Structure and function of the fungal cell wall

Influence of the variation of the alkyl chain length of N-alkyl-β-D-glycosylamine derivatives on antifungal properties

Twelve new glucosidic and galactosidic derivatives of N-alkylaminosugars with different alkylamines from 6 to 18 carbons were synthesized and characterized by (1)H and (13)C NMR. Their antifungal activity against the food fungal pathogen Aspergillus niger was evaluated using the radial growth assay. The influence of the variation of the alkyl chain length of N-alkylaminosugars on the mycelium growth was then discussed. Inhibition by the different alkylamines is shown as a biostatic effect rather than a biocidal effect. It was observed that alkylamines keep their antifungal properties after a thermal treatment compatible with food packaging and processing.

Efficacy of Origanum essential oils for inhibition of potentially pathogenic fungi

This study aimed to assess the efficacy of O. vulgare L. and O. majorana L. essential oil in inhibiting the growth and survival of potentially pathogenic fungal strains and also sought to evaluate the possible mechanisms involved in the establishment of the antifungal property of the tested essential oils through assays of osmotic stability and morphogenesis. Test strains included in this study were Candida albicans ATCC 7645, C. tropicalis LM-14, C. krusei LM-09, Cryptococcus neoformans FGF-5, Aspergillus flavus LM-02, A. fumigatus IPP-21, T. rubrum ATCC 28184, T. mentagrophytes LM-64, Microsporum gypseum ATCC 184, M. canis LM-36 and Cladosporium herbarium ATCC 26362. O. vulgare essential oil presented a MIC value of 80 µL/mL, while for O. majorana this was 160 µL/mL. C. krusei LM-09 was the only strain resistant to all assayed concentrations of both essential oils. O. vulgare and O. majorana essential oil at their MIC values provided a cidal effect against C. albicans ATCC 7645 after 4 h of exposure. O. vulgare essential oil at 80 µL/mL exhibited 100 % inhibition of the radial mycelia growth of T. rubrum ATCC 28184 and M. canis LM-36 for 14 days. Assayed fungus strain protected by sorbitol (osmo-protectant agent) grew in media containing higher concentrations of O. vulgare and O. majorana essential oil in comparison to media without sorbitol, suggesting some specificity of these essential oils for targeting cell wall in the fungi cell. Main morphological changes observed under light microscopy provided by the essential oil of O. vulgare in A. flavus LM-02 were decreased conidiation, leakage of cytoplasm, loss of pigmentation and disrupted cell structure indicating fungal wall degeneration. These results suggest that essential oils from Origanum could be regarded as a potential antifungal compound for controlling the growth of pathogen fungi and the occurrence of mycoses.

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.

Fungal cell wall septation and cytokinesis are inhibited by bleomycins

When the essential and distinctive cell walls of either pathogenic or nonpathogenic fungi break, cytoplasmic membranes rupture and fungi die. This fungicidal activity was discovered previously on nonproliferating Saccharomyces cerevisiae cells treated briefly with the oxidative tool and anticancer drug family of bleomycins. The present studies investigated effects of bleomycin on growing fungal organisms. These included the medically important Aspergillus fumigatus and Cryptococcus neoformans, as well as the emerging human pathogen and fungal model, S. cerevisiae. Bleomycin had its highest potency against A. fumigatus. Scanning electron microscopy and thin-section transmission electron microscopy were used to study morphological growth characteristics. Killing and growth inhibition were also measured. Long, thin, and segmented hyphae were observed when A. fumigatus was grown without bleomycin but were never observed when the mold was grown with the drug. Bleomycin arrested conidial germination, hyphal development, and the progression and completion of cell wall septation. Similarly, the drug inhibited the construction of yeast cell wall septa, preventing cytokinesis and progression in the cell division cycle of S. cerevisiae. Even when cytoplasms of mother and daughter cells separated, septation and cell division did not necessarily occur. Bizarre cell configurations, abnormally thickened cell walls at mother-daughter necks, abnormal polarized growth, large undivided cells, fragmented cells, and empty cell ghosts were also produced. This is the first report of a fungicidal agent that arrests fungal growth and development, septum formation, and cytokinesis and that also preferentially localizes to cell walls and alters isolated cell walls as well as intact cell walls on nongrowing cells.

Candida albicans cell wall antigens for serological diagnosis of candidemia

Serological tests for diagnosis of disseminated fungal infections in the immunocompromised host are used with varying results. In the present study, the relative ability of antibodies to specifically recognize Candida albicans cell wall components was evaluated in order to find antigenic markers for serological diagnosis of candidemia. Native C. albicans cell wall fragments (CW), periodate- (CWIO4) and proteinase-K- (CWP) treated CW, a mildly extracted phosphopeptidomannan (PPM), and beta(1-3)(1-6)-glucan were used as antigens in ELISA with sera from rabbits immunized with C. albicans (n = 10), patients with culture proven candidemia (n = 8) and healthy individuals (n = 8). The antibody response in rabbits consisted predominantly of anti-PPM antibodies, a finding that was substantiated by inhibition-ELISA. Consistently, periodate treatment (CW104) destroyed a major proportion of the antigenic epitopes. Low rabbit antibody levels were found against glucan, the major Candida cell wall component. These results supported the conclusion that glucan is localized mainly in the inner part of the C. albicans cell wall. In contrast to rabbits' serum IgG antibody response against PPM, which was at least tenfold higher than that raised against CW, patients with candidemia had similar IgG antibody levels against both antigens. These levels were significantly higher than those seen in healthy controls (CW, P = 0.0005 and PPM, P < 0.0001). Although the human anti-glucan and anti-CWIO4 IgG antibody levels were low overall, they were nonetheless significantly increased in the patient group (P = 0.0159 for antiglucan and P = 0.0491 for anti-CWIO4). In addition, a correlation was noticed between levels of these antibodies. No significant differences were found between patients and controls for IgM antibodies when CW, CWIO4, PPM and Glu were used as antigens. In conclusion, IgG antibodies to PPM and native cell wall fragments (CW) were highly discriminatory for recognition of candidemia and these antigens are thus promising candidates for use in serodiagnosis.

Relationship between the physical properties of Candida albicans cell well beta-glucan and activation of leukocytes in vitro

We previously reported that the fungal particle 1,3-beta-D-glucan derived from Candida albicans, a pathogenic fungus, was obtained by oxidation of the cell wall with sodium hypochlorite (NaClO). It could be solubilized by treatment with dimethylsulfoxide (DMSO). In the present study, we prepared Candida 1,3-beta-D-glucan having different physical properties, and examined the relationship between leukocyte activation and the physicochemical properties. Beta-glucan activated leukocytes significantly more effectively in a particulate than solubilized form in terms of TNF-alpha production by RAW 264.7 cells, hydrogen peroxide production by murine PEC and IL-8 production by human PBMC. Furthermore, we compared the biological activity of the glucan particles oxidized under various conditions. Interestingly, inactive and antagonistic particles were obtained under strong oxidation conditions. However, the inactive particles showed significant agonistic activity on dissolution in DMSO and following lyophilization. These facts strongly suggested that the solubility and assembly of the components influence the immunopharmacological activities of 1,3-beta-D-glucans.

Complications in cell-surface labelling by biotinylation of Candida albicans due to avidin conjugate binding to cell-wall proteins

Initial contact between the opportunistic fungal pathogen Candida albicans and host tissue occurs at the cell surface. Biotin derivatives have been used to label the cell-surface proteins of yeasts, with labelled proteins subsequently detected by avidin-reporter conjugates. Previous work has indicated that avidin can bind to C. albicans proteins in the absence of biotin, suggesting a possible host-cell-recognition mechanism by fungal cell-surface proteins. To investigate this mechanism, Western blots of proteins extracted from biotinylated and mock-treated cells were probed with avidin or modified-avidin reagents. Each avidin reagent bound to cell-wall proteins extracted from non-biotinylated cells. Binding did not appear to be due to the lectin-like activity of the cell-wall proteins of C. albicans or to the presence of biotin in the sample itself. Binding was inhibited by added biotin, by the chaotrope KSCN and by NaCl in a concentration-dependent manner, although inhibition varied among the avidin conjugates tested. Thus, the non-specific binding of avidin to the cell-wall proteins of C. albicans appears to involve hydrophobic and electrostatic interactions, depending on the particular avidin species. These observations demonstrate potential pitfalls in the use of avidin-biotin complexes to identify cell-surface molecules and could provide insights into protein-protein interactions at the C. albicans cell wall.

Use of recombinant mitogillin for improved serodiagnosis of Aspergillus fumigatus-associated diseases

During human infection, Aspergillus fumigatus secretes a 18-kDa protein that can be detected as an immunodominant antigen in the urine of infected patients. Recently, this protein was shown to be mitogillin, a ribotoxin that cleaves a single phosphodiester bond of the 29S rRNA of eukaryotic ribosomes. We proved the immunogenic capacity of mitogillin in a rabbit animal model, indicating its usefulness as an antigen for serological diagnosis of invasive aspergillosis. The mitogillin gene from A. fumigatus was transferred from plasmid pMIT+ to expression vector pQE30 and expressed in Escherichia coli as a fusion protein. Purified recombinant mitogillin was recognized by serum immunoglobulin G (IgG) of polyclonal rabbit sera that were obtained by immunization with purified native mitogillin. Consequently, we developed an enzyme-linked immunosorbent assay for detection of IgG, IgM, and IgA antibodies to recombinant mitogillin. In serum samples of patients suffering from aspergilloma (AO; n = 32), invasive pulmonary aspergillosis (IPA; n = 42), or invasive disseminated aspergillosis (IDA; n = 40), a good correlation of production of IgG antibody against mitogillin and clinical disease was observed (for patients with AO, 100% [32 of 32] were positive; for patients with IPA, 64% [31 of 42] were positive; for patients with IDA, 60% [24 of 40] were positive). In contrast, positive titers for serum IgG and IgM antibodies against mitogillin were found in only 1.3% of the serum samples of healthy volunteers and positive titers for IgA antibody were found in only 1.0% of the serum samples of healthy volunteers (n = 307; specificity = 95.4%). These results indicate that recombinant mitogillin expressed in E. coli can be used for improvement of the serodiagnosis of A. fumigatus-associated diseases.

Definitive chemical evidence for the constitutive ability of Candida albicans serotype A strains to synthesize beta-1,2 linked oligomannosides containing up to 14 mannose residues

We have previously reported the presence of phosphate bound beta-1,2 linked oligomannosides with unusually high degrees of polymerization (DP > 7) in the mannan of Candida albicans strain VW32. To confirm this observation, we have prepared these oligomannosides from the mannan of C. albicans strain NIH A 207. Gel filtration chromatography and TLC analysis revealed DP up to 14. For both strains, NMR analysis confirmed the exclusive presence of beta-1,2 linkages in the pools of oligomannosides with a DP higher than 6 which presented an average DP of 10.6 (VW32) and 10.4 (NIH A 207). These results are important to consider in relation with the ability of these C. albicans derived oligomannosides to trigger TNFalpha synthesis according to their DP.

Mechanisms of Giardia lamblia differentiation into cysts

Microbiologists have long been intrigued by the ability of parasitic organisms to adapt to changes in the environment. Since most parasites occupy several niches during their journey between vectors and hosts, they have developed adaptive responses which allow them to survive under adverse conditions. Therefore, the life cycles of protozoan and helminthic parasites are excellent models with which to study numerous mechanisms involved in cell differentiation, such as the regulation of gene expression, signal transduction pathways, and organelle biogenesis. Unfortunately, many of these studies are very difficult because the conditions needed to elicit developmental changes in parasites remain undetermined in most cases. Recently, several interesting findings were reported on the process of differentiation of Giardia lamblia trophozoites into cysts. G. lamblia is a flagellated protozoan that inhabits the upper small intestine of its vertebrate host and is a major cause of enteric disease worldwide. It belongs to the earliest identified lineage among eukaryotes and therefore offers a unique insight into the progression from primitive to more complex eukaryotic cells. The discovery of a specific stimulus that induces trophozoites to differentiate into cysts, the identification and characterization of encystation-specific molecules, the elucidation of novel biochemical pathways, and the development of useful reagents and techniques have made this parasite an excellent model with which to study differentiation in eukaryotic cells. In this review, we summarize the most recent fundings on several aspects of Giardia differentiation and discuss the significance of these findings within the context of current knowledge in the field.

Molecular bases of adhesion of Candida albicans

The purpose of this review is to focus on the location and the adhesion activity of the protein (peptide) and the mannan moieties of the mannoprotein in the outer surface of the Candida albicans cell wall. A macromolecule of the mannoprotein located on the outermost surface is undoubtedly a strong adhesin comprising several adhesion molecules including protein and mannan. Mannoproteins can be divided into two classes, higher molecular weight peptidomannans (260 kDa) and lower molecular weight mannoproteins (50-66 kDa), both of which consist of similar mannans and disparate proteins or peptides which have distinct adhesion specificities. The protein moiety of mannoprotein can be divided functionally into two groups, lectin-like proteins and proteins recognizing arginine-glycine-aspartic acid (RGD) ligands. The latter proteins are further subdivided into two groups, CR2/CR3-like proteins and proteins binding extracellular matrix (ECM) proteins. Hydrophobicity of the cell surface of C. albicans influences adhesion of the organisms to epithelial cells. Degree of glycosylation of cell surface mannoproteins that affect yeast cell surface hydrophobicity affects adhesion of C. albicans to epithelial cells. The hydrophobic proteins may have low levels of glycosylation, and changes in glycosylation may determine exposure of hydrophobic protein regions at the cell surface. The serotype A-specific oligosaccharide of antigen 6 (pentaose or hexaose of mannan moiety) has been shown to exhibit marked adhesion ability for epithelial cells, and mannotetraose related to antigenic factor 5 which is present in both serotypes A and B showed adhesive activity for tissue macrophages. Proteinoceous adhesins of C. albicans are expressed preferably on the mycelial form. It is suggested that several of the adhesion molecules of C. albicans described above appear to complementarily utilize multiple adhesion mechanisms.

Phenotype in Candida albicans of a disruption of the BGL2 gene encoding a 1,3-beta-glucosyltransferase

The BGL2 gene encodes a unique 1,3-beta-glucosyltransferase (Bgl2p) present in the cell wall of Candida albicans and other fungi. Although believed to be involved in cell wall assembly, disruption of the gene in saccharomyces cerevisiae showed no apparent phenotype. We performed sequential disruptions of the BGL2 loci in a homozygous ura3 clinical isolate of C. albicans using the URA3 blaster method, in order to investigate the role of Bgl2p in this dimorphic, pathogenic fungus. Strain CACW-1 contained disruptions of both homologues of the BGL2 gene and lacked Bgl2p, as assessed by protein extraction, SDS-PAGE and Western blot analysis, and enzyme assay; however, residual non-Bgl2p transferase activity was detected. CACW-1 was attenuated in virulence for mice when compared to an isogenic parent strain, and fewer organisms were recovered from the kidneys of infected animals. Additional phenotypic changes included: (1) a dramatic increase in the sensitivity to the chitin synthesis inhibitor nikkomycin Z when CACW-1 cells were incubated at 37 or 42 degrees C; (2) an 8.7 +/- 1.6% slower growth rate at 37 degrees C for CACW-1 when compared to its isogenic parent; and (3) aggregation of CACW-1 cells during stationary phase and/or incubation of stationary phase cells in phosphate buffer. Characterization of SDS-extracted cell walls did not reveal any significant differences in the levels of 1,3-beta- or 1,6-beta-glucan. These data reveal that loss of Bgl2p does have a phenotype in C. albicans, and indicate that (1) loss of Bgl2p function renders cells more dependent on chitin for wall integrity, and attenuates virulence (probably due to subtle changes in wall structure), and (2) that additional 1,3-beta-glucosyltransferases are present in the C. albicans BGL2 disruptant.

Design and synthesis of amphipathic antimicrobial peptides

A large proportion of antimicrobial peptides share a common structural feature that is critical to their antimicrobial activity, i.e. amphipathic alpha-helices. The amphipathy of a polypeptide chain can be quantitated through the value of the hydrophobic moment. Generally, antimicrobial peptides are characterized by high hydrophobic moment and low hydrophobicity values. Using these criteria we have identified two short segments that possess hydrophobic moment properties associated with known antimicrobial peptides. Using in vitro assays the segment derived from the protein perforin displays no antifungal or antibacterial activity and, while showing no alpha-helicity in buffer or liposomes, exhibits a modest degree of alpha-helical structure in the presence of the alpha-helical inducer, 2,2,2-trifluoroethanol. However, rational modifications result in a derivative which assumes an alpha-helical conformation in the presence of liposomes, exhibits potent antifungal activity against plant fungal pathogens, has significant antibacterial activity, effects leakage of a fluorescent dye from acidic liposomes and is devoid of hemolytic activity. Results are also presented for a segment derived from the human immunodeficiency virus envelope protein. We suggest that the identification of putative amphipathic structures in proteins may provide a useful starting strategy in the design and synthesis of antimicrobial peptides.

Production of polyclonal and monoclonal antibodies against hyphae from arbuscular mycorrhizal fungi

Polyclonal and monoclonal antibodies were produced against hyphae of the arbuscular mycorrhizal fungus Glomus monosporum. The polyclonal antibodies (pAbs) were raised in a rabbit by immunizing with hyphae. They were tested for their specificity by a dot-immunoblot assay (DIBA). After the third immunization, a distinct difference in the signal strength was observed between the antisera and the preimmune serum. The pAbs showed cross-reactions to a number of fungal species, both mycorrhizal and other. For the production of monoclonal antibodies (mAbs), mice were immunized intraperitoneally with hyphae. The resulting hybridoma cell culture supernatants were tested by an indirect immunolabeling procedure. For this purpose the hyphae were immobilized on silane-coated microscopic slides. The mAb 8A7 reacted with hyphae from all Glomus isolates tested so far. Cross-reactivities were not observed with hyphae from fungi of the family Acaulosporaceae, phytopathogenic fungi tested so far, or from spores from Glomus species.