Uncertainty surrounding the mechanism and safety of the post-harvest fungicide fludioxonil

Fludioxonil is a phenylpyrrole pesticide that is applied to fruit and vegetable crops post-harvest to minimize losses to mold, both during transport and at point of sale. Its effectiveness is reflected in the dramatic increase in its production/usage since its introduction in 1994, an increase that has peaked in recent years as it became licenced for use abroad. Recently, doubts as to the nature of its mechanism of action have been raised. Given that the pesticide has long been known to induce stress intermediates in target and non-target organisms alike, the lack of a firmly established mechanism might be cause for concern. Troubling reports further delineate a capacity to disrupt hepatic, endocrine and neurological systems, indicating that fludioxonil may represent a health threat to consumers. In the absence of a clear, safe mechanism of action, fludioxonil should be re-evaluated for its potential to impact human health.

Detection of the oomycete Pythium insidiosum by real-time PCR targeting the gene coding for exo-1,3-β-glucanase

Pythiosis is a life-threatening infectious disease caused by Pythium insidiosum. Early and accurate diagnosis is the key to prompt treatment and an improved prognosis for patients with pythiosis. An alternative to microbiological and immunological approaches for facilitating diagnosis of pythiosis is the PCR-based assay. Until recently, the ribosomal DNA (rDNA) region was the only target available for PCR-based detection of P. insidiosum. Failure to detect P. insidiosum by PCR amplification using the rDNA-specific primers has been reported. PinsEXO1, encoding an exo-1,3-β-glucanase, is an alternative, novel and efficient target for identification of P. insidiosum by conventional PCR. In this study, we aimed to develop a real-time (RT)-PCR approach targeting PinsEXO1 and compare its performance with conventional PCR for the detection of P. insidiosum. Both conventional and RT-PCR assays were positive for all 35 P. insidiosum strains tested, whilst all 58 control fungi were negative. The turnaround time for conventional PCR was 10 h, whilst that for RT-PCR was 7.5 h. The lowest amounts of genomic DNA template required for successful amplification by conventional and RT-PCR were 1 and 1 × 10(-4) ng, respectively. In conclusion, the RT-PCR assay retained 100% sensitivity and 100% specificity for detection of P. insidiosum. It showed a substantially improved analytical sensitivity and turnaround time that could improve diagnosis of pythiosis. The assay could also facilitate quantitative DNA analysis and epidemiological studies of P. insidiosum.

Structure and function of a fungal adhesin that binds heparin and mimics thrombospondin-1 by blocking T cell activation and effector function

Blastomyces adhesin-1 (BAD-1) is a 120-kD surface protein on B. dermatitidis yeast. We show here that BAD-1 contains 41 tandem repeats and that deleting even half of them impairs fungal pathogenicity. According to NMR, the repeats form tightly folded 17-amino acid loops constrained by a disulfide bond linking conserved cysteines. Each loop contains a highly conserved WxxWxxW motif found in thrombospondin-1 (TSP-1) type 1 heparin-binding repeats. BAD-1 binds heparin specifically and saturably, and is competitively inhibited by soluble heparin, but not related glycosaminoglycans. According to SPR analysis, the affinity of BAD-1 for heparin is 33 nM±14 nM. Putative heparin-binding motifs are found both at the N-terminus and within each tandem repeat loop. Like TSP-1, BAD-1 blocks activation of T cells in a manner requiring the heparan sulfate-modified surface molecule CD47, and impairs effector functions. The tandem repeats of BAD-1 thus confer pathogenicity, harbor motifs that bind heparin, and suppress T-cell activation via a CD47-dependent mechanism, mimicking mammalian TSP-1.

SREB, a GATA transcription factor that directs disparate fates in Blastomyces dermatitidis including morphogenesis and siderophore biosynthesis

Blastomyces dermatitidis belongs to a group of human pathogenic fungi that exhibit thermal dimorphism. At 22 degrees C, these fungi grow as mold that produce conidia or infectious particles, whereas at 37 degrees C they convert to budding yeast. The ability to switch between these forms is essential for virulence in mammals and may enable these organisms to survive in the soil. To identify genes that regulate this phase transition, we used Agrobacterium tumefaciens to mutagenize B. dermatitidis conidia and screened transformants for defects in morphogenesis. We found that the GATA transcription factor SREB governs multiple fates in B. dermatitidis: phase transition from yeast to mold, cell growth at 22 degrees C, and biosynthesis of siderophores under iron-replete conditions. Insertional and null mutants fail to convert to mold, do not accumulate significant biomass at 22 degrees C, and are unable to suppress siderophore biosynthesis under iron-replete conditions. The defect in morphogenesis in the SREB mutant was independent of exogenous iron concentration, suggesting that SREB promotes the phase transition by altering the expression of genes that are unrelated to siderophore biosynthesis. Using bioinformatic and gene expression analyses, we identified candidate genes with upstream GATA sites whose expression is altered in the null mutant that may be direct or indirect targets of SREB and promote the phase transition. We conclude that SREB functions as a transcription factor that promotes morphogenesis and regulates siderophore biosynthesis. To our knowledge, this is the first gene identified that promotes the conversion from yeast to mold in the dimorphic fungi, and may shed light on environmental persistence of these pathogens.

Calcium binding by the essential virulence factor BAD-1 of Blastomyces dermatitidis

BAD-1 (Blastomyces adhesin 1), a 120-kDa protein of Blastomyces dermatitidis, functions as an adhesin, immune modulator, and essential virulence factor. Structurally, BAD-1 is composed of a short N-terminal region, a core of 30 tandem repeats critical for virulence, and a C-terminal epidermal growth factor domain that binds the protein to yeast cell surface chitin. Each of the 30 acidic residue-rich tandem repeats contains a sequence that resembles the calcium-binding loop of the EF-hand domain found in many calcium-binding proteins. Here, we investigated the binding of calcium by BAD-1 and its biological significance. Yeast washed with double distilled H2O released surface-bound BAD-1, but EGTA washes were an order of magnitude more efficient, suggesting an interaction between BAD-1 and calcium. Immobilized BAD-1 was stained with ruthenium red dye, an indicator of calcium-binding proteins. In equilibrium dialysis, BAD-1 bound 45Ca2+ with an affinity of 0.41 x 10(-5) m and a capacity of 27 calcium/mol. Mass spectrometry confirmed this capacity. Elevated [Ca2+] diminished BAD-1 solubility. Upon deletion of its C-terminal epidermal growth factor-like domain, BAD-1 resisted aggregation by elevated [Ca2+] but retained its affinity and capacity for calcium. Removing 20 copies of the tandem repeat, however, sharply reduced the capacity of BAD-1 for calcium. Growth of the bad-1 null yeast was inhibited by 5 mm EGTA, and re-expression of BAD-1 in trans or the addition of exogenous purified BAD-1 restored growth. Thus, BAD-1 is a high capacity calcium-binding protein. This property contributes to the structure and function of BAD-1, as well as to B. dermatitidis acquisition of calcium from the environment.

Exploiting type 3 complement receptor for TNF-alpha suppression, immune evasion, and progressive pulmonary fungal infection

TNF-alpha is crucial in defense against intracellular microbes. Host immune cells use type 3 complement receptors (CR3) to regulate excess TNF-alpha production during physiological clearance of apoptotic cells. BAD1, a virulence factor of Blastomyces dermatitidis, is displayed on yeast and released during infection. BAD1 binds yeast to macrophages (Mphi) via CR3 and CD14 and suppresses TNF-alpha, which is required for resistance. We investigated whether blastomyces adhesin 1 (BAD1) exploits host receptors for immune deviation and pathogen survival. Soluble BAD1 rapidly entered Mphi, accumulated intracellularly by 10 min after introduction to cells, and trafficked to early and late endosomes. Inhibition of receptor recycling by monodansyl cadaverine blocked association of BAD1 with Mphi and reversed TNF-alpha suppression in vitro. Inhibition of BAD1 uptake with cytochalasin D and FcR-redirected delivery of soluble BAD1 as Ag-Ab complexes or of wild-type yeast opsonized with IgG similarly reversed TNF-alpha suppression. Hence, receptor-mediated entry of BAD1 is requisite in TNF-alpha suppression, and the route of entry is critical. Binding of soluble BAD1 to Mphi of CR3(-/-) and CD14(-/-) mice was reduced to 50 and 33%, respectively, of that in wild-type mice. Mphi of CR3(-/-) and CD14(-/-) mice resisted soluble BAD1 TNF-alpha suppression in vitro, but, in contrast to CR3(-/-) cells, CD14(-/-) cells were still subject to suppression mediated by surface BAD1 on wild-type yeast. CR3(-/-) mice resisted both infection and TNF-alpha suppression in vivo, in contrast to wild-type and CD14(-/-) mice. BAD1 of B. dermatitidis thus co-opts normal host cell physiology by exploiting CR3 to subdue TNF-alpha production and foster pathogen survival.

Molecular genetic analysis of Blastomyces dermatitidis reveals new insights about pathogenic mechanisms

Fungal pathogens have emerged as a public health menace owing to the expanding population of vulnerable patients and to a heightened exposure to fungi in our environment, particularly for the systemic dimorphic fungi that inhabit soil worldwide. A better understanding of these microbes and their pathogenic mechanisms is badly needed to further research into therapeutic options. Advances in the molecular tools for genetic manipulation of Blastomyces dermatitidis have enhanced our ability to study this poorly understood dimorphic fungal pathogen. Recent refinements in gene-transfer technique, new selection markers, reliable reporter fusions and successes in gene targeting have shed light upon the importance of the mycelium-to-yeast transition and the crucial and complex role the BAD1 adhesin plays in pathogenesis.

Using new genetic tools to study the pathogenesis of Blastomyces dermatitidis

Fungal pathogens have emerged as a public health menace owing to the expanding population of vulnerable patients and a heightened exposure to fungi in our environment, particularly for the systemic dimorphic fungi that inhabit soil worldwide. A better understanding of these invaders and their pathogenic mechanisms is badly needed to further research into therapeutic options. Advances in the molecular tools available for genetic manipulation of Blastomyces dermatitidis have enhanced our ability to study this poorly understood dimorphic fungal pathogen. Recent refinements in gene-transfer techniques, new selection markers, reliable reporter fusions and successes in gene targeting have shed light upon the importance of the mycelium-to-yeast transition and the crucial and complex role the BAD1 adhesin plays in pathogenesis.

Role of glucan and surface protein BAD1 in complement activation by Blastomyces dermatitidis yeast

Our previous studies showed that Blastomyces dermatitidis yeast activates the human complement system, leading to deposition of opsonic complement fragments onto the yeast surface. This report examines the influence of altered surface expression of glucan or BAD1 protein (formerly WI-1) on the yeast's ability to activate and bind C3. Compared to the wild type, a glucan-deficient mutant yeast delayed initiation of C3 deposition and reduced C3-binding capacity by 50%. Linkage of baker's-yeast beta-glucan to the glucan-deficient yeast restored initial C3 deposition kinetics to the wild-type level and partially restored C3-binding capacity, suggesting that beta-glucan is an initiator of complement activation and a C3 acceptor. The role of BAD1 in B. dermatitidis yeast-complement interaction was also assessed. BAD1 knockout yeast initiated faster C3 deposition and increased C3-binding capacity compared to the wild-type yeast or a BAD1-reconstituted yeast, suggesting either a lack of an intrinsic ability in BAD1 or an inhibitory role of BAD1 in complement activation and binding. However, both complement activation and the capacity for C3 binding by the wild-type yeast were enhanced in normal human serum supplemented with an anti-BAD1 monoclonal antibody (MAb) or in immune sera from blastomycosis patients. Microscopic analysis revealed that more initial C3-binding sites were formed on yeast in the presence of both naturally occurring complement initiators and exogenous anti-BAD1 MAb, suggesting that anti-BAD1 antibody enhanced the ability of B. dermatitidis yeast to interact with the host complement system. Thus, glucan and BAD1 have distinctly different regulatory effects on complement activation by B. dermatitidis.

Targeted gene disruption reveals an adhesin indispensable for pathogenicity of Blastomyces dermatitidis

Systemic fungal infections are becoming more common and difficult to treat, yet the pathogenesis of these infectious diseases remains poorly understood. In many cases, pathogenicity can be attributed to the ability of the fungi to adhere to target tissues, but the lack of tractable genetic systems has limited progress in understanding and interfering with the offending fungal products. In Blastomyces dermatitidis, the agent of blastomycosis, a respiratory and disseminated mycosis of people and animals worldwide, expression of the putative adhesin encoded by the WI-1 gene was investigated as a possible virulence factor. DNA-mediated gene transfer was used to disrupt the WI-1 locus by allelic replacement, resulting in impaired binding and entry of yeasts into macrophages, loss of adherence to lung tissue, and abolishment of virulence in mice; each of these properties was fully restored after reconstitution of WI-1 by means of gene transfer. These findings establish the pivotal role of WI-1 in adherence and virulence of B. dermatitidis yeasts. To our knowledge, they offer the first example of a genetically proven virulence determinant among systemic dimorphic fungi, and underscore the value of reverse genetics for studies of pathogenesis in these organisms.

Purification in quantity of the secreted form of WI-1: a major adhesin on Blastomyces dermatitidis yeasts

WI-1, a 120-kDa adhesin on Blastomyces dermatitidis, binds the yeast to macrophages and is a major target antigen of immune recognition in acquired resistance to the fungus. In past studies, WI-1 has been purified by extracting the protein from the yeast cell wall, which yields microgram quantities for biological assays. We report a strategy for generating and purifying the secreted form of WI-1 in quantity. Yeasts of B. dermatitidis ATCC strain 60636 cultured in HMM medium were found to secrete 10 microg/ml of WI-1 into a supernate relatively free of other medium and yeast components. Using a two-step method of ion exchange and hydrophobic interaction chromatography, we achieved a 7.1-fold purification of WI-1. Purified WI-1 was sequenced at the N-terminus which revealed that the secreted protein exists in two different forms. In functional assays, purified WI-1 also retained its adhesivity for human macrophages, and its antigenicity in binding anti-WI-1 antibodies and stimulating T-cells to proliferate, but it lost some capacity to elicit delayed-type hypersensitivity in mice. These findings advance our understanding of the WI-1 adhesin/antigen and our ability to express and purify WI-1 in quantity and will permit a study of the relationship between three-dimensional structure and activity of the molecule.

Production and localization of restrictocin in Aspergillus restrictus

The production and secretion of restrictocin (a cytotoxin that cleaves ribosomal RNA) by cultures of the fungus Aspergillus restrictus was investigated. Previous studies have indicated that restrictocin production in liquid culture coincides with the appearance of differentiated cell structures. A study of the correlation between the appearance of differentiated structures and restrictocin production was conducted with A. restrictus grown on agar medium. Restrictocin was found to be associated with the cell mass of the agar-grown culture (in contrast to liquid cultures), and was first observed when aerial hyphae emerged. Restrictocin levels increased until the time of conidiation, after which they fell off sharply. No restrictocin could be found in the agar medium. The presence of restrictocin upon and within various cell structures was determined by immunofluorescent laser microscopy. This study showed that restrictocin became localized to the conidiophores and phialides during the process of conidiation. Prior to this, restrictocin was found within the hyphae in localized concentrations that may correspond to secretory vesicles.