Control of dimorphism in a biochemical variant of Candida albicans

Combined transcriptomic and metabolomic analysis revealed that pH changes affected the expression of carbohydrate and ribosome biogenesis-related genes in Aspergillus niger SICU-33

The process of carbohydrate metabolism and genetic information transfer is an important part of the study on the effects of the external environment on microbial growth and development. As one of the most significant environmental parameters, pH has an important effect on mycelial growth. In this study, the effects of environmental pH on the growth and nutrient composition of Aspergillus niger (A. niger) filaments were determined. The pH values of the medium were 5, 7, and 9, respectively, and the molecular mechanism was further investigated by transcriptomics and metabolomics methods. The results showed that pH 5 and 9 significantly inhibited filament growth and polysaccharide accumulation of A. niger. Further, the mycelium biomass of A. niger and the crude polysaccharide content was higher when the medium's pH was 7. The DEGs related to ribosome biogenesis were the most abundant, and the downregulated expression of genes encoding XRN1, RRM, and RIO1 affected protein translation, modification, and carbohydrate metabolism in fungi. The dynamic changes of pargyline and choline were in response to the oxidative metabolism of A. niger SICU-33. The ribophorin_I enzymes and DL-lactate may be important substances related to pH changes during carbohydrate metabolism of A.niger SICU-33. The results of this study provide useful transcriptomic and metabolomic information for further analyzing the bioinformatic characteristics of A. niger and improving the application in ecological agricultural fermentation.

Convergent and divergent roles of the glucose-responsive kinase SNF4 in Candida tropicalis

The sucrose non-fermenting 1 (SNF1) complex is a heterotrimeric protein kinase complex that is an ortholog of the mammalian AMPK complex and is evolutionally conserved in most eukaryotes. This complex contains a catalytic subunit (Snf1), a regulatory subunit (Snf4) and a scaffolding subunit (Sip1/Sip2/Gal73) in budding yeast. Although the function of AMPK has been well studied in Saccharomyces cerevisiae and Candida albicans, the role of AMPK in Candida tropicalis has never been investigated. In this study, we focused on SNF4 in C. tropicalis as this fungus cannot produce a snf1Δ mutant. We demonstrated that C. tropicalis SNF4 shares similar roles in glucose derepression and is necessary for cell wall integrity and virulence. The expression of both SNF1 and SNF4 was significantly induced when glucose was limited. Furthermore, snf4Δ strains exhibited high sensitivity to many surface-perturbing agents because the strains contained lower levels of glucan, chitin and mannan. Interestingly, in contrast to C. albicans sak1Δ and snf4Δ, C. tropicalis snf4Δ exhibited phenotypes for cell aggregation and pseudohypha production. These data indicate that SNF4 performs convergent and divergent roles in C. tropicalis and possibly other unknown roles in the C. tropicalis SNF1-SNF4 AMPK pathway.

Candida albicans Hyphal Morphogenesis within Macrophages Does Not Require Carbon Dioxide or pH-Sensing Pathways

The opportunistic fungal pathogen Candida albicans has evolved a variety of mechanisms for surviving inside and escaping macrophages, including the initiation of filamentous growth. Although several distinct models have been proposed to explain this process at the molecular level, the signals driving hyphal morphogenesis in this context have yet to be clarified. Here, we evaluate the following three molecular signals as potential hyphal inducers within macrophage phagosomes: CO2, intracellular pH, and extracellular pH. Additionally, we revisit previous work suggesting that the intracellular pH of C. albicans fluctuates in tandem with morphological changes in vitro. Using time-lapse microscopy, we observed that C. albicans mutants lacking components of the CO2-sensing pathway were able to undergo hyphal morphogenesis within macrophages. Similarly, a rim101Δ strain was competent in hyphal induction, suggesting that neutral/alkaline pH sensing is not necessary for the initiation of morphogenesis within phagosomes either. Contrary to previous findings, single-cell pH-tracking experiments revealed that the cytosolic pH of C. albicans remains tightly regulated both within macrophage phagosomes and under a variety of in vitro conditions throughout the process of morphogenesis. This finding suggests that intracellular pH is not a signal contributing to morphological changes.

MSS2 maintains mitochondrial function and is required for chitosan resistance, invasive growth, biofilm formation and virulence in Candida albicans

Candida albicans is the most prevalent fungal pathogen in humans, particularly in immunocompromised patients. In this study, by screening a C. albicans mutant library, we first identified that the MSS2 gene, an ortholog of Saccharomyces cerevisiae MSS2 required for mitochondrial respiration, mediates chitosan resistance. Upon treatment with 0.2% chitosan, the growth of mss2Δ strains was strikingly impaired, and MSS2 expression was significantly repressed by chitosan. Furthermore, mss2Δ strains exhibited slow growth on medium supplemented with glycerol as the sole carbon source. Similar to the chitosan-treated wild-type strain, the mss2Δ strain exhibited a significantly impaired ATP production ability. These data suggest that an antifungal mechanism of chitosan against C. albicans acts by inhibiting MSS2 gene expression, leading to repression of mitochondrial function. Normal respiratory function is suggested to be required for fungal virulence. Interestingly, the mss2Δ mutant strains exhibited significantly impaired invasive ability in vitro and ex vivo but retained normal hyphal development ability in liquid medium. Furthermore, the MSS2 deletion strains could not form robust biofilms and exhibited significantly reduced virulence. Collectively, these results demonstrated that the antifungal effect of chitosan against C. albicans is mediated via inhibition of mitochondrial biogenesis. These data may provide another strategy for antifungal drug development via inhibition of fungal mitochondria.

N-acetylglucosamine-mediated morphological transition in Candida albicans and Candida tropicalis

Morphological transitions in Candida species are key factors in facilitating invasion and adapting to environmental changes. N-acetylglucosamine (GlcNAc) is a monosaccharide signalling molecule that can regulate morphological transitions in Candida albicans and Candida tropicalis. Interestingly, although the uptake and metabolic pathways of GlcNAc and GlcNAc-mediated white-to-opaque cell switching are similar between the two Candida species, GlcNAc induces hyphal development in C. albicans, whereas it suppresses hyphal development in C. tropicalis. These findings indicate that the characteristics of C. albicans and C. tropicalis in response to GlcNAc are remarkably different. Here, we compare the conserved and divergent GlcNAc-mediated signalling pathways and catabolism between the two Candida species. Deletion of NGT1, a GlcNAc transportation gene, inhibited hyphal formation in C. albicans but promoted hyphal development in C. tropicalis. To further understand these opposite effects on filamentous growth in response to GlcNAc in the two Candida species, the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) signalling pathways in both C. albicans and C. tropicalis were compared. Interestingly, GlcNAc activated the cAMP/PKA signalling pathway of the two Candida species, suggesting that the hyphal development-regulated circuit is remarkably diverse between the two species. Indeed, the Ndt80-like gene REP1, which is critical for regulating GlcNAc catabolism, exhibits distinct roles in the hyphal development of C. albicans and C. tropicalis. These data suggest possible reasons for the divergent hyphal growth response in C. albicans and C. tropicalis upon GlcNAc induction.

Candida tropicalis RON1 is required for hyphal formation, biofilm development, and virulence but is dispensable for N-acetylglucosamine catabolism

NDT80-like family genes are highly conserved across a large group of fungi, but the functions of each Ndt80 protein are diverse and have evolved differently among yeasts and pathogens. The unique NDT80 gene in budding yeast is required for sexual reproduction, whereas three NDT80-like genes, namely, NDT80, REP1, and RON1, found in Candida albicans exhibit distinct functions. Notably, it was suggested that REP1, rather than RON1, is required for N-acetylglucosamine (GlcNAc) catabolism. Although Candida tropicalis, a widely dispersed fungal pathogen in tropical and subtropical areas, is closely related to Candida albicans, its phenotypic, pathogenic and environmental adaptation characteristics are remarkably divergent. In this study, we focused on the Ron1 transcription factor in C. tropicalis. Protein alignment showed that C. tropicalis Ron1 (CtRon1) shares 39.7% identity with C. albicans Ron1 (CaRon1). Compared to the wild-type strain, the C. tropicalis ron1Δ strains exhibited normal growth in different carbon sources and had similar expression levels of several GlcNAc catabolic genes during GlcNAc treatment. In contrast, C. tropicalis REP1 is responsible for GlcNAc catabolism and is involved in GlcNAc catabolic gene expressions, similar to C. albicans Rep1. However, REP1 deletion strains in C. tropicalis promote hyphal development in GlcNAc with low glucose content. Interestingly, CtRON1, but not CaRON1, deletion mutants exhibited significantly impaired hyphal growth and biofilm formation. As expected, CtRON1 was required for full virulence. Together, the results of this study showed divergent functions of CtRon1 compared to CaRon1; CtRon1 plays a key role in yeast-hyphal dimorphism, biofilm formation and virulence.

LAY ABSTRACT

In this study, we identified the role of RON1, an NDT80-like gene, in Candida tropicalis. Unlike the gene in Candida albicans, our studies showed that RON1 is a key regulator of hyphal formation, biofilm development and virulence but is dispensable for N-acetylglucosamine catabolism in C. tropicalis.

Identification and characterization of ORF19.1725, a novel gene contributing to the white cell pheromone response and virulence-associated functions in Candida albicans

An epigenetic transition between white cells and opaque cells influences several properties of Candida albicans biology, including cellular morphology, biofilm formation, virulence, and sexual mating. In particular, these two cell types exhibit marked differences in their ability to undergo sex. A previous study identified the transcriptional regulator of pheromone response in both the white and opaque states as Cph1 because deletion of this gene abolished both pheromone-induced cell adhesion in white cells and sexual mating in opaque cells. To further explore how these cell types exhibit distinct biological outputs upon pheromone stimulation, we selected five Cph1-regulated genes with significant expression during the pheromone response in the white state but not the opaque state. These phase-specific pheromone-induced genes are ORF19.1539, ORF19.1725, ORF19.2430, ORF19.2691 and ORF19.5557. Deletion of each gene revealed that orf19.1539Δ, orf19.1725Δ, orf19.2430Δ and orf19.5557Δ showed significant decreases in pheromone-stimulated cell adhesion in the white state but retained normal mating competency in the opaque state, indicating that a particular role in white cell pheromone response is mediated by these four genes. Interestingly, the defects of orf19.1725Δ in pheromone-stimulated cell adhesion also abolished conventional biofilms and hyphal growth. Zebrafish egg infection assays further demonstrated that ORF19.1725 is involved in cell adhesion, penetration and virulence. Overall, four Cph1-regulated downstream targets were identified in the regulation of white cell pheromone response. We also clarified the roles of C. albicans ORF19.1725 in cell adhesion, hyphal growth, biofilm formation and virulence.

A comprehensive analysis of Candida albicans phosphoproteome reveals dynamic changes in phosphoprotein abundance during hyphal morphogenesis

The morphological plasticity of Candida albicans is a virulence determinant as the hyphal form has significant roles in the infection process. Recently, phosphoregulation of proteins through phosphorylation and dephosphorylation events has gained importance in studying the regulation of pathogenicity at the molecular level. To understand the importance of phosphorylation in hyphal morphogenesis, global analysis of the phosphoproteome was performed after hyphal induction with elevated temperature, serum, and N-acetyl-glucosamine (GlcNAc) treatments. The study identified 60, 20, and 53 phosphoproteins unique to elevated temperature-, serum-, and GlcNAc-treated conditions, respectively. Distribution of unique phosphorylation sites sorted by the modified amino acids revealed that predominant phosphorylation occurs in serine, followed by threonine and tyrosine residues in all the datasets. However, the frequency distribution of phosphorylation sites in the proteins varied with treatment conditions. Further, interaction network-based functional annotation of protein kinases of C. albicans as well as identified phosphoproteins was performed, which demonstrated the interaction of kinases with phosphoproteins during filamentous growth. Altogether, the present findings will serve as a base for further functional studies in the aspects of protein kinase-target protein interaction in effectuating phosphorylation of target proteins, and delineating the downstream signaling networks linked to virulence characteristics of C. albicans.

Cinnamomum zeylanicum bark essential oil induces cell wall remodelling and spindle defects in Candida albicans

Background

Cinnamon (Cinnamomum zeylanicum) bark extract exhibits potent inhibitory activity against Candida albicans but the antifungal mechanisms of this essential oil remain largely unexplored.

Results

We analyzed the impact of cinnamon bark oil on C. albicans RSY150, and clinical strains isolated from patients with candidemia and candidiasis. The viability of RSY150 was significantly compromised in a dose dependent manner when exposed to cinnamon bark oil, with extensive cell surface remodelling at sub inhibitory levels (62.5 μg/mL). Atomic force microscopy revealed cell surface exfoliation, altered ultrastructure and reduced cell wall integrity for both RSY150 and clinical isolates exposed to cinnamon bark oil. Cell wall damage induced by cinnamon bark oil was confirmed by exposure to stressors and the sensitivity of cell wall mutants involved in cell wall organization, biogenesis, and morphogenesis. The essential oil triggered cell cycle arrest by disrupting beta tubulin distribution, which led to mitotic spindle defects, ultimately compromising the cell membrane and allowing leakage of cellular components. The multiple targets of cinnamon bark oil can be attributed to its components, including cinnamaldehyde (74%), and minor components (< 6%) such as linalool (3.9%), cinamyl acetate (3.8%), α-caryophyllene (5.3%) and limonene (2%). Complete inhibition of the mitotic spindle assembly was observed in C. albicans treated with cinnamaldehyde at MIC (112 μg/mL).

Conclusions

Since cinnamaldehyde disrupts both the cell wall and tubulin polymerization, it may serve as an effective antifungal, either by chemical modification to improve its specificity and efficacy or in combination with other antifungal drugs.

Electronic supplementary material

The online version of this article (10.1186/s40694-018-0046-5) contains supplementary material, which is available to authorized users.

β-lactam substituted polycyclic fused pyrrolidine/pyrrolizidine derivatives eradicate C. albicans in an ex vivo human dentinal tubule model by inhibiting sterol 14-α demethylase and cAMP pathway

BACKGROUND

Further quest for new anti-fungal compounds with proven mechanisms of action arises due to resistance and dose limiting toxicity of existing agents. Among the human fungal pathogens C. albicans predominate by infecting several sites in the body and in particular oral cavity and root canals of human tooth.

METHODS

In the present study, we screened a library of β-lactam substituted polycyclic fused pyrrolidine/pyrrolizidine compounds against Candida sp. Detailed molecular studies were carried out with the active compound 3 on C. albicans. Morphological damage and antibiofilm activity of compound 3 on C. albicans was studied using scanning electron microscopy (SEM). Biochemical evidence for membrane damage was studied using flow cytometry. In silico docking studies were carried out to elucidate the mechanism of action of compound 3. Further, the antifungal activity of compound 3 was evaluated in an ex vivo dentinal tubule infection model.

RESULTS

Screening data showed that several new compounds were active against Candida sp. Among them, Compound 3 was most potent and exerted time kill effect at 4h, post antifungal effect up to 6h. When used in combination with fluconazole or nystatin, compound 3 revealed an minimum inhibitory concentration (MIC) decrease by 4 fold for both drugs used. In-depth molecular studies with compound 3 on C. albicans showed that this compound inhibited yeast to hyphae (Y-H) conversion and this involved the cAMP pathway. Further, SEM images of C. albicans showed that compound 3 caused membrane damage and inhibited biofilm formation. Biochemical evidence for membrane damage was confirmed by increased propidium iodide (PI) uptake in flow cytometry. Further, in silico studies revealed that compound 3 docks with the active site of the key enzyme 14-α-demethylase and this might inhibit ergosterol synthesis. In support of this, ergosterol levels were found to be decreased by 32 fold in compound 3 treated samples as analyzed by high performance liquid chromatography (HPLC). Further, the antifungal activity of compound 3 was evaluated in an ex vivo dentinal tubule infection model, which mimics human tooth root canal infection. Confocal laser scanning microscopy studies showed 83% eradication of C. albicans and a 6 log reduction in colony forming unit (CFU) after 24h treatment in the infected tooth samples in this model.

CONCLUSION

Compound 3 was found to be very effective in eradicating C. albicans by inhibiting cAMP pathway and ergosterol biosynthesis.

GENERAL SIGNIFICANCE

The results of this study can pave the way for developing new antifungal agents with well deciphered mechanisms of action and can be a promising antifungal agent or medicament against root canal infection.

A novel function for Hog1 stress-activated protein kinase in controlling white-opaque switching and mating in Candida albicans

Candida albicans is a commensal in heathy people but has the potential to become an opportunistic pathogen and is responsible for half of all clinical infections in immunocompromised patients. Central to understanding C. albicans behavior is the white-opaque phenotypic switch, in which cells can undergo an epigenetic transition between the white state and the opaque state. The phenotypic switch regulates multiple properties, including biofilm formation, virulence, mating, and fungus-host interactions. Switching between the white and opaque states is associated with many external stimuli, such as oxidative stress, pH, and N-acetylglucosamine, and is directly regulated by the Wor1 transcriptional circuit. The Hog1 stress-activated protein kinase (SAPK) pathway is recognized as the main pathway for adapting to environmental stress in C. albicans. In this work, we first show that loss of the HOG1 gene in A: / A: and α/α cells, but not A: /α cells, results in 100% white-to-opaque switching when cells are grown on synthetic medium, indicating that switching is repressed by the A1: /α2 heterodimer that represses WOR1 gene expression. Indeed, switching in the hog1Δ strain was dependent on the presence of WOR1, as a hog1Δ wor1Δ strain did not show switching to the opaque state. Deletion of PBS2 and SSK2 also resulted in C. albicans cells switching from white to opaque with 100% efficiency, indicating that the entire Hog1 SAPK pathway is involved in regulating this unique phenotypic transition. Interestingly, all Hog1 pathway mutants also caused defects in shmoo formation and mating efficiencies. Overall, this work reveals a novel role for the Hog1 SAPK pathway in regulating white-opaque switching and sexual behavior in C. albicans.

Changes in glutathione-dependent redox status and mitochondrial energetic strategies are part of the adaptive response during the filamentation process in Candida albicans

Candida albicans is an opportunist pathogen responsible for a large spectrum of infections, from superficial mycosis to systemic diseases called candidiasis. Its ability to grow in various morphological forms, such as unicellular budding yeast, filamentous pseudohyphae and hyphae, contributes to its survival in the diverse microenvironments it encounters in the host. During infection in vivo, C. albicans is faced with high levels of reactive oxygen species (ROS) generated by phagocytes, and the thiol-dependent redox status of the cells reflects their levels of oxidative stress. We investigated the role of glutathione during the transition between the yeast and hyphal forms of the pathogen, in relation to possible changes in mitochondrial bioenergetic pathways. Using various growth media and selective mutations affecting the filamentation process, we showed that C. albicans filamentation was always associated with a depletion of intracellular glutathione levels. Moreover, the induction of hypha formation resulted in general changes in thiol metabolism, including the oxidation of cell surface -SH groups and glutathione excretion. Metabolic adaptation involved tricarboxylic acid (TCA) cycle activation, acceleration of mitochondrial respiration and a redistribution of electron transfer pathways, with an increase in the contribution of the alternative oxidase and rotenone-insensitive dehydrogenase. Changes in redox status and apparent oxidative stress may be necessary to the shift to adaptive metabolic pathways, ensuring normal mitochondrial function and adenosine triphosphate (ATP) levels. The consumption of intracellular glutathione levels during the filamentation process may thus be the price paid by C. albicans for survival in the conditions encountered in the host.

Fungal invasion of epithelial cells

Interaction between host cells and invasive Candida plays a large role in the pathogenicity of Candida species. Fungal-induced endocytosis and active penetration are the two distinct, yet complementary invasion mechanisms of invasive candidiasis. Induced endocytosis is a microorganism-triggered, epithelial-driven, clathrin-mediated and actin-dependent process. During the fundamental pathological process of induced endocytosis, invasins (Als3 and Ssa1), which mediate the binding of host epithelial surface proteins, are expressed by Candida species on the hyphal surface. Sequentially, the interaction between invasins and host epithelial surface proteins stimulates the recruitment of clathrin, dynamin and cortactin to the sites where Candida enters epithelial cells, which in turn induce the actin cytoskeleton reorganization. Actin cytoskeleton provides the force required for fungal internalization. Parallely, active penetration of Candida can directly pass through epithelial cells possibly due to progressive elongation of hyphae and physical forces. Several molecules, such as secreted hydrolases and Als3, can affect the protective barrier of the epithelium and make Candida actively penetrate into epithelial cells through intercellular gaps of epithelial layers.

Identification of the cell targets important for propolis-induced cell death in Candida albicans

Candida albicans is the most common fungal pathogen of humans, forming both commensal and opportunistic pathogenic interactions, causing a variety of skin and soft tissue infections in healthy people. In immunocompromised patients C. albicans can result in invasive, systemic infections that are associated with a high incidence of mortality. Propolis is a complex mixture of several resinous substances which are collected from plants by bees. Here, we demonstrated the fungicidal activity of propolis against all three morphogenetic types of C. albicans and that propolis-induced cell death was mediated via metacaspase and Ras signaling. To identify genes that were involved in propolis tolerance, we screened ~800 C. albicans homozygous deletion mutants for decreased tolerance to propolis. Fifty-one mutant strains were identified as being hypersensitive to propolis including seventeen genes involved in cell adhesion, biofilm formation, filamentous growth, phenotypic switching and pathogenesis (HST7, GIN4, VPS34, HOG1, ISW2, SUV3, MDS3, HDA2, KAR3, YHB1, NUP85, CDC10, MNN9, ACE2, FKH2, and SNF5). We validated these results by showing that propolis inhibited the transition from yeast-like to hyphal growth. Propolis was shown to contain compounds that conferred fluorescent properties to C. albicans cells. Moreover, we have shown that a topical pharmaceutical preparation, based upon propolis, was able to control C. albicans infections in a mouse model for vulvovaginal candidiasis. Our results strongly indicate that propolis could be used as a strategy for controlling candidiasis.

The transcription factor Flo8 mediates CO2 sensing in the human fungal pathogen Candida albicans

CO₂ is a critical signaling molecule in a variety of biological processes. The transcription factor Flo8 is identified as a key regulator of CO₂ sensing, which governs CO₂-induced phenotypic transitions in Candida albicans. These findings provide new insights into the understanding of CO₂ sensing in pathogenic fungi.

Physiological levels of CO₂ have a profound impact on prominent biological attributes of the major fungal pathogen of humans, Candida albicans. Elevated CO₂ induces filamentous growth and promotes white-to-opaque switching. However, the underlying molecular mechanisms of CO₂ sensing in C. albicans are insufficiently understood. Here we identify the transcription factor Flo8 as a key regulator of CO₂-induced morphogenesis in C. albicans by screening a gene null mutant library. We show that Flo8 is required for CO₂-induced white-to-opaque switching, as well as for filamentous growth. Ectopic expression of FLO8 hypersensitizes C. albicans cells to the elevated CO₂ levels. Furthermore, we demonstrate that CO₂ signaling in C. albicans involves two pathways: the already reported cAMP/protein kinase A and another major one that is unidentified. The two pathways converge on the transcription factor Flo8, which is the master regulator of CO₂ sensing in C. albicans and plays a critical role in regulation of white-to-opaque switching and filamentous growth. Our findings provide new insights into the understanding of CO₂ sensing in pathogenic fungi that have important implications for higher organisms.

Rfg1, a protein related to the Saccharomyces cerevisiae hypoxic regulator Rox1, controls filamentous growth and virulence in Candida albicans

Candida albicans, the major fungal pathogen in humans, can undergo a reversible transition from ellipsoidal single cells (blastospores) to filaments composed of elongated cells attached end to end. This transition is thought to allow for rapid colonization of host tissues, facilitating the spread of infection. Here, we report the identification of Rfg1, a transcriptional regulator that controls filamentous growth of C. albicans in an environment-dependent manner. Rfg1 is important for virulence of C. albicans in a mouse model and is shown to control a number of genes that have been implicated in this process. The closest relative to Rfg1 in Saccharomyces cerevisiae is Rox1, a key repressor of hypoxic genes. However, Rfg1 does not appear to play a role in the regulation of hypoxic genes in C. albicans. These results demonstrate that a regulatory protein that controls the hypoxic response in S. cerevisiae controls filamentous growth and virulence in C. albicans. The observations described in this paper raise new and intriguing questions about the evolutionary relationship between these processes.

Dimorphism in Yarrowia lipolytica: filament formation is suppressed by nitrogen starvation and inhibition of respiration

In contrast to Saccharomyces cerevisiae, nitrogen starvation inhibited formation of hyphae in liquid cultures of Y. lipolytica, while carbon source did not seem to be important for filament formation. Inhibitors of mitochondrial respiration strongly suppressed the development of hyphae, indicating that energy conversion processes, and thus carbon metabolism, may be involved. pH of the medium also strongly affected the morphology, but only in the presence of a complex nitrogen source, implying that the cells respond to altered nutrition in media with different pH rather than to pH itself. The results suggest that the XPR2 gene encoding Y. lipolytica alkaline extracellular proteinase is involved in the regulation of dimorphism in this species.

Spiral hyphae of Candida albicans formed in anaerobic culture

Candida albicans from a patient with dental caries grew on minimal medium consisting of agar supplemented with magnesium chloride and sodium phosphate. Hyphal growth was observed when the yeast was cultured between 26 degrees C and 28 degrees C under aerobic conditions, and typical chlamydospores were formed. However, when the yeast was cultured at the same temperature under anaerobic conditions, curly hyphae developed on the surface of the medium, but no chlamydospores were formed. This phenomenon was also observed if the culture was started under aerobic conditions but was continued under anaerobic conditions.

High-frequency switching in Candida albicans

Most strains of Candida albicans are capable of switching frequently and reversibly between a number of phenotypes distinguishable by colony morphology. A number of different switching systems have been defined according to the limited set of phenotypes in each switching repertoire, and each strain appears to possess a single system. Switching can affect many aspects of cellular physiology and morphology and appears to be a second level of phenotypic variability superimposed upon the bud-hypha transition. The most dramatic switching system so far identified is the "white-opaque transition." This system dramatizes the extraordinary effects switching can have on the budding cell phenotype, including the synthesis of opaque-specific antigens, the expression of white-specific and opaque-specific genes, and the genesis of unique cell wall structures. Switching has been demonstrated to occur at sites of infection and between episodes of recurrent vaginitis, and it may function to generate variability in commensal and infecting populations for adaptive reasons. Although the molecular mechanisms involved in the switch event are not understood, recent approaches to its elucidation are discussed and an epigenetic mechanism is proposed.

Anaerobiosis and serum promote mycelium formation by Candida albicans in colonies on TSBV agar

Following long-term periodontal treatment with tetracycline a superinfection with Candida may arise. The reduced environment and the serum transudate of the periodontal pocket may promote such infection. The present in vitro study was performed to ascertain whether yeast-mycelium transformation in a fresh periodontal isolate was promoted under anaerobic conditions and in the presence of serum. C. albicans, isolated from a patient with tetracycline-treated refractory periodontitis, was cultured anaerobically or aerobically on TSBV or Sabouraud's dextrose agar at 29 degrees C or 37 degrees C for 72 h, with the pH of the medium being 5.6 or 7.2. TSBV medium was also tested with its horse serum or yeast extract removed. Mycelial growth was recorded visually and by stereo and scanning electron microscopy. Anaerobic culture at 29 degrees C or 37 degrees C on TSBV provided abundant mycelium at both pHs. After aerobic culture the mycelial phase was less pronounced and more abundant at pH 7.2 than at 5.6. TSBV without serum or yeast extract yielded more mycelium after anaerobic than after aerobic culture, although less than when both components were included. Sabouraud's medium provided sparse mycelium after anaerobic culture irrespective of the pH, and no mycelium after aerobic culture.

Importance of some factors on the dimorphism of Candida albicans

The passage between the yeast and mycelial forms of Candida albicans B 311-10 was studied by using the minimal synthetic medium of Shepherd et al. modified without biotin and with low glucose concentrations. It was observed that biotin, aminoacids and particularly pH are not important factors in the dimorphism of C. albicans. The only factor of notable importance in the passage of yeast form to mycelial form in C. albicans was glucose concentration.

Cyclic adenosine 3',5' monophosphate (cAMP) and dimorphism in the pathogenic fungus Paracoccidioides brasiliensis

Exogenous cAMP or its analogs inhibit the mycelium transformation of yeast and induce bulging of the apex of mycelia. But intracellular cAMP levels of yeast and mycelial cells are not significantly different.

Effects of carbon dioxide and pH on adhesion of Candida albicans to vaginal epithelial cells

A controlled-environment membrane model for use in vitro was developed and employed in an attempt to mimic the environment of the vagina in order to study yeast-vaginal cell adhesion. Adhesion in vitro of four strains of Candida albicans (NIH 3181A, NIH 526B, ATCC 18804, and MCO 2400) to vaginal epithelial cells (VEC) appeared to be affected by the pH and the level of carbon dioxide that have been found to be present in the vagina in vivo. Strain 3181A had a greater adhesion ability than 526B when the concentration of yeast cells was increased and when the yeast cells were incubated with VEC at pH 5 in sodium phosphate buffer in ambient air supplemented with 10% CO2. Of the four strains of C. albicans used, 3181A had the greatest adhesion ability, with strains 2400, 18804, and 526B ranked in order of decreasing adhesion ability. Also, an enhanced, electron-dense, matted outer region of the cell walls of the yeasts was observed frequently when they were incubated in ambient air supplemented with 10% CO2. In addition, of the vaginal cells that had yeast cells attached to them, an average of 94.4% of the total yeast cells were attached to the microridge side of the VEC, whereas an average of only 5.6% of the total were found on the nonmicroridge side of the VEC. The results from this study indicate that adhesion of C. albicans to the VEC surface was affected by the strain of yeast used, by the side of the vaginal cell exposed, and by the pH and CO2 levels present in the adhesion assay.

Nutritional studies on Paracoccidioides brasiliensis: the role of organic sulfur in dimorphism

The nutritional requirements of the mycelial and yeast-like phases of the dimorphic fungus Paracoccidioides brasiliensis, a human pathogen, were investigated. For all nine isolates tested, mycelial cells were prototrophic, whereas yeast-like cells required a sulfur-containing amino acid for growth. Moreover, changing the source of nitrogen greatly affected the morphology of the yeast-like cells.

The role of zinc in Candida dimorphism

By analyzing the effects of zinc on growth and dimorphism, it has become clear that there exists at least two modes, or "pathways," of mycelium formation in C. albicans (7). Paradoxically, even though the characteristics for the two modes appear to be opposite in nature, the mycelium that form appear to be superficially similar. Unfortunately, it may be difficult to compare the two modes unambiguously at the molecular level for two reasons. First, the physiology of cells resuming growth after release from stationary phase will undoubtedly differ drastically from the physiology of cells exiting from the growth cycle, regardless of phenotype. Therefore, most molecular or physiologic differences probably will represent differences in growth rate or position in the cell cycle, rather than alternate molecular mechanisms that are basic to the alternate modes of mycelium formation. Second, it has been observed that during release from stationary phase, a prescribed program of gene expression accompanies commitment to the mycelial and budding forms (11). This program was demonstrable because of the excellent synchrony and homogeneity of released cultures (60), which is a characteristic lacking in cultures entering stationary phase in the M10 mode. Even so, a comparison at the molecular level between the two modes of mycelium formation should be undertaken with the above reservations in mind. Perhaps the most attractive aspect of alternate modes of mycelium formation in Candida is at the genetic level of analysis. The hypothesis of homozygosis in the expression of the M10 phenotype is testable, as is the possible role of the M10 phenotype in tissue penetration. If the hypothesis is true and if the M10 phenotype predominates in infected tissue, it would represent a new mechanism of opportunism in infectious fungi that may be used by other systems as well as Candida. If it is not true, a detailed analysis of the differences between the two modes of mycelium formation will still be valuable in our understanding of both the mechanisms regulating phenotypic transitions in Candida and the more general question of cell divergence in developing systems.

Morphogenesis in Candida albicans

This review will survey environmental controls on the morphology of Candida albicans, describe the cellular and ultrastructural events associated with morphological transitions in this fungus, and attempt to relate biochemical phenomena that have been reported to be associated with dimorphic change to C. albicans cell biology. The synthesis of the cell wall of C. albicans and its control remain largely undiscovered, but it is clear that the cell wall is the principal component involved in shape determination. Possible models for C. albicans dimorphism will be critically reviewed.

Reorganization of membrane ergosterol during cell fission events of Candida albicans: a freeze-fracture study of distribution of filipin-ergosterol complexes

The redistribution of ergosterol molecules which occurs during bud and germ tube formation (dimorphism) in Candida albicans was studied using filipin, a sterol-specific antibiotic, and examined by the freeze-fracture technique. When cells were fixed in a glutaraldehyde solution containing 50 micrograms/ml of filipin, filipin-ergosterol complexes, which were recognized as either pits on the exoplasmic fracture face or protuberances on the protoplasmic fracture face, were homogeneously distributed on the yeast plasma membranes. The plasma membrane of young budding yeast cells demonstrated few filipin-ergosterol complexes compared to the parent yeast plasma membrane. In addition, at a certain time during enlargement of budding yeast cells, the complexes became virtually absent from the constricted region between daughter and parent yeast cell. On the other hand, when germ tubes emerged as cylindrical outgrowths from the parent yeast cells, filipin-ergosterol complexes were heterogeneously redistributed on the plasma membrane. These results suggest that ergosterol molecules may be in lower concentration in the plasma membrane at the constricted region of yeast cell than elsewhere on the plasmalemma of the yeast cell.

An effective medium for the selective growth of yeast or mycelial forms of Candida albicans: biochemical aspects of the two forms

A new synthetic medium, based on a modification of a commercially available tissue culture medium, allows Candida albicans to be grown in the yeast or mycelial form. Salient features of the system are described and comparisons with previous physiological investigations are discussed. A concise biochemical profile of these two forms of C. albicans is also presented. The results indicate vast metabolic differences between the two forms.

Effect of antibodies on the respiration and morphology of Candida Albicans

Respirometry was used to study the effect of antibodies to Candida albicans on the gowth of this species in vitro. Blastospore suspensions were pre-incubated with rabbit hyperimmune and normal serum and rabbit antibody and normal gamma globulin at 37 degrees C for 30 minutes. They were then inoculated into respirometer flasks containing a semi-synthetic medium and oxygen consumption was measured at 27 degrees C. Rabbit antiserum to C. albicans serotype A inhibited the growth, as measured by O2 consumption, of serotyped A and B strains of C. albicans. The amount of inhibition appeared to be related to the prevention or retardation of germ tube formation. The growth of blastospores incubated with normal rabbit serum was filamentous, whereas, blastospores incubated with antiserum remained in the yeast phase. Inhibition of O2 consumption and retardation of germ tube formation was also demonstrated with the purified gamma-globulin fraction of the antiserum.

Germination of Candida albicans induced by proline

Blastospores of Candida albicans germinated in proline-biotin-buffer medium incubated at 37 C. Certain other amino acids in the glatamate, asparate, and pyruvate families also fostered germinaton but generally to a lesser extent than did proline. L-Cysteine, D-proline, and certain structural analogues of L-proline inhibited proline-stimualted germination. The concentration of phosphate and glucose was crucial to amino acid-stimulated germination of C. albicans. Clinical isolates and stock cultures varied in their response to the germ tube-inducing activity of proline or other amino acids. The proline-buffer medium cannot be used in a diagnostic test for production of germ tubes by isolates of yeasts.

Factors affecting filamentation in Candida albicans: changes in respiratory activity of Candida albicans during filamentation

Glucose metabolism and respiration of Candida albicans were compared under conditions which permitted either maximal filamentous or maximal yeast growth. Changes in metabolism were monitored by comparing the quantities of ethanol produced, CO2 evolved, and oxygen consumed. Filamenting cultures produced more ethanol and less CO2 than yeasts, with oxygen consumption in the former concomitantly slower than that of the latter. Studies involving cofactors and inhibitors associated with electron transport imply that a transfer of electrons away from flavoprotein is required for maintenance of yeast morphology. Conditions consistent with a buildup of reduced flavoprotein, however, favored filament formation. These changes were expressed metabolically as a shift from an aerobic to a fermentative metabolism. The results presented are consistent with hypotheses correlating filament production with changes in carbohydrate metabolism and an interruption of electron transfer within the cell.

Factors affecting filamentation in Candida albicans: relationship of the uptake and distribution of proline to morphogenesis

When glucose was present in high concentration, Candida albicans formed filaments in a phosphate-buffered medium, regardless of the nitrogen source. In lower concentrations of glucose, filamentation occurred only when various members of the glutamate, succinyl, or acetoacetyl-coenzyme A families of amino acids were used as sole nitrogen sources. Yeast morphology could be maintained either by replacing the amino acids in the medium with ammonium chloride or by making the medium high in phosphate or biotin. Studies using [U-14C]proline indicated that proline was catabolized in a manner consistent with the generation of increased cellular reducing potential and that the proline label entered into the Kreb's cycle. A reduction in Kreb's cycle activity was evidenced by an initial increase and then a rapid drop of the total organic acid content of the cells as well as in specific Kreb's cycle intermediates. Filamentation under conditions of low phosphate, high glucose, and increased cellular reduction potential, accompanied by a decrease in Kreb's cycle activity, suggests that morphogenesis in C. albicans is correlated with a Crabtree-like effect, i.e., repression of mitochondrial activity.