Virulence determinants of the human pathogenic fungus Aspergillus fumigatus protect against soil amoeba predation

A Timeline of Biosynthetic Gene Cluster Discovery in Aspergillus fumigatus: From Characterization to Future Perspectives

In 1999, the first biosynthetic gene cluster (BGC), synthesizing the virulence factor DHN melanin, was characterized in Aspergillus fumigatus. Since then, 19 additional BGCs have been linked to specific secondary metabolites (SMs) in this species. Here, we provide a comprehensive timeline of A. fumigatus BGC discovery and find that initial advances centered around the commonly expressed SMs where chemical structure informed rationale identification of the producing BGC (e.g., gliotoxin, fumigaclavine, fumitremorgin, pseurotin A, helvolic acid, fumiquinazoline). Further advances followed the transcriptional profiling of a ΔlaeA mutant, which aided in the identification of endocrocin, fumagillin, hexadehydroastechrome, trypacidin, and fumisoquin BGCs. These SMs and their precursors are the commonly produced metabolites in most A. fumigatus studies. Characterization of other BGC/SM pairs required additional efforts, such as induction treatments, including co-culture with bacteria (fumicycline/neosartoricin, fumigermin) or growth under copper starvation (fumivaline, fumicicolin). Finally, four BGC/SM pairs were discovered via overexpression technologies, including the use of heterologous hosts (fumicycline/neosartoricin, fumihopaside, sphingofungin, and sartorypyrone). Initial analysis of the two most studied A. fumigatus isolates, Af293 and A1160, suggested that both harbored ca. 34-36 BGCs. However, an examination of 264 available genomes of A. fumigatus shows up to 20 additional BGCs, with some strains showing considerable variations in BGC number and composition. These new BGCs present a new frontier in the future of secondary metabolism characterization in this important species.

The Aspergillus fumigatus maiA gene contributes to cell wall homeostasis and fungal virulence

In this study, two distinct in vitro infection models of Aspergillus fumigatus, using murine macrophages (RAW264.7) and human lung epithelial cells (A549), were employed to identify the genes important for fungal adaptation during infection. Transcriptomic analyses of co-incubated A. fumigatus uncovered 140 fungal genes up-regulated in common between both models that, when compared with a previously published in vivo transcriptomic study, allowed the identification of 13 genes consistently up-regulated in all three infection conditions. Among them, the maiA gene, responsible for a critical step in the L-phenylalanine degradation pathway, was identified. Disruption of maiA resulted in a mutant strain unable to complete the Phe degradation pathway, leading to an excessive production of pyomelanin when this amino acid served as the sole carbon source. Moreover, the disruption mutant exhibited noticeable cell wall abnormalities, with reduced levels of β-glucans within the cell wall but did not show lack of chitin or mannans. The maiA-1 mutant strain induced reduced inflammation in primary macrophages and displayed significantly lower virulence in a neutropenic mouse model of infection. This is the first study linking the A. fumigatus maiA gene to fungal cell wall homeostasis and virulence.

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.

Ergot Alkaloids Contribute to the Pathogenic Potential of the Fungus Aspergillus leporis

Opportunistically pathogenic fungi have varying potential to cause disease in animals. Factors contributing to their virulence include specialized metabolites, which in some cases evolved in contexts unrelated to pathogenesis. Specialized metabolites that increase fungal virulence in the model insect Galleria mellonella include the ergot alkaloids fumigaclavine C in Aspergillus fumigatus (syn. Neosartorya fumigata) and lysergic acid α-hydroxyethylamide (LAH) in the entomopathogen Metarhizium brunneum. Three species of Aspergillus recently found to accumulate high concentrations of LAH were investigated for their pathogenic potential in G. mellonella. Aspergillus leporis was most virulent, A. hancockii was intermediate, and A. homomorphus had very little pathogenic potential. Aspergillus leporis and A. hancockii emerged from and sporulated on dead insects, thus completing their asexual life cycles. Inoculation by injection resulted in more lethal infections than did topical inoculation, indicating that A. leporis and A. hancockii were preadapted for insect pathogenesis but lacked an effective means to breach the insect's cuticle. All three species accumulated LAH in infected insects, with A. leporis accumulating the most. Concentrations of LAH in A. leporis were similar to those observed in the entomopathogen M. brunneum. LAH was eliminated from A. leporis through a CRISPR/Cas9-based gene knockout, and the resulting strain had reduced virulence to G. mellonella. The data indicate that A. leporis and A. hancockii have considerable pathogenic potential and that LAH increases the virulence of A. leporis. IMPORTANCE Certain environmental fungi infect animals occasionally or conditionally, whereas others do not. Factors that affect the virulence of these opportunistically pathogenic fungi may have originally evolved to fill some other role for the fungus in its primary environmental niche. Among the factors that may improve the virulence of opportunistic fungi are specialized metabolites--chemicals that are not essential for basic life functions but provide producers with an advantage in particular environments or under specific conditions. Ergot alkaloids are a large family of fungal specialized metabolites that contaminate crops in agriculture and serve as the foundations of numerous pharmaceuticals. Our results show that two ergot alkaloid-producing fungi that were not previously known to be opportunistic pathogens can infect a model insect and that, in at least one of the species, an ergot alkaloid increases the virulence of the fungus.

"Under Pressure" - How fungi evade, exploit, and modulate cells of the innate immune system

The human immune system uses an arsenal of effector mechanisms to prevent and counteract infections. Yet, some fungal species are extremely successful as human pathogens, which can be attributed to a wide variety of strategies by which these fungi evade, exploit, and modulate the immune system. These fungal pathogens normally are either harmless commensals or environmental fungi. In this review we discuss how commensalism, but also life in an environmental niche without human contact, can drive the evolution of diverse and specialized immune evasion mechanisms. Correspondingly, we discuss the mechanisms contributing to the ability of these fungi to cause superficial to life-threatening infections.

Fungal resilience and host-pathogen interactions: Future perspectives and opportunities

We are constantly exposed to the threat of fungal infection. The outcome-clearance, commensalism or infection-depends largely on the ability of our innate immune defences to clear infecting fungal cells versus the success of the fungus in mounting compensatory adaptive responses. As each seeks to gain advantage during these skirmishes, the interactions between host and fungal pathogen are complex and dynamic. Nevertheless, simply compromising the physiological robustness of fungal pathogens reduces their ability to evade antifungal immunity, their virulence, and their tolerance against antifungal therapy. In this article I argue that this physiological robustness is based on a 'Resilience Network' which mechanistically links and controls fungal growth, metabolism, stress resistance and drug tolerance. The elasticity of this network probably underlies the phenotypic variability of fungal isolates and the heterogeneity of individual cells within clonal populations. Consequently, I suggest that the definition of the fungal Resilience Network represents an important goal for the future which offers the clear potential to reveal drug targets that compromise drug tolerance and synergise with current antifungal therapies.

The Toll pathway mediates Drosophila resilience to Aspergillus mycotoxins through specific Bomanins

Host defense against infections encompasses both resistance, which targets microorganisms for neutralization or elimination, and resilience/disease tolerance, which allows the host to withstand/tolerate pathogens and repair damages. In Drosophila, the Toll signaling pathway is thought to mediate resistance against fungal infections by regulating the secretion of antimicrobial peptides, potentially including Bomanins. We find that Aspergillus fumigatus kills Drosophila Toll pathway mutants without invasion because its dissemination is blocked by melanization, suggesting a role for Toll in host defense distinct from resistance. We report that mutants affecting the Toll pathway or the 55C Bomanin locus are susceptible to the injection of two Aspergillus mycotoxins, restrictocin and verruculogen. The vulnerability of 55C deletion mutants to these mycotoxins is rescued by the overexpression of Bomanins specific to each challenge. Mechanistically, flies in which BomS6 is expressed in the nervous system exhibit an enhanced recovery from the tremors induced by injected verruculogen and display improved survival. Thus, innate immunity also protects the host against the action of microbial toxins through secreted peptides and thereby increases its resilience to infection.

Microbial polyketides and their roles in insect virulence: from genomics to biological functions

Covering: May 1966 up to January 2022Entomopathogenic microorganisms have potential for biological control of insect pests. Their main secondary metabolites include polyketides, nonribosomal peptides, and polyketide-nonribosomal peptide (PK-NRP) hybrids. Among these secondary metabolites, polyketides have mainly been studied for structural identification, pathway engineering, and for their contributions to medicine. However, little is known about the function of polyketides in insect virulence. This review focuses on the role of bacterial and fungal polyketides, as well as PK-NRP hybrids in insect infection and killing. We also discuss gene distribution and evolutional relationships among different microbial species. Further, the role of microbial polyketides and the hybrids in modulating insect-microbial symbiosis is also explored. Understanding the mechanisms of polyketides in insect pathogenesis, how compounds moderate the host-fungus interaction, and the distribution of PKS genes across different fungi and bacteria will facilitate the discovery and development of novel polyketide-derived bio-insecticides.

A bacterial endosymbiont of the fungus Rhizopus microsporus drives phagocyte evasion and opportunistic virulence

Opportunistic infections by environmental fungi are a growing clinical problem, driven by an increasing population of people with immunocompromising conditions. Spores of the Mucorales order are ubiquitous in the environment but can also cause acute invasive infections in humans through germination and evasion of the mammalian host immune system. How they achieve this and the evolutionary drivers underlying the acquisition of virulence mechanisms are poorly understood. Here, we show that a clinical isolate of Rhizopus microsporus contains a Ralstonia pickettii bacterial endosymbiont required for virulence in both zebrafish and mice and that this endosymbiosis enables the secretion of factors that potently suppress growth of the soil amoeba Dictyostelium discoideum, as well as their ability to engulf and kill other microbes. As amoebas are natural environmental predators of both bacteria and fungi, we propose that this tri-kingdom interaction contributes to establishing endosymbiosis and the acquisition of anti-phagocyte activity. Importantly, we show that this activity also protects fungal spores from phagocytosis and clearance by human macrophages, and endosymbiont removal renders the fungal spores avirulent in vivo. Together, these findings describe a new role for a bacterial endosymbiont in Rhizopus microsporus pathogenesis in animals and suggest a mechanism of virulence acquisition through environmental interactions with amoebas.

PLB-985 Neutrophil-Like Cells as a Model To Study Aspergillus fumigatus Pathogenesis

Fungal infections remain a major global concern. Emerging fungal pathogens and increasing rates of resistance mean that additional research efforts and resources must be allocated to advancing our understanding of fungal pathogenesis and developing new therapeutic interventions. Neutrophilic granulocytes are a major cell type involved in protection against the important fungal pathogen Aspergillus fumigatus, where they employ numerous defense mechanisms, including production of antimicrobial extracellular vesicles. A major drawback to work with neutrophils is the lack of a suitable cell line system for the study of fungal pathogenesis. To address this problem, we assessed the feasibility of using differentiated PLB-985 neutrophil-like cells as an in vitro model to study A. fumigatus infection. We find that dimethylformamide-differentiated PLB-985 cells provide a useful recapitulation of many aspects of A. fumigatus interactions with primary human polymorphonuclear leukocytes. We show that differentiated PLB-985 cells phagocytose fungal conidia and acidify conidia-containing phagolysosomes similar to primary neutrophils, release neutrophil extracellular traps, and also produce antifungal extracellular vesicles in response to infection. In addition, we provide an improved method for the isolation of extracellular vesicles produced during infection by employing a size exclusion chromatography-based approach. Advanced liquid chromatography-tandem mass spectrometry (LC-MS/MS) proteomics revealed an enrichment of extracellular vesicle marker proteins and a decrease of cytoplasmic proteins in extracellular vesicles isolated using this improved method. Ultimately, we find that differentiated PLB-985 cells can serve as a genetically tractable model to study many aspects of A. fumigatus pathogenesis. IMPORTANCE Polymorphonuclear leukocytes are an important defense against human fungal pathogens, yet our model systems to study this group of cells remain very limited in scope. In this study, we established that differentiated PLB-985 cells can serve as a model to recapitulate several important aspects of human polymorphonuclear leukocyte interactions with the important human fungal pathogen Aspergillus fumigatus. The proposed addition of a cultured neutrophil-like cell line to the experimental toolbox to study fungal pathogenesis will allow for a more mechanistic description of neutrophil antifungal biology. In addition, the easier handling of the cell line compared to primary human neutrophils allowed us to use PLB-985 cells to provide an improved method for isolation of neutrophil-derived extracellular vesicles using size exclusion chromatography. Together, these results provide significant tools and a baseline knowledge for the future study of neutrophil-derived extracellular vesicles in the laboratory.

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.

Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Differential Antimicrobial Efficacy of Multipurpose Solutions against Acanthamoeba Trophozoites

SIGNIFICANCE

This investigation examines the effectiveness of several common contact lens solutions in the disinfection of Acanthamoeba, which causes a serious eye infection most often resulting from dysfunctional or improper use of contact lens products.

PURPOSE

Acanthamoeba keratitis is an eye infection caused by a free-living amoeba, which can lead to extensive corneal damage and frequently blindness. Acanthamoeba keratitis is linked with contact lens use combined with noncompliance with contact lens care cleaning regimens. The patient's choice and use of multipurpose solutions (MPSs) continue to be a risk factor for Acanthamoeba keratitis. Thus, it is critical that the Acanthamoeba disinfection efficacy of the popular MPSs be determined. Here we compare the efficacy of seven major MPSs on the global market.

METHODS

Using standard methods of Acanthamoeba disinfection and quantification, Acanthamoeba ATCC 30461, 30868, 50370, and 50676 trophozoites were inoculated into each MPS and held for the manufacturer's recommended disinfection time. Acanthamoeba recovery plates were incubated for 14 days, after which positive wells were identified and cell concentrations determined using the 50% endpoint method.

RESULTS

Members of the OPTI-FREE products (Express, Replenish, and Puremoist [Alcon, Fort Worth, TX]) demonstrated significantly higher percentages of antimicrobial activity compared with the renu Advanced Formula (Bausch + Lomb, Rochester, NY), Biotrue (Bausch + Lomb), Acuvue RevitaLens (Johnson & Johnson, Santa Ana, CA), and Lite products (Cooper Vision, Scottsville, NY) for four of the trophozoite strains tested.

CONCLUSIONS

Many of the popular MPS biocides maintain little or no antimicrobial activity against Acanthamoeba trophozoites, and the number of biocides in an MPS does not necessarily indicate its antimicrobial activity.

Aspergillus fumigatus versus Genus Aspergillus: Conservation, Adaptive Evolution and Specific Virulence Genes

Aspergillus is an important fungal genus containing economically important species, as well as pathogenic species of animals and plants. Using eighteen fungal species of the genus Aspergillus, we conducted a comprehensive investigation of conserved genes and their evolution. This also allows us to investigate the selection pressure driving the adaptive evolution in the pathogenic species A. fumigatus. Among single-copy orthologs (SCOs) for A. fumigatus and the closely related species A. fischeri, we identified 122 versus 50 positively selected genes (PSGs), respectively. Moreover, twenty conserved genes of unknown function were established to be positively selected and thus important for adaption. A. fumigatus PSGs interacting with human host proteins show over-representation of adaptive, symbiosis-related, immunomodulatory and virulence-related pathways, such as the TGF-β pathway, insulin receptor signaling, IL1 pathway and interfering with phagosomal GTPase signaling. Additionally, among the virulence factor coding genes, secretory and membrane protein-coding genes in multi-copy gene families, 212 genes underwent positive selection and also suggest increased adaptation, such as fungal immune evasion mechanisms (aspf2), siderophore biosynthesis (sidD), fumarylalanine production (sidE), stress tolerance (atfA) and thermotolerance (sodA). These genes presumably contribute to host adaptation strategies. Genes for the biosynthesis of gliotoxin are shared among all the close relatives of A. fumigatus as an ancient defense mechanism. Positive selection plays a crucial role in the adaptive evolution of A. fumigatus. The genome-wide profile of PSGs provides valuable targets for further research on the mechanisms of immune evasion, antimycotic targeting and understanding fundamental virulence processes.

The fungivorous amoeba Protostelium aurantium targets redox homeostasis and cell wall integrity during intracellular killing of Candida parapsilosis

Predatory interactions among microbes are major evolutionary driving forces for biodiversity. The fungivorous amoeba Protostelium aurantium has a wide fungal food spectrum including foremost pathogenic members of the genus Candida. Here we show that upon phagocytic ingestion by the amoeba, Candida parapsilosis is confronted with an oxidative burst and undergoes lysis within minutes of processing in acidified phagolysosomes. On the fungal side, a functional genomic approach identified copper and redox homeostasis as primary targets of amoeba predation, with the highly expressed copper exporter gene CRP1 and the peroxiredoxin gene PRX1 contributing to survival when encountered with P. aurantium. The fungicidal activity was largely retained in intracellular vesicles of the amoebae. Following their isolation, the content of these vesicles induced immediate killing and lysis of C. parapsilosis in vitro. Proteomic analysis identified 56 vesicular proteins from P. aurantium. Although completely unknown proteins were dominant, many of them could be categorised as hydrolytic enzymes targeting the fungal cell wall, indicating that fungal cell wall structures are under selection pressure by predatory phagocytes in natural environments. TAKE AWAY: The amoeba Protostelium aurantium feeds on fungi, such as Candida parapsilosis. Ingested yeast cells are exposed to reactive oxygen species. A copper exporter and a peroxiredoxin contribute to fungal defence. Yeast cells undergo intracellular lysis. Lysis occurs via a cocktail of hydrolytic enzymes from intracellular vesicles.

Amoebae as Targets for Toxins or Effectors Secreted by Mammalian Pathogens

Numerous microorganisms, pathogenic for mammals, come from the environment where they encounter predators such as free-living amoebae (FLA). The selective pressure due to this interaction could have generated virulence traits that are deleterious for amoebae and represents a weapon against mammals. Toxins are one of these powerful tools that are essential for bacteria or fungi to survive. Which amoebae are used as a model to study the effects of toxins? What amoeba functions have been reported to be disrupted by toxins and bacterial secreted factors? Do bacteria and fungi effectors affect eukaryotic cells similarly? Here, we review some studies allowing to answer these questions, highlighting the necessity to extend investigations of microbial pathogenicity, from mammals to the environmental reservoir that are amoebae.

Natural products in the predatory defence of the filamentous fungal pathogen Aspergillus fumigatus

The kingdom of fungi comprises a large and highly diverse group of organisms that thrive in diverse natural environments. One factor to successfully confront challenges in their natural habitats is the capability to synthesize defensive secondary metabolites. The genetic potential for the production of secondary metabolites in fungi is high and numerous potential secondary metabolite gene clusters have been identified in sequenced fungal genomes. Their production may well be regulated by specific ecological conditions, such as the presence of microbial competitors, symbionts or predators. Here we exemplarily summarize our current knowledge on identified secondary metabolites of the pathogenic fungus Aspergillus fumigatus and their defensive function against (microbial) predators.

Variation of virulence of five Aspergillus fumigatus isolates in four different infection models

Conidia of Aspergillus fumigatus are inhaled by humans on daily basis. As a consequence, these conidia can cause infections that differ in severity ranging from allergic bronchopulmonary aspergillosis to invasive aspergillosis. In this study we compared virulence of five A. fumigatus isolates in four different infection models to address the predictive value of different model systems. Two of the A. fumigatus strains were isolated from dogs with a non-invasive sino-nasal aspergillosis (DTO271-B5 and DTO303-F3), while three strains were isolated from human patients with invasive aspergillosis (Af293, ATCC46645 and CEA10). Infection models used encompassed cultured type II A549 lung epithelial cells, Protostelium aurantium amoeba, Galleria melonella larvae and zebrafish embryos. No major differences in virulence between these five strains were observed in the lung epithelial cell model. In contrast, strain ATCC46645 was most virulent in the amoeba and zebrafish model, whereas it was much less virulent in the Galleria infection model. DTO303-F3 was most virulent in the latter model. In general, reference strain Af293 was less virulent as compared to the other strains. Genome sequence analysis showed that this latter strain differed from the other four strains in 136 SNPs in virulence-related genes. Together, our results show that virulence of individual A. fumigatus strains show significant differences between infection models. We conclude that the predictive value of different model systems varies since the relative virulence across fungal strains does not hold up across different infection model systems.

Nitrogen, Iron and Zinc Acquisition: Key Nutrients to Aspergillus fumigatus Virulence

Aspergillus fumigatus is a ubiquitous soil decomposer and an opportunistic pathogen that is characterized by its large metabolic machinery for acquiring nutrients from media. Lately, an ever-increasing number of genes involved in fungal nutrition has been associated with its virulence. Of these, nitrogen, iron, and zinc metabolism-related genes are particularly noteworthy, since 78% of them have a direct implication in virulence. In this review, we describe the sensing, uptake and regulation process of the acquisition of these nutrients, the connections between pathways and the virulence-implicated genes. Nevertheless, only 40% of the genes mentioned in this review have been assayed for roles in virulence, leaving a wide field of knowledge that remains uncertain and might offer new therapeutic and diagnostic targets.

Host-pathogen interactions: lessons from phagocytic predation on fungi

Free living amoebae share striking similarities with innate immune cells in terms of cell morphology, motility and phagocytic processing of microbes. Their abilities to find, ingest and kill bacteria and fungi in their natural habitats have fostered the hypothesis that amoebae could have served as a training ground for environmentally acquired pathogens. What may have been more obvious for intracellular bacteria, becomes increasingly clear also for several fungal pathogens: a number of virulence determinants of human pathogenic fungi such as Cryptococcus neoformans or Aspergillus fumigatus are equally relevant to resist innate immune cells and environmental phagocytic predators. Here, we summarize the most recent experimental examples underlining the concept of amoeba models to study fungal pathogens.

The impact of the Fungus-Host-Microbiota interplay upon Candida albicans infections: current knowledge and new perspectives

Candida albicans is a major fungal pathogen of humans. It exists as a commensal in the oral cavity, gut or genital tract of most individuals, constrained by the local microbiota, epithelial barriers and immune defences. Their perturbation can lead to fungal outgrowth and the development of mucosal infections such as oropharyngeal or vulvovaginal candidiasis, and patients with compromised immunity are susceptible to life-threatening systemic infections. The importance of the interplay between fungus, host and microbiota in driving the transition from C. albicans commensalism to pathogenicity is widely appreciated. However, the complexity of these interactions, and the significant impact of fungal, host and microbiota variability upon disease severity and outcome, are less well understood. Therefore, we summarise the features of the fungus that promote infection, and how genetic variation between clinical isolates influences pathogenicity. We discuss antifungal immunity, how this differs between mucosae, and how individual variation influences a person's susceptibility to infection. Also, we describe factors that influence the composition of gut, oral and vaginal microbiotas, and how these affect fungal colonisation and antifungal immunity. We argue that a detailed understanding of these variables, which underlie fungal-host-microbiota interactions, will present opportunities for directed antifungal therapies that benefit vulnerable patients.

An evolutionary genomic approach reveals both conserved and species-specific genetic elements related to human disease in closely related Aspergillus fungi

Aspergillosis is an important opportunistic human disease caused by filamentous fungi in the genus Aspergillus. Roughly 70% of infections are caused by Aspergillus fumigatus, with the rest stemming from approximately a dozen other Aspergillus species. Several of these pathogens are closely related to A. fumigatus and belong in the same taxonomic section, section Fumigati. Pathogenic species are frequently most closely related to nonpathogenic ones, suggesting Aspergillus pathogenicity evolved multiple times independently. To understand the repeated evolution of Aspergillus pathogenicity, we performed comparative genomic analyses on 18 strains from 13 species, including 8 species in section Fumigati, which aimed to identify genes, both ones previously connected to virulence as well as ones never before implicated, whose evolution differs between pathogens and nonpathogens. We found that most genes were present in all species, including approximately half of those previously connected to virulence, but a few genes were section- or species-specific. Evolutionary rate analyses identified over 1700 genes whose evolutionary rate differed between pathogens and nonpathogens and dozens of genes whose rates differed between specific pathogens and the rest of the taxa. Functional testing of deletion mutants of 17 transcription factor-encoding genes whose evolution differed between pathogens and nonpathogens identified eight genes that affect either fungal survival in a model of phagocytic killing, host survival in an animal model of fungal disease, or both. These results suggest that the evolution of pathogenicity in Aspergillus involved both conserved and species-specific genetic elements, illustrating how an evolutionary genomic approach informs the study of fungal disease.

Transcriptional Control of the Production of Aspergillus fumigatus Conidia-Borne Secondary Metabolite Fumiquinazoline C Important for Phagocytosis Protection

Aspergillus fumigatus produces diverse secondary metabolites whose biological functions and regulation remain to be understood. Despite the importance of the conidia for this fungus, the role of the conidia-born metabolite fumiquinazoline C (FqC) is unclear. Here, we describe a dual function of the cell-wall integrity pathway in regulating FqC biosynthesis dictated by the MAPK kinase MpkA, which phosphorylates one of the nonribosomal peptide synthetases enzymes of the cluster (FmqC), and the transcription factor RlmA, which directly regulates the expression of fmq genes. Another level of crosstalk between the FqC regulation and the cell physiology is described since the deletion of the stress-responsive transcription factor sebA provokes derepression of the fmq cluster and overproduction of FqC. Thus, we describe a mechanism by which A. fumigatus controls FqC biosynthesis orchestrated by MpkA-RlmA and SebA and hence enabling survival and adaptation to the environmental niche, given that FqC is a deterrent of ameba predation.

Evaluating Alternate Methods of Determining the Antimicrobial Efficacy of Contact Lens Care Products against Acanthamoeba Trophozoites

Acanthamoeba keratitis (AK) is a serious ocular infection caused by a ubiquitous free-living amoeba, Acanthamoeba. This infection often results in extensive corneal damage and blindness, and is notoriously difficult to cure. While Acanthamoeba is an abundant organism, AK is most associated with contact lens hygiene noncompliance and inadequate contact lens care (CLC) disinfection regimens. Thus, accurate and timely antimicrobial efficacy testing of CLC solutions is paramount. Published methods for antimicrobial efficacy testing of Acanthamoeba trophozoites requires 14 days for results. Presently, alternate and/or rapid methods for evaluating CLC products rarely demonstrate equivalent results compared to commonly-reported methods. Propidium iodide is a cellular stain that can only bind to cells with damaged outer membranes. We evaluated propidium iodide staining as an alternative method for determining the relative antimicrobial efficacy of 11 different CLC products against Acanthamoeba trophozoites. Following exposure to a CLC product, the fluorescence intensity of propidium iodide in an Acanthamoeba population demonstrated a strong correlation to the log reduction determined by established, growth-based Acanthamoeba testing used to evaluate the antimicrobial efficacy of CLC products. Thus, propidium iodide was found to be an effective rapid tool for determining cell death in Acanthamoeba trophozoites following exposure to CLC solutions.

Scedosporium Cell Wall: From Carbohydrate-Containing Structures to Host-Pathogen Interactions

Scedosporium species are filamentous fungi usually found in sewage and soil from human-impacted areas. They cause a wide range of diseases in humans, from superficial infections, such as mycetoma, to invasive and disseminated cases, especially associated in immunocompromised patients. Scedosporium species are also related to lung colonization in individuals presenting cystic fibrosis and are considered one of the most frequent fungal pathogens associated to this pathology. Scedosporium cell wall contains glycosylated molecules involved in important biological events related to virulence and pathogenicity and represents a significant source of antigens. Polysaccharides, peptidopolysaccharides, O-linked oligosaccharides and glycosphingolipids have been identified on the Scedosporium surface. Their primary structures were determined based on a combination of techniques including gas chromatography, ESI-MS, and ¹H and ¹³C nuclear magnetic resonance. Peptidorhamnnomannans are common cell wall components among Scedosporium species. Comparing different species, minor structural differences in the carbohydrate portions were detected which could be useful to understand variations in virulence observed among the species. N- and O-linked peptidorhamnomannans are major pathogen-associated molecular patterns and, along with α-glucans, play important roles in triggering host innate immunity. Glycosphingolipids, such as glucosylceramides, have highly conserved structures in Scedosporium species and are crucial for fungal growth and virulence. The present review presents current knowledge on structural and functional aspects of Scedosporium glycoconjugates that are relevant for understanding pathogenicity mechanisms and could contribute to the design of new agents capable of inhibiting growth and differentiation of Scedosporium species. Other cell components such as melanin and ectophosphatases will be also included.

Sensing the threat posed by Aspergillus infection

The mammalian immune system can tune its inflammatory response to the threat level posed by an invading pathogen. It is well established that the host utilizes numerous 'patterns of pathogenicity', such as microbial growth, invasion, and viability, to achieve this tuning during bacterial infections. This review discusses how this notion fits during fungal infection, particularly regarding Aspergillus fumigatus infection. Moreover, how the environmental niches filled by A. fumigatus may drive the evolution of the fungal traits responsible for inducing the strain-specific inflammatory responses that have been experimentally observed will be discussed. Moving forward understanding the mechanisms of the fungal strain-specific inflammatory response due to the initial interactions with the host innate immune system will be essential for enhancing our therapeutic options for the treatment of invasive fungal infections.

Conidial Melanin of the Human-Pathogenic Fungus Aspergillus fumigatus Disrupts Cell Autonomous Defenses in Amoebae

The human-pathogenic fungus Aspergillus fumigatus is a ubiquitous saprophyte that causes fatal lung infections in immunocompromised individuals. Following inhalation, conidia are ingested by innate immune cells and can arrest phagolysosome maturation. How this virulence trait could have been selected for in natural environments is unknown. Here, we found that surface exposure of the green pigment 1,8-dihydroxynaphthalene-(DHN)-melanin can protect conidia from phagocytic uptake and intracellular killing by the fungivorous amoeba Protostelium aurantium and delays its exocytosis from the nonfungivorous species Dictyostelium discoideum To elucidate the antiphagocytic properties of the surface pigment, we followed the antagonistic interactions of A. fumigatus conidia with the amoebae in real time. For both amoebae, conidia covered with DHN-melanin were internalized at far lower rates than were seen with conidia lacking the pigment, despite high rates of initial attachment to nonkilling D. discoideum When ingested by D. discoideum, the formation of nascent phagosomes was followed by transient acidification of phagolysosomes, their subsequent neutralization, and, finally, exocytosis of the conidia. While the cycle was completed in less than 1 h for unpigmented conidia, the process was significantly prolonged for conidia covered with DHN-melanin, leading to an extended intracellular residence time. At later stages of this cellular infection, pigmented conidia induced enhanced damage to phagolysosomes and infected amoebae failed to recruit the ESCRT (endosomal sorting complex required for transport) membrane repair machinery or the canonical autophagy pathway to defend against the pathogen, thus promoting prolonged intracellular persistence in the host cell and the establishment of a germination niche in this environmental phagocyte.IMPORTANCE Infections with Aspergillus fumigatus are usually acquired by an inhalation of spores from environmental sources. How spores of a saprophytic fungus have acquired abilities to withstand and escape the phagocytic attacks of innate immune cells is not understood. The fungal surface pigment dihydroxynaphtalene-melanin has been shown to be a crucial factor for the delay in phagosome maturation. Here, we show that this pigment also has a protective function against environmental phagocytes. Pigmented conidia escaped uptake and killing by the fungus-eating amoeba Protostelium aurantium When ingested by the nonfungivorous phagocyte Dictyostelium discoideum, the pigment attenuated the launch of cell autonomous defenses against the fungal invader, such as membrane repair and autophagy, leading to prolonged intracellular retention. Membrane damage and cytoplasmic leakage may result in an influx of nutrients and thus may further promote intracellular germination of the fungus, indicating that A. fumigatus has acquired some of the basic properties of intracellular pathogens.

Threats Posed by the Fungal Kingdom to Humans, Wildlife, and Agriculture

The fungal kingdom includes at least 6 million eukaryotic species and is remarkable with respect to its profound impact on global health, biodiversity, ecology, agriculture, manufacturing, and biomedical research. Approximately 625 fungal species have been reported to infect vertebrates, 200 of which can be human associated, either as commensals and members of our microbiome or as pathogens that cause infectious diseases. These organisms pose a growing threat to human health with the global increase in the incidence of invasive fungal infections, prevalence of fungal allergy, and the evolution of fungal pathogens resistant to some or all current classes of antifungals. More broadly, there has been an unprecedented and worldwide emergence of fungal pathogens affecting animal and plant biodiversity. Approximately 8,000 species of fungi and Oomycetes are associated with plant disease. Indeed, across agriculture, such fungal diseases of plants include new devastating epidemics of trees and jeopardize food security worldwide by causing epidemics in staple and commodity crops that feed billions. Further, ingestion of mycotoxins contributes to ill health and causes cancer. Coordinated international research efforts, enhanced technology translation, and greater policy outreach by scientists are needed to more fully understand the biology and drivers that underlie the emergence of fungal diseases and to mitigate against their impacts. Here, we focus on poignant examples of emerging fungal threats in each of three areas: human health, wildlife biodiversity, and food security.

The role of melanins in melanotic fungi for pathogenesis and environmental survival

Melanins provide fungi protection from environmental stressors, support their ecological roles, and can confer virulence in pathogens. While the function, structure, and synthesis of melanins in fungi are not fully understood, they have been shown to have varied roles. Recent research has revealed a wide range of functions, from radiation resistance to increasing virulence, shedding light on fungal diversity. Understanding fungal melanins can provide useful information, from harnessing the properties of these various melanins to targeting fungal infections.Key Points• Melanotic fungi are widespread in nature. • Melanin functions to protect fungi in the environment from a range of stresses. • Melanin contributes to pathogenesis and drug resistance of pathogenic fungi.

The Aspergilli and Their Mycotoxins: Metabolic Interactions With Plants and the Soil Biota

Species of the highly diverse fungal genus Aspergillus are well-known agricultural pests, and, most importantly, producers of various mycotoxins threatening food safety worldwide. Mycotoxins are studied predominantly from the perspectives of human and livestock health. Meanwhile, their roles are far less known in nature. However, to understand the factors behind mycotoxin production, the roles of the toxins of Aspergilli must be understood from a complex ecological perspective, taking mold-plant, mold-microbe, and mold-animal interactions into account. The Aspergilli may switch between saprophytic and pathogenic lifestyles, and the production of secondary metabolites, such as mycotoxins, may vary according to these fungal ways of life. Recent studies highlighted the complex ecological network of soil microbiotas determining the niches that Aspergilli can fill in. Interactions with the soil microbiota and soil macro-organisms determine the role of secondary metabolite production to a great extent. While, upon infection of plants, metabolic communication including fungal secondary metabolites like aflatoxins, gliotoxin, patulin, cyclopiazonic acid, and ochratoxin, influences the fate of both the invader and the host. In this review, the role of mycotoxin producing Aspergillus species and their interactions in the ecosystem are discussed. We intend to highlight the complexity of the roles of the main toxic secondary metabolites as well as their fate in natural environments and agriculture, a field that still has important knowledge gaps.

Fumagillin, a Mycotoxin of Aspergillus fumigatus: Biosynthesis, Biological Activities, Detection, and Applications

Fumagillin is a mycotoxin produced, above all, by the saprophytic filamentous fungus Aspergillus fumigatus. This mold is an opportunistic pathogen that can cause invasive aspergillosis, a disease that has high mortality rates linked to it. Its ability to adapt to environmental stresses through the production of secondary metabolites, including several mycotoxins (gliotoxin, fumagillin, pseurotin A, etc.) also seem to play an important role in causing these infections. Since the discovery of the A. fumigatus fumagillin in 1949, many studies have focused on this toxin and in this review we gather all the information currently available. First of all, the structural characteristics of this mycotoxin and the different methods developed for its determination are given in detail. Then, the biosynthetic gene cluster and the metabolic pathway involved in its production and regulation are explained. The activity of fumagillin on its target, the methionine aminopeptidase type 2 (MetAP2) enzyme, and the effects of blocking this enzyme in the host are also described. Finally, the applications that this toxin and its derivatives have in different fields, such as the treatment of cancer and its microsporicidal activity in the treatment of honeybee hive infections with Nosema spp., are reviewed. Therefore, this work offers a complete review of all the information currently related to the fumagillin mycotoxin secreted by A. fumigatus, important because of its role in the fungal infection process but also because it has many other applications, notably in beekeeping, the treatment of infectious diseases, and in oncology.

A hidden battle in the dirt: Soil amoebae interactions with Paracoccidioides spp

Paracoccidioides spp. are thermodimorphic fungi that cause a neglected tropical disease (paracoccidioidomycosis) that is endemic to Latin America. These fungi inhabit the soil, where they live as saprophytes with no need for a mammalian host to complete their life cycle. Despite this, they developed sophisticated virulence attributes allowing them not only to survive in host tissues but also to cause disease. A hypothesis for selective pressures driving the emergence or maintenance of virulence of soil fungi is their interaction with soil predators such as amoebae and helminths. We evaluated the presence of environmental amoeboid predators in soil from armadillo burrows where Paracoccidioides had been previously detected and tested if the interaction of Paracoccidioides with amoebae selects for fungi with increased virulence. Nematodes, ciliates, and amoebae-all potential predators of fungi-grew in cultures from soil samples. Microscopical observation and ITS sequencing identified the amoebae as Acanthamoeba spp, Allovahlkampfia spelaea, and Vermamoeba vermiformis. These three amoebae efficiently ingested, killed and digested Paracoccidioides spp. yeast cells, as did laboratory adapted axenic Acanthamoeba castellanii. Sequential co-cultivation of Paracoccidioides with A. castellanii selected for phenotypical traits related to the survival of the fungus within a natural predator as well as in murine macrophages and in vivo (Galleria mellonella and mice). These changes in virulence were linked to the accumulation of cell wall alpha-glucans, polysaccharides that mask recognition of fungal molecular patterns by host pattern recognition receptors. Altogether, our results indicate that Paracoccidioides inhabits a complex environment with multiple amoeboid predators that can exert selective pressure to guide the evolution of virulence traits.

A sphingolipid synthesis-related protein OrmA in Aspergillus fumigatus is responsible for azole susceptibility and virulence

Previous studies identified that the budding yeast Saccharomyces cerevisiae have two sphingolipid synthesis-related proteins, Orm1p and Orm2p, that negatively regulate the activities of SPT, which is a key rate-limiting enzyme in sphingolipid synthesis. However, little is known about whether sphingolipids in the cell membrane, which are closely related to ergosterols, could affect the efficacy of azole drugs, which target to the ergosterol biosynthesis. In this study, through genome-wide homologue search analysis, we found that the Aspergillus fumigatus genome only contains one Orm homologue, referred to as OrmA for which the protein expression could be induced by azole antifungals in a dose-dependent manner. Deletion of ormA caused hypersensitivity to azoles, and adding the sphingolipid synthesis inhibitor myriocin rescued the azole susceptibility induced by lack of ormA. In contrast, overexpression of OrmA resulted in azole resistance, indicating that OrmA is a positive azole-response regulator. Further mechanism analysis verified that OrmA is related to drug susceptibility by affecting endoplasmic reticulum stress responses in an unfolded protein response pathway-HacA-dependent manner. Lack of ormA led to an abnormal profile of sphingolipid ceramide components accompanied by hypersensitivity to low temperatures. Furthermore, deletion of OrmA significantly reduced virulence in an immunosuppressed mouse model. The findings in this study collectively suggest that the sphingolipid metabolism pathway in A. fumigatus plays a critical role in azole susceptibility and fungal virulence.

Bis-naphthopyrone pigments protect filamentous ascomycetes from a wide range of predators

It is thought that fungi protect themselves from predation by the production of compounds that are toxic to soil-dwelling animals. Here, we show that a nontoxic pigment, the bis-naphthopyrone aurofusarin, protects Fusarium fungi from a wide range of animal predators. We find that springtails (primitive hexapods), woodlice (crustaceans), and mealworms (insects) prefer feeding on fungi with disrupted aurofusarin synthesis, and mealworms and springtails are repelled by wheat flour amended with the fungal bis-naphthopyrones aurofusarin, viomellein, or xanthomegnin. Predation stimulates aurofusarin synthesis in several Fusarium species and viomellein synthesis in Aspergillus ochraceus. Aurofusarin displays low toxicity in mealworms, springtails, isopods, Drosophila, and insect cells, contradicting the common view that fungal defence metabolites are toxic. Our results indicate that bis-naphthopyrones are defence compounds that protect filamentous ascomycetes from predators through a mechanism that does not involve toxicity.

Population growth of the stored product pest Tyrophagus putrescentiae (Acari: Acaridae) on environmentally and medically important fungi

The stored food mite Tyrophagus putrescentiae (Schrank) (Acari: Acaridae) has been associated with the presence of several fungal species. The aims of this work were to evaluate T. putrescentiae population growth associated to environmental and medically important fungal species to determine on which fungal species populations of T. putrescentiae performs best, and to evaluate their ability to disperse each fungal species. First, 24 fungal species were inoculated separately in Petri dishes containing Sabouraud agar medium. One week after inoculation, 50 mites were added to each plate. On the 28th evaluation day, mites and eggs were counted in each plate, and 50 mites randomly collected from each replicate were transferred to new plates containing only Sabouraud agar medium. Then, mites, eggs, and fungal population were evaluated in each plate on day 28 again. The highest population increases were on Trichophyton mentagrophytes, Alternaria sp., Microsporum gypseum, and Aspergillus chevalieri. With Fusarium guttiforme and the medically important fungi Microsporum canis, M. gypseum, T. mentagrophytes, and Sporothrix sp., mites were observed to feed on whole mycelium. Only eight fungal species were dispersed by T. putrescentiae to the new Petri dishes: Aspergillus clavatus, Candida tropicalis, Candida albicans, Fusarium guttiforme, Hyphopichia burtonii, Penicillium citrinum, Rhizophus azygosporus, and Trichophyton mentagrophytes. The best performance of T. putrescentiae was found feeding on F. guttiforme, P. citrinum, and T. mentagrophytes. In conclusion, T. putrescentiae successfully used fungi as a food source, and it proved to be an important tool for disseminating both environmental and medically important fungi.

Melanin and pyomelanin in Aspergillus fumigatus: from its genetics to host interaction

Aspergillus fumigatus is a worldwide-distributed saprophytic fungus and the major cause of invasive aspergillosis. This fungus can produce two types of melanin-dihydroxynaphthalene melanin (DHN-melanin) and pyomelanin. These pigments are considered important resistance mechanisms to stress, as well as virulence factors. The aim of this review is to present the current knowledge of the genetic basis and metabolic pathways of melanin production, their activation, function, and interaction with the host immune system. The DHN-melanin pathway is encoded in a cluster that includes six genes (abr1, abr2, ayg1, arp1, arp2, and pksP/alb1 genes) whose encoded proteins seem to be the origin of the pigment in endosomes. These vesicles are secreted and the pigment is subsequently located in the wall of the conidium beneath the rodlet layer. Unlike DHN-melanin, pyomelanin does not have its own biosynthetic pathway but is related to the activation of the L-tyrosine/L-phenylalanine degradation pathway that includes a cluster of six genes (hppD, hmgX, hmgA, fahA, maiA, and hmgR). Its production is due to the polymerization of homogentisic acid and is linked to conidial germination. Despite the knowledge gained in recent years, further studies will be necessary to confirm the pathways that produce these pigments and their role in the virulence mechanisms of A. fumigatus.

The different morphologies of yeast and filamentous fungi trigger distinct killing and feeding mechanisms in a fungivorous amoeba

Size and diverse morphologies pose a primary challenge for phagocytes such as innate immune cells and predatory amoebae when encountering fungal prey. Although filamentous fungi can escape phagocytic killing by pure physical constraints, unicellular spores and yeasts can mask molecular surface patterns or arrest phagocytic processing. Here, we show that the fungivorous amoeba Protostelium aurantium was able to adjust its killing and feeding mechanisms to these different cell shapes. Yeast-like fungi from the major fungal groups of basidiomycetes and ascomycetes were readily internalized by phagocytosis, except for the human pathogen Candida albicans whose mannoprotein coat was essential to escape recognition by the amoeba. Dormant spores of the filamentous fungus Aspergillus fumigatus also remained unrecognized, but swelling and the onset of germination induced internalization and intracellular killing by the amoeba. Mature hyphae of A. fumigatus were mostly attacked from the hyphal tip and killed by an actin-mediated invasion of fungal filaments. Our results demonstrate that predatory pressure imposed by amoebae in natural environments selects for distinct survival strategies in yeast and filamentous fungi but commonly targets the fungal cell wall as a crucial molecular pattern associated to prey and pathogens.

Gliotoxin from Aspergillus fumigatus Abrogates Leukotriene B4 Formation through Inhibition of Leukotriene A4 Hydrolase

The epidithiodioxopiperazine gliotoxin is a virulence factor of Aspergillus fumigatus, the most important airborne fungal pathogen of humans. Gliotoxin suppresses innate immunity in invasive aspergillosis, particularly by compromising neutrophils, but the underlying molecular mechanisms remain elusive. Neutrophils are the first responders among innate immune cells recruited to sites of infection by the chemoattractant leukotriene (LT)B₄ that is biosynthesized by 5-lipoxygenase and LTA₄ hydrolase (LTA₄H). Here, we identified gliotoxin as inhibitor of LTA₄H that selectively abrogates LTB₄ formation in human leukocytes and in distinct animal models. Gliotoxin failed to inhibit the formation of other eicosanoids and the aminopeptidase activity of the bifunctional LTA₄H. Suppression of LTB₄ formation by gliotoxin required the cellular environment and/or reducing conditions, and only the reduced form of gliotoxin inhibited LTA₄H activity. Conclusively, gliotoxin suppresses the biosynthesis of the potent neutrophil chemoattractant LTB₄ by direct interference with LTA₄H thereby impairing neutrophil functions in invasive aspergillosis.

The 'Amoeboid Predator-Fungal Animal Virulence' Hypothesis

The observation that some aspects of amoeba-fungal interactions resemble animal phagocytic cell-fungal interactions, together with the finding that amoeba passage can enhance the virulence of some pathogenic fungi, has stimulated interest in the amoeba as a model system for the study of fungal virulence. Amoeba provide a relatively easy and cheap model system where multiple variables can be controlled for the study of fungi-protozoal (amoeba) interactions. Consequently, there have been significant efforts to study fungal⁻amoeba interactions in the laboratory, which have already provided new insights into the origin of fungal virulence as well as suggested new avenues for experimentation. In this essay we review the available literature, which highlights the varied nature of amoeba-fungal interactions and suggests some unsolved questions that are potential areas for future investigation. Overall, results from multiple independent groups support the 'amoeboid predator⁻fungal animal virulence hypothesis', which posits that fungal cell predation by amoeba can select for traits that also function during animal infection to promote their survival and thus contribute to virulence.

Dictyostelium Host Response to Legionella Infection: Strategies and Assays

The professional phagocyte Dictyostelium discoideum is a well-established model organism to study host-pathogen interactions. Dictyostelium amoebae grow as separate, independent cells; they divide by binary fission and take up bacteria and yeast via phagocytosis. In the year 2000, D. discoideum was described by two groups as a novel system for genetic analysis of host-pathogen interactions for the intracellular pathogen Legionella pneumophila. Since then additional microbial pathogens that can be studied in D. discoideum have been reported. The organism has various advantages for the dissection of the complex cross-talk between a host and a pathogen. A fully sequenced and well-curated genome is available, there are excellent molecular genetic tools on the market, and the generation of targeted multiple gene knock-outs as well as the realization of untargeted genetic screens is generally straightforward. Dictyostelium also offers easy cultivation, and the cells are suitable for cell biological studies, which in combination with in vivo expression of fluorescence-tagged proteins allows the investigation of the dynamics of bacterial uptake and infection. Furthermore, a large mutant collection is available at the Dictyostelium stock center, favoring the identification of host resistance or susceptibility genes. Here, we briefly describe strategies to identify host cell factors important during an infection, followed by protocols for cell culture and storage, uptake and infection, and confocal microscopy of infected cells.

Mitogen activated protein kinases (MAPK) and protein phosphatases are involved in Aspergillus fumigatus adhesion and biofilm formation

The main characteristic of biofilm formation is extracellular matrix (ECM) production. The cells within the biofilm are surrounded by ECM which provides structural integrity and protection. During an infection, this protection is mainly against cells of the immune system and antifungal drugs. A. fumigatus forms biofilms during static growth on a solid substratum and in chronic aspergillosis infections. It is important to understand how, and which, A. fumigatus signal transduction pathways are important for the adhesion and biofilm formation in a host during infection. Here we investigated the role of MAP kinases and protein phosphatases in biofilm formation. The loss of the MAP kinases MpkA, MpkC and SakA had an impact on the cell surface and the ECM during biofilm formation and reduced the adherence of A. fumigatus to polystyrene and fibronectin-coated plates. The phosphatase null mutants ΔsitA and ΔptcB, involved in regulation of MpkA and SakA phosphorylation, influenced cell wall carbohydrate exposure. Moreover, we characterized the A. fumigatus protein phosphatase PphA. The ΔpphA strain was more sensitive to cell wall-damaging agents, had increased β-(1,3)-glucan and reduced chitin, decreased conidia phagocytosis by Dictyostelium discoideum and reduced adhesion and biofilm formation. Finally, ΔpphA strain was avirulent in a murine model of invasive pulmonary aspergillosis and increased the released of tumor necrosis factor alpha (TNF-α) from bone marrow derived macrophages (BMDMs). These results show that MAP kinases and phosphatases play an important role in signaling pathways that regulate the composition of the cell wall, extracellular matrix production as well as adhesion and biofilm formation in A. fumigatus.

Divalent Metal Cations Potentiate the Predatory Capacity of Amoeba for Cryptococcus neoformans

Among the best-studied interactions between soil phagocytic predators and a human-pathogenic fungus is that of Acanthamoeba castellanii and Cryptococcus neoformans The experimental conditions used in amoeba-fungus confrontation assays can have major effects on whether the fungus or the protozoan is ascendant in the interaction. In the presence of Mg²⁺ and Ca²⁺ in phosphate-buffered saline (PBS), C. neoformans was consistently killed when incubated with A. castellaniiA. castellanii survived better in the presence of Mg²⁺ and Ca²⁺, even when incubated with C. neoformans In the absence of Mg²⁺ and Ca²⁺, C. neoformans survived when incubated with A. castellanii, and the percentage of dead amoebae was higher than when incubated without yeast cells. These results show that the presence of Mg²⁺ and Ca²⁺ can make a decisive contribution toward tilting the outcome of the interaction in favor of the amoeba. Of the two metals, Mg²⁺ had a stronger effect than Ca²⁺ The cations enhanced A. castellanii activity against C. neoformans via enhanced phagocytosis, which is the major mechanism by which amoebae kill fungal cells. We found no evidence that amoebae use extracellular killing mechanisms in their interactions with C. neoformans In summary, the presence of Mg²⁺ and Ca²⁺ enhanced the cell adhesion on the surfaces and the motility of the amoeba, thus increasing the chance for contact with C. neoformans and the frequency of phagocytosis. Our findings imply that the divalent cation concentration in soils could be an important variable for whether amoebae can control C. neoformans in the environment.IMPORTANCE The grazing of soil organisms by phagocytic predators such as amoebae is thought to select for traits that enable some of them to acquire the capacity for virulence in animals. Consequently, knowledge about the interactions between amoebae and soil microbes, such as pathogenic fungi, is important for understanding how virulence can emerge. We show that the interaction between an amoeba and the pathogenic fungus C. neoformans is influenced by the presence in the assay of magnesium and calcium, which potentiate amoebae. The results may also have practical applications, since enriching soils with divalent cations may reduce C. neoformans numbers in contaminated soils.

The Cell Wall Integrity Signaling Pathway and Its Involvement in Secondary Metabolite Production

The fungal cell wall is the external and first layer that fungi use to interact with the environment. Every stress signal, before being translated into an appropriate stress response, needs to overtake this layer. Many signaling pathways are involved in translating stress signals, but the cell wall integrity (CWI) signaling pathway is the one responsible for the maintenance and biosynthesis of the fungal cell wall. In fungi, the CWI signal is composed of a mitogen-activated protein kinase (MAPK) module. After the start of the phosphorylation cascade, the CWI signal induces the expression of cell-wall-related genes. However, the function of the CWI signal is not merely the activation of cell wall biosynthesis, but also the regulation of expression and production of specific molecules that are used by fungi to better compete in the environment. These molecules are normally defined as secondary metabolites or natural products. This review is focused on secondary metabolites affected by the CWI signal pathway with a special focus on relevant natural products such as melanins, mycotoxins, and antibacterial compounds.

Exploring Virulence Determinants of Filamentous Fungal Pathogens through Interactions with Soil Amoebae

Infections with filamentous fungi are common to all animals, but attention is rising especially due to the increasing incidence and high mortality rates observed in immunocompromised human individuals. Here, Aspergillus fumigatus and other members of its genus are the leading causative agents. Attributes like their saprophytic life-style in various ecological niches coupled with nutritional flexibility and a broad host range have fostered the hypothesis that environmental predators could have been the actual target for some of their virulence determinants. In this mini review, we have merged the recent findings focused on the potential dual-use of fungal defense strategies against innate immune cells and soil amoebae as natural phagocytes. Well-established virulence attributes like the melanized surface of fungal conidia or their capacity to produce toxic secondary metabolites have also been found to be protective against the model amoeba Dictyostelium discoideum. Some of the recent advances during interaction studies with human cells have further promoted the adaptation of other amoeba infection models, including the wide-spread generalist Acanthamoeba castellanii, or less prominent representatives like Vermamoeba vermiformis. We further highlight prospects and limits of these natural phagocyte models with regard to the infection biology of filamentous fungi and in comparison to the phagocytes of the innate immune system.

New insights from molecular phylogenetics of amoebophagous fungi (Zoopagomycota, Zoopagales)

Amoebophagous fungi are represented in all fungal groups: Basidiomycota, Ascomycota, Zygomycota, and Chytridiomycota. The amoebophagous fungi, within the zygomycota (Zoopagales, Zoopagomycota), mainly affect naked amoebae as ectoparasites or endoparasites. It is rather difficult to isolate members of the Zoopagales, because of their parasitic lifestyle, and to bring them into culture. Consequently, gene sequences of this group are undersampled, and its species composition and phylogeny are relatively unknown. In the present study, we were able to isolate amoebophagous fungi together with their amoeba hosts from various habitats (moss, pond, bark, and soil). Altogether, four fungal strains belonging to the genera Acaulopage and Stylopage plus one unidentified isolate were detected. Sequences of the 18S rDNA and the complete ITS region and partial 28S (LSU) rDNA were generated. Subsequent phylogenetic analyses showed that all new isolates diverge at one branch together with two environmental clonal sequences within the Zoopagomycota. Here, we provide the first molecular characterization of the genus Stylopage. Stylopage is closely related to the genus Acaulopage. In addition, taxonomy and phylogeny of amoebophagous fungi and their ecological importance are reviewed based on new sequence data, which includes environmental clonal sequences.