Virulence studies of Aspergillus nidulans mutants requiring lysine or p-aminobenzoic acid in invasive pulmonary aspergillosis

Vitamin Biosynthesis as an Antifungal Target

The large increase in the population of immunosuppressed patients, coupled with the limited efficacy of existing antifungals and rising resistance toward them, have dramatically highlighted the need to develop novel drugs for the treatment of invasive fungal infections. An attractive possibility is the identification of possible drug targets within essential fungal metabolic pathways not shared with humans. Here, we review the vitamin biosynthetic pathways (vitamins A⁻E, K) as candidates for the development of antifungals. We present a set of ranking criteria that identify the vitamin B2 (riboflavin), B5 (pantothenic acid), and B9 (folate) biosynthesis pathways as being particularly rich in new antifungal targets. We propose that recent scientific advances in the fields of drug design and fungal genomics have developed sufficiently to merit a renewed look at these pathways as promising sources for the development of novel classes of antifungals.

TrpE feedback mutants reveal roadblocks and conduits toward increasing secondary metabolism in Aspergillus fumigatus

Small peptides formed from non-ribosomal peptide synthetases (NRPS) are bioactive molecules produced by many fungi including the genus Aspergillus. A subset of NRPS utilizes tryptophan and its precursor, the non-proteinogenic amino acid anthranilate, in synthesis of various metabolites such as Aspergillus fumigatus fumiquinazolines (Fqs) produced by the fmq gene cluster. The A. fumigatus genome contains two putative anthranilate synthases - a key enzyme in conversion of anthranilic acid to tryptophan - one beside the fmq cluster and one in a region of co-linearity with other Aspergillus spp. Only the gene found in the co-linear region, trpE, was involved in tryptophan biosynthesis. We found that site-specific mutations of the TrpE feedback domain resulted in significantly increased production of anthranilate, tryptophan, p-aminobenzoate and fumiquinazolines FqF and FqC. Supplementation with tryptophan restored metabolism to near wild type levels in the feedback mutants and suggested that synthesis of the tryptophan degradation product kynurenine could negatively impact Fq synthesis. The second putative anthranilate synthase gene next to the fmq cluster was termed icsA for its considerable identity to isochorismate synthases in bacteria. Although icsA had no impact on A. fumigatus Fq production, deletion and over-expression of icsA increased and decreased respectively aromatic amino acid levels suggesting that IcsA can draw from the cellular chorismate pool.

Inhibitors of amino acids biosynthesis as antifungal agents

Fungal microorganisms, including the human pathogenic yeast and filamentous fungi, are able to synthesize all proteinogenic amino acids, including nine that are essential for humans. A number of enzymes catalyzing particular steps of human-essential amino acid biosynthesis are fungi specific. Numerous studies have shown that auxotrophic mutants of human pathogenic fungi impaired in biosynthesis of particular amino acids exhibit growth defect or at least reduced virulence under in vivo conditions. Several chemical compounds inhibiting activity of one of these enzymes exhibit good antifungal in vitro activity in minimal growth media, which is not always confirmed under in vivo conditions. This article provides a comprehensive overview of the present knowledge on pathways of amino acids biosynthesis in fungi, with a special emphasis put on enzymes catalyzing particular steps of these pathways as potential targets for antifungal chemotherapy.

Phenotypic consequences of LYS4 gene disruption in Candida albicans

A BLAST search of the Candida Genome Database with the Saccharomyces cerevisiae LYS4 sequence known to encode homoaconitase (HA) revealed ORFs 19.3846 and 19.11327. Both alleles of the LYS4 gene were sequentially disrupted in Candida albicans BWP17 cells using PCR-based methodology. The null lys4Δ mutant exhibited lysine auxotrophy in minimal medium but was able to grow in the presence of l-Lys and α-aminoadipate, an intermediate of the α-aminoadipate pathway, at millimolar concentrations. The presence of d-Lys and pipecolic acid did not trigger lys4Δ growth. The C. albicans lys4Δ mutant cells demonstrated diminished germination ability. However, their virulence in vivo in a murine model of disseminated neonatal candidiasis appeared identical to that of the wild-type strain. Moreover, there was no statistically significant difference in fungal burden of infected tissues between the strains.

Analysis of gene expression changes in Trichophyton rubrum after skin interaction

Trichophyton rubrum, an anthropophilic and cosmopolitan fungus, is the most common agent of superficial mycoses. In this study, T. rubrum infection was modelled by adding human skin sections to a limited medium containing glucose and cDNA microarrays were used to monitor T. rubrum gene expression patterns on a global level. We observed that exposure to human skin resulted in upregulation of the expression levels of T. rubrum genes related to many cellular and biological processes, including transcription and translation, metabolism and secondary transport, the stress response, and signalling pathways. These results provide a reference set of T. rubrum genes whose expression patterns change upon infection and reveal previously unknown genes that most likely correspond to proteins that should be considered as virulence factor candidates and potential new drug targets for T. rubrum infection.

The fungal α-aminoadipate pathway for lysine biosynthesis requires two enzymes of the aconitase family for the isomerization of homocitrate to homoisocitrate

Fungi produce α-aminoadipate, a precursor for penicillin and lysine via the α-aminoadipate pathway. Despite the biotechnological importance of this pathway, the essential isomerization of homocitrate via homoaconitate to homoisocitrate has hardly been studied. Therefore, we analysed the role of homoaconitases and aconitases in this isomerization. Although we confirmed an essential contribution of homoaconitases from Saccharomyces cerevisiae and Aspergillus fumigatus, these enzymes only catalysed the interconversion between homoaconitate and homoisocitrate. In contrast, aconitases from fungi and the thermophilic bacterium Thermus thermophilus converted homocitrate to homoaconitate. Additionally, a single aconitase appears essential for energy metabolism, glutamate and lysine biosynthesis in respirating filamentous fungi, but not in the fermenting yeast S. cerevisiae that possesses two contributing aconitases. While yeast Aco1p is essential for the citric acid cycle and, thus, for glutamate synthesis, Aco2p specifically and exclusively contributes to lysine biosynthesis. In contrast, Aco2p homologues present in filamentous fungi were transcribed, but enzymatically inactive, revealed no altered phenotype when deleted and did not complement yeast aconitase mutants. From these results we conclude that the essential requirement of filamentous fungi for respiration versus the preference of yeasts for fermentation may have directed the evolution of aconitases contributing to energy metabolism and lysine biosynthesis.

Evaluation of lysine biosynthesis as an antifungal drug target: biochemical characterization of Aspergillus fumigatus homocitrate synthase and virulence studies

Aspergillus fumigatus is the main cause of severe invasive aspergillosis. To combat this life-threatening infection, only limited numbers of antifungals are available. The fungal alpha-aminoadipate pathway, which is essential for lysine biosynthesis, has been suggested as a potential antifungal drug target. Here we reanalyzed the role of this pathway for establishment of invasive aspergillosis in murine models. We selected the first pathway-specific enzyme, homocitrate synthase (HcsA), for biochemical characterization and for study of its role in virulence. A. fumigatus HcsA was specific for the substrates acetyl-coenzyme A (acetyl-CoA) and alpha-ketoglutarate, and its activity was independent of any metal ions. In contrast to the case for other homocitrate synthases, enzymatic activity was hardly affected by lysine and gene expression increased under conditions of lysine supplementation. An hcsA deletion mutant was lysine auxotrophic and unable to germinate on unhydrolyzed proteins given as a sole nutrient source. However, the addition of partially purified A. fumigatus proteases restored growth, confirming the importance of free lysine to complement auxotrophy. In contrast to lysine-auxotrophic mutants from other fungal species, the mutant grew on blood and serum, indicating the existence of high-affinity lysine uptake systems. In agreement, although the virulence of the mutant was strongly attenuated in murine models of bronchopulmonary aspergillosis, virulence was partially restored by lysine supplementation via the drinking water. Additionally, in contrast to the case for attenuated pulmonary infections, the mutant retained full virulence when injected intravenously. Therefore, we concluded that inhibition of fungal lysine biosynthesis, at least for disseminating invasive aspergillosis, does not appear to provide a suitable target for new antifungals.

Identification of ENA1 as a virulence gene of the human pathogenic fungus Cryptococcus neoformans through signature-tagged insertional mutagenesis

A library of more than 4,500 signature-tagged insertion mutants of the human pathogenic fungus Cryptococcus neoformans was generated, and a subset was screened in a murine inhalation model to identify genes required for virulence. New genes that regulate aspects of C. neoformans virulence were also identified by screening the entire library for in vitro phenotypes related to the ability to cause disease, including melanin production, growth at high temperature, and growth under conditions of nutrient limitation. A screen of 10% of the strain collection in mice identified an avirulent mutant strain with an insertion in the ENA1 gene, which is predicted to encode a fungus-specific sodium or potassium P-type ATPase. The results of the deletion of the gene and complementation experiments confirmed its key role in mammalian virulence. ena1 mutant strains exhibited no change in sensitivity to high salt concentrations but were sensitive to alkaline pH conditions, providing evidence that the fungus may have to survive at elevated pH during infection of the mammalian host. The mutation of the well-characterized virulence factor calcineurin (CNA1) also rendered C. neoformans strains sensitive to elevated pH. ENA1 transcripts in wild-type and cna1 mutant strains were upregulated in response to high pH, and cna1 ena1 double mutant strains exhibited increased sensitivity to elevated pH, indicating that at least two pathways in the fungus mediate survival under alkaline conditions. Signature-tagged mutagenesis is an effective strategy for the discovery of new virulence genes in fungal pathogens of animals.

Aspergillus fumigatus: virulence genes in a street-smart mold

Infections with the filamentous fungus Aspergillus fumigatus are among the most devastating of the systemic mycoses. Unlike most primary pathogens, which possess virulence traits that developed in association with a host organism, evidence suggests that the virulence of A. fumigatus entails a collection of 'street-smart' attributes that have evolved to resist the adverse selection pressures encountered in decaying vegetation. These features enhance the overall competitiveness of the organism in its environmental niche but are also thought to promote growth and survival in a human host. Although many of the genes that are responsible for these characteristics do not fit into the classical definition of a virulence factor, they are nonetheless important to the pathogenesis of aspergillosis and may therefore provide novel opportunities for antifungal development.

Fungal Genomics: A Tool to Explore Central Metabolism of Aspergillus fumigatus and Its Role in Virulence

Aspergillus fumigatus is an opportunistic pathogenic fungus that primarily infects neutropenic animal hosts. This fungus is found throughout the world, can utilize a wide range of substrates for carbon and nitrogen sources, and is capable of growing at elevated temperatures. The ability to grow at high temperatures and utilize a range of nutrient substrates for growth potentially contributes to this being the number one human pathogenic mold worldwide. The recently completed genome sequence for this fungus creates an opportunity to examine how central metabolic pathways and their regulation contribute to pathogenesis. A review of the existing literature illustrates that genes involved in the biosynthesis of key nutrients are essential for pathogenesis in A. fumigatus. In addition, nutrient sensing and regulation of biosynthetic pathways also contribute to fungal pathogenesis. The advent of improved methods for manipulating the genome of A. fumigatus, along with the completed genome sequence, now make it feasible to investigate the role of all metabolic pathways and control of these pathways in fungal virulence.

Tools to study molecular mechanisms of Aspergillus pathogenicity

The unique nature of Aspergillus fungi represents a challenge for scrutinizing the attributes that render these saprophytic microorganisms pathogenic or allergenic under certain environmental circumstances. Recent publication of the genomic sequence from an isolate of the major pathogen Aspergillus fumigatus denotes enormous progress in aiming at cellular features and gene products that contribute to its pathogenicity. Latest developments to study virulence-related characteristics comprise profiling techniques, conditional gene inactivation and precise manipulation of the genome by means of gene targeting. Advances in assessing the virulence potential of particular mutant strains in alternative test systems complement these approaches.

Nitrogen metabolism of Aspergillus and its role in pathogenicity

Aspergilli represent unique pathogens. Based on their saprophytic life style they are able to colonize a variety of ecological niches, among them the immunocompromised individual. Distinct fungal attributes that play a role in pathogenicity of aspergilli have been described, and primary metabolism indisputably has to be taken into account for contributing to the virulence potential of this fungal genus. Here we present an overview of studies that focus on this aspect of nutritional versatility. In the predominant pathogenic representative Aspergillus fumigatus regulation of nitrogen utilization and sensing of nitrogen sources have been scrutinized with respect to pathogenicity. The impact of distinct metabolic pathways on virulence capacities could be evaluated by inspection of auxotrophic mutant strains. Among them, para-aminobenzoic acid-requiring mutants revealed that this biosynthetic route is strictly required for pathogenicity. For amino acid anabolism only lysine biosynthesis has been investigated in this regard. Fungal amino acid biosynthesis is generally subject to strict regulation mediated by the Cross-Pathway Control system, a conserved regulatory circuit evolved to counteract conditions of nutritional stress. A clear influence of the system on pathogenicity could be observed by targeting its transcriptional activator CpcA. However, additional metabolic characteristics as well as regulatory instruments that compensate environmental challenges need to be addressed in future research with the aim to assess the significance of fungal primary metabolism for pathogenicity of aspergillus species.

The contribution of animal models of aspergillosis to understanding pathogenesis, therapy and virulence

Animal models of aspergillosis have been used extensively to study various aspects of pathogenesis, innate and acquired host-response, disease transmission and therapy. Several different animal models of aspergillosis have been developed. Because aspergillosis is an important pulmonary disease in birds, avian models have been used successfully to study preventative vaccines. Studies done to emulate human disease have relied on models using common laboratory animal species. Guinea pig models have primarily been used in therapy studies of invasive pulmonary aspergillosis (IPA). Rabbits have been used to study IPA and systemic disease, as well as fungal keratitis. Rodent, particularly mouse, models of aspergillosis predominate as the choice for most investigators. The availability of genetically defined strains of mice, immunological reagents, cost and ease of handling are factors. Both normal and immunosuppressed animals are used routinely. These models have been used to determine efficacy of experimental therapeutics, comparative virulence of different isolates of Aspergillus, genes involved in virulence, and susceptibility to infection with Aspergillus. Mice with genetic immunological deficiency and cytokine gene-specific knockout mice facilitate studies of the roles cells, and cytokines and chemokines, play in host-resistance to Aspergillus. Overall, these models have been critical to the advancement of therapy, and our current understanding of pathogenesis and host-resistance.

Virulence comparisons of Aspergillus nidulans mutants are confounded by the inflammatory response of p47phox-/- mice

While investigating the requirement for phagosomal alkalinization in the host defense against pulmonary aspergillosis, we observed high morbidity of p47(phox)(-/-) mice infected with pH-insensitive Aspergillus nidulans mutants despite a paucity of fungal growth. Fatal infection also resulted from a normally avirulent p-aminobenzoate auxotroph. This demonstrates that p47(phox)(-/-) murine immunity contributes significantly to A. nidulans lethality. These data have wider implications for microbial virulence studies with p47(phox)(-/-) mice.

The Aspergillus pH-responsive transcription factor PacC regulates virulence

The ability of a pathogen to adapt to the host environment is usually required for the initiation of disease. Here we have investigated the importance of the Aspergillus nidulans PacC-mediated pH response in the pathogenesis of pulmonary aspergillosis. Using mutational analysis, we demonstrate that, in neutropenic mice, elimination of the A. nidulans pH-responsive transcription factor PacC, blocking the ambient pH signal transduction pathway or prevention of PacC proteolytic processing acutely attenuates virulence. Infections caused by these alkali-sensitive mutants are characterized by limited growth in vivo and a reduction of inflammatory cell infiltration. In stark contrast, constitutive activation of PacC causes increased mortality marked by extensive fungal invasive growth. PacC action is therefore required for, and able to enhance virulence, demonstrating that the A. nidulans pH-responsive transcription factor PacC plays a pivotal role in pulmonary pathogenesis.

Anti-fungal Activity of Sulfamethoxazole toward Aspergillus Species

Invasive mycosis has significantly increased in frequency among immunocompromised hosts leading to excessive morbidity and mortality. The combination of sulfamethoxazole (SMX) and trimethoprim (TMP) has been used extensively for the treatment and prophylaxis of infections by various microbes. The purpose of this study is to estimate the anti-fungal activity of SMX-TMP and examine the mechanism of activity. To investigate the antimicrobial activity of SMX-TMP in vitro, a mixture of SMX and TMP at 5:1 was serially diluted and added to potato dextrose agar medium or C-limiting agar medium. Aspergillus species were inoculated on the medium plate with SMX-TMP. The growth of A. fumigatus and A. oryzae was inhibited by addition of SMX-TMP. The anti-Aspergillus effect depended on not TMP but SMX and that was inhibited by p-aminobenzoic acid (PABA). A. niger was not sensitive against SMX-TMP in PDA medium, but sensitive in C-limiting medium. Those results showed that the activity depends on culture medium. Furthermore, addition of human serum did not influence the activity of SMX. The finding in this study suggested that SMX might be effective against Aspergillus species in clinical practice and prophylaxis treatment.

Isogenic auxotrophic mutant strains in the Aspergillus fumigatus genome reference strain AF293

Aspergillus fumigatus is a ubiquitous fungus that is a frequent opportunistic pathogen in immunosuppressed patients. Because of its role as a pathogen, it is of considerable experimental interest. A set of auxotrophic isogenic strains in the A. fumigatus genome reference strain AF293 has been developed. Using molecular genetic methods, arginine and lysine auxotrophs were made by deletion of argB and lysB, respectively. Transformation of these auxotrophic strains with plasmids carrying argB or lysB, respectively, results in efficient integration at these loci. Finally, these strains are able to form stable diploids, which should further facilitate analysis of gene functions in this fungus. Furthermore, the development of this isogenic set of auxotrophic strains in the AF293 background will enable investigators to study this important opportunistic human pathogen with greater facility.

Menacing mold: the molecular biology of Aspergillus fumigatus

Infections with mold pathogens have emerged as an increasing risk faced by patients under sustained immunosuppression. Species of the Aspergillus family account for most of these infections, and in particular Aspergillus fumigatus may be regarded as the most important airborne pathogenic fungus. The improvement in transplant medicine and the therapy of hematological malignancies is often complicated by the threat of invasive aspergillosis. Specific diagnostic methods are still limited as are the possibilities of therapeutic intervention, leading to the disappointing fact that invasive aspergillosis is still associated with a high mortality rate that ranges from 30% to 90%. In recent years considerable progress has been made in understanding the genetics of A. fumigatus, and molecular techniques for the manipulation of the fungus have been developed. Molecular genetics offers not only approaches for the detailed characterization of gene products that appear to be key components of the infection process but also selection strategies that combine classical genetics and molecular biology to identify virulence determinants of A. fumigatus. Moreover, these methods have a major impact on the development of novel strategies leading to the identification of antimycotic drugs. This review summarizes the current knowledge on the biology, molecular genetics, and genomics of A. fumigatus.

fos-1, a putative histidine kinase as a virulence factor for systemic aspergillosis

In fungi, two-component histidine kinases have various functions including regulation of osmosensitivity, and of cell-wall assembly. Furthermore, one of these proteins, cos-1, has been shown to be important for virulence of Candida albicans. Recently, a putative histidine kinase, fos-1, has been isolated and partially characterized from Aspergillus fumigatus. Here we compare the virulence of a fos-1 deletion strain with that of the parental wild-type strain in a murine model of systemic aspergillosis. Our results show that the fos-1 deletion strain has significantly reduced virulence as compared with the parental wild-type strain. Thus, we propose that the fos-1 two-component histidine kinase is a virulence factor of A. fumigatus.

Aspects of fungal pathogenesis in humans

Fungal diseases have become increasingly important in the past few years. Because few fungi are professional pathogens, fungal pathogenic mechanisms tend to be highly complex, arising in large part from adaptations of preexisting characteristics of the organisms' nonparasitic lifestyles. In the past few years, genetic approaches have elucidated many fungal virulence factors, and increasing knowledge of host reactions has also clarified much about fungal diseases. The literature on fungal pathogenesis has grown correspondingly; this review, therefore, will not attempt to provide comprehensive coverage of fungal disease but focuses on properties of the infecting fungus and interactions with the host. These topics have been chosen to make the review most useful to two kinds of readers: fungal geneticists and molecular biologists who are interested in learning about the biological problems posed by infectious diseases, and physicians who want to know the kinds of basic approaches available to study fungal virulence.

Landmarks in the early duplication cycles of Aspergillus fumigatus and Aspergillus nidulans: polarity, germ tube emergence and septation

When the spores of filamentous fungi break dormancy, nuclear division is accompanied by a series of ordered morphological events including the switch from isotropic to polar growth, the emergence of a second germ tube from the conidium and septation. Correlation of these morphological events with nuclear number allows them to serve as duplication cycle landmarks. Early duplication cycle landmarks have been characterized in Aspergillus nidulans, but not in other filamentous fungi. To learn more about duplication cycle control in filamentous fungi, a study was undertaken to compare the timing of landmarks in Aspergillus fumigatus and A. nidulans. Nuclear duplication took approximately 45 min in A. fumigatus, with mitosis occupying roughly 5% of this period. Under the same conditions, nuclear duplication in A. nidulans took approximately 60 min, with mitosis occupying roughly 4% of this period. In A. fumigatus the isotropic to polar switch preceded the first mitosis in 22% of cells, while in A. nidulans the isotropic to polar switch did not occur until after the first mitosis. In both A. fumigatus and A. nidulans the earliest emergence of a second germ tube from the conidium occurred after the third mitotic division. However, by the fifth mitosis only 19% of A. fumigatus conidia had a second germ tube, compared to 98% of A. nidulans conidia. In both A. fumigatus and A. nidulans, formation of the first septum occurred after the fourth mitotic division. In all experiments a few cells lagged behind the others in nuclear number. In this delayed group, it was common to see landmark events at an earlier mitotic division. Differences in nuclear number when identical landmarks occur in A. fumigatus versus A. nidulans, and uncoupling of mitotic division and landmarks in delayed cells suggest that nuclear division and morphogenesis lie in parallel pathways, perhaps coordinated by checkpoints.

Signature-tagged and directed mutagenesis identify PABA synthetase as essential for Aspergillus fumigatus pathogenicity

Signature-tagged mutagenesis (STM) is a method that has been used to screen for genes required for in vivo survival of pathogenic bacteria, but has not been used to investigate a eukaryotic pathogen in an animal model of disease. We have adapted STM to identify genes required for in vivo growth of the opportunistic fungal pathogen Aspergillus fumigatus. Using a mouse model of invasive pulmonary aspergillosis, we have isolated several mutant strains with defects in their ability to replicate in vivo. One strain unable to cause lethal infection was further characterized and found to have an insertion into the promoter of a gene (pabaA) encoding para-aminobenzoic acid synthetase, an enzyme catalyzing a late step in the biosynthesis of folate. The complete inability of this strain, and other pabaA- strains constructed in this study by targeted gene deletion, to cause lethal infection in mice confirms the importance of the folate synthesis pathway for in vivo survival of this pathogen. The successful application of STM to A. fumigatus demonstrates that in vivo genetic analysis of eukaryotic pathogens is feasible and could result in the identification of potential targets, such as para-aminobenzoic acid synthetase, for novel antifungal therapies.

Heterotrimeric G-proteins of a filamentous fungus regulate cell wall composition and susceptibility to a plant PR-5 protein

Membrane permeabilizing plant defensive proteins first encounter the fungal cell wall that can harbor specific components that facilitate or prevent access to the plasma membrane. However, signal transduction pathways controlling cell wall composition in filamentous fungi are largely unknown. We report here that the deposition of cell wall constituents that block the action of osmotin (PR-5), an antifungal plant defense protein, against Aspergillus nidulans requires the activity of a heterotrimeric G-protein mediated signaling pathway. The guanidine nucleotide GDPbetaS, that locks G-proteins in a GDP-bound inactive form, inhibits osmotin-induced conidial lysis. A dominant interfering mutation in FadA, the alpha-subunit of a heterotrimeric G-protein, confers resistance to osmotin. A deletion mutation in SfaD, the beta-subunit of a heterotrimeric G-protein also increases osmotin resistance. Aspergillus nidulans strains bearing these mutations also have increased tolerance to SDS, reduced cell wall porosity and increased chitin content in the cell wall.

Aspergillus fumigatus and aspergillosis

Aspergillus fumigatus is one of the most ubiquitous of the airborne saprophytic fungi. Humans and animals constantly inhale numerous conidia of this fungus. The conidia are normally eliminated in the immunocompetent host by innate immune mechanisms, and aspergilloma and allergic bronchopulmonary aspergillosis, uncommon clinical syndromes, are the only infections observed in such hosts. Thus, A. fumigatus was considered for years to be a weak pathogen. With increases in the number of immunosuppressed patients, however, there has been a dramatic increase in severe and usually fatal invasive aspergillosis, now the most common mold infection worldwide. In this review, the focus is on the biology of A. fumigatus and the diseases it causes. Included are discussions of (i) genomic and molecular characterization of the organism, (ii) clinical and laboratory methods available for the diagnosis of aspergillosis in immunocompetent and immunocompromised hosts, (iii) identification of host and fungal factors that play a role in the establishment of the fungus in vivo, and (iv) problems associated with antifungal therapy.

Attenuated virulence of uridine-uracil auxotrophs of Aspergillus fumigatus

Aspergillus fumigatus mutants that are deficient in the de novo UMP biosynthesis pathway because of a mutation in the pyrG gene encoding orotidine-5'-phosphate decarboxylase (and therefore auxotrophic for uridine or uracil) were evaluated in a murine model of invasive aspergillosis. These mutants were entirely nonpathogenic, and mutant conidia remained ungerminated in alveolar macrophages. Both the germination and virulence defects could be restored by supplementing the drinking water of the animals with uridine. DNA-mediated transformation of one of the pyrG mutants with the Aspergillus niger pyrG gene also restored virulence. These results suggest that uridine and uracil are limiting in the lung environment, thus preventing conidium germination and hence virulence of the pyrG mutants.

Virulence factors of medically important fungi

Human fungal pathogens have become an increasingly important medical problem with the explosion in the number of immunocompromised patients as a result of cancer, steroid therapy, chemotherapy, and AIDS. Additionally, the globalization of travel and expansion of humankind into previously undisturbed habitats have led to the reemergence of old fungi and new exposure to previously undescribed fungi. Until recently, relatively little was known about virulence factors for the medically important fungi. With the advent of molecular genetics, rapid progress has now been made in understanding the basis of pathogenicity for organisms such as Aspergillus species and Cryptococcus neoformans. The twin technologies of genetic transformation and "knockout" deletion construction allowed for genetic tests of virulence factors in these organisms. Such knowledge will prove invaluable for the rational design of antifungal therapies. Putative virulence factors and attributes are reviewed for Aspergillus species, C. neoformans, the dimorphic fungal pathogens, and others, with a focus upon a molecular genetic approach. Candida species are excluded from coverage, having been the subject of numerous recent reviews. This growing body of knowledge about fungal pathogens and their virulence factors will significantly aid efforts to treat the serious diseases they cause.

Fungal virulence genes as targets for antifungal chemotherapy

Fungal virulence genes have now met the age of molecular pathogenesis. The definition of virulence genes needs to be broad so that it encompasses the focus on molecular antifungal targets and vaccine epitopes. However, in the broad but simple definition of a virulence gene, there will be many complex genetic and host interactions which investigators will need to carefully define. Nevertheless, with the increasing numbers of serious fungal infections produced by old and newly reported organisms, the paucity of present antifungal drugs, and the likelihood of increasing drug resistance, the need for investigations into understanding fungal virulence at the molecular level has never been more important.

Virulence of Aspergillus fumigatus double mutants lacking restriction and an alkaline protease in a low-dose model of invasive pulmonary aspergillosis

To investigate the pathogenicity of Aspergillus fumigatus mutants lacking putative virulence factors, we have developed a new murine model of invasive pulmonary aspergillosis based on neutropenia, the major factor predisposing patients to this infection. Mice were treated with cyclophosphamide and inoculated by the intranasal route with 5 x 10(3) conidia, a significant reduction from inoculum levels used in previous models. Evidence for the production of the extracellular alkaline protease (Alp) in lung tissue was obtained by using a fungal transformant harboring an alp::lacZ reporter gene fusion. The pathogenicities of single mutant strains lacking either Alp or the ribotoxin restrictocin and of a double mutant strain lacking both proteins were assessed in this infection model. There were no significant differences between the mutant and the wild-type strains in terms of mortality or histological-features. Inoculations with mixtures of conidia showed that the double mutant strain is slightly less virulent than the wild-type strain. We conclude that Alp and restrictocin are not important virulence determinants in pulmonary infection.