Developmental expression of a tandemly repeated, proline-and glutamine-rich amino acid motif on hyphal surfaces on Candida albicans

Fungal Cell Wall-Associated Effectors: Sensing, Integration, Suppression, and Protection

The cell wall (CW) of plant-interacting fungi, as the direct interface with host plants, plays a crucial role in fungal development. A number of secreted proteins are directly associated with the fungal CW, either through covalent or non-covalent interactions, and serve a range of important functions. In the context of plant-fungal interactions many are important for fungal development in the host environment and may therefore be considered fungal CW-associated effectors (CWAEs). Key CWAE functions include integrating chemical/physical signals to direct hyphal growth, interfering with plant immunity, and providing protection against plant defenses. In recent years, a diverse range of mechanisms have been reported that underpin their roles, with some CWAEs harboring conserved motifs or functional domains, while others are reported to have novel features. As such, the current understanding regarding fungal CWAEs is systematically presented here from the perspective of their biological functions in plant-fungal interactions. An overview of the fungal CW architecture and the mechanisms by which proteins are secreted, modified, and incorporated into the CW is first presented to provide context for their biological roles. Some CWAE functions are reported across a broad range of pathosystems or symbiotic/mutualistic associations. Prominent are the chitin interacting-effectors that facilitate fungal CW modification, protection, or suppression of host immune responses. However, several alternative functions are now reported and are presented and discussed. CWAEs can play diverse roles, some possibly unique to fungal lineages and others conserved across a broad range of plant-interacting fungi. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Promising immunotherapeutic targets for treating candidiasis

In the last twenty years, there has been a significant increase in invasive fungal infections, which has corresponded with the expanding population of individuals with compromised immune systems. As a result, the mortality rate linked to these infections remains unacceptably high. The currently available antifungal drugs, such as azoles, polyenes, and echinocandins, face limitations in terms of their diversity, the escalating resistance of fungi and the occurrence of significant adverse effects. Consequently, there is an urgent need to develop new antifungal medications. Vaccines and antibodies present a promising avenue for addressing fungal infections due to their targeted antifungal properties and ability to modulate the immune response. This review investigates the structure and function of cell wall proteins, secreted proteins, and functional proteins within C. albicans. Furthermore, it seeks to analyze the current advancements and challenges in macromolecular drugs to identify new targets for the effective management of candidiasis.

Macrophage pyroptosis induced by Candida albicans

Candida albicans (C. albicans) is a prevalent opportunistic pathogen that causes mucocutaneous and systemic infections, particularly in immunocompromised individuals. Macrophages play a crucial role in eliminating C. albicans in local and bloodstream contexts, while also regulating antifungal immune responses. However, C. albicans can induce macrophage lysis through pyroptosis, a type of regulated cell death. This process can enable C. albicans to escape from immune cells and trigger the release of IL-1β and IL-18, which can impact both the host and the pathogen. Nevertheless, the mechanisms by which C. albicans triggers pyroptosis in macrophages and the key factors involved in this process remain unclear. In this review, we will explore various factors that may influence or trigger pyroptosis in macrophages induced by C. albicans, such as hypha, ergosterol, cell wall remodeling, and other virulence factors. We will also examine the possible immune response following macrophage pyroptosis.

Development and Use of a Monoclonal Antibody Specific for the Candida albicans Cell-Surface Protein Hwp1

The Candida albicans cell-surface protein Hwp1 functions in adhesion to the host and in biofilm formation. A peptide from the Gln-Pro-rich adhesive domain of Hwp1 was used to raise monoclonal antibody (MAb) 2-E8. MAb 2-E8 specificity for Hwp1 was demonstrated using a hwp1/hwp1 C. albicans isolate and strains that expressed at least one HWP1 allele. Immunofluorescence and atomic force microscopy experiments using MAb 2-E8 confirmed C. albicans germ-tube-specific detection of the Hwp1 protein. MAb 2-E8 also immunolabeled the tips of some Candida dubliniensis germ tubes grown under conditions that maximized HWP1 expression. The phylogeny of HWP1 and closely related genes suggested that the Gln-Pro-rich adhesive domain was unique to C. albicans and C. dubliniensis focusing the utility of MAb 2-E8 on these species. This new reagent can be used to address unanswered questions about Hwp1 and its interactions with other proteins in the context of C. albicans biology and pathogenesis.

Calpain cleavage of Junctophilin-2 generates a spectrum of calcium-dependent cleavage products and DNA-rich NT1-fragment domains in cardiomyocytes

Calpains are calcium-activated neutral proteases involved in the regulation of key signaling pathways. Junctophilin-2 (JP2) is a Calpain-specific proteolytic target and essential structural protein inside Ca²⁺ release units required for excitation-contraction coupling in cardiomyocytes. While downregulation of JP2 by Calpain cleavage in heart failure has been reported, the precise molecular identity of the Calpain cleavage sites and the (patho-)physiological roles of the JP2 proteolytic products remain controversial. We systematically analyzed the JP2 cleavage fragments as function of Calpain-1 versus Calpain-2 proteolytic activities, revealing that both Calpain isoforms preferentially cleave mouse JP2 at R565, but subsequently at three additional secondary Calpain cleavage sites. Moreover, we identified the Calpain-specific primary cleavage products for the first time in human iPSC-derived cardiomyocytes. Knockout of RyR2 in hiPSC-cardiomyocytes destabilized JP2 resulting in an increase of the Calpain-specific cleavage fragments. The primary N-terminal cleavage product NT₁ accumulated in the nucleus of mouse and human cardiomyocytes in a Ca²⁺-dependent manner, closely associated with euchromatic chromosomal regions, where NT₁ is proposed to function as a cardio-protective transcriptional regulator in heart failure. Taken together, our data suggest that stabilizing NT₁ by preventing secondary cleavage events by Calpain and other proteases could be an important therapeutic target for future studies.

Adhesins in the virulence of opportunistic fungal pathogens of human

Aspergillosis, candidiasis, and cryptococcosis are the most common cause of mycoses-related disease and death among immune-compromised patients. Adhesins are cell-surface exposed proteins or glycoproteins of pathogens that bind to the extracellular matrix (ECM) constituents or mucosal epithelial surfaces of the host cells. The forces of interaction between fungal adhesins and host tissues are accompanied by ligand binding, hydrophobic interactions and protein-protein aggregation. Adherence is the primary and critical step involved in the pathogenesis; however, there is limited information on fungal adhesins compared to that on the bacterial adhesins. Except a few studies based on screening of proteome for adhesin identification, majority are based on characterization of individual adhesins. Recently, based on their characteristic signatures, many putative novel fungal adhesins have been predicted using bioinformatics algorithms. Some of these novel adhesin candidates have been validated by in-vitro studies; though, most of them are yet to be characterised experimentally. Morphotype specific adhesin expression as well as tissue tropism are the crucial determinants for a successful adhesion process. This review presents a comprehensive overview of various studies on fungal adhesins and discusses the targetability of the adhesins and adherence phenomenon, for combating the fungal infection in a preventive or therapeutic mode.

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.

Gene expression of Candida albicans strains isolates from patients with denture stomatitis submitted to treatments with photodynamic therapy and nystatin

The study evaluated the effect of antimicrobial photodynamic therapy (aPDT) and nystatin (NYS) in the expression of genes (ACT1, ALS1, CAP1, CAT1, EFG1, HWP1, LIP3, PLB1, SAP1, and SOD1) involved in the virulence of Candida albicans strains recovered from patients with denture stomatitis (DS). These strains were isolated from the patients before (initial) and after treatment (final), and 45 days after the treatments (follow-up). For gene expression analyses, RNA was isolated from the clinical strains, followed by cDNA synthesis and qPCR using specific primers for each target gene. The samples that present integrity were pooled to increase the RNA yield. In the end, four patients treated with aPDT and five patients treated with NYS had the clinical isolates of C. albicans submitted to gene expression evaluation. The data demonstrated a statistical difference in the expression of PLB1 and ACT1 for the different therapies (aPDT versus NYS). Also, there was a statistical difference in the expression of CAT1, SOD1, and LIP3 at the time intervals assessed (initial, final, and follow-up). In contrast, no statistical difference was found in the expression of ALS1, HWP1, EFG1, CAP1, CAT1, SOD1, LIP3, and SAP1 between the therapies, while no significant difference was detected at the time intervals evaluated for ALS1, HWP1, EFG1, CAP1, and SAP1. Therefore, the topical treatments for DS with aPDT or NYS did not effect the expression of most C. albicans virulence genes evaluated.

Consecutive treatments with photodynamic therapy and nystatin altered the expression of virulence and ergosterol biosynthesis genes of a fluconazole-resistant Candida albicans in vivo

This investigation assessed the effect of five consecutive daily topical treatments of antimicrobial photodynamic therapy (aPDT), nystatin (NYS), and an association of treatments on a fluconazole-resistant strain of Candida albicans colonizing the tongues of mice. After the last treatments application, colonies of C. albicans were recovered from the tongues and used to determine their fluconazole susceptibility. After 24 hours of the last treatment, the mice tongues were processed to evaluate the expression of C. albicans genes related to the virulence and ergosterol production. The fluconazole susceptibility test yielded a resistance profile similar for all treatment groups and the control group (no treatment). The treatments aPDT, NYS, NYS+aPDT, and aPDT+NYS promoted a reduction in ALS1, EFG1, CAP1, SOD1, SAP1, and LIP3 expression. The expression of HWP1 was higher in the three groups containing nystatin. In contrast, the treatments produced a significative increase in CAT1 gene expression, mainly in the groups in which aPDT was performed. The expression of genes related to ergosterol production was significantly reduced by the treatments evaluated (aPDT, NYS, NYS+aPDT, and aPDT+NYS). Thus, the consecutive topical treatments performed on mice tongues promoted a reduction in the expression of virulence and ergosterol biosynthesis genes of a fluconazole-resistant C. albicans.

Candida albicans adhesion to central venous catheters: Impact of blood plasma-driven germ tube formation and pathogen-derived adhesins

Candida albicans-related bloodstream infections are often associated with infected central venous catheters (CVC) triggered by microbial adhesion and biofilm formation. We utilized single-cell force spectroscopy (SCFS) and flow chamber models to investigate the adhesion behavior of C. albicans yeast cells and germinated cells to naïve and human blood plasma (HBP)-coated CVC tubing. Germinated cells demonstrated up to 56.8-fold increased adhesion forces to CVC surfaces when compared to yeast cells. Coating of CVCs with HBP significantly increased the adhesion of 60-min germinated cells but not of yeast cells and 30-min germinated cells. Under flow conditions comparable to those in major human veins, germinated cells displayed a flow directional-orientated adhesion pattern to HBP-coated CVC material, suggesting the germ tip to serve as the major adhesive region. None of the above-reported phenotypes were observed with germinated cells of an als3Δ deletion mutant, which displayed similar adhesion forces to CVC surfaces as the isogenic yeast cells. Germinated cells of the als3Δ mutant also lacked a clear flow directional-orientated adhesion pattern on HBP-coated CVC material, indicating a central role for Als3 in the adhesion of germinated C. albicans cells to blood exposed CVC surfaces. In the common model of C. albicans, biofilm formation is thought to be mediated primarily by yeast cells, followed by surface-triggered the formation of hyphae. We suggest an extension of this model in which C. albicans germ tubes promote the initial adhesion to blood-exposed implanted medical devices via the germ tube-associated adhesion protein Als3.

Antimicrobial photodynamic therapy reduces gene expression of Candida albicans in biofilms

The present study evaluated whether the oxidative stress caused by antimicrobial photodynamic therapy (aPDT) affects the expression of C. albicans genes related to adhesion and biofilm formation (ALS1 and HPW1) and oxidative stress response (CAP1, CAT1, and SOD1). The aPDT was mediated by two photosensitizing agents (PSs) Photodithazine® (PDZ at 100 and 200 mg/L) or Curcumin (CUR at 40 and 80 μM) and LED (37.5 J/cm² or 50 J/cm²). The quantification of the expression was performed by Reverse Transcription-Quantitative Polymerase Chain Reaction (RT-qPCR) using specific primers for the target genes. The data were analyzed by Analysis of Variance (α = 0.05), followed by Tukey's post-test. It was observed reduction in the expression of ALS1, HWP1, CAP1, CAT1, and SOD1 when aPDT was performed using 200 mg/L PDZ and 80 μM CUR associated to LED (37.7 and 50 J/cm², respectively) and using 100 mg/L PDZ and 40 μM CUR with LED of 50 J/cm² (versus control). Also, the expression of CAP1 and SOD1 genes was reduced after aPDT using 100 mg/L PDZ and LED of 37.5 J/cm². There was a significant reduction in the expression of genes HWP1, CAP1, and SOD1 after aPDT using 40 μM CUR and 37.5 J/cm² (versus the control group). The application of LED only at 37.5 and 50 J/cm² promoted down-regulation of ALS1, CAP1, CAT1, and SOD1 genes (versus the control group). Therefore, aPDT mediated by LED -associated PSs PDZ and CUR promoted a reduction in the expression of the five C. albicans genes evaluated.

To Be or Not to Be a Pathogen: Candida albicans and Celiac Disease

Celiac disease (CD) is an immune-mediated disorder triggered by the ingestion of gluten and characterized by reversible small-bowel mucosal atrophy in genetically predisposed subjects. Although the prevalence of CD has increased, many aspects of this pathology are still unrecognized. Candida albicans, a commensal of the human gastrointestinal tract, has been linked to CD for a long time based, among others, upon the observation of similarity between the fungal wall component, hyphal wall protein 1, and CD-related gliadin T-cell epitopes. We have recently demonstrated that Candida may switch from commensal to pathogen contingent upon several players, including mast cells, key sentinels of the immune system at the interface between the environment and the host, and the pleiotropic cytokine IL-9. However, other factors are likely to play a role by altering the balance between inflammation and tolerance. In this regard, tryptophan and its metabolites are increasingly being recognized in promoting mucosal homeostasis by balancing the immune response to external cues. Based on these premises, we will discuss how the output of Candida colonization in the gut is highly contextual, being determined at the intersection of many immunological (IL-9/mast cells) and metabolic (tryptophan) pathways that ultimately dictate the Candida commensalism vs. pathogenicity in CD, thus paving the way for novel therapeutic opportunities in CD.

Efficacy of Compounds Isolated from Streptomyces olivaceus against the Morphogenesis and Virulence of Candida albicans

Candida albicans is a type of commensal fungi which causes serious infections in immunocompromised patients and contributes to high mortality. In the present study, we identified that the extract from Streptomyces olivaceus SCSIO T05 inhibited hypha and biofilm formation of C. albicans. Seven compounds were isolated and evaluated for their effects on the biological functions and virulence of C. albicans. Two leading compounds, compound 1 (sorbicillin) and compound 2 (3-methyl-N-(2′-phenethyl)-butyrylamide) were identified as exhibiting strong activity against C. albicans morphological transition, adhesion activity, cytotoxicity, and adhesion to human cells, in a dose-dependent manner. Notably, compound 2 inhibited C. albicans infection in mouse oral mucosal models. Transcriptomic analysis and real-time PCR results revealed that compound 2 most likely inhibited the biological functions of C. albicans cells by regulating the expression levels of HWP1, TEC1, ALS1, IFD6, and CSH1, which are associated with filament formation and cell adhesion. Our results suggest that the candidate compounds present excellent efficacy against C. albicans pathogenicity and that they can be developed as potential options for the clinical treatment of candidiasis.

Targeting the fungal cell wall: current therapies and implications for development of alternative antifungal agents

Fungal infections are a worldwide problem associated with high morbidity and mortality. There are relatively few antifungal agents, and resistance has emerged within these pathogens for the newest antifungal drugs. As the fungal cell wall is critical for growth and development, it is one of the most important targets for drug development. In this review, the currently available cell wall inhibitors and suitable drug candidates for the treatment of fungal infections are explored. Future studies of the fungal cell wall and compounds that have detrimental effects on this important outer structural layer could aid in antifungal drug discovery and lead to the development of alternative cell wall inhibitors to fill gaps in clinical therapies for difficult-to-treat fungal infections.

A new prokaryotic expression vector for the expression of antimicrobial peptide abaecin using SUMO fusion tag

Background

Despite the growing demand for antimicrobial peptides (AMPs) for clinical use as an alternative approach against antibiotic-resistant bacteria, the manufacture of AMPs relies on expensive, small-scale chemical methods. The small ubiquitin-related modifier (SUMO) tag is industrially practical for increasing the yield of recombinant proteins by increasing solubility and preventing degradation in expression systems.

Results

A new vector system, pKSEC1, was designed to produce AMPs, which can work in prokaryotic systems such as Escherichia coli and plant chloroplasts. 6xHis was tagged to SUMO for purification of SUMO-fused AMPs. Abaecin, a 34-aa-long antimicrobial peptide from honeybees, was expressed in a fusion form to 6xHis-SUMO in a new vector system to evaluate the prokaryotic expression platform of the antimicrobial peptides. The fusion sequences were codon-optimized in three different combinations and expressed in E. coli. The combination of the native SUMO sequence with codon-optimized abaecin showed the highest expression level among the three combinations, and most of the expressed fusion proteins were detected in soluble fractions. Cleavage of the SUMO tag by sumoase produced a 29-aa-long abaecin derivative with a C-terminal deletion. However, this abaecin derivative still retained the binding sequence for its target protein, DnaK. Antibacterial activity of the 29-aa long abaecin was tested against Bacillus subtilis alone or in combination with cecropin B. The combined treatment of the abaecin derivative and cecropin B showed bacteriolytic activity 2 to 3 times greater than that of abaecin alone.

Conclusions

Using a SUMO-tag with an appropriate codon-optimization strategy could be an approach for the production of antimicrobial peptides in E.coli without affecting the viability of the host cell.

Electronic supplementary material

The online version of this article (10.1186/s12896-019-0506-x) contains supplementary material, which is available to authorized users.

Candida innate immunity at the mucosa

The tremendous diversity in microbial species that colonise the mucosal surfaces of the human body is only now beginning to be fully appreciated. Distinguishing between the behaviour of commensal microbes and harmful pathogens that reside at mucosal sites in the body is a complex, and exquisitely fine-tuned process central to mucosal health. The fungal pathobiont Candida albicans is frequently isolated from mucosal surfaces with an asymptomatic carriage rate of approximately 60% in the human population. While normally a benign member of the microbiota, overgrowth of C. albicans often results in localised mucosal infection causing morbidity in otherwise healthy individuals, and invasive infection that often causes death in the absence of effective immune defence. C. albicans triggers numerous innate immune responses at mucosal surfaces, and detection of C. albicans hyphae in particular, stimulates the production of antimicrobial peptides, danger-associated molecular patterns and cytokines that function to reduce fungal burdens during infection. This review will summarise our current understanding of innate immune responses to C. albicans at mucosal surfaces.

Release of transcriptional repression through the HCR promoter region confers uniform expression of HWP1 on surfaces of Candida albicans germ tubes

The mechanisms that fungi use to co-regulate subsets of genes specifically associated with morphogenic states represent a basic unsolved problem in fungal biology. Candida albicans is an important model of fungal differentiation both for rapid interconversion between yeast and hyphal growth forms and for white/opaque switching mechanisms. The Sundstrom lab is interested in mechanisms regulating hypha-specific expression of adhesin genes that are critical for C. albicans hyphal growth phenotypes and pathogenicity. Early studies on hypha-specific genes such as HWP1 and ALS3 reported 5’ intergenic regions that are larger than those typically found in an average promoter and are associated with hypha-specific expression. In the case of HWP1, activation and repression involves a 368 bp region, denoted the HWP1 control region (HCR), located 1410 bp upstream of its transcription start site. In previous work we showed that HCR confers developmental regulation to a heterologous ENO1 promoter, indicating that HCR by itself contains sufficient information to couple gene expression to morphology. Here we show that the activation and repression mediated by HCR are localized to distinct HCR regions that are targeted by the transcription factors Nrg1p and Efg1p. The finding that Efg1p mediates both repression via HCR under yeast morphological conditions and activation conditions positions Efg1p as playing a central role in coupling HWP1 expression to morphogenesis through the HCR region. These localization studies revealed that the 120 terminal base pairs of HCR confer Efg1p-dependent repressive activity in addition to the Nrg1p repressive activity mediated by DNA upstream of this subregion. The 120 terminal base pair subregion of HCR also contained an initiation site for an HWP1 transcript that is specific to yeast growth conditions (HCR-Y) and may function in the repression of downstream DNA. The detection of an HWP1 mRNA isoform specific to hyphal growth conditions (HWP1-H) showed that morphology-specific mRNA isoforms occur under both yeast and hyphal growth conditions. Similar results were found at the ALS3 locus. Taken together, these results, suggest that the long 5’ intergenic regions upstream of hypha-specific genes function in generating mRNA isoforms that are important for morphology-specific gene expression. Additional complexity in the HWP1 promoter involving HCR-independent activation was discovered by creating a strain lacking HCR that exhibited variable HWP1 expression during hyphal growth conditions. These results show that while HCR is important for ensuring uniform HWP1 expression in cell populations, HCR independent expression also exists. Overall, these results elucidate HCR-dependent mechanisms for coupling HWP1-dependent gene expression to morphology uniformly in cell populations and prompt the hypothesis that mRNA isoforms may play a role in coupling gene expression to morphology in C. albicans.

Candida Biofilms: Threats, Challenges, and Promising Strategies

Candida species are fungal pathogens known for their ability to cause superficial and systemic infections in the human host. These pathogens are able to persist inside the host due to the development of pathogenicity and multidrug resistance traits, often leading to the failure of therapeutic strategies. One specific feature of Candida species pathogenicity is their ability to form biofilms, which protects them from external factors such as host immune system defenses and antifungal drugs. This review focuses on the current threats and challenges when dealing with biofilms formed by Candida albicans, Candida glabrata, Candida tropicalis, and Candida parapsilosis, highlighting the differences between the four species. Biofilm characteristics depend on the ability of each species to produce extracellular polymeric substances (EPS) and display dimorphic growth, but also on the biofilm substratum, carbon source availability and other factors. Additionally, the transcriptional control over processes like adhesion, biofilm formation, filamentation, and EPS production displays great complexity and diversity within pathogenic yeasts of the Candida genus. These differences not only have implications in the persistence of colonization and infections but also on antifungal resistance typically found in Candida biofilm cells, potentiated by EPS, that functions as a barrier to drug diffusion, and by the overexpression of drug resistance transporters. The ability to interact with different species in in vivo Candida biofilms is also a key factor to consider when dealing with this problem. Despite many challenges, the most promising strategies that are currently available or under development to limit biofilm formation or to eradicate mature biofilms are discussed.

Accessibility and contribution to glucan masking of natural and genetically tagged versions of yeast wall protein 1 of Candida albicans

Yeast wall protein 1 (Ywp1) is an abundant glycoprotein of the cell wall of the yeast form of Candida albicans, the most prevalent fungal pathogen of humans. Antibodies that bind to the polypeptide backbone of isolated Ywp1 show little binding to intact yeast cells, presumably because the Ywp1 epitopes are masked by the polysaccharides of the mannoproteins that form the outer layer of the cell wall. Rare cells do exhibit much greater anti-Ywp1 binding, however, and one of these was isolated and characterized. No differences were seen in its Ywp1, but it exhibited greater adhesiveness, sensitivity to wall perturbing agents, and exposure of its underlying β-1,3-glucan layer to external antibodies. The molecular basis for this greater epitope accessibility has not been determined, but has facilitated exploration of how these properties change as a function of cell growth and morphology. In addition, previously engineered strains with reduced quantities of Ywp1 in their cell walls were also found to have greater β-1,3-glucan exposure, indicating that Ywp1 itself contributes to the masking of wall epitopes, which may be important for understanding the anti-adhesive effect of Ywp1. Ectopic production of Ywp1 by hyphae, which reduces the adhesivity of these filamentous forms of C. albicans, was similarly found to reduce exposure of the β-1,3-glucan in their walls. To monitor Ywp1 in the cell wall irrespective of its accessibility, green fluorescent protein (Gfp) was genetically inserted into wall-anchored Ywp1 using a bifunctional cassette that also allowed production from a single transfection of a soluble, anchor-free version. The wall-anchored Ywp1-Gfp-Ywp1 accumulated in the wall of the yeast forms but not hyphae, and appeared to have properties similar to native Ywp1, including its adhesion-inhibiting effect. Some pseudohyphal walls also detectably accumulated this probe. Strains of C. albicans with tandem hemagglutinin (HA) epitopes inserted into wall-anchored Ywp1 were previously created by others, and were further explored here. As above, rare cells with much greater accessibility of the HA epitopes were isolated, and also found to exhibit greater exposure of Ywp1 and β-1,3-glucan. The placement of the HA cassette inhibited the normal N-glycosylation and propeptide cleavage of Ywp1, but the wall-anchored Ywp1-HA-Ywp1 still accumulated in the cell wall of yeast forms. Bifunctional transformation cassettes were used to additionally tag these molecules with Gfp, generating soluble Ywp1-HA-Gfp and wall-anchored Ywp1-HA-Gfp-Ywp1 molecules. The former revealed unexpected electrophoretic properties caused by the HA insertion, while the latter further highlighted differences between the presence of a tagged Ywp1 molecule (as revealed by Gfp fluorescence) and its accessibility in the cell wall to externally applied antibodies specific for HA, Gfp and Ywp1, with accessibility being greatest in the rapidly expanding walls of budding daughter cells. These strains and results increase our understanding of cell wall properties and how C. albicans masks itself from recognition by the human immune system.

Transcriptional Control of Drug Resistance, Virulence and Immune System Evasion in Pathogenic Fungi: A Cross-Species Comparison

Transcription factors are key players in the control of the activation or repression of gene expression programs in response to environmental stimuli. The study of regulatory networks taking place in fungal pathogens is a promising research topic that can help in the fight against these pathogens by targeting specific fungal pathways as a whole, instead of targeting more specific effectors of virulence or drug resistance. This review is focused on the analysis of regulatory networks playing a central role in the referred mechanisms in the human fungal pathogens Aspergillus fumigatus, Cryptococcus neoformans, Candida albicans, Candida glabrata, Candida parapsilosis, and Candida tropicalis. Current knowledge on the activity of the transcription factors characterized in each of these pathogenic fungal species will be addressed. Particular focus is given to their mechanisms of activation, regulatory targets and phenotypic outcome. The review further provides an evaluation on the conservation of transcriptional circuits among different fungal pathogens, highlighting the pathways that translate common or divergent traits among these species in what concerns their drug resistance, virulence and host immune evasion features. It becomes evident that the regulation of transcriptional networks is complex and presents significant variations among different fungal pathogens. Only the oxidative stress regulators Yap1 and Skn7 are conserved among all studied species; while some transcription factors, involved in nutrient homeostasis, pH adaptation, drug resistance and morphological switching are present in several, though not all species. Interestingly, in some cases not very homologous transcription factors display orthologous functions, whereas some homologous proteins have diverged in terms of their function in different species. A few cases of species specific transcription factors are also observed.

Role of SFP1 in the Regulation of Candida albicans Biofilm Formation

Candida albicans is a major human fungal pathogen. One of the important features of C. albicans pathogenicity is the ability to form biofilms on mucosal surfaces and indwelling medical devices. Biofilm formation involves complex processes in C. albicans, including cell adhesion, filamentous growth, extracellular matrix secretion and cell dispersion. In this work, we characterized the role of the transcription factor Sfp1, particularly with respect to its function in the regulation of biofilm formation. The deletion of the SFP1 gene enhanced cell adhesion and biofilm formation in comparison to the wild-type strain. Interestingly, the sfp1-deleted mutant also exhibited an increase in the expression of the ALS1, ALS3 and HWP1 genes, which encode adhesin proteins. In addition, Sfp1 was demonstrated to function downstream of the Rhb1-TOR signaling pathway. Bcr1 and Efg1 are transcription factors that are critical for controlling biofilm formation, and Efg1 is also required for hyphal growth. Deleting either the BCR1 or EFG1 gene in the sfp1-null background led to reduced adhesin gene expression. As a result, the bcr1/sfp1 or efg1/sfp1 double deletion mutants exhibited dramatically reduced biofilm formation. The results indicated that Sfp1 negatively regulates the ALS1, ALS3 and HWP1 adhesin genes and that the repression of these genes is mediated by the inhibition of Bcr1 and Efg1.

Adhesion molecules from the diatom Phaeodactylum tricornutum (Bacillariophyceae): genomic identification by amino-acid profiling and in vivo analysis

Cell adhesion molecules (CAMs) are important in prokaryotes and eukaryotes for cell-cell and cell-substratum interactions. The characteristics of adhesive proteins in the model diatom Phaeodactylum tricornutum were investigated by bioinformatic analysis and in vivo characterization. Bioinformatic analysis of the protein coding potential of the P. tricornutum genome used an amino-acid profile that we developed as a new system to identify uncharacterized or novel CAMs. Putative diatom CAMs were identified and seven were characterized in vivo, by generation of transgenic diatom lines overexpressing genes encoding C-terminal yellow fluorescent protein (YFP) fusion proteins. Three of these selected genes encode proteins with weak similarity to characterized proteins, a c-type lectin and two fasciclins, whereas the others are novel. The resultant cell lines were investigated for alterations in their adhesive ability. Whole cell-substratum adhesion strength was measured in a fully turbulent flow chamber, while atomic force microscopy was used to quantify the relative frequency of adhesion, as well as the length and strength of single molecules in the secreted mucilage. Finally, quartz crystal microbalance analysis characterized the visco-elastic properties and interaction of the mucilage-substratum interface. These combined studies revealed a range of phenotypes affecting adhesion, and led to the identification of candidate proteins involved in diatom adhesion. In summary, our study has for the first time combined bioinformatics and molecular physiological studies to provide new insights into diatom adhesive molecules.

Comparative adherence of Candida albicans and Candida dubliniensis to human buccal epithelial cells and extracellular matrix proteins

Candida albicans and Candida dubliniensis are very closely related pathogenic yeast species. Despite their close relationship, C. albicans is a far more successful colonizer and pathogen of humans. The purpose of this study was to determine if the disparity in the virulence of the two species is attributed to differences in their ability to adhere to human buccal epithelial cells (BECs) and/or extracellular matrix proteins. When grown overnight at 30°C in yeast extract peptone dextrose, genotype 1 C. dubliniensis isolates were found to be significantly more adherent to human BECs than C. albicans or C. dubliniensis genotypes 2-4 (P < 0.001). However, when the yeast cells were grown at 37°C, no significant difference between the adhesion of C. dubliniensis genotype 1 and C. albicans to human BECs was observed, and C. dubliniensis genotype 1 and C. albicans adhered to BECs in significantly greater numbers than the other C. dubliniensis genotypes (P < 0.001). Using surface plasmon resonance analysis, C. dubliniensis isolates were found to adhere in significantly greater numbers than C. albicans to type I and IV collagen, fibronectin, laminin, vitronectin, and proline-rich peptides. These data suggest that C. albicans is not more adherent to epithelial cells or matrix proteins than C. dubliniensis and therefore other factors must contribute to the greater levels of virulence exhibited by C. albicans.

Towards non-invasive monitoring of pathogen-host interactions during Candida albicans biofilm formation using in vivo bioluminescence

Candida albicans is a major human fungal pathogen causing mucosal and deep tissue infections of which the majority is associated with biofilm formation on medical implants. Biofilms have a huge impact on public health, as fungal biofilms are highly resistant against most antimycotics. Animal models of biofilm formation are indispensable for improving our understanding of biofilm development inside the host, their antifungal resistance and their interaction with the host immune defence system. In currently used models, evaluation of biofilm development or the efficacy of antifungal treatment is limited to ex vivo analyses, requiring host sacrifice, which excludes longitudinal monitoring of dynamic processes during biofilm formation in the live host. In this study, we have demonstrated for the first time that non-invasive, dynamic imaging and quantification of in vitro and in vivo C. albicans biofilm formation including morphogenesis from the yeast to hyphae state is feasible by using growth-phase dependent bioluminescent C. albicans strains in a subcutaneous catheter model in rodents. We have shown the defect in biofilm formation of a bioluminescent bcr1 mutant strain. This approach has immediate applications for the screening and validation ofantimycotics under in vivo conditions, for studying host–biofilm interactions in different transgenic mouse models and for testing the virulence of luminescent C. albicans mutants, hereby contributing to a better understanding of the pathogenesis of biofilm-associated yeast infections.

A sticky situation: untangling the transcriptional network controlling biofilm development in Candida albicans

Candida albicans is a commensal microorganism of the human microbiome; it is also the most prevalent fungal pathogen of humans. Many infections caused by C. albicans are a direct consequence of its proclivity to form biofilms—resilient, surface-associated communities of cells where individual cells acquire specialized properties that are distinct from those observed in suspension cultures. We recently identified the transcriptional network that orchestrates the formation of biofilms in C. albicans. These results set the stage for understanding how biofilms are formed and, once formed, how the specialized properties of biofilms are elaborated. This information will provide new insight for understanding biofilms in more detail and may lead to improvements in preventing and treating biofilm-based infections in the future.

A recently evolved transcriptional network controls biofilm development in Candida albicans

A biofilm is an organized, resilient group of microbes in which individual cells acquire properties, such as drug resistance, that are distinct from those observed in suspension cultures. Here, we describe and analyze the transcriptional network controlling biofilm formation in the pathogenic yeast Candida albicans, whose biofilms are a major source of medical device-associated infections. We have combined genetic screens, genome-wide approaches, and two in vivo animal models to describe a master circuit controlling biofilm formation, composed of six transcription regulators that form a tightly woven network with ∼1,000 target genes. Evolutionary analysis indicates that the biofilm network has rapidly evolved: genes in the biofilm circuit are significantly weighted toward genes that arose relatively recently with ancient genes being underrepresented. This circuit provides a framework for understanding many aspects of biofilm formation by C. albicans in a mammalian host. It also provides insights into how complex cell behaviors can arise from the evolution of transcription circuits.

Candida albicans morphogenesis and host defence: discriminating invasion from colonization

Candida albicans is a common fungal pathogen of humans that colonizes the skin and mucosal surfaces of most healthy individuals. Until recently, little was known about the mechanisms by which mucosal antifungal defences tolerate colonizing C. albicans but react strongly when hyphae of the same microorganism attempt to invade tissue. In this Review, we describe the properties of yeast cells and hyphae that are relevant to their interaction with the host, and the immunological mechanisms that differentially recognize colonizing versus invading C. albicans.

Rapid screening method for compounds that affect the growth and germination of Candida albicans, using a real-time PCR thermocycler

We propose a screening method for compounds affecting growth and germination in Candida albicans using a real-time PCR thermocycler to quantify green fluorescent protein (GFP) fluorescence. Using P(ACT1)-GFP and P(HWP1)-GFP reporter strains, the effects of a wide range of compounds on growth and hyphal formation were quantitatively assessed within 3 h after inoculation.

Effects of fluconazole on the secretome, the wall proteome, and wall integrity of the clinical fungus Candida albicans

Fluconazole is a commonly used antifungal drug that inhibits Erg11, a protein responsible for 14α-demethylation during ergosterol synthesis. Consequently, ergosterol is depleted from cellular membranes and replaced by toxic 14α-methylated sterols, which causes increased membrane fluidity and drug permeability. Surface-grown and planktonic cultures of Candida albicans responded similarly to fluconazole at 0.5 mg/liter, showing reduced biomass formation, severely reduced ergosterol levels, and almost complete inhibition of hyphal growth. There was no evidence of cell leakage. Mass spectrometric analysis of the secretome showed that its composition was strongly affected and included 17 fluconazole-specific secretory proteins. Relative quantification of (14)N-labeled query walls relative to a reference standard mixture of (15)N-labeled yeast and hyphal walls in combination with immunological analysis revealed considerable fluconazole-induced changes in the wall proteome as well. They were, however, similar for both surface-grown and planktonic cultures. Two major trends emerged: (i) decreased incorporation of hypha-associated wall proteins (Als3, Hwp1, and Plb5), consistent with inhibition of hyphal growth, and (ii) increased incorporation of putative wall repair-related proteins (Crh11, Pga4, Phr1, Phr2, Pir1, and Sap9). As exposure to the wall-perturbing drug Congo red led to a similar response, these observations suggested that fluconazole affects the wall. In keeping with this, the resistance of fluconazole-treated cells to wall-perturbing compounds decreased. We propose that fluconazole affects the integrity of both the cellular membranes and the fungal wall and discuss its potential consequences for antifungal therapy. We also present candidate proteins from the secretome for clinical marker development.

Cell wall integrity is linked to mitochondria and phospholipid homeostasis in Candida albicans through the activity of the post-transcriptional regulator Ccr4-Pop2

The cell wall is essential for viability of fungi and is an effective drug target in pathogens such as Candida albicans. The contribution of post-transcriptional gene regulators to cell wall integrity in C. albicans is unknown. We show that the C. albicans Ccr4-Pop2 mRNA deadenylase, a regulator of mRNA stability and translation, is required for cell wall integrity. The ccr4/pop2 mutants display reduced wall β-glucans and sensitivity to the echinocandin caspofungin. Moreover, the deadenylase mutants are compromised for filamentation and virulence. We demonstrate that defective cell walls in the ccr4/pop2 mutants are linked to dysfunctional mitochondria and phospholipid imbalance. To further understand mitochondrial function in cell wall integrity, we screened a Saccharomyces cerevisiae collection of mitochondrial mutants. We identify several mitochondrial proteins required for caspofungin tolerance and find a connection between mitochondrial phospholipid homeostasis and caspofungin sensitivity. We focus on the mitochondrial outer membrane SAM complex subunit Sam37, demonstrating that it is required for both trafficking of phospholipids between the ER and mitochondria and cell wall integrity. Moreover, in C. albicans also Sam37 is essential for caspofungin tolerance. Our study provides the basis for an integrative view of mitochondrial function in fungal cell wall biogenesis and resistance to echinocandin antifungal drugs.

A novel allele of HWP1, isolated from a clinical strain of Candida albicans with defective hyphal growth and biofilm formation, has deletions of Gln/Pro and Ser/Thr repeats involved in cellular adhesion

Gene HWP1 encodes a major Candida albicans hyphae cell wall protein which is a substrate of mammalian transglutaminases, promoting the cross-link of the fungus to epithelial cells. Here, we describe a novel HWP1 allele, isolated from C. albicans blood isolates. Analysis of the translated sequence shows that three important regions are absent in the novel allele, HWP1-2, relative to the previously described allele, HWP1-1. Regions 1 and 2 consist of 10 amino acid repeats important for functional conformation of peptide chains and attachment of C. albicans cells to the mammalian epithelia. Region 3 consists of 34 amino acid residues rich in threonine and serine, with O-glycosylation sites that promote the cross-linking with other proteins on C. albicans surface. The HWP1-2 homozygous strain L757 and the heterozygous strain L296 (HWP1-1/HWP1-2) have significantly lower levels of HWP1 expression during hyphal growth and biofilm formation compared to strain SC5314 (HWP1-1/HWP1-1). However, strain L296 properly forms hyphae and biofilms in vitro while strain L757 has reduced hyphal growth (40.4%) and biofilm formation (90.8%). Our results indicate that the HWP1 locus has biofilm specific allelic differential expression and suggest that the HWP1-2 encoded protein is less efficient to maintain cell-to-cell and cell-to-surface adhesion during biofilm formation. This is the first report of a natural variant of HWP1.

Fungal Biofilms: Relevance in the Setting of Human Disease

The use of indwelling medical devices is rapidly growing and is often complicated by infections with biofilm-forming microbes that are resistant to antimicrobial agents and host defense mechanisms. Fungal biofilms have emerged as a clinical problem associated with these medical device infections, causing significant morbidity and mortality. This review discusses the recent advances in the understanding of fungal biofilms, including the role of fungal surface components in adherence, gene expression, and quorum sensing in biofilm formation. We propose novel strategies for the prevention or eradication of microbial colonization of medical prosthetic devices.

Mass spectrometric quantification of the adaptations in the wall proteome of Candida albicans in response to ambient pH

The mucosal layers colonized by the pathogenic fungus Candida albicans differ widely in ambient pH. Because the properties and functions of wall proteins are probably pH dependent, we hypothesized that C. albicans adapts its wall proteome to the external pH. We developed an in vitro system that mimics colonization of mucosal surfaces by growing biomats at pH 7 and 4 on semi-solid agarose containing mucin as the sole nitrogen source. The biomats expanded radially for at least 8 days at a rate of ~30 μm h(-1). At pH 7, hyphal growth predominated and growth was invasive, whereas at pH 4 only yeast and pseudohyphal cells were present and growth was noninvasive. Both qualitative mass spectrometric analysis of the wall proteome by tandem mass spectrometry and relative quantification of individual wall proteins (pH 7/pH 4), using Fourier transform mass spectrometry (FT-MS) and a reference mixture of (15)N-labelled yeast and hyphal walls, identified similar sets of >20 covalently linked wall proteins. The adhesion proteins Als1 and Als3, Hyr1, the transglucosidase Phr1, the detoxification enzyme Sod5 and the mammalian transglutaminase substrate Hwp1 (immunological detection) were only present at pH 7, whereas at pH 4 the level of the transglucosidase Phr2 was >35-fold higher than at pH 7. Sixteen out of the 22 proteins identified by FT-MS showed a greater than twofold change. These results demonstrate that ambient pH strongly affects the wall proteome of C. albicans, show that our quantitative approach can give detailed insights into the dynamics of the wall proteome, and point to potential vaccine targets.

A multifunctional, synthetic Gaussia princeps luciferase reporter for live imaging of Candida albicans infections

Real-time monitoring of the spatial and temporal progression of infection/gene expression in animals will contribute greatly to our understanding of host-pathogen interactions while reducing the number of animals required to generate statistically significant data sets. Sensitive in vivo imaging technologies can detect low levels of light emitted from luciferase reporters in vivo, but the existing reporters are not optimal for fungal infections. Therefore, our aim was to develop a novel reporter system for imaging Candida albicans infections that overcomes the limitations of current luciferase reporters for this major fungal pathogen. This luciferase reporter was constructed by fusing a synthetic, codon-optimized version of the Gaussia princeps luciferase gene to C. albicans PGA59, which encodes a glycosylphosphatidylinositol-linked cell wall protein. Luciferase expressed from this PGA59-gLUC fusion (referred to as gLUC59) was localized at the C. albicans cell surface, allowing the detection of luciferase in intact cells. The analysis of fusions to strong (ACT1 and EFT3), oxidative stress-induced (TRX1, TRR1, and IPF9996), and morphogenesis-dependent (HWP1) promoters confirmed that gLUC59 is a convenient and sensitive reporter for studies of gene regulation in yeast or hyphal cells, as well as a flexible screening tool. Moreover, the ACT1-gLUC59 fusion represented a powerful tool for the imaging of disease progression in superficial and subcutaneous C. albicans infections. gLUC59 and related cell surface-exposed luciferase reporters might find wide applications in molecular biology, cell biology, pathobiology, and high-throughput screens.

Genome-wide expression profiling of the osmoadaptation response of Debaryomyces hansenii

The euryhaline marine yeast Debaromyces hansenii is a model system for the study of processes related to osmoadaptation. In this study, microarray-based gene expression analyses of the entire genome of D. hansenii was used to study its response to osmotic stress. Differential gene expression, compared to control, was examined at three time points (0.5, 3 and 6 h) after exposure of D. hansenii cultures to high salt concentration. Among the 1.72% of genes showing statistically significant differences in expression, only 65 genes displayed at least three-fold increases in mRNA levels after treatment with 2 M NaCl. On the other hand, 44 genes showed three-fold repression. Upregulated as well as the downregulated genes were grouped into functional categories to identify biochemical processes possibly affected by osmotic stress and involved in osmoadaptation. The observation that only a limited number of genes are upregulated in D. hansenii in response to osmotic stress supports the notion that D. hansenii is pre-adapted to survive in extreme saline environments. In addition, since more than one-half of the upregulated genes encode for ribosomal proteins, it is possible that a translational gene regulatory mechanism plays a key role in D. hansenii's osmoregulatory response. Validation studies for ENA1 and for hyphal wall/cell elongation protein genes, using real-time PCR, confirmed patterns of gene expression observed in our microarray experiments. To our knowledge, this study is the first of its kind in this organism and provides the foundation for future molecular studies assessing the significance of the genes identified here in D. hansenii's osmoadaptation.

Conserved WCPL and CX4C domains mediate several mating adhesin interactions in Saccharomyces cerevisiae

Several adhesins are induced by pheromones during mating in Saccharomyces cerevisiae, including Aga1p, Aga2p, Sag1p (Agalpha1p), and Fig2p. These four proteins all participate in or influence a well-studied agglutinin interaction mediated by Aga1p-Aga2p complexes and Sag1p; however, they also play redundant and essential roles in mating via an unknown mechanism. Aga1p and Fig2p both contain repeated, conserved WCPL and CX(4)C domains. This study was directed toward understanding the mechanism underlying the collective requirement of agglutinins and Fig2p for mating. Apart from the well-known agglutinin interaction between Aga2p and Sag1p, three more pairs of interactions in cells of opposite mating type were revealed by this study, including bilateral heterotypic interactions between Aga1p and Fig2p and a homotypic interaction between Fig2p and Fig2p. These four pairs of adhesin interactions are collectively required for maximum mating efficiency and normal zygote morphogenesis. GPI-less, epitope-tagged forms of Aga1p and Fig2p can be co-immunoprecipitated from the culture medium of mating cells in a manner dependent on the WCPL and CX(4)C domains in the R1 repeat of Aga1p. Using site-directed mutagenesis, the conserved residues in Aga1p that interact with Fig2p were identified. Aga1p is involved in two distinct adhesive functions that are independent of each other, which raises the possibility for combinatorial interactions of this protein with its different adhesion receptors, Sag1 and Fig2p, a property of many higher eukaryotic adhesins.

The protein kinase Tor1 regulates adhesin gene expression in Candida albicans

Eukaryotic cell growth is coordinated in response to nutrient availability, growth factors, and environmental stimuli, enabling cell–cell interactions that promote survival. The rapamycin-sensitive Tor1 protein kinase, which is conserved from yeasts to humans, participates in a signaling pathway central to cellular nutrient responses. To gain insight into Tor-mediated processes in human fungal pathogens, we have characterized Tor signaling in Candida albicans. Global transcriptional profiling revealed evolutionarily conserved roles for Tor1 in regulating the expression of genes involved in nitrogen starvation responses and ribosome biogenesis. Interestingly, we found that in C. albicans Tor1 plays a novel role in regulating the expression of several cell wall and hyphal specific genes, including adhesins and their transcriptional repressors Nrg1 and Tup1. In accord with this transcriptional profile, rapamycin induced extensive cellular aggregation in an adhesin-dependent fashion. Moreover, adhesin gene induction and cellular aggregation of rapamycin-treated cells were strongly dependent on the transactivators Bcr1 and Efg1. These findings support models in which Tor1 negatively controls cellular adhesion by governing the activities of Bcr1 and Efg1. Taken together, these results provide evidence that Tor1-mediated cellular adhesion might be broadly conserved among eukaryotic organisms.

Living organisms have an intrinsic ability to coordinate their growth and proliferation in response to nutrient availability. In organisms ranging from yeasts to humans, the Tor1 signaling pathway responds to nutrient-derived signals and orchestrates cell growth. Accordingly, we find that in the human fungal pathogen Candida albicans, Tor1 signaling also functions to promote growth. We also uncovered a novel role for the Tor1 molecular pathway in promoting hyphal growth of C. albicans on semi-solid surfaces and in controlling cell–cell adherence. Gene expression analysis and genetic manipulations implicate the known cell surface adhesins Als1 and Als3 as mediators of Tor1-regulated cellular adhesion. Further genetic analysis identified the transcriptional regulators Bcr1, Efg1, Nrg1, and Tup1 that together with Tor1 compose a regulatory network governing adhesin gene expression and cellular adhesion. Given that the Tor pathway is the target of several small molecule inhibitors including rapamycin, a versatile pharmacological drug used in medicine, there is considerable interest in Tor signaling pathways and their function. Moreover, given the potent fungicidal activity of rapamycin against C. albicans, novel antifungal therapies remain to be developed, which may also include novel antifungal therapies with less immunosuppressive rapamycin analogs.

Genome-wide analysis of Candida albicans gene expression patterns during infection of the mammalian kidney

Global analysis of the molecular responses of microbial pathogens to their mammalian hosts represents a major challenge. To date few microarray studies have been performed on Candida albicans cells derived from infected tissues. In this study we examined the C. albicans SC5314 transcriptome from renal infections in the rabbit. Genes involved in adhesion, stress adaptation and the assimilation of alternative carbon sources were up-regulated in these cells compared with control cells grown in RPMI 1640, whereas genes involved in morphogenesis, fermentation and translation were down-regulated. When we compared the congenic virulent C. albicans strains NGY152 and SC5314, there was minimal overlap between their transcriptomes during kidney infections. This suggests that much of the gene regulation observed during infections is not essential for virulence. Indeed, we observed a poor correlation between the transcriptome and phenome for those genes that were regulated during kidney infection and that have been virulence tested.

Candida albicans cell wall proteins

The Candida albicans cell wall maintains the structural integrity of the organism in addition to providing a physical contact interface with the environment. The major components of the cell wall are fibrillar polysaccharides and proteins. The proteins of the cell wall are the focus of this review. Three classes of proteins are present in the candidal cell wall. One group of proteins attach to the cell wall via a glycophosphatidylinositol remnant or by an alkali-labile linkage. A second group of proteins with N-terminal signal sequences but no covalent attachment sequences are secreted by the classical secretory pathway. These proteins may end up in the cell wall or in the extracellular space. The third group of proteins lack a secretory signal, and the pathway(s) by which they become associated with the surface is unknown. Potential constituents of the first two classes have been predicted from analysis of genome sequences. Experimental analyses have identified members of all three classes. Some members of each class selected for consideration of confirmed or proposed function, phenotypic analysis of a mutant, and regulation by growth conditions and transcription factors are discussed in more detail.

Phenotypic screening, transcriptional profiling, and comparative genomic analysis of an invasive and non-invasive strain of Candida albicans

Background

Invasion of host tissue by the human fungal pathogen Candida albicans is an important step during the development of candidosis. However, not all C. albicans strains possess the same invasive and virulence properties. For example, the two clinical isolates SC5314 and ATCC10231 differ in their ability to invade host tissue and cause experimental infections. Strain SC5314 is invasive whereas strain ATCC10231 is non-invasive and strongly attenuated in virulence compared to SC5314. In this study we compare the in vitro phenotypic, transcriptional and genomic profiles of these two widely used laboratory strains in order to determine the principal biological and genetic properties responsible for their differential virulence.

Results

In all media tested, the two strains showed the same metabolic flexibility, stress resistance, adhesion properties and hydrolytic enzyme secretion in vitro. However, differences were observed in response to cell-surface disturbing agents and alkaline pH. Furthermore, reduced hyphal formation in strain ATCC10231 under certain conditions correlated with reduced invasive properties in an in vitro invasion assay and a reduced ability to invade epithelial tissue. Despite these diverse phenotypic properties, no substantial genomic differences were detected by comparative genome hybridisation within the open reading frames. However, in vitro transcriptional profiling displayed major differences in the gene expression of these two strains, even under normal in vitro growth conditions.

Conclusion

Our data suggest that the reason for differential virulence of C. albicans strains is not due to the absence of specific genes, but rather due to differences in the expression, function or activity of common genes.

Use of recombinant antigens for the diagnosis of invasive candidiasis

Invasive candidiasis is a frequent and often fatal complication in immunocompromised and critically ill patients. Unfortunately, the diagnosis of invasive candidiasis remains difficult due to the lack of specific clinical symptoms and a definitive diagnostic method. The detection of antibodies against different Candida antigens may help in the diagnosis. However, the methods traditionally used for the detection of antibodies have been based on crude antigenic fungal extracts, which usually show low-reproducibility and cross-reactivity problems. The development of molecular biology techniques has allowed the production of recombinant antigens which may help to solve these problems. In this review we will discuss the usefulness of recombinant antigens in the diagnosis of invasive candidiasis.