Molecular basis of pathogenicity in Blastomyces dermatitidis: the importance of adhesion

Selection on plastic adherence leads to hyper-multicellular strains and incidental virulence in the budding yeast

Many disease-causing microbes are not obligate pathogens; rather, they are environmental microbes taking advantage of an ecological opportunity. The existence of microbes whose life cycle does not require a host and are not normally pathogenic, yet are well-suited to host exploitation, is an evolutionary puzzle. One hypothesis posits that selection in the environment may favor traits that incidentally lead to pathogenicity and virulence, or serve as pre-adaptations for survival in a host. An example of such a trait is surface adherence. To experimentally test the idea of 'accidental virulence', replicate populations of Saccharomyces cerevisiae were evolved to attach to a plastic bead for hundreds of generations. Along with plastic adherence, two multicellular phenotypes- biofilm formation and flor formation- increased; another phenotype, pseudohyphal growth, responded to the nutrient limitation. Thus, experimental selection led to the evolution of highly-adherent, hyper-multicellular strains. Wax moth larvae injected with evolved hyper-multicellular strains were significantly more likely to die than those injected with evolved non-multicellular strains. Hence, selection on plastic adherence incidentally led to the evolution of enhanced multicellularity and increased virulence. Our results support the idea that selection for a trait beneficial in the open environment can inadvertently generate opportunistic, 'accidental' pathogens.

Endemic mycoses in children in North America: a review of radiologic findings

Clinically significant endemic mycoses (fungal infections) in the United States (U.S.) include Blastomyces dermatitidis, Histoplasma capsulatum, and Coccidioides immitis/posadasii. While the majority of infections go clinically unnoticed, symptomatic disease can occur in immunocompromised or hospitalized patients, and occasionally in immune-competent individuals. Clinical manifestations vary widely and their diagnosis may require fungal culture, making the rapid diagnosis a challenge. Imaging can be helpful in making a clinical diagnosis prior to laboratory confirmation, as well as assist in characterizing disease extent and severity. In this review, we discuss the three major endemic fungal infections that occur in the U.S., including mycology, epidemiology, clinical presentations, and typical imaging features with an emphasis on the pediatric population.

Secretome Profiling by Proteogenomic Analysis Shows Species-Specific, Temperature-Dependent, and Putative Virulence Proteins of Pythium insidiosum

In contrast to most pathogenic oomycetes, which infect plants, Pythium insidiosum infects both humans and animals, causing a difficult-to-treat condition called pythiosis. Most patients undergo surgical removal of an affected organ, and advanced cases could be fetal. As a successful human/animal pathogen, P. insidiosum must tolerate body temperature and develop some strategies to survive and cause pathology within hosts. One of the general pathogen strategies is virulence factor secretion. Here, we used proteogenomic analysis to profile and validate the secretome of P. insidiosum, in which its genome contains 14,962 predicted proteins. Shotgun LC–MS/MS analysis of P. insidiosum proteins prepared from liquid cultures incubated at 25 and 37 °C mapped 2980 genome-predicted proteins, 9.4% of which had a predicted signal peptide. P. insidiosum might employ an alternative secretory pathway, as 90.6% of the validated secretory/extracellular proteins lacked the signal peptide. A comparison of 20 oomycete genomes showed 69 P. insidiosum–specific secretory/extracellular proteins, and these may be responsible for the host-specific infection. The differential expression analysis revealed 14 markedly upregulated proteins (particularly cyclophilin and elicitin) at body temperature which could contribute to pathogen fitness and thermotolerance. Our search through a microbial virulence database matched 518 secretory/extracellular proteins, such as urease and chaperones (including heat shock proteins), that might play roles in P. insidiosum virulence. In conclusion, the identification of the secretome promoted a better understanding of P. insidiosum biology and pathogenesis. Cyclophilin, elicitin, chaperone, and urease are top-listed secreted/extracellular proteins with putative pathogenicity properties. Such advances could lead to developing measures for the efficient detection and treatment of pythiosis.

Immunity to Invasive Fungal Diseases

Invasive fungal diseases are rare in individuals with intact immunity. This, together with the fact that there are only a few species that account for most mycotic diseases, implies a remarkable natural resistance to pathogenic fungi. Mammalian immunity to fungi rests on two pillars, powerful immune mechanisms and elevated temperatures that create a thermal restriction zone for most fungal species. Conditions associated with increased susceptibility generally reflect major disturbances of immune function involving both the cellular and humoral innate and adaptive arms, which implies considerable redundancy in host defense mechanisms against fungi. In general, tissue fungal invasion is controlled through either neutrophil or granulomatous inflammation, depending on the fungal species. Neutrophils are critical against Candida spp. and Aspergillus spp. while macrophages are essential for controlling mycoses due to Cryptococcus spp., Histoplasma spp., and other fungi. The increasing number of immunocompromised patients together with climate change could significantly increase the prevalence of fungal diseases.

Transcriptome Analysis of Dimorphic Fungus Sporothrix schenckii Exposed to Temperature Stress

PURPOSE

Sporothrix schenckii is a thermally dimorphic fungus. In a saprotrophic environment or culturing at 25 °C, it grows as mycelia, whereas in host tissues or culturing at 37 °C, it undergoes dimorphic transition and division into pathogenic yeast cells. S. schenckii can cause serious disseminated sporotrichosis in immunocompromised hosts and presents an emerging global health problem. The mycelium-to-yeast transition was a consequence of the adaptive process to different environment. Some studies showed that the transition was significantly related to the virulence and pathogenesis of dimorphic fungi. However the genetic mechanisms of this complicated biological process are poorly understood.

METHOD

Our study presented a comparative transcriptomic analysis perspective on temperature stress in a visceral isolates of S. schenckii, obtaining more genetic information related to dimorphic transition.

RESULTS

The 9.38 Gbp dataset was generated and assembled into 14,423 unigenes. Compared with gene and protein databases, 9561 unigenes were annotated. Comparative analysis identified 1259 genes expressed differentially in mycelium and yeast phase, and were categorized into a number of important biological processes, such as synthesis and metabolism, transmembrane transport, biocatalysis, oxidation reduction, and cellular signal transduction.

CONCLUSIONS

The findings suggested that temperature-dependent transition was tightly associated with stress adaptation, growth and development, signal regulation, adhesion, and colonization, which was predicted to be related with virulence and pathogenesis. Collection of these data should offer fine-scale insights into the mechanisms of dimorphism and pathogenesis of S. schenckii, and meanwhile facilitate the evolutionary and function studies of other dimorphic fungi.

Fungal vaccines, mechanism of actions and immunology: A comprehensive review

Fungal infections include a wide range of opportunistic and invasive diseases. Two of four major fatal diseases in patients with human immunodeficiency virus (HIV) infection are related to the fungal infections, cryptococcosis, and pneumocystosis. Disseminated candidiasis and different clinical forms of aspergillosis annually impose expensive medical costs to governments and hospitalized patients and ultimately lead to high mortality rates. Therefore, urgent implementations are necessary to prevent the expansion of these diseases. Designing an effective vaccine is one of the most important approaches in this field. So far, numerous efforts have been carried out in developing an effective vaccine against fungal infections. Some of these challenges engaged in different stages of clinical trials but none of them could be approved by the United States Food and Drug Administration (FDA). Here, in addition to have a comprehensive overview on the data from studied vaccine programs, we will discuss the immunology response against fungal infections. Moreover, it will be attempted to clarify the underlying immune mechanisms of vaccines targeting different fungal infections that are crucial for designing an effective vaccination strategy.

Quantitative Proteomic Analysis of Four Developmental Stages of Saprolegnia parasitica

Several water mold species from the Saprolegnia genus infect fish, amphibians, and crustaceans in natural ecosystems and aquaculture farms. Saprolegnia parasitica is one of the most severe fish pathogens. It is responsible for millions of dollars of losses to the aquaculture industry worldwide. Here, we have performed a proteomic analysis, using gel-based and solution (iTRAQ) approaches, of four defined developmental stages of S. parasitica grown in vitro, i.e., the mycelium, primary cysts, secondary cysts and germinated cysts, to gain greater insight into the types of proteins linked to the different stages. A relatively high number of kinases as well as virulence proteins, including the ricin B lectin, disintegrins, and proteases were identified in the S. parasitica proteome. Many proteins associated with various biological processes were significantly enriched in different life cycle stages of S. parasitica. Compared to the mycelium, most of the proteins in the different cyst stages showed similar enrichment patterns and were mainly related to energy metabolism, signal transduction, protein synthesis, and post-translational modifications. The proteins most enriched in the mycelium compared to the cyst stages were associated with amino acid metabolism, carbohydrate metabolism, and mitochondrial energy production. The data presented expand our knowledge of metabolic pathways specifically linked to each developmental stage of this pathogen.

The Immunoreactive Exo-1,3-β-Glucanase from the Pathogenic Oomycete Pythium insidiosum Is Temperature Regulated and Exhibits Glycoside Hydrolase Activity

The oomycete organism, Pythium insidiosum, is the etiologic agent of the life-threatening infectious disease called "pythiosis". Diagnosis and treatment of pythiosis is difficult and challenging. Novel methods for early diagnosis and effective treatment are urgently needed. Recently, we reported a 74-kDa immunodominant protein of P. insidiosum, which could be a diagnostic target, vaccine candidate, and virulence factor. The protein was identified as a putative exo-1,3-ß-glucanase (Exo1). This study reports on genetic, immunological, and biochemical characteristics of Exo1. The full-length exo1 coding sequence (2,229 bases) was cloned. Phylogenetic analysis showed that exo1 is grouped with glucanase-encoding genes of other oomycetes, and is far different from glucanase-encoding genes of fungi. exo1 was up-regulated upon exposure to body temperature, and its gene product is predicted to contain BglC and X8 domains, which are involved in carbohydrate transport, binding, and metabolism. Based on its sequence, Exo1 belongs to the Glycoside Hydrolase family 5 (GH5). Exo1, expressed in E. coli, exhibited ß-glucanase and cellulase activities. Exo1 is a major intracellular immunoreactive protein that can trigger host immune responses during infection. Since GH5 enzyme-encoding genes are not present in human genomes, Exo1 could be a useful target for drug and vaccine development against this pathogen.

Blastomycosis mortality rates, United States, 1990-2010

Risks for illness and death caused by the Blastomyces dermatitidis fungus are affected by demographic, geographic, and behavioral factors.

Blastomycosis is a potentially fatal fungal infection endemic to parts of North America. We used national multiple-cause-of-death data and census population estimates for 1990–2010 to calculate age-adjusted mortality rates and rate ratios (RRs). We modeled trends over time using Poisson regression. Death occurred more often among older persons (RR 2.11, 95% confidence limit [CL] 1.76, 2.53 for those 75–84 years of age vs. 55–64 years), men (RR 2.43, 95% CL 2.19, 2.70), Native Americans (RR 4.13, 95% CL 3.86, 4.42 vs. whites), and blacks (RR 1.86, 95% CL 1.73, 2.01 vs. whites), in notably younger persons of Asian origin (mean = 41.6 years vs. 64.2 years for whites); and in the South (RR 18.15, 95% CL 11.63, 28.34 vs. West) and Midwest (RR 23.10, 95% CL14.78, 36.12 vs. West). In regions where blastomycosis is endemic, we recommend that the diagnosis be considered in patients with pulmonary disease and that it be a reportable disease.

Fungal morphogenesis

Morphogenesis in fungi is often induced by extracellular factors and executed by fungal genetic factors. Cell surface changes and alterations of the microenvironment often accompany morphogenetic changes in fungi. In this review, we will first discuss the general traits of yeast and hyphal morphotypes and how morphogenesis affects development and adaptation by fungi to their native niches, including host niches. Then we will focus on the molecular machinery responsible for the two most fundamental growth forms, yeast and hyphae. Last, we will describe how fungi incorporate exogenous environmental and host signals together with genetic factors to determine their morphotype and how morphogenesis, in turn, shapes the fungal microenvironment.

A large community outbreak of blastomycosis in Wisconsin with geographic and ethnic clustering

BACKGROUND

Blastomycosis is a potentially life-threatening infection caused by the soil-based dimorphic fungus Blastomyces dermatitidis, which is endemic throughout much of the Midwestern United States. We investigated an increase in reported cases of blastomycosis that occurred during 2009-2010 in Marathon County, Wisconsin.

METHODS

Case detection was conducted using the Wisconsin Electronic Disease Surveillance System (WEDSS). WEDSS data were used to compare demographic, clinical, and exposure characteristics between outbreak-related and historical case patients, and to calculate blastomycosis incidence rates. Because initial mapping of outbreak case patients' homes and recreational sites demonstrated unusual neighborhood and household case clustering, we conducted a 1:3 matched case-control study to identify factors associated with being in a geographic cluster.

RESULTS

Among the 55 patients with outbreak-related cases, 33 (70%) were hospitalized, 2 (5%) died, 30 (55%) had cluster-related cases, and 20 (45%) were Hmong. The overall incidence increased significantly since 2005 (average 11% increase per year, P < .001), and incidence during 2005-2010 was significantly higher among Asians than non-Asians (2010 incidence: 168 vs 13 per 100 000 population). Thirty of the outbreak cases grouped into 5 residential clusters. Outdoor activities were not risk factors for blastomycosis among cluster case patients or when comparing outbreak cases to historical cases.

CONCLUSIONS

This outbreak of blastomycosis, the largest ever reported, was characterized by unique household and neighborhood clustering likely related to multifocal environmental sources. The reasons for the large number of Hmong affected are unclear, but may involve genetic predisposition.

The two-component histidine kinases DrkA and SlnA are required for in vivo growth in the human pathogen Penicillium marneffei

In order to cause disease fungal pathogens must be capable of evading or tolerating the host immune defence system. One commonly utilized evasion mechanism is the ability to continually reside within macrophages of the innate immune system and survive subsequent phagocytic destruction. For intracellular growth to occur, fungal pathogens which typically grow in a filamentous hyphal form in the environment must be able to switch growth to a unicellular yeast growth form in a process known as dimorphic switching. The cue to undergo dimorphic switching relies on the recognition of, and response to, the intracellular host environment. Two-component signalling systems are utilized by eukaryotes to sense and respond to changes in the external environment. This study has investigated the role of the hybrid histidine kinase components encoded by drkA and slnA, in the dimorphic pathogen Penicillium marneffei. Both SlnA and DrkA are required for stress adaptation but are uniquely required for different aspects of asexual development, hyphal morphogenesis and cell wall integrity. Importantly, slnA and drkA are both essential for the generation of yeast cells in vivo, with slnA required for the germination of conidia and drkA required for dimorphic switching during macrophage infection.

Primary blastomycosis of oral cavity

BACKGROUND

Blastomycosis is an uncommon male-predominant disease caused by the fungus Blastomyces dermatitidis. The lungs are most commonly affected, and other organs are usually involved by dissemination. Clinical feature and pathohistologic findings are similar to the appearance of squamous cell carcinoma.

METHODS AND RESULTS

A 52-year-old male patient who has lived as a farmer on the countryside in Argentina for 35 years presented with an initial histopathologic diagnosis of a squamous cell carcinoma of the right lower jaw. There was no history of pulmonary disease, in particular fever, coughing, or hemoptysis. Final pathohistologic evaluation after resection revealed B. dermatitidis infection. This article presents the first described case of oral manifestation of B. dermatitidis infection in Switzerland.

CONCLUSIONS

Manifestation of blastomycosis in oral tissue can mimic the feature of a squamous cell carcinoma and can therefore be a diagnostic pitfall that head and neck surgeons and a pathologist should be aware of.

Identification and characterization of a laminin-binding protein of Aspergillus fumigatus: extracellular thaumatin domain protein (AfCalAp)

Aspergillus fumigatus, an opportunistic fungal pathogen, infects the human host via inhalation of airborne conidia. Adhesion of fungal conidia, to host cells and extracellular matrix (ECM) components associated with host tissue surfaces, is thought to be the primary step in the pathogenesis and dissemination of infection. To identify novel adhesion proteins (adhesins) of A. fumigatus, we screened its proteome in silico using spaan (software program for prediction of adhesins and adhesin-like proteins using neural networks). One of the predicted adhesin-encoding genes with a P ad (probability of being adhesin) value >0.9, the gene encoding extracellular thaumatin domain protein (AfCalA), was cloned and expressed in Escherichia coli. Recombinant AfCalAp showed significant binding with laminin and murine lung cells. Anti-AfCalAp antibodies inhibited the binding of AfCalAp to laminin in a dose-dependent manner. Significant binding of anti-AfCalAp antibodies to 2 h swollen conidia suggests the presence of AfCalAp on the conidial surface. AfCalA transcript was not detectable in resting conidia but was detected in conidia incubated with RPMI 1640 medium in the presence and absence of lung epithelial cell line (A539)-derived ECM. Elevated levels of IgE antibodies specific to AfCalAp were observed in the sera of two out of seven patients with allergic bronchopulmonary aspergillosis. The study confirms the relevance of the bioinformatic approach for predicting fungal adhesins and establishes AfCalAp as a novel laminin-binding protein of A. fumigatus.

Localized adhesion of nongerminated Venturia inaequalis conidia to leaves and artificial surfaces

Adhesion to the host surface is the first step for successful plant pathogen development and has been reported to be associated with both passive and active processes. For conidia of Venturia inaequalis, which depend on leaf wetness for germination, this process has not yet been described. Conidia of V. inaequalis adhered to wet hydrophobic surfaces immediately after contact to the surface, hours before initiation of germination. Attachment of nongerminated conidia was much better on hydrophobic surfaces, such as apple leaves and polystyrene, than on hydrophilic glass. Conidia released adhesive material localized in a droplet named spore tip glue (STG) at the spore apex which interacted with a contact surface only when water was present. Histochemical investigations indicated the presence of proteins and carbohydrates in STG, lectin labeling the presence of beta-galactose and N-acetylglucosaminyl residues. Transmission electron microscopy revealed two phases in the STG at the tip of dry mature conidia; as STG was present on the outer side of the intact fungal cell wall its formation should be associated with the secretion of glue through pores of the conidial wall. Surface-active substances affected the adhesion of conidia to hydrophobic surfaces stressing the importance of hydrophobic interactions. The use of protein biosynthesis inhibitors did not affect adhesion of conidia indicating that the adhesive material was preformed. It is concluded that the coincidence of STG, contact to a hydrophobic surface, and free water are essential for the adhesion of V. inaequalis conidia.

Isolation and some properties of a glycoprotein of 70 kDa (Gp70) from the cell wall of Sporothrix schenckii involved in fungal adherence to dermal extracellular matrix

Sporothrix schenckii is the etiological agent of sporotrichosis, a subcutaneous mycosis and an emerging disease in immunocompromised patients. Adherence to target cells is a prerequisite for fungal dissemination and systemic complications. However, information on the cell surface components involved in this interaction is rather scarce. In this investigation, the extraction of isolated cell walls from the yeast phase of S. schenckii with SDS and separation of proteins by SDS-PAGE led to the identification of a periodic acid-Schiff (PAS)-reacting 70 kDa glycoprotein (Gp70) that was purified by elution from electrophoresis gels. The purified glycopeptide exhibited a pI of 4.1 and about 5.7% of its molecular mass was contributed by N-linked glycans with no evidence for O-linked oligosaccharides. Confocal analysis of immunofluorescence assays with polyclonal antibodies directed towards Gp70 revealed a rather uniform distribution of the antigen at the cell surface with no distinguishable differences among three different isolates. Localization of Gp70 at the cell surface was confirmed by immunogold staining. Gp70 seems specific for S. schenckii as no immunoreaction was observed in SDS-extracts from other pathogenic and non-pathogenic fungi. Yeast cells of the fungus abundantly adhered to the dermis of mouse tails and the anti-Gp70 serum reduced this process in a concentration-dependent manner. Results are discussed in terms of the potential role of Gp70 in the host-pathogen interaction.

Blastomycosis: Contributions of Morphology to Diagnosis

Blastomycosis is caused by the inhalation of spores of the dimorphic fungus, Blastomyces dermatitidis. The reporting of this disease is not required by all states. The diagnosis is established by culture or by identification of broad budding yeast forms in tissue or cytology samples. A retrospective review of blastomycosis was conducted using surgical pathology and cytopathology records of a large community based general hospital, for the years 1982 to 2002; the autopsy records of a university referral center were searched for the years 1992 to 2004. Thirteen surgical/cytology cases were retrieved: 8 localized to the lung (group 1) and 5 with extrapulmonary presentation (group 2). Three of the former were clinically thought to be tumors. Broad-based budding yeast forms with thick cell walls were seen in all but 1 case and identified on conventional routinely stained preparations. Microbiologic culture was positive in 2 lung cases only, in 1 of which it was the sole means of diagnosis. Culture was negative for Blastomyces in the only extrapulmonary case for which a specimen was submitted. One patient in group 2 died, but had a coexistent disseminated gastric adenocarcinoma. No autopsy was performed. Three autopsies of blastomycosis were recovered (group 3). Two patients were on corticosteroids and 1 was diabetic. Premortem diagnoses were established only within a few days of death in 2 patients. Blastomycosis is seldom a fatal disease. Most patients are immune competent; immune compromise favors an aggressive course. Microbiologic culture and conventional morphologic assessment of routine samples have redundant utility in diagnosis.

Chondroitin sulfate B and heparin mediate adhesion of Penicillium marneffei conidia to host extracellular matrices

Penicilliosis is a disseminated infection in immunocompromised individuals caused by the dimorphic fungus, Penicillium marneffei. Very little is known about its route of infection, however, it is thought that initial infection occurs through inhalation of conidia. We investigated the role played by various extracellular matrix glycosaminoglycans (GAGs) in the initial adherence of P. marneffei conidia using a direct adhesion assay. GAGs were further used to block the binding of fungal spores to human lung epithelial cells and highly sulfated GAGs were tested for their inhibitory effects owing to their degree of sulfation. Our results demonstrated high levels of conidial adhesion to chondroitin sulfate B, heparin and highly sulfated chitosan (CP-3). No direct adherence was observed to immobilized chondroitin sulfate (CS) A, CSC, CSD and hyaluronic acid, as well as chitosans with low sulfate content. The results suggested that P. marneffei conidia bind to iduronic acid (IdoA) of the polysaccharide chains. Involvement of negatively charged sulfate groups in adhesion was also indicated. Furthermore, significant inhibition of conidial adherence to A549 cells was observed in the presence of CSB, heparan sulfate (HS), heparin and CP-3. It was further demonstrated that GAGs can affect the adhesion of conidia to fibronectin and laminin, glycoproteins that have previously been implicated as adhesive receptors for fungal conidia. CSB and HS could partially inhibit the adhesion of fungal conidia to laminin and fibronectin implying that conidia can weakly interact with the IdoA GAG-binding domain(s) of these molecules. The data indicated that, in addition to fibronectin and laminin, IdoA-containing GAGs may play an important role in fungal adherence to the surface of human lung epithelium.

Interaction of pathogenic fungi with host cells: Molecular and cellular approaches

This review provides an overview of several molecular and cellular approaches that are likely to supply insights into the host-fungus interaction. Fungi present intra- and/or extracellular host-parasite interfaces, the parasitism phenomenon being dependent on complementary surface molecules. The entry of the pathogen into the host cell is initiated by the fungus adhering to the cell surface, which generates an uptake signal that may induce its cytoplasmatic internalization. Furthermore, microbial pathogens use a variety of their surface molecules to bind to host extracellular matrix (ECM) components to establish an effective infection. On the other hand, integrins mediate the tight adhesion of cells to the ECM at sites referred to as focal adhesions and also play a role in cell signaling. The phosphorylation process is an important mechanism of cell signaling and regulation; it has been implicated recently in defense strategies against a variety of pathogens that alter host-signaling pathways in order to facilitate their invasion and survival within host cells. The study of signal transduction pathways in virulent fungi is especially important in view of their putative role in the regulation of pathogenicity. This review discusses fungal adherence, changes in cytoskeletal organization and signal transduction in relation to host-fungus interaction.

During attachment Phytophthora spores secrete proteins containing thrombospondin type 1 repeats

Adhesion is a key aspect of disease establishment in animals and plants. Adhesion anchors the parasite to the host surface and is a prerequisite for further development and host cell invasion. Although a number of adhesin molecules produced by animal pathogens have been characterised, molecular details of adhesins of plant pathogens, especially fungi, are largely restricted to general descriptions of the nature of heterogeneous secreted materials. In this paper, we report the cloning of a gene, PcVsv1, encoding a protein secreted during attachment of spores of Phytophthora, a genus of highly destructive plant pathogens. PcVsv1 contains 47 copies of the thrombospondin type 1 repeat, a motif found in adhesins of animals and malarial parasites but not in plants, green algae or true fungi. Our results suggest that PcVsv1 is a spore adhesin and highlight intriguing similarities in structural and molecular features of host attachment in oomycete and malarial parasites.

Fungal virulence, vertebrate endothermy, and dinosaur extinction: is there a connection?

Fungi are relatively rare causes of life-threatening systemic disease in immunologically intact mammals despite being frequent pathogens in insects, amphibians, and plants. Given that virulence is a complex trait, the capacity of certain soil fungi to infect, persist, and cause disease in animals despite no apparent requirement for animal hosts in replication or survival presents a paradox. In recent years studies with amoeba, slime molds, and worms have led to the proposal that interactions between fungi and other environmental microbes, including predators, select for characteristics that are also suitable for survival in animal hosts. Given that most fungal species grow best at ambient temperatures, the high body temperature of endothermic animals must provide a thermal barrier for protection against infection with a large number of fungi. Fungal disease is relatively common in birds but most are caused by only a few thermotolerant species. The relative resistance of endothermic vertebrates to fungal diseases is likely a result of higher body temperatures combined with immune defenses. Protection against fungal diseases could have been a powerful selective mechanism for endothermy in certain vertebrates. Deforestation and proliferation of fungal spores at cretaceous-tertiary boundary suggests that fungal diseases could have contributed to the demise of dinosaurs and the flourishing of mammalian species.

Posttranslational modifications required for cell surface localization and function of the fungal adhesin Aga1p

Adherence of fungal cells to host substrates and each other affects their access to nutrients, sexual conjugation, and survival in hosts. Adhesins are cell surface proteins that mediate these different cell adhesion interactions. In this study, we examine the in vivo functional requirements for specific posttranslational modifications to these proteins, including glycophosphatidylinositol (GPI) anchor addition and O-linked glycosylation. The processing of some fungal GPI anchors, creating links to cell wall beta-1,6 glucans, is postulated to facilitate postsecretory traffic of proteins to cell wall domains conducive to their functions. By studying the yeast sexual adhesin subunit Aga1p, we found that deletion of its signal sequence for GPI addition eliminated its activity, while deletions of different internal domains had various effects on function. Substitution of the Aga1p GPI signal domain with those of other GPI-anchored proteins, a single transmembrane domain, or a cysteine capable of forming a disulfide all produced functional adhesins. A portion of the cellular pool of Aga1p was determined to be cell wall resident. Aga1p and the alpha-agglutinin Agalpha1p were shown to be under glycosylated in cells lacking the protein mannosyltransferase genes PMT1 and PMT2, with phenotypes manifested only in MATalpha cells for single mutants but in both cell types when both genes are absent. We conclude that posttranslational modifications to Aga1p are necessary for its biogenesis and activity. Our studies also suggest that in addition to GPI-glucan linkages, other cell surface anchorage mechanisms, such as transmembrane domains or disulfides, may be employed by fungal species to localize adhesins.

Comparison of histologic lesions of endophthalmitis induced byBlastomyces dermatitidisin untreated and treated dogs: 36 cases (1986–2001)

OBJECTIVE

To compare prevalence of organisms and histologic changes in eyes from dogs with blastomycosis that were either untreated or undergoing treatment with itraconazole.

DESIGN

Retrospective study.

ANIMALS

36 dogs with endophthalmitis associated with blastomycosis.

PROCEDURE

Signalment, results of ophthalmic examination, and duration of treatment with itraconazole were extracted from medical records. Histologic sections from eyes were examined for prevalence and viability (ie, budding) of fungal organisms. A scoring system was devised to assess the degree of inflammation.

RESULTS

Clinically, all eyes were blind and had signs of severe endophthalmitis. Histologically, the type and degree of inflammation and prevalence of Blastomyces dermatitidis were not significantly different between dogs treated with itraconazole and untreated dogs or among groups of dogs treated for different time periods (4 to 14, 15 to 28, or 29 to 72 days). Replication of the organisms in vascular tissues as well as avascular spaces in the eyes was similar in treated and untreated dogs. Lens rupture was seen in 12 of 29 (41%) eyes.

CONCLUSIONS AND CLINICAL RELEVANCE

Persistence of inflammation in eyes of dogs with naturally occurring blastomycosis is likely attributable to the continued presence of B. dermatitidis, regardless of the duration of treatment with itraconazole. Lens capsule rupture, a common and previously unreported histologic finding, may contribute to cataract formation and continued inflammation.

Histoplasma capsulatum molecular genetics, pathogenesis, and responsiveness to its environment

Histoplasma capsulatum is a thermally dimorphic ascomycete that is a significant cause of respiratory and systemic disease in mammals including humans, especially immunocompromised individuals such as AIDS patients. As an environmental mold found in the soil, it is a successful member of a competitive polymicrobial ecosystem. Its host-adapted yeast form is a facultative intracellular pathogen of mammalian macrophages. H. capsulatum faces a variety of environmental changes during the course of infection and must survive under harsh conditions or modulate its microenvironment to achieve success as a pathogen. Histoplasmosis may be considered the fungal homolog of the bacterial infection tuberculosis, since both H. capsulatum and Mycobacterium tuberculosis exploit the macrophage as a host cell and can cause acute or persistent pulmonary and disseminated infection and reactivation disease. The identification and functional analysis of biologically or pathogenically important H. capsulatum genes have been greatly facilitated by the development of molecular genetic experimental capabilities in this organism. This review focuses on responsiveness of this fungus to its environment, including differential expression of genes and adaptive phenotypic traits.

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.

Involvement of fungal cell wall components in adhesion of Sporothrix schenckii to human fibronectin

Systemic sporotrichosis is an emerging infection potentially fatal for immunocompromised patients. Adhesion to extracellular matrix proteins is thought to play a crucial role in invasive fungal diseases. Here we report studies of the adhesion of Sporothrix schenckii to the extracellular protein fibronectin (Fn). Both yeast cells and conidia of S. schenckii were able to adhere to Fn as detected by enzyme-linked immunosorbent binding assays. Adhesion of yeast cells to Fn is dose dependent and saturable. S. schenckii adheres equally well to 40-kDa and 120-kDa Fn proteolytic fragments. While adhesion to Fn was increased by Ca(2+), inhibition assays demonstrated that it was not RGD dependent. A carbohydrate-containing cell wall neutral fraction blocked up to 30% of the observed adherence for the yeast cells. The biochemical nature of this fraction suggests the participation of cell surface glycoconjugates in binding by their carbohydrate or peptide moieties. These results provide new data concerning S. schenckii adhesion mechanisms, which could be important in host-fungus interactions and the establishment of sporotrichosis.