Candida dubliniensis: phylogeny and putative virulence factors

Asynchronous cell cycle and asymmetric vacuolar inheritance in true hyphae of Candida albicans

Candida albicans forms unconstricted hyphae in serum-containing medium that are divided into discrete compartments. Time-lapse photomicroscopy, flow cytometry, and a novel three-dimensional imaging system were used to demonstrate that the kinetics and cell cycle events accompanying hyphal development were correlated with dynamic changes in vacuole morphology and the pattern of vacuole inheritance. Apical cells of hyphae underwent continuous extension before and after the first cytokinesis event. However, the resulting mother cell and sub-apical compartments did not immediately reenter the cell cycle and instead underwent cell cycle arrest before reentering the cycle. Vacuole was inherited asymmetrically at cytokinesis so that the distal, arrested compartments inherited most vacuole and the growing apical cell inherited most cytoplasm. Hydroxyurea release experiments demonstrated that the arrested, vacuolated hyphal compartments were in the G(1) phase of the cycle. The period of cell cycle arrest was decreased by the provision of assimilatable forms of nitrogen, suggesting that the hyphal cell cycle is regulated by nitrogen limitation that results in sup-apical cell cycle arrest. This pattern of growth is distinct from that of the synchronous, symmetrical development of pseudohyphae of C. albicans and other yeast species. These observations suggest that the cellular vacuole space correlates with alterations in the cell cycles of different cell types and that the total organelle space may influence size-regulated functions and hence the timing of the eukaryotic cell cycle.

The use of Niger seed agar to screen for Candida dubliniensis in the clinical microbiology laboratory

Candida dubliniensis is a recently described pathogenic yeast that is closely related to C. albicans. The germ tube test is used routinely in diagnostic laboratories for the identification of C. albicans, and C. dubliniensis may also produce germ tubes under the same conditions. We evaluated a previously described method for differentiating between the two species using Niger seed agar (Staib agar). The aim was to find a useful, user-friendly and cost-effective method for use in diagnostic work. C. albicans produces only yeast cells on this medium after 24 h at 37 degrees C, while C. dubliniensis produces extensive hyphal and pseudohyphal growth that is easily observed. Of 495 yeasts isolated in, or sent for identification to, a diagnostic mycology laboratory 9 isolates (1.8%) were found to be C. dubliniensis. The method was found to be valuable for screening yeasts before proceeding to further identification if positive for hyphal/pseudohyphal growth on Niger seed agar. This method is therefore suitable for the screening of selected yeast isolates in order to identity C. dubliniensis and will further our understanding of the clinical importance of this species.

Erythromycin, an inhibitor of mitoribosomal protein biosynthesis, alters the amphotericin B susceptibility of Candida albicans

Exposure of the yeast Candida albicans to the macrolide antibiotic erythromycin (C(37)H(67)NO(13)) results in elevated tolerance to the polyene antifungal amphotericin B. Erythromycin displays no fungistatic activity against C. albicans but inhibits the synthesis of cytochromes, particularly cytochrome aa(3). Consequently there is a reduction in aerobic respiration by up to 90% when cells are exposed to 10 mg mL(-1) erythromycin. Cellular ergosterol levels are also severely reduced. Erythromycin inhibits protein biosynthesis in ribosomes (mitoribosomes) located within the mitochondrion of the yeast cell, which results in a disruption of cytochrome biosynthesis with an adverse effect on respiration. The synthesis of ergosterol is oxygen dependent and consequently ergosterol levels are depleted in erythromycin-treated C. albicans. Ergosterol is the target for amphotericin B and since there is less of this sterol in erythromycin-treated cells, there is an increase in tolerance of the antifungal agent. Our work indicates that co-administration of erythromycin and amphotericin B to control bacterial and fungal infections, respectively, may inadvertently lead to an elevation in the tolerance of C. albicans for this antifungal agent.

Simple method for screening Candida species isolates for the presence of secreted proteinases: a tool for the prediction of successful inhibitory treatment

The yeasts of the genus Candida are opportunistic pathogens associated with the rising incidence of life-threatening infections in immunocompromised individuals. Secretion of aspartic proteinases has been determined to be one of the virulence factors of the pathogenic Candida species. To analyze the extracellular proteolytic activities of a large number of Candida clinical isolates, we developed a screening system based on a solid medium containing hemoglobin as the sole nitrogen source. The cleavage of hemoglobin by the secreted proteinases results in formation of clearance zones. The visibility of such zones was enhanced by addition of an acid-base indicator. Using this system, we assessed 245 clinical isolates of Candida from patients in the hospital of the Faculty of Medicine, Palacky University, Olomouc, Czech Republic, for the presence of secreted aspartic proteases (Saps). We also used the test plates for rapid semiquantitative testing of Sap inhibitors. Most of the pepstatin analogs affected the formation of the zones of clearance as well as the growth of Candida albicans, C. tropicalis, and C. parapsilosis colonies. By contrast, the human immunodeficiency virus proteinase inhibitors saquinavir, ritonavir, nelfinavir, and indinavir had no effect on the Candida strains tested. These results are in agreement with the inhibition constants obtained for the individual inhibitors with purified Saps. Thus, the plates containing hemoglobin proved to be an appropriate tool for the rapid and reliable assessment of Sap production and inhibition.

Secreted aspartic proteases as virulence factors of Candida species

Candida infections have emerged as a significant medical problem during the last few decades. Among the different virulence traits of C. albicans, secreted proteolytic activity has been intensively investigated. Pathogenesis of the various forms of candidiasis was shown to be associated with the differential and temporal regulation of the expression of genes coding for secreted aspartic proteases (Sap). These enzymes act as cytolysins in macrophages after phagocytosis of Candida, are present in tissue penetration and are also involved in adherence to epithelial cells. Since the introduction of new antiretroviral therapeutics such as HIV protease inhibitors, oropharyngeal candidiasis is less often observed in AIDS patients. Different HIV aspartic protease inhibitors were able to inhibit the C. albicans Saps involved in adherence. The lower rates of oropharyngeal candidiasis observed in individuals receiving antiretroviral combination therapy could reflect not only an improvement in the immune system but also direct inhibition of Candida Saps by HIV protease inhibitors. Therefore, the development of specific aspartic protease inhibitors might be of interest for the inhibition of candidiasis.

Adherence of different Candida dubliniensis isolates in the presence of fluconazole

BACKGROUND

The recently described yeast species Candida dubliniensis is closely related to C. albicans and has been recovered predominantly from the oral cavities of HIV-infected individuals and AIDS patients who are often receiving fluconazole as prophylactic or therapeutic treatment for oropharyngeal candidiasis. Like C. albicans, C. dubliniensis secretes aspartic proteinases which in C. albicans have been shown to be involved in adherence.

OBJECTIVE

To explain the increasing prevalence of C. dubliniensis in AIDS patients and to investigate the virulence factors of this yeast.

METHODS

An in vitro assay was developed to compare the adherence to epithelial cells of C. dubliniensis from HIV-patients with that of C. albicans.

RESULTS

All C. albicans isolates adhered better than the 22 C. dubliniensis isolates. In the presence of fluconazole, the C. dubliniensis isolates tested showed increased adherence as compared with controls without fluconazole. In contrast, all C. albicans isolates decreased in adherence to epithelial cells in the presence of fluconazole independently of their in vitro susceptibility to this drug. Proteinase antigens are present on the surface of C. dubliniensis cells adherent to epithelial target cells. In the presence of fluconazole this proteinase antigen was more strongly expressed.

CONCLUSION

Increased adherence of C. dubliniensis strains in the presence of fluconazole could explain its high recovery rate from HIV-positive patients in recent years. The induction of proteinase secretion in the presence of fluconazole found for most of the C. dubliniensis isolates could be one of the factors involved in adherence.

Pathogenicity and virulence of Candida dubliniensis: comparison with C. albicans

Candida dubliniensis is a newly described fungus that is frequently isolated from the oral cavities of HIV-positive patients. Although extensive studies have been performed on the phylogeny of C. dubliniensis, little is known about the pathogenic ecology of this yeast. Here we examined aspects related to C. dubliniensis in comparison with those of C. albicans. When injected intravenously into mice, C. dubliniensis had a higher survival rate than C. albicans. Histopathological analysis disclosed that C. dubliniensis remained mostly in the yeast form in the infected organs, whereas C. albicans changed into the mycelial form. The host inflammatory reaction was aggressive with C. dubliniensis infection and mild with C. albicans infection. Co-culture of the yeasts with human polymorphonuclear leukocytes disclosed that C. dubliniensis is more vulnerable to the fungicidal activity of leukocytes than C. albicans. C. dubliniensis was also more susceptible to the toxic effect of hydrogen peroxide. When cultured in vitro, C. dubliniensis grew more slowly than C. albicans, but the formation of germ tubes was faster. When the fungi were cultured in RPMI 1640, a fetal bovine serum supplement suppressed the growth of C. dubliniensis but enhanced that of C. albicans. These results clearly indicated that C. dubliniensis is less virulence than C. albicans.

Molecular mechanisms of fluconazole resistance in Candida dubliniensis isolates from human immunodeficiency virus-infected patients with oropharyngeal candidiasis

Candida dubliniensis is a newly identified species of Candida that is phenotypically similar to but genetically distinct from C. albicans. This organism has been recovered with increasing frequency from the oral cavities of human immunodeficiency virus (HIV)-infected and AIDS patients and has been implicated as a causative agent of oral candidiasis and systemic disease. In the present study we characterized the molecular mechanisms of resistance to fluconazole (FLC) in C. dubliniensis clinical isolates from two different HIV-infected patients with oropharyngeal candidiasis. Isolates were identified to the species level by phenotypic and genotypic tests. DNA-typing techniques were used to assess strain identity. Antifungal susceptibility testing was performed by NCCLS techniques. Northern blotting analysis was used to monitor the expression of genes encoding lanosterol demethylase (ERG11) and efflux transporters (CDR and MDR1) in matched sets of C. dubliniensis-susceptible and -resistant isolates by using probes generated from their homologous C. albicans sequences. In addition, ERG11 genes were amplified by PCR, and their nucleotide sequences were determined in order to detect point mutations with a possible effect in the affinity for azoles. Decreasing susceptibilities to FLC were detected in C. dubliniensis isolates recovered from both patients during the course of treatment. FLC-resistant C. dubliniensis isolates from one patient demonstrated combined upregulation of the MDR1, CDR1, and ERG11 genes. Among the isolates from the second patient, all isolates showing decreased susceptibility to FLC demonstrated upregulation of MDR1, whereas the levels of mRNA for the ERG11 genes remained constant and the expression of CDR genes was negligible. Fourteen point mutations were found in the ERG11 genes of the isolates with decreased susceptibility to FLC. These data demonstrate that the development of azole resistance in C. dublinensis clinical isolates from HIV-infected patients treated with FLC is mediated by multiple molecular mechanisms of resistance, similar to the observations found in the case of C. albicans.

Candida africana sp. nov., a new human pathogen or a variant of Candida albicans?

Atypical Candida strains were isolated from patients in Madagascar, Angola and Germany. These isolates were slow growing and were unable to produce chlamydospores. They had atypical carbohydrate assimilation profiles. All strains were unable to assimilate the amino sugars N-acteylglucosamine and glucosamine as well as the disaccharide trehalose and the organic acid DL-lactate. They were germ-tube-positive in serum, but only some of these organisms produced pseudohyphae after a long incubation. As shown by Fourier transform infrared spectroscopy the atypical Candida isolates clustered as a monophyletic group different from C. albicans and C. dubliniensis. All strains belonged to C. albicans serotype B. Considering all data presented here, this group of Candida strains differs from any other known member of the genus Candida. Therefore, it is suggested to represent a new species within the genus Candida for which the name Candida africana is proposed.

Recovery of Candida dubliniensis and other yeasts from human immunodeficiency virus-associated periodontal lesions

Oral and subgingival samples from periodontal lesions were collected from 54 human immunodeficiency virus (HIV)-positive and 20 HIV-negative patients and cultured for yeast species. Of the 54 samples cultured from HIV-positive patients, 44 (82%) were positive for yeast species, of which 29 (66%) were subgingival. A total of 19 (48%) patients were positive for Candida dubliniensis, of which 15 (79%) were colonized in subgingival sites. Seven isolates of Candida glabrata, two isolates of Candida parapsilosis, and one isolate of Saccharomyces cerevisiae were recovered. This study reports for the first time the recovery of C. dubliniensis from subgingival intraoral sites and confirms the presence of Candida species in sites of periodontal disease associated with HIV.

Virulence in Candida species

Candida species other than Candida albicans now account for up to 50% of deep candidiasis cases, yet little attention has been paid to the virulence attributes of these fungi. Adherence to host tissues, response to environmental changes and the secretion of hydrolases are all thought to be important in Candida virulence. The identification of virulence attributes unique to a particular Candida species could provide powerful insights into the pathogenic process but will require the use of genome-wide approaches such as transcript profiling, signature-tagged mutagenesis and in vivo expression technology.

Polymorphic internal transcribed spacer region 1 DNA sequences identify medically important yeasts

Species-specific polymorphisms in the noncoding internal transcribed spacer 2 (ITS2) region of the rRNA operon provide accurate identification of clinically significant yeasts. In this study, we tested the hypothesis that ITS1 noncoding regions contain diagnostically useful alleles. The length of ITS1 region PCR products amplified from 40 species (106 clinical strains, 5 reference strains, and 30 type strains) was rapidly determined with single-base precision by automated capillary electrophoresis. Polymorphisms in the PCR product length permitted 19 species to be distinguished by ITS1 alone, compared with 16 species distinguished by using only ITS2. However, combination of both ITS alleles permitted identification of 30 species (98% of clinical isolates). The remaining 10 species with PCR products of similar sizes contained unique ITS alleles distinguishable by restriction enzyme analysis. DNA sequence analysis of amplified ITS1 region DNA from 79 isolates revealed species-specific ITS1 alleles for each of the 40 pathogenic species examined. This provided identification of unusual clinical isolates, and 53 diagnostic ITS1 sequences were deposited in GenBank. Phylogenetic analyses based on ITS sequences showed a similar overall topology to 26S rRNA gene-based trees. However, different species with identical 26S sequences contained distinct ITS alleles that provided species identification with strong statistical support. Together, these data indicate that the analysis of ITS polymorphisms can reliably identify 40 species of clinically significant yeasts and that the capacity for identifying potentially new pathogenic species by using this database holds significant promise.

Biofilm formation by Candida dubliniensis

Candida dubliniensis is an opportunistic yeast closely related to Candida albicans that has been recently implicated in oropharyngeal candidiasis in human immunodeficiency virus-infected patients. Most manifestations of candidiasis are associated with biofilm formation, with cells in biofilms displaying properties dramatically different from free-living cells grown under normal laboratory conditions. Here, we report on the development of in vitro models of C. dubliniensis biofilms on the surfaces of biomaterials (polystyrene and acrylic) and on the characteristics associated with biofilm formation by this newly described species. Time course analysis using a formazan salt reduction assay to monitor metabolic activities of cells within the biofilm, together with microscopy studies, revealed that biofilm formation by C. dubliniensis occurred after initial focal adherence, followed by growth, proliferation, and maturation over 24 to 48 h. Serum and saliva preconditioning films enhanced the initial attachment of C. dubliniensis and subsequent biofilm formation. Scanning electron microscopy and confocal scanning laser microscopy were used to further characterize C. dubliniensis biofilms. Mature C. dubliniensis biofilms consisted of a dense network of yeasts cells and hyphal elements embedded within exopolymeric material. C. dubliniensis biofilms displayed spatial heterogeneity and an architecture showing microcolonies with ramifying water channels. Antifungal susceptibility testing demonstrated the increased resistance of sessile C. dubliniensis cells, including the type strain and eight different clinical isolates, against fluconazole and amphotericin B compared to their planktonic counterparts. C. dubliniensis biofilm formation may allow this species to maintain its ecological niche as a commensal and during infection with important clinical repercussions.

Isogenic strain construction and gene targeting in Candida dubliniensis

Candida dubliniensis is a recently described opportunistic fungal pathogen that is closely related to Candida albicans but differs from it with respect to epidemiology, certain virulence characteristics, and the ability to develop fluconazole resistance in vitro. A comparison of C. albicans and C. dubliniensis at the molecular level should therefore provide clues about the mechanisms used by these two species to adapt to their human host. In contrast to C. albicans, no auxotrophic C. dubliniensis strains are available for genetic manipulations. Therefore, we constructed homozygous ura3 mutants from a C. dubliniensis wild-type isolate by targeted gene deletion. The two URA3 alleles were sequentially inactivated using the MPA(R)-flipping strategy, which is based on the selection of integrative transformants carrying a mycophenolic acid resistance marker that is subsequently deleted again by site-specific, FLP-mediated recombination. The URA3 gene from C. albicans (CaURA3) was then used as a selection marker for targeted integration of a fusion between the C. dubliniensis MDR1 (CdMDR1) promoter and a C. albicans-adapted GFP reporter gene. Uridine-prototrophic transformants were obtained with high frequency, and all transformants of two independent ura3-negative parent strains had correctly integrated the reporter gene fusion into the CdMDR1 locus, demonstrating that the CaURA3 gene can be used for efficient and specific targeting of recombinant DNA into the C. dubliniensis genome. Transformants carrying the reporter gene fusion did not exhibit detectable fluorescence during growth in yeast extract-peptone-dextrose medium in vitro, suggesting that CdMDR1 is not significantly expressed under these conditions. Fluconazole had no effect on MDR1 expression, but the addition of the drug benomyl strongly activated the reporter gene fusion in a dose-dependent fashion, demonstrating that the CdMDR1 gene, which encodes an efflux pump mediating resistance to toxic compounds, is induced by the presence of certain drugs.

Characterization of agglutinin-like sequence genes from non-albicans Candida and phylogenetic analysis of the ALS family

The ALS (agglutinin-like sequence) gene family of Candida albicans encodes cell-surface glycoproteins implicated in adhesion of the organism to host surfaces. Southern blot analysis with ALS-specific probes suggested the presence of ALS gene families in C. dubliniensis and C. tropicalis; three partial ALS genes were isolated from each organism. Northern blot analysis demonstrated that mechanisms governing expression of ALS genes in C. albicans and C. dubliniensis are different. Western blots with an anti-Als serum showed that cross-reactive proteins are linked by beta 1,6-glucan in the cell wall of each non-albicans Candida, suggesting similar cell wall architecture and conserved processing of Als proteins in these organisms. Although an ALS family is present in each organism, phylogenetic analysis of the C. albicans, C. dubliniensis, and C. tropicalis ALS genes indicated that, within each species, sequence diversification is extensive and unique ALS sequences have arisen. Phylogenetic analysis of the ALS and SAP (secreted aspartyl proteinase) families show that the ALS family is younger than the SAP family. ALS genes in C. albicans, C. dubliniensis, and C. tropicalis tend to be located on chromosomes that also encode genes from the SAP family, yet the two families have unexpectedly different evolutionary histories. Homologous recombination between the tandem repeat sequences present in ALS genes could explain the different histories for co-localized genes in a predominantly clonal organism like C. albicans.

The ALS gene family of Candida albicans

The ALS gene family of Candida albicans encodes large cell-surface glycoproteins that are implicated in the process of adhesion to host surfaces. ALS genes are also found in other Candida species that are isolated from cases of clinical disease. Genes in the ALS family are differentially regulated by physiologically relevant mechanisms. ALS genes exhibit several levels of variability including strain- and allele-specific size differences for the same gene, strain-specific differences in gene regulation, the absence of particular ALS genes in certain isolates, and additional ALS coding regions in others. The differential regulation and genetic variability of the ALS genes results in a diverse cell-surface Als protein profile that is also affected by growth conditions. The ALS genes are one example of a gene family associated with pathogenicity mechanisms in C. albicans and other Candida species.

Typing of Candida albicans isolates by sequence analysis of the cytochrome b gene and differentiation from Candida stellatoidea

Including type strains, mitochondrial cytochrome b genes of 32 strains of Candida albicans and 6 strains of Candida stellatoidea, presently treated as a synonym for C. albicans, were partially sequenced. Analysis of 396-bp nucleotide sequences of the strains under investigation divided C. albicans isolates into three types: type I, type II, and type III; however, strains of C. stellatoidea represented distinct type IV isolates. Deduced amino acid sequences of type I, type II, and type III were identical and differed from that of type IV by one amino acid. Genotypes (rDNA type) of the test strains were also checked. Cytochrome b typing did not correlate with genotyping, and different genotypes occurred for one cytochrome b type. This study shows that cytochrome b gene sequences are useful for analyzing the genetic relatedness of C. albicans isolates and effective for differentiating C. stellatoidea from C. albicans.

PCR fingerprinting: a convenient molecular tool to distinguish between Candida dubliniensis and Candida albicans

Candida dubliniensis was recently identified as a germ-tube- and chlamydospore-positive yeast, phenotypically and morphologically indistinguishable from the phylogenetically closely related yeast species C. albicans and its synonymized variant C. stellatoidea. The high similarity between these yeast species causes significant problems in the correct identification of C. dubliniensis in a standard clinical mycology laboratory. Polymerase chain reaction (PCR) fingerprinting was successfully applied here to distinguish between clinical isolates of the two closely related species. Microsatellite ([GACA]4) and minisatellite ([5'-GAGGGTGGCGGTTCT-3'], derived from the core-sequence of the wild-type phage M13) specific oligonucleotides were used as single primers in PCR to amplify hypervariable inter-repeat DNA sequences from 16 C. dubliniensis strains and 11 C. albicans strains. Each species, represented by its ex-type strain, could be identified by a distinct species-specific multilocus pattern, allowing identification to species level for all clinical isolates. In addition, the PCR fingerprinting generated strain-specific profiles, making this method applicable to epidemiological investigations. PCR fingerprinting using the primer M13 is proposed here as a simple, reliable and highly reproducible molecular tool to differentiate between strains of C. albicans and C. dubliniensis.

Comparison of the hydrophobic properties of Candida albicans and Candida dubliniensis

Although Candida dubliniensis is a close genetic relative of Candida albicans, it colonizes and infects fewer sites. Nearly all instances of candidiasis caused by C. dubliniensis are restricted to the oral cavity. As cell surface hydrophobicity (CSH) influences virulence of C. albicans, CSH properties of C. dubliniensis were investigated and compared to C. albicans. Growth temperature is one factor which affects the CSH status of stationary-phase C. albicans. However, C. dubliniensis, similar to other pathogenic non-albicans species of Candida, was hydrophobic regardless of growth temperature. For all Candida species tested in this study (C. albicans, C. dubliniensis, C. glabrata, C. krusei, C. parapsilosis, and C. tropicalis), CSH status correlated with coaggregation with the anaerobic oral bacterium Fusobacterium nucleatum. Previous studies have shown that CSH status of C. albicans involves multiple surface proteins and surface protein N-glycans. The hydrophobic surface glycoprotein CAgp38 appears to be expressed by C. albicans constitutively regardless of growth temperature and medium. C. dubliniensis expresses a 38-kDa protein that cross-reacts with the anti-CAgp38 monoclonal antibody; however, expression of the protein was growth medium and growth temperature dependent. The anti-CAgp38 monoclonal antibody has been shown to inhibit adhesion of C. albicans to extracellular matrix proteins and to vascular endothelial cells. Since protein glycosylation influences the CSH status of C. albicans, we compared the cell wall mannoprotein content and composition between C. albicans and C. dubliniensis. Similar bulk compositional levels of hexose, phosphate, and protein in their N-glycans were determined. However, a component of the C. albicans N-glycan, acid-labile phosphooligomannoside, is expressed much less or negligibly by C. dubliniensis, and when present, the oligomannosides are predominantly less than five mannose residues in length. In addition, the acid-labile phosphooligomannoside profiles varied among the three strains of C. dubliniensis we tested, indicating the N-glycan of C. dubliniensis differs from C. albicans. For C. albicans, the acid-labile phosphooligomannoside influences virulence and surface fibrillar conformation, which affects exposure of hydrophobic surface proteins. Given the combined role in C. albicans of expression of specific surface hydrophobic proteins in pathogenesis and of surface protein glycosylation on exposure of the proteins, the lack of these virulence-associated CSH entities in C. dubliniensis could contribute to its limited ability to cause disseminated infections.

Identification and phylogenetic relationship of the most common pathogenic Candida species inferred from mitochondrial cytochrome b gene sequences

We sequenced a 396-bp region of the mitochondrial cytochrome b gene of the most common clinically important Candida species: Candida albicans, C. glabrata, C. parapsilosis, C. tropicalis, C. krusei, and C. lusitaniae. The recently described species of Candida, C. dubliniensis, associated with mucosal candidiasis in human immunodeficiency virus-infected individuals, was also included. Two to five strains of each species were examined. Some species represented intraspecies variation, which was not more than 1.8% (DNA). However, interspecies variations were more than 10 and 7%, respectively, for DNA and amino acid sequences. Multiple alignments of nucleotide and deduced amino acid sequences revealed species-specific nucleotides and amino acids. Nucleotide- and amino acid-based phylogenetic trees were constructed and are discussed. Using the database, it is possible to identify presumptive Candida species within a working day.

Phagocytosis, oxidative burst, and killing of Candida dubliniensis and Candida albicans by human neutrophils

Candida dubliniensis is a phylogenetically closely related species to Candida albicans. So far virtually nothing is known about the virulence factors of C. dubliniensis. Cell surface hydrophobicity (CSH) plays a critical role in adhesion of microorganisms to phagocytic cells; hydrophobic cells of C. albicans have been reported to be less sensitive to phagocytic killing than hydrophilic cells. C. dubliniensis displays CSH at 37 degrees C in contrast to C. albicans. To elucidate this issue, we determined levels of phagocytosis, oxidative burst and killing by human neutrophils of C. dubliniensis (n=10) compared to C. albicans (n=10) both cultured at 37 degrees C. Obtained test results revealed no statistically significant differences between these two yeast species for the level of phagocytosis (77.3 vs. 76.2% after 60 min), evoked oxidative burst (64.5 vs. 67.3% after 30 min) and killing (72.7 vs. 73.1% after 240 min). Therefore, human neutrophils can be considered to be equally efficient against these two yeast species.

Comparison of virulence factors of oral Candida dubliniensis and Candida albicans isolates in healthy people and patients with chronic candidosis

We determined differences in the expression of certain virulence factors between oral Candida dubliniensis and Candida albicans species. In addition, clonal differences were sought among C. albicans isolates recovered from patients with and without compromised immune system. The material comprised 93 clinical yeast isolates originated in 40 subjects (1-5 isolates per subject). All 26 C. dubliniensis isolates and 46 C. albicans isolates originated from healthy routine dental clinic patients. Additionally, 21 C. albicans isolates were collected from patients with autoimmune polyendocrinopathy-candidosis-ectodermal dystrophy (APECED), who have chronic candidosis as one manifestation of their immunocompromising disease. Polymerase chain reaction amplification using the random sequence primer OPE-03 enabled grouping of the C. dubliniensis isolates in 2 genotypes (I and II) and C. albicans isolates in 15 genotypes (I-XV). No significant difference was found in the distribution of genotypes between the patients with APECED and the healthy subjects. C. dubliniensis isolates exhibited high-frequency phenotypic switching significantly more frequently than did C. albicans isolates, and vice versa regarding phospholipase and proteinase production. Proteinase production was significantly more frequent among C. albicans genotype V than genotype IX isolates. No significant difference was found in expression of virulence factors of C. albicans isolates between the patients with APECED and the healthy subjects.

Molecular responses to changes in the environmental pH are conserved between the fungal pathogens Candida dubliniensis and Candida albicans

In this work we cloned CdPHR1 and CdPHR2 from the human fungal pathogen Candida dubliniensis. The two genes are homologues to the pH-regulated genes PHR1 and PHR2 from Candida albicans. The pH-dependent pattern of expression of CdPHR1 and CdPHR2 was conserved in C. dubliniensis. CdPHR1 could be shown to be functionally equivalent to PHR1. The pH-regulated mode of expression was maintained when CdPHR1 was integrated in C. albicans. This indicates a fundamentally similar mode of expressional regulation in the two species. CdPHR1 was furthermore capable of reversing the aberrant phenotype of a Saccharomyces cerevisiae GAS1 deletion mutant. In this species, however, expression of CdPHR1 was no longer under control of the external pH. Expression of CdPHR1 was not detected when it was introduced into Aspergillus nidulans. In conclusion, C. dubliniensis and C. albicans respond to changes in the environmental pH with a change in cell shape and differential gene expression.

Retrospective identification and characterization of Candida dubliniensis isolates among Candida albicans clinical laboratory isolates from human immunodeficiency virus (HIV)-infected and non-HIV-infected individuals

Fungal opportunistic infections, and in particular those caused by the various Candida species, have gained considerable significance as a cause of morbidity and, often, mortality. The newly described species Candida dubliniensis phenotypically resembles Candida albicans so closely that it is easily misidentified as such. The present study was designed to determine the frequency at which this new species is not recognized in the clinical laboratory, to determine the patient populations with which C. dubliniensis is associated, to determine colonization versus infection frequency, and to assess fluconazole resistance. Over a 2-year period, 1,251 isolates that were initially identified as C. albicans by a hospital clinical laboratory were reevaluated for C. dubliniensis by inability to grow at 45 degrees C, colony color on CHROMagar Candida medium, coaggregation assay with Fusobacterium nucleatum, and sugar assimilation profiles (API 20C AUX yeast identification system). A total of 15 (1.2%) isolates from 12 patients were identified as C. dubliniensis. Ten of the patients were found to be immunocompromised (these included patients with human immunodeficiency virus infection or AIDS, cancer patients receiving chemotherapy, and patients awaiting transplantation). Thirteen isolates were highly susceptible to fluconazole (MIC, <0.5 microgram/ml). Three isolates from one patient, genotypically confirmed as the same strain, showed variable susceptibility to fluconazole. The first isolate was susceptible, whereas the other two isolates were dose-dependent susceptible (MIC, 16.0 microgram/ml). These data confirm the close association of C. dubliniensis with immunocompromised states and that increased fluconazole MICs may develop in vivo. This study emphasizes the importance of screening germ-tube-positive yeasts for the inability to grow at 45 degrees C followed by confirmatory tests in order to properly identify this species.

Evaluation of phenotypic markers for selection and identification of Candida dubliniensis

Candida dubliniensis is often associated with C. albicans in cultures. Easy-to-perform selective isolation procedures for these closely related species do not exist. Therefore, we evaluated previously described discriminatory phenotypic markers for C. dubliniensis. A total of 150 oral rinses from human immunodeficiency virus (HIV)-infected patients were cultured on CHROMagar Candida. Dark green colonies described as being indicative of C. dubliniensis and other green colonies, 170 in total, were isolated. Chlamydospore formation, intracellular beta-D-glucosidase activity, ability to grow at 42 degrees C, carbohydrate assimilation pattern obtained by the API ID 32C, and Fourier transform infrared (FT-IR) spectroscopy were used for phenotypic characterization. Sequencing of the 5' end of the nuclear large-subunit (26S) ribosomal DNA gene was used for definitive species identification for C. dubliniensis. C. dubliniensis was found in 34% of yeast-colonized HIV-infected patients. The color of the colonies on CHROMagar Candida proved to be insufficient for selecting C. dubliniensis, since only 30 of 53 proven C. dubliniensis isolates showed a dark green color in primary cultures. The described typical chlamydospore formation can give only some indication of C. dubliniensis. The assimilation pattern proved to be insufficient to discriminate C. dubliniensis from C. albicans. All C. dubliniensis strains showed no or highly restricted growth at 42 degrees C and a lack of beta-D-glucosidase activity. Unfortunately, atypical C. albicans strains can also exhibit these phenotypic traits. FT-IR spectroscopy combined with hierarchical clustering proved to be as reliable as genotyping for discriminating the two species.

Initial case of Candida dubliniensis infection from Asia: non-mucosal infection

A yeast, repeatedly isolated from a post-surgical abdominal infection of a 75-year-old Japanese man, was genotyped as Candida dubliniensis. This is the first reported case in Asia of this recently described fungus.

Growth competition between Candida dubliniensis and Candida albicans under broth and biofilm growing conditions

Seven isolates each of Candida albicans and Candida dubliniensis were paired (11 pairs) and examined for competitive interaction. Equal numbers of CFU of each competitor were inoculated into Sabouraud dextrose broth and incubated at 37 degrees C with vigorous shaking under conditions favorable to either broth or biofilm growth. Surviving proportions of each competitor were calculated from the broth culture at 24 and 96 h and the biofilm culture at 96 h, with species differentiation done on CHROMagar Candida medium. C. albicans had a competitive advantage over C. dubliniensis in broth culture and under biofilm growing conditions; however, with the presence of a supporting structure for biofilm formation, C. dubliniensis was able to better withstand the competitive pressures from C. albicans.

A molecular genetic system for the pathogenic yeast Candida dubliniensis

Candida dubliniensis is a recently described pathogenic yeast of the genus Candida that is closely related to Candida albicans but differs from it in several phenotypic and genotypic characteristics, including putative virulence traits, which may explain differences in the spectrum of diseases caused by the two species. In contrast to C. albicans, a molecular genetic system to study virulence of C. dubliniensis is lacking. We have developed a system for the genetic transformation of C. dubliniensis that is based on the use of the dominant selection marker MPA(R) from C. albicans that confers resistance to mycophenolic acid (MPA). Using this transformation system, a GFP (green fluorescent protein) reporter gene that was genetically engineered for functional expression in C. albicans and placed under control of the inducible C. albicans SAP2 (secreted aspartic proteinase) promoter was integrated into the C. dubliniensis genome. MPA-resistant transformants containing the SAP2P-GFP fusion fluoresced under SAP2-inducing conditions but not under SAP2-repressing conditions. These results demonstrate that the MPA(R) selection marker is useful for transformation of C. dubliniensis wild-type strains, that the GFP reporter gene is functionally expressed in C. dubliniensis, and that the C. albicans SAP2 promoter can be used for controlled gene expression in C. dubliniensis. These genetic tools will allow the dissection of the differences in virulence characteristics between the two pathogenic yeast species at the molecular level.

New monoclonal antibody specific for Candida albicans germ tube

Hydrophobic components of the germ tube of the dimorphic pathogenic fungus Candida albicans were used as immunogens to prepare monoclonal antibodies (MAbs). Among the resulting MAbs, one (MAb 16B1-F10) was shown by indirect immunofluorescence to be specific to the surface of the mycelium phase of the C. albicans and C. stellatoidea species. No labeling of any other genera and Candida species tested was observed, including C. dubliniensis, a newly described species which has many phenotypic similarities to C. albicans. This phase-specific epitope resides on a protein moiety. The molecular mass of the antigen released by Zymolyase digestion was determined by gel filtration and ranges from 25 to 166 kDa. The antigen was also shown to be highly hydrophobic. This anti-C. albicans cell wall surface-specific MAb may be a good candidate for use in tests for the rapid differentiation of the two closely related species C. albicans and C. dubliniensis.

[Proteinases of pathogenic fungi]

The present knowledge on the pathogenetic relevance of extracellular and cell wall-attached proteinases from fungal pathogens is briefly reviewed.

Rapid identification of Candida dubliniensis with commercial yeast identification systems

Candida dubliniensis is a newly described species that is closely related phylogenetically to Candida albicans and that is commonly associated with oral candidiasis in human immunodeficiency virus-positive patients. Several recent studies have attempted to elucidate phenotypic and genotypic characteristics of use in separating the two species. However, results obtained with simple phenotypic tests were too variable and tests that provided more definitive data were too complex for routine use in the clinical laboratory setting. The objective of this study was to determine if reproducible identification of C. dubliniensis could be obtained with commercial identification kits. The substrate reactivity profiles of 80 C. dubliniensis isolates were obtained by using the API 20C AUX, ID 32 C, RapID Yeast Plus, VITEK YBC, and VITEK 2 ID-YST systems. The percentages of C. dubliniensis isolates capable of assimilating or hydrolyzing each substrate were compared with the percentages from the C. albicans profiles in each kit's database, and the results were expressed as percent C. dubliniensis and percent C. albicans. Any substrate that showed >50% difference in reactivity was considered useful in differentiating the species. In addition, assimilation of methyl-alpha-D-glucoside (MDG), D-trehalose (TRE), and D-xylose (XYL) by the same isolates was investigated by the traditional procedure of Wickerham and Burton (L. J. Wickerham and K. A. Burton, J. Bacteriol. 56:363-371, 1948). At 48 h (the time recommended by the manufacturer for its new database), we found that the assimilation of four carbohydrates in the API 20C AUX system could be used to distinguish the species, i.e., glycerol (GLY; 88 and 14%), XYL (0 and 88%), MDG (0 and 85%), and TRE (15 and 97%). Similarly, results with the ID 32 C system at 48 h showed that XYL (0 and 98%), MDG (0 and 98%), lactate (LAT; 0 and 96%), and TRE (30 and 96%) could be used to separate the two species. Phosphatase (PHS; 9 and 76%) and alpha-D-glucosidase (23 and 94%) proved to be the most useful for separation of the species in the RapID Yeast Plus system. While at 24 h the profiles obtained with the VITEK YBC system showed that MDG (10 and 95%), XYL (0 and 95%), and GLY (26 and 80%) could be used to separate the two species, at 48 h only XYL (6 and 95%) could be used to separate the two species. The most useful substrates in the VITEK 2 ID-YST system were TRE (1 and 89%), MDG (1 and 99%), LAT (4 and 98%), and PHS (83 and 1%). While the latter kit was not yet commercially available at the time of the study, it would appear to be the most valuable for the identification of C. dubliniensis. Although assimilation of MDG, TRE, and XYL proved to be the most useful for species differentiation by the majority of commercial systems, the results with these carbohydrates by the Wickerham and Burton procedure were essentially the same for both species, albeit following protracted incubation. Thus, it is the rapidity of the assimilation achieved with the commercial systems that allows the differentiation of C. dubliniensis from C. albicans.

Oral Candida dubliniensis as a clinically important species in HIV-seropositive patients in the United States

OBJECTIVE

Interest in Candida dubliniensis has led to renewed clinical investigations regarding incidence, drug resistance, pathogenesis, and epidemiology of fungal infections in patients with HIV. C dubliniensis phenotypically resembles Candida albicans in many respects, yet it can be identified and differentiated as a unique Candida species by its phenotypic and genetic profiles. The purpose of this study was to prospectively evaluate the prevalence of C dubliniensis in clinical isolates and determine the clinical and demographic characteristics of patients harboring C dubliniensis.

STUDY DESIGN

Over a 6-week period, 24 yeast-positive isolates from HIV-positive dental patients were screened for C dubliniensis through use of phenotypic criteria. HIV viral load, CD4 count, and complete oral health evaluations were performed on each patient at the same visit during which the oral fungal surveillance culture was taken.

RESULTS

Six isolates from 24 HIV-seropositive and yeast-positive patients were shown to be consistent phenotypically and by electrophoretic karyotyping with the European reference strain of C dubliniensis. Dose-dependent susceptibility to fluconazole was shown in one of the C dubliniensis isolates. Five of the 6 patients demonstrated moderate to high viral loads. General oral health, as evidenced by the presence of advanced periodontal lesions and a high decayed, missing, and filled teeth index (>20), was poor in 3 of the 6 patients with C dubliniensis and 7 of the 18 patients with C albicans. A history of intravenous drug abuse was present in 50% of the C dubliniensis -positive patients, which is representative of the HIV-positive population at the hospital.

CONCLUSIONS

In this small sample, C dubliniensis represented 25% of the yeast-positive cultures. The clinical significance of this interesting species in the United States may be related to high viral load, rapid AIDS progression, and/or concomitant oral disease, such as a high caries index or periodontal disease.

Molecular epidemiology of Candida albicans isolates from AIDS and cancer patients using a novel standardized CARE-2 DNA fingerprinting technique

A total of 277 Candida isolates from various body sites of 149 AIDS and cancer patients treated in four different university clinics in Würzburg, Germany were collected over a period of 27 months and phenotypically and genotypically characterized. The fingerprinting patterns of 194 Candida albicans isolates obtained with the moderately repetitive, C. albicans-specific DNA fragment CARE-2 were digitized and retrospectively compared with a highly accurate computer-assisted standardization method. A total of 168 different genotypic patterns (< 100% identity) could be differentiated using this technique. Although comparative analysis of C. albicans subsets revealed a pronounced tendency of C. albicans isolates from HIV patients to form clusters, the mean genetic variability in HIV and cancer patient isolates was virtually identical. Patients with a specific disease condition or in a certain age group were not found to harbour C. albicans isolates displaying a characteristic "signature genotype". Micro-evolutionary changes were detected by CARE-2 fingerprinting in temporal successive isolates of one patient, but nosocomial transmission of identical isolates between unrelated patients was never seen. Genotyping showed that patient isolates can replace one another; occasionally also species switches were observed. Secreted aspartic protease (SAP) production was not correlated with a specific C. albicans banding pattern; isolates obtained from HIV patients and from an internal control group secreted comparable amounts of SAP. Candida dubliniensis isolates in this study showed an elevated level of SAP production. When used under standardized conditions, CARE-2 fingerprinting is an efficient, reproducible and sensitive technique to characterize C. albicans isolates.

Rapid PCR test for discriminating between Candida albicans and Candida dubliniensis isolates using primers derived from the pH-regulated PHR1 and PHR2 genes of C. albicans

The development of a satisfactory means to reliably distinguish between the two closely related species Candida albicans and Candida dubliniensis in the clinical mycology laboratory has proved difficult because these two species are phenotypically so similar. In this study, we have detected homologues of the pH-regulated C. albicans PHR1 and PHR2 genes in C. dubliniensis. Restriction fragment length polymorphism analysis suggests that there are significant sequence differences between the genes of the two species. In order to exploit this apparent difference, oligonucleotide primers based on the coding sequence of the C. albicans PHR1 structural gene were designed and used in PCR experiments. Use of these primers with C. albicans template DNA from 17 strains yielded a predicted 1.6-kb product, while C. dubliniensis template DNA from 19 strains yielded no product. We therefore propose that PCR using these primers is a rapid and reliable means of distinguishing the two germ tube- and chlamydospore-producing species C. albicans and C. dubliniensis.

In vivo analysis of secreted aspartyl proteinase expression in human oral candidiasis

Secreted aspartyl proteinases are putative virulence factors in Candida infections. Candida albicans possesses at least nine members of a SAP gene family, all of which have been sequenced. Although the expression of the SAP genes has been extensively characterized under laboratory growth conditions, no studies have analyzed in detail the in vivo expression of these proteinases in human oral colonization and infection. We have developed a reliable and sensitive procedure to detect C. albicans mRNA from whole saliva of patients with oral C. albicans infection and those with asymptomatic Candida carriage. The reverse transcription-PCR protocol was used to determine which of the SAP1 to SAP7 genes are expressed by C. albicans during colonization and infection of the oral cavity. SAP2 and the SAP4 to SAP6 subfamily were the predominant proteinase genes expressed in the oral cavities of both Candida carriers and patients with oral candidiasis; SAP4, SAP5, or SAP6 mRNA was detected in all subjects. SAP1 and SAP3 transcripts were observed only in patients with oral candidiasis. SAP7 mRNA expression, which has never been demonstrated under laboratory conditions, was detected in several of the patient samples. All seven SAP genes were simultaneously expressed in some patients with oral candidiasis. This is the first detailed study showing that the SAP gene family is expressed by C. albicans during colonization and infection in humans and that C. albicans infection is associated with the differential expression of individual SAP genes which may be involved in the pathogenesis of oral candidiasis.

Coaggregation of Candida dubliniensis with Fusobacterium nucleatum

The binding of microorganisms to each other and oral surfaces contributes to the progression of microbial infections in the oral cavity. Candida dubliniensis, a newly characterized species, has been identified in human immunodeficiency virus-seropositive patients and other immunocompromised individuals. C. dubliniensis phenotypically resembles Candida albicans in many respects yet can be identified and differentiated as a unique Candida species by phenotypic and genetic profiles. The purpose of this study was to determine oral coaggregation (CoAg) partners of C. dubliniensis and to compare these findings with CoAg of C. albicans under the same environmental conditions. Fifteen isolates of C. dubliniensis and 40 isolates of C. albicans were tested for their ability to coaggregate with strains of Fusobacterium nucleatum, Peptostreptococcus micros, Peptostreptococcus magnus, Peptostreptococcus anaerobius, Porphyromonas gingivalis, and Prevotella intermedia. When C. dubliniensis and C. albicans strains were grown at 37 degrees C on Sabouraud dextrose agar, only C. dubliniensis strains coaggregated with F. nucleatum ATCC 49256 and no C. albicans strains showed CoAg. However, when the C. dubliniensis and C. albicans strains were grown at 25 or 45 degrees C, both C. dubliniensis and C. albicans strains demonstrated CoAg with F. nucleatum. Heating the C. albicans strains (grown at 37 degrees C) at 85 degrees C for 30 min or treating them with dithiothreitol allowed the C. albicans strains grown at 37 degrees C to coaggregate with F. nucleatum. CoAg at all growth temperatures was inhibited by mannose and alpha-methyl mannoside but not by EDTA or arginine. The CoAg reaction between F. nucleatum and the Candida species involved a heat-labile component on F. nucleatum and a mannan-containing heat-stable receptor on the Candida species. The CoAg reactions between F. nucleatum and the Candida species may be important in the colonization of the yeast in the oral cavity, and the CoAg of C. dubliniensis by F. nucleatum when grown at 37 degrees C provides a rapid, specific, and inexpensive means to differentiate C. dubliniensis from C. albicans isolates in the clinical laboratory.

Identification and expression of multidrug transporters responsible for fluconazole resistance in Candida dubliniensis

Candida dubliniensis is a recently described Candida species associated with oral candidosis in human immunodeficiency virus (HIV)-infected and AIDS patients, from whom fluconazole-resistant clinical isolates have been previously recovered. Furthermore, derivatives exhibiting a stable fluconazole-resistant phenotype have been readily generated in vitro from fluconazole-susceptible isolates following exposure to the drug. In this study, fluconazole-resistant isolates accumulated up to 80% less [3H] fluconazole than susceptible isolates and also exhibited reduced susceptibility to the metabolic inhibitors 4-nitroquinoline-N-oxide and methotrexate. These findings suggested that C. dubliniensis may encode multidrug transporters similar to those encoded by the C. albicans MDR1, CDR1, and CDR2 genes (CaMDR1, CaCDR1, and CaCDR2, respectively). A C. dubliniensis homolog of CaMDR1, termed CdMDR1, was cloned; its nucleotide sequence was found to be 92% identical to the corresponding CaMDR1 sequence, while the predicted CdMDR1 protein was found to be 96% identical to the corresponding CaMDR1 protein. By PCR, C. dubliniensis was also found to encode homologs of CDR1 and CDR2, termed CdCDR1 and CdCDR2, respectively. Expression of CdMDR1 in a fluconazole-susceptible delta pdr5 null mutant of Saccharomyces cerevisiae conferred a fluconazole-resistant phenotype and resulted in a 75% decrease in accumulation of [3H]fluconazole. Northern analysis of fluconazole-susceptible and -resistant isolates of C. dubliniensis revealed that fluconazole resistance was associated with increased expression of CdMDR1 mRNA. In contrast, most studies showed that overexpression of CaCDR1 was associated with fluconazole resistance in C. albicans. Increased levels of the CdMdr1p protein were also detected in fluconazole-resistant isolates. Similar results were obtained with fluconazole-resistant derivatives of C. dubliniensis generated in vitro, some of which also exhibited increased levels of CdCDR1 mRNA and CdCdr1p protein. These results demonstrate that C. dubliniensis encodes multidrug transporters which mediate fluconazole resistance in clinical isolates and which can be rapidly mobilized, at least in vitro, on exposure to fluconazole.