Cellular responses of Candida albicans to phagocytosis and the extracellular activities of neutrophils are critical to counteract carbohydrate starvation, oxidative and nitrosative stress

Modulatory immune responses in fungal infection associated with organ transplant - advancements, management, and challenges

Organ transplantation stands as a pivotal achievement in modern medicine, offering hope to individuals with end-stage organ diseases. Advancements in immunology led to improved organ transplant survival through the development of immunosuppressants, but this heightened susceptibility to fungal infections with nonspecific symptoms in recipients. This review aims to establish an intricate balance between immune responses and fungal infections in organ transplant recipients. It explores the fundamental immune mechanisms, recent advances in immune response dynamics, and strategies for immune modulation, encompassing responses to fungal infections, immunomodulatory approaches, diagnostics, treatment challenges, and management. Early diagnosis of fungal infections in transplant patients is emphasized with the understanding that innate immune responses could potentially reduce immunosuppression and promise efficient and safe immuno-modulating treatments. Advances in fungal research and genetic influences on immune-fungal interactions are underscored, as well as the potential of single-cell technologies integrated with machine learning for biomarker discovery. This review provides a snapshot of the complex interplay between immune responses and fungal infections in organ transplantation and underscores key research directions.

Reprogramming in Candida albicans Gene Expression Network under Butanol Stress Abrogates Hyphal Development

Candida albicans is the causative agent of invasive fungal infections. Its hyphae-forming ability is regarded as one of the important virulence factors. To unravel the impact of butanol on Candida albicans, it was placed in O+ve complete human serum with butanol (1% v/v). The Candida transcriptome under butanol stress was then identified by mRNA sequencing. Studies including electron microscopy demonstrated the inhibition of hyphae formation in Candida under the influence of butanol, without any significant alteration in growth rate. The numbers of genes upregulated in the butanol in comparison to the serum alone were 1061 (20 min), 804 (45 min), and 537 (120 min). Candida cells exhibited the downregulation of six hypha-specific transcription factors and the induction of four repressor/regulator genes. Many of the hypha-specific genes exhibited repression in the medium with butanol. The genes related to adhesion also exhibited repression, whereas, among the heat-shock genes, three showed inductions in the presence of butanol. The fungal-specific genes exhibited induction as well as repression in the butanol-treated Candida cells. Furthermore, ten upregulated genes formed the core stress gene set in the presence of butanol. In the gene ontology analysis, enrichment of the processes related to non-coding RNA, ribosome biosynthesis, and metabolism was observed in the induced gene set. On the other side, a few GO biological process terms, including biofilm formation and filamentous growth, were enriched in the repressed gene set. Taken together, under butanol stress, Candida albicans is unable to extend hyphae and shows growth by budding. Many of the genes with perturbed expression may have fitness or virulence attributes and may provide prospective sites of antifungal targets against C. albicans.

Candida auris-macrophage cellular interactions and transcriptional response

The pathogenic yeast Candida auris represents a global threat of the utmost clinical relevance. This emerging fungal species is remarkable in its resistance to commonly used antifungal agents and its persistence in the nosocomial settings. The innate immune system is one the first lines of defense preventing the dissemination of pathogens in the host. C. auris is susceptible to circulating phagocytes, and understanding the molecular details of these interactions may suggest routes to improved therapies. In this work, we examined the interactions of this yeast with macrophages. We found that macrophages avidly phagocytose C. auris; however, intracellular replication is not inhibited, indicating that C. auris resists the killing mechanisms imposed by the phagocyte. Unlike Candida albicans, phagocytosis of C. auris does not induce macrophage lysis. The transcriptional response of C. auris to macrophage phagocytosis is very similar to other members of the CUG clade (C. albicans, C. tropicalis, C. parapsilosis, C. lusitaniae), i.e., downregulation of transcription/translation and upregulation of alternative carbon metabolism pathways, transporters, and induction of oxidative stress response and proteolysis. Gene family expansions are common in this yeast, and we found that many of these genes are induced in response to macrophage co-incubation. Among these, amino acid and oligopeptide transporters, as well as lipases and proteases, are upregulated. Thus, C. auris shares key transcriptional signatures shared with other fungal pathogens and capitalizes on the expansion of gene families coding for potential virulence attributes that allow its survival, persistence, and evasion of the innate immune system.

Fungal extracellular vesicle-mediated regulation: from virulence factor to clinical application

Invasive fungal disease (IFD) poses a significant threat to immunocompromised patients and remains a global challenge due to limited treatment options, high mortality and morbidity rates, and the emergence of drug-resistant strains. Despite advancements in antifungal agents and diagnostic techniques, the lack of effective vaccines, standardized diagnostic tools, and efficient antifungal drugs contributes to the ongoing impact of invasive fungal infections (IFI). Recent studies have highlighted the presence of extracellular vesicles (EVs) released by fungi carrying various components such as enzymes, lipids, nucleic acids, and virulence proteins, which play roles in both physiological and pathological processes. These fungal EVs have been shown to interact with the host immune system during the development of fungal infections whereas their functional role and potential application in patients are not yet fully understood. This review summarizes the current understanding of the biologically relevant findings regarding EV in host-pathogen interaction, and aim to describe our knowledge of the roles of EV as diagnostic tools and vaccine vehicles, offering promising prospects for the treatment of IFI patients.

Living together: The role of Candida albicans in the formation of polymicrobial biofilms in the oral cavity

The oral cavity of humans is colonized by diversity of microbial community, although dominated by bacteria, it is also constituted by a low number of fungi, often represented by Candida albicans. Although in the vast minority, this usually commensal fungus under certain conditions of the host (e.g., immunosuppression or antibiotic therapy), can transform into an invasive pathogen that adheres to mucous membranes and also to medical or dental devices, causing mucosal infections. This transformation is correlated with changes in cell morphology from yeast-like cells to hyphae and is supported by numerous virulence factors exposed by C. albicans cells at the site of infection, such as multifunctional adhesins, degradative enzymes, or toxin. All of them affect the surrounding host cells or proteins, leading to their destruction. However, at the site of infection, C. albicans can interact with different bacterial species and in its filamentous form may produce biofilms-the elaborated consortia of microorganisms, that present increased ability to host colonization and resistance to antimicrobial agents. In this review, we highlight the modification of the infectious potential of C. albicans in contact with different bacterial species, and also consider the mutual bacterial-fungal relationships, involving cooperation, competition, or antagonism, that lead to an increase in the propagation of oral infection. The mycofilm of C. albicans is an excellent hiding place for bacteria, especially those that prefer low oxygen availability, where microbial cells during mutual co-existence can avoid host recognition or elimination by antimicrobial action. However, these microbial relationships, identified mainly in in vitro studies, are modified depending on the complexity of host conditions and microbial dominance in vivo.

The interactions of Candida albicans with gut bacteria: a new strategy to prevent and treat invasive intestinal candidiasis

BACKGROUND

The gut microbiota plays an important role in human health, as it can affect host immunity and susceptibility to infectious diseases. Invasive intestinal candidiasis is strongly associated with gut microbiota homeostasis. However, the nature of the interaction between Candida albicans and gut bacteria remains unclear.

OBJECTIVE

This review aimed to determine the nature of interaction and the effects of gut bacteria on C. albicans so as to comprehend an approach to reducing intestinal invasive infection by C. albicans.

METHODS

This review examined 11 common gut bacteria's interactions with C. albicans, including Escherichia coli, Pseudomonas aeruginosa, Acinetobacter baumannii, Enterococcus faecalis, Staphylococcus aureus, Salmonella spp., Helicobacter pylori, Lactobacillus spp., Bacteroides spp., Clostridium difficile, and Streptococcus spp.

RESULTS

Most of the studied bacteria demonstrated both synergistic and antagonistic effects with C. albicans, and just a few bacteria such as P. aeruginosa, Salmonella spp., and Lactobacillus spp. demonstrated only antagonism against C. albicans.

CONCLUSIONS

Based on the nature of interactions reported so far by the literature between gut bacteria and C. albicans, it is expected to provide new ideas for the prevention and treatment of invasive intestinal candidiasis.

The role of manganese in morphogenesis and pathogenesis of the opportunistic fungal pathogen Candida albicans

Metals such as Fe, Cu, Zn, and Mn are essential trace nutrients for all kingdoms of life, including microbial pathogens and their hosts. During infection, the mammalian host attempts to starve invading microbes of these micronutrients through responses collectively known as nutritional immunity. Nutritional immunity for Zn, Fe and Cu has been well documented for fungal infections; however Mn handling at the host-fungal pathogen interface remains largely unexplored. This work establishes the foundation of fungal resistance against Mn associated nutritional immunity through the characterization of NRAMP divalent metal transporters in the opportunistic fungal pathogen, Candida albicans. Here, we identify C. albicans Smf12 and Smf13 as two NRAMP transporters required for cellular Mn accumulation. Single or combined smf12Δ/Δ and smf13Δ/Δ mutations result in a 10-80 fold reduction in cellular Mn with an additive effect of double mutations and no losses in cellular Cu, Fe or Zn. As a result of low cellular Mn, the mutants exhibit impaired activity of mitochondrial Mn-superoxide dismutase 2 (Sod2) and cytosolic Mn-Sod3 but no defects in cytosolic Cu/Zn-Sod1 activity. Mn is also required for activity of Golgi mannosyltransferases, and smf12Δ/Δ and smf13Δ/Δ mutants show a dramatic loss in cell surface phosphomannan and in glycosylation of proteins, including an intracellular acid phosphatase and a cell wall Cu-only Sod5 that is key for oxidative stress resistance. Importantly, smf12Δ/Δ and smf13Δ/Δ mutants are defective in formation of hyphal filaments, a deficiency rescuable by supplemental Mn. In a disseminated mouse model for candidiasis where kidney is the primary target tissue, we find a marked loss in total kidney Mn during fungal invasion, implying host restriction of Mn. In this model, smf12Δ/Δ and smf13Δ/Δ C. albicans mutants displayed a significant loss in virulence. These studies establish a role for Mn in Candida pathogenesis.

The adaptive response to alternative carbon sources in the pathogen Candida albicans involves a remodeling of thiol- and glutathione-dependent redox status

Candida albicans is an opportunist pathogen responsible for a large spectrum of infections, from superficial mycosis to systemic diseases known as candidiasis. During infection in vivo, Candida albicans must adapt to host microenvironments and this adaptive response is crucial for the survival of this organism, as it facilitates the effective assimilation of alternative carbon sources others than glucose. We performed a global proteomic analysis on the global changes in protein abundance in response to changes in micronutrient levels, and, in parallel, explored changes in the intracellular redox and metabolic status of the cells. We show here that each of the carbon sources considered - glucose, acetate and lactate - induces a unique pattern of response in C. albicans cells, and that some conditions trigger an original and specific adaptive response involving the adaptation of metabolic pathways, but also a complete remodeling of thiol-dependent antioxidant defenses. Protein S-thiolation and the overproduction of reduced glutathione are two components of the response to high glucose concentration. In the presence of acetate, glutathione-dependent oxidative stress occurs, reduced thiol groups bind to proteins, and glutathione is exported out of the cells, these changes probably being triggered by an increase in glutathione-S-transferases. Overall, our results suggest that the role of cellular redox status regulation and defenses against oxidative stress, including the thiol- and glutathione-dependent response, in the adaptive response of C. albicans to alternative carbon sources should be reconsidered.

Evaluation of the antifungal effect of chlorogenic acid against strains of Candida spp. resistant to fluconazole: apoptosis induction and in silico analysis of the possible mechanisms of action

Introduction. Candida spp. are commensal fungal pathogens of humans, but when there is an imbalance in the microbiota, or weak host immunity, these yeasts can become pathogenic, generating high medical costs.Gap Statement. With the increase in resistance to conventional antifungals, the development of new therapeutic strategies is necessary. This study evaluated the in vitro antifungal activity of chlorogenic acid against fluconazole-resistant strains of Candida spp. Mechanism of action through flow cytometry and in silico analyses, as well as molecular docking assays with ALS3 and SAP5, important proteins in the pathogenesis of Candida albicans associated with the adhesion process and biofilm formation.Results. The chlorogenic acid showed in vitro antifungal activity against the strains tested, causing reduced cell viability, increased potential for mitochondrial depolarization and production of reactive oxygen species, DNA fragmentation and phosphatidylserine externalization, indicating an apoptotic process. Concerning the analysis through docking, the complexes formed between chlorogenic acid and the targets Thymidylate Kinase, CYP51, 1Yeast Cytochrome BC1 Complex e Exo-B-(1,3)-glucanase demonstrated more favourable binding energy. In addition, chlorogenic acid presented significant interactions with the ALS3 active site residues of C. albicans, important in the adhesion process and resistance to fluconazole. Regarding molecular docking with SAP5, no significant interactions were found between chlorogenic acid and the active site of the enzyme.Conclusion. We concluded that chlorogenic acid has potential use as an adjuvant in antifungal therapies, due to its anti-Candida activity and ability to interact with important drug targets.

The Role of the Oral Immune System in Oropharyngeal Candidiasis-Facilitated Invasion and Dissemination of Staphylococcus aureus

Candida albicans and Staphylococcus aureus account for most invasive fungal and bacterial bloodstream infections (BSIs), respectively. However, the initial point of invasion responsible for S. aureus BSIs is often unclear. Recently, C. albicans has been proposed to mediate S. aureus invasion of immunocompromised hosts during co-colonization of oral mucosal surfaces. The status of the oral immune system crucially contributes to this process in two distinct ways: firstly, by allowing invasive C. albicans growth during dysfunction of extra-epithelial immunity, and secondly following invasion by some remaining function of intra-epithelial immunity. Immunocompromised individuals at risk of developing invasive oral C. albicans infections could, therefore, also be at risk of contracting concordant S. aureus BSIs. Considering the crucial contribution of both oral immune function and dysfunction, the aim of this review is to provide an overview of relevant aspects of intra and extra-epithelial oral immunity and discuss predominant immune deficiencies expected to facilitate C. albicans induced S. aureus BSIs.

From environmental adaptation to host survival: Attributes that mediate pathogenicity of Candida auris

Candida species are a major cause of invasive fungal infections. While Candida albicans, C. glabrata, C. parapsilosis, and C. tropicalis are the most dominant species causing life-threatening candidiasis, C. auris recently emerged as a new species causing invasive infections with high rates of clinical treatment failures. To mimic initial phases of systemic Candida infections with dissemination via the bloodstream and to elucidate the pathogenic potential of C. auris, we used an ex vivo whole blood infection model. Similar to other clinically relevant Candida spp., C. auris is efficiently killed in human blood, but showed characteristic patterns of immune cell association, survival rates, and cytokine induction. Dual-species transcriptional profiling of C. auris-infected blood revealed a unique C. auris gene expression program during infection, while the host response proofed similar and conserved compared to other Candida species. C. auris-specific responses included adaptation and survival strategies, such as counteracting oxidative burst of immune cells, but also expression of potential virulence factors, (drug) transporters, and cell surface-associated genes. Despite comparable pathogenicity to other Candida species in our model, C. auris-specific transcriptional adaptations as well as its increased stress resistance and long-term environmental survival, likely contribute to the high risk of contamination and distribution in a nosocomial setting. Moreover, infections of neutrophils with pre-starved C. auris cells suggest that environmental preconditioning can have modulatory effects on the early host interaction. In summary, we present novel insights into C. auris pathogenicity, revealing adaptations to human blood and environmental niches distinctive from other Candida species.

A dynamically evolving war between autophagy and pathogenic microorganisms

Autophagy is an intracellular degradation process that maintains cellular homeostasis. It is essential for protecting organisms from environmental stress. Autophagy can help the host to eliminate invading pathogens, including bacteria, viruses, fungi, and parasites. However, pathogens have evolved multiple strategies to interfere with autophagic signaling pathways or inhibit the fusion of autophagosomes with lysosomes to form autolysosomes. Moreover, host cell matrix degradation by different types of autophagy can be used for the proliferation and reproduction of pathogens. Thus, determining the roles and mechanisms of autophagy during pathogen infections will promote understanding of the mechanisms of pathogen‍‒‍host interactions and provide new strategies for the treatment of infectious diseases.

Pathogenesis and virulence of Candida albicans

Candida albicans is a commensal yeast fungus of the human oral, gastrointestinal, and genital mucosal surfaces, and skin. Antibiotic-induced dysbiosis, iatrogenic immunosuppression, and/or medical interventions that impair the integrity of the mucocutaneous barrier and/or perturb protective host defense mechanisms enable C. albicans to become an opportunistic pathogen and cause debilitating mucocutaneous disease and/or life-threatening systemic infections. In this review, we synthesize our current knowledge of the tissue-specific determinants of C. albicans pathogenicity and host immune defense mechanisms.

Interplay between Candida albicans and Lactic Acid Bacteria in the Gastrointestinal Tract: Impact on Colonization Resistance, Microbial Carriage, Opportunistic Infection, and Host Immunity

Emerging studies have highlighted the disproportionate role of Candida albicans in influencing both early community assembly of the bacterial microbiome and dysbiosis during allergic diseases and intestinal inflammation. Nonpathogenic colonization of the human gastrointestinal (GI) tract by C. albicans is common, and the role of this single fungal species in modulating bacterial community reassembly after broad-spectrum antibiotics can be readily recapitulated in mouse studies. One of the most notable features of C. albicans-associated dysbiotic states is a marked change in the levels of lactic acid bacteria (LAB). C. albicans and LAB share metabolic niches throughout the GI tract, and in vitro studies have identified various interactions between these microbes. The two predominant LAB affected are Lactobacillus species and Enterococcus species. Lactobacilli can antagonize enterococci and C. albicans, while Enterococcus faecalis and C. albicans have been reported to exhibit a mutualistic relationship. E. faecalis and C. albicans are also causative agents of a variety of life-threatening infections, are frequently isolated together from mixed-species infections, and share certain similarities in clinical presentation-most notably their emergence as opportunistic pathogens following disruption of the microbiota. In this review, we discuss and model the mechanisms used by Lactobacillus species, E. faecalis, and C. albicans to modulate each other's growth and virulence in the GI tract. With multidrug-resistant E. faecalis and C. albicans strains becoming increasingly common in hospital settings, examining the interplay between these three microbes may provide novel insights for enhancing the efficacy of existing antimicrobial therapies.

Interesting antifungal drug targets in the central metabolism of Candida albicans

To treat infections caused by Candida albicans, azoles, polyenes, and echinocandins are used. However, resistance occurs against all three, so there is an urgent need for new antifungal drugs with a novel mode of action. Recently, it became clear that central metabolism plays an important role in the virulence of C. albicans. Glycolysis is, for example, upregulated during virulence conditions, whereas the glyoxylate cycle is important upon phagocytosis by host immune cells. These findings indicate that C. albicans adapts its metabolism to the environment for maximal virulence. In this review, we provide an overview of the potency of different central metabolic pathways and their key enzymes as potential antifungal drug targets.

Nutritional immunity: targeting fungal zinc homeostasis

Transition metals, such as Zn²⁺, are essential dietary constituents of all biological life, including mammalian hosts and the pathogens that infect them. Therefore, to thrive and cause infection, pathogens must successfully assimilate these elements from the host milieu. Consequently, mammalian immunity has evolved to actively restrict and/or pool metals to toxic concentrations in an effort to attenuate microbial pathogenicity - a process termed nutritional immunity. Despite host-induced Zn²⁺ nutritional immunity, pathogens such as Candida albicans, are still capable of causing disease and thus must be equipped with robust Zn²⁺ sensory, uptake and detoxification machinery. This review will discuss the strategies employed by mammalian hosts to limit Zn²⁺ during infection, and the subsequent fungal interventions that counteract Zn²⁺ nutritional immunity.

The Glycosylphosphatidylinositol-Anchored Superoxide Dismutase of Scedosporium apiospermum Protects the Conidia from Oxidative Stress

Scedosporium species are common fungal pathogens in patients with cystic fibrosis (CF). To colonize the CF lungs, fungi must cope with the host immune response, especially the reactive oxygen species (ROS) released by phagocytic cells. To this aim, pathogens have developed various antioxidant systems, including superoxide dismutases (SODs) which constitute the first-line protection against oxidative stress. Interestingly, one of the S. apiospermum SOD-encoding genes (SODD gene) exhibits a glycosylphosphatidylinositol (GPI) anchor-binding site and encodes a conidial-specific surface SOD. In this study, a SODDΔ mutant was engineered from a non-homologous end joining-deficient strain (KU70Δ) of S. apiospermum. Compared to its parent strain, the double mutant KU70Δ/SODDΔ exhibited increased susceptibility to various oxidizing agents and triazole antifungals. In addition, the loss of SodD resulted in an increased intracellular killing of the conidia by M1 macrophages derived from human blood monocytes, suggesting the involvement of this superoxide dismutase in the evasion to the host defenses. Nevertheless, one cannot disregard an indirect role of the enzyme in the synthesis or assembly of the cell wall components since transmission electron microscopic analysis revealed a thickening of the inner cell wall layer of the conidia. Further studies are needed to confirm the role of this enzyme in the pathogenesis of Scedosporium infections, including the production of a recombinant protein and study of its protective effect against the infection in a mouse model of scedosporiosis.

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.

Fingolimod Potentiates the Antifungal Activity of Amphotericin B

Candida albicans (C. albicans) is an opportunistic human fungal pathogen that can cause severe infection in clinic. Its incidence and mortality rate has been increasing rapidly. Amphotericin B (AMB), the clinical golden standard antifungal agent, has severe side effects that limit its clinical application. Thus, lowering the concentration and increasing the efficacy of AMB in a combinatorial antifungal therapy have been pursued by both industry and academia. Here we identify that fingolimod (FTY720), an immunomodulatory drug used for oral treatment of relapsing-remitting multiple sclerosis, can potentiate the efficacy of AMB against C. albicans growth synergistically. Furthermore, we observe an antifungal efficacy of FTY720 in combination with AMB against diverse fungal pathogens. Intriguingly, cells treated with both drugs are hypersensitive to endothelial endocytosis and macrophage killing. This is later found to be due to the hyperaccumulation of reactive oxygen species and the corresponding increase in activities of superoxide dismutase and catalase in the cells that received combinatorial treatment. Therefore, the combination of AMB and FTY720 provides a promising antifungal strategy.

Modulation of antioxidant defence system in response to berberine in Candida albicans

Emergence of multidrug resistant species of Candida is evolving, which advocates an urgent need for the development of new therapeutic strategies and antifungal drugs. Activation of antioxidant defence system in Candida albicans is known as forefront mechanism to escape drug toxicity. This study evaluated the role of antioxidant defence genes in the susceptibility to fluconazole in C. albicans and also determined the effect of berberine on growth, antioxidant enzymes and the expression of their genes in C. albicans isolates. Expression of major antioxidant genes was significantly increased in fluconazole-resistant isolates in comparison with the susceptible group. Antifungal susceptibility against berberine showed MIC values ranging from 125 to 500 μg/ml. Berberine treatment caused upregulation of mRNA expression and enzymatic activities of the targeted major antioxidants. Interestingly, C. albicans exhibited efficient antioxidant response at lower concentrations but could not sufficiently alleviate berberine-induced oxidative stress occurring at concentrations greater than 250 μg/ml. Therefore, berberine could serve as a potent Reactive Oxygen Species (ROS)-inducing agent, disrupting the antioxidant system especially in fluconazole-resistant C. albicans to overcome antifungal drug resistance. TAKE AWAYS: Evaluated the role of antioxidant enzymes in FLC resistance in C. albicans Studied the effect of berberine on growth of different C. albicans isolates Investigated the modulation of antioxidant enzymes by berberine in C. albicans Studied the effect of berberine on antioxidant gene expression in C. albicans.

A TRP1-marker-based system for gene complementation, overexpression, reporter gene expression and gene modification in Candida glabrata

Although less prevalent than its relative Candida albicans, the yeast Candida glabrata is a successful pathogen of humans, which causes life-threatening candidiasis. It is thus vital to understand the pathogenicity mechanisms and contributing genes in C. glabrata. However, gene complementation as a tool for restoring the function of a previously deleted gene is not standardized in C. glabrata, and it is less frequently used than in C. albicans.

In this study, we established a gene complementation strategy using genomic integration at the TRP1 locus. We prove that our approach can not only be used for integration of complementation cassettes, but also for overexpression of markers like fluorescent proteins and the antigen ovalbumin, or of potential pathogenicity-related factors like the biotin transporter gene VHT1. With urea amidolyase Dur1,2 as an example, we demonstrate the application of the gene complementation approach for the expression of sequence-modified genes. With this approach, we found that a lysine-to-arginine mutation in the biotinylation motif of Dur1,2 impairs urea-dependent growth of C. glabrata and C. albicans. Taken together, the TRP1-based gene complementation approach is a valuable tool for investigating novel gene functions and for elucidating their role in the pathobiology of C. glabrata.

The RSC (Remodels the Structure of Chromatin) complex of Candida albicans shows compositional divergence with distinct roles in regulating pathogenic traits

Regulation of gene expression programs is crucial for the survival of microbial pathogens in host environments and for their ability to cause disease. Here we investigated the epigenetic regulator RSC (Remodels the Structure of Chromatin) in the most prevalent human fungal pathogen Candida albicans. Biochemical analysis showed that CaRSC comprises 13 subunits and contains two novel non-essential members, which we named Nri1 and Nri2 (Novel RSC Interactors) that are exclusive to the CTG clade of Saccharomycotina. Genetic analysis showed distinct essentiality of C. albicans RSC subunits compared to model fungal species suggesting functional and structural divergence of RSC functions in this fungal pathogen. Transcriptomic and proteomic profiling of a conditional mutant of the essential catalytic subunit gene STH1 demonstrated global roles of RSC in C. albicans biology, with the majority of growth-related processes affected, as well as mis-regulation of genes involved in morphotype switching, host-pathogen interaction and adaptive fitness. We further assessed the functions of non-essential CaRSC subunits, showing that the novel subunit Nri1 and the bromodomain subunit Rsc4 play roles in filamentation and stress responses; and also interacted at the genetic level to regulate cell viability. Consistent with these roles, Rsc4 is required for full virulence of C. albicans in the murine model of systemic infection. Taken together, our data builds the first comprehensive study of the composition and roles of RSC in C. albicans, showing both conserved and distinct features compared to model fungal systems. The study illuminates how C. albicans uses RSC-dependent transcriptional regulation to respond to environmental signals and drive survival fitness and virulence in mammals.

Fungal factors involved in host immune evasion, modulation and exploitation during infection

Human and plant pathogenic fungi have a major impact on public health and agriculture. Although these fungi infect very diverse hosts and are often highly adapted to specific host niches, they share surprisingly similar mechanisms that mediate immune evasion, modulation of distinct host targets and exploitation of host nutrients, highlighting that successful strategies have evolved independently among diverse fungal pathogens. These attributes are facilitated by an arsenal of fungal factors. However, not a single molecule, but rather the combined effects of several factors enable these pathogens to establish infection. In this review, we discuss the principles of human and plant fungal pathogenicity mechanisms and discuss recent discoveries made in this field.

Survival Strategies of Pathogenic Candida Species in Human Blood Show Independent and Specific Adaptations

To ensure their survival, pathogens have to adapt immediately to new environments in their hosts, for example, during the transition from the gut to the bloodstream. Here, we investigated the basis of this adaptation in a group of fungal species which are among the most common causes of hospital-acquired infections, the Candida species. On the basis of a human whole-blood infection model, we studied which genes and processes are active over the course of an infection in both the host and four different Candida pathogens. Remarkably, we found that, while the human host response during the early phase of infection is predominantly uniform, the pathogens pursue largely individual strategies and each one regulates genes involved in largely disparate processes in the blood. Our results reveal that C. albicans, C. glabrata, C. parapsilosis, and C. tropicalis all have developed individual strategies for survival in the host. This indicates that their pathogenicity in humans has evolved several times independently and that genes which are central for survival in the host for one species may be irrelevant in another.

Only four species, Candida albicans, C. glabrata, C. parapsilosis, and C. tropicalis, together account for about 90% of all Candida bloodstream infections and are among the most common causes of invasive fungal infections of humans. However, virulence potential varies among these species, and the phylogenetic tree reveals that their pathogenicity may have emerged several times independently during evolution. We therefore tested these four species in a human whole-blood infection model to determine, via comprehensive dual-species RNA-sequencing analyses, which fungal infection strategies are conserved and which are recent evolutionary developments. The ex vivo infection progressed from initial immune cell interactions to nearly complete killing of all fungal cells. During the course of infection, we characterized important parameters of pathogen-host interactions, such as fungal survival, types of interacting immune cells, and cytokine release. On the transcriptional level, we obtained a predominantly uniform and species-independent human response governed by a strong upregulation of proinflammatory processes, which was downregulated at later time points after most of the fungal cells were killed. In stark contrast, we observed that the different fungal species pursued predominantly individual strategies and showed significantly different global transcriptome patterns. Among other findings, our functional analyses revealed that the fungal species relied on different metabolic pathways and virulence factors to survive the host-imposed stress. These data show that adaptation of Candida species as a response to the host is not a phylogenetic trait, but rather has likely evolved independently as a prerequisite to cause human infections.

Natural Variation in Clinical Isolates of Candida albicans Modulates Neutrophil Responses

Neutropenia predisposes patients to life-threatening infection with Candida albicans, a commensal and opportunistic fungal pathogen. How phenotypic variation in C. albicans isolates dictates neutrophil responses is poorly understood. By using a panel of clinical C. albicans strains, here we report that the prototype strain SC5314 induces the most potent accumulation of reactive oxygen species (ROS) and neutrophil extracellular traps (NETs) by human neutrophils of all tested isolates. ROS and NET accumulation positively correlated with the degree of hyphal formation by the isolates, the hypha being the fungal morphotype that promotes pathogenesis. However, there was no correlation of ROS and NET accumulation with fungal killing by neutrophils. Fungal killing was also not correlated with phagocytosis levels or oxidative stress susceptibility of the isolates. The bloodstream isolate P94015 cannot make hyphae and was previously shown to be hyperfit in the murine gut commensalism model. Our results show that P94015 displays poor phagocytosis by neutrophils, the least ROS and NET accumulation of all tested isolates, and resistance to neutrophil-mediated killing. Our data suggest that reduced susceptibility to neutrophils is likely to be independent from a previously described genetic mutation in P94015 that promotes commensalism. Reduced clearance by neutrophils could benefit commensal fitness of C. albicans and could also have promoted the virulence of P94015 in the human patient in the absence of hyphal morphogenesis. Collectively, our study provides new insights into neutrophil interactions with C. albicans and suggests that studying diverse isolates informs knowledge of the relevant aspects of this key immune interaction.IMPORTANCE Neutrophils are the key immune cell type for host defenses against infections with Candida albicansC. albicans strains isolated from patients display large phenotypic diversity, but how this diversity impacts host-pathogen interactions with neutrophils is incompletely defined. Here, we show that important neutrophil responses, such as accumulation of reactive oxygen species and neutrophil extracellular traps, as well as the levels of phagocytosis and killing of the pathogen, differ when comparing diverse C. albicans isolates. A bloodstream patient isolate previously described as more suited to commensalism than pathogenesis in animal models is relatively "silent" to neutrophils and resistant to killing. Our findings illuminate the relationships between fungal morphogenesis, neutrophil responses, and C. albicans survival. Our findings suggest that host phenotypes of a commensally adapted strain could be driven by resistance to immune clearance and indicate that we should extend our studies beyond the "prototype" strain SC5314 for deeper understanding of Candida-neutrophil interactions.

Copper-only superoxide dismutase enzymes and iron starvation stress in Candida fungal pathogens

Copper (Cu)-only superoxide dismutases (SOD) represent a newly characterized class of extracellular SODs important for virulence of several fungal pathogens. Previous studies of the Cu-only enzyme SOD5 from the opportunistic fungal pathogen Candida albicans have revealed that the active-site structure and Cu binding of SOD5 strongly deviate from those of Cu/Zn-SODs in its animal hosts, making Cu-only SODs a possible target for future antifungal drug design. C. albicans also expresses a Cu-only SOD4 that is highly similar in sequence to SOD5, but is poorly characterized. Here, we compared the biochemical, biophysical, and cell biological properties of C. albicans SOD4 and SOD5. Analyzing the recombinant proteins, we found that, similar to SOD5, Cu-only SOD4 can react with superoxide at rates approaching diffusion limits. Both SODs were monomeric and they exhibited similar binding affinities for their Cu cofactor. In C. albicans cultures, SOD4 and SOD5 were predominantly cell wall proteins. Despite these similarities, the SOD4 and SOD5 genes strongly differed in transcriptional regulation. SOD5 was predominantly induced during hyphal morphogenesis, together with a fungal burst in reactive oxygen species. Conversely, SOD4 expression was specifically up-regulated by iron (Fe) starvation and controlled by the Fe-responsive transcription factor SEF1. Interestingly, Candida tropicalis and the emerging fungal pathogen Candida auris contain a single SOD5-like SOD rather than a pair, and in both fungi, this SOD was induced by Fe starvation. This unexpected link between Fe homeostasis and extracellular Cu-SODs may help many fungi adapt to Fe-limited conditions of their hosts.

Exploiting the vulnerable active site of a copper-only superoxide dismutase to disrupt fungal pathogenesis

Copper-only superoxide dismutases (SODs) represent a new class of SOD enzymes that are exclusively extracellular and unique to fungi and oomycetes. These SODs are essential for virulence of fungal pathogens in pulmonary and disseminated infections, and we show here an additional role for copper-only SODs in promoting survival of fungal biofilms. The opportunistic fungal pathogen Candida albicans expresses three copper-only SODs, and deletion of one of them, SOD5, eradicated candidal biofilms on venous catheters in a rodent model. Fungal copper-only SODs harbor an irregular active site that, unlike their Cu,Zn-SOD counterparts, contains a copper co-factor unusually open to solvent and lacks zinc for stabilizing copper binding, making fungal copper-only SODs highly vulnerable to metal chelators. We found that unlike mammalian Cu,Zn-SOD1, C. albicans SOD5 indeed rapidly loses its copper to metal chelators such as EDTA, and binding constants for Cu(II) predict that copper-only SOD5 has a much lower affinity for copper than does Cu,Zn-SOD1. We screened compounds with a variety of indications and identified several metal-binding compounds, including the ionophore pyrithione zinc (PZ), that effectively inhibit C. albicans SOD5 but not mammalian Cu,Zn-SOD1. We observed that PZ both acts as an ionophore that promotes uptake of toxic metals and inhibits copper-only SODs. The pros and cons of a vulnerable active site for copper-only SODs and the possible exploitation of this vulnerability in antifungal drug design are discussed.

Candida albicans Ras1 Inactivation Increases Resistance to Phagosomal Killing by Human Neutrophils

Host phagocytic cells are crucial players in initial defense against Candida albicans infection. C. albicans utilizes MAP kinases and Ras1 stress response signaling pathways to protect itself from killing by immune cells. In this study, we tested the importance of these pathways in C. albicans phagocytosis by neutrophils and subsequent phagosomal survival. Phagocytosis was influenced by C. albicans morphology, so hyphal length of >10 μm reduced the phagocytic index (PI) 2- to 3-fold in human neutrophils. Primary human neutrophils killed 81% of phagocytosed C. albicans, while primary mouse neutrophils killed 63% of yeasts. We found that both the C. albicans Cek1 and Hog1 pathways were required for survival of phagocytosed yeast, whereas deletion of C. albicans RAS1 resulted in an 84% increase in survival within neutrophils compared to that of the wild type (WT). The absence of Ras1 did not alter reactive oxygen species (ROS) production by C. albicans; however, phagocytosed C. albicans Δ/Δras1 cells reduced ROS release by neutrophils by 86%. Moreover, C. albicans Δ/Δras1 cells had increased resistance to hydrogen peroxide as a result of high levels of catalase activity. This phenotype was specific to Ras1, since these effects were not observed in the absence of its partner Cyr1 or with its downstream target Efg1. In addition, C. albicans Δ/Δras1 cells had a significantly increased resistance to nonoxidative killing by human neutrophil peptide 1 (HNP-1) that was reversed by restoring cellular cAMP levels. These data show that C. albicans Ras1 inactivation leads to fungal resistance to both oxidative and nonoxidative mechanisms of neutrophil phagosomal killing.

Innate Immune and Fungal Model of Alzheimer's Disease

Various fungi and bacteria can colonize in the brain and produce physical alterations seen in Alzheimer’s disease (AD). Environmental and genetic factors affect the occurrence of fungal colonization, and how fungi can grow, enter the brain, and interact with the innate immune system. The essence of AD development is the defeat of the innate immune system, whether through vulnerable patient health status or treatment that suppresses inflammation by suppressing the innate immune system. External and mechanical factors that lead to inflammation are a door for pathogenic opportunity. Current research associates the presence of fungi in the etiology of AD and is shown in cerebral tissue at autopsy. From the time of the discovery of AD, much speculation exists for an infective cause. Identifying any AD disease organism is obscured by processes that can take place over years. Amyloid protein deposits are generally considered to be evidence of an intrinsic response to stress or imbalance, but instead amyloid may be evidence of the innate immune response which exists to destroy fungal colonization through structural interference and cytotoxicity. Fungi can remain ensconced for a long time in niches or inside cells, and it is the harboring of fungi that leads to repeated reinfection and slow wider colonization that eventually leads to a grave outcome. Although many fungi and bacteria are associated with AD affected tissues, discussion here focuses on Candida albicans as the archetype of human fungal pathology because of its wide proliferation as a commensal fungus, extensive published research, numerous fungal morphologies, and majority proliferation in AD tissues.

Phagocytes as central players in the defence against invasive fungal infection

Fungal pathogens cause severe and life-threatening infections worldwide. The majority of invasive infections occurs in immunocompromised patients and is based on acquired as well as congenital defects of innate and adaptive immune responses. In many cases, these defects affect phagocyte functions. Consequently, professional phagocytes - mainly monocytes, macrophages, dendritic cells and polymorphonuclear neutrophilic granulocytes - have been shown to act as central players in initiating and modulating antifungal immune responses as well as elimination of fungal pathogens. In this review we will summarize our current understanding on the role of these professional phagocytes in invasive fungal infection to emphasize two important aspects. (i) Analyses on the interaction between fungi and phagocytes have contributed to significant new insights into phagocyte biology. Important examples for this include the identification of pattern recognition receptors for β-glucan, a major cell wall component of many fungal pathogens, as well as the identification of genetic polymorphisms that determine individual host responses towards invading fungi. (ii) At the same time it was shown that fungal pathogens have evolved sophisticated mechanisms to counteract the attack of professional phagocytes. These mechanisms range from complete mechanical destruction of phagocytes to exquisite adaptation of some fungi to the hostile intracellular environment, enabling them to grow and replicate inside professional phagocytes.

Redox Regulation, Rather than Stress-Induced Phosphorylation, of a Hog1 Mitogen-Activated Protein Kinase Modulates Its Nitrosative-Stress-Specific Outputs

In all eukaryotic kingdoms, mitogen-activated protein kinases (MAPKs) play critical roles in cellular responses to environmental cues. These MAPKs are activated by phosphorylation at highly conserved threonine and tyrosine residues in response to specific inputs, leading to their accumulation in the nucleus and the activation of their downstream targets. A specific MAP kinase can regulate different downstream targets depending on the nature of the input signal, thereby raising a key question: what defines the stress-specific outputs of MAP kinases? We find that the Hog1 MAPK contributes to nitrosative-stress resistance in Candida albicans even though it displays minimal stress-induced phosphorylation under these conditions. We show that Hog1 becomes oxidized in response to nitrosative stress, accumulates in the nucleus, and regulates the nitrosative stress-induced transcriptome. Mutation of specific cysteine residues revealed that C156 and C161 function together to promote stress resistance, Hog1-mediated nitrosative-stress-induced gene expression, resistance to phagocytic killing, and C. albicans virulence. We propose that the oxidation of Hog1, rather than its phosphorylation, contributes to the nitrosative-stress-specific responses of this MAP kinase.

Mitogen-activated protein kinases play key roles in the responses of eukaryotic cells to extracellular signals and are critical for environmental-stress resistance. The widely accepted paradigm is that MAP kinases are activated by phosphorylation, which then triggers their nuclear accumulation and the activation of target proteins and genes that promote cellular adaptation. Our data suggest that alternative forms of posttranslational modification can modulate MAP kinase functionality in Candida albicans. We demonstrate that Hog1 is not significantly phosphorylated in response to nitrosative stress, yet it displays nuclear accumulation and contributes to the global transcriptional response to this stress, as well as promoting nitrosative-stress resistance. Instead, nitrosative stress triggers changes in the redox status of Hog1. We also show that specific Hog1 cysteine residues influence its activation of stress genes. Therefore, alternative posttranslational modifications appear to regulate the stress-specific outputs of MAP kinases.

Patient Susceptibility to Candidiasis-A Potential for Adjunctive Immunotherapy

Candida spp. are colonizing fungi of human skin and mucosae of the gastrointestinal and genitourinary tract, present in 30-50% of healthy individuals in a population at any given moment. The host defense mechanisms prevent this commensal fungus from invading and causing disease. Loss of skin or mucosal barrier function, microbiome imbalances, or defects of immune defense mechanisms can lead to an increased susceptibility to severe mucocutaneous or invasive candidiasis. A comprehensive understanding of the immune defense against Candida is essential for developing adjunctive immunotherapy. The important role of underlying genetic susceptibility to Candida infections has become apparent over the years. In most patients, the cause of increased susceptibility to fungal infections is complex, based on a combination of immune regulation gene polymorphisms together with other non-genetic predisposing factors. Identification of patients with an underlying genetic predisposition could help determine which patients could benefit from prophylactic antifungal treatment or adjunctive immunotherapy. This review will provide an overview of patient susceptibility to mucocutaneous and invasive candidiasis and the potential for adjunctive immunotherapy.

Eukaryotic copper-only superoxide dismutases (SODs): A new class of SOD enzymes and SOD-like protein domains

The copper-containing superoxide dismutases (SODs) represent a large family of enzymes that participate in the metabolism of reactive oxygen species by disproportionating superoxide anion radical to oxygen and hydrogen peroxide. Catalysis is driven by the redox-active copper ion, and in most cases, SODs also harbor a zinc at the active site that enhances copper catalysis and stabilizes the protein. Such bimetallic Cu,Zn-SODs are widespread, from the periplasm of bacteria to virtually every organelle in the human cell. However, a new class of copper-containing SODs has recently emerged that function without zinc. These copper-only enzymes serve as extracellular SODs in specific bacteria (i.e. Mycobacteria), throughout the fungal kingdom, and in the fungus-like oomycetes. The eukaryotic copper-only SODs are particularly unique in that they lack an electrostatic loop for substrate guidance and have an unusual open-access copper site, yet they can still react with superoxide at rates limited only by diffusion. Copper-only SOD sequences similar to those seen in fungi and oomycetes are also found in the animal kingdom, but rather than single-domain enzymes, they appear as tandem repeats in large polypeptides we refer to as CSRPs (copper-only SOD-repeat proteins). Here, we compare and contrast the Cu,Zn versus copper-only SODs and discuss the evolution of copper-only SOD protein domains in animals and fungi.

Nanoparticle formulation increases Syzygium cumini antioxidant activity in Candida albicans-infected diabetic rats

CONTEXT

Syzygium cumini (L.) Skeels (Myrtaceae) is a medicinal plant widely used in folk medicine for the treatment of diabetes mellitus (DM). However, studies on the use of this plant and of nanoparticle formulations against DM-related fungal infections are scarce.

OBJECTIVE

To evaluate the effect of the treatments with aqueous seed extract of S. cumini (ASc) and ASc-loaded polymeric nanoparticles (NPASc) on biochemical parameters in Candida albicans-infected diabetic rats.

MATERIALS AND METHODS

Male Wistar rats were divided into eight groups: Control, DM, C. albicans, C. albicans + ASc, C. albicans + NPASc, DM + C. albicans, DM + C. albicans + ASc and DM + C. albicans + NPASc. Rats were daily treated with ASc or NPASc (100 mg/kg) for 21 days. Biochemical parameters in serum and urine, advanced oxidation protein product (AOPP) and TBARS levels in the serum, kidney, liver and pancreas and N-acetyl-β-d-glucosaminidase (NAG) activities in kidney and urine were evaluated.

RESULTS

Biochemical and oxidative stress parameters increased in rats with DM and/or candidiasis. NPASc was more effective than ASc in decreasing glucose (56%), cholesterol (33%) and creatinine (51%) levels; serum (16%) and pancreatic (46%) AOPP and renal (48%) TBARS levels when compared with DM + C. albicans group. In C. albicans group, both treatments decreased NAG activity but did not decrease creatinine levels.

CONCLUSIONS

These data suggest that the use of nanotechnology is able to improve plant extract properties such as antioxidant activity that may be useful in diabetes-related complications.

Candida albicans FRE8 encodes a member of the NADPH oxidase family that produces a burst of ROS during fungal morphogenesis

Until recently, NADPH oxidase (NOX) enzymes were thought to be a property of multicellularity, where the reactive oxygen species (ROS) produced by NOX acts in signaling processes or in attacking invading microbes through oxidative damage. We demonstrate here that the unicellular yeast and opportunistic fungal pathogen Candida albicans is capable of a ROS burst using a member of the NOX enzyme family, which we identify as Fre8. C. albicans can exist in either a unicellular yeast-like budding form or as filamentous multicellular hyphae or pseudohyphae, and the ROS burst of Fre8 begins as cells transition to the hyphal state. Fre8 is induced during hyphal morphogenesis and specifically produces ROS at the growing tip of the polarized cell. The superoxide dismutase Sod5 is co-induced with Fre8 and our findings are consistent with a model in which extracellular Sod5 acts as partner for Fre8, converting Fre8-derived superoxide to the diffusible H₂O₂ molecule. Mutants of fre8Δ/Δ exhibit a morphogenesis defect in vitro and are specifically impaired in development or maintenance of elongated hyphae, a defect that is rescued by exogenous sources of H₂O₂. A fre8Δ/Δ deficiency in hyphal development was similarly observed in vivo during C. albicans invasion of the kidney in a mouse model for disseminated candidiasis. Moreover C. albicans fre8Δ/Δ mutants showed defects in a rat catheter model for biofilms. Together these studies demonstrate that like multicellular organisms, C. albicans expresses NOX to produce ROS and this ROS helps drive fungal morphogenesis in the animal host.

We demonstrate here that the opportunistic human fungal pathogen Candida albicans uses a NADPH oxidase enzyme (NOX) and reactive oxygen species (ROS) to control morphogenesis in an animal host. C. albicans was not previously known to express NOX enzymes as these were thought to be a property of multicellular organisms, not unicellular yeasts. We describe here the identification of C. albicans Fre8 as the first NOX enzyme that can produce extracellular ROS in a unicellular yeast. C. albicans can exist as either a unicellular yeast or as multicellular elongated hyphae, and Fre8 is specially expressed during transition to the hyphal state where it works to produce ROS at the growing tip of the polarized cell. C. albicans cells lacking Fre8 exhibit a deficiency in elongated hyphae during fungal invasion of the kidney in a mouse model for systemic candidiasis. Moreover, Fre8 is required for fungal survival in a rodent model for catheter biofilms. These findings implicate a role for fungal derived ROS in controlling morphogenesis of this important fungal pathogen for public health.

Differential Metabolism of a Two-Carbon Substrate by Members of the Paracoccidioides Genus

The genus Paracoccidioides comprises known fungal pathogens of humans and can be isolated from different infection sites. Metabolic peculiarities in different members of the Paracoccidioides led us to perform proteomic studies in the presence of the two-carbon molecule acetate, which predominates in the nutrient-poor environment of the phagosome. To investigate the expression rates of proteins of different members of Paracoccidioides, including one isolate of P. lutzii (Pb01) and three isolates of P. brasiliensis (Pb03, Pb339, and PbEPM83), using sodium acetate as a carbon source, proteins were quantified using label-free and data-independent liquid chromatography-mass spectrometry. Protein profiles of the isolates were statistically analyzed, revealing proteins that were differentially expressed when the fungus was cultivated in a non-preferential carbon source rather than glucose. A total of 1,160, 1,211, 1,280, and 1,462 proteins were reproducibly identified and relatively quantified in P. lutzii and the P. brasiliensis isolates Pb03, Pb339, and PbEPM83, respectively. Notably, 526, 435, 744, and 747 proteins were differentially expressed among P. lutzii and the P. brasiliensis isolates Pb03, Pb339, and PbEPM83, respectively, with a fold-change equal to or higher than 1.5. This analysis revealed that reorganization of metabolism occurred through the induction of proteins related to gluconeogenesis, glyoxylic/glyoxylate cycle, response to stress, and degradation of amino acids in the four isolates. The following differences were observed among the isolates: higher increases in the expression levels of proteins belonging to the TCA and respiratory chain in PbEPM83 and Pb01; increase in ethanol production in Pb01; utilization of cell wall components for gluconeogenesis in Pb03 and PbEPM83; and increased β-oxidation and methylcitrate cycle proteins in Pb01and PbEPM83. Proteomic profiles indicated that the four isolates reorganized their metabolism in different manners to use acetate as a carbon source.

The carboxylic acid transporters Jen1 and Jen2 affect the architecture and fluconazole susceptibility of Candida albicans biofilm in the presence of lactate

Candida albicans has the ability to adapt to different host niches, often glucose-limited but rich in alternative carbon sources. In these glucose-poor microenvironments, this pathogen expresses JEN1 and JEN2 genes, encoding carboxylate transporters, which are important in the early stages of infection. This work investigated how host microenvironments, in particular acidic containing lactic acid, affect C. albicans biofilm formation and antifungal drug resistance. Multiple components of the extracellular matrix were also analysed, including their impact on antifungal drug resistance, and the involvement of both Jen1 and Jen2 in this process. The results show that growth on lactate affects biofilm formation, morphology and susceptibility to fluconazole and that both Jen1 and Jen2 might play a role in these processes. These results support the view that the adaptation of Candida cells to the carbon source present in the host niches affects their pathogenicity.

Dual transcriptome of the immediate neutrophil and Candida albicans interplay

Background

Neutrophils are traditionally considered transcriptionally inactive. Compared to other immune cells, little is known about their transcriptional profile during interaction with pathogens.

Methods

We analyzed the meta-transcriptome of the neutrophil-Candida albicans interplay and the transcriptome of C. albicans challenged with neutrophil extracellular traps (NETs) by RNA-Seq, considering yeast and hypha individually in each approach.

Results

The neutrophil response to C. albicans yeast and hyphae was dominated by a morphotype-independent core response. However, 11 % of all differentially expressed genes were regulated in a specific manner when neutrophils encountered the hyphal form of C. albicans. While involving genes for transcriptional regulators, receptors, and cytokines, the neutrophil core response lacked typical antimicrobial effectors genes. Genes of the NOD-like receptor pathway, including NLRP3, were enriched.

Neutrophil- and NET-provoked responses in C. albicans differed. At the same time, the Candida transcriptome upon neutrophil encounter and upon NET challenge included genes from various metabolic processes and indicate a mutual role of the regulators Tup1p, Efg1p, Hap43p, and Cap1p. Upon challenge with neutrophils and NETs, the overall Candida response was partially morphotype-specific. Yet again, actual oppositional regulation in yeasts and hyphae was only detected for the arginine metabolism in neutrophil-infecting C. albicans.

Conclusions

 Taken together, our study provides a comprehensive and quantitative transcript profile of the neutrophil–C. albicans interaction. By considering the two major appearances of both, neutrophils and C. albicans, our study reveals yet undescribed insights into this medically relevant encounter. Hence, our findings will facilitate future research and potentially inspire novel therapy developments.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4097-4) contains supplementary material, which is available to authorized users.

Sfp1 and Rtg3 reciprocally modulate carbon source-conditional stress adaptation in the pathogenic yeast Candida albicans

The pathogenicity of the clinically important yeast, Candida albicans, is dependent on robust responses to host-imposed stresses. These stress responses have generally been dissected in vitro at 30°C on artificial growth media that do not mimic host niches. Yet host inputs, such as changes in carbon source or temperature, are known to affect C. albicans stress adaptation. Therefore, we performed screens to identify novel regulators that promote stress resistance during growth on a physiologically relevant carboxylic acid and at elevated temperatures. These screens revealed that, under these 'non-standard' growth conditions, numerous uncharacterised regulators are required for stress resistance in addition to the classical Hog1, Cap1 and Cta4 stress pathways. In particular, two transcription factors (Sfp1 and Rtg3) promote stress resistance in a reciprocal, carbon source-conditional manner. SFP1 is induced in stressed glucose-grown cells, whereas RTG3 is upregulated in stressed lactate-grown cells. Rtg3 and Sfp1 regulate the expression of key stress genes such as CTA4, CAP1 and HOG1 in a carbon source-dependent manner. These mechanisms underlie the stress sensitivity of C. albicans sfp1 cells during growth on glucose, and rtg3 cells on lactate. The data suggest that C. albicans exploits environmentally contingent regulatory mechanisms to retain stress resistance during host colonisation.

Elevated catalase expression in a fungal pathogen is a double-edged sword of iron

Most fungal pathogens of humans display robust protective oxidative stress responses that contribute to their pathogenicity. The induction of enzymes that detoxify reactive oxygen species (ROS) is an essential component of these responses. We showed previously that ectopic expression of the heme-containing catalase enzyme in Candida albicans enhances resistance to oxidative stress, combinatorial oxidative plus cationic stress, and phagocytic killing. Clearly ectopic catalase expression confers fitness advantages in the presence of stress, and therefore in this study we tested whether it enhances fitness in the absence of stress. We addressed this using a set of congenic barcoded C. albicans strains that include doxycycline-conditional tetON-CAT1 expressors. We show that high basal catalase levels, rather than CAT1 induction following stress imposition, reduce ROS accumulation and cell death, thereby promoting resistance to acute peroxide or combinatorial stress. This conclusion is reinforced by our analyses of phenotypically diverse clinical isolates and the impact of stochastic variation in catalase expression upon stress resistance in genetically homogeneous C. albicans populations. Accordingly, cat1Δ cells are more sensitive to neutrophil killing. However, we find that catalase inactivation does not attenuate C. albicans virulence in mouse or invertebrate models of systemic candidiasis. Furthermore, our direct comparisons of fitness in vitro using isogenic barcoded CAT1, cat1Δ and tetON-CAT1 strains show that, while ectopic catalase expression confers a fitness advantage during peroxide stress, it confers a fitness defect in the absence of stress. This fitness defect is suppressed by iron supplementation. Also high basal catalase levels induce key iron assimilatory functions (CFL5, FET3, FRP1, FTR1). We conclude that while high basal catalase levels enhance peroxide stress resistance, they place pressure on iron homeostasis through an elevated cellular demand for iron, thereby reducing the fitness of C. albicans in iron-limiting tissues within the host.

The pathogenic yeast Candida albicans faces multiple challenges within its human host. These include the need to protect itself against the toxic oxidants used by the host to kill invading microbes, and the need to scavenge iron, an essential micronutrient that is limiting in certain tissues. The iron-containing enzyme, catalase, detoxifies hydrogen peroxide, thereby playing a major role in protecting C. albicans against reactive oxygen species and neutrophil killing. Indeed, we show that high basal catalase expression increases the resistance of this yeast to oxidative and combinatorial (oxidative plus cationic) stresses. Yet, rather than enhancing the virulence of C. albicans as had been predicted, high basal catalase expression decreases fungal colonisation in certain iron-limiting tissues. Furthermore, we demonstrate that catalase inactivation does not significantly perturb the virulence of C. albicans in models of systemic infection. We also show that ectopic catalase expression increases the demand for iron in C. albicans, thereby reducing the fitness of this pathogen in the absence of stress under iron-limiting conditions. Therefore, high basal catalase expression is a double-edged sword: it enhances the fitness of C. albicans in the presence of stress, but reduces fitness in the absence of stress. This explains why catalase overexpression reduces rather than enhances virulence.

Adaptation of Candida albicans to Reactive Sulfur Species

Candida albicans is an opportunistic fungal pathogen that is highly resistant to different oxidative stresses. How reactive sulfur species (RSS) such as sulfite regulate gene expression and the role of the transcription factor Zcf2 and the sulfite exporter Ssu1 in such responses are not known. Here, we show that C. albicans specifically adapts to sulfite stress and that Zcf2 is required for that response as well as induction of genes predicted to remove sulfite from cells and to increase the intracellular amount of a subset of nitrogen metabolites. Analysis of mutants in the sulfate assimilation pathway show that sulfite conversion to sulfide accounts for part of sulfite toxicity and that Zcf2-dependent expression of the SSU1 sulfite exporter is induced by both sulfite and sulfide. Mutations in the SSU1 promoter that selectively inhibit induction by the reactive nitrogen species (RNS) nitrite, a previously reported activator of SSU1, support a model for C. albicans in which Cta4-dependent RNS induction and Zcf2-dependent RSS induction are mediated by parallel pathways, different from S. cerevisiae in which the transcription factor Fzf1 mediates responses to both RNS and RSS. Lastly, we found that endogenous sulfite production leads to an increase in resistance to exogenously added sulfite. These results demonstrate that C. albicans has a unique response to sulfite that differs from the general oxidative stress response, and that adaptation to internal and external sulfite is largely mediated by one transcription factor and one effector gene.

Robust Extracellular pH Modulation by Candida albicans during Growth in Carboxylic Acids

The opportunistic fungal pathogen Candida albicans thrives within diverse niches in the mammalian host. Among the adaptations that underlie this fitness is an ability to utilize a wide array of nutrients, especially sources of carbon that are disfavored by many other fungi; this contributes to its ability to survive interactions with the phagocytes that serve as key barriers against disseminated infections. We have reported that C. albicans generates ammonia as a byproduct of amino acid catabolism to neutralize the acidic phagolysosome and promote hyphal morphogenesis in a manner dependent on the Stp2 transcription factor. Here, we report that this species rapidly neutralizes acidic environments when utilizing carboxylic acids like pyruvate, α-ketoglutarate (αKG), or lactate as the primary carbon source. Unlike in cells growing in amino acid-rich medium, this does not result in ammonia release, does not induce hyphal differentiation, and is genetically distinct. While transcript profiling revealed significant similarities in gene expression in cells grown on either carboxylic or amino acids, genetic screens for mutants that fail to neutralize αKG medium identified a nonoverlapping set of genes, including CWT1, encoding a transcription factor responsive to cell wall and nitrosative stresses. Strains lacking CWT1 exhibit retarded αKG-mediated neutralization in vitro, exist in a more acidic phagolysosome, and are more susceptible to macrophage killing, while double cwt1Δ stp2Δ mutants are more impaired than either single mutant. Together, our observations indicate that C. albicans has evolved multiple ways to modulate the pH of host-relevant environments to promote its fitness as a pathogen.

IMPORTANCE

The fungal pathogen Candida albicans is a ubiquitous and usually benign constituent of the human microbial ecosystem. In individuals with weakened immune systems, this organism can cause potentially life-threatening infections and is one of the most common causes of hospital-acquired infections. Understanding the interactions between C. albicans and immune phagocytic cells, such as macrophages and neutrophils, will define the mechanisms of pathogenesis in this species. One such adaptation is an ability to make use of nonstandard nutrients that we predict are plentiful in certain niches within the host, including within these phagocytic cells. We show here that the metabolism of certain organic acids enables C. albicans to neutralize acidic environments, such as those within macrophages. This phenomenon is distinct in several significant ways from previous reports of similar processes, indicating that C. albicans has evolved multiple mechanisms to combat the harmful acidity of phagocytic cells.

Anti-Immune Strategies of Pathogenic Fungi

Pathogenic fungi have developed many strategies to evade the host immune system. Multiple escape mechanisms appear to function together to inhibit attack by the various stages of both the adaptive and the innate immune response. Thus, after entering the host, such pathogens fight to overcome the immune system to allow their survival, colonization and spread to different sites of infection. Consequently, the establishment of a successful infectious process is closely related to the ability of the pathogen to modulate attack by the immune system. Most strategies employed to subvert or exploit the immune system are shared among different species of fungi. In this review, we summarize the main strategies employed for immune evasion by some of the major pathogenic fungi.

Cell biology of Candida albicans-host interactions

•

The cell biology of Candida albicans is adapted both for life as a commensal and as a pathogen.

•

C. albicans can either downregulate or upregulate virulence properties in the human host.

•

This fungus modulates the activity of phagocytes to enable its own survival.

•

Candida is metabolically flexible enabling it to survive in multiple niches in the host.

Candida albicans is a commensal coloniser of most people and a pathogen of the immunocompromised or patients in which barriers that prevent dissemination have been disrupted. Both the commensal and pathogenic states involve regulation and adaptation to the host microenvironment. The pathogenic potential can be downregulated to sustain commensalism or upregulated to damage host tissue and avoid and subvert immune surveillance. In either case it seems as though the cell biology of this fungus has evolved to enable the establishment of different types of relationships with the human host. Here we summarise latest advances in the analysis of mechanisms that enable C. albicans to occupy different body sites whilst avoiding being eliminated by the sentinel activities of the human immune system.

The Phylogeny and Active Site Design of Eukaryotic Copper-only Superoxide Dismutases

In eukaryotes the bimetallic Cu/Zn superoxide dismutase (SOD) enzymes play important roles in the biology of reactive oxygen species by disproportionating superoxide anion. Recently, we reported that the fungal pathogen Candida albicans expresses a novel copper-only SOD, known as SOD5, that lacks the zinc cofactor and electrostatic loop (ESL) domain of Cu/Zn-SODs for substrate guidance. Despite these abnormalities, C. albicans SOD5 can disproportionate superoxide at rates limited only by diffusion. Here we demonstrate that this curious copper-only SOD occurs throughout the fungal kingdom as well as in phylogenetically distant oomycetes or "pseudofungi" species. It is the only form of extracellular SOD in fungi and oomycetes, in stark contrast to the extracellular Cu/Zn-SODs of plants and animals. Through structural biology and biochemical approaches we demonstrate that these copper-only SODs have evolved with a specialized active site consisting of two highly conserved residues equivalent to SOD5 Glu-110 and Asp-113. The equivalent positions are zinc binding ligands in Cu/Zn-SODs and have evolved in copper-only SODs to control catalysis and copper binding in lieu of zinc and the ESL. Similar to the zinc ion in Cu/Zn-SODs, SOD5 Glu-110 helps orient a key copper-coordinating histidine and extends the pH range of enzyme catalysis. SOD5 Asp-113 connects to the active site in a manner similar to that of the ESL in Cu/Zn-SODs and assists in copper cofactor binding. Copper-only SODs are virulence factors for certain fungal pathogens; thus this unique active site may be a target for future anti-fungal strategies.

Innate immune cell response upon Candida albicans infection

Candida albicans is a polymorphic fungus which is the predominant cause of superficial and deep tissue fungal infections. This microorganism has developed efficient strategies to invade the host and evade host defense systems. However, the host immune system will be prepared for defense against the microbe by recognition of receptors, activation of signal transduction pathways and cooperation of immune cells. As a consequence, C. albicans could either be eliminated by immune cells rapidly or disseminate hematogenously, leading to life-threatening systemic infections. The interplay between Candida albicans and the host is complex, requiring recognition of the invaded pathogens, activation of intricate pathways and collaboration of various immune cells. In this review, we will focus on the effects of innate immunity that emphasize the first line protection of host defense against invaded C. albicans including the basis of receptor-mediated recognition and the mechanisms of cell-mediated immunity.

The Rewiring of Ubiquitination Targets in a Pathogenic Yeast Promotes Metabolic Flexibility, Host Colonization and Virulence

Efficient carbon assimilation is critical for microbial growth and pathogenesis. The environmental yeast Saccharomyces cerevisiae is “Crabtree positive”, displaying a rapid metabolic switch from the assimilation of alternative carbon sources to sugars. Following exposure to sugars, this switch is mediated by the transcriptional repression of genes (carbon catabolite repression) and the turnover (catabolite inactivation) of enzymes involved in the assimilation of alternative carbon sources. The pathogenic yeast Candida albicans is Crabtree negative. It has retained carbon catabolite repression mechanisms, but has undergone posttranscriptional rewiring such that gluconeogenic and glyoxylate cycle enzymes are not subject to ubiquitin-mediated catabolite inactivation. Consequently, when glucose becomes available, C. albicans can continue to assimilate alternative carbon sources alongside the glucose. We show that this metabolic flexibility promotes host colonization and virulence. The glyoxylate cycle enzyme isocitrate lyase (CaIcl1) was rendered sensitive to ubiquitin-mediated catabolite inactivation in C. albicans by addition of a ubiquitination site. This mutation, which inhibits lactate assimilation in the presence of glucose, reduces the ability of C. albicans cells to withstand macrophage killing, colonize the gastrointestinal tract and cause systemic infections in mice. Interestingly, most S. cerevisiae clinical isolates we examined (67%) have acquired the ability to assimilate lactate in the presence of glucose (i.e. they have become Crabtree negative). These S. cerevisiae strains are more resistant to macrophage killing than Crabtree positive clinical isolates. Moreover, Crabtree negative S. cerevisiae mutants that lack Gid8, a key component of the Glucose-Induced Degradation complex, are more resistant to macrophage killing and display increased virulence in immunocompromised mice. Thus, while Crabtree positivity might impart a fitness advantage for yeasts in environmental niches, the more flexible carbon assimilation strategies offered by Crabtree negativity enhance the ability of yeasts to colonize and infect the mammalian host.

Most yeast species occupy environmental niches, but some infect humans. All species must assimilate carbon to grow and colonize their niche, but carbon source availability differs significantly between niches. The environmental yeast Saccharomyces cerevisiae is thought to have evolved under conditions of sugar feast and famine because it has evolved mechanisms to exploit energetically favourable sugars first, and then switch to using alternative carbon sources. These mechanisms depend on catabolite inactivation—the degradation of enzymes involved in the assimilation of alternative carbon sources when glucose is present. In the pathogenic yeast Candida albicans, which has evolved in sugar-poor host niches, these enzymes are not subject to catabolite inactivation. Consequently, C. albicans can simultaneously exploit sugars and alternative carbon sources. We demonstrate that this metabolic flexibility promotes resistance to macrophage killing, gut colonization, and the ability to cause systemic infection. We also show that many S. cerevisiae clinical isolates have lost catabolite inactivation, and hence can simultaneously assimilate sugars and alternative carbon sources. The disruption of catabolite inactivation in S. cerevisiae renders it more resistant to phagocytic killing, and more virulent. We conclude that virulence is enhanced by metabolic flexibility.

Mechanisms Underlying the Delayed Activation of the Cap1 Transcription Factor in Candida albicans following Combinatorial Oxidative and Cationic Stress Important for Phagocytic Potency

Following phagocytosis, microbes are exposed to an array of antimicrobial weapons that include reactive oxygen species (ROS) and cationic fluxes. This is significant as combinations of oxidative and cationic stresses are much more potent than the corresponding single stresses, triggering the synergistic killing of the fungal pathogen Candida albicans by “stress pathway interference.” Previously we demonstrated that combinatorial oxidative plus cationic stress triggers a dramatic increase in intracellular ROS levels compared to oxidative stress alone. Here we show that activation of Cap1, the major regulator of antioxidant gene expression in C. albicans, is significantly delayed in response to combinatorial stress treatments and to high levels of H₂O₂. Cap1 is normally oxidized in response to H₂O₂; this masks the nuclear export sequence, resulting in the rapid nuclear accumulation of Cap1 and the induction of Cap1-dependent genes. Here we demonstrate that following exposure of cells to combinatorial stress or to high levels of H₂O₂, Cap1 becomes trapped in a partially oxidized form, Cap1^OX-1. Notably, Cap1-dependent gene expression is not induced when Cap1 is in this partially oxidized form. However, while Cap1^OX-1 readily accumulates in the nucleus and binds to target genes following high-H₂O₂ stress, the nuclear accumulation of Cap1^OX-1 following combinatorial H₂O₂ and NaCl stress is delayed due to a cationic stress-enhanced interaction with the Crm1 nuclear export factor. These findings define novel mechanisms that delay activation of the Cap1 transcription factor, thus preventing the rapid activation of the stress responses vital for the survival of C. albicans within the host.

Combinatorial stress-mediated synergistic killing represents a new unchartered area in the field of stress signaling. This phenomenon contrasts starkly with “stress cross-protection,” where exposure to one stress protects against subsequent exposure to a different stress. Previously we demonstrated that the pathogen Candida albicans is acutely sensitive to combinations of cationic and oxidative stresses, because the induction of H₂O₂-responsive genes is blocked in the presence of cationic stress. We reveal that this is due to novel mechanisms that delay activation of the Cap1 AP-1-like transcription factor, the major regulator of the H₂O₂-induced regulon. Cap1 becomes trapped in a partially oxidized form following simultaneous exposure to oxidative and cationic stresses. In addition, cationic stress promotes the interaction of Cap1 with the Crm1 nuclear export factor, thus inhibiting its nuclear accumulation. These mechanisms probably explain the potency of neutrophils, which employ multiple stresses to kill fungal pathogens.