Potential role of phospholipases in virulence and fungal pathogenesis

mRNA trafficking in fungi

Fungal growth depends on active transport of macromolecules along the actin and/or microtubule cytoskeleton. Thereby, molecular cargo such as proteins, lipids, and mRNAs is targeted to defined subcellular regions. Active transport and localisation of mRNAs mediate localised translation so that protein synthesis occurs where protein function is required. In Saccharomyces cerevisiae, actomyosin-dependent mRNA trafficking participates in polar growth, asymmetric cell division, targeting of membrane proteins and import of mitochondrial proteins. The best-understood example is transport of ASH1 mRNA to the distal pole of the incipient daughter cell. cis-acting RNA sequences are recognised by the RNA-binding protein She2p that is connected via the adaptor She3p to the molecular motor Myo4p. Local translation at the poles of daughter cells causes Ash1p to accumulate predominantly in nuclei of daughter cells, where this transcription factor inhibits mating-type switching. Recently, it was also shown that actomyosin-dependent ASH1 mRNA transport directs tip cell-specific gene expression in filaments of the human pathogen Candida albicans. Furthermore, in the plant pathogen Ustilago maydis microtubule-dependent shuttling of the RNA-binding protein Rrm4 is essential to determine the axis of polarity in infectious filaments. Thus, mRNA trafficking appears to be universally required for polar growth of fungi.

Isolation and characterization of a haemolysin from Trichophyton mentagrophytes

Haemolytic activities of Trichophyton (T.) mentagrophytes were detected and characterized by qualitative and quantitative assays. On Columbia agar supplemented with blood from horses, cattle or sheep, T. mentagrophytes expressed a strong zone of complete haemolysis. No haemolytic activities could be detected in the closely related T. verrucosum var. ochraceum. The same results were obtained after cultivation of the fungi on sterile cellulose acetate filters placed on the surface on Columbia blood agar. After removal of the filter, complete haemolysis was detected below the colony of T. mentagrophytes. A soluble haemolysin from culture supernatant of this strain was isolated and partially purified. Specific haemolytic activity per mg protein was enriched 2.6-fold in filtrate F(1), a fraction obtained as filtrate after filtration through 3kDa cut-off membranes. The partially purified haemolysin was neither affected by proteinase K treatment, nor by high and low temperatures, suggesting that it represents a small peptide haemolysin. Accordingly, in a commercial enzymatic activity test only the crude culture filtrate, but none of the subsequent purification fractions showed reactivity. Evaluation of the specificity of the haemolysin using erythrocytes from different mammalian species revealed that sensitivity was highest to those of equines, followed by erythrocytes from sheep, cattle, swine, dogs and humans. None of the erythrocytes was lysed by filtrate F(1) from T. verrucosum var. ochraceum. Furthermore, different eukaryotic cell lines from different species were tested in their sensitivity to cytolytic activities of the haemolysin, but no membrane damage could be detected.

Synthesis and in vitro activity of 2-thiazolylhydrazone derivatives compared with the activity of clotrimazole against clinical isolates of Candida spp

In this paper, we report on the synthesis of a novel series of 2-thiazolylhydrazone derivatives and the influence of the substituents on the thiazole ring on antifungal activity. All synthesized compounds were screened for their in vitro activities against 22 clinical isolates of Candida spp., representing six different species, compared to clotrimazole as a reference compound. Some of the tested compounds were found to possess significant antifungal activity when compared to clotrimazole, in particular compound 14 which exhibited higher potency against most of the Candida spp. considered. The compounds that were most active as anti-Candida agents were also submitted to cytotoxic screening by the Trypan Blue dye exclusion assay and in general they were shown to induce low cytotoxic effects.

Isolation of transcripts over-expressed in human pathogen Trichophyton rubrum during growth in keratin

Trichophyton rubrum is a cosmopolitan and anthropophilic fungus able to invade keratinized tissue, causing infection in human skin and nails. This work evaluated the changes in the extracellular pH during its growth in keratin (after 6, 12, 24, 48, 72h and 7 days) at initial pH 5.0. We observed a gradual increase of basal pH under keratin exposure when compared to glucose condition. Also, we identified 576T. rubrum transcripts differentially expressed by subtractive suppression hybridization (SSH) using conidia cultivated for 72h in keratin as tester, and cultivated in glucose as driver. The over-expression of 238 transcripts obtained under keratin condition was confirmed by macro-array dot-blot, revealing 28 unigenes. Putative proteins encoded by these genes showed similarity to fungi proteins involved in basic metabolism, growth and virulence, i.e., transporters ABC-MDR, MFS and ATPase of copper, NIMA interactive protein, Gag-Pol polyprotein, virulence factors serine-protease subtilisin and metalloprotease, cytochrome P450, GlcN-6-phosphate deaminase and Hsp30. The upregulation of T. rubrum genes encoding subtilisin, metalloprotease and Gag-Pol polyprotein was also validated by northern blot. The results of this study provide the first insight into genes differentially expressed during T. rubrum grown in keratin that may be involved in fungal pathogenesis.

Synthesis, antifungal and haemolytic activity of a series of bis(pyridinium)alkanes

A series of bis(pyridinium)alkanes have been prepared and their antifungal activity, haemolytic activity and ability to inhibit fungal phospholipase B1 have been investigated, together with those of the commercially available antiseptics octenidine and dequalinium. Removal of the amino substituents from the pyridinium rings resulted in a significant decrease in antifungal activity. However, shortening or removing the alkyl chains attached to the amino groups had little effect on antifungal activity and significantly reduced haemolytic activity. Only octenidine was a strong inhibitor of fungal phospholipase B1.

Clonorchis sinensis: expression, characterization, immunolocalization and serological reactivity of one excretory/secretory antigen-LPAP homologue

From a Clonorchis sinensis adult cDNA plasmid library, a cDNA clone encoding a novel lysophosphatidic acid phosphatase (LPAP) homologue was isolated. The predicted molecular weight of putative protein was 48.8 kDa and the deduced amino acid sequence had 45%, 32%, and 29% identity with LPAP of Schistosoma japonicum, Danio rerio, and Homo sapiens, respectively. Prediction of signal peptide and Western blot analysis indicated that the CsLPAP homologue was an excretory-secretory antigen (ES antigen) of C. sinensis. Immunostaining revealed that the CsLPAP was markedly localized in the intestinal cecum, seminal receptacle and eggs of the adult worm. The recombinant CsLPAP showed slightly higher sensitivity (82.14%) and specificity (85.86%) than the crude worm antigen by enzyme-linked immunosorbent assay (ELISA), a result which suggested that the recombinant antigen might be valuable in the serodiagnosis of human clonorchiasis.

Candida glabrata: an emerging oral opportunistic pathogen

Following the widespread use of immunosuppressive therapy and broad-spectrum antimycotic prophylaxis, C. glabrata has emerged as an important opportunistic pathogen in the oral mucosa. In the past, studies on the virulence factors and host-pathogen interactions of this organism were scarce, but continued to rise in recent years. Denture-wearing, immunosuppression, antibiotic therapy, and aging are risk factors for oral colonization or infection with C. glabrata. Compared with C. albicans, C. glabrata exhibits lower oral keratinocyte-adherence capacity, but higher denture-surface-adherence ability. The role of extracellular hydrolase production in the virulence of this organism does not appear to be as important as it is in C. albicans pathogenesis. Although traditionally thought of as a non-transforming yeast organism, both phenotypic switching and pseudohyphal formation have recently been identified in C. glabrata, but their role in pathogenesis is not known. With the exception of granulocyte monocyte colony-stimulating factor, C. glabrata triggers a lower proinflammatory cytokine response in oral epithelial cells than does C. albicans, in a strain-dependent manner. C. glabrata is less susceptible to killing by human beta-defensins than is C. albicans and exhibits various degrees of resistance to the antifungal activity of salivary histatins and mucins. In addition, C. glabrata possesses both innate and acquired resistance against antifungal drugs, due to its ability to modify ergosterol biosynthesis, mitochondrial function, or antifungal efflux. This resistance allows for its relative overgrowth over other susceptible species and may contribute to the recent emergence of C. glabrata infections in chronically immunocompromised populations. Further investigations on the virulence and host-pathogen interactions of C. glabrata are needed to better define the pathogenesis of oral C. glabrata infection in susceptible hosts.

Secretory aspartyl peptidase activity from mycelia of the human fungal pathogen Fonsecaea pedrosoi: effect of HIV aspartyl proteolytic inhibitors

Fonsecaea pedrosoi is the principal causative agent of chromoblastomycosis, which is a chronic, often debilitating, suppurative and granulomatous mycosis. Very little is known about the hydrolytic enzymes produced by this human fungal pathogen. In the present study, we have identified extracellular proteolytic activity from F. pedrosoi mycelial forms when grown in chemically defined conditions. Secretory aspartyl peptidase activity was measured during 15 days of fungal growth in vitro using bovine serum albumin (BSA) as the soluble substrate and extreme acidic pH (2.0). This activity was totally inhibited by pepstatin A, a classic aspartyl peptidase inhibitor. Conversely, metallo (o-phenanthroline), cysteine (E-64) and serine (PMSF) proteolytic inhibitors failed to restrain proteolytic activity. We also evaluated the effect of four distinct HIV aspartyl peptidase inhibitors on the secretory proteolytic activity of F. pedrosoi mycelia. Indinavir, ritonavir and nelfinavir powerfully inhibited extracellular aspartyl proteolytic activity by approximately 97, 96 and 87%, respectively, whereas saquinavir did not significantly interfere with BSA hydrolysis. Mycelial-derived secretory aspartyl peptidase activity cleaved other proteinaceous substrates, including human albumin, fibrinogen, fibronectin, laminin and type I collagen. As previously reported by our group, conidia also produce secretory aspartyl peptidase. In this sense, we investigated the effect of pepstatin A on F. pedrosoi development. Pepstatin A was able to inhibit the growth of conidium and its transformation into mycelium. Taken together, our results suggest a possible participation of aspartyl peptidases in the essential fungal processes, such as growth, differentiation, nutrition and cleavage of relevant host proteinaceous components.

Correlation between adhesion, enzyme production, and susceptibility to fluconazole in Candida albicans obtained from denture wearers

OBJECTIVE

The aim of this study was to assess the contribution and the correlation between the virulence factors of Candida albicans in denture stomatitis.

STUDY DESIGN

Thirty C. albicans strains obtained from the oral cavity of patients with denture stomatitis and 30 C. albicans obtained from the oral cavity of denture wearers with normal palatal mucosa were compared for adhesion ability to buccal epithelial cells (BEC), reduction in adhesion after exposure to fluconazole, and enzyme production. The correlation between these virulence factors was assessed by Pearson's correlation coefficient.

RESULTS

C. albicans obtained from denture stomatitis patients were more adherent to BEC and higher enzyme producers than those obtained from healthy patients. Our results demonstrated that exposure to fluconazole reduces C. albicans adherence to BEC. This study could also provide evidence of correlation between virulence factors.

CONCLUSION

Our results suggest that adhesion and enzyme production could be factors that, along with predisposing conditions related to the host, determine if an individual will develop disease or remain as a healthy carrier and confirm that fluconazole has an impact on the adherence ability and enzyme production in C. albicans.

Inactivation of the phospholipase B gene PLB5 in wild-type Candida albicans reduces cell-associated phospholipase A2 activity and attenuates virulence

Phospholipases are critical for modification and redistribution of lipid substrates, membrane remodeling and microbial virulence. Among the many different classes of phospholipases, fungal phospholipase B (Plb) proteins show the broadest range of substrate specificity and hydrolytic activity, hydrolyzing acyl ester bonds in phospholipids and lysophospholipids and further catalyzing lysophospholipase-transacylase reactions. The genome of the opportunistic fungal pathogen Candida albicans encodes a PLB multigene family with five putative members; we present the first characterization of this group of potential virulence determinants. CaPLB5, the third member of this multigene family characterized herein is a putative secretory protein with a predicted GPI-anchor attachment site. Real-time RT-PCR gene expression analysis of CaPLB5 and the additional CaPLB gene family members revealed that filamentous growth and physiologically relevant environmental conditions are associated with increased PLB gene activity. The phenotypes expressed by null mutant and revertant strains of CaPLB5 indicate that this lipid hydrolase plays an important role for cell-associated phospholipase A(2) activity and in vivo organ colonization.

Enhanced Enzymatic Activity of Yeast-like Fungi Responsible for Onychomycosis in Renal Transplant Recipients

BACKGROUND

Renal transplant recipients (RTR) are regarded as a group especially predisposed to onychomycosis. The exact mechanism of increased frequency of onychomycosis in RTR is however not fully understood.

OBJECTIVES

This study was undertaken to evaluate activity of hydrolitic enzymes of fungi most commonly causing fungal nail infections in RTR and to compare it with enzymatic activity of the same fungi isolated from lesional nails in immunocompetent patients.

MATERIAL AND METHODS

28 strains of yeast-like fungi cultured from lesional nails in RTR and 25 strains of yeasts isolated from changed nails in immunocompetent patients were included into the study. All fungi were identified on the basis of routine mycological procedures. Activity of 19 hydrolytic enzymes was assessed by API ZYMÒ test (bioMerieux).

RESULTS

Fungi cultured from RTR showed activity of 16 out of 19 enzymes, whereas fungi isolated from immunocompetent patients only 11 out of 19 enzymes. Moreover, yeast-like fungi isolated from RTR showed higher generally higher activity of detected enzymes compared to yeast strains obtained from the lesional nails of immunocompetent patients.

CONCLUSIONS

This study shows for the first time enhanced enzymatic activity of yeast-like fungi isolated from lesional nails in RTR in comparison to fungi cultured from changed nails in immunocompetent patients. It is hypothesized that this enhanced enzymatic activity may be responsible for higher incidence of onychomycosis in RTR.

Alternative Candida albicans lifestyles: growth on surfaces

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

Interaction of serotonin with Candida albicans selectively attenuates fungal virulence in vitro

In this study we investigated whether the direct interaction between Candida albicans CBS 5982 and 5-hydroxytryptamine (5-HT) alters candidial virulence. Hyphae elongation, phospholipase activity and the production of secreted aspartyl proteinases (Saps) following 5-HT treatment were investigated. 5-HT treatment of C. albicans significantly (P < 0.05) affected hyphal extension, phospholipase activity and the production of Saps at concentrations of 118-0.46 mM. In conclusion, our findings suggest that the interaction between 5-HT and C. albicans may diminish the virulence properties of this fungal pathogen.

Enzymatic Variability of Candida krusei Isolates in a Course of Fungal Infection in a Liver Transplant Recipient

Transplant recipients are at high risk of fungal infections. The main site of fungal infections in patients undergoing liver transplantation is the abdominal cavity. One factor determining the pathogenicity of fungi is their ability to secrete hydrolytic enzymes. The aim of this study was to assess the enzymatic activity of Candida krusei, which caused an infection in a liver transplant recipient. The clinical specimens included swabs of throat, nose, and two drains, as well as bile, stool, and abdominal cavity aspirate. The yeast-like fungi isolated were identified by an ID 32 C test (bioMérieux) and their enzymatic activity assayed with the use of an API-ZYM test. Two biotypes of C. krusei were identified, depending on the source of the clinical specimen. The C. krusei isolates cultured from a throat swab, a nasal swab, and one of the drains secreted esterase lipase C8 (enzyme IV) and valine arylamidase (enzyme VII), in contrast to those isolated from the bile, abdominal cavity fluid, another drain, and stool. Characterization of two biotypes of C. krusei isolates cultured from different clinical samples from several infection sites indicated an ability of C. krusei to adapt to variable environmental conditions.

Intracellular trafficking of fluorescently tagged proteins associated with pathogenesis in Candida albicans

Proteins that enter the secretory pathway play important roles in virulence and pathogenesis in Candida albicans, but our understanding of the trafficking of these proteins is in its early stages. In Saccharomyces cerevisiae, dominant negative alleles of YPT1 and SEC4 interrupt secretory traffic at pre- and post-Golgi steps, respectively. We therefore used a dominant negative genetic approach to examine the intracellular trafficking of several proteins associated with virulence or azole resistance. When the dominant negative ypt1(N121I) allele of C. albicans was overexpressed, yellow-fluorescent protein (YFP) tagged forms of two plasma membrane transporters (Cdrlp and Ftrlp) and the vacuolar membrane ABC transporter Mltlp accumulated in intracellular structures that appeared related to the ER, but localization of Cdc10p and Int1p was unaffected. When the dominant negative sec4(S28N) allele of C. albicans was overexpressed, Cdrlp and Ftrlp accumulated intracellularly, and localization of Mltlp, Cdc10p and Int1p was unaffected. These results imply that (i) Cdrlp and Ftrlp are transported to the plasma membrane by the general secretory pathway, (ii) Mlt1p enters the secretory pathway but is diverted to the vacuole at an early post-Golgi step, and (iii) like Cdc10p, Int1p does not enter the general secretory pathway.

The contribution of animal models of aspergillosis to understanding pathogenesis, therapy and virulence

Animal models of aspergillosis have been used extensively to study various aspects of pathogenesis, innate and acquired host-response, disease transmission and therapy. Several different animal models of aspergillosis have been developed. Because aspergillosis is an important pulmonary disease in birds, avian models have been used successfully to study preventative vaccines. Studies done to emulate human disease have relied on models using common laboratory animal species. Guinea pig models have primarily been used in therapy studies of invasive pulmonary aspergillosis (IPA). Rabbits have been used to study IPA and systemic disease, as well as fungal keratitis. Rodent, particularly mouse, models of aspergillosis predominate as the choice for most investigators. The availability of genetically defined strains of mice, immunological reagents, cost and ease of handling are factors. Both normal and immunosuppressed animals are used routinely. These models have been used to determine efficacy of experimental therapeutics, comparative virulence of different isolates of Aspergillus, genes involved in virulence, and susceptibility to infection with Aspergillus. Mice with genetic immunological deficiency and cytokine gene-specific knockout mice facilitate studies of the roles cells, and cytokines and chemokines, play in host-resistance to Aspergillus. Overall, these models have been critical to the advancement of therapy, and our current understanding of pathogenesis and host-resistance.

Detection of antibodies to phospholipase B in patients infected with Cryptococcus neoformans by enzyme-linked immunosorbent assay (ELISA)

Secreted phospholipases are virulence factors of several fungi, including Cryptococcus neoformans. We describe for the first time the detection by ELISA of antibodies to cryptococcal phospholipase B in the serum of patients infected with C. neoformans or C. gattii. Sixty-nine sera from 25 patients with cryptococcosis, 23 patients with Candida infections and 26 with bacterial or viral infections were tested. The sensitivity of the ELISA in patients with cryptococcosis was 100% in immunocompetent hosts and 64.3% in immunosuppressed patients with cryptococcal meningitis. Absorbance readings were significantly higher in immunocompetent patients (P<0.001). Titres remained positive for up to 2 years. Positive results were noted in 10 episodes of invasive candidiasis, three patients colonised with Candida, and three cases of bacterial infection. Mean absorbance readings were significantly lower in patients with bacterial infection (P <0.001). We conclude that phospholipase B is produced in vivo during cryptococcal infection and that serum phospholipase B antibodies are a sensitive marker of present or past infection. Cross-reactivity of the ELISA with sera from patients with candidiasis indicates that common epitopes are present on cryptococcal and candidal phospholipase B, hence fungal phospholipase B may constitute a new therapeutic target.

Phosphatidylinositol-dependent phospholipases C Plc2 and Plc3 of Candida albicans are dispensable for morphogenesis and host–pathogen interaction

Phospholipases play an important role as virulence factors in human pathogens. Candida albicans, the major fungal pathogen of humans, encodes phospholipases of type A, B, C and D. Type B Plb2 and type D Pld1 phospholipases have been shown to contribute to virulence in this organism. We analyzed, in C. albicans, PLC2 and PLC3, two highly conserved genes coding for phosphatidylinositol-dependent phospholipases C with homology to the known virulence factor PlcA in the human pathogen Listeria monocytogenes. We show that expression of PLC2 and PLC3 is upregulated under different filament-inducing conditions and in the constitutive filamentous mutant tup1Delta. In order to analyze PLC2 and PLC3 function in C. albicans, we constructed strains that carry PLC2 or PLC3 under a constitutive promoter and strains that lack all four PLC2/3 alleles. These strains were not affected in their ability to produce filaments under non-inducing conditions, nor was filamentation modified under inducing conditions, suggesting that PLC2/3 are not critical determinants of the yeast-to-hypha switch. In a cell culture model for macrophage interaction, phagocytosis of C. albicans and subsequent killing were not influenced by PLC2/3. These results demonstrate that C. albicans PLC2 and PLC3 are dispensable for virulence; moreover, they underline the sharp contrast with the function of plcA in L. monocytogenes.

A yeast by any other name: Candida glabrata and its interaction with the host

Well-characterized traits important to Candida albicans virulence, such as hyphal formation or secreted proteinase activity, play no known role in Candida glabrata virulence. Likewise, some C. glabrata characteristics, such as chromatin-based regulation of the large telomeric family of lectins encoded by the EPA (epithelial adhesin) genes, have no precise parallels in C. albicans. However, similarities between the two species, for example in population structure, in the large numbers of (putative) adhesins that they encode, and in phenotypic plasticity conferred by phenotypic switching, suggest that they share general strategies in adaptation to an opportunistic lifestyle.

Fluconazole-resistant pathogens Candida inconspicua and C. norvegensis: DNA sequence diversity of the rRNA intergenic spacer region, antifungal drug susceptibility, and extracellular enzyme production

The opportunistic fungal pathogens Candida inconspicua and C. norvegensis are very rarely isolated from patients and are resistant to fluconazole. We collected 38 strains of the two microorganisms isolated from Europe and Japan, and compared the polymorphism of the rRNA intergenic spacer (IGS) and internal transcribed spacer (ITS) regions, antifungal drug susceptibility, and extracellular enzyme production as a potential virulence factor. While the IGS sequences of C. norvegensis were not very divergent (more than 96.7% sequence similarity among the strains), those of C. inconspicua showed remarkable diversity, and were divided into four genotypes with three subtypes. In the ITS region, no variation was found in either species. Since the sequence similarity of the two species is approximately 70% at the ITS region, they are closely related phylogenetically. Fluconazole resistance was reconfirmed for the two microorganisms but they were susceptible to micafungin and amphotericin B. No strain of either species secreted aspartyl proteinase or phospholipase B. These results provide basal information for accurate identification, which is of benefit to global molecular epidemiological studies and facilitates our understanding of the medical mycological characteristics of C. inconspicua and C. norvegensis.

A preliminary analysis of the process of protein secretion and the diversity of putative secreted hydrolases encoded inAspergillus fumigatus: insights from the genome

The genome sequence of Aspergillus fumigatus has enabled the annotation of genes likely to encode secreted enzymes that may be important in underpinning the natural lifestyle of the fungus and its pathogenicity. We summarize the data from the genome sequence relevant to both the process of protein secretion and the predicted hydrolase enzymes secreted by A. fumigatus.

Differential invasion of Candida albicans isolates in an in vitro model of oral candidosis

The study assessed the ability of Candida albicans isolates to invade an in vitro oral tissue model. The extent and pattern of isolate invasion was then correlated with the infection origin of the isolate to identify characteristics that may be restricted to specific forms of oral infection, particularly chronic hyperplastic candidosis (CHC). Reconstituted human oral epithelium was infected with C. albicans isolated from normal oral mucosa (n = 4), CHC (n = 7), non-CHC oral candidoses (n = 4) and squamous cell carcinoma (SCC; n = 4). After infection for 24 h, histological analysis revealed yeast adhesion, hyphal extension, and invasion of the epithelium. Differential patterns of invasion were evident and, whilst consistent for a given isolate, did not relate to the infection origin of the isolate. Two principal patterns of invasion were evident and described as either a 'localised' or a 'uniform' distribution of invading hyphae. Several isolates also exhibited superficial infection with limited hyphal invasion. In conclusion, the use of the in vitro tissue model allowed the assessment of the invasive capabilities of isolates of C. albicans. However, the apparent differences in invasive characteristics did not appear to be related to the clinical origin of isolates.

Characterisation and expression of phospholipases B from the opportunistic fungusAspergillus fumigatus

The phospholipase B family (PLB) are enzymes sharing phospholipase (PL), lysophospholipase (LPL) and lysophospholipase-transacylase (LPTA) activities. They have been shown to be important virulence factors in several human fungal pathogens including Candida albicans and Cryptococcus neoformans. Aspergillus fumigatus, a human opportunistic fungal pathogen leading to a high rate of mortality in immunosuppressed patients is known to possess an extracellular phospholipase B activity. In this paper, we report the molecular characterisation of three PLB genes from A. fumigatus (afplb) using degenerate primers in PCR amplification and data from the A. fumigatus genome project. They are expressed at 37 degrees C, and two of them (afplb1 and afplb3) are induced by lecithin. They encode proteins of 633, 588 and 630 amino acids, respectively, presenting together a T-Coffee score of 81. They also possess the amino acid triad responsible for enzymatic activity in the mammalian cytosolic PLA2 and other fungal PLBs. AfPLB1 and afPLB3 are secreted with a cleaved signal peptide. The complete cDNA sequences were obtained by RACE-PCR for the two secreted afPLBs and probably account for the extracellular phospholipase activity previously reported in the culture media of A. fumigatus.

A synthetic peptide as a novel anticryptococcal agent

An engineered, killer decapeptide (KP) has been synthesized based on the sequence of a recombinant, single-chain anti-idiotypic antibody (KT-scFv) acting as a functional internal image of a yeast killer toxin. Killer decapeptide exerted a strong fungicidal activity against Candida albicans, which was attributed to peptide interaction with beta-glucan. As this polysaccharide is also a critical component of the cryptococcal cell wall, we wondered whether KP was also active against Cryptococcus neoformans, a human pathogen of increasing medical importance. We found that KP was able to kill both capsular and acapsular C. neoformans cells in vitro. Furthermore, KP impaired the production of specific C. neoformans virulence factors including protease and urease activity and capsule formation, rendering the fungus more susceptible to natural effector cells. In vivo treatment with KP significantly reduced fungal burden in mice with cryptococcosis and, importantly, protected the majority of immunosuppressed animals from an otherwise lethal infection. Given the relevance of cryptococcosis in immunocompromised individuals and the inability of conventional drugs to completely resolve the infection, the results of the present study indicate KP as an ideal candidate for further studies on novel anticryptococcal agents.

Role of Candida albicans polymorphism in interactions with oral epithelial cells

BACKGROUND

Candida albicans is a polymorphic organism which undergoes morphologic transition between yeast, pseudohyphal and hyphal forms. The ability of C. albicans to change from yeast to filamentous types is a major virulence determinant of this organism. However, the exact role of hyphal transformation in establishing oral mucosal infection is still poorly understood.

METHODS

In this study we used mutants with defects in filamentation, as well as oral strains, which differ in their capacity to form true hyphae, to examine the role of hyphal transformation in the interactions of C. albicans with oral epithelial cells in vitro. These interactions included the ability of these strains to adhere to and injure epithelial cells, as well as their ability to trigger a proinflammatory cytokine response.

RESULTS

We found that strains SC5314 and ATCC28366 formed true hyphae on epithelial cells, whereas strain ATCC32077 and the tup1/tup1 mutant formed only pseudohyphae. Double mutant efg1/efg1cph1/cph1 grew exclusively as blastospores. We also found that yeast and pseudohyphal strains showed reduced adherence capacity to oral keratinocytes and caused minimal cell damage. Moreover, we showed that both yeast and pseudohyphal forms have a strongly attenuated proinflammatory phenotype, since they failed to induce significant interleukin (IL)-1alpha and IL-8 responses by oral epithelial cells.

CONCLUSIONS

Germination of C. albicans into true hyphae is particularly important in the interactions with oral epithelial cells in vitro.

Candida parapsilosis characterization in an outbreak setting

Candida parapsilosis is an important non-albicans species which infects hospitalized patients. No studies have correlated outbreak infections of C. parapsilosis with multiple virulence factors. We used DNA fingerprinting to determine genetic variability among isolates from a C. parapsilosis outbreak and from our clinical database. We compared phenotypic markers of pathogenesis, including adherence, biofilm formation, and protein secretion (secretory aspartic protease [SAP] and phospholipase). Adherence was measured as colony counts on silicone elastomer disks immersed in agar. Biofilms formed on disks were quantified by dry weight. SAP expression was measured by hydrolysis of bovine albumin; a colorimetric assay was used to quantitate phospholipase. DNA fingerprinting indicated that the outbreak isolates were clonal and genetically distinct from our database. Biofilm expression by the outbreak clone was greater than that of sporadic isolates (p < 0.0005). Adherence and protein secretion did not correlate with strain pathogenicity. These results suggest that biofilm production plays a role in C. parapsilosis outbreaks.

Phenotypic switching and beta-N-acetylhexosaminidase activity of the pathogenic yeast Trichosporon asahii

The pathogenic yeast Trichosporon asahii is the major causative agent of deep-seated trichosporonosis in immunocompromised patients. Although infection by this microorganism is becoming increasingly frequent, information related to its pathogenicity and virulence factors is still limited. Therefore, we investigated phenotypic switching in colony morphology, and the production of extracellular enzymes as a virulence factor. Sixty-one clinical isolates of T. asahii produced four different morphological types on Sabouraud dextrose agar (SDA): 69% WF (white farinose), 18% WP (white pustular), 10% Y (yellowish white), and 3% WC (white cerebriform). Strains of the three major types (WF, WP, and Y) produced two to five colony types when cultured on SDA at 37 C. The frequency of switching between colony types was 10(-2) to 10(-4), as in Candida albicans and Cryptococcus neoformans. Notably, most of the colonies switched to the smooth (S) type irreversibly, at frequencies of 10(-2) to 10(-3). No secreted aspartic proteinase or phospholipase activity was detected in T. asahii, while beta-N-acetylhexosaminidase activity, which catalyzes the hydrolysis of terminal nonreducing N-acetyl-D-hexosamine residues in N-acetyl-beta-D-hexosaminides, was found. Furthermore, enzymatic activity of the S type was significantly greater than that of the parent type in all strains. No other clinically relevant Trichosporon species (T. mucoides, T. inkin, and T. ovoides ) produced this enzyme. These results provide basal information for understanding the pathogenic potential of T. asahii.

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.

Vaccines and immunotherapies for the prevention of infectious diseases having cutaneous manifestations

Although the development of antimicrobial drugs has advanced rapidly in the past several years, such agents act against only certain groups of microbes and are associated with increasing rates of resistance. These limitations of treatment force physicians to continue to rely on prevention, which is more effective and cost-effective than therapy. From the use of the smallpox vaccine by Jenner in the 1700s to the current concerns about biologic warfare, the technology for vaccine development has seen numerous advances. The currently available vaccines for viral illnesses include Dryvax for smallpox; the combination measles, mumps, and rubella vaccine; inactivated vaccine for hepatitis A; plasma-derived vaccine for hepatitis B; and the live attenuated Oka strain vaccine for varicella zoster. Vaccines available against bacterial illnesses include those for anthrax, Haemophilus influenzae, and Neisseria meningitidis. Currently in development for both prophylactic and therapeutic purposes are vaccines for HIV, herpes simplex virus, and human papillomavirus. Other vaccines being investigated for prevention are those for cytomegalovirus, respiratory syncytial virus, parainfluenza virus, hepatitis C, and dengue fever, among many others. Fungal and protozoan diseases are also subjects of vaccine research. Among immunoglobulins approved for prophylactic and therapeutic use are those against cytomegalovirus, hepatitis A and B, measles, rabies, and tetanus. With this progress, it is hoped that effective vaccines soon will be developed for many more infectious diseases with cutaneous manifestations.

Haemolytic activities of Trichophyton species

Dermatophytes are keratinophilic fungi able to invade the stratum corneum of the skin and other keratinized structures. The pathogenic interactions between host and fungus are poorly understood. Some enzymes, especially keratinases, have previously been taken into consideration as virulence factors. Haemolysins have not been evaluated in this regard, though they are known to play an important role in the host-parasite interaction in bacterial infections. We investigate the haemolytic activity of four Trichophyton species: T. rubrum, T. mentagrophytes complex, Tequinum and T. verrucosum. The strains were tested on Columbia agar with 5% blood from horses, cattle and sheep. They show different haemolytic activities. T. rubrum and T. equinum produce a zone of complete haemolysis followed by a small zone of incomplete haemolysis around the colony. T. mentagrophytes and T. verrucosum produce a zone of complete haemolysis. Haemolytic activity is pronounced in dermatophytes and may play an important role as a virulence factor.

DNA array analysis of altered gene expression in human leukocytes stimulated with soluble and particulate forms of Candida cell wall β-glucan

We previously reported that 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). We found that OX-CA and CSBG showed significantly different levels of activity toward leukocytes. Here, we have used cDNA microarrays to analyze the mRNA expression of 1176 genes in PBMCs stimulated with Candida cell wall glucan and considered the difference in the activation mechanism of OX-CA and CSBG. Total mRNA showed a significant change for 147 out of 1176 arrayed genes on stimulation with OX-CA and CSBG for 4 h. Among those genes, 62 were common, 26 were OX-CA-specific and 59 were CSBG-specific. Many of these up-regulated genes encode effectors with well-characterized proinflammatory properties. The expression of genes related with signal transduction differed in the particulate and soluble glucans derived from C. albicans having exactly the same primary structure. This fact suggested that each glucan induced specific biological activity through a different activation mechanism. This study using cDNA microarrays to analyze a broad spectrum of mRNA expression provides information on the biological activity of Candida cell wall glucan as a potential pathogenic factor.

Comparison of extracellular phospholipase activities in clinical and environmentalAspergillus fumigatusisolates

Extracellular phospholipase production by environmental and clinical isolates of Aspergillus fumigatus collected from several centres world-wide were compared. All isolates produced extracellular phospholipases which included phospholipase C and a phospholipid acyl hydrolase (phospholipase A and/or phospholipase B) activity. Clinical isolates of A. fumigatus produced the largest zone sizes in a diffusion assay and clinical isolates produced more extracellular phospholipase C than environmental isolates. However, environmental isolates of A. fumigatus showed increased acyl hydrolase activity compared to clinical isolates of A. fumigatus. This study suggests that extracellular phospholipase C activity, but not extracellular acyl hydrolase activity may be important in the pathogenicity of A. fumigatus.

Epidemiology of Candida species infections in critically ill non-immunosuppressed patients

A substantial proportion of patients become colonised with Candida spp during hospital stay, but only few subsequently develop severe infection. Clinical signs of severe infection manifest early but lack specificity until late in the course of the disease, thus representing a particular challenge for diagnosis. Mostly nosocomial, invasive candidiasis occurs in only 1-8% of patients admitted to hospitals, but in around 10% of patients housed in intensive care units where it can represent up to 15% of all nosocomial infections. We review the epidemiology of invasive candidiasis in non-immunocompromised, critically ill patients with special emphasis on disease trends over time, pathophysiology, diagnostic approach, risk factors, and impact. Recent epidemiological data suggesting that the emergence of non-albicans candida strains with reduced susceptibility to azoles, previously linked to the use of new antifungals for empiric and prophylactic therapy in immunocompromised patients, may not have occurred in the critically ill. Management of invasive candidiasis in these patients will be addressed in the December issue of The Lancet Infectious Diseases.

Disruption of the human pathogenic yeast Candida albicans catalase gene decreases survival in mouse-model infection and elevates susceptibility to higher temperature and to detergents

Catalase-deficient strains of the human pathogenic yeast Candida albicans were constructed using the URA-blaster method. The disruptant was viable and grew normally in an ordinary culture condition, but became extremely sensitive to treatment with hydrogen peroxide. No catalase activity was observed in a catalase (CCT)-gene-disrupted strain, 1F5-4-1, suggesting that there were no other catalase or catalase-like enzymes in this yeast. The disruptant was shown to be sensitive to higher temperature and to low concentrations of SDS, NP-40, or Triton X-100. After a wild-type CCT gene was reintroduced into the disruptant, catalase activity was restored and the strain became moderately sensitive to treatment with hydrogen peroxide. However, neither the temperature sensitivity nor the susceptibility to SDS observed in the disruptant was restored in the CCT-reintroduced strain. A model infection experiment using wild-type and dCCT strains showed that the disruptants disappeared more rapidly than the wild-type strain in mouse liver, lung, and spleen. These results suggest that the catalase plays a significant role in survival in the host immune system and thus leads this organism to establish infection in the host.

Colonization and epithelial adhesion in the pathogenesis of neonatal candidiasis

Candida species are important nosocomial pathogens in the newborn population, particularly among the premature very-low-birth-weight infants in neonatal intensive care units. Candida colonization of the neonatal skin and gastrointestinal tract is an important first step in the pathogenesis of invasive disease. C albicans is the most commonly isolated species in colonized or infected infants. Over the past decade the incidence of both colonization and infection with other Candida species, particularly C parapsilosis, has risen dramatically. Colonization of the infant occurs early in life and is affected by a variety of common practices in neonatal intensive care. Microbial factors also augment colonization, including the ability of Candida to adhere to human epithelium. A better understanding of the complex interactions between host risk factors and virulence traits of colonizing yeast may allow the risk of systemic spread to be reduced in the population of premature infants.

Chronic hyperplastic candidosis/candidiasis (candidal leukoplakia)

Chronic hyperplastic candidosis/candidiasis (CHC; syn. candidal leukoplakia) is a variant of oral candidosis that typically presents as a white patch on the commissures of the oral mucosa. The major etiologic agent of the disease is the oral fungal pathogen Candida predominantly belonging to Candida albicans, although other systemic co-factors, such as vitamin deficiency and generalized immune suppression, may play a contributory role. Clinically, the lesions are symptomless and regress after appropriate antifungal therapy and correction of underlying nutritional or other deficiencies. If the lesions are untreated, a minor proportion may demonstrate dysplasia and develop into carcinomas. This review outlines the demographic features, etiopathogenesis, immunological features, histopathology, and the role of Candida in the disease process. In the final part of the review, newer molecular biological aspects of the disease are considered together with the management protocols that are currently available, and directions for future research.

'Ready made' virulence and 'dual use' virulence factors in pathogenic environmental fungi--the Cryptococcus neoformans paradigm

Environmental pathogenic fungi present a paradox in that they are virulent in animals without requiring animal hosts for replication or survival, a phenomenon we call 'ready-made' virulence. In the human pathogenic fungus Cryptococcus neoformans, the capacity for virulence in animals may originate from environmental selective pressures imposed by such organisms as amoeboid and nematode predators. Many C. neoformans virulence factors appear to have 'dual use' capabilities that confer survival advantages in both animal hosts and in the environment. The findings with C. neoformans may provide a paradigm for understanding the origin and maintenance of virulence in other pathogenic environmental fungi.

The origin and maintenance of virulence for the human pathogenic fungus Cryptococcus neoformans

The origin of virulence in environmental fungi that have no requirement for animal hosts in their life cycle is enigmatic. Cryptococcus neoformans is a human pathogenic fungus with virulence factors for mammalian pathogenesis that also contribute to environmental survival. C. neoformans virulence may originate from selection pressures imposed by environmental predators.

An analysis of the Candida albicans genome database for soluble secreted proteins using computer-based prediction algorithms

We sought to identify all genes in the Candida albicans genome database whose deduced proteins would likely be soluble secreted proteins (the secretome). While certain C. albicans secretory proteins have been studied in detail, more data on the entire secretome is needed. One approach to rapidly predict the functions of an entire proteome is to utilize genomic database information and prediction algorithms. Thus, we used a set of prediction algorithms to computationally define a potential C. albicans secretome. We first assembled a validation set of 47 C. albicans proteins that are known to be secreted and 47 that are known not to be secreted. The presence or absence of an N-terminal signal peptide was correctly predicted by SignalP version 2.0 in 47 of 47 known secreted proteins and in 47 of 47 known non-secreted proteins. When all 6165 C. albicans ORFs from CandidaDB were analysed with SignalP, 495 ORFs were predicted to encode proteins with N-terminal signal peptides. In the set of 495 deduced proteins with N-terminal signal peptides, 350 were predicted to have no transmembrane domains (or a single transmembrane domain at the extreme N-terminus) and 300 of these were predicted not to be GPI-anchored. TargetP was used to eliminate proteins with mitochondrial targeting signals, and the final computationally-predicted C. albicans secretome was estimated to consist of up to 283 ORFs. The C. albicans secretome database is available at http://info.med.yale.edu/intmed/infdis/candida/

Intestinal Candida phospholipase is not elevated in patients with antibiotic-associated diarrhea

In order to assess the role of Candida-secreted phospholipase in antibiotic-associated diarrhea (AAD), 43 fecal Candida isolates from patients with AAD and from controls were tested on egg yolk agar for production of phospholipase. Phospholipase zones did not differ between the isolates from patients with AAD and from controls. The data indicate that the fungal virulence factor phospholipase may not be responsible for AAD in adults.

Extracellular proteinase and phospholipase activity of three genotypic strains of a human pathogenic yeast, Candida albicans

Strains of a human pathogenic yeast, Candida albicans, have (A) intronless, (B) intron-containing, and (C) a mixture of intron-containing and intronless 26S rRNA genes. To elucidate the significance of these three genotypes in pathogenesis, we measured two major virulence factors, extracellular proteinase and phospholipase activity, in 56 clinical isolates of C. albicans, and investigated the relationship between genotype and enzymatic activity. The genotype B strains had significantly higher proteinase and phospholipase activities than genotypes A or C. These results suggest that to understand the pathogenesis of C. albicans, the genotypes should be considered.

Fungal infection of the radicular dentin

Although fungi have been detected in infected root canals, their precise role as endodontic pathogens has not yet been elucidated. The purpose of this study was to investigate the pattern of radicular dentin colonization by five fungal species. Bovine root sections were infected with each of the following fungal species: Candida albicans, Candida glabrata, Candida guilliermondii, Candida parapsilosis, and Saccharomyces cerevisiae. After 14 days, the sections were fixed in glutaraldehyde, split into two halves, critical point-dried in CO2, sputter-coated with gold, and examined under scanning electron microscopy. Regardless of the species, single or budding yeast cells were the only fungal forms observed. C. albicans colonized most of the specimens. On the other hand, the other four fungal species presented discrete or no colonization of the radicular dentin. C. albicans showed different patterns of dentin infection. In some specimens, colonization of the dentinal surface was slight and no penetration within dentinal tubules was observed. In the other specimens, some areas of the root canal walls were covered with large colonies of yeast cells and some dentinal tubules were heavily infected. The results suggested that whereas C. albicans showed the ability to colonize dentin, the other four fungal species did not. This can help to explain why C. albicans is the fungal species most often found in endodontic infections.

Phospholipase and protease activities in clinical Candida isolates with reference to the sources of strains

The present study was aimed at determining in vitro phospholipase and protease activities in 95 clinical isolates of various Candida species (C. albicans, C. glabrata, C. guilliermondii,C.kefyr, C. krusei, C. lipolytica, C. lusitaniae, C. parapsilosis, C.rugosa and C. tropicalis). 59 (62.1%) of isolates examined were phospholipase positive and 75 (78.9.9%) were protease positive. 56 (93.3%) of C.albicans isolates tested were phospholipase producers - however only a few strains of C. glabrata and C. kefyr behaved in the same way. Protease activity was detected in 57 (95%) of C. albicans strains tested and in a few strains of C. kefyr, C.lipolytica, C. parapsilosis and C. tropicalis- the remaining isolates were negative. 56 strains of C.albicans and 2 strains of C. kefyr tested produced both phospholipase and protease.

fos-1, a putative histidine kinase as a virulence factor for systemic aspergillosis

In fungi, two-component histidine kinases have various functions including regulation of osmosensitivity, and of cell-wall assembly. Furthermore, one of these proteins, cos-1, has been shown to be important for virulence of Candida albicans. Recently, a putative histidine kinase, fos-1, has been isolated and partially characterized from Aspergillus fumigatus. Here we compare the virulence of a fos-1 deletion strain with that of the parental wild-type strain in a murine model of systemic aspergillosis. Our results show that the fos-1 deletion strain has significantly reduced virulence as compared with the parental wild-type strain. Thus, we propose that the fos-1 two-component histidine kinase is a virulence factor of A. fumigatus.