The 'Amoeboid Predator-Fungal Animal Virulence' Hypothesis

Unraveling the interplay between unicellular parasites and bacterial biofilms: Implications for disease persistence and antibiotic resistance

Bacterial biofilms have attracted significant attention due to their involvement in persistent infections, food and water contamination, and infrastructure corrosion. This review delves into the intricate interactions between bacterial biofilms and unicellular parasites, shedding light on their impact on biofilm formation, structure, and function. Unicellular parasites, including protozoa, influence bacterial biofilms through grazing activities, leading to adaptive changes in bacterial communities. Moreover, parasites like Leishmania and Giardia can shape biofilm composition in a grazing independent manner, potentially influencing disease outcomes. Biofilms, acting as reservoirs, enable the survival of protozoan parasites against environmental stressors and antimicrobial agents. Furthermore, these biofilms may influence parasite virulence and stress responses, posing challenges in disease treatment. Interactions between unicellular parasites and fungal-containing biofilms is also discussed, hinting at complex microbial relationships in various ecosystems. Understanding these interactions offers insights into disease mechanisms and antibiotic resistance dissemination, paving the way for innovative therapeutic strategies and ecosystem-level implications.

Comprehensive characterization of extracellular vesicles produced by environmental (Neff) and clinical (T4) strains of Acanthamoeba castellanii

We conducted a comprehensive comparative analysis of extracellular vesicles (EVs) from two Acanthamoeba castellanii strains, Neff (environmental) and T4 (clinical). Morphological analysis via transmission electron microscopy revealed slightly larger Neff EVs (average = 194.5 nm) compared to more polydisperse T4 EVs (average = 168.4 nm). Nanoparticle tracking analysis (NTA) and dynamic light scattering validated these differences. Proteomic analysis of the EVs identified 1,352 proteins, with 1,107 common, 161 exclusive in Neff, and 84 exclusively in T4 EVs. Gene ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) mapping revealed distinct molecular functions and biological processes and notably, the T4 EVs enrichment in serine proteases, aligned with its pathogenicity. Lipidomic analysis revealed a prevalence of unsaturated lipid species in Neff EVs, particularly triacylglycerols, phosphatidylethanolamines (PEs), and phosphatidylserine, while T4 EVs were enriched in diacylglycerols and diacylglyceryl trimethylhomoserine, phosphatidylcholine and less unsaturated PEs, suggesting differences in lipid metabolism and membrane permeability. Metabolomic analysis indicated Neff EVs enrichment in glycerolipid metabolism, glycolysis, and nucleotide synthesis, while T4 EVs, methionine metabolism. Furthermore, RNA-seq of EVs revealed differential transcript between the strains, with Neff EVs enriched in transcripts related to gluconeogenesis and translation, suggesting gene regulation and metabolic shift, while in the T4 EVs transcripts were associated with signal transduction and protein kinase activity, indicating rapid responses to environmental changes. In this novel study, data integration highlighted the differences in enzyme profiles, metabolic processes, and potential origins of EVs in the two strains shedding light on the diversity and complexity of A. castellanii EVs and having implications for understanding host-pathogen interactions and developing targeted interventions for Acanthamoeba-related diseases.IMPORTANCEA comprehensive and fully comparative analysis of extracellular vesicles (EVs) from two Acanthamoeba castellanii strains of distinct virulence, a Neff (environmental) and T4 (clinical), revealed striking differences in their morphology and protein, lipid, metabolites, and transcripts levels. Data integration highlighted the differences in enzyme profiles, metabolic processes, and potential distinct origin of EVs from both strains, shedding light on the diversity and complexity of A. castellanii EVs, with direct implications for understanding host-pathogen interactions, disease mechanisms, and developing new therapies for the clinical intervention of Acanthamoeba-related diseases.

Convergent evolution of innate immune-modulating effectors in invasive fungal pathogens

Invasive fungal infections pose a major threat to human health. Bacterial and protozoan pathogens secrete protein effectors that overcome innate immune barriers to promote microbial colonization, yet few such molecules have been identified in human fungal pathogens. Recent studies have begun to reveal these long-sought effectors and have illuminated how they subvert key cellular pathways, including apoptosis, myeloid cell polarization, Toll-like receptor signaling, and phagosome action. Thus, despite lacking the specialized secretion systems of bacteria and parasites, it is increasingly clear that fungi independently evolved effectors targeting pathways often subverted by other classes of pathogens. These findings demonstrate the remarkable power of convergent evolution to enable diverse microbes to infect humans while also setting the stage for detailed dissection of fungal disease mechanisms.

Modelling dynamics between free-living amoebae and bacteria

Free-living amoebae (FLA) serve as hosts for a variety of endosymbionts, which are microorganisms that reside and multiply within the FLA. Some of these endosymbionts pose a pathogenic threat to humans, animals, or both. The symbiotic relationship with FLA not only offers these microorganisms protection but also enhances their survival outside their hosts and assists in their dispersal across diverse habitats, thereby escalating disease transmission. This review is intended to offer an exhaustive overview of the existing mathematical models that have been applied to understand the dynamics of FLA, especially concerning their interactions with bacteria. An extensive literature review was conducted across Google Scholar, PubMed, and Scopus databases to identify mathematical models that describe the dynamics of interactions between FLA and bacteria, as published in peer-reviewed scientific journals. The literature search revealed several FLA-bacteria model systems, including Pseudomonas aeruginosa, Pasteurella multocida, and Legionella spp. Although the published mathematical models account for significant system dynamics such as predator-prey relationships and non-linear growth rates, they generally overlook spatial and temporal heterogeneity in environmental conditions, such as temperature, and population diversity. Future mathematical models will need to incorporate these factors to enhance our understanding of FLA-bacteria dynamics and to provide valuable insights for future risk assessment and disease control measures.

Adaptation to an amoeba host drives selection of virulence-associated traits and genetic variation in saprotrophic Candida albicans

Amoebae are micropredators that play an important role in controlling fungal populations in ecosystems. However, the interaction between fungi and their amoebic predators suggests that the pressure from predatory selection can significantly influence the development of fungal virulence and evolutionary processes. Thus, the purpose of this study was to investigate the adaptation of saprotrophic Candida albicans strains during their interactions with Acanthamoeba castellanii. We conducted a comprehensive analysis of survival after co-culture by colony counting of the yeast cells and examining yeast cell phenotypic and genetic characteristics. Our results indicated that exposure to amoebae enhanced the survival capacity of environmental C. albicans and induced visible morphological alterations in C. albicans, particularly by an increase in filamentation. These observed phenotypic changes were closely related to concurrent genetic variations. Notably, mutations in genes encoding transcriptional repressors (TUP1 and SSN6), recognized for their negative regulation of filamentous growth, were exclusively identified in amoeba-passaged isolates, and absent in unexposed isolates. Furthermore, these adaptations increased the exposed isolates' fitness against various stressors, simultaneously enhancing virulence factors and demonstrating an increased ability to invade A549 lung human epithelial cells. These observations indicate that the sustained survival of C. albicans under ongoing amoebic predation involved a key role of mutation events in microevolution to modulate the ability of these isolates to change phenotype and increase their virulence factors, demonstrating an enhanced potential to survive in diverse environmental niches.

β-1,3-Glucan recognition by Acanthamoeba castellanii as a putative mechanism of amoeba-fungal interactions

In this study, we conducted an in-depth analysis to characterize potential Acanthamoeba castellanii (Ac) proteins capable of recognizing fungal β-1,3-glucans. Ac specifically anchors curdlan or laminarin, indicating the presence of surface β-1,3-glucan-binding molecules. Using optical tweezers, strong adhesion of laminarin- or curdlan-coated beads to Ac was observed, highlighting their adhesive properties compared to controls (characteristic time τ of 46.9 and 43.9 s, respectively). Furthermore, Histoplasma capsulatum (Hc) G217B, possessing a β-1,3-glucan outer layer, showed significant adhesion to Ac compared to a Hc G186 strain with an α-1,3-glucan outer layer (τ of 5.3 s vs τ 83.6 s). The addition of soluble β-1,3-glucan substantially inhibited this adhesion, indicating the involvement of β-1,3-glucan recognition. Biotinylated β-1,3-glucan-binding proteins from Ac exhibited higher binding to Hc G217B, suggesting distinct recognition mechanisms for laminarin and curdlan, akin to macrophages. These observations hinted at the β-1,3-glucan recognition pathway's role in fungal entrance and survival within phagocytes, supported by decreased fungal viability upon laminarin or curdlan addition in both phagocytes. Proteomic analysis identified several Ac proteins capable of binding β-1,3-glucans, including those with lectin/glucanase superfamily domains, carbohydrate-binding domains, and glycosyl transferase and glycosyl hydrolase domains. Notably, some identified proteins were overexpressed upon curdlan/laminarin challenge and also demonstrated high affinity to β-1,3-glucans. These findings underscore the complexity of binding via β-1,3-glucan and suggest the existence of alternative fungal recognition pathways in Ac.IMPORTANCEAcanthamoeba castellanii (Ac) and macrophages both exhibit the remarkable ability to phagocytose various extracellular microorganisms in their respective environments. While substantial knowledge exists on this phenomenon for macrophages, the understanding of Ac's phagocytic mechanisms remains elusive. Recently, our group identified mannose-binding receptors on the surface of Ac that exhibit the capacity to bind/recognize fungi. However, the process was not entirely inhibited by soluble mannose, suggesting the possibility of other interactions. Herein, we describe the mechanism of β-1,3-glucan binding by A. castellanii and its role in fungal phagocytosis and survival within trophozoites, also using macrophages as a model for comparison, as they possess a well-established mechanism involving the Dectin-1 receptor for β-1,3-glucan recognition. These shed light on a potential parallel evolution of pathways involved in the recognition of fungal surface polysaccharides.

Alternative in-vivo models of mucormycosis

Mucormycosis is still regarded a rare fungal infection, but the high incidences of COVID-associated cases in India and other countries have shown its potential threat to large patient cohorts. In addition, infections by these fast-growing fungi are often fatal and cause disfigurement, badly affecting patients' lives. In advancing our understanding of pathogenicity factors involved in this disease, to enhance the diagnostic toolset and to evaluate novel treatment regimes, animal models are indispensable. As ethical and practical considerations typically favor the use of alternative model systems, this review provides an overview of alternative animal models employed for mucormycosis and discusses advantages and limitations of the respective model.

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.

Kicking sleepers out of bed: Macrophages promote reactivation of dormant Cryptococcus neoformans by extracellular vesicle release and non-lytic exocytosis

Macrophages play a key role in disseminated cryptococcosis, a deadly fungal disease caused by Cryptococcus neoformans. This opportunistic infection can arise following the reactivation of a poorly characterized latent infection attributed to dormant C. neoformans. Here, we investigated the mechanisms underlying reactivation of dormant C. neoformans using an in vitro co-culture model of viable but non-culturable (VBNC; equivalent of dormant) yeast cells with bone marrow-derived murine macrophages (BMDMs). Comparative transcriptome analysis of BMDMs incubated with log, stationary phase or VBNC cells of C. neoformans showed that VBNC cells elicited a reduced transcriptional modification of the macrophage but retaining the ability to regulate genes important for immune response, such as NLRP3 inflammasome-related genes. We further confirmed the maintenance of the low immunostimulatory capacity of VBNC cells using multiplex cytokine profiling, and analysis of cell wall composition and dectin-1 ligands exposure. In addition, we evaluated the effects of classic (M1) or alternative (M2) macrophage polarization on VBNC cells. We observed that intracellular residence sustained dormancy, regardless of the polarization state of macrophages and despite indirect detection of pantothenic acid (or its derivatives), a known reactivator for VBNC cells, in the C. neoformans-containing phagolysosome. Notably, M0 and M2, but not M1 macrophages, induced extracellular reactivation of VBNC cells by the secretion of extracellular vesicles and non-lytic exocytosis. Our results indicate that VBNC cells retain the low immunostimulatory profile required for persistence of C. neoformans in the host. We also describe a pro-pathogen role of macrophage-derived extracellular vesicles in C. neoformans infection and reinforce the impact of non-lytic exocytosis and the macrophage profile on the pathophysiology of cryptococcosis.

Amoeba predation of Cryptococcus: A quantitative and population genomic evaluation of the accidental pathogen hypothesis

The "Amoeboid Predator-Fungal Animal Virulence Hypothesis" posits that interactions with environmental phagocytes shape the evolution of virulence traits in fungal pathogens. In this hypothesis, selection to avoid predation by amoeba inadvertently selects for traits that contribute to fungal escape from phagocytic immune cells. Here, we investigate this hypothesis in the human fungal pathogens Cryptococcus neoformans and Cryptococcus deneoformans. Applying quantitative trait locus (QTL) mapping and comparative genomics, we discovered a cross-species QTL region that is responsible for variation in resistance to amoeba predation. In C. neoformans, this same QTL was found to have pleiotropic effects on melanization, an established virulence factor. Through fine mapping and population genomic comparisons, we identified the gene encoding the transcription factor Bzp4 that underlies this pleiotropic QTL and we show that decreased expression of this gene reduces melanization and increases susceptibility to amoeba predation. Despite the joint effects of BZP4 on amoeba resistance and melanin production, we find no relationship between BZP4 genotype and escape from macrophages or virulence in murine models of disease. Our findings provide new perspectives on how microbial ecology shapes the genetic architecture of fungal virulence, and suggests the need for more nuanced models for the evolution of pathogenesis that account for the complexities of both microbe-microbe and microbe-host interactions.

On predatory fungi feeding on free-living amoebae harbouring yeast-like endoparasites

Amoebae of the genus Vannella isolated from an ornamental fish aquarium were found to be infected with fungi. Upon plate culture, amoeba-trapping hyphal filaments were developed, and the amoeba trophozoites were found to harbour yeast-like parasites in their cytoplasm. Transfection of hyphae to a laboratory strain of Vannella resulted in the formation of conidia indicating the possible presence of zygomycetes of the genus Acaulopage, while efforts to culture the endoparasite remained unsuccessful. Biomolecular analysis based on rDNA revealed the presence of two distinct types of fungi, confirming the filamentous form as Acaulopage sp. (Zoopagomycota, Zoopagales) and identifying the yeast-like endoparasite as Cladosporium sp. (Ascomycota, Cladosporiales). To our knowledge, this is the first report of amoebae infected with Cladosporium.

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.

Long-Term Persistence of <i>Yersinia pestis</i> in Association with Acanthamoeba castellanii in Experiment

The aim of the study was to test the feasibility of long-term survival and preservation of the properties of Yersinia pestis in association with soil amoeba Acanthamoeba castellanii. Materials and methods. Y. pestis strains and acanthamoeba isolated in the common area of the Gorno-Altai high-mountain plague focus were used for the study. The systematic affiliation of protozoa was determined through analyzing the 18S rRNA gene fragment sequencing data, followed by alignment with amoeba sequences from the NCBI GenBank database. A fluorescent Y. pestis strain was obtained by electroporation using the pTurboGFP-B plasmid. Co-cultivation was carried out in saline buffer in the absence of nutrients for the cells of plague pathogen. The influence of co-culturing with protozoa on Y. pestis properties was determined using microbiological, biological, and molecular-genetic methods. Results and discussion. The cell viability preservation for 22 months of the experiment in Y. pestis strain belonging to the main subspecies of the antique biovar, the 4.ANT phylogenetic line in co-culture with amoeba cells in the absence of additional nutrients has been established. Co-cultivation with amoebae did not lead to a change in the cultural, morphological, genetic and virulent properties of the plague pathogen strain. The data obtained confirm the possibility of using Acanthamoeba castellanii by the plague microbe to persist in soil biocenoses and open up the prospect of studying the mechanisms of plague pathogen surviving during extended inter-epizootic periods.

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.

Isolation of a fungal calcineurin A mutant suggests that amoebae can counter-select virulence attributes of microbes

Evolutionary selection pressures that resulted in microbes found within environmental reservoirs that can cause diseases in animals are unknown. One hypothesis is that predatory organisms select microbes able to counteract animal immune cells. Here, a non-pathogenic yeast, Sporobolomyces primogenomicus, was exposed to predation by Acanthamoeba castellanii. Strains emerged that were resistant to being killed by this amoeba. All these strains had altered morphology, growing as pseudohyphae. The mutation in one strain was identified: CNA1 encodes the calcineurin A subunit that is highly conserved in fungi and where it is essential for their virulence in hosts including mammals, insects, and plants.

Interaction of Talaromyces marneffei with free living soil amoeba as a model of fungal pathogenesis

Talaromyces (Penicillium) marneffei is an important dimorphic mycosis endemic in Southeast Asia and Southern China, but the origin and maintenance of virulence traits in this organism remains obscure. Several pathogenic fungi, including Cryptococcus neoformans, Aspergillus fumigatus, Blastomyces dermatitidis, Sporothrix schenckii, Histoplasma capsulatum and Paracoccidioides spp. interact with free living soil amoebae and data suggests that fungal pathogenic strategies may emerge from environmental interactions of these fungi with ubiquitous phagocytic microorganisms. In this study, we examined the interactions of T. marneffei with the soil amoeba Acanthamoeba castellanii. T. marneffei was rapidly ingested by A. castellanii and phagocytosis of fungal cells resulted in amoeba death after 24 h of contact. Co-culture also resulted in a rapid transition for conidia to the fission-yeast form. In addition, well-established virulence factors such as melanin and a yeast specific mannoprotein of T. marneffei were expressed during interaction with A. castellanii at 37°C. Our findings support the assumption that soil amoebae environmental predators play a role in the selection and maintenance of particular features in T. marneffei that impart virulence to this clinically important dimorphic fungus in mammalian hosts.

Novel Environmental Niches for Candida auris: Isolation from a Coastal Habitat in Colombia

Climate change has been proposed as a set of factors that affect the frequency, distribution, and morbimortality of many infectious diseases, in which mycosis has also been impacted. Most fungi have the ability to cause disease in mammalian hosts as a result of their competitive fitness advantages that allow adaptation to diverse ecological niches. Candida auris has burst in the infectious disease scenario, and it has been hypothesized that a combination of stress adaptation and biotic predation has driven this fungus in the evolution of thermotolerance and halotolerance mechanisms to adapt to different environmental niches, which have resulted in the capacity to cross the thermal infection barrier in humans. Consequently, the isolation of C. auris from estuaries in Colombia adds to the evidence that suggests that this fungus existed in the environment previously to being recognized as a human pathogen, and promotes the need for further investigations to identify additional ecological niches.

Recognition of Cell Wall Mannosylated Components as a Conserved Feature for Fungal Entrance, Adaptation and Survival Within Trophozoites of Acanthamoeba castellanii and Murine Macrophages

Acanthamoeba castellanii (Ac) is a species of free-living amoebae (FLAs) that has been widely applied as a model for the study of host-parasite interactions and characterization of environmental symbionts. The sharing of niches between Ac and potential pathogens, such as fungi, favors associations between these organisms. Through predatory behavior, Ac enhances fungal survival, dissemination, and virulence in their intracellular milieu, training these pathogens and granting subsequent success in events of infections to more evolved hosts. In recent studies, our group characterized the amoeboid mannose binding proteins (MBPs) as one of the main fungal recognition pathways. Similarly, mannose-binding lectins play a key role in activating antifungal responses by immune cells. Even in the face of similarities, the distinct impacts and degrees of affinity of fungal recognition for mannose receptors in amoeboid and animal hosts are poorly understood. In this work, we have identified high-affinity ligands for mannosylated fungal cell wall residues expressed on the surface of amoebas and macrophages and determined the relative importance of these pathways in the antifungal responses comparing both phagocytic models. Mannose-purified surface proteins (MPPs) from both phagocytes showed binding to isolated mannose/mannans and mannosylated fungal cell wall targets. Although macrophage MPPs had more intense binding when compared to the amoeba receptors, the inhibition of this pathway affects fungal internalization and survival in both phagocytes. Mass spectrometry identified several MPPs in both models, and in silico alignment showed highly conserved regions between spotted amoeboid receptors (MBP and MBP1) and immune receptors (Mrc1 and Mrc2) and potential molecular mimicry, pointing to a possible convergent evolution of pathogen recognition mechanisms.

Evolution of the human pathogenic lifestyle in fungi

Fungal pathogens cause more than a billion human infections every year, resulting in more than 1.6 million deaths annually. Understanding the natural history and evolutionary ecology of fungi is helping us understand how disease-relevant traits have repeatedly evolved. Different types and mechanisms of genetic variation have contributed to the evolution of fungal pathogenicity and specific genetic differences distinguish pathogens from non-pathogens. Insights into the traits, genetic elements, and genetic and ecological mechanisms that contribute to the evolution of fungal pathogenicity are crucial for developing strategies to both predict emergence of fungal pathogens and develop drugs to combat them.

Phenotypic Diversity of C. neoformans var. neoformans Clinical Isolates from Localized and Disseminated Infections

Cryptococcus neoformans var. neoformans is the second most prevalent agent of cryptococcosis in central Europe. Infections mostly present with localized skin and disseminated infections. Previous studies did not find these presentations to be determined by the fungal genotype as detected by multilocus sequence typing (MLST). However, phenotypic fungal traits may impact clinical presentation. Here, we studied the growth and virulence factors of C. neoformans var. neoformans isolates from disseminated and localized infections and an environmental isolate. We used coincubation with Acanthamoeba castellanii and the Galleria mellonella infection model to identify phenotypic characteristics potentially associated with clinical presentation. Clinical isolates of C. neoformans var. neoformans present a substantial phenotypic variability. Median survival of G. mellonella varied between 6 and 14 days. C. neoformans var. neoformans isolates from disseminated infections showed stronger melanization and larger capsules. They demonstrated superior uptake into an amoeba and increased cytotoxicity for the amoeba. Differences of strains from localized and disseminated infections in coincubation with amoeba are in line with the importance of phagocytes in the pathogenesis of disseminated cryptococcosis. Phenotypic traits and non-vertebrate infection models may help understand the virulence potential of C. neoformans var. neoformans isolates.

Adaptation to an amoeba host leads to Pseudomonas aeruginosa isolates with attenuated virulence

The opportunistic pathogen Pseudomonas aeruginosa is ubiquitous in the environment, and in humans, it is capable of causing acute or chronic infections. In the natural environment, predation by bacterivorous protozoa represents a primary threat to bacteria. Here, we determined the impact of long-term exposure of P. aeruginosa to predation pressure. P. aeruginosa persisted when coincubated with the bacterivorous Acanthamoeba castellanii for extended periods and produced genetic and phenotypic variants. Sequencing of late-stage amoeba-adapted P. aeruginosa isolates demonstrated single nucleotide polymorphisms within genes that encode known virulence factors, and this correlated with a reduction in expression of virulence traits. Virulence for the nematode Caenorhabditis elegans was attenuated in late-stage amoeba-adapted P. aeruginosa compared to early-stage amoeba-adapted and nonadapted counterparts. Further, late-stage amoeba-adapted P. aeruginosa showed increased competitive fitness and enhanced survival in amoebae as well as in macrophage and neutrophils. Interestingly, our findings indicate that the selection imposed by amoebae resulted in P. aeruginosa isolates with reduced virulence and enhanced fitness, similar to those recovered from chronic cystic fibrosis infections. Thus, predation by protozoa and long-term colonization of the human host may represent similar environments that select for similar losses of gene function.

IMPORTANCEPseudomonas aeruginosa is an opportunistic pathogen that causes both acute infections in plants and animals, including humans, and chronic infections in immunocompromised and cystic fibrosis patients. This bacterium is commonly found in soils and water, where bacteria are constantly under threat of being consumed by bacterial predators, e.g., protozoa. To escape being killed, bacteria have evolved a suite of mechanisms that protect them from being consumed or digested. Here, we examined the effect of long-term predation on the genotypes and phenotypes expressed by P. aeruginosa. We show that long-term coincubation with protozoa gave rise to mutations that resulted in P. aeruginosa becoming less pathogenic. This is particularly interesting as similar mutations arise in bacteria associated with chronic infections. Importantly, the genetic and phenotypic traits possessed by late-stage amoeba-adapted P. aeruginosa are similar to those observed in isolates obtained from chronic cystic fibrosis infections. This notable overlap in adaptation to different host types suggests similar selection pressures among host cell types as well as similar adaptation strategies.

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.

Inositol Metabolism Regulates Capsule Structure and Virulence in the Human Pathogen Cryptococcus neoformans

The environmental yeast Cryptococcus neoformans is the most common cause of deadly fungal meningitis in primarily immunocompromised populations. A number of factors contribute to cryptococcal pathogenesis. Among them, inositol utilization has been shown to promote C. neoformans development in nature and invasion of central nervous system during dissemination. The mechanisms of the inositol regulation of fungal virulence remain incompletely understood. In this study, we analyzed inositol-induced capsule growth and the contribution of a unique inositol catabolic pathway in fungal development and virulence. We found that genes involved in the inositol catabolic pathway are highly induced by inositol, and they are also highly expressed in the cerebrospinal fluid of patients with meningoencephalitis. This pathway in C. neoformans contains three genes encoding myo-inositol oxygenases that convert myo-inositol into d-glucuronic acid, a substrate of the pentose phosphate cycle and a component of the polysaccharide capsule. Our mutagenesis analysis demonstrates that inositol catabolism is required for C. neoformans virulence and deletion mutants of myo-inositol oxygenases result in altered capsule growth as well as the polysaccharide structure, including O-acetylation. Our study indicates that the ability to utilize the abundant inositol in the brain may contribute to fungal pathogenesis in this neurotropic fungal pathogen. IMPORTANCE The human pathogen Cryptococcus neoformans is the leading cause of fungal meningitis in primarily immunocompromised populations. Understanding how this environmental organism adapts to the human host to cause deadly infection will guide our development of novel disease control strategies. Our recent studies revealed that inositol utilization by the fungus promotes C. neoformans development in nature and invasion of the central nervous system during infection. The mechanisms of the inositol regulation in fungal virulence remain incompletely understood. In this study, we found that C. neoformans has three genes encoding myo-inositol oxygenase, a key enzyme in the inositol catabolic pathway. Expression of these genes is highly induced by inositol, and they are highly expressed in the cerebrospinal fluid of patients with meningoencephalitis. Our mutagenesis analysis indeed demonstrates that inositol catabolism is required for C. neoformans virulence by altering the growth and structure of polysaccharide capsule, a major virulence factor. Considering the abundance of free inositol and inositol-related metabolites in the brain, our study reveals an important mechanism of host inositol-mediated fungal pathogenesis for this neurotropic fungal pathogen.

Biosynthesis, evolution and ecology of microbial terpenoids

Covering: through June 2021Terpenoids are the largest class of natural products recognised to date. While mostly known to humans as bioactive plant metabolites and part of essential oils, structurally diverse terpenoids are increasingly reported to be produced by microorganisms. For many of the compounds biological functions are yet unknown, but during the past years significant insights have been obtained for the role of terpenoids in microbial chemical ecology. Their functions include stress alleviation, maintenance of cell membrane integrity, photoprotection, attraction or repulsion of organisms, host growth promotion and defense. In this review we discuss the current knowledge of the biosynthesis and evolution of microbial terpenoids, and their ecological and biological roles in aquatic and terrestrial environments. Perspectives on their biotechnological applications, knowledge gaps and questions for future studies are discussed.

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.

Microbial warfare in the wild-the impact of protists on the evolution and virulence of bacterial pathogens

During the long history of co-evolution with protists, bacteria have evolved defense strategies to avoid grazing and survive phagocytosis. These mechanisms allow bacteria to exploit phagocytic cells as a protective niche in which to escape from environmental stress and even replicate. Importantly, these anti-grazing mechanisms can function as virulence factors when bacteria infect humans. Here, we discuss how protozoan predation exerts a selective pressure driving bacterial virulence and shaping their genomes, and how bacteria-protist interactions might contribute to the spread of antibiotic resistance as well. We provide examples to demonstrate that besides being voracious bacterial predators, protozoa can serve as melting pots where intracellular organisms exchange genetic information, or even "training grounds" where some pathogens become hypervirulent after passing through. In this special issue, we would like to emphasize the tremendous impact of bacteria-protist interactions on human health and the potential of amoebae as model systems to study biology and evolution of a variety of pathogens. Besides, a better understanding of bacteria-protist relationships will help us expand our current understanding of bacterial virulence and, likely, how pathogens emerge.

The Consequences of Our Changing Environment on Life Threatening and Debilitating Fungal Diseases in Humans

Human activities have significantly impacted the environment and are changing our climate in ways that will have major consequences for ourselves, and endanger animal, plant and microbial life on Earth. Rising global temperatures and pollution have been highlighted as potential drivers for increases in infectious diseases. Although infrequently highlighted, fungi are amongst the leading causes of infectious disease mortality, resulting in more than 1.5 million deaths every year. In this review we evaluate the evidence linking anthropomorphic impacts with changing epidemiology of fungal disease. We highlight how the geographic footprint of endemic mycosis has expanded, how populations susceptible to fungal infection and fungal allergy may increase and how climate change may select for pathogenic traits and indirectly contribute to the emergence of drug resistance.

Amoeba Predation of Cryptococcus neoformans Results in Pleiotropic Changes to Traits Associated with Virulence

Amoeboid predators, such as amoebae, are proposed to select for survival traits in soil microbes such as Cryptococcus neoformans; these traits can also function in animal virulence by defeating phagocytic immune cells, such as macrophages. Consistent with this notion, incubation of various fungal species with amoebae enhanced their virulence, but the mechanisms involved are unknown. In this study, we exposed three strains of C. neoformans (1 clinical and 2 environmental) to predation by Acanthamoeba castellanii for prolonged times and then analyzed surviving colonies phenotypically and genetically. Surviving colonies comprised cells that expressed either pseudohyphal or yeast phenotypes, which demonstrated variable expression of traits associated with virulence, such as capsule size, urease production, and melanization. Phenotypic changes were associated with aneuploidy and DNA sequence mutations in some amoeba-passaged isolates, but not in others. Mutations in the gene encoding the oligopeptide transporter (CNAG_03013; OPT1) were observed among amoeba-passaged isolates from each of the three strains. Isolates derived from environmental strains gained the capacity for enhanced macrophage toxicity after amoeba selection and carried mutations on the CNAG_00570 gene encoding Pkr1 (AMP-dependent protein kinase regulator) but manifested reduced virulence in mice because they elicited more effective fungal-clearing immune responses. Our results indicate that C. neoformans survival under constant amoeba predation involves the generation of strains expressing pleiotropic phenotypic and genetic changes. Given the myriad potential predators in soils, the diversity observed among amoeba-selected strains suggests a bet-hedging strategy whereby variant diversity increases the likelihood that some will survive predation.IMPORTANCECryptococcus neoformans is a ubiquitous environmental fungus that is also a leading cause of fatal fungal infection in humans, especially among immunocompromised patients. A major question in the field is how an environmental yeast such as C. neoformans becomes a human pathogen when it has no need for an animal host in its life cycle. Previous studies showed that C. neoformans increases its pathogenicity after interacting with its environmental predator amoebae. Amoebae, like macrophages, are phagocytic cells that are considered an environmental training ground for pathogens to resist macrophages, but the mechanism by which C. neoformans changes its virulence through interactions with protozoa is unknown. Our study indicates that fungal survival in the face of amoeba predation is associated with the emergence of pleiotropic phenotypic and genomic changes that increase the chance of fungal survival, with this diversity suggesting a bet-hedging strategy to ensure that some forms survive.

Environmental Candida auris and the Global Warming Emergence Hypothesis

Global warming was proposed to be a contributing cause for the nearly simultaneous emergence of different clades of Candida auris as a nosocomial pathogen in different continents. The global warming emergence hypothesis posits that C. auris existed in the environment prior to its clinical recognition and became pathogenic for humans because of thermal adaptation in response to climate change. The isolation of C. auris from two sites in the remote Andaman Islands establishes it as an environmental organism, a necessary condition for the hypothesis.

Global warming was proposed to be a contributing cause for the nearly simultaneous emergence of different clades of Candida auris as a nosocomial pathogen in different continents. The global warming emergence hypothesis posits that C. auris existed in the environment prior to its clinical recognition and became pathogenic for humans because of thermal adaptation in response to climate change. The isolation of C. auris from two sites in the remote Andaman Islands establishes it as an environmental organism, a necessary condition for the hypothesis. The observation that one environmental isolate grew slower at mammalian temperatures than clinical strains is consistent with the notion that their ancestor recently adapted to higher temperatures. The knowledge that C. auris can be recovered from the environment should prompt additional searches to define its ecological niches, and the analysis of future environmental isolates will provide evidence for validating or refuting the global warming emergence hypothesis.

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.

Cryptococcus in Wildlife and Free-Living Mammals

Cryptococcosis is typically a sporadic disease that affects a broad range of animal species globally. Disease is a consequence of infection with members of the Cryptococcus neoformans or Cryptococcus gattii species complexes. Although cryptococcosis in many domestic animals has been relatively well-characterized, free-living wildlife animal species are often neglected in the literature outside of occasional case reports. This review summarizes the clinical presentation, pathological findings and potential underlying causes of cryptococcosis in various other animals, including terrestrial wildlife species and marine mammals. The evaluation of the available literature supports the hypothesis that anatomy (particularly of the respiratory tract), behavior and environmental exposures of animals play vital roles in the outcome of host–pathogen–environment interactions resulting in different clinical scenarios. Key examples range from koalas, which exhibit primarily C. gattii species complex disease presumably due to their behavior and environmental exposure to eucalypts, to cetaceans, which show predominantly pulmonary lesions due to their unique respiratory anatomy. Understanding the factors at play in each clinical scenario is a powerful investigative tool, as wildlife species may act as disease sentinels.

Effects of Acanthamoeba castellanii on the dissolved oxygen and the microbial community under the experimental aquatic model

Acanthamoeba castellanii is a protist that has a high predation efficiency for bacteria in a number of monoxenic culture experiments. However, the role of A. castellanii in the microbial community is still unknown because of the lack of studies on multiple-species interactions. The aim of this study was to investigate the change of bacterial composition after A. castellanii emerges in a water environment. We added A. castellanii to an environmental water sample and incubated it for two days. Then, we performed 16S ribosomal RNA sequencing techniques to analyze the changes in bacterial composition. In this study, A. castellanii slightly increased the relative abundance of a few opportunistic pathogens, such as Legionella, Roseomonas, and Haemophilus. This result may be related to the training ground hypothesis. On the other hand, the growth of some bacteria was inhibited, such as Cyanobacteria and Firmicutes. Although A. castellanii did not drastically change the whole bacterial community, we surprisingly found the dissolved oxygen concentration was increased after incubation with A. castellanii. We applied environmental water at the laboratory scale to investigate the interactions among A. castellanii, complex microbial communities and the environment. We identified the bacteria that are sensitive to A. castellanii and further found the novel relationship between dissolved oxygen and microbial interaction. Our results helped to clarify the role of A. castellanii in microbial communities.

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.

Variation in Cell Surface Hydrophobicity among Cryptococcus neoformans Strains Influences Interactions with Amoebas

Cryptococcus neoformans and Cryptococcus gattii are pathogenic fungi that cause significant morbidity and mortality. Cell surface hydrophobicity (CSH) is a biophysical parameter that influences the adhesion of fungal cells or spores to biotic and abiotic surfaces. C. neoformans is encased by polysaccharide capsule that is highly hydrophilic and is a critical determinant of virulence. In this study, we report large differences in the CSH of some C. neoformans and C. gattii strains. The capsular polysaccharides of C. neoformans strains differ in repeating motifs and therefore vary in the number of hydroxyl groups, which, along with higher-order structure of the capsule, may contribute to the variation in hydrophobicity that we observed. We found that cell wall composition, in the context of chitin-chitosan content, does not influence CSH. For C. neoformans, CSH correlated with phagocytosis by natural soil predator Acanthamoeba castellanii Furthermore, capsular binding of the protective antibody (18B7), but not the nonprotective antibody (13F1), altered the CSH of C. neoformans strains. Variability in CSH could be an important characteristic in comparing the biological properties of cryptococcal strains.IMPORTANCE The interaction of a microbial cell with its environment is influenced by the biophysical properties of a cell. The affinity of the cell surface for water, defined by the cell surface hydrophobicity (CSH), is a biophysical parameter that varies among different strains of Cryptococcus neoformans The CSH influences the phagocytosis of the yeast by its natural predator in the soil, the amoeba. Studying variation in biophysical properties like CSH gives us insight into the dynamic host-predator interaction and host-pathogen interaction in a damage-response framework.

Pathogen and host genetics underpinning cryptococcal disease

Cryptococcosis is a severe fungal disease causing 220,000 cases of cryptococcal meningitis yearly. The etiological agents of cryptococcosis are taxonomically grouped into at least two species complexes belonging to the genus Cryptococcus. All of these yeasts are environmentally ubiquitous fungi (often found in soil, leaves and decaying wood, tree hollows, and associated with bird feces especially pigeon guano). Infection in a range of animals including humans begins following inhalation of spores or aerosolized yeasts. Recent advances provide fundamental insights into the factors from both the pathogen and its hosts which influence pathogenesis and disease. The complex interactions leading to disease in mammalian hosts have also updated from the availability of better genomic tools and datasets. In this review, we discuss recent genetic research on Cryptococcus, covering the epidemiology, ecology, and evolution of Cryptococcus pathogenic species. We also discuss the insights into the host immune response obtained from the latest genetic modified host models as well as insights from monogenic disorders in humans. Finally we highlight outstanding questions that can be answered in the near future using bioinformatics and genomic tools.

Legionnaires’ Disease: State of the Art Knowledge of Pathogenesis Mechanisms of Legionella

Legionella species are environmental gram-negative bacteria able to cause a severe form of pneumonia in humans known as Legionnaires’ disease. Since the identification of Legionella pneumophila in 1977, four decades of research on Legionella biology and Legionnaires’ disease have brought important insights into the biology of the bacteria and the molecular mechanisms that these intracellular pathogens use to cause disease in humans. Nowadays, Legionella species constitute a remarkable model of bacterial adaptation, with a genus genome shaped by their close coevolution with amoebae and an ability to exploit many hosts and signaling pathways through the secretion of a myriad of effector proteins, many of which have a eukaryotic origin. This review aims to discuss current knowledge of Legionella infection mechanisms and future research directions to be taken that might answer the many remaining open questions. This research will without a doubt be a terrific scientific journey worth taking.

Polymicrobial interactions involving fungi and their importance for the environment and in human disease

Understanding polymicrobial interactions involving fungi in the environment and the human mycobiome is necessary to address environmental and medically related problems such as drought or antimicrobial resistance. The diversity of these interactions highlights the complexity of fungi, considering how some interactions can be antagonistic, while others synergistic. Over the years, an increase in studies on the mycobiome have revealed similarities between the human and environmental hosts. More recently, studies have focused on microbial commensal relationships and identifying causative agents of human disease. The overlap of some of these interactions is impossible to ignore, indicating that there are areas for medical exploitation that need to be further investigated. This review provides the latest advances in polymicrobial interactions involving fungi and discusses the importance of the fungal lifestyle in the environment and in human disease.

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.

On the Emergence of Cryptococcus gattii in the Pacific Northwest: Ballast Tanks, Tsunamis, and Black Swans

The appearance of Cryptococcus gattii in the North American Pacific Northwest (PNW) in 1999 was an unexpected and is still an unexplained event. Recent phylogenomic analyses strongly suggest that this pathogenic fungus arrived in the PNW approximately 7 to 9 decades ago. In this paper, we theorize that the ancestors of the PNW C. gattii clones arrived in the area by shipborne transport, possibly in contaminated ballast, and established themselves in coastal waters early in the 20th century. In 1964, a tsunami flooded local coastal regions, transporting C. gattii to land. The occurrence of cryptococcosis in animals and humans 3 decades later suggests that adaptation to local environs took time, possibly requiring an increase in virulence and further dispersal. Tsunamis as a mechanism for the seeding of land with pathogenic waterborne microbes may have important implications for our understanding of how infectious diseases emerge in certain regions. This hypothesis suggests experimental work for its validation or refutation.

Benefits and Costs of Animal Virulence for Microbes

This essay is written from the vantage point of the microbial world. While the focus of much thought in the microbial pathogenesis and infectious diseases fields has been on the impact of host-microbe interaction on the host, here we ask questions about what happens to the microbe.

This essay is written from the vantage point of the microbial world. While the focus of much thought in the microbial pathogenesis and infectious diseases fields has been on the impact of host-microbe interaction on the host, here we ask questions about what happens to the microbe. What are the costs and benefits for microbes of having the capacity for virulence? Our exploration of this topic leads us to conclude that virulence confers very few benefits for microbes, unless disease is necessary for microbial survival through host-to-host spread. In fact, the capacity for virulence is often fraught with risk for microbes, including host dependence and the threat of extinction. The costs of virulence may explain why, relative to their enormous numbers in nature, very few microbes are actually associated with human and animal disease.

Intracellular parasitism, the driving force of evolution of Legionella pneumophila and the genus Legionella

Legionella pneumophila is an intracellular pathogen that causes a severe pneumonia called Legionnaires' disease that is often fatal when not promptly diagnosed and treated. However, L. pneumophila is mainly an environmental pathogen of protozoa. This bacterium parasitizes free-living amoeba and other aquatic protozoa with which it co-evolved over an evolutionary long time. Due to the close relationship between hosts and pathogens, their co-evolution leads to molecular interactions such as the exchange of genetic material through horizontal gene transfer (HGT). Those genes that confer an advantage to the bacteria were fixed in their genomes and help these pathogens to subvert host functions to their advantage. Genome sequencing of L. pneumophila and recently of the entire genus Legionella that comprises over 60 species revealed that Legionellae have co-opted genes and thus cellular functions from their eukaryotic hosts to a surprisingly high extent never observed before for an prokaryotic organism. Acquisition and loss of these eukaryotic-like genes and eukaryotic domains is an ongoing process underlining the highly dynamic nature of the Legionella genomes. Although the large amount and diversity of HGT that occurred between Legionella and their protozoan hosts seems to be unique in the prokaryotic world, the analyses of more and more genomes from environmental organisms and symbionts of amoeba revealed that such genetic exchanges occur among all amoeba-associated bacteria and also among the different microorganisms that infect amoeba such as viruses. This dynamic reshuffling and gene-acquisition has led to the emergence of major human pathogens such as Legionella and may lead to the emergence of new human pathogens from the environment.

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.

Current Status and Trends in Alternative Models to Study Fungal Pathogens

Fungal infections affect over a billion people, with mortality rates estimated at 1⁻2 million per year [...].