Yeast wall protein 1 of Candida albicans

The development of single-domain VHH nanobodies that target the Candida albicans cell surface

Candida albicans causes life-threatening invasive infections that are hard to diagnose and treat, with drug resistance leading to treatment failure. The goal of this study was to develop VHH (single variable domain on a heavy chain) nanobodies to detect drug-resistant infections. Llamas were immunized with a mixture of heat killed and fixed C. albicans cells of different morphologies. Llama lymphocyte RNA was used to generate phage display libraries that were tested for binding to C. albicans cells or cell wall fractions, and single antibody domains were isolated. The libraries were panned against echinocandin-resistant C. albicans isolates and counter-selected against echinocandin-susceptible isolates with the aim of isolating binding domains specific for antigens on drug-resistant cells. Thirty diverse VHH nanobodies were selected, and binding characteristics were assessed via dose-response ELISA. Binding was tested against a variety of C. albicans isolates and other Candida species, indicating that the VHHs were specific for C. albicans. The VHH nanobodies were sorted into four distinct groups based on their binding patterns. Two of the groups bound preferentially to the yeast cell poles and hyphae, respectively. Nanobody binding to C. albicans deletion mutants was tested by fluorescence microscopy and ELISA to identify the antigen targets. VHH19 nanobody, belonging to the largest group, recognized the Als4 adhesin. VHH14 antibody in the hyphae-specific group recognized Als3. None of the isolated VHH nanobodies was selective for drug-resistant clinical isolates. Our data indicate that this approach can generate valuable single-domain antibodies specific to C. albicans proteins.IMPORTANCEThe human fungal pathogen Candida albicans causes a range of diseases from superficial mucosal infections such as oral and vaginal thrush to life-threatening, systemic infections. Accurate and rapid diagnosis of these infections remains challenging, and currently, there are no rapid ways to diagnose drug-resistant infections without performing drug susceptibility testing from blood culture, which can take several days. In this proof-of-concept study, we have generated a diverse set of single domain VHH antibodies (nanobodies) from llamas that recognize and bind specifically to C. albicans cell surface. The nanobodies were classified into four groups based on their binding patterns, for example, cell poles or hyphae. Specific nanobodies were verified as recognizing the important adhesin Als4 or the hyphae associated invasin Als3, respectively. The data validate the approach that small VHH antibody domains hold future promise for diagnostic applications and as probes to study the fungal cell surface.

Sinefungin, a natural nucleoside analog of S-adenosyl methionine, impairs the pathogenicity of Candida albicans

Candida albicans, an opportunistic fungal pathogen, causes life-threatening infections in immunocompromised patients. Current antifungals are limited by toxicity, drug-drug interactions, and emerging resistance, underscoring the importance of identifying novel treatment approaches. Here, we elucidate the impact of sinefungin, an analog of S-adenosyl methionine, on the virulence of C. albicans strain SC5314 and clinical isolates. Our data indicate that sinefungin impairs pathogenic traits of C. albicans including hyphal morphogenesis, biofilm formation, adhesion to epithelial cells, and virulence towards Galleria mellonella, highlighting sinefungin as an avenue for therapeutic intervention. We determine that sinefungin particularly disturbs N6-methyladenosine (m6A) formation. Transcriptome analysis of C. albicans hyphae upon sinefungin treatment reveals an increase in transcripts related to the yeast form and decrease in those associated with hyphae formation and virulence. Collectively, our data propose sinefungin as a potent molecule against C. albicans and emphasize further exploration of post-transcriptional control mechanisms of pathogenicity for antifungal design.

Temporal dynamics of Candida albicans morphogenesis and gene expression reveals distinctions between in vitro and in vivo filamentation

Candida albicans is a common human fungal pathogen that is also a commensal of the oral cavity and gastrointestinal tract. C. albicans pathogenesis is linked to its transition from budding yeast to filamentous morphologies including hyphae and pseudohyphae. The centrality of this virulence trait to C. albicans pathobiology has resulted in extensive characterization of a wide range of factors associated with filamentation with a strong focus on transcriptional regulation. The vast majority of these experiments have used in vitro conditions to induce the yeast-to-filament transition. Taking advantage of in vivo approaches to quantitatively characterize both morphology and gene expression during filamentation during mammalian infection, we have investigated the dynamics of these two aspects of filamentation in vivo and compared them to in vitro filament induction with "host-like" tissue culture media supplemented with serum at mammalian body temperature. Although filamentation shares many common features in the two conditions, we have found two significant differences. First, alternative carbon metabolism genes are expressed early during in vitro filamentation and late in vivo, suggesting significant differences in glucose availability. Second, C. albicans begins a hyphae-to-yeast transition after 4-h incubation while we find little evidence of hyphae-to-yeast transition in vivo up to 24 h post-infection. We show that the low rate of in vivo hyphae-to-yeast transition is likely due to the very low expression of PES1, a key driver of lateral yeast in vitro and that heterologous expression of PES1 is sufficient to trigger lateral yeast formation in vivo.IMPORTANCECandida albicans filamentation is correlated with virulence and is an intensively studied aspect of C. albicans biology. The vast majority of studies on C. albicans filamentation are based on in vitro induction of hyphae and pseudohyphae. Here we used an in vivo filamentation assay and in vivo expression profiling to compare the tempo of morphogenesis and gene expression between in vitro and in vivo filamentation. Although the hyphal gene expression profile is induced rapidly in both conditions, it remains stably expressed over a 12-h time course in vivo while it peaks after 4 h in vitro and is reduced. This reduced hyphal gene expression in vitro correlates with reduced hyphae and increased hyphae-to-yeast transition. By contrast, there is little evidence of hyphae-to-yeast transition in vivo.

The role of the C. albicans transcriptional repressor NRG1 during filamentation and disseminated candidiasis is strain dependent

Candida albicans is one of the most common causes of superficial and invasive fungal diseases in humans. Its ability to cause disease is closely linked to its ability to undergo a morphological transition from budding yeast to filamentous forms (hyphae and pseudohyphae). The extent to which C. albicans strains isolated from patients undergo filamentation varies significantly. In addition, the filamentation phenotypes of mutants involving transcription factors that positively regulate hyphal morphogenesis can also vary from strain to strain. Here, we characterized the virulence, in vitro and in vivo filamentation, and in vitro and in vivo hypha-associated gene expression profiles for four poorly filamenting C. albicans isolates and their corresponding deletion mutants of the repressor of filamentation NRG1. The two most virulent strains, 57055 and 78048, show robust in vivo filamentation but are predominately yeast phase under in vitro hypha induction; the two low-virulence strains (94015 and 78042) do not undergo filamentation well under either condition. In vitro, deletion of NRG1 increases hyphae formation in the SC5314 derivative SN250, but only pseudohyphae are formed in the clinical isolates. Deletion of NRG1 modestly increased the virulence of 78042, which was accompanied by increased expression of hypha-associated genes without an increase in filamentation. Strikingly, deletion of NRG1 in 78048 reduced filamentation in vivo, expression of candidalysin (ECE1), and virulence without dramatically altering establishment of infection. Thus, the function of the conserved repressor NRG1 in C. albicans shows strain-based heterogeneity during infection.IMPORTANCEClinical isolates of the human fungal pathogen Candida albicans show significant variation in their ability to undergo in vitro filamentation and in the function of well-characterized transcriptional regulators of filamentation. Here, we show that Nrg1, a key repressor of filamentation and filament specific gene expression in standard reference strains, has strain-dependent functions, particularly during infection. Most strikingly, loss of NRG1 function can reduce filamentation, hypha-specific gene expression such as the toxin candidalysin, and virulence in some strains. Our data emphasize that the functions of seemingly fundamental and well-conserved transcriptional regulators such as Nrg1 are contextual with respect to both environment and genetic backgrounds.

Heat Shock Protein SSA1 Enriched in Hypoxic Secretome of Candida albicans Exerts an Immunomodulatory Effect via Regulating Macrophage Function

Candida albicans is an opportunistic pathogenic yeast that can survive in both normoxic and hypoxic environments. The involvement of C. albicans secretome on host biological processes has been demonstrated. However, the immunoregulatory function of C. albicans secretome released under hypoxic condition remains unclear. This study demonstrated the differences in cytokine responses and protein profiles between secretomes prepared under normoxic and hypoxic conditions. Furthermore, the immunoregulatory effects of heat shock protein SSA1(Ssa1), a protein candidate enriched in the hypoxic secretome, were investigated. Stimulation of mouse bone marrow-derived macrophages (BMMs) with Ssa1 resulted in the significant production of interleukin (IL)-10, IL-6, and tumor necrosis factor (TNF)-α as well as the significant expression of M2b macrophage markers (CD86, CD274 and tumor necrosis factor superfamily member 14), suggesting that C. albicans Ssa1 may promote macrophage polarization towards an M2b-like phenotype. Proteomic analysis of Ssa1-treated BMMs also revealed that Ssa1 reduced inflammation-related factors (IL-18-binding protein, IL-1 receptor antagonist protein, OX-2 membrane glycoprotein and cis-aconitate decarboxylase) and enhanced the proteins involved in anti-inflammatory response (CMRF35-like molecule 3 and macrophage colony-stimulating factor 1 receptor). Based on these results, we investigated the effect of Ssa1 on C. albicans infection and showed that Ssa1 inhibited the uptake of C. albicans by BMMs. Taken together, our results suggest that C. albicans alters its secretome, particularly by promoting the release of Ssa1, to modulate host immune response and survive under hypoxic conditions.

The good, the bad, and the hazardous: comparative genomic analysis unveils cell wall features in the pathogen Candidozyma auris typical for both baker's yeast and Candida

The drug-resistant pathogenic yeast Candidozyma auris (formerly named Candida auris) is considered a critical health problem of global importance. As the cell wall plays a crucial role in pathobiology, here we performed a detailed bioinformatic analysis of its biosynthesis in C. auris and related Candidozyma haemuli complex species using Candida albicans and Saccharomyces cerevisiae as references. Our data indicate that the cell wall architecture described for these reference yeasts is largely conserved in Candidozyma spp.; however, expansions or reductions in gene families point to subtle alterations, particularly with respect to β--1,3--glucan synthesis and remodeling, phosphomannosylation, β-mannosylation, and glycosylphosphatidylinositol (GPI) proteins. In several aspects, C. auris holds a position in between C. albicans and S. cerevisiae, consistent with being classified in a separate genus. Strikingly, among the identified putative GPI proteins in C. auris are adhesins typical for both Candida (Als and Hyr/Iff) and Saccharomyces (Flo11 and Flo5-like flocculins). Further, 26 putative C. auris GPI proteins lack homologs in Candida genus species. Phenotypic analysis of one such gene, QG37_05701, showed mild phenotypes implicating a role associated with cell wall β-1,3-glucan. Altogether, our study uncovered a wealth of information relevant for the pathogenicity of C. auris as well as targets for follow-up studies.

The role of the C. albicans transcriptional repressor NRG1 during filamentation and disseminated candidiasis is strain-dependent

Candida albicans is one of the most common causes of superficial and invasive fungal disease in humans. Its ability to cause disease has been closely linked to its ability to undergo a morphological transition from budding yeast to filamentous forms (hyphae and pseudohyphae). The ability of C. albicans strains isolated from patients to undergo filamentation varies significantly. In addition, the filamentation phenotypes of mutants involving transcription factors that positively regulate hyphal morphogenesis can also vary from strain to strain. Here, we characterized the virulence, in vitro and in vivo filamentation, and in vitro and in vivo hypha-associated gene expression profiles of four poorly filamenting C. albicans isolates and their corresponding deletion mutants of the repressor of filamentation NRG1. The two most virulent strains, 57055 and 78048, show robust in vivo filamentation while remaining predominately yeast phase exposed to RPMI+10% bovine calf serum at 37°C; the two low virulence strains (94015 and 78042) do not filament well under either condition. Deletion of NRG1 increases hyphae formation in the SC5314 derivative SN250 but only pseudohyphae are formed in the clinical isolates in vivo. Deletion of NRG1 modestly increased the virulence of 78042 which was accompanied by increased expression of hyphae-associated genes without an increase in filamentation. Strikingly, deletion of NRG1 in 78048 reduced filamentation, expression of candidalysin (ECE1) and virulence in vivo without dramatically altering establishment of infection. Thus, the function of NRG1 varies significantly within this set of C. albicans isolates and can actually suppress filamentation in vivo.

Significant variation of filamentation phenotypes in clinical Candida albicans strains

Introduction

Candida albicans is an opportunistic human pathogen that typically resides as part of the microbiome in the gastrointestinal and genitourinary tracts of a large portion of the human population. This fungus lacks a true sexual cycle and evolves in a largely clonal pattern. The ability to cause disease is consistent across the species as strains causing systemic infections appear across the known C. albicans intra-species clades.

Methods

In this work, strains collected from patients with systemic C. albicans infections isolated at the Nebraska Medicine clinical laboratory were typed by MLST analysis. Since the ability to form filaments has been linked to pathogenesis in C. albicans, these clinical strains, as well as a previously genotyped set of clinical strains, were tested for their ability to filament across a variety of inducing conditions.

Results

Genotyping of the clinical strains demonstrated that the strains isolated at one of the major medical centers in our region were as diverse as strains collected across the United States. We demonstrated that clinical strains exhibit a variety of filamentation patterns across differing inducing conditions. The only consistent pattern observed in the entire set of clinical strains tested was an almost universal inability to filament in standard solid inducing conditions used throughout the C. albicans field. A different solid filamentation assay that produces more robust filamentation profiles from clinical strains is proposed in this study, although not all strains expected to filament in vivo were filamentous in this assay.

Discussion

Our data supports growing evidence that broad phenotypic diversity exists between the C. albicans type strain and clinical strains, suggesting that the type strain poorly represents filamentation patterns observed in most clinical isolates. These data further highlight the need to use diverse clinical strains in pathogenesis assays.

Comparative dynamics of gene expression during in vitro and in vivo Candida albicans filamentation

Candida albicans is one of them most common causes of fungal disease in humans and is a commensal member of the human microbiome. The ability of C. albicans to cause disease is tightly correlated with its ability to undergo a morphological transition from budding yeast to a filamentous form (hyphae and pseudohyphae). This morphological transition is accompanied by the induction of a set of well characterized hyphae-associated genes and transcriptional regulators. To date, the vast majority of data regarding this process has been based on in vitro studies of filamentation using a range of inducing conditions. Recently, we developed an in vivo imaging approach that allows the direct characterization of morphological transition during mammalian infection. Here, we couple this imaging assay with in vivo expression profiling to characterize the time course of in vivo filamentation and the accompanying changes in gene expression. We also compare in vivo observations to in vitro filamentation using a medium (RPMI 1640 tissue culture medium with 10% bovine calf serum) widely used to mimic host conditions. From these data, we make the following conclusions regarding in vivo and in vitro filamentation. First, the transcriptional programs regulating filamentation are rapidly induced in vitro and in vivo. Second, the tempo of filamentation in vivo is prolonged relative to in vitro filamentation and the period of high expression of genes associated with that process is also prolonged. Third, hyphae are adapting to changing infection environments after filamentation has reached steady-state.

Candida albicans Oropharyngeal Infection Is an Exception to Iron-Based Nutritional Immunity

Candida albicans is a commensal of the human gastrointestinal tract and a common cause of human fungal disease, including mucosal infections, such as oropharyngeal candidiasis and disseminated infections of the bloodstream and deep organs. We directly compared the in vivo transcriptional profile of C. albicans during oral infection and disseminated infection of the kidney to identify niche specific features. Overall, 97 genes were differentially expressed between the 2 infection sites. Virulence-associated genes, such as hyphae-specific transcripts, were expressed similarly in the 2 sites. Genes expressed during growth in a poor carbon source (ACS1 and PCK1) were upregulated in oral tissue relative to kidney. Most strikingly, C. albicans in oral tissue shows the transcriptional hallmarks of an iron replete state while in the kidney it is in the expected iron starved state. Interestingly, C. albicans expresses genes associated with a low zinc environment in both niches. Consistent with these expression data, strains lacking transcription factors that regulate iron responsive genes (SEF1, HAP5) have no effect on virulence in a mouse model of oral candidiasis. During microbial infection, the host sequesters iron, zinc, and other metal nutrients to suppress growth of the pathogen in a process called nutritional immunity. Our results indicate that C. albicans is subject to iron and zinc nutritional immunity during disseminated infection but not to iron nutritional immunity during oral infection. IMPORTANCE Nutritional immunity is a response by which infected host tissue sequesters nutrients, such as iron, to prevent the microbe from efficiently replicating. Microbial pathogens subjected to iron nutritional immunity express specific genes to compensate for low iron availability. By comparing the gene expression profiles of the common human fungal pathogen Candida albicans in 2 infection sites, we found that C. albicans infecting the kidney has the transcriptional profile of iron starvation. By contrast, the C. albicans expression profile during oropharyngeal infection indicates the fungus is not iron starved. Two transcription factors that activate the transcriptional response to iron starvation are not required for C. albicans virulence during oral infection but are required for disseminated infection of the kidney. Thus, our results indicate that C. albicans is subject to nutritional iron immunity during disseminated infection but not during oropharyngeal infection, and highlight niche specific differences in the host-Candida albicans interaction.

Atg8 and Ire1 in combination regulate the autophagy-related endoplasmic reticulum stress response in Candida albicans

The unfolded protein response (UPR) is an important pathway to prevent endoplasmic reticulum (ER) stress in eukaryotic cells. In Saccharomyces cerevisiae, Ire1 is a key regulatory factor required for HAC1 gene splicing for further production of functional Hac1 and activation of UPR gene expression. Autophagy is another mechanism involved in the attenuation of ER stress by ER-phagy, and Atg8 is a core protein in autophagy. Both autophagy and UPR are critical for ER stress response, but whether they act individually or in combination in Candida albicans is unknown. In this study, we explored the interaction between Ire1 and the autophagy protein Atg8 for the ER stress response by constructing the atg8Δ/Δire1Δ/Δ double mutant in the pathogenic fungus C. albicans. Compared to the single mutants atg8Δ/Δ or ire1Δ/Δ, atg8Δ/Δire1Δ/Δ exhibited much higher sensitivity to various ER stress-inducing agents and more severe attenuation of UPR gene expression under ER stress. Further investigations showed that the double mutant had a defect in ER-phagy, which was associated with attenuated vacuolar fusion under ER stress. This study revealed that Ire1 and Atg8 in combination function in the activation of the UPR and ER-phagy to maintain ER homeostasis under ER stress in C. albicans.

QCR7 affects the virulence of Candida albicans and the uptake of multiple carbon sources present in different host niches

Background

Candida albicans is a commensal yeast that may cause life-threatening infections. Studies have shown that the cytochrome b-c1 complex subunit 7 gene (QCR7) of C. albicans encodes a protein that forms a component of the mitochondrial electron transport chain complex III, making it an important target for studying the virulence of this yeast. However, to the best of our knowledge, the functions of QCR7 have not yet been characterized.

Methods

A QCR7 knockout strain was constructed using SN152, and BALb/c mice were used as model animals to determine the role of QCR7 in the virulence of C. albicans. Subsequently, the effects of QCR7 on mitochondrial functions and use of carbon sources were investigated. Next, its mutant biofilm formation and hyphal growth maintenance were compared with those of the wild type. Furthermore, the transcriptome of the qcr7Δ/Δ mutant was compared with that of the WT strain to explore pathogenic mechanisms.

Results

Defective QCR7 reduced recruitment of inflammatory cells and attenuated the virulence of C. albicans infection in vivo. Furthermore, the mutant influenced the use of multiple alternative carbon sources that exist in several host niches (GlcNAc, lactic acid, and amino acid, etc.). Moreover, it led to mitochondrial dysfunction. Furthermore, the QCR7 knockout strain showed defects in biofilm formation or the maintenance of filamentous growth. The overexpression of cell-surface-associated genes (HWP1, YWP1, XOG1, and SAP6) can restore defective virulence phenotypes and the carbon-source utilization of qcr7Δ/Δ.

Conclusion

This study provides new insights into the mitochondria-based metabolism of C. albicans, accounting for its virulence and the use of variable carbon sources that promote C. albicans to colonize host niches.

Alpha-Hemolysin from Staphylococcus aureus Obstructs Yeast-Hyphae Switching and Diminishes Pathogenicity in Candida albicans

The use of antibiotics can disrupt the body's natural balance and increase the susteptibility of patients towards fungal infections. Candida albicans is a dimorphic opportunistic fungal pathogen with niches similar to those of bacteria. Our aim was to study the interaction between this pathogen and bacteria to facilitate the control of C. albicans infection. Alpha-hemolysin (Hla), a protein secreted from Staphylococcus aureus, causes cell wall damage and impedes the yeast-hyphae transition in C. albicans. Mechanistically, Hla stimulation triggered the formation of reactive oxygen species that damaged the cell wall and mitochondria of C. albicans. The cell cycle was arrested in the G0/G1 phase, CDC42 was downregulated, and Ywp1 was upregulated, disrupting yeast hyphae switching. Subsequently, hyphae development was inhibited. In mouse models, C. albicans pretreated with Hla reduced the C. albicans burden in skin and vaginal mucosal infections, suggesting that S. aureus Hla can inhibit hyphal development and reduce the pathogenicity of candidiasis in vivo.

Candida albicans oropharyngeal infection is an exception to iron-based nutritional immunity

Candida albicans is a commensal of the human gastrointestinal tract and one of the most causes of human fungal disease, including mucosal infections such as oropharyngeal candidiasis and disseminated infections of the bloodstream and deep organs. We directly compared the in vivo transcriptional profile of C. albicans during oral infection and disseminated infection of the kidney to identify niche specific features. Although the expression of a set of environmentally responsive genes were correlated in the two infection sites (Pearson R ² , 0.6), XXX genes were differentially expressed. Virulence associated genes such as hyphae-specific transcripts were expressed similarly in the two sites. Genes expressed during growth in a poor carbon source ( ACS1 and PCK1 ) were upregulated in oral tissue relative to kidney. Most strikingly, C. albicans in oral tissue shows the transcriptional hallmarks of an iron-replete state while in the kidney it is in the expected iron starved state. Interestingly, C. albicans expresses genes associated with a low zinc environment in both niches. Consistent with these expression data, deletion of two transcription factors that activate iron uptake genes ( SEF1 , HAP5 ) have no effect on virulence in a mouse model of oral candidiasis. During microbial infection, the host sequesters iron and other metal nutrients to suppress growth of the pathogen in a process called nutritional immunity. Our results indicate that C. albicans is subject to iron and zinc nutritional immunity during disseminated infection but is exempted from iron nutritional immunity during oral infection.

Comprehensive Interactome Analysis for the Sole Adenylyl Cyclase Cyr1 of Candida albicans

Cyr1, the sole adenylyl cyclase of the fungal pathogen Candida albicans, is a central component of the cAMP/protein kinase A signaling pathway that controls the yeast-to-hypha transition. Cyr1 is a multivalent sensor and integrator of various external and internal signals. To better understand how these signals are relayed to Cyr1 to regulate its activity, we sought to establish the interactome of Cyr1 by using stable isotope labeling by amino acids in cell culture (SILAC)-based quantitative proteomics to identify the proteins that coimmunoprecipitated with Cyr1. The method identified 36 proteins as candidates for authentic Cyr1-interacting partners, together with two known Cyr1-binding proteins, Cap1 and Act1. Fourteen identified proteins belonged to three functional groups, including actin regulation, cell wall components, and mitochondrial activities, that are known to play important roles in cell morphogenesis. To validate the proteomics data, we used biochemical and genetic methods to characterize two cell wall-related proteins, Mp65 and Sln1. First, coimmunoprecipitation confirmed their physical association with Cyr1. Second, deleting either MP65 or SLN1 resulted in severe defects in filamentation on serum plates. This study establishes the first Cyr1 interactome and uncovers a potential role for cell wall proteins in directly regulating Cyr1 activity to determine growth forms in C. albicans. IMPORTANCE A critical virulence trait of the human fungal pathogen Candida albicans is its ability to undergo the yeast-to-hypha transition in response to diverse environmental and cellular stimuli. Previous studies suggested that the sole adenylyl cyclase of C. albicans, Cyr1, is a multivalent signal sensor and integrator synthesizing cAMP to activate the downstream hypha-promoting events through the cAMP/protein kinase A pathway. To fully understand how Cyr1 senses and processes multiple stimuli to generate appropriate signal outputs, it was necessary to identify and characterize Cyr1-interacting partners. This study employed SILAC-based quantitative proteomic approaches and identified 36 Cyr1-associated proteins, many having functions associated with hyphal morphogenesis. Coimmunoprecipitation verified two cell surface proteins, Mp65 and Sln1. Furthermore, genetic and phenotypic analyses demonstrated the cAMP-dependent roles of these two proteins in determining hyphal growth. Our study establishes the first Cyr1 interactome and uncovers new Cyr1 regulators that mediate cell surface signals to influence the growth mode of C. albicans.

Hydroquinones Including Tetrachlorohydroquinone Inhibit Candida albicans Biofilm Formation by Repressing Hyphae-Related Genes

Candida albicans is an opportunistic pathogenic fungus responsible for candidiasis. The pathogen readily forms antifungal agent-resistant biofilms on implanted medical devices or human tissue. Morphologic transition from yeast to filamentous cells and subsequent biofilm formation is a key virulence factor and a prerequisite for biofilm development by C. albicans. We investigated the antibiofilm and antifungal activities of 18 hydroquinones against fluconazole-resistant C. albicans. Tetrachlorohydroquinone (TCHQ) at subinhibitory concentrations (2 to 10 μg/mL) significantly inhibited C. albicans biofilm formation with an MIC of 50 μg/mL, whereas the backbone hydroquinone did not (MIC > 400 μg/mL), and it markedly inhibited cell aggregation and hyphal formation. Transcriptomic analyses showed that TCHQ downregulated the expressions of several hyphae-forming and biofilm-related genes (ALS3, ECE1, HWP1, RBT5, and UME6) but upregulated hyphae- and biofilm-inhibitory genes (IFD6 and YWP1). Furthermore, it prevented C. albicans biofilm development on porcine skin and at concentrations of 20 to 50 μg/mL was nontoxic to the nematode Caenorhabditis elegans and did not adversely affect Brassica rapa seed germination and growth. This study indicates that hydroquinones, particularly TCHQ, diminish the virulence, biofilm formation, and animal tissue adhesion of C. albicans, which suggests hydroquinones should be considered potential candidate antifungal agents against drug-resistant C. albicans strains. IMPORTANCE Persistence in chronic infections by Candida albicans is due to its ability of biofilm formation that endures conventional antifungals and host immune systems. Hence, the inhibition of biofilm formation and virulence characteristics is another mean of addressing infections. This study is a distinctive one since 18 hydroquinone analogues were screened and TCHQ efficiently inhibited the biofilm formation by C. albicans with significantly changed expressional profile of hyphae-forming and biofilm-related genes. The antibiofilm efficacy was confirmed using a porcine skin model and chemical toxicity was investigated using plant seed germination and nematode models. Our findings reveal that TCHQ can efficiently control the C. albicans biofilms and virulence characteristics.

Mucin O-glycans are natural inhibitors of Candida albicans pathogenicity

Mucins are large gel-forming polymers inside the mucus barrier that inhibit the yeast-to-hyphal transition of Candida albicans, a key virulence trait of this important human fungal pathogen. However, the molecular motifs in mucins that inhibit filamentation remain unclear despite their potential for therapeutic interventions. Here, we determined that mucins display an abundance of virulence-attenuating molecules in the form of mucin O-glycans. We isolated and cataloged >100 mucin O-glycans from three major mucosal surfaces and established that they suppress filamentation and related phenotypes relevant to infection, including surface adhesion, biofilm formation and cross-kingdom competition between C. albicans and the bacterium Pseudomonas aeruginosa. Using synthetic O-glycans, we identified three structures (core 1, core 1 + fucose and core 2 + galactose) that are sufficient to inhibit filamentation with potency comparable to the complex O-glycan pool. Overall, this work identifies mucin O-glycans as host molecules with untapped therapeutic potential to manage fungal pathogens.

TMT-based quantitative proteomic analysis of the effects of novel antimicrobial peptide AMP-17 against Candida albicans

Candida albicans is the most common human fungal pathogen in immunocompromised individuals. With the emergence of clinical fungal resistance, there is an urgent need to develop novel antifungal agents. AMP-17, a novel antimicrobial peptide from Musca domestica, has an antifungal effect against C. albicans, but its mechanism of antifungal action remains unclear. In the current study, we performed a proteomics analysis in C. albicans using TMT technique under the treatment of AMP-17. A total of 3931 proteins were identified, of which 3600 included quantitative information. With a 1.5-fold change threshold and a t-test p-value < 0.05 as standard, 423 differentially expressed proteins (DEPs) were up-regulated and 180 DEPs were down-regulated in the AMP-17/control. Notably, GO enrichment revealed that DEPs associated with the cell wall, RNA and oxidative stress were significantly up-regulated, while DEPs involved in ergosterol metabolism and membrane were significantly down-regulated in the AMP-17/control. KEGG pathway enrichment revealed that DEPs involved seven significant metabolic pathways, mainly involved oxidative phosphorylation, RNA degradation, propanoate metabolism and fatty acid metabolism. These results show that AMP-17 induces a complex organism response in C. albicans, indicating that AMP-17 may inhibit growth by affecting multiple targets in C. albicans cells. SIGNIFICANCE: Antimicrobial peptides (AMPs) are an important part of the innate immune system of organisms and having broad range of activity against fungi, bacteria and viruses. These AMPs are considered as probable candidate for forthcoming drugs, due to their broad range of activity, lesser toxicity and decreased resistance development by target cells. AMP-17, a novel antimicrobial peptide from M. domestica, has significant antifungal activity against C. albicans. It has been confirmed that AMP-17 can play an antifungal effect by destroying the cell wall and cell membrane of C. albicans in previous studies, but its mechanism of action at the protein level is currently unclear. In the current study, using the TMT-based quantitative proteomics method, 603 differentially expressed proteins were identified in the cells of C. albicans treated with AMP-17 for 12 h, and these DEPs were closely related to cell wall, cell membrane, RNA degradation and oxidative stress. The results provide new insights into the potential mechanism of action of AMP- 17 against C. albicans. Meanwhile, it provides certain technical support and theoretical basis for the research and development of novel peptide drugs.

Mycobiota-induced IgA antibodies regulate fungal commensalism in the gut and are dysregulated in Crohn's disease

Secretory immunoglobulin A (sIgA) plays an important role in gut barrier protection by shaping the resident microbiota community, restricting the growth of bacterial pathogens and enhancing host protective immunity via immunological exclusion. Here, we found that a portion of the microbiota-driven sIgA response is induced by and directed towards intestinal fungi. Analysis of the human gut mycobiota bound by sIgA revealed a preference for hyphae, a fungal morphotype associated with virulence. Candida albicans was a potent inducer of IgA class-switch recombination among plasma cells, via an interaction dependent on intestinal phagocytes and hyphal programming. Characterization of sIgA affinity and polyreactivity showed that hyphae-associated virulence factors were bound by these antibodies and that sIgA influenced C. albicans morphotypes in the murine gut. Furthermore, an increase in granular hyphal morphologies in patients with Crohn's disease compared with healthy controls correlated with a decrease in antifungal sIgA antibody titre with affinity to two hyphae-associated virulence factors. Thus, in addition to its importance in gut bacterial regulation, sIgA targets the uniquely fungal phenomenon of hyphal formation. Our findings indicate that antifungal sIgA produced in the gut can play a role in regulating intestinal fungal commensalism by coating fungal morphotypes linked to virulence, thereby providing a protective mechanism that might be dysregulated in patients with Crohn's disease.

The Anti-Biofilm Efficacy of Caffeic Acid Phenethyl Ester (CAPE) In Vitro and a Murine Model of Oral Candidiasis

Candida albicans is the main fungal species associated with the development of oral candidiasis. Currently, therapeutic options for these infections are limited by the adverse effects of antifungal drugs and by the emergence of drug resistant strains. Thus, the development of new antifungal agents is needed for the prevention and treatment of oral Candida infections. Caffeic acid phenethyl ester (CAPE) is a natural compound from propolis polyphenolic groups that exhibits many pharmacological properties. In this study, we investigated whether CAPE can have antifungal and immunomodulatory effects on oral candidiasis. Preliminary tests to assess the antifungal activity of CAPE were performed using the Minimum Inhibitory Concentration (MIC) assay that demonstrated inhibition in a range from 16 to 32 μg/mL, confirming its antifungal activity on several C. albicans strains isolated from the oral cavity. Subsequently, we analyzed Candida spp biofilms formed in vitro, in which CAPE treatment at 5 x MIC caused a reduction of 68.5% in the total biomass and ~2.60 Log in the viable cell count (CFU/mL) in relation to the untreated biofilm (p<0.0001). Next, RNA was extracted from untreated and CAPE-treated biofilms and analyzed by real-time qPCR. A series of genes analyzed (ALS1, ECE1, EPA1, HWP1, YWP1, BCR1, BGR1, CPH1, EFG1, NDT80, ROB1, TEC1, UME6, SAP2, SAP5, PBL2, and LIP9) were downregulated by CAPE compared to the untreated control group (p<0.0001). In in vivo studies using Galleria mellonella, the treatment with CAPE prolonged survival of larvae infected by C. albicans by 44.5% (p < 0.05) and accompanied by a 2.07-fold increase in the number of hemocytes. Flow cytometry revealed the most prominent increases were in types P2 and P3 hemocytes, granular cells, which phagocytize pathogens. In addition, CAPE treatment decreased the fungal load in the hemolymph and stimulated the expression of antifungal peptide genes such as galiomicin and gallerimycin. The antifungal and immunomodulatory activities observed in G. mellonella were extended to a murine model of oral candidiasis, in which CAPE decreased the levels of C. albicans colonization (~2 log CFU/mL) in relation to the untreated control group. In addition, CAPE treatment significantly reduced pseudomembranous lesions, invasion of hyphae on epithelium surfaces, tissue damage and inflammatory infiltrate (p < 0.05). CAPE was also able to increase the expression of β-defensin 3 compared to the infected and untreated group by 3.91-fold (p < 0.0001). Taken together, these results show that CAPE has both antifungal and immunomodulatory effects, making it a promising natural antifungal agent for the treatment and prevention of candidiasis and shows impact to oral candidiasis.

Cell Wall Proteome Profiling of a Candida albicans Fluconazole-Resistant Strain from a Lebanese Hospital Patient Using Tandem Mass Spectrometry-A Pilot Study

Candida albicans is an opportunistic pathogenic fungus responsible for high mortality rates in immunocompromised individuals. Azole drugs such as fluconazole are the first line of therapy in fungal infection treatment. However, resistance to azole treatment is on the rise. Here, we employ a tandem mass spectrometry approach coupled with a bioinformatics approach to identify cell wall proteins present in a fluconazole-resistant hospital isolate upon drug exposure. The isolate was previously shown to have an increase in cell membrane ergosterol and cell wall chitin, alongside an increase in adhesion, but slightly attenuated in virulence. We identified 50 cell wall proteins involved in ergosterol biosynthesis such as Erg11, and Erg6, efflux pumps such as Mdr1 and Cdr1, adhesion proteins such as Als1, and Pga60, chitin deposition such as Cht4, and Crh11, and virulence related genes including Sap5 and Lip9. Candidial proteins identified in this study go a long way in explaining the observed phenotypes. Our pilot study opens the way for a future large-scale analysis to identify novel proteins involved in drug-resistance mechanisms.

High Biofilm Formation of Non-Smooth Candida parapsilosis Correlates with Increased Incorporation of GPI-Modified Wall Adhesins

Candida parapsilosis is among the most frequent causes of candidiasis. Clinical isolates of this species show large variations in colony morphotype, ranging from round and smooth to a variety of non-smooth irregular colony shapes. A non-smooth appearance is related to increased formation of pseudohyphae, higher capacity to form biofilms on abiotic surfaces, and invading agar. Here, we present a comprehensive study of the cell wall proteome of C. parapsilosis reference strain CDC317 and seven clinical isolates under planktonic and sessile conditions. This analysis resulted in the identification of 40 wall proteins, most of them homologs of known Candida albicans cell wall proteins, such as Gas, Crh, Bgl2, Cht2, Ecm33, Sap, Sod, Plb, Pir, Pga30, Pga59, and adhesin family members. Comparative analysis of exponentially growing and stationary phase planktonic cultures of CDC317 at 30 °C and 37 °C revealed only minor variations. However, comparison of smooth isolates to non-smooth isolates with high biofilm formation capacity showed an increase in abundance and diversity of putative wall adhesins from Als, Iff/Hyr, and Hwp families in the latter. This difference depended more strongly on strain phenotype than on the growth conditions, as it was observed in planktonic as well as biofilm cells. Thus, in the set of isolates analyzed, the high biofilm formation capacity of non-smooth C. parapsilosis isolates with elongated cellular phenotypes correlates with the increased surface expression of putative wall adhesins in accordance with their proposed cellular function.

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

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

Targeting Candida albicans in dual-species biofilms with antifungal treatment reduces Staphylococcus aureus and MRSA in vitro

Polymicrobial biofilms consisting of fungi and bacteria are frequently formed on endotracheal tubes and may contribute to development of ventilator associated pneumonia (VAP) in critically ill patients. This study aimed to determine the role of early Candida albicans biofilms in supporting dual-species (dual-kingdom) biofilm formation with respiratory pathogens in vitro, and investigated the effect of targeted antifungal treatment on bacterial cells within the biofilms. Dual-species biofilm formation between C. albicans and three respiratory pathogens commonly associated with VAP (Pseudomonas aeruginosa, Escherichia coli and Staphylococcus aureus) was studied using quantitative PCR. It was shown that early C. albicans biofilms enhanced the numbers of E. coli and S. aureus (including methicillin resistant S. aureus; MRSA) but not P. aeruginosa within dual-species biofilms. Transwell assays demonstrated that contact with C. albicans was required for the increased bacterial cell numbers observed. Total Internal Reflection Fluorescence microscopy showed that both wild type and hyphal-deficient C. albicans provided a scaffold for initial bacterial adhesion in dual species biofilms. qPCR results suggested that further maturation of the dual-species biofilm significantly increased bacterial cell numbers, except in the case of E.coli with hyphal-deficient C. albicans (Ca_gcn5Δ/Δ). A targeted preventative approach with liposomal amphotericin (AmBisome®) resulted in significantly decreased numbers of S. aureus in dual-species biofilms, as determined by propidium monoazide-modified qPCR. Similar results were observed when dual-species biofilms consisting of clinical isolates of C. albicans and MRSA were treated with liposomal amphotericin. However, reductions in E. coli numbers were not observed following liposomal amphotericin treatment. We conclude that early C. albicans biofilms have a key supporting role in dual-species biofilms by enhancing bacterial cell numbers during biofilm maturation. In the setting of increasing antibiotic resistance, an important and unexpected consequence of antifungal treatment of dual-species biofilms, is the additional benefit of decreased growth of multi-drug resistant bacteria such as MRSA, which could represent a novel future preventive strategy.

Cell Surface Expression of Nrg1 Protein in Candida auris

Candida auris is an emerging antifungal resistant human fungal pathogen increasingly reported in healthcare facilities. It persists in hospital environments, and on skin surfaces, and can form biofilms readily. Here, we investigated the cell surface proteins from C. auris biofilms grown in a synthetic sweat medium mimicking human skin conditions. Cell surface proteins from both biofilm and planktonic control cells were extracted with a buffer containing β-mercaptoethanol and resolved by 2-D gel electrophoresis. Some of the differentially expressed proteins were excised and identified by mass spectrometry. C. albicans orthologs Spe3p, Tdh3p, Sod2p, Ywp1p, and Mdh1p were overexpressed in biofilm cells when compared to the planktonic cells of C. auris. Interestingly, several proteins with zinc ion binding activity were detected. Nrg1p is a zinc-binding transcription factor that negatively regulates hyphal growth in C. albicans. C. auris does not produce true hypha under standard in vitro growth conditions, and the role of Nrg1p in C. auris is currently unknown. Western blot analyses of cell surface and cytosolic proteins of C. auris against anti-CalNrg1 antibody revealed the Nrg1p in both locations. Cell surface localization of Nrg1p in C. auris, an unexpected finding, was further confirmed by immunofluorescence microscopy. Nrg1p expression is uniform across all four clades of C. auris and is dependent on growth conditions. Taken together, the data indicate that C. auris produces several unique proteins during its biofilm growth, which may assist in the skin-colonizing lifestyle of the fungus during its pathogenesis.

Candida albicans biofilms and polymicrobial interactions

Candida albicans is a common fungus of the human microbiota. While generally a harmless commensal in healthy individuals, several factors can lead to its overgrowth and cause a range of complications within the host, from localized superficial infections to systemic life-threatening disseminated candidiasis. A major virulence factor of C. albicans is its ability to form biofilms, a closely packed community of cells that can grow on both abiotic and biotic substrates, including implanted medical devices and mucosal surfaces. These biofilms are extremely hard to eradicate, are resistant to conventional antifungal treatment and are associated with high morbidity and mortality rates, making biofilm-associated infections a major clinical challenge. Here, we review the current knowledge of the processes involved in C. albicans biofilm formation and development, including the central processes of adhesion, extracellular matrix production and the transcriptional network that regulates biofilm development. We also consider the advantages of the biofilm lifestyle and explore polymicrobial interactions within multispecies biofilms that are formed by C. albicans and selected microbial species.

Onychomycosis: An Updated Review

Background: Onychomycosis is a common fungal infection of the nail.

Objective: The study aimed to provide an update on the evaluation, diagnosis, and treatment of onychomycosis.

Methods: A PubMed search was completed in Clinical Queries using the key term “onychomycosis”. The search was conducted in May 2019. The search strategy included meta-analyses, randomized controlled trials, clinical trials, observational studies, and reviews published within the past 20 years. The search was restricted to English literature. Patents were searched using the key term “onychomycosis” in www.freepatentsonline.com.

Results: Onychomycosis is a fungal infection of the nail unit. Approximately 90% of toenail and 75% of fingernail onychomycosis are caused by dermatophytes, notably Trichophyton mentagrophytes and Trichophyton rubrum. Clinical manifestations include discoloration of the nail, subungual hyperkeratosis, onycholysis, and onychauxis. The diagnosis can be confirmed by direct microscopic examination with a potassium hydroxide wet-mount preparation, histopathologic examination of the trimmed affected nail plate with a periodic-acid-Schiff stain, fungal culture, or polymerase chain reaction assays. Laboratory confirmation of onychomycosis before beginning a treatment regimen should be considered. Currently, oral terbinafine is the treatment of choice, followed by oral itraconazole. In general, topical monotherapy can be considered for mild to moderate onychomycosis and is a therapeutic option when oral antifungal agents are contraindicated or cannot be tolerated. Recent patents related to the management of onychomycosis are also discussed.

Conclusion: Oral antifungal therapies are effective, but significant adverse effects limit their use.Although topical antifungal therapies have minimal adverse events, they are less effective than oral antifungal therapies, due to poor nail penetration. Therefore, there is a need for exploring more effective and/or alternative treatment modalities for the treatment of onychomycosis which are safer and more effective.

Sodium New Houttuyfonate Inhibits Candida albicans Biofilm Formation by Inhibiting the Ras1-cAMP-Efg1 Pathway Revealed by RNA-seq

Here, we aim to investigate the antifungal effect and mechanism of action of sodium new houttuyfonate (SNH) against Candida albicans. Microdilution analysis results showed that SNH possesses potent inhibitory activity against C. albicans SC5314, with a MIC₈₀ of 256 μg/mL. Furthermore, we found that SNH can effectively inhibit the initial adhesion of C. albicans. Inverted microscopy, crystal violet staining, scanning electron microscopy and confocal laser scanning microscopy results showed that morphological changes during the transition from yeast to hypha and the biofilm formation of C. albicans are repressed by SNH treatment. We also found that SNH can effectively inhibit the biofilm formation of clinical C. albicans strains (Z103, Z3044, Z1402, and Z1407) and SNH in combination with fluconazole, berberine chloride, caspofungin and itraconazole antifungal agents can synergistically inhibit the biofilm formation of C. albicans. Eukaryotic transcriptome sequencing and qRT-PCR results showed that SNH treatment resulted in significantly down-regulated expression in several biofilm formation related genes in the Ras1-cAMP-Efg1 pathway (ALS1, ALA1, ALS3, EAP1, RAS1, EFG1, HWP1, and TEC1) and significantly up-regulated expression in yeast form-associated genes (YWP1 and RHD1). We also found that SNH can effectively reduce the production of key messenger cAMP in the Ras1-cAMP-Efg1 pathway. Furthermore, using Galleria mellonella as an in vivo model we found that SNH can effectively treat C. albicans infection in vivo. Our presented results suggest that SNH exhibits potential antibiofilm effects related to inhibiting the Ras1-cAMP-Efg1 pathway in the biofilm formation of C. albicans.

Characteristics of Extracellular Vesicles Released by the Pathogenic Yeast-Like Fungi Candida glabrata, Candida parapsilosis and Candida tropicalis

Candida spp. yeast-like fungi are opportunistic pathogens in humans and have been recently found to release extracellular vesicles (EVs) that are involved in many vital biological processes in fungal cells. These include communication between microorganisms and host–pathogen interactions during infection. The production of EVs and their content have been significantly characterized in the most common candidal species Candida albicans, including the identification of numerous virulence factors and cytoplasmic proteins in the EV cargo. We have here conducted the isolation and proteomic characterization of EVs produced by the clinically important non-albicans Candida species C. glabrata, C. tropicalis and C. parapsilosis. With the use of ultracentrifugation of the cell-free culture supernatant, the candidal EVs were collected and found to be a heterogeneous population of particles for each species with sizes ranging from 60–280 nm. The proteinaceous contents of these vesicles were analyzed using LC-MS/MS, with particular attention paid to surface-expressed proteins that would come into immediate and direct contact with host cells. We thereby identified 42 extracellular and surface-connected proteins from C. glabrata, 33 from C. parapsilosis, and 34 from C. tropicalis, including membrane-associated transporters, glycoproteins and enzymes involved in the organization of the fungal cell wall, as well as several cytoplasmic proteins, including alcohol dehydrogenase, enolase, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase and pyruvate kinase, for which the vesicular transport is a possible mechanism underlying their non-classical secretion.

11g, a Potent Antifungal Candidate, Enhances Candida albicans Immunogenicity by Unmasking β-Glucan in Fungal Cell Wall

In the course of optimizing GPI biosynthesis inhibitors, we designed and synthetized a 2-aminonicotinamide derivative named 11g. After evaluating the antifungal activity of compound 11g in vitro, we investigated the influences of 11g on fungi immunogenicity. In addition, we also took advantage of murine systemic candidiasis model to investigate the protective effects of 11g in vivo. Results show that 11g exhibited potent antifungal activity both in vitro and in vivo. Further study shows that 11g caused the unmasking of fungi β-glucan layer, leading to stronger immune responses in macrophages through Dectin-1. These results suggest that 11g is a very promising antifungal candidate, which assists in eliciting stronger immune responses to help host immune system disposing pathogens. The discovery of 11g might expand the toolbox of fungal infection treatment.

Caspofungin Induced Cell Wall Changes of Candida Species Influences Macrophage Interactions

Candida species are known to differ in their ability to cause infection and have been shown to display varied susceptibilities to antifungal drugs. Treatment with the echinocandin, caspofungin, leads to compensatory alterations in the fungal cell wall. This study was performed to compare the structure and composition of the cell walls of different Candida species alone and in response to caspofungin treatment, and to evaluate how changes at the fungal cell surface affects interactions with macrophages. We demonstrated that the length of the outer fibrillar layer varied between Candida species and that, in most cases, reduced fibril length correlated with increased exposure of β-1,3-glucan on the cell surface. Candida glabrata and Candida guilliermondii, which had naturally more β-1,3-glucan exposed on the cell surface, were phagocytosed significantly more efficiently by J774 macrophages. Treatment with caspofungin resulted in increased exposure of chitin and β-1,3-glucan on the surface of the majority of Candida species isolates that were tested, with the exception of C. glabrata and Candida parapsilosis isolates. This increase in exposure of the inner cell wall polysaccharides, in most cases, correlated with reduced uptake by macrophages and in turn, a decrease in production of TNFα. Here we show that differences in the exposure of cell wall carbohydrates and variations in the repertoire of covalently attached surface proteins of different Candida species contributes to their recognition by immune cells.

In vitro and in vivo evaluation of the antifungal activity of fluoxetine combined with antifungals against Candida albicans biofilms and oral candidiasis

Candida albicans biofilms are responsible for oral candidiasis. Fluoxetine is a widely used antidepressant, with certain anti-Candida activities. The antifungal activity of fluoxetine combined with various antifungals against C. albicans biofilms and oral candidiasis was evaluated in this study. The morphological change in the inhibition of fluoxetine on C. albicans biofilms was observed using SEM. The interactions between fluoxetine and antifungals against C. albicans biofilms were evaluated using microdilution checkerboard methods, FICI and the ΔE model. The synergistic combination was tested in vivo on the mice model of oral candidiasis. SEM imaging showed fluoxetine inhibited hyphal growth and biofilm formation. Fluoxetine combined with caspofungin exhibited synergistic effects against C. albicans biofilms. Antagonistic effects occurred when fluoxetine was combined with amphotericin B or terbinafine. Further, the fluoxetine combined with caspofungin significantly reduced the lesion score and CFU of C. albicans on the murine tongue (p < 0.05), and relieved oral candidiasis of the infected mice.

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

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

Candida albicans biofilm development is governed by cooperative attachment and adhesion maintenance proteins

The opportunistic fungal pathogen Candida albicans is capable of adhering to the oral mucosa despite forces created by salivary flow. Although many fungal adhesion proteins have been identified, less is known about the temporal development of cell adhesion and biofilm growth in a flow environment. In this study, we use a flow system with real-time imaging of C. albicans cells as they adhere and grow. Rates of cell attachment and dispersion of C. albicans knockout strains of putative adhesins, transcription factors, and deletions with a hyperfilamentous phenotype were quantified during 18 h of biofilm development. Cell adhesion under flow is a multi-phase process initiated with cell rolling, then an initial firm attachment to the substrate occurs. After attachment, cells enter a growth phase where cells either commit to adherence or disperse. C. albicans Δeap1, Δhwp2, Δhyr1, and Δihd1 cells had significantly reduced initial attachment and subsequent adhesion, while Δals1/Δals3 had no change in initial attachment but reduced adhesion maintenance. WT cells had increased adhesion during the late growth phase when hyphae were more highly expressed. Hyperfilamentous strains had 10-fold higher total biofilm growth, a result of significantly reduced detachment rates, showing that hyphal morphogenesis is important for adhesion maintenance in the developing biofilm. The rate of C. albicans biomass dispersion was most important for determining the density of the mature biomass. Adhesion maintenance was mediated in part by Ywp1, a protein previously thought to regulate dispersion, thus it functions as an adhesion maintenance protein in C. albicans.

Serial Systemic Candida albicans Infection Highlighted by Proteomics

Candida albicans is the major pathogen isolated from nosocomial bloodstream infections, leading to higher mortality rates. Thus, due to its clinical relevance, studies aiming to understand host–pathogen interactions in C. albicans infection are necessary. Therefore, we performed proteomic analysis using a murine model of serial systemic infection by C. albicans to evaluate possible changes in the protein profile of the pathogen over time. Firstly, we observed a reduction in the median survival time of infected animals with increasing passage number, suggesting a higher pathogenicity acquired during repeated infections. By LC-MS/MS, it was possible to obtain protein profiles from the wild-type strain (WT) and compare them to proteins extracted from Candida cells recovered from infected tissues during passages one, three, and four (P1, P3, and P4). We obtained 56, 29, and 97 proteins in P1, P3, P4, respectively, all varying in abundance. Regarding biological processes, the majority of proteins were related to carbohydrate metabolism, stress responses and amino acid metabolism. The proteins were also categorized according to their potential role in virulence traits, such as biofilm production, yeast-to-hyphae transition, phenotypic switching, proteins related to stress responses, and uncharacterized proteins. Therefore, serial infection in combination with proteomic approach enabled us to deepen the existing knowledge about host-pathogen interactions.

Candida albicans Interactions with Mucosal Surfaces during Health and Disease

Flexible adaptation to the host environment is a critical trait that underpins the success of numerous microbes. The polymorphic fungus Candida albicans has evolved to persist in the numerous challenging niches of the human body. The interaction of C. albicans with a mucosal surface is an essential prerequisite for fungal colonisation and epitomises the complex interface between microbe and host. C. albicans exhibits numerous adaptations to a healthy host that permit commensal colonisation of mucosal surfaces without provoking an overt immune response that may lead to clearance. Conversely, fungal adaptation to impaired immune fitness at mucosal surfaces enables pathogenic infiltration into underlying tissues, often with devastating consequences. This review will summarise our current understanding of the complex interactions that occur between C. albicans and the mucosal surfaces of the human body.

Genetic Analysis of Candida auris Implicates Hsp90 in Morphogenesis and Azole Tolerance and Cdr1 in Azole Resistance

Candida auris is an emerging fungal pathogen and a serious global health threat as the majority of clinical isolates display elevated resistance to currently available antifungal drugs. Despite the increased prevalence of C. auris infections, the mechanisms governing drug resistance remain largely elusive. In diverse fungi, the evolution of drug resistance is enabled by the essential molecular chaperone Hsp90, which stabilizes key regulators of cellular responses to drug-induced stress. Hsp90 also orchestrates temperature-dependent morphogenesis in Candida albicans, a key virulence trait. However, the role of Hsp90 in the pathobiology of C. auris remains unknown. In order to study regulatory functions of Hsp90 in C. auris, we placed HSP90 under the control of a doxycycline-repressible promoter to enable transcriptional repression. We found that Hsp90 is essential for growth in C. auris and that it enables tolerance of clinical isolates with respect to the azoles, which inhibit biosynthesis of the membrane sterol ergosterol. High-level azole resistance was independent of Hsp90 but dependent on the ABC transporter CDR1, deletion of which resulted in abrogated resistance. Strikingly, we discovered that C. auris undergoes a morphogenetic transition from yeast to filamentous growth in response to HSP90 depletion or cell cycle arrest but not in response to other cues that induce C. albicans filamentation. Finally, we observed that this developmental transition is associated with global transcriptional changes, including the induction of cell wall-related genes. Overall, this report provides a novel insight into mechanisms of drug tolerance and resistance in C. auris and describes a developmental transition in response to perturbation of a core regulator of protein homeostasis.IMPORTANCE Fungal pathogens pose a serious threat to public health. Candida auris is an emerging fungal pathogen that is often resistant to commonly used antifungal drugs. However, the mechanisms governing drug resistance and virulence in this organism remain largely unexplored. In this study, we adapted a conditional expression system to modulate the transcription of an essential gene, HSP90, which regulates antifungal resistance and virulence in diverse fungal pathogens. We showed that Hsp90 is essential for growth in C. auris and is important for tolerance of the clinically important azole antifungals, which block ergosterol biosynthesis. Further, we established that the Cdr1 efflux transporter regulates azole resistance. Finally, we discovered that C. auris transitions from yeast to filamentous growth in response to Hsp90 inhibition, accompanied by global transcriptional remodeling. Overall, this work provides a novel insight into mechanisms regulating azole resistance in C. auris and uncovers a distinct developmental program regulated by Hsp90.

Propeptide genesis by Kex2-dependent cleavage of yeast wall protein 1 (Ywp1) of Candida albicans

Candida albicans is a prevalent fungal resident and opportunistic pathogen of humans, exhibiting a variety of ovoid and filamentous morphologies. Anchored within the cell wall of the ovoid yeast form of C. albicans is an abundant glycoprotein termed yeast wall protein 1 (Ywp1). Ywp1 has an antiadhesive effect that may facilitate yeast cell dispersal; it also contributes to the masking of the glucan matrix of the yeast cell wall, potentially providing shielding from recognition by the human immune system. Mature Ywp1 consists of an O-glycosylated core of 378 amino acids associated with an N-glycosylated propeptide that originates from an N-terminal segment of Ywp1. A tribasic (-RRR-) sequence in the immature Ywp1 polypeptide is separated by 8 amino acids from a dibasic (-KR-) sequence that is a canonical site for cleavage by the intracellular endopeptidase Kex2, and cleavage occurs at both of these sites to generate an 11 kilodalton (kDa) propeptide that remains strongly associated with the mature core of Ywp1. Previous studies demonstrated an absence of the 11 kDa propeptide in strains lacking Kex2, but the presence of lesser amounts of a 12 kDa propeptide ostensibly (and paradoxically) arising from cleavage at the dibasic site. Subsequent studies of wild type strains, however, suggested that post-secretion cleavages were carried out in vitro by acid proteases in unbuffered cultures to generate the 12 kDa propeptide. Here, intact and Gfp-tagged Ywp1 are utilized to show that neither of the two multibasic sites is normally cleaved in the absence of Kex2, but that uncleaved Ywp1 is still N-glycosylated and subsequently anchored to the cell wall. This furthers our understanding of the multistep cleavage of this highly conserved sequence, as well as the possible contributions of the cleaved propeptide to the maturation and functioning of Ywp1.

Candida albicans Dispersed Cells Are Developmentally Distinct from Biofilm and Planktonic Cells

Candida albicans surface-attached biofilms such as those formed on intravenous catheters with direct access to the bloodstream often serve as a nidus for continuous release of cells capable of initiating new infectious foci. We previously reported that cells dispersed from a biofilm are yeast cells that originate from the top-most hyphal layers of the biofilm. Compared to their planktonic counterparts, these biofilm dispersal yeast cells displayed enhanced virulence-associated characteristics and drug resistance. However, little is known about their molecular properties. To address that issue, in this study we aimed to define the molecular characteristics of these biofilm dispersal cells. We found that the inducer of dispersal, PES1, genetically interacts with the repressor of filamentation, NRG1, in a manner consistent with the definition of dispersed cells as yeast cells. Further, using a flow biofilm model, we performed comprehensive comparative RNA sequencing on freshly dispersed cells in order to identify unique transcriptomic characteristics. Gene expression analysis demonstrated that dispersed cells largely inherit a biofilm-like mRNA profile. Strikingly, however, dispersed cells seemed transcriptionally reprogrammed to acquire nutrients such as zinc and amino acids and to metabolize alternative carbon sources, while their biofilm-associated parent cells did not induce the same high-affinity transporters or express gluconeogenetic genes, despite exposure to the same nutritional signals. Collectively, the findings from this study characterize cell dispersal as an intrinsic step of biofilm development which generates propagules more adept at colonizing distant host sites. This developmental step anticipates the need for virulence-associated gene expression before the cells experience the associated external signals.

Candida albicans surface-attached biofilms serve as a reservoir of cells to perpetuate and expand an infection; cells released from biofilms on catheters have direct access to the bloodstream. Biofilm dispersal yeast cells exhibit enhanced adhesion, invasion, and biofilm formation compared to their planktonic counterparts. Here, we show using transcriptome sequencing (RNA-seq) that dispersed yeast cells are developmentally distinct from the cells in their parent biofilms as well as from planktonic yeast cells. Dispersal cells possess an anticipatory expression pattern that primes them to infect new sites in the host, to survive in nutrient-starved niches, and to invade new sites. These studies identified dispersal cells as a unique proliferative cell type of the biofilm and showed that they could serve as targets for antibiofilm drug development in the future.

Antifungal activity of clinical Lactobacillus strains against Candida albicans biofilms: identification of potential probiotic candidates to prevent oral candidiasis

This study isolated Lactobacillus strains from caries-free subjects and evaluated the inhibitory effects directly on three strains of C. albicans, two clinical strains and one reference strain. Thirty Lactobacillus strains were isolated and evaluated for antimicrobial activity against in vitro C. albicans biofilms. L. paracasei 28.4, L. rhamnosus 5.2 and L. fermentum 20.4 isolates exhibited the most significant inhibitory activity against C. albicans. Co-incubation between these microorganisms resulted in deterrence of biofilm development and retardation of hyphal formation. The hindrance of biofilm development was characterized by the downregulated expression of C. albicans biofilm-specific genes (ALS3, HWP1, EFG1 and CPH1). L. paracasei 28.4, L. rhamnosus 5.2 and L. fermentum 20.4 demonstrated the ability to exert antifungal activity through the inhibition of C. albicans biofilms.

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.

Candida albicans-derived mannoproteins activate NF-κB in reporter cells expressing TLR4, MD2 and CD14

The ability of soluble C. albicans 20A (serotype A) mannoprotein (CMP) to serve as a ligand for toll-like receptor 4 (TLR4) and its co-receptors was examined using commercially available and stably-transfected HEK293 cells that express human TLR4, MD2 and CD14, but not MR. These TLR4 reporter cells also express an NF-κB-dependent, secreted embryonic alkaline phosphatase (SEAP) reporter gene. TLR4-reporter cells exhibited a dose-dependent SEAP response to both LPS and CMP, wherein peak activation was achieved after stimulation with 40–50 μg/mL of CMP. Incubation on polymyxin B resin had no effect on CMP’s ligand activity, but neutralized LPS-spiked controls. HEK293 Null cells lacking TLR4 and possessing the same SEAP reporter failed to respond to LPS or CMP, but produced SEAP when activated with TNFα. Reporter cell NF-κB responses were accompanied by transcription of IL-8, TNFα, and COX-2 genes. Celecoxib inhibited LPS-, CMP-, and TNFα-dependent NF-κB responses; whereas, indomethacin had limited effect on LPS and CMP responses. SEAP production in response to C. albicans A9 mnn4Δ mutant CMP, lacking phosphomannosylations on N-linked glycans, was significantly greater (p ≤ 0.005) than SEAP responses to CMP derived from parental A9 (both serotype B). These data confirm that engineered human cells expressing TLR4, MD2 and CD14 can respond to CMP with NF-κB activation and the response can be influenced by variations in CMP-mannosylation. Future characterizations of CMPs from other sources and their application in this model may provide further insight into variations observed with TLR4 dependent innate immune responses targeting different C. albicans strains.

Filamentation Involves Two Overlapping, but Distinct, Programs of Filamentation in the Pathogenic Fungus Candida albicans

The ability of the human pathogenic fungus Candida albicans to switch between yeast-like and filamentous forms of growth has long been linked to pathogenesis. Numerous environmental conditions, including growth at high temperatures, nutrient limitation, and exposure to serum, can trigger this morphological switch and are frequently used in in vitro models to identify genes with roles in filamentation. Previous work has suggested that differences exist between the various in vitro models both in the genetic requirements for filamentation and transcriptional responses to distinct filamentation-inducing media, but these differences had not been analyzed in detail. We compared 10 in vitro models for filamentation and found broad genetic and transcriptomic differences between model systems. The comparative analysis enabled the discovery of novel media-independent genetic requirements for filamentation as well as a core filamentation transcriptional profile. Our data also suggest that the physical environment drives distinct programs of filamentation in C. albicans, which has significant implications for filamentation in vivo.

Three Fusarium oxysporum mitogen-activated protein kinases (MAPKs) have distinct and complementary roles in stress adaptation and cross-kingdom pathogenicity

Mitogen-activated protein kinase (MAPK) cascades mediate cellular responses to environmental signals. Previous studies in the fungal pathogen Fusarium oxysporum have revealed a crucial role of Fmk1, the MAPK orthologous to Saccharomyces cerevisiae Fus3/Kss1, in vegetative hyphal fusion and plant infection. Here, we genetically dissected the individual and combined contributions of the three MAPKs Fmk1, Mpk1 and Hog1 in the regulation of development, stress response and virulence of F. oxysporum on plant and animal hosts. Mutants lacking Fmk1 or Mpk1 were affected in reactive oxygen species (ROS) homeostasis and impaired in hyphal fusion and aggregation. Loss of Mpk1 also led to increased sensitivity to cell wall and heat stress, which was exacerbated by simultaneous inactivation of Fmk1, suggesting that both MAPKs contribute to cellular adaptation to high temperature, a prerequisite for mammalian pathogens. Deletion of Hog1 caused increased sensitivity to hyperosmotic stress and resulted in partial rescue of the restricted colony growth phenotype of the mpk1Δ mutant. Infection assays on tomato plants and the invertebrate animal host Galleria mellonella revealed distinct and additive contributions of the different MAPKs to virulence. Our results indicate that positive and negative cross-talk between the three MAPK pathways regulates stress adaptation, development and virulence in the cross-kingdom pathogen F. oxysporum.

The Candida albicans HIR histone chaperone regulates the yeast-to-hyphae transition by controlling the sensitivity to morphogenesis signals

Morphological plasticity such as the yeast-to-hyphae transition is a key virulence factor of the human fungal pathogen Candida albicans. Hyphal formation is controlled by a multilayer regulatory network composed of environmental sensing, signaling, transcriptional modulators as well as chromatin modifications. Here, we demonstrate a novel role for the replication-independent HIR histone chaperone complex in fungal morphogenesis. HIR operates as a crucial modulator of hyphal development, since genetic ablation of the HIR complex subunit Hir1 decreases sensitivity to morphogenetic stimuli. Strikingly, HIR1-deficient cells display altered transcriptional amplitudes upon hyphal initiation, suggesting that Hir1 affects transcription by establishing transcriptional thresholds required for driving morphogenetic cell-fate decisions. Furthermore, ectopic expression of the transcription factor Ume6, which facilitates hyphal maintenance, rescues filamentation defects of hir1Δ/Δ cells, suggesting that Hir1 impacts the early phase of hyphal initiation. Hence, chromatin chaperone-mediated fine-tuning of transcription is crucial for driving morphogenetic conversions in the fungal pathogen C. albicans.

A Family of Secretory Proteins Is Associated with Different Morphotypes in Cryptococcus neoformans

Cryptococcus neoformans, an opportunistic human fungal pathogen, can undergo a yeast-to-hypha transition in response to environmental cues. This morphological transition is associated with changes in the expression of cell surface proteins. The Cryptococcus cell surface and secreted protein Cfl1 was the first identified adhesin in the Basidiomycota. Cfl1 has been shown to regulate morphology, biofilm formation, and intercellular communication. Four additional homologs of CFL1 are harbored by the Cryptococcus genome: DHA1, DHA2, CPL1, and CFL105 The common features of this gene family are the conserved C-terminal SIGC domain and the presence of an N-terminal signal peptide. We found that all these Cfl1 homolog proteins are indeed secreted extracellularly. Interestingly, some of these secretory proteins display cell type-specific expression patterns: Cfl1 is hypha specific, Dha2 is yeast specific, and Dha1 (delayed hypersensitivity antigen 1) is expressed in all cell types but is particularly enriched at basidia. Interestingly, Dha1 is induced by copper limitation and suppressed by excessive copper in the medium. This study further attests to the physiological heterogeneity of the Cryptococcus mating colony, which is composed of cells with heterogeneous morphotypes. The differential expression of these secretory proteins contributes to heterogeneity, which is beneficial for the fungus to adapt to changing environments.IMPORTANCE Heterogeneity in physiology and morphology is an important bet-hedging strategy for nonmobile microbes such as fungi to adapt to unpredictable environmental changes. Cryptococcus neoformans, a ubiquitous basidiomycetous fungus, is known to switch from the yeast form to the hypha form during sexual development. However, in a mating colony, only a subset of yeast cells switch to hyphae, and only a fraction of the hyphal subpopulation will develop into fruiting bodies, where meiosis and sporulation occur. Here, we investigated a basidiomycete-specific secretory protein family. We found that some of these proteins are cell type specific, thus contributing to the heterogeneity of a mating colony. Our study also demonstrates the importance of examining the protein expression pattern at the individual-cell level in addition to population gene expression profiling for the investigation of a heterogeneous community.

Bypass of Candida albicans Filamentation/Biofilm Regulators through Diminished Expression of Protein Kinase Cak1

Biofilm formation on implanted medical devices is a major source of lethal invasive infection by Candida albicans. Filamentous growth of this fungus is tied to biofilm formation because many filamentation-associated genes are required for surface adherence. Cell cycle or cell growth defects can induce filamentation, but we have limited information about the coupling between filamentation and filamentation-associated gene expression after cell cycle/cell growth inhibition. Here we identified the CDK activating protein kinase Cak1 as a determinant of filamentation and filamentation-associated gene expression through a screen of mutations that diminish expression of protein kinase-related genes implicated in cell cycle/cell growth control. A cak1diminished expression (DX) strain displays filamentous growth and expresses filamentation-associated genes in the absence of typical inducing signals. In a wild-type background, expression of filamentation-associated genes depends upon the transcription factors Bcr1, Brg1, Efg1, Tec1, and Ume6. In the cak1 DX background, the dependence of filamentation-associated gene expression on each transcription factor is substantially relieved. The unexpected bypass of filamentation-associated gene expression activators has the functional consequence of enabling biofilm formation in the absence of Bcr1, Brg1, Tec1, Ume6, or in the absence of both Brg1 and Ume6. It also enables filamentous cell morphogenesis, though not biofilm formation, in the absence of Efg1. Because these transcription factors are known to have shared target genes, we suggest that cell cycle/cell growth limitation leads to activation of several transcription factors, thus relieving dependence on any one.

The ability of the pathogen Candida albicans to grow on surfaces as biofilms is a determinant of infection ability, because biofilms on implanted medical devices seed infections. Biofilm formation by this organism requires growth in the form of filamentous cells and the expression of filamentation-associated genes. Inhibition of cell proliferation can induce filamentous cell formation, as we find here for strains that express greatly reduced levels of the cell cycle regulator Cak1. Surprisingly, biofilm formation occurs independently of many central biofilm regulatory genes when Cak1 levels are reduced. This response to proliferation inhibition may reflect the activation of numerous biofilm regulators, thus relieving the dependence on any one regulator. The stimulation of biofilm formation by proliferation inhibition, a property of many bacterial pathogens as well, may contribute to the limited effectiveness of antimicrobials against biofilms.

ZCF32, a fungus specific Zn(II)2 Cys6 transcription factor, is a repressor of the biofilm development in the human pathogen Candida albicans

As a human fungal pathogen, Candida albicans can cause a wide variety of disease conditions ranging from superficial to systemic infections. Many of these infections are caused by an inherent ability of the pathogen to form biofilms on medical devices resulting in high mortality. Biofilms formed by C. albicans are a complex consortium of yeast and hyphal cells embedded in an extracellular matrix and are regulated by a network of transcription factors. Here, we report the role of a novel Zn(II)2-Cys6 binuclear cluster transcription factor, ZCF32, in the regulation of biofilm formation. Global transcriptome analysis reveals that biofilm development is the most altered pathway in the zcf32 null mutant. To delineate the functional correlation between ZCF32 and biofilm development, we determined the set of genes directly regulated by Zcf32. Our data suggests that Zcf32 regulates biofilm formation by repressing the expression of adhesins, chitinases and a significant number of other GPI-anchored proteins. We establish that there is the lesser recruitment of Zcf32 on the promoters of biofilm genes in biofilm condition compared to the planktonic mode of growth. Taking together, we propose that the transcription factor ZCF32 negatively regulates biofilm development in C. albicans.