Candida dubliniensis: phylogeny and putative virulence factors

Comparative transcript profiling of Candida albicans and Candida dubliniensis identifies SFL2, a C. albicans gene required for virulence in a reconstituted epithelial infection model

Candida albicans and Candida dubliniensis are closely related species displaying differences in virulence and genome content, therefore providing potential opportunities to identify novel C. albicans virulence genes. C. albicans gene arrays were used for comparative analysis of global gene expression in the two species in reconstituted human oral epithelium (RHE). C. albicans (SC5314) showed upregulation of hypha-specific and virulence genes within 30 min postinoculation, coinciding with rapid induction of filamentation and increased RHE damage. C. dubliniensis (CD36) showed no detectable upregulation of hypha-specific genes, grew as yeast, and caused limited RHE damage. Several genes absent or highly divergent in C. dubliniensis were upregulated in C. albicans. One such gene, SFL2 (orf19.3969), encoding a putative heat shock factor, was deleted in C. albicans. DeltaDeltasfl2 cells failed to filament under a range of hypha-inducing conditions and exhibited greatly reduced RHE damage, reversed by reintroduction of SFL2 into the DeltaDeltasfl2 strain. Moreover, SFL2 overexpression in C. albicans triggered hyphal morphogenesis. Although SFL2 deletion had no apparent effect on host survival in the murine model of systemic infection, DeltaDeltasfl2 strain-infected kidney tissues contained only yeast cells. These results suggest a role for SFL2 in morphogenesis and an indirect role in C. albicans pathogenesis in epithelial tissues.

Monoclonal antibody-mediated inhibition of adhesion of Candida albicans and Candida dubliniensis to human epithelial cells

The aim of this study was to assess the role of whole saliva, four saliva-derived preparations, and six monoclonal antibodies (mAbs), directed against components of the cell wall of Candida albicans, on the adhesion of C. albicans and Candida dubliniensis to human epithelial cells (HEC). C. albicans serotype A NCPF 3153 and C. albicans serotype B ATCC 90028 showed higher adhesion to HEC than C. dubliniensis NCPF 3949. Pooled whole saliva was more efficient than salivary secretory immunoglobulin A, partially purified by chromatography, at inhibiting the adhesion of C. albicans serotype A NCPF 3153 to HEC. Monoclonal antibodies C7, 14-8, and 26G7 were the most potent inhibitors of adhesion. Our results show that mAbs can mimic the inhibition of adhesion of C. albicans to HEC that is mediated by human saliva.

Genetic relatedness of subgingival and buccal Candida dubliniensis isolates in immunocompetent subjects assessed by RAPD-PCR

Background

It is recognized that Candida dubliniensis commonly colonizes oral and subgingival sites in immunocompetent subjects with periodontal disease.

Objective

Since there are few data available on genetic characterization of C. dubliniensis in periodontal pockets and other oral sites, the aim of this study was to characterize subgingival and mucosal C. dubliniensis isolates recovered from immunocompetent subjects and to assay the genetic similarity of such isolates from both niches in the same patient by random amplified polymorphic DNA (RAPD).

Design

C. dubliniensis recovered from subgingival plaque and from buccal cavity samples were studied in 240 immunocompetent non-smoking individuals. Arbitrary amplification was carried out by RAPD-polymerase chain reaction (PCR).

Results

RAPD analysis showed identical genotypes of C. dubliniensis in different sampling sites (buccal cavity and subgingival areas) in eight of 10 patients except for those derived from two participants who presented presumably unrelated isolates.

Conclusions

On the basis of the findings presented, the origin of the colonization of C. dubliniensis in subgingival biofilm seems to be the buccal cavity in a single patient. Consequently, it may be assumed that most of C. dubliniensis in these sites arise from the endogenous commensal strains.

Comparative genomics of the fungal pathogens Candida dubliniensis and Candida albicans

Candida dubliniensis is the closest known relative of Candida albicans, the most pathogenic yeast species in humans. However, despite both species sharing many phenotypic characteristics, including the ability to form true hyphae, C. dubliniensis is a significantly less virulent and less versatile pathogen. Therefore, to identify C. albicans-specific genes that may be responsible for an increased capacity to cause disease, we have sequenced the C. dubliniensis genome and compared it with the known C. albicans genome sequence. Although the two genome sequences are highly similar and synteny is conserved throughout, 168 species-specific genes are identified, including some encoding known hyphal-specific virulence factors, such as the aspartyl proteinases Sap4 and Sap5 and the proposed invasin Als3. Among the 115 pseudogenes confirmed in C. dubliniensis are orthologs of several filamentous growth regulator (FGR) genes that also have suspected roles in pathogenesis. However, the principal differences in genomic repertoire concern expansion of the TLO gene family of putative transcription factors and the IFA family of putative transmembrane proteins in C. albicans, which represent novel candidate virulence-associated factors. The results suggest that the recent evolutionary histories of C. albicans and C. dubliniensis are quite different. While gene families instrumental in pathogenesis have been elaborated in C. albicans, C. dubliniensis has lost genomic capacity and key pathogenic functions. This could explain why C. albicans is a more potent pathogen in humans than C. dubliniensis.

Isolation of Candida dubliniensis from denture wearers

Candida albicans is considered the most important Candida species able to cause oral infections in denture wearers. In recent years, Candida dubliniensis has emerged as a pathogenic yeast in humans. The close phenotypic similarities of C. albicans and C. dubliniensis have led to the misidentification of these species. In this work, our aim was to verify through PCR the presence of C. dubliniensis in palate and maxillary denture samples from 112 denture wearers presenting with or without denture-related stomatitis (DRS). C. dubliniensis was isolated at low rates from both palate (5.3 % and 10.7 %) and maxillary denture (5.3 % and 8.9 %) samples from wearers regardless of the presence of the disease. However, when C. dubliniensis was detected in individuals with DRS, it was always associated with C. albicans. In addition, our results showed that C. albicans was the most commonly identified candidal species in maxillary denture and hard palate samples from DRS patients (78.5 % and 89.2 %, respectively) as well as from controls (31.2 % and 28.5 %, respectively). In conclusion, C. dubliniensis was detected in the oral environment of denture wearers. The association of C. dubliniensis with C. albicans occurred in approximately 10 % of the DRS cases.

Differential expression of Candida dubliniensis-secreted aspartyl proteinase genes (CdSAP1-4) under different physiological conditions and during infection of a keratinocyte culture

The in vitro and keratinocyte (HaCAT cells) culture expression of four putative genes coding for secreted aspartyl proteases of Candida dubliniensis-CdSAP1, CdSAP2, CdSAP3, and CdSAP4 (CdSAP1-4) - is reported for the first time. In addition, CdSAP7, 8, 9, and 10, orthologous genes of Candida albicans, were recognized in C. dubliniensis genome. There are no orthologs of C. albicans SAP5 and 6 in C. dubliniensis. The expression of CdSAP1 and 2 was independent of the morphological stage of C. dubliniensis; they are expressed at both pH 4 and pH 7, and were induced with albumin as nitrogen source. CdSAP3 expression was regulated by the pH, and was related to the infection process of keratinocytes. Expression of CdSAP4 predominated during the mycelial phase and the initial stage of keratinocyte infection. During infection of the HaCaT cell line, only genes CdSAP3-4 were expressed, and keratinocytes were affected in their number and shape by the infection with C. dubliniensis; however, this effect decreased in the presence of pepstatin A (aspartyl protease inhibitor). Pepstatin A was not able to inhibit keratinocyte damage. Based on the aforementioned, we suggest that the Saps from C. dubliniensis could be considered a virulence factor just as those from C. albicans, and participants in the nitrogen metabolism of the yeast for nutrient acquisition.

Effect of propranolol on hyphae formation signal in Candida albicans

Candida albicans (C. albicans) is known as an opportunistic pathogen that changes from a yeast form to a hyphae form in response to various outside environmental signals. The addition of propranolol inhibited hyphae formation of C. albicans. Propranolol inhibited the expression of agglutinin like sequence 3 (ALS3) and ALS8mRNA, which are regulated by the cAMP-EFG1 pathway in C. albicans. Propranolol did not affect the expression of CST20, HST7 or CPH1mRNA, which are components of the mitogen-activated protein (MAP) kinase cascade in C. albicans. The expression of CYR1mRNA, which encodes adenylate cyclase of C. albicans, was not affected by propranolol. These findings indicated that the interruption of hyphae formation by propranolol is caused by inhibition of the cAMP-EFG1 pathway, but not effects on the MAP kinase cascade.

Genome-wide gene expression profiling and a forward genetic screen show that differential expression of the sodium ion transporter Ena21 contributes to the differential tolerance of Candida albicans and Candida dubliniensis to osmotic stress

Candida albicans is more pathogenic than Candida dubliniensis. However, this disparity in virulence is surprising given the high level of sequence conservation and the wide range of phenotypic traits shared by these two species. Increased sensitivity to environmental stresses has been suggested to be a possible contributory factor to the lower virulence of C. dubliniensis. In this study, we investigated, in the first comparison of C. albicans and C. dubliniensis by transcriptional profiling, global gene expression in each species when grown under conditions in which the two species exhibit differential stress tolerance. The profiles revealed similar core responses to stresses in both species, but differences in the amplitude of the general transcriptional responses to thermal, salt and oxidative stress. Differences in the regulation of specific stress genes were observed between the two species. In particular, ENA21, encoding a sodium ion transporter, was strongly induced in C. albicans but not in C. dubliniensis. In addition, ENA21 was identified in a forward genetic screen for C. albicans genomic sequences that increase salt tolerance in C. dubliniensis. Introduction of a single copy of CaENA21 was subsequently shown to be sufficient to confer salt tolerance upon C. dubliniensis.

[Candida dubliniensis: Identification methods and epidemiologic implication]

Candida dubliniensis was recently described (1995) associated with oral candidiasis in HIV-positive patients. This organism is very closely related to the pathogenic human yeast, Candida albicans, and share a great number of phenotypic and genotypic characters. This great similarity limits the discrimination between these two species. Several phenotypic and molecular methods were developed. The phenotypic methods are simply used in routine discrimination between these two species and depend on the growth at high temperature, sugar assimilation, growth on special mediums and chlamydospore production…; but these methods are insensitive in discrimination between these two species. The molecular biology methods are highly reliable and able to confirm rapidly the identification of this species. In this article, we will review the various studies run out concerning the methods deployed for the identification of C. dubliniensis as well as the epidemiological implication of this new pathogen.

Production of monoclonal antibodies for detection of a secreted aspartyl proteinase from Candida spp. in biologic specimens

Secreted acid proteinases (SAP) constitute an important group of virulence factors in Candida albicans. In the present work, an acid proteinase from C. albicans was sequentially purified from the supernatant of a yeast culture by precipitation with ammonium sulfate, ion exchange chromatography, and molecular exclusion chromatography, yielding a specific enzymatic activity of 204.1 IU/mg on bovine serum albumin (BSA). The molecular mass of the purified proteinase was estimated at 43 kd after exclusion chromatography and at 41 kd by nondenaturating sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The purified proteinase was able to degrade BSA at pH 2.5, but was not active on collagen, and it was significantly inhibited by pepstatin A. The immunization of BALB/c mice with the purified proteinase and later fusion of their spleen cells with myeloma cells resulted in 19 monoclonal antibody secreting hybridomas (MAbs) capable of detecting SAP in enzyme-linked immunosorbent assay (ELISA) assays. All MAbs obtained are isotype IgG1 kappa (kappa) immunoglobulins and develop a 41 kd protein band by Western blot (WB) in samples of SAP obtained from C. albicans (12-A) and C. dubliniensis (strain 778) crude extracts. The anti-SAP MAbs were used in capture ELISA and two combinations of these antibodies proved suitable for SAP detection, that is, MAP1 (1B1B3) or MAP2 (2D2C10) as coat antibodies, and biotinylated MAP3 (2A6E8) as detect antibody. Capture ELISA using these sets of MAbs detected over 32 ng/mL protein in purified SAP samples as well as in crude C. albicans and C. dubliniensis extracts. The results herein obtained allow for the prediction of how this set of antibodies can be useful for SAP detection in biologic specimens.

Differential expression of Candida albicans secreted aspartyl proteinase in human vulvovaginal candidiasis

The opportunistic pathogen Candida albicans secretes aspartic proteinases (Saps), which raises a particular interest because of their role as virulence factors. Candida albicans possesses at least 10 members of a SAP gene family, all of which have been sequenced. Although the expression of the SAP genes has been extensively characterised under laboratory growth conditions and Saps contribute to the virulence of C. albicans in animal models of infection, few studies have analysed the difference of the in vivo expression of these proteinases in recurrent vulvovaginal candidiasis (RVVC), vulvovaginal candidiasis (VVC) and asymptomatic Candida carriers. The RT-PCR protocol was used to determine which of the SAP 1 to SAP 7 genes are expressed by C. albicans during asymptomatic Candida carriage, VVC and RVVC infection in this study. We found SAP 2, the SAP 4-6 and SAP 7 were the predominant proteinase genes expressed in the sample of both Candida carriers and patients with VVC and RVVC. SAP 2 and SAP 4-6 mRNA were detected in all subjects. SAP 1 and SAP 3 transcripts were observed only in patients with VVC and RVVC. SAP 7 mRNA expressions were detected in several of the patients and carriers samples. All seven SAP genes were simultaneously expressed in some patients with VVC and RVVC. This study demonstrates the differential expression of the secreted aspartic proteinases (Saps) genes during colonisation and VVC and RVVC infection in humans and correlates the expression of specific Candida species virulence genes with active disease and anatomical location.

The involvement of physico-chemical interactions in the adhesion of Candida albicans and Candida dubliniensis to epithelial cells

Candida albicans and Candida dubliniensis are two pathogenic yeasts particularly hazardous to immunocompromised patients. Adhesion of yeast cells to epithelium is considered one of the virulence factors and its study is of major importance. The main aim of this study was the comparison of the influence of physico-chemical properties on the adhesion of C. albicans and C. dubliniensis to epithelium. Two strains of each Candida species were used in the adhesion assays to HeLa cells. Adhered cells were enumerated by direct microscopic images observation. Yeast cell surface tension parameters and degree of hydrophobicity were determined by contact angle measurement. Pseudohyphae and hyphae formation was analysed by scanning electron microscopy. Yeast cells presented no statistical differences concerning their physico-chemical surface properties. However, the extent of adhesion to epithelium was different among the four strains. As general conclusion, yeast adhesion to epithelium seems to be strain-dependent and not directly correlated with pseudohyphae formation.

Structure-based specificity mapping of secreted aspartic proteases of Candida parapsilosis, Candida albicans, and Candida tropicalis using peptidomimetic inhibitors and homology modeling

Secreted aspartic proteases (Saps) of pathogenic Candida spp. represent a specific target for antifungal drug development. We synthesized a series of peptidomimetic inhibitors with different isosteric groups and modifications at individual positions and tested them with purified Saps from C. albicans (Sap2p), C. tropicalis (Sapt1p), and C. parapsilosis (Sapp1p). The kinetic parameters indicated that all three proteases prefer binding of inhibitors containing bulky hydrophobic residues between positions P3 and P3'. The most divergent specificity was found for Sapp1p. The sequence alignment of Sap2p, Sapt1p, and Sapp1p, and homology modeling of Sapp1p with the crystal structure of Sapt1p and the complex of Sap2p with a peptidomimetic inhibitor showed that the overall folds of Sap2p, Sapt1p, and Sapp1p are similar. However, the N- and C-terminal loops formed by disulfide bonds between residues 47-53 and 258-292 are significantly shorter in Sapp1p, and a unique insertion following Tyr 129 in Sapp1p results in the formation of a loop that can interact with inhibitor residues. These Sapp1p structural differences might lead to its altered susceptibility to inhibition.

[An update on Candida dubliniensis]

Eleven years ago, Irish authors, using molecular biology, demonstrated the existence of Candida dubliniensis, a new species of Candida close to Candida albicans. Initially isolated from AIDS patients with oral candidiasis, this species was detected, even in immunocompetent patients. Recently, with new, easy to implement identification tests (latex, immunochromatography), numerous epidemiological studies were undertaken. In most studies, C. dubliniensis was most often identified in the oral cavity. In the absence of HIV infection, the proportion C. dubliniensis/C. albicans ranged from 1 to 5% but it increased to 15-20% in case of HIV infection. It should be stressed that, from an experimental point of view, the acquisition of a secondary resistance to fluconazole is more quickly obtained with C. dubliniensis that with C. albicans, this resistance remains exceptionally observed in clinical observations.

Multilocus sequence typing reveals that the population structure of Candida dubliniensis is significantly less divergent than that of Candida albicans

The pathogenic yeast Candida dubliniensis is phylogenetically very closely related to Candida albicans, and both species share many phenotypic and genetic characteristics. DNA fingerprinting using the species-specific probe Cd25 and sequence analysis of the internal transcribed spacer (ITS) region of the ribosomal gene cluster previously showed that C. dubliniensis is comprised of three major clades comprising four distinct ITS genotypes. Multilocus sequence typing (MLST) has been shown to be very useful for investigating the epidemiology and population biology of C. albicans and has identified many distinct major and minor clades. In the present study, we used MLST to investigate the population structure of C. dubliniensis for the first time. Combinations of 10 loci previously tested for MLST analysis of C. albicans were assessed for their discriminatory ability with 50 epidemiologically unrelated C. dubliniensis isolates from diverse geographic locations, including representative isolates from the previously identified three Cd25-defined major clades and the four ITS genotypes. Dendrograms created by using the unweighted pair group method with arithmetic averages that were generated using the data from all 10 loci revealed a population structure which supports that previously suggested by DNA fingerprinting and ITS genotyping. The MLST data revealed significantly less divergence within the C. dubliniensis population examined than within the C. albicans population. These findings show that MLST can be used as an informative alternative strategy for investigating the population structure of C. dubliniensis. On the basis of the highest number of genotypes per variable base, we recommend the following eight loci for MLST analysis of C. dubliniensis: CdAAT1b, CdACC1, CdADP1, CdMPIb, CdRPN2, CdSYA1, exCdVPS13, and exCdZWF1b, where "Cd" indicates C. dubliniensis and "ex" indicates extended sequence.

Immune evasion by acquisition of complement inhibitors: the mould Aspergillus binds both factor H and C4b binding protein

Pathogenic fungi represent a major threat particularly to immunocompromised hosts, leading to severe, and often lethal, systemic opportunistic infections. Although the impaired immune status of the host is clearly the most important factor leading to disease, virulence factors of the fungus also play a role. Factor H (FH) and its splice product FHL-1 represent the major fluid phase inhibitors of the alternative pathway of complement, whereas C4b-binding protein (C4bp) is the main fluid phase inhibitor of the classical and lectin pathways. Both proteins can bind to the surface of various human pathogens conveying resistance to complement destruction and thus contribute to their pathogenic potential. We have recently shown that Candida albicans evades complement by binding both Factor H and C4bp. Here we show that moulds such as Aspergillus spp. bind Factor H, the splicing variant FHL-1 and also C4bp. Immunofluorescence and flow cytometry studies show that the binding of Factor H and C4bp to Aspergillus spp. appears to be even stronger than to Candida spp. and that different, albeit possibly nearby, binding moieties mediate this surface attachment.

Differential regulation of the transcriptional repressor NRG1 accounts for altered host-cell interactions in Candida albicans and Candida dubliniensis

Candida dubliniensis is genetically closely related to Candida albicans, but causes fewer infections in humans and exhibits reduced virulence and filamentation in animal models of infection. We investigated the role of the C. dubliniensis transcriptional repressor-encoding gene CdNRG1 in regulating this phenotype. Deletion of both copies of CdNRG1 increased the formation of true hyphae by C. dubliniensis in response to serum, exogenous cAMP and CO2. In addition, deletion of CdNRG1 greatly enhanced filamentation and survival of C. dubliniensis in co-culture with murine macrophages. In the reconstituted human oral epithelium infection model, the nrg1Delta mutant caused increased tissue damage relative to the wild-type strain. However, deletion of CdNRG1 did not change the virulence of C. dubliniensis in the systemic mouse model of infection. The increased rate of hypha formation in C. albicans relative to C. dubliniensis in response to phagocytosis by macrophages and serum was associated with rapid downregulation of NRG1 expression in C. albicans. This study demonstrates that the reduced virulence and host cell damage elicited by C. dubliniensis may in part be due to the inability of this species to modulate NRG1 expression in response to the same environmental signals that promote filamentation in C. albicans.

Lower filamentation rates of Candida dubliniensis contribute to its lower virulence in comparison with Candida albicans

Candida albicans and C. dubliniensis are very closely related yeast species. In this study, we have conducted a thorough comparison of the ability of the two species to produce hyphae and their virulence in two infection models. Under all induction conditions tested C. albicans consistently produced hyphae more efficiently than C. dubliniensis. In the oral reconstituted human epithelial model, C. dubliniensis isolates grew exclusively in the yeast form, while the C. albicans strains produced abundant hyphae that invaded and caused significant damage to the epithelial tissue. In the oral-intragastric infant mouse infection model, C. dubliniensis strains were more rapidly cleared from the gastrointestinal tract than C. albicans. Immunosuppression of Candida-infected mice caused dissemination to internal organs by both species, but C. albicans was found to be far more effective at dissemination than C. dubliniensis. These data suggest that a major reason for the comparatively low virulence of C. dubliniensis is its lower capacity to produce hyphae.

Enzymatic and hemolytic activities of Candida dubliniensis strains

Candida dubliniensis is an opportunistic yeast that has been recovered from several body sites in many populations; it is most often recovered from the oral cavities of human immunodeficiency virus-infected patients. Although extensive studies on epidemiology and phylogeny of C. dubliniensis have been performed, little is known about virulence factors such as exoenzymatic and hemolytic activities. In this study we compared proteinase, hyaluronidase, chondroitin sulphatase and hemolytic activities in 18 C. dubliniensis and 30 C. albicans strains isolated from AIDS patients. C. albicans isolates produced higher amounts of proteinase than C. dubliniensis (p < 0.05). All the tested C. dubliniensis strains expressed hyaluronidase and chondroitin sulphatase activities, but none of them were significantly different from those observed with C. albicans (p > 0.05). Hemolytic activity was affected by CaCl2; when this component was absent, we did not notice any significant difference between C. albicans and C. dubliniensis hemolytic activities. On the contrary, when we added 2.5 g% CaCl2, the hemolytic activity was reduced on C. dubliniensis and stimulated on C. albicans tested strains (p < 0.05).

Extensive chromosome rearrangements distinguish the karyotype of the hypovirulent species Candida dubliniensis from the virulent Candida albicans

Candida dubliniensis and Candida albicans, the most common human fungal pathogen, have most of the same genes and high sequence similarity, but C. dubliniensis is less virulent. C. albicans causes both mucosal and hematogenously disseminated disease, C. dubliniensis mostly mucosal infections. Pulse-field electrophoresis, genomic restriction enzyme digests, Southern blotting, and the emerging sequence from the Wellcome Trust Sanger Institute were used to determine the karyotype of C. dubliniensis type strain CD36. Three chromosomes have two intact homologues. A translocation in the rDNA repeat on chromosome R exchanges telomere-proximal regions of R and chromosome 5. Translocations involving the remaining chromosomes occur at the Major Repeat Sequence. CD36 lacks an MRS on chromosome R but has one on 3. Of six other C. dubliniensis strains, no two had the same electrophoretic karyotype. Despite extensive chromosome rearrangements, karyotypic differences between C. dubliniensis and C. albicans are unlikely to affect gene expression. Karyotypic instability may account for the diminished pathogenicity of C. dubliniensis.

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.

Effect of farnesol on Candida dubliniensis morphogenesis

AIMS

Cell-cell signalling in Candida albicans is a known phenomenon and farnesol was identified as a quorum sensing molecule determining the yeast morphology. The aim of this work was to verify if farnesol had a similar effect on Candida dubliniensis, highlighting the effect of farnesol on Candida spp. morphogenesis.

METHODS AND RESULTS

Two different strains of C. dubliniensis and one of C. albicans were grown both in RPMI 1640 and in serum in the presence of absence of farnesol. At 150 micromol l(-1) farnesol the growth rate of both Candida species was not affected. On the contrary, farnesol inhibited hyphae and pseudohyphae formation in C. dubliniensis.

CONCLUSION

Farnesol seems to mediate cell morphology in both Candida species.

SIGNIFICANCE AND IMPACT OF THE STUDY

The effect of farnesol on C. dubliniensis morphology was not reported previously.

Prevalence and phenotypic evaluation of Candida dubliniensis in pregnant women with vulvovaginal candidosis in a university hospital in Ankara

Candida dubliniensis is very similar to Candida albicans in terms of genotypic and phenotypic characteristics. As the hormonal milieu of the vagina during pregnancy, characterised by a lack of maternal cell-mediated immunity, enhances Candida colonisation and serves as a risk factor for symptomatic expression, investigation into the isolation of C. dubliniensis in vaginal discharges of pregnant women with vulvovaginal candidosis was made. A total of 77 Candida isolates obtained from 60 patients positive for vulvovaginal candidosis collected from 218 pregnant women were investigated for C. dubliniensis subsistence. In total 41 Candida species phenotypically identified as C. albicans on the basis of a positive germ tube test and carbohydrate assimilation tests were screened for the presence of C. dubliniensis. Phenotypic tests for differentiation of C. dubliniensis from C. albicans, such as growth at 42 and 45 degrees C on Sabouraud dextrose agar, appearance on CHROMagar and colony morphology on Cornmeal-Tween-80 agar and Staib agar were carried out. Only one strain (2.43%) was phenotypically identified as C. dubliniensis. According to our study, a combination of at least five phenotypic methods is necessary for an exact diagnosis of C. dubliniensis. Large-scale studies of pregnant women are required to discover the aetiological importance of this yeast.

Candida species adhesion to oral epithelium: factors involved and experimental methodology used

Due to the increasing prevalence and emergence of Non-Candida albicans Candida (NCAC) species, especially in immunosupressed patients, it is becoming urgent to deepen the current knowledge about virulence factors of these species. Adhesion of cells to epithelium is considered one of the major virulence factors of Candida species. However, relatively little is known concerning the adhesion mechanisms of NCAC species to epithelium, as well as about the factors affecting the adhesion process. This review focuses both the mechanisms that regulate the adhesion interactions and the factors involved and the description of the experimental methodology that has been used to perform the adhesion assays.

Rapid PCR-based test for identifying Candida albicans by using primers derived from the pH-regulated KER1 gene

A PCR-based method in combination with a simple, reliable and inexpensive DNA extraction procedure for rapid detection of Candida albicans clinical isolates is described here. The extraction protocol is based on a combination of chemical (NaOH and detergents) and physical (boiling) treatments, thus avoiding many of the problems inherent in the currently available DNA extraction protocols (basically the use of expensive and/or toxic chemical reagents), and may be useful for daily clinical routine. The PCR-based system described here uses a single pair of primers (SC1F and SC1R) deduced from the C. albicans-specific KER1 gene sequence. These primers amplify a 670-bp fragment of the KER1 gene. All the clinical C. albicans isolates generated the expected 670-bp amplicon. Other non-albicans Candida species, including the azole-resistant C. krusei and C. glabrata, and the very closely related C. dubliniensis, failed to amplify any DNA fragment. The PCR results reported here suggest that amplification with SC1F and SC1R primers is species specific and, consequently, may be useful for specifically identifying C. albicans strains.

In vitro evaluation of virulence attributes of Candida spp. isolated from patients affected by diabetes mellitus

BACKGROUND

Diabetes mellitus is a common disease found worldwide and it has been previously suggested that oral candidal infections may be more frequent or severe in patients with this disease. Systemic and local factors may influence the balance between the host and yeasts, and favour the transformation of Candida isolates from commensal to pathogenic microorganisms. Candida species have developed specific virulence mechanisms that confer the ability to colonise host surfaces, to invade deeper host tissue, or to evade host defences. Few studies have investigated the expression of the virulence attributes of oral Candida isolates in patients with diabetes mellitus.

MATERIAL AND METHODS

The in vitro extracellular proteinase production and the in vitro ability to adhere to fibronectin of 229 Candida isolates of two geographic different groups of patients with diabetes mellitus and of healthy subjects were assessed.

RESULTS

Candida isolates of patients with diabetes mellitus expressed a higher ability to adhere than those of healthy subjects. Higher levels of adhesion were also recorded in patients with a lower oral Candida colonisation. No differences were observed in the in vitro expression of extracellular proteinase of Candida isolates of patients with diabetes mellitus and those of non-diabetic subjects. Isolates of patients with type 2 diabetes mellitus expressed greater levels of proteinase than isolates of type 1 diabetes mellitus.

CONCLUSIONS

Diabetes mellitus could be considered as an additional variable that may influence not only oral Candida carriage but also the ability of isolates to enhance the expression of virulence attributes.

Phenotypic methods and commercial systems for the discrimination between C. albicans and C. dubliniensis

Candida dubliniensis is a recently described Candida species associated with oral candidosis that exhibits a high degree of phenotypic similarity to Candida albicans. However, these species show differences in levels of resistance to antimycotic agents and ability to cause infections. Therefore, accurate clinical identification of C. dubliniensis and C. albicans species is important in order to treat oral candidal infections. Phenotypic identification methods are easy-to-use procedures for routine discrimination of oral isolates in the clinical microbiology laboratory. However, C. dubliniensis may be so far underreported in clinical samples because most currently used identification methods fail to recognize this yeast. Phenotypic methods depend on growth temperature, carbon source assimilation, chlamydospore and hyphal growth production, positive or negative growth on special media and intracellular enzyme production, among others. In this review, some phenotypic methods are presented with a special emphasis on the discrimination of C. dubliniensis and C. albicans.

Hydrolytic enzymes as virulence factors of Candida albicans

Candida albicans is a facultative pathogenic micro-organism that has developed several virulence traits enabling invasion of host tissues and avoidance of host defence mechanisms. Virulence factors that contribute to this process are the hydrolytic enzymes. Most of them are extracellularly secreted by the fungus. The most discussed hydrolytic enzymes produced by C. albicans are secreted aspartic proteinases (Saps). The role of these Saps for C. albicans infections was carefully evaluated in numerous studies, whereas only little is known about the physiological role of the secreted phospholipases (PL) and almost nothing about the involvement of lipases (Lip) in virulence. They may play an important role in the pathogenicity of candidosis and their hydrolytic activity probably has a number of possible functions in addition to the simple role of digesting molecules for nutrition. Saps as the best-studied member of this group of hydrolytic enzymes contribute to host tissue invasion by digesting or destroying cell membranes and by degrading host surface molecules. There is also some evidence that hydrolytic enzymes are able to attack cells and molecules of the host immune system to avoid or resist antimicrobial activity. High hydrolytic activity with broad substrate specificity has been found in several Candida species, most notably in C. albicans. This activity is attributed to multigene families with at least 10 members for Saps and Lips and several members for PL B. Distinct members of these gene families are differentially regulated in various Candida infections. In future, prevention and control of Candida infections might be achieved by pharmacological or immunological tools specifically modulated to inhibit virulence factors, e.g. the family of Saps.

Antimicrobial peptides enhance the candidacidal activity of antifungal drugs by promoting the efflux of ATP from Candida cells

OBJECTIVES

To establish a novel strategy of fungal infection control.

METHODS

We examined the influences of antimicrobial peptides including a synthesized short lactoferrin peptide (FKCRRWQWRM, Peptide 2; Pep2) on the synthesis of Candida cell wall polysaccharides, ergosterol synthesis, membrane permeability and the efflux of ATP.

RESULTS

Colony formation of Candida albicans was synergistically suppressed by a combination of low concentrations of each drug and peptide. All peptides and amphotericin B, but not itraconazole, revealed weak inhibitory activities against ergosterol synthesis and the peptides weakly suppressed the synthesis of Candida cell wall components, glucan, mannan and chitin. Cell membrane permeability was not only increased by these peptides but also clearly increased by both amphotericin B and itraconazole. ATP efflux was however up-regulated by low concentrations of the peptides, especially by Pep2 and Hst5, although both antifungal drugs did not exert any influence on ATP efflux. The expression of the Candida drug resistance genes 1 and 2 (CDR1 and CDR2) was increased by both drugs, but this increase was suppressed by each peptide. In addition, larger amounts of amphotericin B and itraconazole remained in Candida cells in the presence of Pep2 or Hst5 due to the lower excretion. The effects of both peptides on ATP efflux and increase of intercellular amphotericin B and itraconazole were blocked by anion channel inhibitors 4,4'-diisothiocyanatestilbene-2, 2'-disulphonic acid and 5-nitro-2-(3-phenylpropylamino) benzoic acid.

CONCLUSIONS

The examined peptides, especially Pep2 and Hst5, enhance the candidacidal activity of antifungal drugs by promoting anion channel-associated ATP efflux from Candida cells and decreasing efflux of the drugs, which could be useful clinical applications.

Analysis and expression of STE13ca gene encoding a putative X-prolyl dipeptidyl aminopeptidase from Candida albicans

Candida albicans STE13ca gene was identified by its homology to the Saccharomyces cerevisiae STE13 gene that encodes for the dipeptidyl aminopeptidase A (DAP A) involved in the maturation of alpha-factor mating pheromone. Our study revealed that C. albicans ATCC 10231 depicts dipeptidyl aminopeptidase activity. We also analyzed the expression of the STE13ca gene homologue from this pathogenic yeast. This gene of 2793 pb is homozygotic and encodes for a predicted protein of 930 amino acids with a molecular weight of 107,035 Da. The predicted protein displays significant sequence similarity to S. cerevisiae Ste13p. This C. albicans gene is located in chromosome R. STE13ca gene increases its levels of expression in conditions of nutritional stress (proline as nitrogen source) and during formation of the germinal tube, suggesting a basic biological function for the STE13ca in this yeast.

Candida dubliniensis: ten years on

Candida dubliniensis was first described as a novel species in 1995. This organism is very closely related to the important human yeast pathogen, Candida albicans. However, despite the very close phylogenetic relationship between C. albicans and C. dubliniensis and the fact that they share a large number of phenotypic traits, epidemiological and virulence model data indicate that the former is a far more successful pathogen. In order to investigate the molecular basis of the lower virulence of C. dubliniensis recent comparative genomic hybridisation studies have revealed the absence and divergence of specific genes implicated in candidal virulence. Data from the C. dubliniensis genome sequencing project will allow a complete comparison between the genomes of the two species to be performed and thus enhance our understanding of candidal virulence and how virulence has evolved in Candida species.

Prevalence of Candida dubliniensis among germ tube-positive Candida isolates in a maternity hospital in Kuwait

In this study, 1644 germ tube-positive Candida isolates from a maternity hospital was prospectively examined for the prevalence of Candida dubliniensis. Candida species were isolated from different clinical specimens, but majority (>90%) of them came from high vaginal swabs and urine specimens. The phenotypic and molecular identification methods for C. dubliniensis included production of rough colonies and chlamydospores on simplified sunflower seed agar, determination of assimilation profile by Vitek 2 yeast identification system, specific amplification of rDNA of internally transcribed spacer (ITS)-2 region by semi-nested PCR and direct DNA sequencing of the ITS-1 and ITS-2 regions. Three germ tube-positive Candida isolates were identified as C. dubliniensis with an overall prevalence of 0.2%. Of these, two came from urine specimens and one from a vaginal swab. None of the C. dubliniensis isolates showed resistance against fluconazole, voriconazole and amphotericin B. The study reinforces the usefulness of sunflower seed agar in presumptive identification of C. dubliniensis and confirms the prevailing view that this species forms only a minor constituent of Candida species occurring in vagina or other anatomic sites of non-HIV/AIDS-infected individuals.

Differential expression of the NRG1 repressor controls species-specific regulation of chlamydospore development in Candida albicans and Candida dubliniensis

Candida albicans and Candida dubliniensis are opportunistic fungal pathogens that are closely related but differ in their epidemiology and in some phenotypic characteristics, including certain virulence-related traits. A comparison of these two species at the molecular level could therefore provide new insights into the biology and pathogenicity of Candida. Both species share the ability to produce chlamydospores, but only C. dubliniensis forms pseudohyphae with abundant chlamydospores on Staib agar (syn. Guizotia abyssinica creatinine agar), on which C. albicans grows as a budding yeast. To understand the basis of this species-specific, differential regulation of morphogenetic development, we set out to identify C. albicans genes that repress chlamydospore formation under these conditions. A C. albicans genomic library was integrated into the C. dubliniensis genome and transformants were screened for clones in which filamentation and/or chlamydospore production on Staib agar was suppressed. This screen identified two genes, CaNRG1 and CaPDE2, encoding a general transcriptional repressor and a high affinity cAMP phosphodiesterase, respectively. Expression of CaNRG1 in C. dubliniensis repressed pseudohyphae and chlamydospore formation, whereas expression of CaPDE2 only reduced the extent of filamentous growth but did not affect chlamydospore formation. We found that C. dubliniensis, but not C. albicans, specifically downregulates NRG1 expression on Staib medium to allow chlamydospore development. Artificial overexpression of CdNRG1 suppressed pseudohyphal growth and production of chlamydospores in C. dubliniensis. Conversely, deletion of CaNRG1 in C. albicans resulted in chlamydospore formation on Staib agar, confirming its central role in the regulation of this morphogenetic process. Our results demonstrate that differential regulation of a single gene, NRG1, in C. albicans and C. dubliniensis is responsible for their species-specific response to environmental signals that induce chlamydospore development.

Evaluation of API ID 32C and VITEK-2 to identify Candida dubliniensis

We have compared two diagnostic systems, API-ID32C (bioMerieux) and VITEK-2ID-YST (bioMerieux), in their ability to diagnose 50 clinical isolates of Candida dubliniensis. API identified 48 isolates and VITEK-2 identified 33 of the 50 isolates. Lactic acid assimilation showed highly dispersed results with API, being of special importance, since this test is one of the four that makes it possible to differentiate C. dubliniensis from C. albicans. The detection of one enzyme, Phosphate-4MU, was always positive with VITEK-2 when it should have been negative. Overall, API ID 32C obtained better results than VITEK-2. However, the latter is simpler to use and has a greater database.

Differential recovery of Candida species from subgingival sites in human immunodeficiency virus-positive and healthy children from Rio de Janeiro, Brazil

The prevalence of subgingival Candida species was studied in 52 human immunodeficiency virus (HIV)-positive and 42 HIV-negative children. Candida was cultured from 22 (42.3%) and 3 (7.1%) HIV-infected and control children, respectively. C. albicans was the most common Candida species isolated from HIV-infected children, followed by C. dubliniensis, C. glabrata, and C. tropicalis. In the HIV-positive group, the prevalence of Candida isolation was significantly higher in children who presented with low CD4(+)-T-lymphocyte counts, elevated viral loads, and gingivitis.

Comparison of phospholipase and proteinase activity in Candida albicans and C. dubliniensis

Although the production of virulence enzymes by Candida albicans has been extensively explored, little attention has been given to the virulence factors of C. dubliniensis. In the present study, an attempt was made to investigate phospholipase activity (Pz value) and secretory aspartyl proteinase production of C. dubliniensis and compare it with C. albicans. None of the 87 C. dubliniensis isolates tested, produced phosholipases whereas, in contrast all the 52 (100%) C. albicans isolates tested demonstrated varying degree of phospholipase activity (Pz value: 0.37-0.74), with 35 (67.3%) of them eliciting a higher phospholipase activity (Pz values between 0.37 and 0.50). Only 32% of the C. dubliniensis isolates exhibited moderate activity (score of 1+) of secretory aspartyl proteinase whereas a vast majority (68%) of them were non-proteolytic. On the contrary, a strong proteinase activity (score of 2+) was observed for 79% of C. albicans while the remaining 21% isolates showed moderate proteinase activity (score of 1+). As phospholipases and aspartyl proteinases of C. albicans are considered important virulence factors, the absence or lowered expression of these enzymes in C. dubliniensis may indicate the less virulent nature of this novel yeast species when compared with C. albicans.

Isolation and molecular identification of Candida dubliniensis from non-human immunodeficiency virus-infected patients in Kuwait

Candida dubliniensis is an emerging pathogen capable of causing oropharyngeal, vaginal and bloodstream infections. Although C. dubliniensis is similar to Candida albicans in several phenotypic characteristics, it differs from it with respect to epidemiology, certain virulence factors and the ability to develop resistance to fluconazole rapidly. In this study, the first seven isolations of C. dubliniensis from Kuwait are described, all originating from non-human immunodeficiency virus (HIV)-infected patients. The isolates were initially identified by the Vitek 2 yeast identification system, positive germ tube test, production of rough colonies and chlamydospores on Staib agar and by their inability to assimilate xylose, trehalose or methyl alpha-D-glucoside. The species identity of the isolates was subsequently confirmed by specific amplification of rDNA targeting the internally transcribed spacer 2 (ITS2), restriction endonuclease digestion of the amplified DNA and direct DNA sequencing of the ITS2. Using the E-test method, the MICs of C. dubliniensis test isolates were in the range 0.125-0.75 microg ml(-1) for fluconazole, 0.002-0.75 microg ml(-1) for itraconazole, 0.006-0.125 microg ml(-1) for ketoconazole, 0.002-0.5 microg ml(-1) for amphotericin B and 0.002-0.016 microg ml(-1) for voriconazole. Two of the isolates were resistant to 5-flucytosine (>32 microg ml(-1)), but none against fluconazole. The study reinforces the current view that C. dubliniensis has a much wider geographical and epidemiological distribution.

Interactions of Candida albicans with other Candida spp. and bacteria in the biofilms

AIMS

To study the interactions between Candida albicans and 12 other species of Candida and bacteria in biofilms.

METHODS AND RESULTS

The number of cells within growing biofilms in a polystyrene tube model was measured after adding C. albicans to preformed biofilms of other micro-organisms and vice versa. It was also measured after simultaneous biofilm formation of C. albicans and other micro-organisms. The number of cells of C. albicans within the growing biofilms decreased significantly (P < 0.05) when the fungus was added to preformed biofilms of Candida spp. and bacteria except, with C. parapsilosis, Torulopsis glabrata and the glycocalyx producer Pseudomonas aeruginosa. When C. parapsilosis, Staphylococcus epidermidis (nonglycocalyx producer) or Serratia marcescens was added to preformed biofilms of C. albicans, the number of cells of these micro-organisms increased in the growing biofilms.

CONCLUSIONS

Biofilms of C. albicans are capable of holding other micro-organisms and more likely to be heterogeneous with other bacteria and fungi in the environment and on medical devices.

SIGNIFICANCE AND IMPACT OF THE STUDY

Recognition of the heterogeneity of biofilm-associated organisms can influence treatment decisions, particularly in patients who do not respond to initial appropriate therapy.

Comparison of the epidemiology, drug resistance mechanisms, and virulence of and

Candida dubliniensis is a pathogenic yeast species that was first identified as a distinct taxon in 1995. Epidemiological studies have shown that C. dubliniensis is prevalent throughout the world and that it is primarily associated with oral carriage and oropharyngeal infections in human immunodeficiency virus (HIV)-infected and acquired immune deficiency syndrome (AIDS) patients. However, unlike Candida albicans, C. dubliniensis is rarely found in the oral microflora of normal healthy individuals and is responsible for as few as 2% of cases of candidemia (compared to approximately 65% for C. albicans). The vast majority of C. dubliniensis isolates identified to date are susceptible to all of the commonly used antifungal agents, however, reduced susceptibility to azole drugs has been observed in clinical isolates and can be readily induced in vitro. The primary mechanism of fluconazole resistance in C. dubliniensis has been shown to be overexpression of the major facilitator efflux pump Mdr1p. It has also been observed that a large number of C. dubliniensis strains express a non-functional truncated form of Cdr1p, and it has been demonstrated that this protein does not play a significant role in fluconazole resistance in the majority of strains examined to date. Data from a limited number of infection models reflect findings from epidemiological studies and suggest that C. dubliniensis is less pathogenic than C. albicans. The reasons for the reduced virulence of C. dubliniensis are not clear as it has been shown that the two species express a similar range of virulence factors. However, although C. dubliniensis produces hyphae, it appears that the conditions and dynamics of induction may differ from those in C. albicans. In addition, C. dubliniensis is less tolerant of environmental stresses such as elevated temperature and NaCl and H(2)O(2) concentration, suggesting that C. albicans may have a competitive advantage when colonising and causing infection in the human body. It is our hypothesis that a genomic comparison between these two closely-related species will help to identify virulence factors responsible for the far greater virulence of C. albicans and possibly identify factors that are specifically implicated in either superficial or systemic candidal infections.

Fluctuations in haemocyte density and microbial load may be used as indicators of fungal pathogenicity in larvae of Galleria mellonella

A positive correlation exists between the pathogenicity of bacteria and fungi when evaluated in the insect Galleria mellonella and mice. This work sought to determine whether fluctuations in the number of haemocytes and the proliferation of yeast cells in infected larvae could be used to determine the relative pathogenicity of a range of yeast isolates. Larvae were inoculated with 1 x 10(6) stationary-phase yeast cells and incubated in the dark at 30 degrees C for 48 h. The results indicated that larvae inoculated with the most pathogenic isolates (i.e. those capable of killing >80% of infected larvae) showed a significant reduction in haemocyte density. Larvae inoculated with isolates of low pathogenicity (i.e. capable of killing <20% of infected larvae) demonstrated only a small fluctuation in haemocyte numbers. The most pathogenic yeast isolates proliferated in the larvae, whereas the isolates of low pathogenicity did not. These results demonstrate a relationship between the ability of yeast isolates to kill larvae and changes in haemocyte density and yeast cell density in infected larvae. These end points may extend the applicability of the G. mellonella system for use with a wider range of microbial isolates.

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.