Candida and Torulopsis: a blinded evaluation of use of pseudohypha formation as basis for identification of medically important yeasts

Characterization of the Dielectrophoretic Response of Different Candida Strains Using 3D Carbon Microelectrodes

Bloodstream infection with Candida fungal cells remains one of the most life-threatening complications among hospitalized patients around the world. Although most of the cases are still due to Candida albicans, the rising incidence of infections caused by other Candida strains that may not respond to traditional anti-fungal treatments merits the development of a method for species-specific isolation of Candida. To this end, here we present the characterization of the dielectrophoresis (DEP) response of Candida albicans, Candida tropicalis and Candida parapsilosis. We complement such characterization with a study of the Candida cells morphology. The Candida strains exhibited subtle differences in their morphology and dimensions. All the Candida strains exhibited positive DEP in the range 10-500 kHz, although the strength of the DEP response was different for each Candida strain at different frequencies. Only Candida tropicalis showed positive DEP at 750 kHz. The current results show potential for manipulation and enrichment of a specific Candida strain at specific DEP conditions towards aiding in the rapid identification of Candida strains to enable the effective and timely treatment of Candida infections.

A nonsense mutation in the ERG6 gene leads to reduced susceptibility to polyenes in a clinical isolate of Candida glabrata

Unlike the molecular mechanisms that lead to azole drug resistance, the molecular mechanisms that lead to polyene resistance are poorly documented, especially in pathogenic yeasts. We investigated the molecular mechanisms responsible for the reduced susceptibility to polyenes of a clinical isolate of Candida glabrata. Sterol content was analyzed by gas-phase chromatography, and we determined the sequences and levels of expression of several genes involved in ergosterol biosynthesis. We also investigated the effects of the mutation harbored by this isolate on the morphology and ultrastructure of the cell, cell viability, and vitality and susceptibility to cell wall-perturbing agents. The isolate had a lower ergosterol content in its membranes than the wild type, and the lower ergosterol content was found to be associated with a nonsense mutation in the ERG6 gene and induction of the ergosterol biosynthesis pathway. Modifications of the cell wall were also seen, accompanied by increased susceptibility to cell wall-perturbing agents. Finally, this mutation, which resulted in a marked fitness cost, was associated with a higher rate of cell mortality. Wild-type properties were restored by complementation of the isolate with a centromeric plasmid containing a wild-type copy of the ERG6 gene. In conclusion, we have identified the molecular event responsible for decreased susceptibility to polyenes in a clinical isolate of C. glabrata. The nonsense mutation detected in the ERG6 gene of this isolate led to a decrease in ergosterol content. This isolate may constitute a useful tool for analysis of the relevance of protein trafficking in the phenomena of azole resistance and pseudohyphal growth.

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.

Reduced susceptibility to polyenes associated with a missense mutation in the ERG6 gene in a clinical isolate of Candida glabrata with pseudohyphal growth

Little information is available about the molecular mechanisms responsible for polyene resistance in pathogenic yeasts. A clinical isolate of Candida glabrata with a poor susceptibility to polyenes, as determined by disk diffusion method and confirmed by determination of MIC, was recovered from a patient treated with amphotericin B. Quantitative analysis of sterols revealed a lack of ergosterol and an accumulation of late sterol intermediates, suggesting a defect in the final steps of the ergosterol pathway. Sequencing of CgERG11, CgERG6, CgERG5, and CgERG4 genes revealed exclusively a unique missense mutation in CgERG6 leading to the substitution of a cysteine by a phenylalanine in the corresponding protein. In addition, real-time reverse transcription-PCR demonstrated an overexpression of genes encoding enzymes involved in late steps of the ergosterol pathway. Moreover, this isolate exhibited a pseudohyphal growth whatever the culture medium used, and ultrastructural changes of the cell wall of blastoconidia were seen consisting in a thinner inner layer. Cell wall alterations were also suggested by the higher susceptibility of growing cells to Calcofluor white. Additionally, complementation of this isolate with a wild-type copy of the CgERG6 gene restored susceptibility to polyenes and a classical morphology. Together, these results demonstrated that mutation in the CgERG6 gene may lead to a reduced susceptibility to polyenes and to a pseudohyphal growth due to the subsequent changes in sterol content of the plasma membrane.

Biology of the pathogenic yeast Candida glabrata

The yeasts, being favorite eukaryotic microorganisms used in food industry and biotechnologies for production of biomass and various substances, are also used as model organisms in genetic manipulation, molecular and biological research. In this respect, Saccharomyces cerevisiae is the best-known species but current situation in medicine and industry requires the use of other species. Here we summarize the basic taxonomic, morphological, physiological, genetic, etc. information about the pathogenic yeast Candida glabrata that is evolutionarily very closely related to baker's yeast.

Restriction enzyme analysis of ribosomal DNA shows that Candida inconspicua clinical isolates can be misidentified as Candida norvegensis with traditional diagnostic procedures

We identified 29 yeast isolates from 22 patients using the API ID32C panel. Twenty-eight of these isolates were Candida norvegensis and one was C. inconspicua. Although C. norvegensis is considered a pseudohypha-producing species, only one isolate produced pseudohyphae. Restriction enzyme analysis of PCR-amplified ribosomal DNA with four different enzymes proved that all isolates were C. inconspicua.

Three mating type-like loci in Candida glabrata

Candida glabrata, the second most prevalent Candida species colonizing humans, possesses three mating type-like (MTL) loci (MTL1, MTL2, and MTL3). These loci contain pairs of MTL genes with their respective coding regions on complementary Crick and Watson DNA strands. Each pair of genes is separated by a shared intergenic promoter region, the same configuration found at the mating type loci of Saccharomyces cerevisiae. Two of the MTL loci, MTL1 and MTL2, contain either the MTLa1/MTLa2 configuration or the MTLalpha1/MTLalpha2 configuration in different strains. All but one of the 38 tested C. glabrata strains were either aaalpha or aalphaalpha. One test strain was alphaalphaalpha. Based on the mating type genotype, the MTL genes at the MTL1 or MTL2 loci, and the size of the XbaI fragment harboring MTL1 or MTL2, four classes of C. glabrata strains (I, II, III, and IV) were distinguished. Northern analysis revealed that strains were either a-expressors or alpha-expressors and that expression always reflected the genotype of either the MTL1 or MTL2 locus, depending on the class. The expression pattern in each class, therefore, is similar to that observed in S. cerevisiae, which harbors two silent cassette loci, HMR and HML, and the expression locus MAT. High-frequency phenotypic switching between core phenotypes in an alpha-expressing, but not in an a-expressing, strain modulated the level of MTL expression, suggesting a possible relationship between core phenotypic switching and mating.

Phenotypic switching and filamentation in Candida glabrata

Candida glabrata switches spontaneously, reversibly and at high frequency among the following four phenotypes distinguishable by graded colony colouration on CuSO(4)-containing agar: white (Wh), light brown (LB), dark brown (DB) and very dark brown (vDB). These phenotypes also differ in a graded fashion in the level of expression of the metallothionein gene MTII (Wh<LB<DB>vDB), the frequency of switching (Wh>LB>DB>vDB) and colouration on phloxine B-containing agar (Wh>LB>DB>vDB). Switching among the four graded phenotypes is referred to as 'the core switching system'. An additional switch phenotype, 'irregular wrinkle' (IWr), has been identified, which exhibits a highly wrinkled colony morphology. The characteristics of IWr suggest that switching to and from this phenotype represents a second high-frequency switching system. A microscopic analysis revealed that during the first 3 days of colony development, cells in the centres of Wh, LB, DB and vDB colonies expressed almost exclusively the budding yeast phenotype. After 3 days, however, pseudohyphae and cells extending tubes accumulated, so that by 7 days the proportions of these two cellular phenotypes reached 40-50% and 10-20%, respectively. In contrast, IWr colonies were composed almost exclusively of pseudohyphae through the first 6 days of colony development. After 6 days, IWr colonies began to accumulate both budding yeast cells and tubes. The tubes formed by C. glabrata reached lengths of up to six cell diameters, but the tubes did not represent traditional compartmentalized hyphae. Tube growth ended when the tube tip expanded to form a bud. Tubes then functioned as corridors for daughter nucleus migration to the apical bud, and were ultimately left uncompartmentalized and nucleus free. Core switching, pseudohypha formation and tube formation occurred in a majority of 62 tested clinical isolates, demonstrating that these developmental programmes are general characteristics of most strains of C. glabrata.

Candida glabrata displays pseudohyphal growth

The ability to undergo morphological change has been reported as an advantageous trait in fungal pathogenesis. Here we demonstrate that Candida glabrata ATCC2001, like diploid Saccharomyces cerevisiae strains, forms elongated chains of pseudohyphal cells on solid nitrogen starvation media (SLAD). Constrictions were apparent between adjoining cells; no parallel-sided hyphae were seen and pseudohyphae invaded the agar. When SLAD was supplemented with ammonium sulfate both C. glabrata and diploid S. cerevisiae strains lost their ability to undergo pseudohyphal growth. However, on this media C. glabrata yeast cells invaded the agar in a similar fashion to the invasive growth mode exhibited by haploid strains of S. cerevisiae cultured on rich media such as YPD. C. glabrata was not capable of invading YPD demonstrating that the process of filamentation is distinct in these two fungi. To our knowledge this is the first report to demonstrate that C. glabrata can undergo morphological change and grow as an invasive filamentous organism.

Evaluation of six commercial systems for identification of medically important yeasts

Six commercially available systems for the identification of yeasts were evaluated using 133 clinical isolates and four reference strains that had been previously identified by conventional methods and 19 recent clinical isolates that had been identified by the ID32C system (bioMérieux, France). The total identification rates (TIR) established for the total number of strains tested and the database identification rates (DBIR) established for the strains included in the respective manufacturer databases were both determined. After incubation for 4 h, the TIR and DBIR were 78% and 84%, respectively, for the RapID Yeast Plus system (Innovative Diagnostic Systems, USA). After incubation for 24 h, the TIR and DBIR were 32% and 32%, respectively, for the ID32C, 65% and 67% for the Auxacolor system (Sanofi Diagnostics Pasteur, France), 62% and 65% for the Fungichrom I system (International Microbio, France), 52% and 65% for the Fungifast I twin system (International Microbio), and 62% and 68% for the API Candida system (bioMérieux). The maximum TIR and DBIR (+/- 1%) obtained after incubation for 48 h were 86% and 88% for the Auxacolor, 85% and 89% for the Fungichrom I, 78% and 98% for the Fungifast I twin, and 82% and 91% for the API Candida. For the ID32C, the maximum TIR and DBIR were 98% and 98%, respectively, but these values were obtained only after 72 h of incubation. In addition, the six systems varied in their ease of use and readings. In conclusion, based on results obtained with 156 strains, the Auxacolor and Fungichrom systems seem the most appropriate for use in a clinical microbiology laboratory, due to their ease of use and reading, their rapidity, their cost per test, and their relatively high TIR results, which indicated acceptable performance with strains frequently isolated in our hospital. For a reference identification, the ID32C remains the sole system usable.