Zebrafish as a model host for Candida albicans infection

Bibliometric analysis and thematic review of Candida pathogenesis: Fundamental omics to applications as potential antifungal drugs and vaccines

Invasive candidiasis caused by the pathogenic Candida yeast species has resulted in elevating global mortality. The pathogenicity of Candida spp. is not only originated from its primary invasive yeast-to-hyphal transition; virulence factors (transcription factors, adhesins, invasins, and enzymes), biofilm, antifungal drug resistance, stress tolerance, and metabolic adaptation have also contributed to a greater clinical burden. However, the current research theme in fungal pathogenicity could hardly be delineated with the increasing research output. Therefore, our study analysed the research trends in Candida pathogenesis over the past 37 years via a bibliometric approach against the Scopus and Web of Science databases. Based on the 3993 unique documents retrieved, significant international collaborations among researchers were observed, especially between Germany (Bernhard Hube) and the UK (Julian Naglik), whose focuses are on Candida proteinases, adhesins, and candidalysin. The prominent researchers (Neils Gow, Alistair Brown, and Frank Odds) at the University of Exeter and the University of Aberdeen (second top performing affiliation) UK contribute significantly to the mechanisms of Candida adaptation, tolerance, and stress response. However, the science mapping of co-citation analysis performed herein could not identify a hub representative of subsequent work since the clusters were semi-redundant. The co-word analysis that was otherwise adopted, revealed three research clusters; the cluster-based thematic analyses indicated the severeness of Candida biofilm and antifungal resistance as well as the elevating trend on molecular mechanism elucidation for drug screening and repurposing. Importantly, the in vivo pathogen adaptation and interactions with hosts are crucial for potential vaccine development.

Chitosan nanogel aqueous treatment improved blood biochemicals, antioxidant capacity, immune response, immune-related gene expression and infection resistance of Nile tilapia

Candida albicans is a pathogenic yeast recently associated with diverse diseases in aquaculture. The present study investigated the efficacy of chitosan nanogel (CNG) in ameliorating effects of C. albicans on Nile tilapia (Oreochromis niloticus). Fish were randomly distributed into four groups (control, waterborne CNG at 75 μg/L, intraperitoneally challenged with C. albicans (1.8 × 107 CFU/mL), and waterborne CNG at 75 μg/L + C. albicans at 1.8 × 107 CFU/mL). Results showed that C. albicans infection reduced survival rate (57.5%) and caused marked clinical symptoms in fish among all infected groups. Substantial declines in immunological (complement 3, lysozyme, and immunoglobulin M), protein (total protein and non-albumin protein), and antioxidant (catalase, glutathione peroxidase, and superoxide dismutase) biochemical endpoints were exhibited, The C. albicans infected group also showed marked down-regulation in the expression of immune-related genes, including toll-like receptor 2 (TLR-2), transforming growth factor beta2, tumor necrosis factor alpha, interleukins (IL-1β, IL-6, and IL-10), and antiapoptotic gene (B-cell lymphoma, BCL-2). The expression of the apoptotic gene (Bcl-2 associated X protein, BAX) was up-regulated in fish challenged by C. albicans. The application of waterborne CNG to fish challenged with C. albicans infection improved fish survival (79.5%) and all other measured parameters. The main finding of this work was that CNG is a nanotechnology with potential for preventing degraded health status by C. albicans infection in tilapia, and thus has promise as an intervention in aquaculture settings.

Farnesol remodels the peritoneal cavity immune environment influencing Candida albicans pathogenesis during intra-abdominal infection

Candida albicans is a lifelong member of the mycobiome causing mucosal candidiasis and life-threatening, systemic, and intra-abdominal disease in immunocompromised and transplant patients. Despite the clinical importance of intra-abdominal candidiasis with mortality rates between 40% and 70%, the contribution of fungal virulence factors and host immune responses to disease has not been extensively studied. Secretion of the quorum-sensing molecule, farnesol, acts as a virulence factor for C. albicans during systemic infection, while inducing local, protective innate immune responses in oral models of infection. Previously, we reported that farnesol recruits macrophages to the peritoneal cavity in mice, suggesting a role for farnesol in innate immune responses. Here, we expand on our initial findings, showing that farnesol profoundly alters the peritoneal cavity microenvironment promoting innate inflammation. Intra-peritoneal injection of farnesol stimulates rapid local death of resident peritoneal cells followed by recruitment of neutrophils and inflammatory macrophages into the peritoneal cavity and peritoneal mesothelium associated with an early increase in chemokines followed by proinflammatory cytokines. These rapid inflammatory responses to farnesol significantly increase morbidity and mortality of mice with intra-abdominal candidiasis associated with increased formation of peritoneal adhesions, despite similar rates of fungal clearance from the peritoneal cavity and retro-peritoneal organs. C. albicans ddp3Δ/ddp3Δ knockout and reconstituted strains recapitulate these findings. This indicates that farnesol may be detrimental to the host during intra-abdominal infections. Importantly, our results highlight a need to understand how C. albicans virulence factors modulate the host immune response within the peritoneum, an exceedingly common site of Candida infection.

Protection against lethal sepsis following immunization with Candida species varies by isolate and inversely correlates with bone marrow tissue damage

Protection against lethal Candida albicans (Ca)/Staphylococcus aureus (Sa) intra-abdominal infection (IAI)-mediated sepsis can be achieved by a novel form of trained innate immunity (TII) involving Gr-1+ myeloid-derived suppressor cells (MDSCs) that are induced by inoculation (immunization) with low virulence Candida species [i.e., Candida dubliniensis (Cd)] that infiltrate the bone marrow (BM). In contrast, more virulent Candida species (i.e., C. albicans), even at sub-lethal inocula, fail to induce similar levels of protection. The purpose of the present study was to test the hypothesis that the level of TII-mediated protection induced by Ca strains inversely correlates with damage in the BM as a reflection of virulence. Mice were immunized by intraperitoneal inoculation with several parental and mutant strains of C. albicans deficient in virulence factors (hyphal formation and candidalysin production), followed by an intraperitoneal Ca/Sa challenge 14 d later and monitored for sepsis and mortality. Whole femur bones were collected 24 h and 13 d after immunization and assessed for BM tissue/cellular damage via ferroptosis and histology. While immunization with standard but not sub-lethal inocula of most wild-type C. albicans strains resulted in considerable mortality, protection against lethal Ca/Sa IAI challenge varied by strain was usually less than that for C. dubliniensis, with no differences observed between parental and corresponding mutants. Finally, levels of protection afforded by the Ca strains were inversely correlated with BM tissue damage (R 2 = -0.773). TII-mediated protection against lethal Ca/Sa sepsis induced by Candida strain immunization inversely correlates with BM tissue/cellular damage as a reflection of localized virulence.

Strain variation in gene expression impact of hyphal cyclin Hgc1 in Candida albicans

Formation of hyphae is a key virulence trait of the fungal pathogen Candida albicans. Hypha morphogenesis depends upon the cyclin Hgc1, which acts together with cyclin-dependent protein kinase Cdc28 to phosphorylate effectors that drive polarized growth. Hgc1 has also been implicated in gene regulation through its effects on 2 transcription factors, Efg1 and Ume6. Here, we report RNA-sequencing (RNA-seq) analysis of 2 pairs of hgc1Δ/Δ mutants and their respective wild-type strains, which lie in 2 different genetic backgrounds. We find that hgc1Δ/Δ mutations alter expression of 271 genes in both genetic backgrounds and 266 of those genes respond consistently with regard to up- or down-regulation. Consistency is similar to what has been observed with efg1Δ/Δ mutations and greater than observed with nrg1Δ/Δ mutations in these 2 backgrounds. The gene expression response includes genes under Efg1 control, as expected from prior studies. Hgc1-responsive genes also include ergosterol biosynthetic genes and bud neck-related genes, which may reflect interactions between Hgc1 and additional transcription factors as well as effects of Hgc1 on cellular length-to-width ratios.

A Comparative Analysis on the Innate Immune Responses of Cirrhinus mrigala Challenged with Pseudomonas aeruginosa and Fusarium oxysporum

Microbes are the most significant ubiquitous pathogens that cause serious infections in freshwater fish, leading to tremendous economic losses. The present study was designed to investigate the extent of changes in cytokine expression, hemato-biochemical parameters, and tissue histology of Cirrhinus mrigala (C. mrigala) challenged with Pseudomonas aeruginosa (P. aeruginosa) and Fusarium oxysporum (F. oxysporum). Fish were divided into three major groups: control, P. aeruginosa-challenged, and F. oxysporum-challenged. The infection in both challenge assays was allowed to progress until 7 days post infection. Upregulated expression of TNF-α and IL-1β was found in blood, gills, livers, and kidneys of the challenged fish. Significant differences were noted in hematological parameters of challenged fish. Alanine aminotransferase, aspartate aminotransferase, and alkaline aminotransferase levels also showed significant differences in infected and control groups. An increase in serum albumin and globulin and a decrease in total protein were noted in infected groups as compared to the control group. Severe histological alterations were noted in gill, liver, and kidney tissues of the infected groups as compared to control. The order of histological alteration index for P. aeruginosa challenge was liver > kidney > gills, and for F. oxysporum challenge it was kidney > liver > gills. These changes in fish infected by P. aeruginosa and F. oxysporum can be used as an effective and subtle index to monitor the physiological and pathological conditions of fish.

Hgc1 Independence of Biofilm Hyphae in Candida albicans

Biofilm and hypha formation are central to virulence of the fungal pathogen Candida albicans. The G1 cyclin gene HGC1 is required for hypha formation under diverse in vitro and in vivo growth conditions. Hgc1 is required for disseminated infection and is a linchpin in the argument that hyphal morphogenesis itself is required for pathogenicity. We report here that HGC1 is dispensable for hypha formation during biofilm formation both in vitro, under strong inducing conditions, and in vivo, in a mouse oropharyngeal candidiasis model. These findings are validated with two or more C. albicans isolates. Systematic screening of overexpressed cyclin genes indicates that CCN1 and CLN3 can compensate partially for Hgc1 function during biofilm growth. This conclusion is also supported by the severity of the hgc1Δ/Δ ccn1Δ/Δ double mutant biofilm defect. Our results suggest that hypha formation in biofilm is accomplished by combined action of multiple cyclins, not solely by Hgc1. IMPORTANCE The HGC1 gene encodes a cyclin that is required for virulence of the fungal pathogen Candida albicans. It is required to produce the elongated hyphal filaments of free-living planktonic cells that are associated with virulence. Here, we show that HGC1 is not required to produce hyphae in the alternative growth form of a biofilm community. We observe Hgc1-independent hyphae in two infection-relevant situations, biofilm growth in vitro and biofilm-like oropharyngeal infection. Our analysis suggests that hypha formation in the biofilm state reflects combined action of multiple cyclins.

Battle royale: Immune response on biofilms – host-pathogen interactions

The research interest of the scientific community in biofilm-forming microorganisms is growing due to the problems caused by their infections affecting humans and animals, mainly because of the difficulty of the host immune system in eradicating these microbial complex communities and the increasing antimicrobial resistance rates worldwide. This review describes the virulence factors and their interaction with the microbial communities of four well-known and highly biofilm-forming pathogens, more exactly, Pseudomonas aeruginosa, Escherichia coli, Staphylococcus spp., and Candida spp. The innate and adaptive immune responses caused by the infection with these microorganisms and their evasion to the host immune system by biofilm formation are discussed in the present work. The relevance of the differences in the expression of certain virulence factors and the immune response in biofilm-associated infections when compared to planktonic infections is usually described as the biofilm architecture protects the pathogen and alters the host immune responses, here we extensively discussed these mechanisms.

Zebrafish as model for studies in dentistry

INTRODUCTION

Over the last years, zebrafish has gained prominence in the biomedical community. It is currently considered one of the best vertebrate animal models for various types of studies, such as toxicology and developmental biology.

OBJECTIVE

The aim of this study was to conduct a literature review on the use of zebrafish in dentistry and whether this animal model could be a viable alternative for performing different types of studies in this area.

METHODS

A literature search was performed using the PubMed, Lilacs, Embase, and Dentistry and Oral Sciences Source. The keywords used as search terms were zebrafish and dentistry. The selection criteria were articles published in English that used zebrafish as an animal model in dentistry, oral health, and craniofacial growth/development.

RESULTS

The electronic search of literature yielded 421 articles. After the analysis of the abstracts, 29 articles were selected for an in-depth analysis and reading of the full text.

CONCLUSIONS

All studies included in this review confirm zebrafish's excellence as an animal model for various types of dentistry studies, as well as assisting and complementing other studies involving mammals.

Integrative analyses of mRNA and microRNA expression profiles reveal the innate immune mechanism for the resistance to Vibrio parahaemolyticus infection in Epinephelus coioides

Vibrio parahaemolyticus, as one of the main pathogens of marine vibriosis, has brought huge losses to aquaculture. However, the interaction mechanism between V. parahaemolyticus and Epinephelus coioides remains unclear. Moreover, there is a lack of comprehensive multi-omics analysis of the immune response of grouper spleen to V. parahaemolyticus. Herein, E. coioides was artificially injected with V. parahaemolyticus, and it was found that the mortality was 16.7% in the early stage of infection, and accompanied by obvious histopathological lesions in the spleen. Furthermore, 1586 differentially expressed genes were screened by mRNA-seq. KEGG analysis showed that genes were significantly enriched in immune-related pathways, Acute-phase immune response, Apoptosis, Complement system and Cytokine-cytokine receptor interaction. As for miRNA-seq analysis, a total of 55 significantly different miRNAs were identified. Further functional annotation analysis indicated that the target genes of differentially expressed miRNAs were enriched in three important pathways (Phosphatidylinositol signaling system, Lysosome and Focal adhesions). Through mRNA-miRNA integrated analysis, 1427 significant miRNA–mRNA pairs were obtained and “p53 signaling pathway”, “Intestinal immune network for IgA production” were considered as two crucial pathways. Finally, miR-144-y, miR-497-x, novel-m0459-5p, miR-7133-y, miR-378-y, novel-m0440-5p and novel-m0084-3p may be as key miRNAs to regulate immune signaling pathways via the miRNA-mRNA interaction network. The above results suggest that the mRNA-miRNA integrated analysis not only sheds new light on the molecular mechanisms underlying the interaction between host and V. parahaemolyticus but also provides valuable and new insights into resistance to vibrio infection.

Design and Husbandry Considerations for a Containment Level 2 Aquatic Facility

The greatly increased use of aquatic species to study disease over the past 20 years necessitates understanding their husbandry and housing requirements to optimize research and welfare and to ensure compliance with regulations. To achieve these goals, aquatic systems have expanded from pet shop and home aquaria to research-grade systems incorporating designs and features to increase their robustness, practicality, and flexibility. Moreover, these last decades have seen the increasing use of aquatic animals for infectious disease research using containment level 2 (CL2)/biosafety level 2 pathogens. In this study, we discuss the facility design requirements and modifications, which must be considered for the planning, construction, and use of an aquatic facility for zebrafish infected with CL2 pathogens. These include decontamination of water and equipment, racking and filtration design, personal protective equipment, and husbandry procedures. This guidance is based on our experience in the design and ongoing management of such facilities.

Expression of immune markers of white sturgeon (Acipenser transmontanus) during Veronaea botryosa infection at different temperatures

Systemic phaeohyphomycosis caused by Veronaea botryosa is one of the most important emergent diseases to affect sturgeon aquaculture in North America. White sturgeon (Acipenser transmontanus) cultured at temperatures above 15 °C are at higher risk of severe disseminated disease and higher mortalities. Despite this, little is known regarding disease pathogenesis and the immune response to infection. The objective of this study was to investigate the acute (2 days post-challenge [dpc]) and chronic (32 dpc) response of white sturgeon at 13 °C and 18 °C challenged with V. botryosa via intramuscular injection, using gene expression analysis of a diverse array of soluble immune and inflammatory mediators. Significantly greater amounts of irf8 (p < 0.05) and tfg-β (p < 0.05) genes were detected in gills of exposed fish at 18 °C when compared to those at 13 °C 32 dpc. Transcript levels of haptoglobin, serotransferrin, serum amyloid, cathelicidin, tnf-α, and il-17 were significantly increased in splenic tissues of challenged fish maintained at 18 °C late in infection (p < 0.05). However, only haptoglobin and serotransferrin transcript abundance were significantly greater in exposed fish when compared to controls 32dpc. Moreover, haptoglobin transcripts at this time point were significantly greater in exposed fish at 18 °C when compared to those challenged at 13 °C. Fewer differences were detected in fish kept at 13 °C. In agreement with transcript quantification, western blot assessment of haptoglobin showed increased levels in the challenged fish maintained at 18 °C.

Analysis of the white sturgeon (Acipenser transmontanus) immune response during immunostimulation and Veronaea botryosa infection

Systemic phaeohyphomycosis caused by Veronaea botryosa is regarded as an important emerging mycotic disease of sturgeon aquaculture. However, no vaccines or treatments are currently available. The effects of dietary β-glucan supplementation on resistance to V. botryosa infection was examined in controlled challenges by exposing immunostimulated and control fish to ~7.25 × 10⁵ fungal spores/fish via intra-muscular injection. Six weeks post-challenge, cumulative mortality was determined and antibodies to acute phase-proteins (APP) were used to quantify the conserved APP peptides in the serum of challenged and control fish using Western blot. Transcript levels for all tested pro-inflammatory cytokines, APP, and regulatory cytokines in the spleen were similar amongst treatments at the end of the three-week feeding period. However, significantly higher survival occurred in fingerlings fed 0.3% β-glucans compared to non-immunostimulated fish groups (p < 0.05) six weeks post-challenge. A strong proinflammatory response was detected in exposed treatment groups, and greater survival at 6 weeks was associated with higher transcript abundance of Il-17 in fish fed β-glucans. Findings support the important role of this cytokine in response to fungal infection.

Immunopathogenesis of Emerging Candida auris and Candida haemulonii Strains

The emergence of a multidrug-resistant Candida species, C. auris and C. haemulonii, has been reported worldwide. In Thailand, information on them is limited. We collected clinical isolates from Thai patients with invasive candidiasis. Both species were compared with a laboratory C. albicans strain. In vitro antifungal susceptibility and thermotolerance, and pathogenesis in the zebrafish model of infection were investigated. Both species demonstrated high minimal inhibitory concentrations to fluconazole and amphotericin B. Only C. auris tolerated high temperatures, like C. albicans. In a zebrafish swim-bladder-inoculation model, the C. auris-infected group had the highest mortality rate and infectivity, suggesting the highest virulence. The case fatality rates of C. auris, C. haemulonii, and C. albicans were 100%, 83.33%, and 51.52%, respectively. Further immunological studies revealed that both emerging Candida species stimulated genes involved in the proinflammatory cytokine group. Interestingly, the genes relating to leukocyte recruitment were downregulated only for C. auris infections. Almost all immune response genes to C. auris had a peak response at an early infection time, which contrasted with C. haemulonii. In conclusion, both emerging species were virulent in a zebrafish model of infection and could activate the inflammatory pathway. This study serves as a stepping stone for further pathogenesis studies of these important emerging species.

Zebrafish Models for the Safety and Therapeutic Testing of Nanoparticles with a Focus on Macrophages

New nanoparticles and biomaterials are increasingly being used in biomedical research for drug delivery, diagnostic applications, or vaccines, and they are also present in numerous commercial products, in the environment and workplaces. Thus, the evaluation of the safety and possible therapeutic application of these nanomaterials has become of foremost importance for the proper progress of nanotechnology. Due to economical and ethical issues, in vitro and in vivo methods are encouraged for the testing of new compounds and/or nanoparticles, however in vivo models are still needed. In this scenario, zebrafish (Danio rerio) has demonstrated potential for toxicological and pharmacological screenings. Zebrafish presents an innate immune system, from early developmental stages, with conserved macrophage phenotypes and functions with respect to humans. This fact, combined with the transparency of zebrafish, the availability of models with fluorescently labelled macrophages, as well as a broad variety of disease models offers great possibilities for the testing of new nanoparticles. Thus, with a particular focus on macrophage-nanoparticle interaction in vivo, here, we review the studies using zebrafish for toxicological and biodistribution testing of nanoparticles, and also the possibilities for their preclinical evaluation in various diseases, including cancer and autoimmune, neuroinflammatory, and infectious diseases.

Anti-inflammatory potential of myristic acid and palmitic acid synergism against systemic candidiasis in Danio rerio (Zebrafish)

Nosocomial Candida colonization causes Systemic candidiasis in human with invasive infections in immunocompromised patients. Of all Candida spp., C. albicans is dominant in morbidity of all systemic candidiasis but C. tropicalis is phenomenal in mortality, virulence aspects and resistance development against antifungal drugs. The present study investigated the synergistic anti-virulent activity of myristic acid (MA) and palmitic acid (PA) against insidious dimorphic Candida spp. (C. albicans and C. tropicalis). In vitro and qPCR results revealed the mechanisms of MA-PA combination effectively inhibiting various virulence aspects such as biofilm, hyphal formation, secreted aspartyl proteases, lipases, ergosterol biosynthesis and drug effluxes. Further, in Danio rerio (Zebrafish), the MA-PA treatment increased the survival of animals and also the treated groups showed decreased level of fungal burden compared to the infected controls, after 3^rd day of post infection. Histopathology of vital organs and SEM analysis of skin revealed a drastic recovery and reduced the inflammation of both Candida spp. infections in MA-PA treated animals. In addition, MA-PA treatment reduced the haemolysin and increased the susceptibility of Candida spp. in human blood model. Hence, this study suggested the therapeutic utilization of MA-PA as synergistic combination for their anti-inflammatory potency against systemic candidiasis and candidemia.

Inhibitory Effect of Morin Against Candida albicans Pathogenicity and Virulence Factor Production: An in vitro and in vivo Approaches

Candida albicans is considered an exclusive etiologic agent of candidiasis, a very common fungal infection in human. The expression of virulence factors contributes highly to the pathogenicity of C. albicans. These factors include biofilm formation, yeast-to-hyphal transition, adhesins, aspartyl proteases, and phospholipases secretion. Moreover, resistance development is a critical issue for the therapeutic failure of antifungal agents against systemic candidiasis. To circumvent resistance development, the present study investigated the virulence targeted therapeutic activity of the phyto-bioactive compound morin against C. albicans. Morin is a natural compound commonly found in medicinal plants and widely used in the pharmaceutical and cosmetic products/industries. The present study explicated the significant inhibitory potential of morin against biofilm formation and other virulence factors' production, such as yeast-hyphal formation, phospholipase, and exopolymeric substances, in C. albicans. Further, qPCR analysis confirmed the downregulation of biofilm and virlence-related genes in C. albicans upon morin treatment, which is in correspondence with the in vitro bioassays. Further, the docking analysis revealed that morin shows strong affinity with Hwp-1 protein, which regulates the expression of biofilm and hyphal formation in C. albicans and, thereby, abolishes fungal pathogenicity. Moreover, the anti-infective potential of morin against C. albicans-associated systemic candidiasis is confirmed through an in vivo approach using biomedical model organism zebrafish (Danio rerio). The outcomes of the in vivo study demonstrate that the morin treatment effectively rescues animals from C. albicans infections and extends their survival rate by inhibiting the internal colonization of C. albicans. Histopathology analysis revealed extensive candidiasis-related pathognomonic changes in the gills, intestine, and kidney of animals infected with C. albicans, while no extensive abnormalities were observed in morin-treated animals. The results evidenced that morin has the ability to protect against the pathognomonic effect and histopathological lesions caused by C. albicans infection in zebrafish. Thus, the present study suggests that the utilization of morin could act as a potent therapeutic medication for C. albicans instigated candidiasis.

Succinyl chitosan gold nanocomposite: Preparation, characterization, in vitro and in vivo anticandidal activity

Herein, we have successfully synthesized a novel N-Succinyl chitosan/gold nanocomposite (N-SuC/Au NC) using N-SuC and gold(III) chloride, and investigated the biocompatibility and antifungal activity. The synthesized N-SuC/Au NC was characterized by UV-visible spectroscopy, X-ray diffraction, field emission scanning electron microscope, and inductively coupled plasma atomic emission spectroscopy. The N-SuC/Au NC exhibited a strong inhibition effect towards pathogenic Candida albicans. Morphological analysis revealed the destruction of C. albicans cell membrane due to N-SuC/Au NC treatment. The in vitro and in vivo toxicity of N-SuC/Au NC was analyzed with HEK293T mammalian cells and zebrafish larvae, respectively. The synthesized N-SuC/Au NC demonstrated no cytotoxicity towards HEK293T cells up to 1200 μg/mL concentration. The survival rate of the zebrafish larvae at 120 hpf, was found as 100% up to 1200 μg/mL of N-SuC/Au NC exposure. The in vivo studies further confirmed the inhibitory effects of N-SuC/Au NC on the formation of C. albicans hyphae in infected zebrafish muscle tissue.

In Vitro or In Vivo Models, the Next Frontier for Unraveling Interactions between Malassezia spp. and Hosts. How Much Do We Know?

Malassezia is a lipid-dependent genus of yeasts known for being an important part of the skin mycobiota. These yeasts have been associated with the development of skin disorders and cataloged as a causal agent of systemic infections under specific conditions, making them opportunistic pathogens. Little is known about the host-microbe interactions of Malassezia spp., and unraveling this implies the implementation of infection models. In this mini review, we present different models that have been implemented in fungal infections studies with greater attention to Malassezia spp. infections. These models range from in vitro (cell cultures and ex vivo tissue), to in vivo (murine models, rabbits, guinea pigs, insects, nematodes, and amoebas). We additionally highlight the alternative models that reduce the use of mammals as model organisms, which have been gaining importance in the study of fungal host-microbe interactions. This is due to the fact that these systems have been shown to have reliable results, which correlate with those obtained from mammalian models. Examples of alternative models are Caenorhabditis elegans, Drosophila melanogaster, Tenebrio molitor, and Galleria mellonella. These are invertebrates that have been implemented in the study of Malassezia spp. infections in order to identify differences in virulence between Malassezia species.

Candida albicans triggers NADPH oxidase-independent neutrophil extracellular traps through dectin-2

Candida albicans is one of the top leading causes of healthcare-associated bloodstream infection. Neutrophil extracellular traps (NET) are known to capture and kill pathogens. It is reported that opsonized C. albicans-triggered NETosis is NADPH oxidase-dependent. We discovered a NADPH oxidase-independent NETosis pathway in neutrophil response to unopsonized C. albicans. While CR3 engagement with opsonized C. albicans triggered NET, dectin-2 recognized unopsonized C. albicans and mediated NET formation. Engagement of dectin-2 activated the downstream Syk-Ca²⁺-PKCδ-protein arginine deiminase 4 (PAD4) signaling pathway which modulated nuclear translocation of neutrophil elastase (NE), histone citrullination and NETosis. In a C. albicans peritonitis model we observed Ki67⁺Ly6G⁺ NETotic cells in the peritoneal exudate and mesenteric tissues within 3 h of infection. Treatment with PAD4 inhibitor GSK484 or dectin-2 deficiency reduced % Ki67⁺Ly6G⁺ cells and the intensity of Ki67 in peritoneal neutrophils. Employing DNA digestion enzyme micrococcal nuclease, GSK484 as well as dectin-2-deficient mice, we further showed that dectin-2-mediated PAD4-dependent NET formation in vivo restrained the spread of C. albicans from the peritoneal cavity to kidney. Taken together, this study reveals that unopsonized C. albicans evokes NADPH oxidase-independent NETosis through dectin-2 and its downstream signaling pathway and dectin-2-mediated NET helps restrain fungal dissemination.

Candida albicans as a dimorphic fungal pathogen is one of the top leading causes of overall healthcare-associated bloodstream infection worldwide. Invasive candidiasis affects more than 250,000 people each year and leads to more than 50,000 deaths. Upon stimulation, neutrophils release nuclear DNA that forms a web-like structure named neutrophil extracellular traps (NET). NET is known to capture pathogens and restrain the spread of infection in the host. It has been reported that opsonized C. albicans induces NET through NADPH oxidase. Here we show a NADPH oxidase-independent NETosis in response to unopsonized C. albicans. Signaling pathway leading to NETosis involves dectin-2 downstream Syk-Ca²⁺-PKCδ-PAD4/NE. In a C. albicans peritonitis model, NETotic cells are found in the peritoneal exudates and they adhere to mesenteric tissue. Treatment with PAD4 inhibitor or dectin-2 deficiency dampens the ability of neutrophil to undergo NETosis and facilitates the spread of fungus from the peritoneal cavity to kidney. Our work defines the molecular mechanism involved in NADPH oxidase-independent NET formation and sheds light on the role of dectin-2 in neutrophil anti-C. albicans function.

Notch1a can widely mediate innate immune responses in zebrafish larvae infected with Vibrio parahaemolyticus

The Notch signaling pathway is known to regulate innate immunity by influencing macrophage function and interacting with the Toll-like receptor (TLR) signaling pathway. However, the comprehensive role of the Notch signaling pathway in the innate immune response remains unknown. To assess the function of Notch1a in immunity, we examined the innate immune responses to Vibrio parahaemolyticus strain Vp13 of wild-type (WT) and notch1a^-/- zebrafish larvae generated using the clustered regularly interspaced short palindromic repeats/CRISPR associated protein 9 (CRISPR/Cas9) system. The median lethal dose (LD₅₀) of V. parahaemolyticus was significantly lower in notch1a^-/- larvae than in WT larvae 3 days post fertilization (dpf). Transcriptome data analysis revealed 359 significantly differentially expressed genes (DEGs), including 246 significantly down-regulated genes and 113 significantly up-regulated genes, in WT infected groups compared with WT control groups. In contrast, 986 significantly DEGs were found in notch1a^-/- infected groups compared with notch1a^-/- control groups, of which 82 genes were significantly down-regulated and 904 genes were significantly up-regulated. These DEGs belonged to the tumor necrosis factor (TNF), complement, nuclear factor kappa B (NF-κB), cathepsin, interleukin (IL), chemokine, serpin peptidase inhibitor, matrix metallopeptidase, innate immune cells, pattern recognition receptor (PRR), and other cytokine families. Our results indicate that Notch1a plays roles in inhibiting many immunity-related genes and could comprehensively mediate the innate immune response by regulating TLRs, nucleotide-binding-oligomerization-domain-like receptors (NLRs), lectins, complement, ILs, chemokines, TNF, cathepsin, and serpin. Further studies are required to understand the specific mechanism of Notch1a in innate immunity.

Candida albicans Infection Model in Zebrafish (Danio rerio) for Screening Anticandidal Drugs

BACKGROUND

Candida albicans is an opportunistic fungal pathogen which causes systemic infections in human. In this study, C. albicans infection model was developed in zebrafish to understand the host-pathogen interactions for straightforward anticandidal drug screening.

METHODS

To develop the infection, 1 × 10⁶ cells of C. albicans suspended in phosphate-buffered saline were deposited in zebrafish dorsal muscle by manually operated syringe. The infection progression was externally assessed by a scale of wound-healing events, based on visible changes of yeast deposited in the muscle tissues. Chemotherapy was carried out with known antifungal drugs (fluconazole, nystatin, and amphotericin B) and a potential antifungal agent, chitosan silver nanocomposites (CAgNC), after the infection as direct exposure in the water. Histopathological analysis was performed to identify the pathogen virulence and the host-pathogen interaction during the infection.

RESULTS

The light microscopic observations and histopathological analysis revealed the yeast-hyphae transition at the site of infection (at 72 hpi) and progression of the infection in the host tissues. The larval survival rate under fluconazole (up to 80 μg mL^-1) and nystatin (up to 20 μg mL^-1) was > 90% and for CAgNC it was 40% at 36 h post-exposure (hpe). The infection progression was suppressed with the fungicidal treatments. Among inflammatory genes, il-1β has been highly upregulated (14.68-fold) at 24 h post infection (hpi). Both il-1β and tnf-α were moderately upregulated in infected fish gills at 72 hpi. Among the C. albicans antioxidant genes, cat1 and sod2 have been upregulated during the infection, and relative expression folds were increased from low to moderate levels with the time.

DISCUSSION

We demonstrate the approach for the development of artificial infection model of zebrafish with C. albicans. By this mini vertebrate zebrafish model, researchers will be able to study novel anticandidal compounds in vivo with respect to the host, pathogen, and their interactions.

DST659 genotype of Candida albicans showing positive association between biofilm formation and dominance in Taiwan

Based on multiple locus sequence typing, we previously found that DST659 and DST693 were dominant genotypes of Candida albicans among the bloodstream isolates at Chang-Gung Memorial Hospital at Linkou. Biofilm-forming activity, which is critical for C. albicans virulence, probably contributed to the dominance of antifungal sensitive isolates in hospital. Both in vitro membrane weighting and in vivo zebrafish egg infection assays were used to evaluate the biofilm-forming activity of DST659 and DST693 genotypes. Medical records of the patients infected by these two genotypes were retrospectively reviewed. High biofilm-forming activity of DST659 isolates was demonstrated in vitro and further proved with the zebrafish egg infection model, which showed a positive correlation between the biofilm-forming extent on chorion and the in vitro biofilm activity. Moreover, significantly less embryos survived when infected with DST659 isolates than those with DST693 (1.25% vs. 11.43%), and the high-biofilm subset of DST659 showed a greater reduction in survival of embryos at 48 h post-infection than the low-biofilm subset (0 vs. 1.92%). Patients infected with DST659 seemed to survive slightly worse than those infected with DST693, although the difference was insignificant. It is noteworthy that DST659-infected patients were associated with a higher incidence in renal insufficiency as compared to those with DST693, the low biofilm genotype. We suggest that a strong biofilm activity of DST659 contributed to a high mortality rate in zebrafish hosts and poor renal function in patients, as well as gaining the dominance in the northern Taiwan.

The Zebrafish as a Model Host for Invasive Fungal Infections

The zebrafish has become a widely accepted model host for studies of infectious disease, including fungal infections. The species is genetically tractable, and the larvae are transparent and amenable to prolonged in vivo imaging and small molecule screening. The aim of this review is to provide a thorough introduction into the published studies of fungal infection in the zebrafish and the specific ways in which this model has benefited the field. In doing so, we hope to provide potential new zebrafish researchers with a snapshot of the current toolbox and prior results, while illustrating how the model has been used well and where the unfulfilled potential of this model can be found.

Recent Advances in the Use of Galleria mellonella Model to Study Immune Responses against Human Pathogens

The use of invertebrates for in vivo studies in microbiology is well established in the scientific community. Larvae of Galleria mellonella are a widely used model for studying pathogenesis, the efficacy of new antimicrobial compounds, and immune responses. The immune system of G. mellonella larvae is structurally and functionally similar to the innate immune response of mammals, which makes this model suitable for such studies. In this review, cellular responses (hemocytes activity: phagocytosis, nodulation, and encapsulation) and humoral responses (reactions or soluble molecules released in the hemolymph as antimicrobial peptides, melanization, clotting, free radical production, and primary immunization) are discussed, highlighting the use of G. mellonella as a model of immune response to different human pathogenic microorganisms.

Saprolegnia parasitica Isolated from Rainbow Trout in Korea: Characterization, Anti-Saprolegnia Activity and Host Pathogen Interaction in Zebrafish Disease Model

Saprolegniasis is one of the most devastating oomycete diseases in freshwater fish which is caused by species in the genus Saprolegnia including Saprolegnia parasitica. In this study, we isolated the strain of S. parasitica from diseased rainbow trout in Korea. Morphological and molecular based identification confirmed that isolated oomycete belongs to the member of S. parasitica, supported by its typical features including cotton-like mycelium, zoospores and phylogenetic analysis with internal transcribed spacer region. Pathogenicity of isolated S. parasitica was developed in embryo, juvenile, and adult zebrafish as a disease model. Host-pathogen interaction in adult zebrafish was investigated at transcriptional level. Upon infection with S. parasitica, pathogen/antigen recognition and signaling (TLR2, TLR4b, TLR5b, NOD1, and major histocompatibility complex class I), pro/anti-inflammatory cytokines (interleukin [IL]-1β, tumor necrosis factor α, IL-6, IL-8, interferon γ, IL-12, and IL-10), matrix metalloproteinase (MMP9 and MMP13), cell surface molecules (CD8⁺ and CD4⁺) and antioxidant enzymes (superoxide dismutase, catalase) related genes were differentially modulated at 3- and 12-hr post infection. As an anti-Saprolegnia agent, plant based lawsone was applied to investigate on the susceptibility of S. parasitica showing the minimum inhibitory concentration and percentage inhibition of radial growth as 200 μg/mL and 31.8%, respectively. Moreover, natural lawsone changed the membrane permeability of S. parasitica mycelium and caused irreversible damage and disintegration to the cellular membranes of S. parasitica. Transcriptional responses of the genes of S. parasitica mycelium exposed to lawsone were altered, indicating that lawsone could be a potential anti-S. parasitica agent for controlling S. parasitica infection.

Macrophage-Microbe Interactions: Lessons from the Zebrafish Model

Macrophages provide front line defense against infections. The study of macrophage-microbe interplay is thus crucial for understanding pathogenesis and infection control. Zebrafish (Danio rerio) larvae provide a unique platform to study macrophage-microbe interactions in vivo, from the level of the single cell to the whole organism. Studies using zebrafish allow non-invasive, real-time visualization of macrophage recruitment and phagocytosis. Furthermore, the chemical and genetic tractability of zebrafish has been central to decipher the complex role of macrophages during infection. Here, we discuss the latest developments using zebrafish models of bacterial and fungal infection. We also review novel aspects of macrophage biology revealed by zebrafish, which can potentiate development of new therapeutic strategies for humans.

The PHR Family: The Role of Extracellular Transglycosylases in Shaping Candida albicans Cells

Candida albicans is an opportunistic microorganism that can become a pathogen causing mild superficial mycosis or more severe invasive infections that can be life-threatening for debilitated patients. In the etiology of invasive infections, key factors are the adaptability of C. albicans to the different niches of the human body and the transition from a yeast form to hypha. Hyphal morphology confers high adhesiveness to the host cells, as well as the ability to penetrate into organs. The cell wall plays a crucial role in the morphological changes C. albicans undergoes in response to specific environmental cues. Among the different categories of enzymes involved in the formation of the fungal cell wall, the GH72 family of transglycosylases plays an important assembly role. These enzymes cut and religate β-(1,3)-glucan, the major determinant of cell shape. In C. albicans, the PHR family encodes GH72 enzymes, some of which work in specific environmental conditions. In this review, we will summarize the work from the initial discovery of PHR genes to the study of the pH-dependent expression of PHR1 and PHR2, from the characterization of the gene products to the recent findings concerning the stress response generated by the lack of GH72 activity in C. albicans hyphae.

Cell Intrinsic Galectin-3 Attenuates Neutrophil ROS-Dependent Killing of Candida by Modulating CR3 Downstream Syk Activation

Invasive candidiasis is a leading cause of nosocomial bloodstream infection. Neutrophils are the important effector cells in host resistance to candidiasis. To investigate the modulation of neutrophil fungicidal function will advance our knowledge on the control of candidiasis. While recombinant galectin-3 enhances neutrophil phagocytosis of Candida, we found that intracellular galectin-3 downregulates neutrophil fungicidal functions. Co-immunoprecipitation and immunofluorescence staining reveal that cytosolic gal3 physically interacts with Syk in neutrophils after Candida stimulation. Gal3^−/− neutrophils have higher level of Syk activation as well as greater abilities to generate reactive oxygen species (ROS) and kill Candida than gal3^+/+ cells. While galectin-3 deficiency modulates neutrophil and macrophage activation and the recruitment of monocytes and dendritic cells, the deficiency does not affect the numbers of infiltrating neutrophils or macrophages. Galectin-3 deficiency ameliorates systemic candidiasis by reducing fungal burden, renal pathology, and mortality. Adoptive transfer experiments demonstrate that cell intrinsic galectin-3 negatively regulates neutrophil effector functions against candidiasis. Reducing galectin-3 expression or activity by siRNA or gal3 inhibitor TD139 enhances human neutrophil ROS production. Mice treated with TD139 have enhanced ability to clear the fungus. Our work unravels the mechanism by which galectin-3 regulates Syk-dependent neutrophil fungicidal functions and raises the possibility that blocking gal3 in neutrophils may be a promising therapeutic strategy for treating systemic candidiasis.

Antifungal Therapy: New Advances in the Understanding and Treatment of Mycosis

The high rates of morbidity and mortality caused by fungal infections are associated with the current limited antifungal arsenal and the high toxicity of the compounds. Additionally, identifying novel drug targets is challenging because there are many similarities between fungal and human cells. The most common antifungal targets include fungal RNA synthesis and cell wall and membrane components, though new antifungal targets are being investigated. Nonetheless, fungi have developed resistance mechanisms, such as overexpression of efflux pump proteins and biofilm formation, emphasizing the importance of understanding these mechanisms. To address these problems, different approaches to preventing and treating fungal diseases are described in this review, with a focus on the resistance mechanisms of fungi, with the goal of developing efficient strategies to overcoming and preventing resistance as well as new advances in antifungal therapy. Due to the limited antifungal arsenal, researchers have sought to improve treatment via different approaches, and the synergistic effect obtained by the combination of antifungals contributes to reducing toxicity and could be an alternative for treatment. Another important issue is the development of new formulations for antifungal agents, and interest in nanoparticles as new types of carriers of antifungal drugs has increased. In addition, modifications to the chemical structures of traditional antifungals have improved their activity and pharmacokinetic parameters. Moreover, a different approach to preventing and treating fungal diseases is immunotherapy, which involves different mechanisms, such as vaccines, activation of the immune response and inducing the production of host antimicrobial molecules. Finally, the use of a mini-host has been encouraging for in vivo testing because these animal models demonstrate a good correlation with the mammalian model; they also increase the speediness of as well as facilitate the preliminary testing of new antifungal agents. In general, many years are required from discovery of a new antifungal to clinical use. However, the development of new antifungal strategies will reduce the therapeutic time and/or increase the quality of life of patients.

Green synthesis, physio-chemical characterization and anti-candidal function of a biocompatible chitosan gold nanocomposite as a promising antifungal therapeutic agent

Green synthesized chitosan gold nanocomposite (CAuNC) and its anti-candidal function.

Modeling Infectious Diseases in the Context of a Developing Immune System

Zebrafish has been used for over a decade to study the mechanisms of a wide variety of inflammatory disorders and infections, with models ranging from bacterial, viral, to fungal pathogens. Zebrafish has been especially relevant to study the differentiation, specialization, and polarization of the two main innate immune cell types, the macrophages and the neutrophils. The optical accessibility and the early appearance of myeloid cells that can be tracked with fluorescent labels in zebrafish embryos and the ability to use genetics to selectively ablate or expand immune cell populations have permitted studying the interaction between infection, development, and metabolism. Additionally, zebrafish embryos are readily colonized by a commensal flora, which facilitated studies that emphasize the requirement for immune training by the natural microbiota to properly respond to pathogens. The remarkable conservation of core mechanisms required for the recognition of microbial and danger signals and for the activation of the immune defenses illustrates the high potential of the zebrafish model for biomedical research. This review will highlight recent insight that the developing zebrafish has contributed to our understanding of host responses to invading microbes and the involvement of the microbiome in several physiological processes. These studies are providing a mechanistic basis for developing novel therapeutic approaches to control infectious diseases.

Dissecting Candida albicans Infection from the Perspective of C. albicans Virulence and Omics Approaches on Host-Pathogen Interaction: A Review

Candida bloodstream infections remain the most frequent life-threatening fungal disease, with Candida albicans accounting for 70% to 80% of the Candida isolates recovered from infected patients. In nature, Candida species are part of the normal commensal flora in mammalian hosts. However, they can transform into pathogens once the host immune system is weakened or breached. More recently, mortality attributed to Candida infections has continued to increase due to both inherent and acquired drug resistance in Candida, the inefficacy of the available antifungal drugs, tedious diagnostic procedures, and a rising number of immunocompromised patients. Adoption of animal models, viz. minihosts, mice, and zebrafish, has brought us closer to unraveling the pathogenesis and complexity of Candida infection in human hosts, leading towards the discovery of biomarkers and identification of potential therapeutic agents. In addition, the advancement of omics technologies offers a holistic view of the Candida-host interaction in a non-targeted and non-biased manner. Hence, in this review, we seek to summarize past and present milestone findings on C. albicans virulence, adoption of animal models in the study of C. albicans infection, and the application of omics technologies in the study of Candida–host interaction. A profound understanding of the interaction between host defense and pathogenesis is imperative for better design of novel immunotherapeutic strategies in future.

Advancing toxicology research using in vivo high throughput toxicology with small fish models

Small freshwater fish models, especially zebrafish, offer advantages over traditional rodent models, including low maintenance and husbandry costs, high fecundity, genetic diversity, physiology similar to that of traditional biomedical models, and reduced animal welfare concerns. The Collaborative Workshop on Aquatic Models and 21st Century Toxicology was held at North Carolina State University on May 5-6, 2014, in Raleigh, North Carolina, USA. Participants discussed the ways in which small fish are being used as models to screen toxicants and understand mechanisms of toxicity. Workshop participants agreed that the lack of standardized protocols is an impediment to broader acceptance of these models, whereas development of standardized protocols, validation, and subsequent regulatory acceptance would facilitate greater usage. Given the advantages and increasing application of small fish models, there was widespread interest in follow-up workshops to review and discuss developments in their use. In this article, we summarize the recommendations formulated by workshop participants to enhance the utility of small fish species in toxicology studies, as well as many of the advances in the field of toxicology that resulted from using small fish species, including advances in developmental toxicology, cardiovascular toxicology, neurotoxicology, and immunotoxicology. We alsoreview many emerging issues that will benefit from using small fish species, especially zebrafish, and new technologies that will enable using these organisms to yield results unprecedented in their information content to better understand how toxicants affect development and health.

Leptospiral outer membrane protein LipL32 induces inflammation and kidney injury in zebrafish larvae

Leptospirosis is an often overlooked cause of acute kidney injury that can lead to multiple organ failure and even death. The principle protein that conserved in many pathogenic leptospires is the outer membrane protein LipL32. However, the role of LipL32 in the pathogenesis of renal injury in leptospirosis is not entirely clear. Here we studied the effects of LipL32 on the developing kidney in zebrafish larvae. Incubation of zebrafish larvae with Leptospira santarosai serovar Shermani induced acute tubular injury predominantly in the proximal pronephric ducts. Furthermore, microinjection of lipl32 mRNA or recombinant LipL32 protein into zebrafish larvae increased macrophage accumulation and disrupted the basolateral location of NA-K-ATPase in pronephric ducts. These changes led to substantial impairment of the pronephric kidney structure. We further demonstrated that morpholino knockdown of tlr2, but not tlr4, reduced the LipL32-induced leukocyte infiltration and kidney injury. These data demonstrate that LipL32 contributes to the renal pathology in leptospirosis and gives some clues to the potential virulence of LipL32. Our results support the use of zebrafish as a model organism for studying the disease mechanism of leptospiral infection. This model might permit the future exploration of the virulence and molecular pathways of different leptospiral outer membrane proteins.

Phenotypic Plasticity Regulates Candida albicans Interactions and Virulence in the Vertebrate Host

Phenotypic diversity is critical to the lifestyles of many microbial species, enabling rapid responses to changes in environmental conditions. In the human fungal pathogen Candida albicans, cells exhibit heritable switching between two phenotypic states, white and opaque, which yield differences in mating, filamentous growth, and interactions with immune cells in vitro. Here, we address the in vivo virulence properties of the two cell states in a zebrafish model of infection. Multiple attributes were compared including the stability of phenotypic states, filamentation, virulence, dissemination, and phagocytosis by immune cells, and phenotypes equated across three different host temperatures. Importantly, we found that both white and opaque cells could establish a lethal systemic infection. The relative virulence of the two cell types was temperature dependent; virulence was similar at 25°C, but at higher temperatures (30 and 33°C) white cells were significantly more virulent than opaque cells. Despite the difference in virulence, fungal burden, and dissemination were similar between cells in the two states. Additionally, both white and opaque cells exhibited robust filamentation during infection and blocking filamentation resulted in decreased virulence, establishing that this program is critical for pathogenesis in both cell states. Interactions between C. albicans cells and immune cells differed between white and opaque states. Macrophages and neutrophils preferentially phagocytosed white cells over opaque cells in vitro, and neutrophils showed preferential phagocytosis of white cells in vivo. Together, these studies distinguish the properties of white and opaque cells in a vertebrate host, and establish that the two cell types demonstrate both important similarities and key differences during infection.

Organism-wide studies into pathogenicity and morphogenesis in Talaromyces marneffei

Organism-wide approaches examining the genetic mechanisms controlling growth and proliferation have proven to be a powerful tool in the study of pathogenic fungi. For many fungal pathogens techniques to study transcription and protein expression are particularly useful, and offer insights into infection processes by these species. Here we discuss the use of approaches such as differential display, suppression subtractive hybridization, microarray, RNA-seq, proteomics, genetic manipulation and infection models for the AIDS-defining pathogen Talaromyces marneffei. Together these methods have broadened our understanding of the biological processes, and genes that underlie them, which are involved in switching between the saprophytic and pathogenic states of T. marneffei, the maintenance of these two specialized cell types and its ability to cause disease.

Hgc1-Cdc28-how much does a single protein kinase do in the regulation of hyphal development in Candida albicans?

The fungal human pathogen Candida albicans can cause invasive infection with high mortality rates. A key virulence factor is its ability to switch between three morphologies: yeast, pseudohyphae and hyphae. In contrast to the ovalshaped unicellular yeast cells, hyphae are highly elongated, tube-like, and multicellular. A long-standing question is what coordinates all the cellular machines to construct cells with distinct shapes. Hyphal-specific genes (HSGs) are thought to hold the answer. Among the numerous HSGs found, only UME6 and HGC1 are required for hyphal development. UME6 encodes a transcription factor that regulates many HSGs including HGC1. HGC1 encodes a G1 cyclin which partners with the Cdc28 cyclin-dependent kinase. Hgc1-Cdc28 simultaneously phosphorylates and regulates multiple substrates, thus controlling multiple cellular apparatuses for morphogenesis. This review is focused on major progresses made in the past decade on Hgc1's roles and regulation in C. albicans hyphal development and other traits important for infection.

A Systems Biology Approach to the Coordination of Defensive and Offensive Molecular Mechanisms in the Innate and Adaptive Host-Pathogen Interaction Networks

Infected zebrafish coordinates defensive and offensive molecular mechanisms in response to Candida albicans infections, and invasive C. albicans coordinates corresponding molecular mechanisms to interact with the host. However, knowledge of the ensuing infection-activated signaling networks in both host and pathogen and their interspecific crosstalk during the innate and adaptive phases of the infection processes remains incomplete. In the present study, dynamic network modeling, protein interaction databases, and dual transcriptome data from zebrafish and C. albicans during infection were used to infer infection-activated host-pathogen dynamic interaction networks. The consideration of host-pathogen dynamic interaction systems as innate and adaptive loops and subsequent comparisons of inferred innate and adaptive networks indicated previously unrecognized crosstalk between known pathways and suggested roles of immunological memory in the coordination of host defensive and offensive molecular mechanisms to achieve specific and powerful defense against pathogens. Moreover, pathogens enhance intraspecific crosstalk and abrogate host apoptosis to accommodate enhanced host defense mechanisms during the adaptive phase. Accordingly, links between physiological phenomena and changes in the coordination of defensive and offensive molecular mechanisms highlight the importance of host-pathogen molecular interaction networks, and consequent inferences of the host-pathogen relationship could be translated into biomedical applications.

Zebrafish Egg Infection Model for Studying Candida albicans Adhesion Factors

Disseminated candidiasis is associated with 30-40% mortality in severely immunocompromised patients. Among the causal agents, Candida albicans is the dominant one. Various animal models have been developed for investigating gene functions in C. albicans. Zebrafish injection models have increasingly been applied in elucidating C. albicans pathogenesis because of the conserved immunity, prolific fecundity of the zebrafish and the low costs of care systems. In this study, we established a simple, noninvasive zebrafish egg bath infection model, defined its optimal conditions, and evaluated the model with various C. albicans mutant strains. The deletion of SAP6 did not have significant effect on the virulence. By contrast, the deletion of BCR1, CPH1, EFG1, or TEC1 significantly reduced the virulence under current conditions. Furthermore, all embryos survived when co-incubated with bcr1/bcr1, cph1/cph1 efg1/efg1, efg1/efg1, or tec1/tec1 mutant cells. The results indicated that our novel zebrafish model is time-saving and cost effective.

Neutrophils in host defense: new insights from zebrafish

Neutrophils are highly motile phagocytic cells that play a critical role in the immune response to infection. Zebrafish (Danio rerio) are increasingly used to study neutrophil function and host-pathogen interactions. The generation of transgenic zebrafish lines with fluorescently labeled leukocytes has made it possible to visualize the neutrophil response to infection in real time by use of optically transparent zebrafish larvae. In addition, the genetic tractability of zebrafish has allowed for the generation of models of inherited neutrophil disorders. In this review, we discuss several zebrafish models of infectious disease, both in the context of immunocompetent, as well as neutrophil-deficient hosts and how these models have shed light on neutrophil behavior during infection.

Candida glabrata: new tools and technologies-expanding the toolkit

In recent years, there has been a noticeable rise in fungal infections related to non-albicans Candida species, including Candida glabrata which has both intrinsic resistance to and commonly acquired resistance to azole antifungals. Phylogenetically, C. glabrata is more closely related to the mostly non-pathogenic model organism Saccharomyces cerevisiae than to other Candida species. Despite C. glabrata's designation as a pathogen by Wickham in 1957, relatively little is known about its mechanism of virulence. Over the past few years, technology to analyse the molecular basis of infection has developed rapidly, and here we briefly review the major advances in tools and technologies available to explore and investigate the virulence of C. glabrata that have occurred over the past decade.

[Looking through zebrafish to study host-pathogen interactions]

The zebrafish offers many advantages that motivated and validated its use to study the virulence of numerous human pathogens, including viruses, bacteria and fungi. Its immune system is homologous to the one of mammals. The optical transparency of zebrafish embryos allows non-invasive and real-time monitoring of the infection processes through the use of imaging techniques. The zebrafish is therefore a useful and powerful model to study host-pathogen interactions at a cellular level. It may be used to describe pathophysiological events and subversion mechanisms that are specific to each pathogen. In addition to increasing our understanding of the host immune defense, this model is of high potential for medical application, being particularly amenable to high-throughput screening for the discovery of new anti-infective molecules.

Myeloperoxidase-deficient zebrafish show an augmented inflammatory response to challenge with Candida albicans

Myeloperoxidase is a key component of neutrophil granules involved in killing engulfed microorganisms. We obtained a zebrafish mutant (smu681) lacking Sudan black staining by large-scale screening, which was a neutrophil-replete but myeloperoxidase-deficient mutant. When infiltrated with Candida albicans, smu681 embryos and sibling embryos showed similar survival after infection. Proliferation of C. albicans was more rapid in smu681 embryos than in sibling embryos, although it was eventually suppressed. In addition, the number of neutrophils accumulating at the site of infection was significantly larger in mutant embryos than in sibling embryos, and mutant embryos showed increased expression of several inflammatory cytokines after C. albicans infection. These findings indicate that myeloperoxidase deficiency alters the inflammatory response to fungal infection.

Immersion infection of germ-free zebrafish with Listeria monocytogenes induces transient expression of innate immune response genes

Zebrafish, Denio rerio, can be an alternative to other classic animal models for human infectious diseases to examine the processes of microbial infections and host-pathogen interactions in vivo because of their small body dimension but large clutch size. We established germ-free zebrafish infection models of Listeria monocytogenes through different routes of infection: oral immersion and injection via yolk sac, brain ventricle and blood island. Immersion of zebrafish larva even with 10(10) CFU/mL L. monocytogenes EGDe strain in egg water was unable to cause mortality, but GFP-expressing bacteria in the gut lumen can be observed in frozen sections. Several selected maker genes of the innate immune system, including cyp1a, irg1l, il1b, and mmp9, were significantly induced by oral immersion not only with strain EGDe, but also with strain M7 and L. innocua, though to a lesser degree (P < 0.01). Such induction appears to be transient with peak at 48 h post-infection, but returned to basal level at 72 h post-infection. Of the three injection routes, mortality after infection by yolk sac was 80% in early stage of infection. Few eggs can survive and hatch. Injection into zebrafish embryos via brain ventricle or blood island led to progressive lethal infection. L. mocytogenes EGDe showed steady replication in the fish embryos and was far more pathogenic than strain M7, which is consistent with findings in the murine model. We conclude that zebrafish can serve as susceptible and microscopically visible infection models for L. monocytogenes via different routes and can be applied to further studies on the interactions between bacterial virulence factors and host immune responses.