Formation of Aerial Hyphae in Candida albicans

Each of 22 isolates of Candida albicans was induced to produce aerial hyphae by culturing on a solid medium containing a peptone, acid-hydrolyzed casein, soluble starch, and agar in an atmosphere of 10% CO(2) at 37 C and room temperatures. Production of aerial hyphae is not diagnostic for C. albicans. Some of the other species of Candida may also produce such structures.     

Lipid Composition of the Arthrospores, Yeastlike Cells, and Mycelium of the Fungus Mucor hiemalis

The fungus Mucor hiemalisF-1156, which is believed to be monomorphic, was found to be able to grow dimorphically in a liquid medium that is free of chemical agents influencing morphogenesis. The growing mycelium produced arthrospores in large amounts. The lipids of the mycelium, yeastlike budding cells, and arthrospores differed in the contents of saturated and unsaturated fatty acids and in the proportion of polar and neutral lipids. The arthrospores contained more monoenoic fatty acids in the total lipids, more triacylglycerides and sterol esters in the neutral lipids, and more phosphatidylcholine and phosphatidylethanolamine in the polar lipids than the yeastlike cells. These differences in the lipid composition of different types of fungal cells should be taken into account in the studies of the lipogenesis of M. hiemalis.     

Gut Bacteria Shape Intestinal Microhabitats Occupied by the Fungus Candida albicans

1. Download : Download high-res image (210KB)
2. Download : Download full-size image

     

Budding in the Dimorphic Fungus Phialophora dermatitidis

Ultrastructural comparisons of yeast and hyphal bud formation in Phialophora dermatitidis reveal that bud initiation is characterized by a blastic rupture of the outer portion of the yeast or hyphal wall and the emergence of a bud protuberance through the resulting opening. The wall of the emerging bud is continuous, with only an inner wall layer of the parental yeast or hypha. The outer, ruptured portion of the parental wall typically forms a collar around the constricted emergence region of the developing bud. The cytoplasm within the very young emerging bud invariably contains a small number of membrane-bound vesicles. The septum formed between the daughter bud and the parental yeast or hypha is a complete septum devoid of a septal pore, septal pore plug, or any associated Woronin bodies characteristic of simple septa of the moniliform or true hyphae. These observations suggest that yeast bud formation and lateral hyphal bud formation in the dimorphic fungus P. dermatitidis involve a growth process which occurs identically in both the yeast and mold phase of this human pathogenic organism.     

Surface Stress Induces a Conserved Cell Wall Stress Response in the Pathogenic Fungus Candida albicans

The human fungal pathogen Candida albicans can grow at temperatures of up to 45°C. Here, we show that at 42°C substantially less biomass was formed than at 37°C. The cells also became more sensitive to wall-perturbing compounds, and the wall chitin levels increased, changes that are indicative of wall stress. Quantitative mass spectrometry of the wall proteome using ¹⁵N metabolically labeled wall proteins as internal standards revealed that at 42°C the levels of the β-glucan transglycosylases Phr1 and Phr2, the predicted chitin transglycosylases Crh11 and Utr2, and the wall maintenance protein Ecm33 increased. Consistent with our previous results for fluconazole stress, this suggests that a wall-remodeling response is mounted to relieve wall stress. Thermal stress as well as different wall and membrane stressors led to an increased phosphorylation of the mitogen-activated protein (MAP) kinase Mkc1, suggesting activation of the cell wall integrity (CWI) pathway. Furthermore, all wall and membrane stresses tested resulted in diminished cell separation. This was accompanied by decreased secretion of the major chitinase Cht3 and the endoglucanase Eng1 into the medium. Consistent with this, cht3 cells showed a similar phenotype. When treated with exogenous chitinase, cell clusters both from stressed cells and mutant strains were dispersed, underlining the importance of Cht3 for cell separation. We propose that surface stresses lead to a conserved cell wall remodeling response that is mainly governed by Mkc1 and is characterized by chitin reinforcement of the wall and the expression of remedial wall remodeling enzymes.     

Pigment Formation for Differentiating Cryptococcus neoformans from Candida albicans

When 2,3- or 3,4-dihydroxybenzoic acid, 3,4-dihydroxyphenylalanine, and 3,4-dihydroxycinnamic acid are added to growth media, they are converted to a characteristic brown pigment by Cryptococcus neoformans. This pigment formation has hitherto been encountered only when this microorganism was cultivated on media containing Guizotia abyssinica seed. This phenomenon can be used for differentiating Cryptococcus neoformans from Candida albicans. Possible precursors of these o-diphenols (quinic acid, aromatic monohydroxy acids, or tyrosine) do not give rise to the brown pigmentation.     

Streptococcus mutans Competence-Stimulating Peptide Inhibits Candida albicans Hypha Formation

The oral cavity is colonized by microorganisms growing in biofilms in which interspecies interactions take place. Streptococcus mutans grows in biofilms on enamel surfaces and is considered one of the main etiological agents of human dental caries. Candida albicans is also commonly found in the human oral cavity, where it interacts with S. mutans. C. albicans is a polymorphic fungus, and the yeast-to-hypha transition is involved in virulence and biofilm formation. The aim of this study was to investigate interkingdom communication between C. albicans and S. mutans based on the production of secreted molecules. S. mutans UA159 inhibited C. albicans germ tube (GT) formation in cocultures even when physically separated from C. albicans. Only S. mutans spent medium collected in the early exponential phase (4-h-old cultures) inhibited the GT formation of C. albicans. During this phase, S. mutans UA159 produces a quorum-sensing molecule, competence-stimulating peptide (CSP). The role of CSP in inhibiting GT formation was confirmed by using synthetic CSP and a comC deletion strain of S. mutans UA159, which lacks the ability to produce CSP. Other S. mutans strains and other Streptococcus spp. also inhibited GT formation but to different extents, possibly reflecting differences in CSP amino acid sequences among Streptococcus spp. or differences in CSP accumulation in the media. In conclusion, CSP, an S. mutans quorum-sensing molecule secreted during the early stages of growth, inhibits the C. albicans morphological switch.The oral cavity is colonized by many different microbial species, where most reside in biofilms. Because of its multispecies nature, the oral microbial community is one of the best biofilm models for studying interspecies interactions (17). The gram-positive bacterium Streptococcus mutans shows a high prevalence in dental biofilms, and it is considered to be the major etiological agent involved in human dental caries (21). The fungal species Candida albicans constitutes a minor part of the total microbial flora (19) and can be isolated as a commensal from the oral cavity of 50% to 60% of healthy adults (33). However, in immunocompromised individuals (for example, due to human immunodeficiency virus infection or as a result of chemotherapy) and elderly patients, this fungus often leads to candidiasis (24). C. albicans is a polymorphic fungus that can exist in three morphotypes: budding yeast, pseudohypha, and true hypha (5). The morphological switch from yeast to hyphal cells is important in many processes, such as virulence (22) and biofilm formation (10, 18), and is therefore the subject of many studies.Bacteria and yeasts are often found together in vivo, and there is growing evidence that interspecies, and even interkingdom, interactions occur within these populations (7). These interactions can be mediated through signaling molecules (40), as recently described for the interaction between C. albicans and Pseudomonas aeruginosa, an opportunistic bacterial pathogen (15). N-3-oxo-C₁₂ homoserine lactone (HSL), a signaling molecule involved in bacterial quorum sensing, completely represses C. albicans hypha formation without altering the growth rate. Although many gram-negative bacteria produce HSLs with shorter acyl chains (e.g., C₄-HSL), the inhibition of C. albicans hypha formation is caused specifically by long-chained HSL molecules. In addition, related, non-HSL molecules with long acyl chains, such as dodecanol and farnesol, also inhibit the hypha formation of C. albicans (8).A recent report described the coculturing of C. albicans and S. mutans in model oral biofilms on hydroxyapatite (26). It was shown that S. mutans increased the growth of C. albicans by stimulating coadhesion while simultaneously suppressing the formation of hyphae. S. mutans is a gram-positive bacterium and does not produce HSL-type molecules, and the nature of the interaction with C. albicans is presently unknown. In this study, the interaction between S. mutans and C. albicans was investigated by studying the effect of secreted molecules of S. mutans on C. albicans hypha formation.     

Chlamydospore Production and Germ-Tube Formation by Auxotrophs of Candida albicans

A prototrophic strain and 21 auxotrophic strains of Candida albicans were assessed for their capacity to produce chlamydospores and germ tubes. All of the mutants were able to produce germ-tubes in human serum but only two mutants produced them in defined medium with L-alpha-amino-n-butyric acid as the sole source of nitrogen. Most auxotrophs were not able to produce chlamydospores on corn meal agar with 1% Tween 80, but they could be induced to do so if the medium was supplemented with their growth requirement(s). Although L-cysteine was able to support the growth of two methionine mutants, it did not support chlamydospore formation when added to corn meal agar with 1% Tween 80. Mutants of C. albicans that do not form chlamydospores could be incorrectly identified in laboratories that rely on chlamydospore formation for identification.     

In Vitro Effect of Amphotericin B on Candida albicans,Candida glabrata and Candida parapsilosis Biofilm Formation

Candida spp. biofilm is considered highly resistant to conventional antifungals. The aim of this study was to investigate the in vitro effect of amphotericin B on Candida spp. biofilms at different stages of maturation. We investigated the activity of amphotericin B against 78 clinical isolates of Candida spp., representing three species, growing as planktonic and sessile cells, by a widely accepted broth microdilution method. The in vitro effect on sessile cell viability was evaluated by MTT reduction assay. All examined strains were susceptible to amphotericin B when grown as free-living cells. At the early stages of biofilm maturation 96.7–100.0 % strains, depending on species, displayed amphotericin B sessile minimal inhibitory concentration (SMIC) ≤1 μg/mL. Mature Candida spp. biofilm of 32.1–90.0 % strains displayed amphotericin B SMIC ≤1 μg/mL. Based on these results, amphotericin B displays species- and strain-depending activity against Candida spp. biofilms.     

Comparison of Switching and Biofilm Formation between MTL-Homozygous Strains of Candida albicans and Candida dubliniensis

Candida albicans and Candida dubliniensis are highly related species that share the same main developmental programs. In C. albicans, it has been demonstrated that the biofilms formed by strains heterozygous and homozygous at the mating type locus (MTL) differ functionally, but studies rarely identify the MTL configuration. This becomes a particular problem in studies of C. dubliniensis, given that one-third of natural strains are MTL homozygous. For that reason, we have analyzed MTL-homozygous strains of C. dubliniensis for their capacity to switch from white to opaque, the stability of the opaque phenotype, CO₂ induction of switching, pheromone induction of adhesion, the effects of minority opaque cells on biofilm thickness and dry weight, and biofilm architecture in comparison with C. albicans. Our results reveal that C. dubliniensis strains switch to opaque at lower average frequencies, exhibit a far lower level of opaque phase stability, are not stimulated to switch by high CO₂, exhibit more variability in biofilm architecture, and most notably, form mature biofilms composed predominately of pseudohyphae rather than true hyphae. Therefore, while several traits of MTL-homozygous strains of C. dubliniensis appear to be degenerating or have been lost, others, most notably several related to biofilm formation, have been conserved. Within this context, the possibility is considered that C. dubliniensis is transitioning from a hypha-dominated to a pseudohypha-dominated biofilm and that aspects of C. dubliniensis colonization may provide insights into the selective pressures that are involved.     

Candida albicans Niche Specialization: Features That Distinguish Biofilm Cells from Commensal Cells

The fungus Candida albicans is a frequent commensal colonizer of the human gastrointestinal (GI) tract, but is also an opportunistic pathogen. This review explores features that distinguish the colonizing and pathogenic forms of C. albicans. Candida albicans in a biofilm is used as an example of a pathogenic form of the organism, because biofilms are a common feature of device-associated C. albicans infections. Biofilms (complex, sessile communities of cells) have been the subject of several large-scale gene expression studies. Biofilms and commensal C. albicans colonizing the murine GI tract show a variety of differentially expressed genes. Cell surface proteins encoded by these differentially expressed genes are especially attractive as targets for new clinical prevention, diagnosis, or treatment tools that are specific for C. albicans in its pathogenic biofilm state.     

Gluconeogenesis in Candida albicans

According to different metabolic situations in various stages of Candida albicans pathogenesis the regulation of carbohydrate metabolism was investigated. We report the genetic characterization of all major C. albicans gluconeogenic and glyoxylate cycle genes (fructose-1,6-bisphosphatase, PEP carboxykinase, malate synthase and isocitrate lyase) which were isolated after functional complementation of the corresponding Saccharomyces cerevisiae deletion mutants. Remarkably, the regulation of the heterologously expressed C. albicans gluconeogenic and glyoxylate cycle genes was similar to that of the homologous S. cerevisiae genes. A C. albicans DeltaCafbp1 deletion strain failed to utilize non-fermentable carbon sources but hyphal growth was not affected. Our results show that regulation of gluconeogenesis in C. albicans is similar to that of S. cerevisiae and that the current knowledge on how gluconeogenesis is regulated will facilitate the physiological understanding of C. albicans.     

Streptococcus mutans Can Modulate Biofilm Formation and Attenuate the Virulence of Candida albicans

Streptococcus mutans and Candida albicans are found together in the oral biofilms on dental surfaces, but little is known about the ecological interactions between these species. Here, we studied the effects of S. mutans UA159 on the growth and pathogencity of C. albicans. Initially, the effects of S. mutans on the biofilm formation and morphogenesis of C. albicans were tested in vitro. Next, we investigate the influence of S. mutans on pathogenicity of C. albicans using in vivo host models, in which the experimental candidiasis was induced in G. mellonella larvae and analyzed by survival curves, C. albicans count in hemolymph, and quantification of hyphae in the host tissues. In all the tests, we evaluated the direct effects of S. mutans cells, as well as the indirect effects of the subproducts secreted by this microorganism using a bacterial culture filtrate. The in vitro analysis showed that S. mutans cells favored biofilm formation by C. albicans. However, a reduction in biofilm viable cells and inhibition of hyphal growth was observed when C. albicans was in contact with the S. mutans culture filtrate. In the in vivo study, injection of S. mutans cells or S. mutans culture filtrate into G. mellonella larvae infected with C. albicans increased the survival of these animals. Furthermore, a reduction in hyphal formation was observed in larval tissues when C. albicans was associated with S. mutans culture filtrate. These findings suggest that S. mutans can secrete subproducts capable to inhibit the biofilm formation, morphogenesis and pathogenicity of C. albicans, attenuating the experimental candidiasis in G. mellonella model.