Deletion of the NOT4 gene impairs hyphal development and pathogenicity in Candida albicans

A first-in-class inhibitor of Hsp110 molecular chaperones of pathogenic fungi

Proteins of the Hsp110 family are molecular chaperones that play important roles in protein homeostasis in eukaryotes. The pathogenic fungus Candida albicans, which causes infections in humans, has a single Hsp110, termed Msi3. Here, we provide proof-of-principle evidence supporting fungal Hsp110s as targets for the development of new antifungal drugs. We identify a pyrazolo[3,4-b] pyridine derivative, termed HLQ2H (or 2H), that inhibits the biochemical and chaperone activities of Msi3, as well as the growth and viability of C. albicans. Moreover, the fungicidal activity of 2H correlates with its inhibition of in vivo protein folding. We propose 2H and related compounds as promising leads for development of new antifungals and as pharmacological tools for the study of the molecular mechanisms and functions of Hsp110s.

The Ccr4-Not Complex: Architecture and Structural Insights

The Ccr4-Not complex is an essential multi-subunit protein complex that plays a fundamental role in eukaryotic mRNA metabolism and has a multitude of different roles that impact eukaryotic gene expression . It has a conserved core of three Not proteins, the Ccr4 protein, and two Ccr4 associated factors, Caf1 and Caf40. A fourth Not protein, Not4, is conserved, but is only a stable subunit of the complex in yeast. Certain subunits have been duplicated during evolution, with functional divergence, such as Not3 in yeast, and Ccr4 or Caf1 in human. However the complex includes only one homolog for each protein. In addition, species-specific subunits are part of the complex, such as Caf130 in yeast or Not10 and Not11 in human. Two conserved catalytic functions are associated with the complex, deadenylation and ubiquitination . The complex adopts an L-shaped structure, in which different modules are bound to a large Not1 scaffold protein. In this chapter we will summarize our current knowledge of the architecture of the complex and of the structure of its constituents.

An integrative genomics approach identifies novel pathways that influence candidaemia susceptibility

Candidaemia is a bloodstream infection caused by Candida species that primarily affects specific groups of at-risk patients. Because only small candidaemia patient cohorts are available, classical genome wide association cannot be used to identify Candida susceptibility genes. Therefore, we have applied an integrative genomics approach to identify novel susceptibility genes and pathways for candidaemia. Candida-induced transcriptome changes in human primary leukocytes were assessed by RNA sequencing. Genetic susceptibility to candidaemia was assessed using the Illumina immunochip platform for genotyping of a cohort of 217 patients. We then integrated genetics data with gene-expression profiles, Candida-induced cytokine production capacity, and circulating concentrations of cytokines. Based on the intersection of transcriptome pathways and genomic data, we prioritized 31 candidate genes for candidaemia susceptibility. This group of genes was enriched with genes involved in inflammation, innate immunity, complement, and hemostasis. We then validated the role of MAP3K8 in cytokine regulation in response to Candida stimulation. Here, we present a new framework for the identification of susceptibility genes for infectious diseases that uses an unbiased, hypothesis-free, systems genetics approach. By applying this approach to candidaemia, we identified novel susceptibility genes and pathways for candidaemia, and future studies should assess their potential as therapeutic targets.

Candida albicans Biofilms and Human Disease

In humans, microbial cells (including bacteria, archaea, and fungi) greatly outnumber host cells. Candida albicans is the most prevalent fungal species of the human microbiota; this species asymptomatically colonizes many areas of the body, particularly the gastrointestinal and genitourinary tracts of healthy individuals. Alterations in host immunity, stress, resident microbiota, and other factors can lead to C. albicans overgrowth, causing a wide range of infections, from superficial mucosal to hematogenously disseminated candidiasis. To date, most studies of C. albicans have been carried out in suspension cultures; however, the medical impact of C. albicans (like that of many other microorganisms) depends on its ability to thrive as a biofilm, a closely packed community of cells. Biofilms are notorious for forming on implanted medical devices, including catheters, pacemakers, dentures, and prosthetic joints, which provide a surface and sanctuary for biofilm growth. C. albicans biofilms are intrinsically resistant to conventional antifungal therapeutics, the host immune system, and other environmental perturbations, making biofilm-based infections a significant clinical challenge. Here, we review our current knowledge of biofilms formed by C. albicans and closely related fungal species.

The GRF10 homeobox gene regulates filamentous growth in the human fungal pathogen Candida albicans

Candida albicans is the most common human fungal pathogen and can cause life-threatening infections. Filamentous growth is critical in the pathogenicity of C. albicans, as the transition from yeast to hyphal forms is linked to virulence and is also a pivotal process in fungal biofilm development. Homeodomain-containing transcription factors have been linked to developmental processes in fungi and other eukaryotes. We report here on GRF10, a homeobox transcription factor-encoding gene that plays a role in C. albicans filamentation. Deletion of the GRF10 gene, in both C. albicans SN152 and BWP17 strain backgrounds, results in mutants with strongly decreased hyphal growth. The mutants are defective in chlamydospore and biofilm formation, as well as showing dramatically attenuated virulence in a mouse infection model. Expression of the GRF10 gene is highly induced during stationary phase and filamentation. In summary, our study emphasizes a new role for the homeodomain-containing transcription factor in morphogenesis and pathogenicity of C. albicans.

AI-2 of Aggregatibacter actinomycetemcomitans inhibits Candida albicans biofilm formation

Aggregatibacter actinomycetemcomitans, a Gram-negative bacterium, and Candida albicans, a polymorphic fungus, are both commensals of the oral cavity but both are opportunistic pathogens that can cause oral diseases. A. actinomycetemcomitans produces a quorum-sensing molecule called autoinducer-2 (AI-2), synthesized by LuxS, that plays an important role in expression of virulence factors, in intra- but also in interspecies communication. The aim of this study was to investigate the role of AI-2 based signaling in the interactions between C. albicans and A. actinomycetemcomitans. A. actinomycetemcomitans adhered to C. albicans and inhibited biofilm formation by means of a molecule that was secreted during growth. C. albicans biofilm formation increased significantly when co-cultured with A. actinomycetemcomitans luxS, lacking AI-2 production. Addition of wild-type-derived spent medium or synthetic AI-2 to spent medium of the luxS strain, restored inhibition of C. albicans biofilm formation to wild-type levels. Addition of synthetic AI-2 significantly inhibited hypha formation of C. albicans possibly explaining the inhibition of biofilm formation. AI-2 of A. actinomycetemcomitans is synthesized by LuxS, accumulates during growth and inhibits C. albicans hypha- and biofilm formation. Identifying the molecular mechanisms underlying the interaction between bacteria and fungi may provide important insight into the balance within complex oral microbial communities.

Impact of environmental conditions on the form and function of Candida albicans biofilms

Candida albicans, like other pathogens, can form complex biofilms on a variety of substrates. However, as the number of studies of gene regulation, architecture, and pathogenic traits of C. albicans biofilms has increased, so have differences in results. This suggests that depending upon the conditions employed, biofilms may vary widely, thus hampering attempts at a uniform description. Gene expression studies suggest that this may be the case. To explore this hypothesis further, we compared the architectures and traits of biofilms formed in RPMI 1640 and Spider media at 37°C in air. Biofilms formed by a/α cells in the two media differed to various degrees in cellular architecture, matrix deposition, penetrability by leukocytes, fluconazole susceptibility, and the facilitation of mating. Similar comparisons of a/a cells in the two media, however, were made difficult given that in air, although a/a cells form traditional biofilms in RPMI medium, they form polylayers composed primarily of yeast cells in Spider medium. These polylayers lack an upper hyphal/matrix region, are readily penetrated by leukocytes, are highly fluconazole susceptible, and do not facilitate mating. If, however, air is replaced with 20% CO2, a/a cells make a biofilm in Spider medium similar architecturally to that of a/α cells, which facilitates mating. A second, more cursory comparison is made between the disparate cellular architectures of a/a biofilms formed in air in RPMI and Lee's media. The results demonstrate that C. albicans forms very different types of biofilms depending upon the composition of the medium, level of CO2 in the atmosphere, and configuration of the MTL locus.

The Hsp90 co-chaperone Sgt1 governs Candida albicans morphogenesis and drug resistance

The molecular chaperone Hsp90 orchestrates regulatory circuitry governing fungal morphogenesis, biofilm development, drug resistance, and virulence. Hsp90 functions in concert with co-chaperones to regulate stability and activation of client proteins, many of which are signal transducers. Here, we characterize the first Hsp90 co-chaperone in the leading human fungal pathogen, Candida albicans. We demonstrate that Sgt1 physically interacts with Hsp90, and that it governs C. albicans morphogenesis and drug resistance. Genetic depletion of Sgt1 phenocopies depletion of Hsp90, inducing yeast to filament morphogenesis and invasive growth. Sgt1 governs these traits by bridging two morphogenetic regulators: Hsp90 and the adenylyl cyclase of the cAMP-PKA signaling cascade, Cyr1. Sgt1 physically interacts with Cyr1, and depletion of either Sgt1 or Hsp90 activates cAMP-PKA signaling, revealing the elusive link between Hsp90 and the PKA signaling cascade. Sgt1 also mediates tolerance and resistance to the two most widely deployed classes of antifungal drugs, azoles and echinocandins. Depletion of Sgt1 abrogates basal tolerance and acquired resistance to azoles, which target the cell membrane. Depletion of Sgt1 also abrogates tolerance and resistance to echinocandins, which target the cell wall, and renders echinocandins fungicidal. Though Sgt1 and Hsp90 have a conserved impact on drug resistance, the underlying mechanisms are distinct. Depletion of Hsp90 destabilizes the client protein calcineurin, thereby blocking crucial responses to drug-induced stress; in contrast, depletion of Sgt1 does not destabilize calcineurin, but blocks calcineurin activation in response to drug-induced stress. Sgt1 influences not only morphogenesis and drug resistance, but also virulence, as genetic depletion of C. albicans Sgt1 leads to reduced kidney fungal burden in a murine model of systemic infection. Thus, our characterization of the first Hsp90 co-chaperone in a fungal pathogen establishes C. albicans Sgt1 as a global regulator of morphogenesis and drug resistance, providing a new target for treatment of life-threatening fungal infections.

The Ccr4--not complex

The Ccr4-Not complex is a unique, essential and conserved multi-subunit complex that acts at the level of many different cellular functions to regulate gene expression. Two enzymatic activities, namely ubiquitination and deadenylation, are provided by different subunits of the complex. However, studies over the last decade have demonstrated a tantalizing multi-functionality of this complex that extends well beyond its identified enzymatic activities. Most of our initial knowledge about the Ccr4-Not complex stemmed from studies in yeast, but an increasing number of reports on this complex in other species are emerging. In this review we will discuss the structure and composition of the complex, and describe the different cellular functions with which the Ccr4-Not complex has been connected in different organisms. Finally, based upon our current state of knowledge, we will propose a model to explain how one complex can provide such multi-functionality. This model suggests that the Ccr4-Not complex might function as a "chaperone platform".

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.

Licorice and its potential beneficial effects in common oro-dental diseases

Licorice, the name given to the roots and stolons of Glycyrrhiza species, has been used since ancient times as a traditional herbal remedy. Licorice contains several classes of secondary metabolites with which numerous human health benefits have been associated. Recent research suggests that licorice and its bioactive ingredients such as glycyrrhizin, glabridin, licochalcone A, licoricidin, and licorisoflavan A possess potential beneficial effects in oral diseases. This paper reviews the effects of licorice and licorice constituents on both the oral microbial pathogens and the host immune response involved in common ora-dental diseases (dental caries, periodontitis, candidiasis, and recurrent aphthous ulcers). It also summarizes results of clinical trials that investigated the potential beneficial effects of licorice and its constituents for preventing/treating oro-dental diseases.

Regulatory circuitry governing fungal development, drug resistance, and disease

Pathogenic fungi have become a leading cause of human mortality due to the increasing frequency of fungal infections in immunocompromised populations and the limited armamentarium of clinically useful antifungal drugs. Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are the leading causes of opportunistic fungal infections. In these diverse pathogenic fungi, complex signal transduction cascades are critical for sensing environmental changes and mediating appropriate cellular responses. For C. albicans, several environmental cues regulate a morphogenetic switch from yeast to filamentous growth, a reversible transition important for virulence. Many of the signaling cascades regulating morphogenesis are also required for cells to adapt and survive the cellular stresses imposed by antifungal drugs. Many of these signaling networks are conserved in C. neoformans and A. fumigatus, which undergo distinct morphogenetic programs during specific phases of their life cycles. Furthermore, the key mechanisms of fungal drug resistance, including alterations of the drug target, overexpression of drug efflux transporters, and alteration of cellular stress responses, are conserved between these species. This review focuses on the circuitry regulating fungal morphogenesis and drug resistance and the impact of these pathways on virulence. Although the three human-pathogenic fungi highlighted in this review are those most frequently encountered in the clinic, they represent a minute fraction of fungal diversity. Exploration of the conservation and divergence of core signal transduction pathways across C. albicans, C. neoformans, and A. fumigatus provides a foundation for the study of a broader diversity of pathogenic fungi and a platform for the development of new therapeutic strategies for fungal disease.

Diverse protein kinase interactions identified by protein microarrays reveal novel connections between cellular processes

Protein kinases are key regulators of cellular processes. In spite of considerable effort, a full understanding of the pathways they participate in remains elusive. We globally investigated the proteins that interact with the majority of yeast protein kinases using protein microarrays. Eighty-five kinases were purified and used to probe yeast proteome microarrays. One-thousand-twenty-three interactions were identified, and the vast majority were novel. Coimmunoprecipitation experiments indicate that many of these interactions occurred in vivo. Many novel links of kinases to previously distinct cellular pathways were discovered. For example, the well-studied Kss1 filamentous pathway was found to bind components of diverse cellular pathways, such as those of the stress response pathway and the Ccr4-Not transcriptional/translational regulatory complex; genetic tests revealed that these different components operate in the filamentation pathway in vivo. Overall, our results indicate that kinases operate in a highly interconnected network that coordinates many activities of the proteome. Our results further demonstrate that protein microarrays uncover a diverse set of interactions not observed previously.

Some biological features of Candida albicans mutants for genes coding fungal proteins containing the CFEM domain

Several biological features of Candida albicans genes (PGA10, RBT5 and CSA1) coding for putative polypeptide species belonging to a subset of fungal proteins containing an eight-cysteine domain referred as common in several fungal extracellular membrane (CFEM) are described. The deletion of these genes resulted in a cascade of pleiotropic effects. Thus, mutant strains exhibited higher cell surface hydrophobicity levels and an increased ability to bind to inert or biological substrates. Confocal scanning laser microscopy using concanavalin A-Alexafluor 488 (which binds to mannose and glucose residues) and FUN-1 (a cytoplasmic fluorescent probe for cell viability) dyes showed that mutant strains formed thinner and more fragile biofilms. These apparently contained lower quantities of extracellular matrix material and less metabolically active cells than their parental strain counterpart, although the relative percentage of mycelial forms was similar in all cases. The cell surface of C. albicans strains harbouring deletions for genes coding CFEM-domain proteins appeared to be severely altered according to atomic force microscopy observations. Assessment of the relative gene expression within individual C. albicans cells revealed that CFEM-coding genes were upregulated in mycelium, although these genes were shown not to affect virulence in animal models. Overall, this study has demonstrated that CFEM domain protein-encoding genes are pleiotropic, influencing cell surface characteristics and biofilm formation.

Polyelectrolyte multilayers fabricated from antifungal β-peptides: design of surfaces that exhibit antifungal activity against Candida albicans

The fungal pathogen Candida albicans can form biofilms on the surfaces of medical devices that are resistant to drug treatment and provide a reservoir for recurrent infections. The use of fungicidal or fungistatic materials to fabricate or coat the surfaces of medical devices has the potential to reduce or eliminate the incidence of biofilm-associated infections. Here we report on (i) the fabrication of multilayered polyelectrolyte thin films (PEMs) that promote the surface-mediated release of an antifungal β-peptide and (ii) the ability of these films to inhibit the growth of C. albicans on film-coated surfaces. We incorporated a fluorescently labeled antifungal β-peptide into the structures of PEMs fabricated from poly-l-glutamic acid (PGA) and poly-l-lysine (PLL) using a layer-by-layer fabrication procedure. These films remained stable when incubated in culture media at 37 °C and released β-peptide gradually into solution for up to 400 h. Surfaces coated with β-peptide-containing films inhibited the growth of C. albicans , resulting in a 20% reduction of cell viability after 2 h and a 74% decrease in metabolic activity after 7 h when compared to cells incubated on PGA/PLL-coated surfaces without β-peptide. In addition, β-peptide-containing films inhibited hyphal elongation by 55%. These results, when combined, demonstrate that it is possible to fabricate β-peptide-containing thin films that inhibit the growth and proliferation of C. albicans and provide the basis of an approach that could be used to inhibit the formation of C. albicans biofilms on film-coated surfaces. The layer-by-layer approach reported here could ultimately be used to coat the surfaces of catheters, surgical instruments, and other devices to inhibit drug-resistant C. albicans biofilm formation in clinical settings.

Our current understanding of fungal biofilms

Fungal biofilms are an escalating clinical problem associated with significant rates of mortality. Candida albicans is the most notorious of all fungal biofilm formers. However, non-Candida species, yeasts such as Cryptococcus neoformans, and filamentous moulds such as Aspergillus fumigatus, have been shown to be implicated in biofilm-associated infections. Fungal biofilms have distinct developmental phases, including adhesion, colonisation, maturation and dispersal, which are governed by complex molecular events. Recalcitrance to antifungal therapy remains the greatest threat to patients with fungal biofilms. This review discusses our current understanding of the basic biology and clinical implications associated with fungal biofilms.

Streptococcus mutans competence-stimulating peptide inhibits Candida albicans hypha formation

The oral cavity is colonized by microorganisms growing in biofilms in which interspecies interactions take place. Streptococcus mutans grows in biofilms on enamel surfaces and is considered one of the main etiological agents of human dental caries. Candida albicans is also commonly found in the human oral cavity, where it interacts with S. mutans. C. albicans is a polymorphic fungus, and the yeast-to-hypha transition is involved in virulence and biofilm formation. The aim of this study was to investigate interkingdom communication between C. albicans and S. mutans based on the production of secreted molecules. S. mutans UA159 inhibited C. albicans germ tube (GT) formation in cocultures even when physically separated from C. albicans. Only S. mutans spent medium collected in the early exponential phase (4-h-old cultures) inhibited the GT formation of C. albicans. During this phase, S. mutans UA159 produces a quorum-sensing molecule, competence-stimulating peptide (CSP). The role of CSP in inhibiting GT formation was confirmed by using synthetic CSP and a comC deletion strain of S. mutans UA159, which lacks the ability to produce CSP. Other S. mutans strains and other Streptococcus spp. also inhibited GT formation but to different extents, possibly reflecting differences in CSP amino acid sequences among Streptococcus spp. or differences in CSP accumulation in the media. In conclusion, CSP, an S. mutans quorum-sensing molecule secreted during the early stages of growth, inhibits the C. albicans morphological switch.

Biofilm formation in clinical Candida isolates and its association with virulence

Biofilm formation, an important virulence trait of Candida species was measured in 107 Candida isolates from 32 candidemic patients by XTT [2,3-bis (2-methoxy-4nitro-5-sulfo-phenyl)-2H-tetra-zolium-5-carboxanilide] activity and compared to biofilm formation of Candida isolates from oropharyngeal lesions of 19 AIDS patients. Biofilm formation by XTT varied among species and C. albicans; C. lusitaniae and C. krusei produced more biofilm than the other Candida species. C. tropicalis was the most dominant species isolated from blood followed by C. albicans, and other non-albicans species whereas only C. albicans was recovered from oral lesions. Importantly, though Biofilm formation was variable within a species it was stable in sequential isolates during chronic infection. Sequential isolates exhibited identical Karyotype pattern or RAPD patterns unless patients were co-infected with more than one strain. High biofilm formation was associated with slow growth rate but not with adherence. Murine infection studies demonstrated that, degree of in-vitro biofilm formation was associated with virulence in mice, as mice infected both with no and low biofilm formers survived longer than mice infected with high biofilm former C. albicans (p< or =0.001). We conclude that biofilm formation is a stable but strain specific characteristic that can greatly vary among C. albicans and non-albicans strains, and plays an important role in persistence of infection.

Hyphal content determines the compression strength of Candida albicans biofilms

Candida albicans is the most frequently isolated human fungal pathogen among species causing biofilm-related clinical infections. Mechanical properties of Candida biofilms have hitherto been given no attention, despite the fact that mechanical properties are important for selection of treatment or dispersal of biofilm organisms due to a bodily fluid flow. The aim of this study was to identify the factors that determine the compression strength of Candida biofilms. Biofilms of C. albicans wild-type parental strain Caf2-1, mutant strain Chk24 lacking Chk1p [known to be involved in regulation of morphogenesis (yeast-to-hyphae transition)] and gene-reconstructed strain Chk23 were evaluated for their resistance to compression, along with biofilms of Candida tropicalis GB 9/9 and Candida parapsilosis GB 2/8, derived from used voice prosthetic biofilms. Additionally, cell morphologies within the biofilm, cell-surface hydrophobicities and extracellular polymeric substance composition were determined. Our results suggest that the hyphae-to-yeast ratio influences the compression strength of C. albicans biofilms. Biofilms with a hyphal content >50 % possessed significantly higher compressive strength and were more difficult to destroy by vortexing and sonication than biofilms with a lower hyphal content. However, when the amount of extracellular DNA (eDNA) in biofilms of C. albicans Caf2-1 and Chk24 increased, biofilm strength declined, suggesting that eDNA may influence biofilm integrity adversely.

Aspergillus fumigatus forms biofilms with reduced antifungal drug susceptibility on bronchial epithelial cells

Aspergillus fumigatus is a leading cause of death in immunocompromised patients and a frequent colonizer of the respiratory tracts of asthma and cystic fibrosis (CF) patients. Biofilms enable bacteria and yeasts to persist in infections and can contribute to antimicrobial resistance. We investigated the ability of A. fumigatus to form biofilms on polystyrene (PS) and human bronchial epithelial (HBE) and CF bronchial epithelial (CFBE) cells. We developed a novel in vitro coculture model of A. fumigatus biofilm formation on HBE and CFBE cells. Biofilm formation was documented by dry weight, scanning electron microscopy (SEM), and confocal scanning laser microscopy (CSLM). The in vitro antifungal activities of seven antifungal drugs were tested by comparing planktonic and sessile A. fumigatus strains. A. fumigatus formed an extracellular matrix on PS and HBE and CFBE cells as evidenced by increased dry weight, SEM, and CSLM. These biofilms exhibited decreased antifungal drug susceptibility and were adherent to the epithelial cells, with fungi remaining viable throughout 3 days. These observations might have implications for treatment of A. fumigatus colonization in chronic lung diseases and for its potential impact on airway inflammation, damage, and infection.

Temporal analysis of Candida albicans gene expression during biofilm development

Microarrays were used to identify changes in gene expression associated with Candida albicans biofilm development. Two biofilm substrates (denture and catheter), and two C. albicans strains for each substrate, were tested to remove model- and strain-dependent variability from the overall dataset. Three biofilm developmental phases were examined: early (6 h), intermediate (12 h), and mature (48 h). Planktonic specimens were collected at the same time points. Data analysis focused primarily on gene expression changes over the time-course of biofilm development. Glycolytic and non-glycolytic carbohydrate assimilation, amino acid metabolism, and intracellular transport mechanisms were important during the early phase of biofilm formation. These early events increase intracellular pools of pyruvate, pentoses and amino acids, which prepare the biofilm for the large biomass increase that begins around 12 h of development. This developmental stage demands energy and utilizes specific transporters for amino acids, sugars, ions, oligopeptides and lactate/pyruvate. At mature phase (48 h), few genes were differentially expressed compared with the 12 h time point, suggesting a relative lack of initiation of new metabolic activity. Data analysis to assess biofilm model-specific gene expression showed more dynamic changes in the denture model than in the catheter model. Data analysis to identify gene expression changes that are associated with each strain/substrate combination identified the same types of genes that were identified in the analysis of the entire dataset. Collectively, these data suggest that genes belonging to different, but interconnected, functional categories regulate the morphology and phenotype of C. albicans biofilm.

Effect of subinhibitory concentration of some established and experimental antifungal compounds on the germ tube formation in Candida albicans

The influence of subinhibitory concentrations of six established and 19 newly synthesized antifungal compounds on the dimorphic transition of three C. albicans strains was evaluated in the filamentation-inducing medium. Amphotericin B was found to produce almost complete inhibition in the germination at a concentration of 1/10 of the corresponding MIC and partial inhibition at a concentration as low as MIC/50. Flucytosine and four azole derivatives were proven ineffective. From the newly synthesized drugs, the incrustoporin derivative LNO6-22, two phenylguanidine derivatives (PG15, PG45), and four thiosalicylanilide derivatives, in particular, showed results comparable to those of amphotericin B, with a high inhibition of germ tube formation at concentrations of MIC/10. In general, concentrations of MIC/50 had no visible effect.

Candida albicans Als3p is required for wild-type biofilm formation on silicone elastomer surfaces

Candida albicans ALS3 encodes a large cell-surface glycoprotein that has adhesive properties. Immunostaining of cultured C. albicans germ tubes showed that Als3p is distributed diffusely across the germ tube surface. Two-photon laser scanning microscopy of model catheter biofilms grown using a PALS3-green fluorescent protein (GFP) reporter strain showed GFP production in hyphae throughout the biofilm structure while biofilms grown using a PTPI1-GFP reporter strain showed GFP in both hyphae and yeast-form cells. Model catheter biofilms formed by an als3 Delta/als3 Delta strain were weakened structurally and had approximately half the biomass of a wild-type biofilm. Reintegration of a wild-type ALS3 allele restored biofilm mass and wild-type biofilm structure. Production of an Als3p-Ag alpha 1p fusion protein under control of the ALS3 promoter in the als3 Delta/als3 Delta strain restored some of the wild-type biofilm structural features, but not the wild-type biofilm mass. Despite its inability to restore wild-type biofilm mass, the Als3p-Ag alpha 1p fusion protein mediated adhesion of the als3 Delta/als3 Delta C. albicans strain to human buccal epithelial cells (BECs). The adhesive role of the Als3p N-terminal domain was further demonstrated by blocking adhesion of C. albicans to BECs with immunoglobulin reactive against the Als3p N-terminal sequences. Together, these data suggest that portions of Als3p that are important for biofilm formation may be different from those that are important in BEC adhesion, and that Als3p may have multiple functions in biofilm formation. Overexpression of ALS3 in an efg1 Delta/efg1 Delta strain that was deficient for filamentous growth and biofilm formation resulted in growth of elongated C. albicans cells, even under culture conditions that do not favour filamentation. In the catheter biofilm model, the ALS3 overexpression strain formed biofilm with a mass similar to that of a wild-type control. However, C. albicans cells in the biofilm had yeast-like morphology. This result uncouples the effect of cellular morphology from biofilm formation and underscores the importance of Als3p in biofilm development on silicone elastomer surfaces.

Alternative Candida albicans lifestyles: growth on surfaces

Candida albicans, an opportunistic fungal pathogen, causes a wide variety of human diseases such as oral thrush and disseminated candidiasis. Many aspects of C. albicans physiology have been studied during liquid growth, but in its natural environment, the gastrointestinal tract of a mammalian host, the organism associates with surfaces. Growth on a surface triggers several behaviors, such as biofilm formation, invasion, and thigmotropism, that are important for infection. Recent discoveries have identified factors that regulate these behaviors and revealed the importance of these behaviors for pathogenesis.

The role of Candida albicans NOT5 in virulence depends upon diverse host factors in vivo

We previously identified Candida albicans Not5p as an immunogenic protein expressed during oropharyngeal candidiasis (OPC). In this study, we demonstrate that C. albicans NOT5 reverses the growth defects of a Saccharomyces cerevisiae not5 mutant strain at 37 degrees C, suggesting that the genes share at least some functional equivalence. We implicate C. albicans NOT5 in the pathogenesis of disseminated candidiasis (DC) induced by intravenous infection among neutropenic and nonimmunosuppressed mice, as well as in that of OPC in mice immunosuppressed with corticosteroids. We find no role in virulence, however, among neutropenic and corticosteroid-suppressed mice with DC resulting from gastrointestinal translocation, nor do we implicate the gene in vulvovaginal candidiasis among mice in pseudoestrus. These findings suggest that the role of NOT5 in virulence depends on the specific in vivo environment and is influenced by diverse factors such as tissue site, portal of entry, and the status of host defenses. NOT5 is necessary for normal adherence to colonic and cervical epithelial cells in vitro, demonstrating that such assays cannot fully replicate disease processes in vivo. Lastly, antibody responses against Not5p do not differ in the sera of patients with OPC, patients with DC, and healthy controls, suggesting that the protein is associated with both commensalism and the pathogenesis of disease.

Differential cytokine production and Toll-like receptor signaling pathways by Candida albicans blastoconidia and hyphae

Toll-like receptors (TLR) are crucial for an efficient antifungal defense. We investigated the differential recognition of blastoconidia and hyphae of Candida albicans by TLRs. In contrast to Candida blastoconidia, which stimulated large amounts of gamma interferon (IFN-gamma), the tissue-invasive Candida hyphae did not stimulate any IFN-gamma by human peripheral blood mononuclear cells (PBMC) or murine splenic lymphocytes. After stimulation with blastoconidia, the production of IFN-gamma was TLR4 dependent, as shown by the significantly decreased IFN-gamma production in anti-TLR4-treated PBMC and in splenic lymphocytes from TLR4-defective ScCr mice. In addition, peritoneal macrophages from ScCr mice produced less tumor necrosis factor alpha (TNF-alpha) than macrophages of control mice did when stimulated with Candida blastoconidia, but not with hyphae, indicating that TLR4-mediated signals are lost during hyphal germination. In contrast, macrophages from TLR2 knockout mice had a decreased production of TNF-alpha in response to both Candida blastoconidia and hyphae. Candida hyphae stimulated production of interleukin-10 through TLR2-dependent mechanisms. In conclusion, TLR4 mediates proinflammatory cytokine induction after Candida stimulation, whereas Candida recognition by TLR2 leads mainly to anti-inflammatory cytokine release. TLR4-mediated proinflammatory signals are lost during germination of Candida blastoconidia into hyphae. Phenotypic switching during germination may be an important escape mechanism of C. albicans, resulting in counteracting host defense.

Demonstration of loss of heterozygosity by single-nucleotide polymorphism microarray analysis and alterations in strain morphology in Candida albicans strains during infection

Candida albicans is a diploid yeast with a predominantly clonal mode of reproduction, and no complete sexual cycle is known. As a commensal organism, it inhabits a variety of niches in humans. It becomes an opportunistic pathogen in immunocompromised patients and can cause both superficial and disseminated infections. It has been demonstrated that genome rearrangement and genetic variation in isolates of C. albicans are quite common. One possible mechanism for generating genome-level variation among individuals of this primarily clonal fungus is mutation and mitotic recombination leading to loss of heterozygosity (LOH). Taking advantage of a recently published genome-wide single-nucleotide polymorphism (SNP) map (A. Forche, P. T. Magee, B. B. Magee, and G. May, Eukaryot. Cell 3:705-714, 2004), an SNP microarray was developed for 23 SNP loci residing on chromosomes 5, 6, and 7. It was used to examine 21 strains previously shown to have undergone mitotic recombination at the GAL1 locus on chromosome 1 during infection in mice. In addition, karyotypes and morphological properties of these strains were evaluated. Our results show that during in vivo passaging, LOH events occur at observable frequencies, that such mitotic recombination events occur independently in different loci across the genome, and that changes in karyotypes and alterations of phenotypic characteristics can be observed alone, in combination, or together with LOH.