Contribution of Fdh3 and Glr1 to Glutathione Redox State, Stress Adaptation and Virulence in Candida albicans

Glutathione is required for growth and cadmium tolerance in the amphibian chytrid fungus, Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidis (Bd) is a lethal amphibian pathogen, partly due to its ability to evade the immune system of susceptible frog species. In many pathogenic fungi, the antioxidant glutathione is a virulence factor that helps neutralise oxidative stressors generated from host immune cells, as well as other environmental stressors such as heavy metals. The role of glutathione in stress tolerance in Bd has not been investigated. Here, we examine the changes in the glutathione pool after stress exposure and quantify the effect of glutathione depletion on cell growth and stress tolerance. Depletion of glutathione repressed growth and release of zoospores, suggesting that glutathione is essential for life cycle completion in Bd. Supplementation with <2 mM exogenous glutathione accelerated zoospore development, but concentrations >2 mM were strongly inhibitory to Bd cells. While hydrogen peroxide exposure lowered the total cellular glutathione levels by 42 %, glutathione depletion did not increase the sensitivity to hydrogen peroxide. Exposure to cadmium increased total cellular glutathione levels by 93 %. Glutathione-depleted cells were more sensitive to cadmium, and this effect was attenuated by glutathione supplementation, suggesting that glutathione plays an important role in cadmium tolerance. The effects of heat and salt were exacerbated by the addition of exogenous glutathione. The impact of glutathione levels on Bd stress sensitivity may help explain differences in host susceptibility to chytridiomycosis and may provide opportunities for synergistic therapeutics.

Insight into metabolic sensors of nitrosative stress protection in Phytophthora infestans

Phytophthora infestans, a representative of phytopathogenic oomycetes, have been proven to cope with redundant sources of internal and host-derived reactive nitrogen species (RNS). To gain insight into its nitrosative stress resistance mechanisms, metabolic sensors activated in response to nitrosative challenge during both in vitro growth and colonization of the host plant were investigated. The conducted analyses of gene expression, protein accumulation, and enzyme activity reveal for the first time that P. infestans (avirulent MP946 and virulent MP977 toward potato cv. Sarpo Mira) withstands nitrosative challenge and has an efficient system of RNS elimination. The obtained data indicate that the system protecting P. infestans against nitric oxide (NO) involved the expression of the nitric oxide dioxygenase (Pi-NOD1) gene belonging to the globin family. The maintenance of RNS homeostasis was also supported by an elevated S-nitrosoglutathione reductase activity and upregulation of peroxiredoxin 2 at the transcript and protein levels; however, the virulence pattern determined the expression abundance. Based on the experiments, it can be concluded that P. infestans possesses a multifarious system of metabolic sensors controlling RNS balance via detoxification, allowing the oomycete to exist in different micro-environments flexibly.

Fungal infections: Immune defense, immunotherapies and vaccines

Invasive fungal infection is an under recognized and emerging global health threat. Recently, the World Health Organization (WHO) released the first ever list of health-threatening fungi to guide research and public health interventions to strengthen global response to fungi infections and antifungal resistance. Currently, antifungal drugs only demonstrate partial success in improving prognosis of infected patients, and this is compounded by the rapid evolution of drug resistance among fungi species. The increased prevalence of fungal infections in individuals with underlying immunological deficiencies reflects the importance of an intact host immune system in controlling mycoses, and further highlights immunomodulation as a potential new avenue for the treatment of disseminated fungal diseases. In this review, we will summarize how host innate immune cells sense invading fungi through their pattern recognition receptors, and subsequently initiate a series of effector mechanisms and adaptive immune responses to mediate fungal clearance. In addition, we will discuss emerging preclinical and clinical data on antifungal immunotherapies and fungal vaccines which can potentially expand our antifungal armamentarium in future.

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 Role of the Glutathione System in Stress Adaptation, Morphogenesis and Virulence of Pathogenic Fungi

Morphogenesis and stress adaptation are key attributes that allow fungal pathogens to thrive and infect human hosts. During infection, many fungal pathogens undergo morphological changes, and this ability is highly linked to virulence. Furthermore, pathogenic fungi have developed multiple antioxidant defenses to cope with the host-derived oxidative stress produced by phagocytes. Glutathione is a major antioxidant that can prevent cellular damage caused by various oxidative stressors. While the role of glutathione in stress detoxification is known, studies of the glutathione system in fungal morphological switching and virulence are lacking. This review explores the role of glutathione metabolism in fungal adaptation to stress, morphogenesis, and virulence. Our comprehensive analysis of the fungal glutathione metabolism reveals that the role of glutathione extends beyond stressful conditions. Collectively, glutathione and glutathione-related proteins are necessary for vitality, cellular development and pathogenesis.

Fungal consortium of two Beauveria bassiana strains increases their virulence, growth, and resistance to stress: A metabolomic approach

The use of two or more microorganisms in a microbial consortium has been increasingly applied in the biological control of diseases and pests. Beauveria bassiana is one of the most widely studied fungal species in biological control, yet little is known about its role in fungal consortiums. In a previous study, our group found that a consortium formed by two strains of B. bassiana had significantly greater biocontrol potential against the polyphagous caterpillars Duponchelia fovealis (Lepidoptera: Crambidae) than either strain on its own. In this study, we use GC-MS and LC-MS/MS to evaluate and discuss the metabolomics of the consortium. A total of 21 consortium biomarkers were identified, corresponding to 14 detected by LC-MS/MS and seven by GC-MS. Antioxidant and anti-inflammatory mechanisms are the main properties of the metabolites produced by the consortium. These metabolites can depress the insect’s immune system, increasing its vulnerability and, hence, the fungal virulence of the consortium. In light of these results, we propose an action model of insect mortality due to the metabolites secreted by the consortium. The model includes the inhibition of defense mechanisms such as pro-inflammatory interleukin secretion, cell migration, cell aggregation, Dif, Dorsal and Relish gene transcription, and JAK/STAT and JNK signaling pathways. It also promotes the cleaning of oxidative molecules, like ROS, NOS, and H₂O₂, and the induction of virulence factors.

The Interactions Between Candida albicans and Mucosal Immunity

Mucosa protects the body against external pathogen invasion. However, pathogen colonies on the mucosa can invade the mucosa when the immunosurveillance is compromised, causing mucosal infection and subsequent diseases. Therefore, it is necessary to timely and effectively monitor and control pathogenic microorganisms through mucosal immunity. Candida albicans is the most prevalent fungi on the mucosa. The C. albicans colonies proliferate and increase their virulence, causing severe infectious diseases and even death, especially in immunocompromised patients. The normal host mucosal immune defense inhibits pathogenic C. albicans through stepwise processes, such as pathogen recognition, cytokine production, and immune cell phagocytosis. Herein, the current advances in the interactions between C. albicans and host mucosal immune defenses have been summarized to improve understanding on the immune mechanisms against fungal infections.

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.

Transcriptional response of Candida albicans to Pseudomonas aeruginosa in a polymicrobial biofilm

Candida albicans is frequently co-isolated with the Gram-negative bacterium, Pseudomonas aeruginosa. In vitro, the interaction is complex, with both species influencing each other. Not only does the bacterium kill hyphal cells of C. albicans through physical interaction, it also affects C. albicans biofilm formation and morphogenesis, through various secreted factors and cell wall components. The present study sought to expand the current knowledge regarding the interaction between C. albicans and P. aeruginosa, using transcriptome analyses of early static biofilms. Under these conditions, a total of 2,537 open reading frames (approximately 40% of the C. albicans transcriptome) was differentially regulated in the presence of P. aeruginosa. Upon deeper analyses it became evident that the response of C. albicans toward P. aeruginosa was dominated by a response to hypoxia, and included those associated with stress as well as iron and zinc homeostasis. These conditions may also lead to the observed differential regulation of genes associated with cell membrane synthesis, morphology, biofilm formation and phenotypic switching. Thus, C. albicans in polymicrobial biofilms with P. aeruginosa have unique transcriptional profiles that may influence commensalism as well as pathogenesis.

Fungal immunity and pathogenesis in mammals versus the invertebrate model organism Galleria mellonella

In recent decades, Galleria mellonella (Lepidoptera: Pyralidae) have emerged as a model system to explore experimental aspects of fungal pathogenesis. The benefits of the G. mellonella model include being faster, cheaper, higher throughput and easier compared with vertebrate models. Additionally, as invertebrates, their use is subject to fewer ethical and regulatory issues. However, for G. mellonella models to provide meaningful insight into fungal pathogenesis, the G. mellonella-fungal interactions must be comparable to mammalian-fungal interactions. Indeed, as discussed in the review, studies suggest that G. mellonella and mammalian immune systems share many similarities, and fungal virulence factors show conserved functions in both hosts. While the moth model has opened novel research areas, many comparisons are superficial and leave large gaps of knowledge that need to be addressed concerning specific mechanisms underlying G. mellonella-fungal interactions. Closing these gaps in understanding will strengthen G. mellonella as a model for fungal virulence in the upcoming years. In this review, we provide comprehensive comparisons between fungal pathogenesis in mammals and G. mellonella from immunological and virulence perspectives. When information on an antifungal immune component is unknown in G. mellonella, we include findings from other well-studied Lepidoptera. We hope that by outlining this information available in related species, we highlight areas of needed research and provide a framework for understanding G. mellonella immunity and fungal interactions.

A first insight into the genome of Prototheca wickerhamii, a major causative agent of human protothecosis

BACKGROUND

Colourless microalgae of the Prototheca genus are the only known plants that have consistently been implicated in a range of clinically relevant opportunistic infections in both animals and humans. The Prototheca algae are emerging pathogens, whose incidence has increased importantly over the past two decades. Prototheca wickerhamii is a major human pathogen, responsible for at least 115 cases worldwide. Although the algae are receiving more attention nowadays, there is still a substantial knowledge gap regarding their biology, and pathogenicity in particular. Here we report, for the first time, the complete nuclear genome, organelle genomes, and transcriptome of the P. wickerhamii type strain ATCC 16529.

RESULTS

The assembled genome size was of 16.7 Mbp, making it the smallest and most compact genome sequenced so far among the protothecans. Key features of the genome included a high overall GC content (64.5%), a high number (6081) and proportion (45.9%) of protein-coding genes, and a low repetitive sequence content (2.2%). The vast majority (90.6%) of the predicted genes were confirmed with the corresponding transcripts upon RNA-sequencing analysis. Most (93.2%) of the genes had their putative function assigned when searched against the InterProScan database. A fourth (23.3%) of the genes were annotated with an enzymatic activity possibly associated with the adaptation to the human host environment. The P. wickerhamii genome encoded a wide array of possible virulence factors, including those already identified in two model opportunistic fungal pathogens, i.e. Candida albicans and Trichophyton rubrum, and thought to be involved in invasion of the host or elicitation of the adaptive stress response. Approximately 6% of the P. wickerhamii genes matched a Pathogen-Host Interaction Database entry and had a previously experimentally proven role in the disease development. Furthermore, genes coding for proteins (e.g. ATPase, malate dehydrogenase) hitherto considered as potential virulence factors of Prototheca spp. were demonstrated in the P. wickerhamii genome.

CONCLUSIONS

Overall, this study is the first to describe the genetic make-up of P. wickerhamii and discovers proteins possibly involved in the development of protothecosis.

An NADPH-independent mechanism enhances oxidative and nitrosative stress tolerance in yeast cells lacking glucose-6-phosphate dehydrogenase activity

The reduced form of nicotinamide adenine dinucleotide phosphate (NADPH), which is required for various redox systems involving antioxidative stress enzymes, is thus important for stress tolerance mechanisms. Here, we analyzed the stress response of the NADPH-depleted cells of Saccharomyces cerevisiae. A cell viability assay showed that the NADPH depletion induced by disruption of the ZWF1 gene encoding glucose-6-phosphate dehydrogenase, which is the major determinant of the intracellular NADPH/NADP⁺ ratio, enhanced the tolerance of S. cerevisiae to both oxidative and nitrosative stresses. The subsequent analyses demonstrated that the antioxidative transcriptional factor Yap1 was activated and the cytosolic catalase Ctt1, whose expression is regulated by Yap1, was upregulated in zwf1Δ cells irrespective of the presence or absence of stress stimuli. Moreover, deletion of the YAP1 or CTT1 gene inhibited the increased stress tolerance of zwf1Δ cells, indicating that Ctt1 dominantly contributed to the higher stress tolerance of zwf1Δ cells. Our findings suggest that an NADPH-independent mechanism enhances oxidative and nitrosative stress tolerance in ZWF1-lacking yeast cells.

The critical role of MetR/MetB/MetC/MetX in cysteine and methionine metabolism, fungal development and virulence of Alternaria alternata

Methionine is a unique sulfur-containing amino acid, which plays an important role in biological protein synthesis and various cellular processes. Here, we characterized the biological functions of AaMetB, AaMetC, and AaMetX in the tangerine pathotype of Alternaria alternata Morphological analysis showed that the mutants lacking AaMetB, AaMetC, or AaMetX resulted in less aerial hypha and fewer conidia in artificial media. Pathogenicity analysis showed that AaMetB, AaMetC, and AaMetX are required for full virulence. The defects in vegetative growth, conidiation and virulence of ΔMetB, ΔMetC, and ΔMetX can be restored by exogenous methionine and homocysteine, indicating that AaMetB, AaMetC, and AaMetX are required for methionine biosynthesis. However, exogenous cysteine only restored the growth and virulence defects of ΔMetR but not ΔMetB/C/X, suggesting that AaMetR is essential for cysteine biosynthesis. Oxidant sensitivity assay showed that only ΔMetR is sensitive to H₂O₂ and many ROS-generating compounds, indicating that AaMetR is essential for oxidative tolerance. Interestingly, fungicides indoor bioassays showed that only the ΔMetR mutants are susceptive to chlorothalonil, a fungicide that could bind to the cysteine of glyceraldehyde-3-phosphate dehydrogenase. Comparative transcriptome analysis showed that the inactivation of MetB, MetC, MetX, or MetR significantly affected the expression of methionine metabolism-related genes. Moreover, the inactivation of AaMetR significantly affected the expression of many genes related to glutathione metabolism, which is essential for ROS tolerance. Taken together, our study provides genetic evidence to define the critical roles of AaMetB, AaMetC, AaMetX, and AaMetR in cysteine and methionine metabolism, fungal development and virulence of Alternaria alternata IMPORTANCE The transcription factor METR regulating methionine metabolism is essential for reactive oxygen species (ROS) tolerance and virulence in many phytopathogenic fungi. However, the underlying regulatory mechanism of METR involved in this process is still unclear. In the present study, we generated AaMetB, AaMetC and AaMetX deletion mutants and compared these mutants with AaMetR disrupted mutants. Interestingly, we found that AaMetB, AaMetC and AaMetX are required for vegetative growth, conidiation, and pathogenicity in Alternaria alternata, but not for ROS tolerance and cysteine metabolism. Furthermore, we found that METR is involved in the biosynthesis of cysteine, which is an essential substrate for the biosynthesis of methionine and glutathione. This study emphasizes the critical roles of MetR, MetB, MetC, MetX in the regulation of cysteine and methionine metabolism, as well as the cross-link with glutathione-mediated ROS tolerance in phytopathogenic fungi, which provides a foundation for future investigations.

Shifts in microbial community, pathogenicity-related genes and antibiotic resistance genes during dairy manure piled up

The uncomposted faeces of dairy cow are usually stacked on cow breeding farms, dried under natural conditions and then used as cow bedding material or they may be continuously piled up. However, no information is available to evaluate variations in the human and animal pathogen genes and antibiotic resistance during the accumulation of fresh faeces of dairy cow to manure. Here, we present the metagenomic analysis of fresh faeces and manure from a dairy farm in Ning Xia, showing a unique enrichment of human and animal pathogen genes and antibiotic resistance genes (ARGs) in manure. We found that manure accumulation could significantly increase the diversity and abundance of the pathogenic constituents. Furthermore, pathogens from manure could spread to the plant environment and enphytotic pathogens could affect the yield and quality of crops during the use of manure as a fertilizer. Levels of virulence genes and ARGs increased with the enrichment of microbes and pathogens when faeces accumulated to manure. Accumulated manure was also the transfer station of ARGs to enrich the ARGs in the environment, indicating the ubiquitous presence of environmental antibiotic resistance genes. Our results demonstrate that manure accumulation and usage without effective manure management is an unreasonable approach that could enrich pathogenic microorganisms and ARGs in the environment. The manure metagenome structure allows us to appreciate the overall influence and interaction of animal waste on water, soil and other areas impacted by faecal accumulation and the factors that influence pathogen occurrence in products from dairy cows.

Transcript profiling reveals potential regulators for oxidative stress response of a necrotrophic chickpea pathogen Ascochyta rabiei

Necrotrophic pathogens experience host-generated oxidative stress during pathogenesis. They overcome such hostile environment by intricate mechanisms which are largely understudied. In this article, reference-based transcriptome analysis of a devastating Ascochyta Blight (AB) disease causing chickpea pathogen Ascochyta rabiei was explored to get insights into survival mechanisms under oxidative stress. Here, expression profiling of mock-treated and menadione-treated fungus was carried out by RNA-Seq approach. A significant number of genes in response to oxidative stress were overrepresented, suggestive of a robust and coordinated defense system of A. rabiei. A total 73 differentially expressed genes were filtered out from both the transcriptomes, among them 64 were up-regulated and 9 were found down-regulated. The gene ontology and KEGG mapping were conducted to comprehend the possible regulatory roles of differentially expressed genes in metabolic networks and biosynthetic pathways. Transcript profiling, KEGG pathway and gene ontology-based enrichment analysis revealed 12 (16.43%) stress responsive factors, 25 (34.24%) virulence associated genes, 10 (13.69%) putative effectors and 28 (38.35%) important interacting proteins associated with various metabolic pathways. In addition, genes with differential expression were further explored for underlying putative pathogenicity factors. We identified five genes ST47_g10291, ST47_g9396, ST47_g10294, ST47_g4395, and ST47_g7191 that were common to stress and fungal pathogenicity. The factors recognized in this work can be used to establish molecular tools to explain the regulatory gene networks engaged in stress response of fungal pathogens and disease management.

Genome-Wide Identification of Myxobacterial Predation Genes and Demonstration of Formaldehyde Secretion as a Potentially Predation-Resistant Trait of Pseudomonas aeruginosa

Despite widespread use in human biology, genome-wide association studies (GWAS) of bacteria are few and have, to date, focused primarily on pathogens. Myxobacteria are predatory microbes with large patchwork genomes, with individual strains secreting unique cocktails of predatory proteins and metabolites. We investigated whether a GWAS strategy could be applied to myxobacteria to identify genes associated with predation. Deduced proteomes from 29 myxobacterial genomes (including eight Myxococcus genomes sequenced for this study), were clustered into orthologous groups, and the presence/absence of orthologues assessed in superior and inferior predators of ten prey organisms. 139 ‘predation genes’ were identified as being associated significantly with predation, including some whose annotation suggested a testable predatory mechanism. Formaldehyde dismutase (fdm) was associated with superior predation of Pseudomonas aeruginosa, and predatory activity of a strain lacking fdm could be increased by the exogenous addition of a formaldehyde detoxifying enzyme, suggesting that production of formaldehyde by P. aeruginosa acts as an anti-predation behaviour. This study establishes the utility of bacterial GWAS to investigate microbial processes beyond pathogenesis, giving plausible and verifiable associations between gene presence/absence and predatory phenotype. We propose that the slow growth rate of myxobacteria, coupled with their predatory mechanism of constitutive secretion, has rendered them relatively resistant to genome streamlining. The resultant genome expansion made possible their observed accumulation of prey-specific predatory genes, without requiring them to be selected for by frequent or recent predation on diverse prey, potentially explaining both the large pan-genome and broad prey range of myxobacteria.

Proteomic analysis of Paracoccidioides brasiliensis complex isolates: Correlation of the levels of differentially expressed proteins with in vivo virulence

BACKGROUND

Paracoccidioidomycosis (PCM) is a systemic mycosis commonly found in Latin America that is caused by distinct species of Paracoccidioides genus: Paracoccidioides brasiliensis complex (S1, PS2, PS3 and PS4) and Paracoccidioides lutzii. Its pathobiology has been recently explored by different approaches to clarify the mechanisms of host-pathogen interactions underpinning PCM. The diversity of clinical forms of this disease has been attributed to both host- and fungus-related factors.

METHODOLOGY/PRINCIPAL FINDINGS

For better understanding of the molecular underpinnings of host-fungus interactions, we evaluated in vivo virulence of nine Paracoccidioides brasiliensis complex isolates and correlated it to protein expression profiles obtained by two-dimensional gel electrophoresis. Based on the recovery of viable fungi from mouse organs, the isolates were classified as those having low, moderate, or high virulence. Highly virulent isolates overexpressed proteins related to adhesion process and stress response, probably indicating important roles of those fungal proteins in regulating the colonization capacity, survival, and ability to escape host immune system reaction. Moreover, highly virulent isolates exhibited enhanced expression of glycolytic pathway enzymes concomitantly with repressed expression of succinyl-CoA ligase beta chain, a protein related to the tricarboxylic acid cycle.

CONCLUSIONS/SIGNIFICANCE

Our findings may point to the mechanisms used by highly virulent P. brasiliensis isolates to withstand host immune reactions and to adapt to transient iron availability as strategies to survive and overcome stress conditions inside the host.

The Transcription Factor Sfp1 Regulates the Oxidative Stress Response in Candida albicans

Candida albicans is a commensal that inhabits the skin and mucous membranes of humans. Because of the increasing immunocompromised population and the limited classes of antifungal drugs available, C. albicans has emerged as an important opportunistic pathogen with high mortality rates. During infection and therapy, C. albicans frequently encounters immune cells and antifungal drugs, many of which exert their antimicrobial activity by inducing the production of reactive oxygen species (ROS). Therefore, antioxidative capacity is important for the survival and pathogenesis of C. albicans. In this study, we characterized the roles of the zinc finger transcription factor Sfp1 in the oxidative stress response against C. albicans. A sfp1-deleted mutant was more resistant to oxidants and macrophage killing than wild-type C. albicans and processed an active oxidative stress response with the phosphorylation of the mitogen-activated protein kinase (MAPK) Hog1 and high CAP1 expression. Moreover, the sfp1-deleted mutant exhibited high expression levels of antioxidant genes in response to oxidative stress, resulting in a higher total antioxidant capacity, glutathione content, and glutathione peroxidase and superoxide dismutase enzyme activity than the wild-type C. albicans. Finally, the sfp1-deleted mutant was resistant to macrophage killing and ROS-generating antifungal drugs. Together, our findings provide a new understanding of the complex regulatory machinery in the C. albicans oxidative stress response.

Transcriptional profiling of Scedosporium apiospermum enzymatic antioxidant gene battery unravels the involvement of thioredoxin reductases against chemical and phagocytic cells oxidative stress

Scedosporium species rank the second, after Aspergillus fumigatus, among the filamentous fungi colonizing the airways of patients with cystic fibrosis (CF). Development of microorganisms in the respiratory tract depends on their capacity to evade killing by the host immune system, particularly through the oxidative response of macrophages and neutrophils, with the release of reactive oxygen species (ROS) and reactive nitrogen species (RNS). This is particularly true in the airways of CF patients which display an exacerbated inflammatory reaction. To protect themselves, pathogens have developed various enzymatic antioxidant systems implicated in ROS degradation, including superoxide dismutases, catalases, cytochrome C peroxidases, chloroperoxidases and enzymes of the glutathione and thioredoxin systems, or in RNS degradation, that is, flavohemoglobins, nitrate reductases, and nitrite reductases. Here we investigated the transcriptional regulation of the enzymatic antioxidant gene battery in 24-h-old hyphae of Scedosporium apiospermum in response to oxidative stress induced chemically or by exposure to activated phagocytic cells. We showed that 21 out of the 33 genes potentially implicated in the oxidative or nitrosative stress response were overexpressed upon exposure of the fungus to various chemical oxidants, while they were only 13 in co-cultures with macrophages or neutrophils. Among them, genes encoding two thioredoxin reductases and to a lesser extent, a peroxiredoxin and one catalase were found to be overexpressed after chemical oxidative stress as well as in co-cultures. These results suggest that thioredoxin reductases, which are known to be virulence factors in other pathogenic fungi, play a key role in pathogenesis of scedosporiosis, and may be new drug targets.

YpdA, a putative bacillithiol disulfide reductase, contributes to cellular redox homeostasis and virulence in Staphylococcus aureus

The intracellular redox environment of Staphylococcus aureus is mainly buffered by bacillithiol (BSH), a low molecular weight thiol. The identity of enzymes responsible for the recycling of oxidized bacillithiol disulfide (BSSB) to the reduced form (BSH) remains elusive. We examined YpdA, a putative bacillithiol reductase, for its role in maintaining intracellular redox homeostasis. The ypdA mutant showed increased levels of BSSB and a lower bacillithiol redox ratio vs. the isogenic parent, indicating a higher level of oxidative stress within the bacterial cytosol. We showed that YpdA consumed NAD(P)H; and YpdA protein levels were augmented in response to stress. Wild type strains overexpressing YpdA showed increased tolerance to oxidants and electrophilic agents. Importantly, YpdA overexpression in the parental strain caused an increase in BSH levels accompanied by a decrease in BSSB concentration in the presence of stress, resulting in an increase in bacillithiol redox ratio vs. the vector control. Additionally, the ypdA mutant exhibited decreased survival in human neutrophils (PMNs) as compared with the parent, while YpdA overexpression protected the resulting strain from oxidative stress in vitro and from killing by human neutrophils ex vivo. Taken together, these data present a new role for YpdA in S. aureus physiology and virulence through the bacillithiol system.

Nitric Oxide Signalling in Yeast

Nitric oxide (NO) is a cellular signalling molecule widely conserved among organisms, including microorganisms such as bacteria, yeasts, and fungi, and higher eukaryotes such as plants and mammals. NO is mainly produced by the activities of NO synthase (NOS) or nitrite reductase (NIR). There are several NO detoxification systems, including NO dioxygenase (NOD) and S-nitrosoglutathione reductase (GSNOR). NO homeostasis, based on the balance between NO synthesis and degradation, is important for regulating its physiological functions, since an excess of NO causes nitrosative stress due to the high reactivity of NO and NO-derived compounds. In yeast, NO may be involved in stress responses, but the role of NO and the mechanism underlying NO signalling are poorly understood due to the lack of mammalian NOS orthologs in the yeast genome. NOS and NIR activities have been observed in yeast cells, but the gene-encoding NOS and the mechanism by which NO production is catalysed by NIR remain unclear. On the other hand, yeast cells employ NOD and GSNOR to maintain intracellular redox balance following endogenous NO production, treatment with exogenous NO, or exposure to environmental stresses. This article reviews NO metabolism (synthesis, degradation) and its regulation in yeast. The physiological roles of NO in yeast, including the oxidative stress response, are also discussed. Such investigations into NO signalling are essential for understanding how NO modulates the genetics and physiology of yeast. In addition to being responsible for the pathology and pharmacology of various degenerative diseases, NO signalling may be a potential target for the construction and engineering of industrial yeast strains.

Stress Adaptation

Fungal species display an extraordinarily diverse range of lifestyles. Nevertheless, the survival of each species depends on its ability to sense and respond to changes in its natural environment. Environmental changes such as fluctuations in temperature, water balance or pH, or exposure to chemical insults such as reactive oxygen and nitrogen species exert stresses that perturb cellular homeostasis and cause molecular damage to the fungal cell. Consequently, fungi have evolved mechanisms to repair this damage, detoxify chemical insults, and restore cellular homeostasis. Most stresses are fundamental in nature, and consequently, there has been significant evolutionary conservation in the nature of the resultant responses across the fungal kingdom and beyond. For example, heat shock generally induces the synthesis of chaperones that promote protein refolding, antioxidants are generally synthesized in response to an oxidative stress, and osmolyte levels are generally increased following a hyperosmotic shock. In this article we summarize the current understanding of these and other stress responses as well as the signaling pathways that regulate them in the fungi. Model yeasts such as Saccharomyces cerevisiae are compared with filamentous fungi, as well as with pathogens of plants and humans. We also discuss current challenges associated with defining the dynamics of stress responses and with the elaboration of fungal stress adaptation under conditions that reflect natural environments in which fungal cells may be exposed to different types of stresses, either sequentially or simultaneously.

Phagosomal Neutralization by the Fungal Pathogen Candida albicans Induces Macrophage Pyroptosis

The interaction of Candida albicans with the innate immune system is the key determinant of the pathogen/commensal balance and has selected for adaptations that facilitate the utilization of nutrients commonly found within the host, including proteins and amino acids; many of the catabolic pathways needed to assimilate these compounds are required for persistence in the host. We have shown that C. albicans co-opts amino acid catabolism to generate and excrete ammonia, which raises the extracellular pH, both in vitro and in vivo and induces hyphal morphogenesis. Mutants defective in the uptake or utilization of amino acids, such as those lacking STP2, a transcription factor that regulates the expression of amino acid permeases, are impaired in multiple aspects of fungus-macrophage interactions resulting from an inability to neutralize the phagosome. Here we identified a novel role in amino acid utilization for Ahr1p, a transcription factor previously implicated in regulation of adherence and hyphal morphogenesis. Mutants lacking AHR1 were defective in growth, alkalinization, and ammonia release on amino acid-rich media, similar to stp2Δ and ahr1Δ stp2Δ cells, and occupied more acidic phagosomes. Notably, ahr1Δ and stp2Δ strains did not induce pyroptosis, as measured by caspase-1-dependent interleukin-1β release, though this phenotype could be suppressed by pharmacological neutralization of the phagosome. Altogether, we show that C. albicans-driven neutralization of the phagosome promotes hyphal morphogenesis, sufficient for induction of caspase-1-mediated macrophage lysis.

Candida albicans glutathione reductase downregulates Efg1-mediated cyclic AMP/protein kinase A pathway and leads to defective hyphal growth and virulence upon decreased cellular methylglyoxal content accompanied by activating alcohol dehydrogenase and glycolytic enzymes

BACKGROUND

Glutathione reductase maintains the glutathione level in a reduced state. As previously demonstrated, glutathione is required for cell growth/division and its biosynthesizing-enzyme deficiency causes methylglyoxal accumulation. However, experimental evidences for reciprocal relationships between Cph1-/Efg1-mediated signaling pathway regulation and methylglyoxal production exerted by glutathione reductase on yeast morphology remain unclear.

METHODS

Glutathione reductase (GLR1) disruption/overexpression were performed to investigate aspects of pathological/morphological alterations in Candida albicans. These assumptions were proved by observations of cellular susceptibility to oxidants and thiols, and measurements of methylglyoxal and glutathione content in hyphal-inducing conditions mainly through the activity of GLR1-overexpressing cells. Additionally, the transcriptional/translational levels of bioenergetic enzymes and dimorphism-regulating protein kinases were examined in the strain.

RESULTS

The GLR1-deficient strain was non-viable when GLR1 expression under the control of a CaMAL2 promoter was conditionally repressed, despite partial rescue of growth by exogenous thiols. During filamentation, non-growing hyphal GLR1-overexpressing cells exhibited resistance against oxidants and cellular methylglyoxal was significantly decreased, which concomitantly increased expressions of genes encoding energy-generating enzymes, including fructose-1,6-bisphosphate aldolase, glyceraldehyde-3-phosphate dehydrogenase, and alcohol dehydrogenase (ADH1), with remarkable repression of Efg1-signaling cascades.

CONCLUSIONS

This is the first report that GLR1-triggered Efg1-mediated signal transduction repression strictly reduces dimorphic switching and virulence by maintaining the basal level of methylglyoxal following the enhanced gene expressions of glycolytic enzymes and ADH1.

GENERAL SIGNIFICANCE

The Efg1 downregulatory mechanism by GLR1 expression has possibilities to involve in other complex network of signal pathways. Understanding how GLR1 overexpression affects multiple signaling pathways can help identify attractive targets for antifungal drugs.

Galleria mellonella as a model host for microbiological and toxin research

Mammals are widely used by microbiologists as a model host species to study infectious diseases of humans and domesticated livestock. These studies have been pivotal for our understanding of mechanisms of virulence and have allowed the development of diagnostics, pre-treatments and therapies for disease. However, over the past decade we have seen efforts to identify organisms which can be used as alternatives to mammals for these studies. The drivers for this are complex and multifactorial and include cost, ethical and scientific considerations. Galleria mellonella have been used as an alternative infection model since the 1980s and its utility for the study of bacterial disease and antimicrobial discovery was recently comprehensively reviewed. The wider applications of G. mellonella as a model host, including its susceptibility to 29 species of fungi, 7 viruses, 1 species of parasite and 16 biological toxins, are described in this perspective. In addition, the latest developments in the standardisation of G. mellonella larvae for research purposes has been reviewed.

Interactions of fungal pathogens with phagocytes

The surveillance and elimination of fungal pathogens rely heavily on the sentinel behaviour of phagocytic cells of the innate immune system, especially macrophages and neutrophils. The efficiency by which these cells recognize, uptake and kill fungal pathogens depends on the size, shape and composition of the fungal cells and the success or failure of various fungal mechanisms of immune evasion. In this Review, we describe how fungi, particularly Candida albicans, interact with phagocytic cells and discuss the many factors that contribute to fungal immune evasion and prevent host elimination of these pathogenic microorganisms.

Nitric Oxide in the Offensive Strategy of Fungal and Oomycete Plant Pathogens

In the course of evolutionary changes pathogens have developed many invasion strategies, to which the host organisms responded with a broad range of defense reactions involving endogenous signaling molecules, such as nitric oxide (NO). There is evidence that pathogenic microorganisms, including two most important groups of eukaryotic plant pathogens, also acquired the ability to synthesize NO via non-unequivocally defined oxidative and/or reductive routes. Although the both kingdoms Chromista and Fungi are remarkably diverse, the experimental data clearly indicate that pathogen-derived NO is an important regulatory molecule controlling not only developmental processes, but also pathogen virulence and its survival in the host. An active control of mitigation or aggravation of nitrosative stress within host cells seems to be a key determinant for the successful invasion of plant pathogens representing different lifestyles and an effective mode of dispersion in various environmental niches.