Adherence and receptor relationships of Candida albicans

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

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

Oligonucleotide delivery to antigen presenting cells by using schizophyllan

Schizophyllan (SPG), a member of the β-glucan family, can form novel complexes with homo-polynucleotides such as poly(dA) through hydrogen bonding between two main chain glucoses and the one nucleotide base. Dectin-1, one of the major receptors for β-glucans, is known to be expressed on antigen presenting cells (APCs) such as macrophages and dendritic cells. This suggests that the above-mentioned complexes could deliver bound functional oligonucleotides (ODNs) including antisense (AS)-ODNs, small interfering RNA, and CpG-ODNs to the APCs. Analysis using a quartz crystal microbalance revealed that a complex consisting of SPG and dA₆₀ with a phosphorothioate backbone was recognized by recombinant Dectin-1 protein. Treatment with this complex containing an AS-ODN for tumor necrosis factor alpha protected mice against lipopolysaccharide-induced hepatitis at a very low AS-ODN dose. Moreover, immunization with CpG-ODN/SPG complex and antigenic proteins induced potent antigen specific immune responses. The present review also represents peptide delivery by conjugation with dA₆₀ and the preparation of a nanogel using DNA-DNA hybridization. These findings indicate that the delivery of a specific ODN using β-glucans could be used for treating various diseases caused by APCs and for activating antigen specific immune responses.

Candida albicans exhibit two classes of cell surface binding sites for serum albumin defined by their affinity, abundance and prospective role in interkingdom signalling

Serum albumin binding to the yeast form of Candida albicans is described. Two populations of binding site are identified using two complementary spectroscopic techniques: an extrinsic fluorescent probe, 3-hexa-decanoyl-7-hydrocoumarin ([HEXCO) added to the C. albicans yeast cell surface that records the electrostatic surface potential and so responds to the surface binding of serum albumin and secondly a light scattering technique that reveals how albumin modulates aggregation of the yeast population. The albumin binding sites are found to possess different binding affinities and relative abundance leading to different total binding capacities. These are characterized as a receptor population with high affinity binding (Kd ~ 17 μM) but relatively low abundance and a separate population with high abundance but much lower affinity (Kd ~ 364 μM). The low-affinity binding sites are shown to be associated with the yeast cell aggregation. These values are found be dependent on the C. albicans strain and the nature of the culture media; some examples of these effects are explored. The possible physiological consequences of the presence of these sites are speculated in terms of evading the host's immune response, biofilm formation and possible interkingdom signaling processes.

Pattern recognition receptors as therapeutic targets for bacterial, viral and fungal sepsis

Sepsis remains to be a significant health care problem associated with high morbidities and mortalities. Recognizing its heterogeneity, it is critical to understand our host immunological responses to develop appropriate therapeutic approaches according to the type of sepsis. Because pattern recognition receptors are largely responsible for the recognition of microbes, we reviewed their role in immunological responses in the setting of bacterial, fungal and viral sepsis. We also considered their therapeutic potentials in sepsis.

Comprehensive genetic analysis of adhesin proteins and their role in virulence of Candida albicans

Candida albicans is a microbial fungus that exists as a commensal member of the human microbiome and an opportunistic pathogen. Cell surface-associated adhesin proteins play a crucial role in C. albicans' ability to undergo cellular morphogenesis, develop robust biofilms, colonize, and cause infection in a host. However, a comprehensive analysis of the role and relationships between these adhesins has not been explored. We previously established a CRISPR-based platform for efficient generation of single- and double-gene deletions in C. albicans, which was used to construct a library of 144 mutants, comprising 12 unique adhesin genes deleted singly, and every possible combination of double deletions. Here, we exploit this adhesin mutant library to explore the role of adhesin proteins in C. albicans virulence. We perform a comprehensive, high-throughput screen of this library, using Caenorhabditis elegans as a simplified model host system, which identified mutants critical for virulence and significant genetic interactions. We perform follow-up analysis to assess the ability of high- and low-virulence strains to undergo cellular morphogenesis and form biofilms in vitro, as well as to colonize the C. elegans host. We further perform genetic interaction analysis to identify novel significant negative genetic interactions between adhesin mutants, whereby combinatorial perturbation of these genes significantly impairs virulence, more than expected based on virulence of the single mutant constituent strains. Together, this study yields important new insight into the role of adhesins, singly and in combinations, in mediating diverse facets of virulence of this critical fungal pathogen.

Comparative effect of osmium tetroxide and ruthenium tetroxide on Lacazia loboi ultrastructure

Lobomycosis is a skin infection produced by the fungus Lacazia loboi, which mainly affects some indigenous and afro-descendant populations in Tropical America. We previously reported the comparative effect of osmium tetroxide (OsO₄ ) and ruthenium tetroxide (RuO₄ ) in the electron microscopy (EM) of other related microorganisms. The objective of this study is to compare the effect of postfixation with OsO₄ and RuO₄ in the ultrastructure of L. loboi yeasts. Skin biopsies on patients diagnosed with lobomycosis were fixed in glutaraldehyde at 3% and postfixed in the following solutions: (a) 1% OsO₄ , (b) 0.2% RuO₄ , and (c) OsO₄ at 1% followed by RuO₄ at 0.2%. They were then processed using the conventional method for EM. Unlike OsO_4, the treatment with RuO₄ revealed different shades of gray and electron dense bands in the cell wall and other cell components of L. loboi. The most notable finding was the presence of radial filamentous structures around the yeast, which made the image look like the sun. Postfixation with RuO₄ revealed ultrastructural details that had not been previously reported for L loboi. The combined use of OsO₄ and RuO₄ in EM of microorganisms with cell walls can be useful to evaluate the effect of microbicide substances.

Antimicrobial Peptides: a New Frontier in Antifungal Therapy

Invasive fungal infections in humans are generally associated with high mortality, making the choice of antifungal drug crucial for the outcome of the patient. The limited spectrum of antifungals available and the development of drug resistance represent the main concerns for the current antifungal treatments, requiring alternative strategies. Antimicrobial peptides (AMPs), expressed in several organisms and used as first-line defenses against microbial infections, have emerged as potential candidates for developing new antifungal therapies, characterized by negligible host toxicity and low resistance rates. Most of the current literature focuses on peptides with antibacterial activity, but there are fewer studies of their antifungal properties. This review focuses on AMPs with antifungal effects, including their in vitro and in vivo activities, with the biological repercussions on the fungal cells, when known. The classification of the peptides is based on their mode of action: although the majority of AMPs exert their activity through the interaction with membranes, other mechanisms have been identified, including cell wall inhibition and nucleic acid binding. In addition, antifungal compounds with unknown modes of action are also described. The elucidation of such mechanisms can be useful to identify novel drug targets and, possibly, to serve as the templates for the synthesis of new antimicrobial compounds with increased activity and reduced host toxicity.

Effect of Silver Diamine Fluoride on Reducing Candida albicans Adhesion on Dentine

BACKGROUND

Candida albicans is the most frequent pathogenic fungus in oral cavities. It adheres to dental tissues as part of dental plaques and contributes to caries formation.

OBJECTIVES

To evaluate the effect of silver diamine fluoride (SDF) on reducing C. albicans adhesion on dentine surfaces.

METHODS

Flat dentine surfaces were prepared from bovine dental disks, and samples were divided into three groups. The first and second groups were pretreated for 3 min with 299 mM or 2.99 M SDF, respectively, and the third group (control) did not undergo any SDF pretreatment. All samples were washed, inoculated with C. albicans suspension onto their dentine surface, incubated at 30 °C for 6 h, and washed again to remove any nonadherent cells. The abundance of adherent cells was investigated using colorimetric and real-time polymerase chain reaction approaches. Subsequently, the morphological changes in C. albicans by pretreatment with SDF were observed under a scanning electron microscope (SEM).

RESULTS

SDF inhibited candidal growth at concentrations as low as 2.99 µM. Dentine disks pretreated with 299 mM or 2.99 M SDF displayed significantly fewer adhered cells as compared with the control group. Upon pretreatment with SDF, SEM images showed severe morphological changes in the cellular walls, in a dose-dependent manner, suggesting a fungicidal effect of SDF against the yeast.

CONCLUSION

SDF should be considered for clinical applications aimed at inhibiting dental plaque caused by C. albicans, particularly in children and elderly individuals.

β-Glucan augments IL-1β production by activating the JAK2/STAT3 pathway in cultured rabbit keratinocytes

Trichophyton mentagrophytes (T. mentagrophytes) is the main cause of rabbit dermatophytosis. As the main pathogen-associated molecular pattern of T. mentagrophytes, the role of β-glucan in the pathogenesis of rabbit dermatophytosis remains elusive. Keratinocytes (KC) are the main cellular component and the first defensive line against fungal pathogens in the skin. Therefore, the present study investigated the effects of β-glucan on rabbit KC from dorsal skin. β-glucan was found to inhibit KC proliferation by 10% at 20 ug/ml and this concentration was thus considered as optimal. Next, 20 ug/ml β-glucan stimulation for 24 h significantly increased CXCL8, CXCL11, and IL-1β secretions in KC. Furthermore, β-glucan exposure induced the expressions of JAK2 mRNA, STAT3 mRNA, and p-STAT3 protein. Silencing JAK2 expression inhibited p-STAT3 protein expression and β-glucan-induced IL-1β secretion. And overexpression of JAK2 further promoted β-glucan-mediated p-STAT3 protein and IL-1β productions. These results suggested that β-glucan-induced CXCL8, CXCL11, and IL-1β secretions in rabbit KC might be involved in the inflammatory response of T. mentagrophytes infected rabbit dorsal skin. However, only IL-1β secretion was promoted by the JAK2/STAT3 signaling pathway. In conclusion, this study is a necessary step toward elucidating the mechanisms that underlie skin immune system injury stimulated by β-glucan.

Candida sp. Infections in Patients with Diabetes Mellitus

Candidiasis has increased substantially worldwide over recent decades and is a significant cause of morbidity and mortality, especially among critically ill patients. Diabetes mellitus (DM) is a metabolic disorder that predisposes individuals to fungal infections, including those related to Candida sp., due to a immunosuppressive effect on the patient. This review aims to discuss the latest studies regarding the occurrence of candidiasis on DM patients and the pathophysiology and etiology associated with these co-morbidities. A comprehensive review of the literature was undertaken. PubMed, Scopus, Elsevier’s ScienceDirect, and Springer’s SpringerLink databases were searched using well-defined search terms. Predefined inclusion and exclusion criteria were applied to classify relevant manuscripts. Results of the review show that DM patients have an increased susceptibility to Candida sp. infections which aggravates in the cases of uncontrolled hyperglycemia. The conclusion is that, for these patients, the hospitalization periods have increased and are commonly associated with the prolonged use of indwelling medical devices, which also increase the costs associated with disease management.

Candida albicans Agglutinin-Like Sequence (Als) Family Vignettes: A Review of Als Protein Structure and Function

Approximately two decades have passed since the description of the first gene in the Candida albicans ALS (agglutinin-like sequence) family. Since that time, much has been learned about the composition of the family and the function of its encoded cell-surface glycoproteins. Solution of the structure of the Als adhesive domain provides the opportunity to evaluate the molecular basis for protein function. This review article is formatted as a series of fundamental questions and explores the diversity of the Als proteins, as well as their role in ligand binding, aggregative effects, and attachment to abiotic surfaces. Interaction of Als proteins with each other, their functional equivalence, and the effects of protein abundance on phenotypic conclusions are also examined. Structural features of Als proteins that may facilitate invasive function are considered. Conclusions that are firmly supported by the literature are presented while highlighting areas that require additional investigation to reveal basic features of the Als proteins, their relatedness to each other, and their roles in C. albicans biology.

Increase in virulence of Sporothrix brasiliensis over five years in a patient with chronic disseminated sporotrichosis

The metropolitan region of Rio de Janeiro is hyperendemic for cat-associated sporotrichosis. This study aimed to assess the virulence of serial Sporothrix isolates from a 61-year-old male patient with chronic, destructive disseminated sporotrichosis. Five Sporothrix isolates were cultured from skin exudates and bone samples over a 5-year period, and all were molecularly identified as Sporothrix brasiliensis. The final isolate was significantly more virulent in Galleria mellonella larvae compared to earlier isolates. We conclude that S. brasiliensis has the capacity to increase in virulence in vivo. This finding is significant to clinicians caring for individuals with S. brasiliensis disease and it suggests that further studies are needed to identify the mechanisms underlying pathogenicity enhancement during chronic disease.

The Candida albicans agglutinin-like sequence family of adhesins: functional insights gained from structural analysis

Candida albicans colonizes many host sites suggesting its interaction with diverse ligands. Candida albicans adhesion is mediated by a number of proteins including those in the Als (agglutinin-like sequence) family, which have been studied intensively. The recent solution of the Als binding domain structure ended years of speculation regarding the molecular mechanism for Als adhesive function. Als adhesins bind flexible C termini from a broad collection of proteins, providing the basis for adhesion to various cell types and perhaps for C. albicans broad tissue tropism. Understanding adhesive functions at the molecular level will reveal the sequence of events in C. albicans pathogenesis, from host recognition to complex interactions such as development of polymicrobial biofilms or disseminated disease.

Cellular Components Mediating Coadherence of Candida albicans and Fusobacterium nucleatum

Candida albicans is an opportunistic fungal pathogen found as part of the normal oral flora. It can be coisolated with Fusobacterium nucleatum, an opportunistic bacterial pathogen, from oral disease sites, such as those involved in refractory periodontitis and pulp necrosis. The physical coadherence between these 2 clinically important microbes has been well documented and suggested to play a role in facilitating their oral colonization and colocalization and contributing to polymicrobial pathogenesis. Previous studies indicated that the physical interaction between C. albicans and F. nucleatum was mediated by the carbohydrate components on the surface of C. albicans and the protein components on the Fusobaterium cell surface. However, the identities of the components involved still remain elusive. This study was aimed at identifying the genetic determinants involved in coaggregation between the 2 species. By screening a C. albicans SN152 mutant library and a panel of F. nucleatum 23726 outer membrane protein mutants, we identified FLO9, which encodes a putative adhesin-like cell wall mannoprotein of C. albicans and radD, an arginine-inhibitable adhesin-encoding gene in F. nucleatum that is involved in interspecies coadherence. Consistent with these findings, we demonstrated that the strong coaggregation between wild-type F. nucleatum 23726 and C. albicans SN152 in an in vitro assay could be greatly inhibited by arginine and mannose. Our study also suggested a complex multifaceted mechanism underlying physical interaction between C. albicans and F. nucleatum and for the first time revealed the identity of major genetic components involved in mediating the coaggregation. These observations provide useful knowledge for developing new targeted treatments for disrupting interactions between these 2 clinically relevant pathogens.

Hyphae-specific genes HGC1, ALS3, HWP1, and ECE1 and relevant signaling pathways in Candida albicans

Fungal virulence mechanisms include adhesion to epithelia, morphogenesis, production of secretory hydrolytic enzymes, and phenotype switching, all of which contribute to the process of pathogenesis. A striking feature of the biology of Candida albicans is its ability to grow in yeast, pseudohyphal, and hyphal forms. The hyphal form plays an important role in causing disease, by invading epithelial cells and causing tissue damage. In this review, we illustrate some of the main hyphae-specific genes, namely HGC1, UME6, ALS3, HWP1, and ECE1, and their relevant and reversed signal transduction pathways in reactions stimulated by environmental factors, including pH, CO2, and serum.

Effectiveness of coating acrylic resin dentures on preventing Candida adhesion

PURPOSE

The aim of this study was to prevent the adhesion of C. albicans on acrylic resin dentures by modifying their surfaces.

MATERIALS AND METHODS

Ninety acrylic resin plates were divided into three groups. Group I: conventionally processed acrylic resin plates. Group II: plates painted with 2-Octyl Cyanoacrylate adhesive. Group III: plates painted with Adper Single Bond Adhesive. All specimens were immersed separately in containers filled with artificial saliva that contained C. albicans and then incubated for 11 days at 37°C. Three methods of evaluation were used to count the adhered Candida: direct culture, slide count, and serial dilutions.

RESULTS

C. albicans in 1/10, 1/10², and 1/10³ dilutions showed overgrowth in group I, while overgrowth was noted only with 1/10 dilution in group III. For group III, mean colony numbers of 123, 22, 3.4, and 0 were found for the 1/10², 1/10³, 1/10⁴, and 1/10⁵ dilutions, respectively. Regarding the slide counts, group I showed a mean fungal count of 166 compared to 40 for group III with 1/10 dilution, 21 compared to 9 with 1/10³ dilution, 8.6 compared to 0.7 with 1/10³ dilution, and 1.2 compared to 0 with 1/10⁴ dilution. No plates in group II showed any candidal colonies regardless of the method of evaluation (0%). These differences were statistically significant (p < 0.0001).

CONCLUSION

Coating the acrylic resin dentures with Adper Single Bond Adhesive was effective in reducing C. albicans adhesion to dentures, while coating with 2-Octyl Cyanoacrylate adhesive completely inhibited such adhesion.

Surface control of blastospore attachment and ligand-mediated hyphae adhesion of Candida albicans

Controlling the adhesion of Candida albicans on surfaces by the selected ligand deconvolutes effects from multiple adhesins and nonspecific interactions.

Carbon source-induced reprogramming of the cell wall proteome and secretome modulates the adherence and drug resistance of the fungal pathogen Candida albicans

The major fungal pathogen Candida albicans can occupy diverse microenvironments in its human host. During colonization of the gastrointestinal or urogenital tracts, mucosal surfaces, bloodstream, and internal organs, C. albicans thrives in niches that differ with respect to available nutrients and local environmental stresses. Although most studies are performed on glucose-grown cells, changes in carbon source dramatically affect cell wall architecture, stress responses, and drug resistance. We show that growth on the physiologically relevant carboxylic acid, lactate, has a significant impact on the C. albicans cell wall proteome and secretome. The regulation of cell wall structural proteins (e.g. Cht1, Phr1, Phr2, Pir1) correlated with extensive cell wall remodeling in lactate-grown cells and with their increased resistance to stresses and antifungal drugs, compared with glucose-grown cells. Moreover, changes in other proteins (e.g. Als2, Gca1, Phr1, Sap9) correlated with the increased adherence and biofilm formation of lactate-grown cells. We identified mating and pheromone-regulated proteins that were exclusive to lactate-grown cells (e.g. Op4, Pga31, Pry1, Scw4, Yps7) as well as mucosa-specific and other niche-specific factors such as Lip4, Pga4, Plb5, and Sap7. The analysis of the corresponding null mutants confirmed that many of these proteins contribute to C. albicans adherence, stress, and antifungal drug resistance. Therefore, the cell wall proteome and secretome display considerable plasticity in response to carbon source. This plasticity influences important fitness and virulence attributes known to modulate the behavior of C. albicans in different host microenvironments during infection.

Candida albicans: A Model Organism for Studying Fungal Pathogens

Candida albicans is an opportunistic human fungal pathogen that causes candidiasis. As healthcare has been improved worldwide, the number of immunocompromised patients has been increased to a greater extent and they are highly susceptible to various pathogenic microbes and C. albicans has been prominent among the fungal pathogens. The complete genome sequence of this pathogen is now available and has been extremely useful for the identification of repertoire of genes present in this pathogen. The major challenge is now to assign the functions to these genes of which 13% are specific to C. albicans. Due to its close relationship with yeast Saccharomyces cerevisiae, an edge over other fungal pathogens because most of the technologies can be directly transferred to C. albicans from S. cerevisiae and it is amenable to mutation, gene disruption, and transformation. The last two decades have witnessed enormous amount of research activities on this pathogen that leads to the understanding of host-parasite interaction, infections, and disease propagation. Clearly, C. albicans has emerged as a model organism for studying fungal pathogens along with other two fungi Aspergillus fumigatus and Cryptococcus neoformans. Understanding its complete life style of C. albicans will undoubtedly be useful for developing potential antifungal drugs and tackling Candida infections. This will also shed light on the functioning of other fungal pathogens.

LL37 and hBD-3 elevate the β-1,3-exoglucanase activity of Candida albicans Xog1p, resulting in reduced fungal adhesion to plastic

The opportunistic fungus Candida albicans causes oral thrush and vaginal candidiasis, as well as candidaemia in immunocompromised patients including those undergoing cancer chemotherapy, organ transplant and those with AIDS. We previously found that the AMPs (antimicrobial peptides) LL37 and hBD-3 (human β-defensin-3) inhibited C. albicans viability and its adhesion to plastic. For the present study, the mechanism by which LL37 and hBD-3 reduced C. albicans adhesion was investigated. After AMP treatment, C. albicans adhesion to plastic was reduced by up to ~60% and was dose-dependent. Our previous study indicated that LL37 might interact with the cell-wall β-1,3-exoglucanase Xog1p, which is involved in cell-wall β-glucan metabolism, and consequently the binding of LL37 or hBD-3 to Xog1p might cause the decrease in adhesion. For the present study, Xog1p(41-438)-6H, an N-terminally truncated, active, recombinant construct of Xog1p and Xog1p fragments were produced and used in pull-down assays and ELISA in vitro, which demonstrated that all constructs interacted with both AMPs. Enzymatic analyses showed that LL37 and hBD-3 enhanced the β-1,3-exoglucanase activity of Xog1p(41-438)-6H approximately 2-fold. Therefore elevated Xog1p activity might compromise cell-wall integrity and decrease C. albicans adhesion. To test this hypothesis, C. albicans was treated with 1.3 μM Xog1p(41-438)-6H and C. albicans adhesion to plastic decreased 47.7%. Taken together, the evidence suggests that Xog1p is one of the LL37/hBD-3 targets, and elevated β-1,3-exoglucanase activity reduces C. albicans adhesion to plastic.

Structural basis for the broad specificity to host-cell ligands by the pathogenic fungus Candida albicans

Candida albicans is the most prevalent fungal pathogen in humans and a major source of life-threatening nosocomial infections. The Als (agglutinin-like sequence) glycoproteins are an important virulence factor for this fungus and have been associated with binding of host-cell surface proteins and small peptides of random sequence, the formation of biofilms and amyloid fibers. High-resolution structures of N-terminal Als adhesins (NT-Als; up to 314 amino acids) show that ligand recognition relies on a motif capable of binding flexible C termini of peptides in extended conformation. Central to this mechanism is an invariant lysine that recognizes the C-terminal carboxylate of ligands at the end of a deep-binding cavity. In addition to several protein-peptide interactions, a network of water molecules runs parallel to one side of the ligand and contributes to the recognition of diverse peptide sequences. These data establish NT-Als adhesins as a separate family of peptide-binding proteins and an unexpected adhesion system for primary, widespread protein-protein interactions at the Candida/host-cell interface.

Photodynamic therapy with Pc 4 induces apoptosis of Candida albicans

The high prevalence of drug resistance necessitates the development of novel antifungal agents against infections caused by opportunistic fungal pathogens, such as Candida albicans. Elucidation of apoptosis in yeast-like fungi may provide a basis for future therapies. In mammalian cells, photodynamic therapy (PDT) has been demonstrated to generate reactive oxygen species, leading to immediate oxidative modifications of biological molecules and resulting in apoptotic cell death. In this report, we assess the in vitro cytotoxicity and mechanism of PDT, using the photosensitizer Pc 4, in planktonic C. albicans. Confocal image analysis confirmed that Pc 4 localizes to cytosolic organelles, including mitochondria. A colony formation assay showed that 1.0 μM Pc 4 followed by light at 2.0 J cm(-2) reduced cell survival by 4 logs. XTT (2,3-bis[2-methoxy-4-nitro-5-sulfophenyl]-2H-tetrazolium-5-carboxyanilide) assay revealed that Pc 4-PDT impaired fungal metabolic activity, which was confirmed using the FUN-1 (2-chloro-4-[2,3-dihydro-3-methyl-(benzo-1,3-thiazol-2-yl)-methylidene]-1-phenylquinolinium iodide) fluorescence probe. Furthermore, we observed changes in nuclear morphology characteristic of apoptosis, which were substantiated by increased externalization of phosphatidylserine and DNA fragmentation following Pc 4-PDT. These data indicate that Pc 4-PDT can induce apoptosis in C. albicans. Therefore, a better understanding of the process will be helpful, as PDT may become a useful treatment option for candidiasis.

In vitro adherence of Candida albicans isolated from patients with chronic periodontitis

Adherence is considered an extremely important virulence factor in yeast.

OBJECTIVE

The aim of this study was to analyze the adherence to epithelial cells of C. albicans isolated from patients with chronic periodontitis in comparison to healthy patients.

MATERIAL AND METHODS

Candida albicans cells isolated from individuals with chronic periodontitis (n=25) and healthy controls (n=25) were included in this study. Suspensions of C. albicans (10(6) cells/mL) and epithelial cells (10(5) cells/mL) were mixed and incubated at 37ºC for 1 h. The number of yeasts adhered to 25 epithelial cells was counted.

RESULTS

The number of C. albicans cells adhered to epithelial cells was statistically higher in the chronic periodontitis group than in the control group (Student's t-test, p=0.000).

CONCLUSION

The results of the present study suggest a higher Candida adherence of samples isolated from patients with chronic periodontitis.

Candida species differ in their interactions with immature human gastrointestinal epithelial cells

Life-threatening gastrointestinal (GI) diseases of prematurity are highly associated with systemic candidiasis. This implicates the premature GI tract as an important site for invasion by Candida. Invasive interactions of Candida spp. with immature enterocytes have heretofore not been analyzed. Using a primary immature human enterocyte line, we compared the ability of multiple isolates of different Candida spp. to penetrate, injure, and induce a cytokine response from host cells. Of all the Candida spp. analyzed, C. albicans had the greatest ability to penetrate and injure immature enterocytes and to elicit IL-8 release (p < 0.01). In addition, C. albicans was the only Candida spp. to form filamentous hyphae when in contact with immature enterocytes. Similarly, a C. albicans mutant with defective hyphal morphogenesis and invasiveness had attenuated cytotoxicity for immature enterocytes (p < 0.003). Thus, hyphal morphogenesis correlates with immature enterocyte penetration, injury, and inflammatory responses. Furthermore, variability in enterocyte injury was observed among hyphal-producing C. albicans strains, suggesting that individual organism genotypes also influence host-pathogen interactions. Overall, the finding that Candida spp. differed in their interactions with immature enterocytes implicates that individual spp. may use different pathogenesis mechanisms.

Activation of focal adhesion kinase enhances the adhesion of Fusarium solani to human corneal epithelial cells via the tyrosine-specific protein kinase signaling pathway

PURPOSE

To determine the role of the integrin-FAK signaling pathway triggered by the adherence of F. solani to human corneal epithelial cells (HCECs).

METHODS

After pretreatment with/without genistein, HCECs were incubated with F. solani spores at different times (0-24 h). Cell adhesion assays were performed by optical microscopy. Changes of the ultrastructure were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The expression of F-actin and Paxillin (PAX) were detected by immunofluorescence and western blotting to detect the expression of these key proteins with/without genistein treatment.

RESULTS

Cell adhesion assays showed that the number of adhered spores began to rise at 6 h after incubation and peaked at 8 h. SEM and TEM showed that the HCECs exhibited a marked morphological alteration induced by the attachment and entry of the spores. The expression of PAX increased, while the expression of F-actin decreased by stimulation with F. solani. The interaction of F. solani with HCECs causes actin rearrangement in HCECs. Genistein strongly inhibited FAK phosphorylation and the activation of the downstream protein (PAX). F. solani-induced enhancement of cell adhesion ability was inhibited along with the inhibition of FAK phosphorylation.

CONCLUSIONS

Our results suggest that the integrin-FAK signaling pathway is involved in the control of F. solani adhesion to HCECs and that the activation of focal adhesion kinase enhances the adhesion of human corneal epithelial cells to F. solani via the tyrosine-specific protein kinase signaling pathway.

Thymol-induced alterations in Candida albicans imaged by atomic force microscopy

Thymol, a constituent of thyme essential oil that has been credited with interesting antimicrobial and antifungal effects, acts by interfering with the envelope of Candida albicans and this activity has been investigated by means of atomic force microscopy (AFM). Candida culture samples incubated with 1, 1/2, and 1/4 MIC of thymol or vehicle were taken at time 0 and after 1, 2, and 4 h, the envelopes of 100 cells in each of five randomly chosen fields were analysed by means of AFM. Our AFM findings show that thymol affects the envelope of C. albicans cells. The cells showed major morphostructural deformities with envelope damage becoming greater at increasing thymol concentrations and longer times of incubation, including the number of flattened cells with surface folds, cells with holes, and collapsed cells and ghosts. Thymol is an amphipathic monoterpene, which suggests that it affects cell membrane structure by generating asymmetries and membrane tensions. This is confirmed by the fact that terpenes alter cell permeability by entering between the fatty acyl chains making up the membrane lipid bilayers, disrupting lipid packing, and changing membrane fluidity. All of these phenomena lead to major surface alterations and deformities that also reduce the ability of fungi to adhere to mucosal cells, and decrease their virulence and infectiousness.

Influence of mannan and glucan on complement activation and C3 binding by Candida albicans

The complement system is important for host resistance to hematogenously disseminated candidiasis. However, modulation of complement activation by cell wall components of Candida albicans has not been characterized. Although intact yeast display mannan on the surface, glucan, typically located in the interior, becomes exposed during C. albicans infection. We show here the distinct effects of mannan and glucan on complement activation and opsonophagocytosis. Previous studies showed that intact cells are resistant to initiation of complement activation through the alternative pathway, and antimannan antibody reverses this resistance via an Fc-independent mechanism. The present study shows that this mannan-dependent resistance can be overcome by periodate-borohydride conversion of mannose polysaccharides to polyalcohols; cells treated with periodate-borohydride initiate the alternative pathway without the need for antibody. These observations identify an inhibitory role for intact mannan in complement activation. Next, removal of the surface-displayed mannan by acid treatment of periodate-borohydride cells exposes glucan. Glucan-displaying cells or purified beta-glucan initiate the alternative pathway when incubated with the purified proteins of the alternative pathway alone, suggesting that C. albicans glucan is a natural activator of the alternative pathway. Finally, ingestion of mannan-displaying cells by human neutrophils requires anti-mannan antibody, whereas ingestion of glucan-displaying cells requires complement. These results demonstrate a contrasting requirement of natural antibody and complement for opsonophagocytosis of C. albicans cells displaying mannan or glucan. Thus, differential surface expression of mannan and glucan may influence recognition of C. albicans by the complement system.

CYP56 (Dit2p) in Candida albicans: characterization and investigation of its role in growth and antifungal drug susceptibility

The complete DNA sequence of Candida albicans DIT2, encoding cytochrome P450 family 56 (CYP56), was obtained, and heterologous expression was achieved in Escherichia coli, where CYP56 was targeted to the membrane fraction. In reconstituted assays with the purified enzyme, CYP56 was shown to catalyze the conversion of N-formyl tyrosine into N,N'-bisformyl dityrosine, a reaction that was dependent on cytochrome P450 reductase, NADPH, and oxygen, yielding a turnover of 21.6 min(-1) and a k(s) of 26 microM. The Hill number was calculated as 1.6, indicating that two molecules of the substrate could bind to the protein. Azole antifungals could bind to the heme of CYP56 as a sixth ligand with high affinity. Both chromosomal alleles of CYP56 were disrupted using the SAT1 flipper technique, and CYP56 was found to be nonessential for cell viability under the culture conditions investigated. Susceptibility to azole drugs that bind to cytochromes P450 was tested, and the mutant showed unaltered susceptibility. However, the mutant showed increased susceptibility to the echinocandin drug caspofungin, suggesting an alteration in 1,3-glucan synthase and/or cell wall structure mediated by the presence of dityrosine. Phenotypically, the wild-type and mutant strains were morphologically similar when cultured in rich yeast extract-peptone-dextrose medium. However in minimal medium, the cyp56Delta mutant strain exhibited hyphal growth, in contrast to the wild-type strain, which grew solely in the yeast form. Furthermore, CYP56 was essential for chlamydospore formation.

Antifungals: need to search for a new molecular target

In the 1990s, drug resistance has become an important problem in a variety of infectious diseases including human immunodeficiency virus infection, tuberculosis, and other bacterial infections which have profound effects on human health. At the same time, there have been dramatic increase in the incidence of fungal infections, which are probably the result of alterations in immune status associated with the acquired immuno deficiency syndrome epidemic, cancer chemotherapy, and organ and bone marrow transplantation. The rise in the incidence of fungal infections has exacerbated the need for the next generation of antifungal agents, since many of the currently available drugs have undesirable side effects, are ineffective against new or reemerging fungi, or lead to the rapid development of the resistance. This review will focus on the pathogenic yeast Candida albicans, since a large body of work on the factors and mechanism associated with antifungal drug resistance in this organism is reported sufficiently. It will certainly elaborate the probable molecular targets for drug design, discovered to date.

Increase of virulence and its phenotypic traits in drug-resistant strains of Candida albicans

There is concern about the rise of antifungal drug resistance, but little is known about comparative biological properties and pathogenicity of drug-resistant strains. We generated fluconazole (FLC; CO23 RFLC)- or micafungin (FK; CO23 RFK)-resistant strains of Candida albicans by treating a FLC- and FK-susceptible strain of this fungus (CO23 S) with stepwise-increasing concentrations of either drug. Molecular analyses showed that CO23 RFLC had acquired markedly increased expression of the drug-resistance efflux pump encoded by the MDR1 gene, whereas CO23 RFK had a homozygous mutation in the FSK1 gene. These genetic modifications did not alter to any extent the growth capacity of the drug-resistant strains in vitro, either at 28 degrees C or at 37 degrees C, but markedly increased their experimental pathogenicity in a systemic mouse infection model, as assessed by the overall mortality and target organ invasion. Interestingly, no apparent increase in the vaginopathic potential of the strains was observed with an estrogen-dependent rat vaginal infection. The increased pathogenicity of drug-resistant strains for systemic infection was associated with a number of biochemical and physiological changes, including (i) marked cellular alterations associated with a different expression and content of major cell wall polysaccharides, (ii) more rapid and extensive hypha formation in both liquid and solid media, and (iii) increased adherence to plastic and a propensity for biofilm formation. Overall, our data demonstrate that experimentally induced resistance to antifungal drugs, irrespective of drug family, can substantially divert C. albicans biology, affecting in particular biological properties of potential relevance for deep-seated candidiasis.

Dectin-1 is required for beta-glucan recognition and control of fungal infection

Beta-glucan is one of the most abundant polysaccharides in fungal pathogens, yet its importance in antifungal immunity is unclear. Here we show that deficiency of dectin-1, the myeloid receptor for beta-glucan, rendered mice susceptible to infection with Candida albicans. Dectin-1-deficient leukocytes demonstrated significantly impaired responses to fungi even in the presence of opsonins. Impaired leukocyte responses were manifested in vivo by reduced inflammatory cell recruitment after fungal infection, resulting in substantially increased fungal burdens and enhanced fungal dissemination. Our results establish a fundamental function for beta-glucan recognition by dectin-1 in antifungal immunity and demonstrate a signaling non-Toll-like pattern-recognition receptor required for the induction of protective immune responses.

Fungal keratitis at the new york eye and ear infirmary

INTRODUCTION

This study was designed to review the clinical experience at our institution with fungal keratitis during a 16-year period.

MATERIALS AND METHODS

A review of the clinical and microbiology records of the New York Eye and Infirmary identified 61cases of fungal keratitis in 57 patients between January 1, 1987 and June 1, 2003. The medical records of all patients were retrospectively reviewed to better delineate patient demographics, risk factors, etiologic organisms, treatment, and outcomes.

RESULTS

A total of 5083 positive corneal cultures were recorded'from January 1, 1987 to June 1, 2003. Sixty-one eyes in 57 patients (37 women) were positive for fungus (1.2%). Three'patients had bilateral simultaneous infections. Candida albicans accounted for 29 of 61 cases (48%). Human immunodeficiency virus (HIV) seropositivity (15 eyes), chronic ocular surface disease (14 eyes), and trauma (7 eyes) were the most commonly associated risk factors.

CONCLUSIONS

Our experience with fungal keratitis in the northeastern United States appears to be different than those reported from other areas of the United States. Serologic positivity for HIV and chronic ocular surface disease were the most common associated risk factors followed by trauma, herpes simplex keratitis, and contact lens use. Candida species predominated, whereas filamentous fungi were uncommon.

MNN5 encodes an iron-regulated alpha-1,2-mannosyltransferase important for protein glycosylation, cell wall integrity, morphogenesis, and virulence in Candida albicans

The cell walls of microbial pathogens mediate physical interactions with host cells and hence play a key role in infection. Mannosyltransferases have been shown to determine the cell wall properties and virulence of the pathogenic fungus Candida albicans. We previously identified a C. albicans alpha-1,2-mannosyltransferase, Mnn5, for its novel ability to enhance iron usage in Saccharomyces cerevisiae. Here we have studied the enzymatic properties of purified Mnn5 and characterized its function in its natural host. Mnn5 catalyzes the transfer of mannose to both alpha-1,2- and alpha-1,6-mannobiose, and this activity requires Mn2+ as a cofactor and is regulated by the Fe2+ concentration. An mnn5Delta mutant showed a lowered ability to extend O-linked, and possibly also N-linked, mannans, hypersensitivity to cell wall-damaging agents, and a reduction of cell wall mannosylphosphate content, phenotypes typical of many fungal mannosyltransferase mutants. The mnn5Delta mutant also exhibited some unique defects, such as impaired hyphal growth on solid media and attenuated virulence in mice. An unanticipated phenotype was the mnn5Delta mutant's resistance to killing by the iron-chelating protein lactoferrin, rendering it the first protein found that mediates lactoferrin killing of C. albicans. In summary, MNN5 deletion impairs a wide range of cellular events, most likely due to its broad substrate specificity. Of particular interest was the observed role of iron in regulating the enzymatic activity, suggesting an underlying relationship between Mnn5 activity and cellular iron homeostasis.

Proteomic analysis ofCandida albicans yeast and hyphal cell wall and associated proteins

Candida albicans is an important human pathogen that causes systemic infections, predominantly among populations with weakened immune systems. The morphological transition from the yeast to the hyphal state is one of the key factors in C. albicans pathogenesis. Owing to their location at the host-pathogen interface, the cell wall and associated proteins are of interest, especially with respect to the yeast to hyphal transition. This study entailed the proteomic analysis of differentially regulated proteins involved in this transition. The protein profiles of C. albicans DTT/SDS-extractible proteins and the cyanogen bromide (CNBr)/trypsin-extractable proteins of a cell wall-enriched fraction from yeast and hyphae were compared. In total, 107 spots were identified from the DTT/SDS-extractible cell wall-enriched fraction, corresponding to 82 unique proteins. Of these DTT/SDS-extractible proteins, 14 proteins were upregulated and 10 were downregulated in response to hyphal induction. Approximately 6-9% of total cell wall-protein-enriched fraction was found to be resistant to DTT/SDS extraction. Analysis of the DTT/SDS-resistant fraction using a CNBr/trypsin extraction resulted in the identification of 29 proteins. Of these, 17 were identified only in the hyphae, four were identified only in the yeast, and eight were identified in both the yeast and hyphae.

Quantification of ALS1 gene expression in Candida albicans biofilms by RT-PCR using hybridisation probes on the LightCycler™

The fungal pathogen Candida albicans has the ability to grow as a biofilm on synthetic materials. This presents a significant problem in the clinical situation when the organism grows as a biofilm on medical devices resulting in infections which are resistant to antifungal agents. Determining the extent to which certain genes are involved in biofilm formation is an important aspect for the development of strategies to control pathogenic biofilms. ALS1 is a member of the ALS (agglutinin-like sequence) family, the protein products of which are implicated in attachment to endothelial cells and biofilm formation. The expression of ALS1 in biofilms grown on silicone elastomer, a material used in the manufacture of medical devices, and planktonically grown cells was investigated using a novel real-time quantitative reverse transcriptase PCR (q-RT PCR) on the LightCycler. This study demonstrates quantitatively that ALS1 is clearly up-regulated during biofilm growth. The real-time q-RT PCR assay described here has the potential to be used as an indicator of biofilm formation on medical devices.

ABG1, a novel and essential Candida albicans gene encoding a vacuolar protein involved in cytokinesis and hyphal branching

Immunoscreening of a Candida albicans expression library resulted in the isolation of a novel gene encoding a 32.9-kDa polypeptide (288 amino acids), with 27.7% homology to the product of Saccharomyces cerevisiae YGR106c, a putative vacuolar protein. Heterozygous mutants in this gene displayed an altered budding growth pattern, characterized by the formation of chains of buds, decreasingly in size towards the apex, without separation of the daughter buds. Consequently, this gene was designated ABG1. A conditional mutant for ABG1 with the remaining allele under the control of the MET3 promoter did not grow in the presence of methionine and cysteine, demonstrating that ABG1 was essential for viability. Western analysis revealed the presence of a major 32.9-kDa band, mainly in a particulate fraction (P40) enriched in vacuoles, and tagging with green fluorescent protein confirmed that Abg1p localized to the vacuole. Vacuole inheritance has been linked to the regulation of branching frequency in C. albicans. Under repressing conditions, the conditional mutant had an increased frequency of branching under hyphal inducing conditions and an altered sensitivity to substances that interfered with cell wall assembly. Repression of ABG1 in the conditional mutant strain caused disturbance of normal size and number of vacuoles both in yeast and mycelial cells and also in the asymmetric vacuole inheritance associated with the characteristic pattern of germ tubes and branching in C. albicans. These observations indicate that ABG1 plays a key role in vacuole biogenesis, cytokinesis, and hyphal branching.

Analysis of the Candida albicans Als2p and Als4p adhesins suggests the potential for compensatory function within the Als family

The ALS (agglutinin-like sequence) gene family encodes eight large cell-surface glycoproteins. The work presented here focuses on Als2p and Als4p, and is part of a larger effort to deduce the function of each Als protein. Both ALS4 alleles were deleted from the Candida albicans genome and the phenotype of the mutant strain (als4Delta/als4Delta; named 2034) studied. Loss of Als4p slowed germ tube formation of cells grown in RPMI 1640 medium and resulted in decreased adhesion of C. albicans to vascular endothelial cells. Loss of Als4p did not affect adhesion to buccal epithelial cells, biofilm formation in a catheter model, or adhesion to or destruction of oral reconstituted human epithelium (RHE). Although deletion of one ALS2 allele was achieved readily, a strain lacking the second allele was not identified despite screening thousands of transformants. The remaining ALS2 allele was placed under control of the C. albicans MAL2 promoter to create an als2Delta/PMAL2-ALS2 strain (named 2342). Real-time RT-PCR analysis of strain 2342 grown in glucose-containing medium (non-inducing conditions) showed that although ALS2 transcript levels were greatly reduced compared to wild-type cells, some ALS2 transcript remained. The decreased ALS2 expression levels were sufficient to slow germ tube formation in RPMI 1640 and Lee medium, reduce adhesion to vascular endothelial cells and to RHE, decrease RHE destruction, and impair biofilm formation. Growth of strain 2342 in maltose-containing medium (inducing conditions) restored the wild-type phenotype in all assays. Real-time RT-PCR analysis demonstrated that in maltose-containing medium, strain 2342 overexpressed ALS2 compared to wild-type cells; however no overexpression phenotype was apparent. Microarray analysis revealed little transcriptional response to ALS4 deletion, but showed twofold up-regulation of orf19.4765 in the glucose-medium-grown als2Delta/PMAL2-ALS2 strain. orf19.4765 encodes a protein with features of a glycosylated cell wall protein with similarity to Saccharomyces cerevisiae Ccw12p, although initial analysis suggested functional differences between the two proteins. Real-time RT-PCR measurement of ALS2 and ALS4 transcript copy number showed a 2.8-fold increase in ALS2 expression in the als4Delta/als4Delta strain and a 3.2-fold increase in ALS4 expression in the als2Delta/PMAL2-ALS2 strain, suggesting the potential for compensatory function between these related proteins.

Dectin-1 mediates macrophage recognition of Candida albicans yeast but not filaments

The ability of Candida albicans to rapidly and reversibly switch between yeast and filamentous morphologies is crucial to pathogenicity, and it is thought that the filamentous morphology provides some advantage during interaction with the mammalian immune system. Dectin-1 is a receptor that binds beta-glucans and is important for macrophage phagocytosis of fungi. The receptor also collaborates with Toll-like receptors for inflammatory activation of phagocytes by fungi. We show that yeast cell wall beta-glucan is largely shielded from Dectin-1 by outer wall components. However, the normal mechanisms of yeast budding and cell separation create permanent scars which expose sufficient beta-glucan to trigger antimicrobial responses through Dectin-1, including phagocytosis and activation of reactive oxygen production. During filamentous growth, no cell separation or subsequent beta-glucan exposure occurs, and the pathogen fails to activate Dectin-1. The data demonstrate a mechanism by which C. albicans shape alone directly contributes to the method by which phagocytes recognize the fungus.

Inhibition of adherence and killing of Candida albicans with a 23-Mer peptide (Fn/23) with dual antifungal properties

Candida albicans adheres to host tissue and then proliferates in order to establish a commensal as well as a pathogenic state. Specific adherence to proteins is provided by several surface adhesins of Candida. Two well-studied proteins, Als1p and Als5p, do not require energy for adherence to occur (dead as well as living cells adhere) and have a multiplier effect of cell-cell aggregation that mediates the formation of microcolonies of Candida cells. The entire process is spontaneous, reversible, and stable for physiologically relevant chemical and physical forces. This adherence process is inhibited by the addition of free peptide ligands, including a 23-mer derived from fibronectin (Fn/23) that binds to the adhesins through H bond formation. Adherence was measured by determining the number of yeast cells that adhered to 90-microm-diameter polyethylene glycol (PEG) beads with a 7-mer peptide (KLRIPSV) synthesized on the surfaces of the beads. The concentration of the Fn/23 peptide that inhibited the adherence of cells to the peptide-coated beads by 50% was 4 to 5 microM, and the magnitudes of adherence were similar regardless of the presence or absence of physiologic salt concentrations. The minimum fungicidal concentration of Fn/23 was 2 to 4 microM in water, but there was no killing in physiologic salt concentrations. Peptides from the C and N termini or the center sequence of Fn/23 had no effect on inhibition of adherence and little effect on fungal viability. The fungicidal effect was similar to that seen with 23-, 19-, and 18-mer peptides derived from porcine myeloid cells, a Helicobacter pylori ribosomal protein, and a hybrid of cecropin and magainin, respectively. However, these fungicidal peptides did not inhibit C. albicans adherence to the peptide-coated PEG beads. This dual property of Fn/23, i.e., inhibition of adherence and killing of C. albicans, may provide important adjuvant effects in the treatment of disease caused by this fungus.

The hyphal and yeast forms of Candida albicans bind the complement regulator C4b-binding protein

Candida albicans, an important pathogenic yeast, activates all three pathways of the complement system. To understand how this yeast evades the effects of the activated system, we have analyzed the binding of the classical pathway inhibitor C4b-binding protein (C4BP) by C. albicans. Purified native as well as recombinant C4BP bound dose dependently to the yeast and hyphal forms, as shown by multiple methods, such as confocal microscopy, flow cytometry, a novel enzyme-linked immunosorbent assay, absorption from human serum, and direct binding assays with purified proteins. A prominent binding site was identified at the tip of the germ tube, a structure that is considered important for tissue penetration and pathogenesis. The binding site in C4BP was localized to the two N-terminal complement control protein domains by using recombinant deletion constructs and site-specific monoclonal antibodies. As the alternative pathway inhibitors factor H and FHL-1 also bind to C. albicans, the binding of all three plasma proteins was compared. Simultaneous binding of the classical regulator C4BP and the alternative pathway regulator factor H was demonstrated by confocal microscopy. In addition, FHL-1 competed for binding with C4BP, suggesting that these two related complement regulators bind to the same structures on the yeast surface. The surface-attached C4BP maintains its complement regulatory activities and inactivates C4b. The surface-attached human C4BP serves multiple functions relevant for immune evasion and likely pathogenicity. It inhibits complement activation at the yeast surface and, in addition, mediates adhesion of C. albicans to host endothelial cells.

The ATC1 Gene Encodes a Cell Wall-linked Acid Trehalase Required for Growth on Trehalose in Candida albicans

After screening a Candida albicans genome data base, the product of an open reading frame (IPF 19760/CA2574) with 41% identity to Saccharomyces cerevisiae vacuolar acid trehalase (Ath1p) was identified and named Atc1p. The deduced amino acid sequence shows that Atc1p contains an N-terminal hydrophobic signal peptide and 20 potential sites for N-glycosylation. C. albicans homozygous mutants that lack acid trehalase activity were constructed by gene disruption at the two ATC chromosomal alleles. Analysis of these null mutants shows that Atc1p is localized in the cell wall and is required for growth on trehalose as a carbon source. An Atc1p endowed with acid trehalase activity was obtained by an in vtro transcription-translation coupled system. These results strongly suggest that ATC1 is the structural gene encoding cell wall acid trehalase in C. albicans. Determinations of ATC1 mRNA expression as well as acid trehalase activity in the presence and absence of glucose point out that ATC1 gene is regulated by glucose repression.

Antibody response toCandida albicanscell wall antigens

The cell wall of Candida albicans is not only the structure where many essential biological functions reside but is also a significant source of candidal antigens. The major cell wall components that elicit a response from the host immune system are proteins and glycoproteins, the latter being predominantly mannoproteins. Both carbohydrate and protein moieties are able to trigger immune responses. Proteins and glycoproteins exposed at the most external layers of the wall structure are involved in several types of interactions of fungal cells with the exocellular environment. Thus, coating of fungal cells with host antibodies has the potential to profoundly influence the host-parasite interaction by affecting antibody-mediated functions such as opsonin-enhanced phagocytosis and blocking the binding activity of fungal adhesins to host ligands. In this review we examine various members of the protein and glycoprotein fraction of the C. albicans cell wall that elicit an antibody response in vivo. Some of the studies demonstrate that certain cell wall antigens and anti-cell wall antibodies may be the basis for developing specific and sensitive serologic tests for the diagnosis of candidiasis, particularly the disseminated form. In addition, recent studies have focused on the potential of antibodies against the cell wall protein determinants in protecting the host against infection. Hence, a better understanding of the humoral response triggered by the cell wall antigens of C. albicans may provide the basis for the development of (i) effective procedures for the serodiagnosis of disseminated candidiasis, and (ii) novel prophylactic (vaccination) and therapeutic strategies to control this type of infections.

Degenerate peptide recognition by Candida albicans adhesins Als5p and Als1p

Candida albicans and Saccharomyces cerevisiae expressing the adhesins Als5p or Als1p adhere to immobilized peptides and proteins that possess appropriate sequences of amino acids in addition to a sterically accessible peptide backbone. In an attempt to further define the nature of these targets, we surveyed the ability of yeast cells to adhere to 90- micro m-diameter polyethylene glycol beads coated with a 7-mer peptide from a library of 19(7) unique peptide-beads. C. albicans bound to ca. 10% of beads from the library, whereas S. cerevisiae expressing Als5p or Als1p bound to ca. 0.1 to 1% of randomly selected peptide-beads. S. cerevisiae expressing Als1p had a distinctly different adherence phenotype than did cells expressing Als5p. The former adhered in groups or clumps of cells, whereas the latter adhered initially as single cells, an event which was followed by the build up of cell-cell aggregates. Beads with adherent cells were removed, and the peptide attached to the bead was determined by amino acid sequencing. All adhesive beads carried a three-amino-acid sequence motif (tau phi+) that possessed a vast combinatorial potential. Adherence was sequence specific and was inhibited when soluble peptide identical to the immobilized peptide was added. The Als5p adhesin recognized some peptides that went unrecognized by Als1p. The sequence motif of adhesive peptides identified by this method is common in proteins and offers so many possible sequence combinations that target recognition by the Als proteins is clearly degenerate. A degenerate recognition system provides the fungi with the potential of adhering to a multitude of proteins and peptides, an advantage for any microorganism attempting to establish a commensal or pathogenic relationship with a host.

Surface glycans of Candida albicans and other pathogenic fungi: physiological roles, clinical uses, and experimental challenges

Although fungi have always been with us as commensals and pathogens, fungal infections have been increasing in frequency over the past few decades. There is a growing body of literature describing the involvement of carbohydrate groups in various aspects of fungal disease. Carbohydrates comprising the cell wall or capsule, or as a component of glycoproteins, are the fungal cell surface entities most likely to be exposed to the surrounding environment. Thus, the fungus-host interaction is likely to involve carbohydrates before DNA, RNA, or even protein. The interaction between fungal and host cells is also complex, and early studies using whole cells or crude cell fractions often produced seemingly conflicting results. What was needed, and what has been developing, is the ability to identify specific glycan structures and determine how they interact with immune system components. Carbohydrate analysis is complicated by the complexity of glycan structures and by the challenges of separating and detecting carbohydrates experimentally. Advances in carbohydrate chemistry have enabled us to move from the foundation of composition analysis to more rapid characterization of specific structures. This, in turn, will lead to a greater understanding of how fungi coexist with their hosts as commensals or exist in conflict as pathogens.

CHS8-a fourth chitin synthase gene of Candida albicans contributes to in vitro chitin synthase activity, but is dispensable for growth

In silico analysis of the genome sequence of the human pathogenic fungus Candida albicans identified an open reading frame encoding a putative fourth member of the chitin synthase gene family. This gene, named CaCHS8, encodes an 1105 amino acid open reading frame with the conserved motifs characteristic of class I zymogenic chitin synthases with closest sequence similarity to the non-essential C. albicans class I CHS2 gene. Although the CaCHS8 gene was expressed in both yeast and hyphal cells, homozygous chs8 Delta null mutants had normal growth rates, cellular morphologies and chitin contents. The null mutant strains had a 25% reduction in chitin synthase activity and were hypersensitive to Calcofluor White. A chs2 Delta chs8 Delta double mutant had less than 3% of normal chitin synthase activity and had increased wall glucan and decreased mannan but was unaffected in growth or cell morphology. The C. albicans class I double mutant did not exhibit a bud-lysis phenotype as found in the class I chs1 Delta mutant of Saccharomyces cerevisiae. Therefore, C. albicans has four chitin synthases with two non-essential class I Chs isoenzymes that contribute collectively to more than 97% of the in vitro chitin synthase activity.

Candida albicans secreted aspartyl proteinases in virulence and pathogenesis

Candida albicans is the most common fungal pathogen of humans and has developed an extensive repertoire of putative virulence mechanisms that allows successful colonization and infection of the host under suitable predisposing conditions. Extracellular proteolytic activity plays a central role in Candida pathogenicity and is produced by a family of 10 secreted aspartyl proteinases (Sap proteins). Although the consequences of proteinase secretion during human infections is not precisely known, in vitro, animal, and human studies have implicated the proteinases in C. albicans virulence in one of the following seven ways: (i) correlation between Sap production in vitro and Candida virulence, (ii) degradation of human proteins and structural analysis in determining Sap substrate specificity, (iii) association of Sap production with other virulence processes of C. albicans, (iv) Sap protein production and Sap immune responses in animal and human infections, (v) SAP gene expression during Candida infections, (vi) modulation of C. albicans virulence by aspartyl proteinase inhibitors, and (vii) the use of SAP-disrupted mutants to analyze C. albicans virulence. Sap proteins fulfill a number of specialized functions during the infective process, which include the simple role of digesting molecules for nutrient acquisition, digesting or distorting host cell membranes to facilitate adhesion and tissue invasion, and digesting cells and molecules of the host immune system to avoid or resist antimicrobial attack by the host. We have critically discussed the data relevant to each of these seven criteria, with specific emphasis on how this proteinase family could contribute to Candida virulence and pathogenesis.

The yeast Candida albicans binds complement regulators factor H and FHL-1

The human facultative pathogenic yeast Candida albicans causes mucocutaneous infections and is the major cause of opportunistic fungal infections in immunocompromised patients. C. albicans activates both the alternative and classical pathway of the complement system. The aim of this study was to assay whether C. albicans binds human complement regulators in order to control complement activation at its surface. We observed binding of two central complement regulators, factor H and FHL-1, from normal human serum to C. albicans by adsorption assays, immunostaining, and fluorescence-activated cell sorter (FACS) analyses. Specificity of acquisition was further confirmed in direct binding assays with purified proteins. The surface-attached regulators maintained their complement regulatory activities and mediated factor I-dependent cleavage of C3b. Adsorption assays with recombinant deletion mutant proteins were used to identify binding domains. Two binding sites were localized. One binding domain common to both factor H and FHL-1 is located in the N-terminal short consensus repeat domains (SCRs) 6 and 7, and the other one located in C-terminal SCRs 19 and 20 is unique to factor H. These data indicate that by surface acquisition of host complement regulators, the human pathogenic yeast C. albicans is able to regulate alternative complement activation at its surface and to inactivate toxic complement activation products.

Candida albicans expresses a focal adhesion kinase-like protein that undergoes increased tyrosine phosphorylation upon yeast cell adhesion to vitronectin and the EA.hy 926 human endothelial cell line

The signaling pathways triggered by adherence of Candida albicans to the host cells or extracellular matrix are poorly understood. We provide here evidence in C. albicans yeasts of a p105 focal adhesion kinase (Fak)-like protein (that we termed CaFak), antigenically related to the vertebrate p125Fak, and its involvement in integrin-like-mediated fungus adhesion to vitronectin (VN) and EA.hy 926 human endothelial cell line. Biochemical analysis with different anti-chicken Fak antibodies identified CaFak as a 105-kDa protein and immunofluorescence and cytofluorimetric analysis on permeabilized cells specifically stain C. albicans yeasts; moreover, confocal microscopy evidences CaFak as a cytosolic protein that colocalizes on the membrane with the integrin-like VN receptors upon yeast adhesion to VN. The protein tyrosine kinase (PTK) inhibitors genistein and herbimycin A strongly inhibited C. albicans yeast adhesion to VN and EA.hy 926 endothelial cells. Moreover, engagement of alpha v beta 3 and alpha v beta 5 integrin-like on C. albicans either by specific monoclonal antibodies or upon adhesion to VN or EA.hy 926 endothelial cells stimulates CaFak tyrosine phosphorylation that is blocked by PTK inhibitor. A role for CaFak in C. albicans yeast adhesion was also supported by the failure of VN to stimulate its tyrosine phosphorylation in a C. albicans mutant showing normal levels of CaFak and VNR-like integrins but displaying reduced adhesiveness to VN and EA.hy 926 endothelial cells. Our results suggest that C. albicans Fak-like protein is involved in the control of yeast cell adhesion to VN and endothelial cells.