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

Emergence of Blastoschizomyces capitatus yeast infections, Central Europe

We report 5 cases of disseminated infection caused by Blastoschizomyces capitatus yeast in central Switzerland. The emergence of this yeast in an area in which it is not known to be endemic should alert clinicians caring for immunocompromised patients outside the Mediterranean region to consider infections caused by unfamiliar fungal pathogens.

Experimental infection of bats with Geomyces destructans causes white-nose syndrome

White-nose syndrome (WNS) has caused recent catastrophic declines among multiple species of bats in eastern North America. The disease's name derives from a visually apparent white growth of the newly discovered fungus Geomyces destructans on the skin (including the muzzle) of hibernating bats. Colonization of skin by this fungus is associated with characteristic cutaneous lesions that are the only consistent pathological finding related to WNS. However, the role of G. destructans in WNS remains controversial because evidence to implicate the fungus as the primary cause of this disease is lacking. The debate is fuelled, in part, by the assumption that fungal infections in mammals are most commonly associated with immune system dysfunction. Additionally, the recent discovery that G. destructans commonly colonizes the skin of bats of Europe, where no unusual bat mortality events have been reported, has generated further speculation that the fungus is an opportunistic pathogen and that other unidentified factors are the primary cause of WNS. Here we demonstrate that exposure of healthy little brown bats (Myotis lucifugus) to pure cultures of G. destructans causes WNS. Live G. destructans was subsequently cultured from diseased bats, successfully fulfilling established criteria for the determination of G. destructans as a primary pathogen. We also confirmed that WNS can be transmitted from infected bats to healthy bats through direct contact. Our results provide the first direct evidence that G. destructans is the causal agent of WNS and that the recent emergence of WNS in North America may represent translocation of the fungus to a region with a naive population of animals. Demonstration of causality is an instrumental step in elucidating the pathogenesis and epidemiology of WNS and in guiding management actions to preserve bat populations against the novel threat posed by this devastating infectious disease.

Experimental models of cryptococcosis

Cryptococcosis is a life-threatening fungal disease that infects around one million people each year. Establishment and progression of disease involves a complex interplay between the fungus and a diverse range of host cell types. Over recent years, numerous cellular, tissue, and animal models have been exploited to probe this host-pathogen interaction. Here we review the range of experimental models that are available for cryptococcosis research and compare the relative advantages and limitations of the different systems.

Characteristics of Paecilomyces lilacinus infection comparing immunocompetent with immunosuppressed murine model

The characteristics of Paecilomyces lilacinus infection were evaluated using two murine experimental models: immunocompetent and immunosuppressed. The evaluation criteria for characteristics of infection were clinical signs, weight loss, survival rates, histopathological alterations and the number of viable fungal cells re-isolated from different organs; and those for immunological status were in vitro lymphoproliferative response, cell surface phenotyping and IFN-γ production. Morphological evaluation showed that P. lilacinus isolates presented morphological characteristics consistent with those described in the literature. The immunocompetent mice could be infected by the fungi, but they did not develop the disease, unlike the immunosuppressed mice, which showed clinical signs of mycosis in an environment of suppressed cellular immune response. The hypothesis of latent infection reactivation in mice was not confirmed. The difference observed in the infection rate of the two fungi isolates points to an intrinsic variation between strains of P. lilacinus and led us to hypothesise that even in the presence of immunosuppressed environment, the fungus virulence can play a role in the pathogenesis of hyalohyphomycosis.

Microbial Pathogens in the Fungal Kingdom

The fungal kingdom is vast, spanning ~1.5 to as many as 5 million species diverse as unicellular yeasts, filamentous fungi, mushrooms, lichens, and both plant and animal pathogens. The fungi are closely aligned with animals in one of the six to eight supergroups of eukaryotes, the opisthokonts. The animal and fungal kingdoms last shared a common ancestor ~1 billion years ago, more recently than other groups of eukaryotes. As a consequence of their close evolutionary history and shared cellular machinery with metazoans, fungi are exceptional models for mammalian biology, but prove more difficult to treat in infected animals. The last common ancestor to the fungal/metazoan lineages is thought to have been unicellular, aquatic, and motile with a posterior flagellum, and certain extant species closely resemble this hypothesized ancestor. Species within the fungal kingdom were traditionally assigned to four phyla, including the basal fungi (Chytridiomycota, Zygomycota) and the more recently derived monophyletic lineage, the dikarya (Ascomycota, Basidiomycota). The fungal tree of life project has revealed that the basal lineages are polyphyletic, and thus there are as many as eight to ten fungal phyla. Fungi that infect vertebrates are found in all of the major lineages, and virulence arose multiple times independently. A sobering recent development involves the species Batrachochytrium dendrobatidis from the basal fungal phylum, the Chytridiomycota, which has emerged to cause global amphibian declines and extinctions. Genomics is revolutionizing our view of the fungal kingdom, and genome sequences for zygomycete pathogens (Rhizopus, Mucor), skin-associated fungi (dermatophytes, Malassezia), and the Candida pathogenic species clade promise to provide insights into the origins of virulence. Here we survey the diversity of fungal pathogens and illustrate key principles revealed by genomics involving sexual reproduction and sex determination, loss of conserved pathways in derived fungal lineages that are retained in basal fungi, and shared and divergent virulence strategies of successful human pathogens, including dimorphic and trimorphic transitions in form. The overarching conclusion is that fungal pathogens of animals have arisen repeatedly and independently throughout the fungal tree of life, and while they share general properties, there are also unique features to the virulence strategies of each successful microbial pathogen.

White-nose syndrome fungus (Geomyces destructans) in bats, Europe

Unlike bats in North America, bats in Europe are not killed by this fungus.

White-nose syndrome is an emerging disease in North America that has caused substantial declines in hibernating bats. A recently identified fungus (Geomyces destructans) causes skin lesions that are characteristic of this disease. Typical signs of this infection were not observed in bats in North America before white-nose syndrome was detected. However, unconfirmed reports from Europe indicated white fungal growth on hibernating bats without associated deaths. To investigate these differences, hibernating bats were sampled in Germany, Switzerland, and Hungary to determine whether G. destructans is present in Europe. Microscopic observations, fungal culture, and genetic analyses of 43 samples from 23 bats indicated that 21 bats of 5 species in 3 countries were colonized by G. destructans. We hypothesize that G. destructans is present throughout Europe and that bats in Europe may be more immunologically or behaviorally resistant to G. destructans than their congeners in North America because they potentially coevolved with the fungus.

Mammalian endothermy optimally restricts fungi and metabolic costs

Endothermy and homeothermy are mammalian characteristics whose evolutionary origins are poorly understood. Given that fungal species rapidly lose their capacity for growth above ambient temperatures, we have proposed that mammalian endothermy enhances fitness by creating exclusionary thermal zones that protect against fungal disease. According to this view, the relative paucity of invasive fungal diseases in immunologically intact mammals relative to other infectious diseases would reflect an inability of most fungal species to establish themselves in a mammalian host. In this study, that hypothesis was tested by modeling the fitness increase with temperature versus its metabolic costs. We analyzed the tradeoff involved between the costs of the excess metabolic rates required to maintain a body temperature and the benefit gained by creating a thermal exclusion zone that protects against environmental microbes such as fungi. The result yields an optimum at 36.7°C, which closely approximates mammalian body temperatures. This calculation is consistent with and supportive of the notion that an intrinsic thermally based resistance against fungal diseases could have contributed to the success of mammals in the Tertiary relative to that of other vertebrates.

Mammals are characterized by both maintaining and closely regulating high body temperatures, processes that are known as endothermy and homeothermy, respectively. The mammalian lifestyle is energy intensive and costly. The evolutionary mechanisms responsible for the emergence and success of these mammalian characteristics are not understood. This work suggests that high mammalian temperatures represent optima in the tradeoff between metabolic costs and the increased fitness that comes with resistance to fungal diseases.

What makes Aspergillus fumigatus a successful pathogen? Genes and molecules involved in invasive aspergillosis

Aspergillus fumigatus is an opportunistic pathogen that causes 90% of invasive aspergillosis (IA) due to Aspergillus genus, with a 50-95% mortality rate. It has been postulated that certain virulence factors are characteristic of A. fumigatus, but the "non-classical" virulence factors seem to be highly variable. Overall, published studies have demonstrated that the virulence of this fungus is multifactorial, associated with its structure, its capacity for growth and adaptation to stress conditions, its mechanisms for evading the immune system and its ability to cause damage to the host. In this review we intend to give a general overview of the genes and molecules involved in the development of IA. The thermotolerance section focuses on five genes related with the capacity of the fungus to grow at temperatures above 30°C (thtA, cgrA, afpmt1, kre2/afmnt1, and hsp1/asp f 12). The following sections discuss molecules and genes related to interaction with the host and with the immune responses. These sections include β-glucan, α-glucan, chitin, galactomannan, galactomannoproteins (afmp1/asp f 17 and afmp2), hydrophobins (rodA/hyp1 and rodB), DHN-melanin, their respective synthases (fks1, rho1-4, ags1-3, chsA-G, och1-4, mnn9, van1, anp1, glfA, pksP/alb1, arp1, arp2, abr1, abr2, and ayg1), and modifying enzymes (gel1-7, bgt1, eng1, ecm33, afpigA, afpmt1-2, afpmt4, kre2/afmnt1, afmnt2-3, afcwh41 and pmi); several enzymes related to oxidative stress protection such as catalases (catA, cat1/catB, cat2/katG, catC, and catE), superoxide dismutases (sod1, sod2, sod3/asp f 6, and sod4), fatty acid oxygenases (ppoA-C), glutathione tranferases (gstA-E), and others (afyap1, skn7, and pes1); and efflux transporters (mdr1-4, atrF, abcA-E, and msfA-E). In addition, this review considers toxins and related genes, such as a diffusible toxic substance from conidia, gliotoxin (gliP and gliZ), mitogillin (res/mitF/asp f 1), hemolysin (aspHS), festuclavine and fumigaclavine A-C, fumitremorgin A-C, verruculogen, fumagillin, helvolic acid, aflatoxin B1 and G1, and laeA. Two sections cover genes and molecules related with nutrient uptake, signaling and metabolic regulations involved in virulence, including enzymes, such as serine proteases (alp/asp f 13, alp2, and asp f 18), metalloproteases (mep/asp f 5, mepB, and mep20), aspartic proteases (pep/asp f 10, pep2, and ctsD), dipeptidylpeptidases (dppIV and dppV), and phospholipases (plb1-3 and phospholipase C); siderophores and iron acquisition (sidA-G, sreA, ftrA, fetC, mirB-C, and amcA); zinc acquisition (zrfA-H, zafA, and pacC); amino acid biosynthesis, nitrogen uptake, and cross-pathways control (areA, rhbA, mcsA, lysF, cpcA/gcn4p, and cpcC/gcn2p); general biosynthetic pathway (pyrG, hcsA, and pabaA), trehalose biosynthesis (tpsA and tpsB), and other regulation pathways such as those of the MAP kinases (sakA/hogA, mpkA-C, ste7, pbs2, mkk2, steC/ste11, bck1, ssk2, and sho1), G-proteins (gpaA, sfaD, and cpgA), cAMP-PKA signaling (acyA, gpaB, pkaC1, and pkaR), His kinases (fos1 and tcsB), Ca(2+) signaling (calA/cnaA, crzA, gprC and gprD), and Ras family (rasA, rasB, and rhbA), and others (ace2, medA, and srbA). Finally, we also comment on the effect of A. fumigatus allergens (Asp f 1-Asp f 34) on IA. The data gathered generate a complex puzzle, the pieces representing virulence factors or the different activities of the fungus, and these need to be arranged to obtain a comprehensive vision of the virulence of A. fumigatus. The most recent gene expression studies using DNA-microarrays may be help us to understand this complex virulence, and to detect targets to develop rapid diagnostic methods and new antifungal agents.

DNA-based detection of the fungal pathogen Geomyces destructans in soils from bat hibernacula

White-nose syndrome (WNS) is an emerging disease causing unprecedented morbidity and mortality among bats in eastern North America. The disease is characterized by cutaneous infection of hibernating bats by the psychrophilic fungus Geomyces destructans. Detection of G. destructans in environments occupied by bats will be critical for WNS surveillance, management and characterization of the fungal lifecycle. We initiated an rRNA gene region-based molecular survey to characterize the distribution of G. destructans in soil samples collected from bat hibernacula in the eastern United States with an existing PCR test. Although this test did not specifically detect G. destructans in soil samples based on a presence/absence metric, it did favor amplification of DNA from putative Geomyces species. Cloning and sequencing of PCR products amplified from 24 soil samples revealed 74 unique sequence variants representing 12 clades. Clones with exact sequence matches to G. destructans were identified in three of 19 soil samples from hibernacula in states where WNS is known to occur. Geomyces destructans was not identified in an additional five samples collected outside the region where WNS has been documented. This study highlights the diversity of putative Geomyces spp. in soil from bat hibernacula and indicates that further research is needed to better define the taxonomy of this genus and to develop enhanced diagnostic tests for rapid and specific detection of G. destructans in environmental samples.

Aspergillus fumigatus: contours of an opportunistic human pathogen

Aspergillus fumigatus is currently the major air-borne fungal pathogen. It is able to cause several forms of disease in humans of which invasive aspergillosis is the most severe. The high mortality rate of this disease prompts increased efforts to disclose the basic principles of A. fumigatus pathogenicity. According to our current knowledge, A. fumigatus lacks sophisticated virulence traits; it is nevertheless able to establish infection due to its robustness and ability to adapt to a wide range of environmental conditions. This review focuses on two crucial aspects of invasive aspergillosis: (i) properties of A. fumigatus that are relevant during infection and may distinguish it from non-pathogenic Aspergillus species and (ii) interactions of the pathogen with the innate and adaptive immune systems.

Uniform categorization of biocommunication in bacteria, fungi and plants

This article describes a coherent biocommunication categorization for the kingdoms of bacteria, fungi and plants. The investigation further shows that, besides biotic sign use in trans-, inter- and intraorganismic communication processes, a common trait is interpretation of abiotic influences as indicators to generate an appropriate adaptive behaviour. Far from being mechanistic interactions, communication processes within organisms and between organisms are sign-mediated interactions. Sign-mediated interactions are the precondition for every cooperation and coordination between at least two biological agents such as cells, tissues, organs and organisms. Signs of biocommunicative processes are chemical molecules in most cases. The signs that are used in a great variety of signaling processes follow syntactic (combinatorial), pragmatic (context-dependent) and semantic (content-specific) rules. These three levels of semiotic rules are helpful tools to investigate communication processes throughout all organismic kingdoms. It is not the aim to present the latest empirical data concerning communication in these three kingdoms but to present a unifying perspective that is able to interconnect transdisciplinary research on bacteria, fungi and plants.

Global warming will bring new fungal diseases for mammals

Fungi are major pathogens of plants, other fungi, rotifers, insects, and amphibians, but relatively few cause disease in mammals. Fungi became important human pathogens only in the late 20th century, primarily in hosts with impaired immunity as a consequence of medical interventions or HIV infection. The relatively high resistance of mammals has been attributed to a combination of a complex immune system and endothermy. Mammals maintain high body temperatures relative to environmental temperatures, creating a thermally restrictive ambient for the majority of fungi. According to this view, protection given by endothermy requires a temperature gradient between those of mammals and the environment. We hypothesize that global warming will increase the prevalence of fungal diseases in mammals by two mechanisms: (i) increasing the geographic range of currently pathogenic species and (ii) selecting for adaptive thermotolerance for species with significant pathogenic potential but currently not pathogenic by virtue of being restricted by mammalian temperatures.

The outcome of Cryptococcus neoformans intracellular pathogenesis in human monocytes

Background

Cryptococcus neoformans is an encapsulated yeast that is a facultative intracellular pathogen. The interaction between macrophages and C. neoformans is critical for extrapulmonary dissemination of this pathogenic yeast. C. neoformans can either lyse macrophages or escape from within them through a process known as phagosomal extrusion. However, most studies of intracellular pathogenesis have been made with mouse cells and their relevance to human infection is uncertain. In this study we extended studies of C. neoformans-macrophage cellular interaction/s to human peripheral blood monocytes.

Results

This study demonstrated that C. neoformans can shed polysaccharide within human monocytes, spread from cell to cell, and be extruded from them. Furthermore, human monocytes responded to ingestion of C. neoformans with cell cycle progression from G1 to S.

Conclusion

Similarities between mouse and human cells support the suitability of mouse cells for the study of intracellular pathogenesis mechanisms. Given that these hosts diverged over 70 million years ago, the similar pathogenic strategies for C. neoformans in murine and human cells supports the hypothesis that the mechanism that underlies the mammalian intracellular pathogenesis of C. neoformans originated from interactions with a third host, possibly soil amoeboid predators, before the mammalian radiation.

The radioprotective properties of fungal melanin are a function of its chemical composition, stable radical presence and spatial arrangement

Melanized microorganisms are often found in environments with very high background radiation levels such as in nuclear reactor cooling pools and the destroyed reactor in Chernobyl. These findings and the laboratory observations of the resistance of melanized fungi to ionizing radiation suggest a role for this pigment in radioprotection. We hypothesized that the radioprotective properties of melanin in microorganisms result from a combination of physical shielding and quenching of cytotoxic free radicals. We have investigated the radioprotective properties of melanin by subjecting the human pathogenic fungi Cryptococcus neoformans and Histoplasma capsulatum in their melanized and non-melanized forms to sublethal and lethal doses of radiation of up to 8 kGy. The contribution of chemical composition, free radical presence, spatial arrangement, and Compton scattering to the radioprotective properties of melanin was investigated by high-performance liquid chromatography, electron spin resonance, transmission electron microscopy, and autoradiographic techniques. Melanin protected fungi against ionizing radiation and its radioprotective properties were a function of its chemical composition, free radical quenching, and spherical spatial arrangement.

Animal models: an important tool in mycology

Animal models of fungal infections are, and will remain, a key tool in the advancement of the medical mycology. Many different types of animal models of fungal infection have been developed, with murine models the most frequently used, for studies of pathogenesis, virulence, immunology, diagnosis, and therapy. The ability to control numerous variables in performing the model allows us to mimic human disease states and quantitatively monitor the course of the disease. However, no single model can answer all questions and different animal species or different routes of infection can show somewhat different results. Thus, the choice of which animal model to use must be made carefully, addressing issues of the type of human disease to mimic, the parameters to follow and collection of the appropriate data to answer those questions being asked. This review addresses a variety of uses for animal models in medical mycology. It focuses on the most clinically important diseases affecting humans and cites various examples of the different types of studies that have been performed. Overall, animal models of fungal infection will continue to be valuable tools in addressing questions concerning fungal infections and contribute to our deeper understanding of how these infections occur, progress and can be controlled and eliminated.

Fusarium solani species complex isolates conspecific with Fusarium solani f. sp. cucurbitae race 2 from naturally infected human and plant tissue and environmental sources are equally virulent on plants, grow at 37 degrees C and are interfertile

In a previous taxonomic study based on multilocus sequencing of Fusarium from clinical specimens and hospital environments, the most common lineage was Fusarium solani species complex group 1 (FSSC 1) which is conspecific with F. solani f. sp. cucurbitae race 2, a pathogen of cucurbit fruits. The aims of our study were to determine if clinical and environmental isolates of FSSC 1 are plant pathogens and members of the same biological species as cucurbit isolates, and to determine if all isolates can germinate, grow and sporulate at 37 degrees C. Isolates from the different sources did not differ in virulence on zucchini fruits. All FSSC 1 isolates were pathogenic and produced more rot than FSSC isolates from plant hosts other than cucurbits. Both mating types were found among isolates from each of the sources, and all isolates were sexually compatible with cucurbit isolates. All isolates germinated, grew and sporulated at 37 degrees C. This is the first report in which plant pathogenicity has been verified for a collection of human clinical isolates. Our data are consistent with the hypothesis that all FSSC 1 isolates, regardless of source, are a single biological species, equally virulent plant pathogens and tolerant of the human body temperature.

Ionizing radiation changes the electronic properties of melanin and enhances the growth of melanized fungi

Background

Melanin pigments are ubiquitous in nature. Melanized microorganisms are often the dominating species in certain extreme environments, such as soils contaminated with radionuclides, suggesting that the presence of melanin is beneficial in their life cycle. We hypothesized that ionizing radiation could change the electronic properties of melanin and might enhance the growth of melanized microorganisms.

Methodology/Principal Findings

Ionizing irradiation changed the electron spin resonance (ESR) signal of melanin, consistent with changes in electronic structure. Irradiated melanin manifested a 4-fold increase in its capacity to reduce NADH relative to non-irradiated melanin. HPLC analysis of melanin from fungi grown on different substrates revealed chemical complexity, dependence of melanin composition on the growth substrate and possible influence of melanin composition on its interaction with ionizing radiation. XTT/MTT assays showed increased metabolic activity of melanized C. neoformans cells relative to non-melanized cells, and exposure to ionizing radiation enhanced the electron-transfer properties of melanin in melanized cells. Melanized Wangiella dermatitidis and Cryptococcus neoformans cells exposed to ionizing radiation approximately 500 times higher than background grew significantly faster as indicated by higher CFUs, more dry weight biomass and 3-fold greater incorporation of ¹⁴C-acetate than non-irradiated melanized cells or irradiated albino mutants. In addition, radiation enhanced the growth of melanized Cladosporium sphaerospermum cells under limited nutrients conditions.

Conclusions/Significance

Exposure of melanin to ionizing radiation, and possibly other forms of electromagnetic radiation, changes its electronic properties. Melanized fungal cells manifested increased growth relative to non-melanized cells after exposure to ionizing radiation, raising intriguing questions about a potential role for melanin in energy capture and utilization.

Interactions between the environmental pathogen Listeria monocytogenes and a free-living protozoan (Acanthamoeba castellanii)

Listeria monocytogenes can cause severe disease in animal hosts, but it has no recognized animal host reservoir. We tested the hypothesis that L. monocytogenes retains virulence traits to survive predation by amoebae and that listeriolysin O plays a crucial role in this process. Co-culturing of L. monocytogenes and Acanthamoeba castellanii demonstrated that L. monocytogenes does not actively kill amoebae, but in the presence of amoebae, high bacterial population densities can be maintained over a period of at least 96 h. A gentamicin protection assay demonstrated that there is no significant difference in the ability to survive predation between serovars (4b versus 1/2a and 1/2c; P = 0.08) and between five species of Listeria (P = 0.14). Three of these species do not harbour the hly gene responsible for listeriolysin O production. A hly knockout strain had poorer survival compared with the parental strain (P = 0.04 at 24 h; P = 0.04 at 48 h; P = 0.02 at 72 h) and electron microscopy was consistent with a wild-type strain being able to escape the phagosome whereas the hly knockout strain did not appear to have this ability. Thus, while there is weak evidence that listeriolysin O can contribute to improved survival after ingestion by amoebae, listeriolysin O does not appear to provide a significant selective advantage under the conditions of this study.

Polyketides, proteins and genes in fungi: programmed nano-machines begin to reveal their secrets

Fungi produce a wide variety of biologically active compounds. Among these, the polyketides form a large and structurally diverse group. These compounds are synthesised by highly programmed and very large iterative multifunctional proteins, the polyketide synthases, with nm dimensions. This review outlines the current state of knowledge regarding the links between gene sequence, protein architecture and biosynthetic programming for fungal polyketide synthases.

Cryptococcus neoformans: the yeast that likes it hot

The ability of fungi to grow well at mammalian body temperatures is a fundamental characteristic of invasive human fungal pathogens. Cryptococcus neoformans, with its genetics, molecular biology, robust animal models and clinical importance, has become a premier fungal model system for molecular fungal pathogenesis studies. There has been over a half century of study into how C. neoformans grows at high temperatures. However, recently the understanding of high-temperature growth at a molecular level has dramatically accelerated. Many strategies have been used to identify genes and over a dozen genes have already been identified to be necessary for high-temperature growth. It is likely that there are many more to discover. It is clear that, as further studies understand how this encapsulated yeast is able to withstand the stresses of high temperature at a genetic and molecular basis, we will also know more about how it and other fungal pathogens have evolved into well-established human pathogens.

Phylogenetic and evolutionary aspects of Paracoccidioides brasiliensis reveal a long coexistence with animal hosts that explain several biological features of the pathogen

The habitat of the mycelial saprobic form of Paracoccidioides brasiliensis, which produces the infectious propagula, has not been determined and has proven difficult for mycologists to describe. The fungus has been rarely isolated from the environment, the disease has a prolonged latency period and no outbreaks have been reported. These facts have precluded the adoption of preventive measures to avoid infection. The confirmation of natural infections in nine-banded armadillos (Dasypus novemcinctus) with P. brasiliensis, in high frequency and wide geographic distribution, has opened new avenues for the study and understanding of its ecology. Armadillos belong to the order Xenarthra, which has existed in South America ever since the Paleocene Era (65 million years ago), when the South American subcontinent was still a detached land, before the consolidation of what is now known as the American continent. On the other hand, strong molecular evidence suggests that P. brasiliensis and other dimorphic pathogenic fungi--such as Blastomyces dermatitidis, Coccidioides immitis and Histoplasma capsulatum--belong to the family Onygenaceae sensu lato (order Onygenales, Ascomycota), which appeared around 150 million years ago. P. brasiliensis ecology and relation to its human host are probably linked to the fungal evolutionary past, especially its long coexistence with and adaptation to animal hosts other than Homo sapiens, of earlier origin. Instead of being a blind alley, the meaning of parasitism for dimorphic pathogenic fungi should be considered as an open two-way avenue, in which the fungus may return to the environment, therefore contributing to preserve its teleomorphic (sexual) and anamorphic (asexual) forms in a defined and protected natural habitat.

Relationship between temperature optima and secreted protease activities of three Pythium species and pathogenicity toward plant and animal hosts

The in vitro physiological characteristics of three species of Pythium (oomycetes) that utilize different food sources were compared with their ecological activities: P. insidiosum is a pathogen of mammals (including humans), P. graminicola infects the roots of graminaceous hosts, and P. grandisporangium is an enigmatic water mold isolated from mangrove leaves and marine algae. P. insidiosum and P. graminicola showed peak growth rates at 37 degrees C before complete inhibition of growth at 40 degrees C; P. grandisporangium grew fastest at 22 degrees C. Differences between the invasive pressures exerted by the hyphae of these microorganisms were not considered significant in relation to the substrates colonized by these water molds. All three species showed substantial secreted protease activity, producing three or more serine proteases with weights ranging from 24-38 kDa. Fastest growth rates were supported when collagen was supplied as the sole carbon source, and none of the species were able to grow on purified plant cell wall polysaccharides. The growth and nutritional characteristics of P. graminicola and P. grandisporangium bear little obvious relationship to the ecological niches that they inhabit. This highlights the caution necessary in extrapolating from laboratory analyses to the natural environment, and points to the potential importance of ecological opportunity in determining the host range and food source of certain microorganisms.