p was investigated. Specific mutations in fks1 genes from Saccharomyces cerevisiae and Candida albicans mutants are described that are necessary and sufficient for reduced susceptibility to the echinocandin drug caspofungin. One group of amino acid changes in ScFks1p, ScFks2p, and CaFks1p defines a conserved region (Phe 641 to Asp 648 of CaFks1p) in the Fks1 family of proteins. The relationship between several of these fks1 mutations and the phenotype of reduced caspofungin susceptibility was confirmed using site-directed mutagenesis or integrative transformation. Glucan synthase activity from these mutants was less susceptible to caspofungin inhibition, and heterozygous and homozygous Cafks1 C. albicans mutants could be distinguished based on the shape of inhibition curves. The C. albicans mutants were less susceptible to caspofungin than wild-type strains in a murine model of disseminated candidiasis. Five Candida isolates with reduced susceptibility to caspofungin were recovered from three patients enrolled in a clinical trial. Four C. albicans strains showed amino acid changes at Ser 645 of CaFks1p, while a single Candida krusei isolate had a deduced R1361G substitution. The clinical C. albicans mutants were less susceptible to caspofungin in the disseminated candidiasis model, and GS inhibition profiles and DNA sequence analyses were consistent with a homozygous fks1 mutation. Our results indicate that substitutions in the Fks1p subunit of GS are sufficient to confer reduced susceptibility to echinocandins in S. cerevisiae and the pathogens C. albicans and C. krusei.

Pneumocystis: immune recognition and evasion

Pulmonary infection caused by the opportunistic fungal organism Pneumocystis continues to be a leading AIDS defining illness. The initiation of highly active antiretroviral therapy (HAART) in the HIV-infected population has led to a significant reduction in the incidence of Pneumocystis pneumonia (PCP), although recent trends suggest the incidence has plateaued rather than decreased. Host defense against Pneumocystis involves a delicate, concerted balance between the inflammatory response and immune-mediated clearance. Innate cellular immunity is a cornerstone in this response as it provides the initial recognition event that precipitates an immune response, ultimately leading to clearance of the organism from the host. This review will focus on carbohydrate moieties found in the Pneumocystis cell wall and the immune events that occur following their recognition.

Yapsins are a family of aspartyl proteases required for cell wall integrity in Saccharomyces cerevisiae

The yeast cell wall is a crucial extracellular organelle that protects the cell from lysis during environmental stress and morphogenesis. Here, we demonstrate that the yapsin family of five glycosylphosphatidylinositol-linked aspartyl proteases is required for cell wall integrity in Saccharomyces cerevisiae. Yapsin null mutants show hypersensitivity to cell wall perturbation, and both the yps1Delta2Delta mutant and the quintuple yapsin mutant (5ypsDelta) undergo osmoremedial cell lysis at 37 degrees C. The cell walls of both 5ypsDelta and yps1Delta2Delta mutants have decreased amounts of 1,3- and 1,6-beta-glucan. Although there is decreased incorporation of both 1,3- and 1,6-beta-glucan in the 5ypsDelta mutant in vivo, in vitro specific activity of both 1,3- and 1,6-beta-glucan synthesis is similar to wild type, indicating that the yapsins affect processes downstream of glucan synthesis and that the yapsins may be involved in the incorporation or retention of cell wall glucan. Presumably as a response to the significant alterations in cell wall composition, the cell wall integrity mitogen-activated kinase signaling cascade (PKC1-MPK pathway) is basally active in 5ypsDelta. YPS1 expression is induced during cell wall stress and remodeling in a PKC1-MPK1-dependent manner, indicating that Yps1p is a direct, and important, output of the cell wall integrity response. The Candida albicans (SAP9) and Candida glabrata (CgYPS1) homologues of YPS1 complement the phenotypes of the yps1Delta mutant. Taken together, these data indicate that the yapsins play an important role in glucan homeostasis in S. cerevisiae and that yapsin homologues may play a similar role in the pathogenic yeasts C. albicans and C. glabrata.

Genes and molecules involved in Aspergillus fumigatus virulence

Aspergillus fumigatus causes a wide range of diseases that include mycotoxicosis, allergic reactions and systemic diseases (invasive aspergillosis) with high mortality rates. Pathogenicity depends on immune status of patients and fungal strain. There is no unique essential virulence factor for development of this fungus in the patient and its virulence appears to be under polygenetic control. The group of molecules and genes associated with the virulence of this fungus includes many cell wall components, such as beta-(1-3)-glucan, galactomannan, galactomannanproteins (Afmp1 and Afmp2), and the chitin synthetases (Chs; chsE and chsG), as well as others. Some genes and molecules have been implicated in evasion from the immune response, such as the rodlets layer (rodA/hyp1 gene) and the conidial melanin-DHN (pksP/alb1 gene). The detoxifying systems for Reactive Oxygen Species (ROS) by catalases (Cat1p and Cat2p) and superoxide dismutases (MnSOD and Cu, ZnSOD), had also been pointed out as essential for virulence. In addition, this fungus produces toxins (14 kDa diffusible substance from conidia, fumigaclavin C, aurasperon C, gliotoxin, helvolic acid, fumagilin, Asp-hemolysin, and ribotoxin Asp fI/mitogilin F/restrictocin), allergens (Asp f1 to Asp f23), and enzymatic proteins as alkaline serin proteases (Alp and Alp2), metalloproteases (Mep), aspartic proteases (Pep and Pep2), dipeptidyl-peptidases (DppIV and DppV), phospholipase C and phospholipase B (Plb1 and Plb2). These toxic substances and enzymes seems to be additive and/or synergistic, decreasing the survival rates of the infected animals due to their direct action on cells or supporting microbial invasion during infection. Adaptation ability to different trophic situations is an essential attribute of most pathogens. To maintain its virulence attributes A. fumigatus requires iron obtaining by hydroxamate type siderophores (ornitin monooxigenase/SidA), phosphorous obtaining (fos1, fos2, and fos3), signal transductional falls that regulate morphogenesis and/or usage of nutrients as nitrogen (rasA, rasB, rhbA), mitogen activated kinases (sakA codified MAP-kinase), AMPc-Pka signal transductional route, as well as others. In addition, they seem to be essential in this field the amino acid biosynthesis (cpcA and homoaconitase/lysF), the activation and expression of some genes at 37 degrees C (Hsp1/Asp f12, cgrA), some molecules and genes that maintain cellular viability (smcA, Prp8, anexins), etc. Conversely, knowledge about relationship between pathogen and immune response of the host has been improved, opening new research possibilities. The involvement of non-professional cells (endothelial, and tracheal and alveolar epithelial cells) and professional cells (natural killer or NK, and dendritic cells) in infection has been also observed. Pathogen Associated Molecular Patterns (PAMP) and Patterns Recognizing Receptors (PRR; as Toll like receptors TLR-2 and TLR-4) could influence inflammatory response and dominant cytokine profile, and consequently Th response to infec tion. Superficial components of fungus and host cell surface receptors driving these phenomena are still unknown, although some molecules already associated with its virulence could also be involved. Sequencing of A. fumigatus genome and study of gene expression during their infective process by using DNA microarray and biochips, promises to improve the knowledge of virulence of this fungus.

Caspofungin inhibits Rhizopus oryzae 1,3-beta-D-glucan synthase, lowers burden in brain measured by quantitative PCR, and improves survival at a low but not a high dose during murine disseminated zygomycosis

Rhizopus oryzae is the most common cause of zygomycosis, a life-threatening infection that usually occurs in patients with diabetic ketoacidosis. Despite standard therapy, the overall rate of mortality from zygomycosis remains >50%, and new strategies for treatment are urgently needed. The activities of caspofungin acetate (CAS) and other echinocandins (antifungal inhibitors of the synthesis of 1,3-beta-D-glucan synthase [GS]) against the agents of zygomycosis have remained relatively unexplored, especially in animal models of infection. We found that R. oryzae has both an FKS gene, which in other fungi encodes a subunit of the GS synthesis complex, and CAS-susceptible, membrane-associated GS activity. Low-dose but not high-dose CAS improved the survival of mice with diabetic ketoacidosis infected with a small inoculum but not a large inoculum of R. oryzae. Fungal burden, assessed by a novel quantitative PCR assay, correlated with increasing inocula and progression of disease, particularly later in the infection, when CFU counts did not. CAS decreased the brain burden of R. oryzae when it was given prophylactically but not when therapy was started after infection. These results indicate that CAS has significant but limited activity against R. oryzae in vivo and demonstrates an inverse dose-response effect. The potential for CAS to play a role in combination therapy against zygomycosis merits further investigation.

Characterization ofAspergillus fumigatusmutants with reduced susceptibility to caspofungin

Caspofungin acetate (CAS) is a member of a new class of clinically-approved echinocandin drugs to treat invasive aspergillosis. CAS inhibits the activity of beta-1,3-D-glucan synthase (GS), thus damaging the fungal cell wall. Although no clinical resistance of Aspergillus to CAS has been reported as yet, the development of in vitro reduced susceptibility is presumed to be inevitable. By contrast, echinocandin resistance in laboratory strains of Candida albicans and Saccharomyces cerevisiae has been well documented. To study the potential for clinical resistance in Aspergillus, two classes of Aspergillus fumigatus mutant strains were isolated that exhibited reduced susceptibility to CAS. In the first class, a site-directed mutation within the target gene (AfFKS1, encoding the putative catalytic subunit of GS) was introduced and shown to confer low-level (16-fold) reduced susceptibility. A second class of spontaneous mutants were sensitive to low levels of drug but displayed nearly normal growth above 0.5 microg/ml, suggesting induction of an unknown resistance mechanism. At higher levels of drug (> or = 16 microg/ml), the mutants displayed partially restored sensitivity. Preliminary studies indicate that neither target site mutations, nor changes in target gene expression are present in these strains, as has been documented for several yeasts. Instead, preliminary results indicate that the molecular mechanism(s) underlying reduced susceptibility of CAS in the A. fumigatus strains is novel, possibly due to remodeling of the cell wall components.

Enzyme encapsulation in permeabilized Saccharomyces cerevisiae cells

The Saccharomyces cerevisiae cell wall provides a semipermeable barrier that can retain intracellular proteins but still permits small molecules to pass through. When S. cerevisiae cells expressing E. coli lacZ are treated with detergent to extract the cell membrane, beta-galactosidase activity in the permeabilized cells is approximately 40% of the activity of the protein in cell extract. However, the permeabilized cells can easily be collected and reused over 15 times without appreciable loss in activity. Cell wall composition and thickness can be modified using different cell strains for enzyme expression or by mutating genes involved in cell wall biosynthesis or degradation. The Sigma1278b strain cell wall is less permeable than the walls of BY4742 and W303 cells, and deleting EXG1, which encodes a 1,3-beta-glucanase, can further reduce permeability. A short Zymolyase treatment can increase cell wall permeability without rupturing the cells. Encapsulating multiple enzymes in permeabilized cells can offer kinetic advantages over the same enzymes in solution. Regeneration of ATP from AMP by adenylate kinase and pyruvate kinase encapsulated in the same cell proceeded more rapidly than regeneration using a cell extract. Combining permeabilized cells containing adenylate kinase with permeabilized cells containing pyruvate kinase can also regenerate ATP from AMP, but the kinetics of this reaction are slower than regeneration using cell extract or permeabilized cells expressing both enzymes.

Genomic approach to identification of mutations affecting caspofungin susceptibility in Saccharomyces cerevisiae

The antifungal agent caspofungin (CAS) specifically interferes with glucan synthesis and cell wall formation. To further study the cellular processes affected by CAS, we analyzed a Saccharomyces cerevisiae mutant collection (4,787 individual knockout mutations) to identify new genes affecting susceptibility to the drug. This collection was screened for increased CAS sensitivity (CAS-IS) or increased CAS resistance (CAS-IR). MICs were determined by the broth microdilution method. Disruption of 20 genes led to CAS-IS (four- to eightfold reductions in the MIC). Eleven of the 20 genes are involved in cell wall and membrane function, notably in the protein kinase C (PKC) integrity pathway (MID2, FKS1, SMI1, and BCK1), chitin and mannan biosynthesis (CHS3, CHS4, CHS7, and MNN10), and ergosterol biosynthesis (ERG5 and ERG6). Four of the 20 genes (TPO1, VPS65, VPS25, and CHC1) are involved in vacuole and transport functions, 3 of the 20 genes (CCR4, POP2, and NPL3) are involved in the control of transcription, and 2 of the 20 genes are of unknown function. Disruption of nine additional genes led to CAS-IR (a fourfold increase of MIC). Five of these nine genes (SLG1, ERG3, VRP1, CSG2, and CKA2) are involved in cell wall function and signal transduction, and two of the nine genes (VPS67 and SAC2) are involved in vacuole function. To assess the specificity of susceptibility to CAS, the MICs of amphotericin B, fluconazole, flucytosine, and calcofluor for the strains were tested. Seven of 20 CAS-IS strains (with disruption of FKS1, SMI1, BCK1, CHS4, ERG5, TPO1, and ILM1) and 1 of 9 CAS-IR strains (with disruption of SLG1) demonstrated selective susceptibility to CAS. To further explore the importance of PKC in CAS susceptibility, the activity of the PKC inhibitor staurosporine in combination with CAS was tested against eight Aspergillus clinical isolates by the microdilution assay. Synergistic or synergistic-to-additive activities were found against all eight isolates by use of both MIC and minimum effective concentration endpoints.

Caspofungin uptake is mediated by a high-affinity transporter in Candida albicans

The uptake of the echinocandin drug caspofungin acetate in Candida albicans was evaluated at drug levels at or near the MIC for the organism. Maximal uptake was achieved in 10 min and was energy independent. A saturable transport system, consistent with a facilitated-diffusion carrier, was observed with the unlabeled drug competing with the labeled drug for uptake and efflux. More than 90% of the transported drug was observed in a single kinetic compartment that was available for efflux, indicating that the drug was free in the cytoplasm following uptake. Efflux was also energy independent but was sensitive to the presence of a fully loaded carrier on both faces of the bilayer. Overall, the data presented are consistent with the presence of a high-affinity facilitated-diffusion transporter that mediates caspofungin uptake and could be a potential source of transport-related reduced susceptibility.

Schizosaccharomyces pombe Pmr1p is essential for cell wall integrity and is required for polarized cell growth and cytokinesis

The cps5-138 fission yeast mutant shows an abnormal lemon-like morphology at 28 degrees C in minimal medium and a lethal thermosensitive phenotype at 37 degrees C. Cell growth is completely inhibited at 28 degrees C in a Ca2+-free medium, in which the wild type is capable of growing normally. Under these conditions, actin patches become randomly distributed throughout the cell, and defects in septum formation and subsequent cytokinesis appear. The mutant cell is hypersensitive to the cell wall-digesting enzymatic complex Novozym234 even under permissive conditions. The gene SPBC31E1.02c, which complements all the mutant phenotypes described above, was cloned and codes for the Ca2+-ATPase homologue Pmr1p. The gene is not essential under optimal growth conditions but is required under conditions of low Ca2+ (<0.1 mM) or high temperature (>35 degrees C). The green fluorescent protein-tagged Cps5 proteins, which are expressed under physiological conditions (an integrated single copy with its own promoter in the cps5Delta strain), display a localization pattern typical of endoplasmic reticulum proteins. Biochemical analyses show that 1,3-beta-D-glucan synthase activity in the mutant is decreased to nearly half that of the wild type and that the mutant cell wall contains no detectable galactomannan when the cells are exposed to a Ca2+-free medium. The mutant acid phosphatase has an increased electrophoretic mobility, suggesting that incomplete protein glycosylation takes place in the mutant cells. These results indicate that S. pombe Pmr1p is essential for the maintenance of cell wall integrity and cytokinesis, possibly by allowing protein glycosylation and the polarized actin distribution to take place normally. Disruption and complementation analyses suggest that Pmr1p shares its function with a vacuolar Ca2+-ATPase homologue, Pmc1p (SPAPB2B4.04c), to prevent lethal activation of calcineurin for cell growth.

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.

Echinocandin antifungal drugs

The echinocandins are large lipopeptide molecules that are inhibitors of beta-(1,3)-glucan synthesis, an action that damages fungal cell walls. In vitro and in vivo, the echinocandins are rapidly fungicidal against most Candida spp and fungistatic against Aspergillus spp. They are not active at clinically relevant concentrations against Zygomycetes, Cryptococcus neoformans, or Fusarium spp. No drug target is present in mammalian cells. The first of the class to be licensed was caspofungin, for refractory invasive aspergillosis (about 40% response rate) and the second was micafungin. Adverse events are generally mild, including (for caspofungin) local phlebitis, fever, abnormal liver function tests, and mild haemolysis. Poor absorption after oral administration limits use to the intravenous route. Dosing is once daily and drug interactions are few. The echinocandins are widely distributed in the body, and are metabolised by the liver. Results of studies of caspofungin in candidaemia and invasive candidiasis suggest equivalent efficacy to amphotericin B, with substantially fewer toxic effects. Absence of antagonism in combination with other antifungal drugs suggests that combination antifungal therapy could become a general feature of the echinocandins, particularly for invasive aspergillosis.

Cell wall-associated enzymes in fungi

This review compiles and discusses previous reports on the identity of wall-associated enzymes (WAEs) in fungi and addresses critically the widely different terminologies used in the literature to specify the type of bonding of WAEs to other entities of the cell wall compartment, the extracellular matrix (ECM). A facile and rapid fractionation protocol for catalytically active WAEs is presented, which uses crude cell walls as the experimental material, a variety of test enzymes (including representatives of polysaccharide synthases and hydrolases, phosphatases, gamma-glutamyltransferases, pyridine-nucleotide dehydrogenases and phenol-oxidising enzymes) and a combination of simple hydrophilic and hydrophobic extractants. The protocol provides four fully operationally defined classes of WAEs, with constituent members of each class displaying the same basic type of physicochemical interaction with binding partners in situ. The routine application of the protocol to different species and cell types could yield easily accessible data useful for building-up a general objective information retrieval system of WAEs, suitable as an heuristic basis both for the unravelling of the role and for the biotechnological potentialities of WAEs. A detailed account is given of the function played in the ECM by WAEs in the metabolism of chitin (chitin synthase, chitinase and beta-N-acetylhexosaminidase) and of phenols (tyrosinase).