Hyphal development in Candida albicans from different cell states

Candida albicans is an important opportunistic fungal pathogen of immunocompromised individuals. The ability to switch between yeast, pseudohyphal, and hyphal growth forms (polymorphism) is one of the most investigated virulence attributes of C. albicans. The usual method for inducing hypha formation in the lab is by diluting cells from a saturated culture into fresh medium at 37 °C. The molecular mechanism at action under these conditions has been previously investigated. C. albicans can also form hyphae in growing cells without dilution. The ability of C. albicans to form hyphae in different cell states facilitates the fungus to adapt varied host environments during infection. A recent study by Su et al. uncovered the molecular mechanism for how C. albicans develops hyphae under the condition without inoculation. N-Acetylglucosamine (GlcNAc) stimulates filamentation in log phase cells through transcriptional down-regulation of NRG1, the major repressor of hyphal development. Instead of cAMP-PKA pathway, GlcNAc sensor Ngs1 is responsible for this process. Ngs1 binds to GlcNAc to activate its N-acetyltransferase activity, leading to the induction of BRG1 expression. The increased level of BRG1 could repress NRG1 transcripts, resulting in hyphal growth. Hyphal development in log phase cells induced by serum or neutral pH also requires activation of BRG1 to down-regulate NRG1 transcription. Therefore, hyphal induction under the condition without inoculation is trigged by Brg1-mediated removal of Nrg1 inhibition. This review describes our current understanding of the molecular mechanism underlying hyphal development, the best studied virulence factor in C. albicans. These will expand the number of potential drug targets with novel modes of action for anti-virulence therapeutics.

Biphasic zinc compartmentalisation in a human fungal pathogen

Nutritional immunity describes the host-driven manipulation of essential micronutrients, including iron, zinc and manganese. To withstand nutritional immunity and proliferate within their hosts, pathogenic microbes must express efficient micronutrient uptake and homeostatic systems. Here we have elucidated the pathway of cellular zinc assimilation in the major human fungal pathogen Candida albicans. Bioinformatics analysis identified nine putative zinc transporters: four cytoplasmic-import Zip proteins (Zrt1, Zrt2, Zrt3 and orf19.5428) and five cytoplasmic-export ZnT proteins (orf19.1536/Zrc1, orf19.3874, orf19.3769, orf19.3132 and orf19.52). Only Zrt1 and Zrt2 are predicted to localise to the plasma membrane and here we demonstrate that Zrt2 is essential for C. albicans zinc uptake and growth at acidic pH. In contrast, ZRT1 expression was found to be highly pH-dependent and could support growth of the ZRT2-null strain at pH 7 and above. This regulatory paradigm is analogous to the distantly related pathogenic mould, Aspergillus fumigatus, suggesting that pH-adaptation of zinc transport may be conserved in fungi and we propose that environmental pH has shaped the evolution of zinc import systems in fungi. Deletion of C. albicans ZRT2 reduced kidney fungal burden in wild type, but not in mice lacking the zinc-chelating antimicrobial protein calprotectin. Inhibition of zrt2Δ growth by neutrophil extracellular traps was calprotectin-dependent. This suggests that, within the kidney, C. albicans growth is determined by pathogen-Zrt2 and host-calprotectin. As well as serving as an essential micronutrient, zinc can also be highly toxic and we show that C. albicans deals with this potential threat by rapidly compartmentalising zinc within vesicular stores called zincosomes. In order to understand mechanistically how this process occurs, we created deletion mutants of all five ZnT-type transporters in C. albicans. Here we show that, unlike in Saccharomyces cerevisiae, C. albicans Zrc1 mediates zinc tolerance via zincosomal zinc compartmentalisation. This novel transporter was also essential for virulence and liver colonisation in vivo. In summary, we show that zinc homeostasis in a major human fungal pathogen is a multi-stage process initiated by Zrt1/Zrt2-cellular import, followed by Zrc1-dependent intracellular compartmentalisation.

The Hyr1 protein from the fungus Candida albicans is a cross kingdom immunotherapeutic target for Acinetobacter bacterial infection

Different pathogens share similar medical settings and rely on similar virulence strategies to cause infections. We have previously applied 3-D computational modeling and bioinformatics to discover novel antigens that target more than one human pathogen. Active and passive immunization with the recombinant N-terminus of Candida albicans Hyr1 (rHyr1p-N) protect mice against lethal candidemia. Here we determine that Hyr1p shares homology with cell surface proteins of the multidrug resistant Gram negative bacterium, Acinetobacter baumannii including hemagglutinin (FhaB) and outer membrane protein A (OmpA). The A. baumannii OmpA binds to C. albicans Hyr1p, leading to a mixed species biofilm. Deletion of HYR1, or blocking of Hyr1p using polyclonal antibodies, significantly reduce A. baumannii binding to C. albicans hyphae. Furthermore, active vaccination with rHyr1p-N or passive immunization with polyclonal antibodies raised against specific peptide motifs of rHyr1p-N markedly improve survival of diabetic or neutropenic mice infected with A. baumannii bacteremia or pneumonia. Antibody raised against one particular peptide of the rHyr1p-N sequence (peptide 5) confers majority of the protection through blocking A. baumannii invasion of host cells and inducing death of the bacterium by a putative iron starvation mechanism. Anti-Hyr1 peptide 5 antibodies also mitigate A. baumannii /C. albicans mixed biofilm formation in vitro. Consistent with our bioinformatic analysis and structural modeling of Hyr1p, anti-Hyr1p peptide 5 antibodies bound to A. baumannii FhaB, OmpA, and an outer membrane siderophore binding protein. Our studies highlight the concept of cross-kingdom vaccine protection against high priority human pathogens such as A. baumannii and C. albicans that share similar ecological niches in immunocompromised patients.

Semicontinuous sophorolipid fermentation using a novel bioreactor with dual ventilation pipes and dual sieve-plates coupled with a novel separation system

Sophorolipids (SLs) are biosurfactants with widespread applications. The yield and purity of SLs are two important factors to be considered during their commercial large-scale production. Notably, SL accumulation causes an increase in viscosity, decrease in dissolved oxygen and product inhibition in the fermentation medium. This inhibits the further production and purification of SLs. This describes the development of a novel integrated system for SL production using Candida albicans O-13-1. Semicontinuous fermentation was performed using a novel bioreactor with dual ventilation pipes and dual sieve-plates (DVDSB). SLs were separated and recovered using a newly designed two-stage separation system. After SL recovery, the fermentation broth containing residual glucose and oleic acid was recycled back into the bioreactor. This novel approach considerably alleviated the problem of product inhibition and accelerated the rate of substrate utilization. Production of SLs achieved was 477 g l-1 , while their productivity was 1.59 g l-1  h-1 . Purity of SLs improved by 23.3%, from 60% to 74%, using DVDSB with the separation system. The conversion rate of carbon source increased from 0.5 g g-1 (in the batch fermentation) to 0.6 g g-1 . These results indicated that the integrated system could improve the efficiency of production and purity of SLs.

Secondary structure of cell-penetrating peptides during interaction with fungal cells

Cell-penetrating peptides (CPPs) are peptides that cross cell membranes, either alone or while carrying molecular cargo. Although their interactions with mammalian cells have been widely studied, much less is known about their interactions with fungal cells, particularly at the biophysical level. We analyzed the interactions of seven CPPs (penetratin, Pep-1, MPG, pVEC, TP-10, MAP, and cecropin B) with the fungal pathogen Candida albicans using experiments and molecular simulations. Circular dichroism (CD) of the peptides revealed a structural transition from a random coil or weak helix to an α-helix occurs for all peptides when the solvent is changed from aqueous to hydrophobic. However, CD performed in the presence of C. albicans cells showed that proximity to the cell membrane is not necessarily sufficient to induce this structural transition, as penetratin, Pep-1, and MPG did not display a structural shift in the presence of cells. Monte Carlo simulations were performed to further probe the molecular-level interaction with the cell membrane, and these simulations suggested that pVEC, TP-10, MAP, and cecropin B strongly penetrate into the hydrophobic domain of the membrane lipid bilayer, inducing a transition to an α-helical conformation. In contrast, penetratin, Pep-1 and MPG remained in the hydrophilic region without a shift in conformation. The experimental data and MC simulations combine to explain how peptide structure affects their interaction with cells and their mechanism of translocation into cells (direct translocation vs. endocytosis). Our work also highlights the utility of combining biophysical experiments, biological experiments, and molecular modeling to understand biological phenomena.

Evaluation of the Efficacy of Different Mixing Techniques and Disinfection on Microbial Colonization of Polyether Impression Materials: A Comparative Study

AIM

This study aims to determine the role of mixing techniques of polyether impression materials and efficacy of disinfection on microbial colonization of these impression materials.

MATERIALS AND METHODS

Polyether impression material was mixed using two methods: First by hand mixing (group I) and second using an automixer (group II) with a total of 100 samples. Four microbial strains were studied, which included Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. After incubation, the bacterial colonies were counted, and then, disinfectant solution was applied. The effect of disinfection solution was evaluated for each specimen.

RESULTS

The surface of polyether impression materials mixed with an automixer has less number of voids and overall a smoother surface as compared with the hand-mixed ones. On comparing the disinfection procedures, i.e., specimens without any disinfection and specimens after disinfection, statistically highly significant difference was seen between all the groups.

CONCLUSION

We can conclude that impression mixing procedures are important in determining the surface characteristics of the impression and ultimately the colonization of bacteria and also determine the importance of disinfection on microbial colonization.

CLINICAL SIGNIFICANCE

This study emphasises the deleterious role of nosocomial infections and specific measures that should be taken regarding the prevention of such diseases. Dental impressions are proved to be a source of such infections and may lead to transmission of such diseases. Thus, proper measures should be taken right from the first step of impression taking to minimizing and preventing such kind of contaminations in clinical practice.

New Application of Neomycin B-Bisbenzimidazole Hybrids as Antifungal Agents

Alkylated aminoglycosides and bisbenzimidazoles have previously been shown to individually display antifungal activity. Herein, we explore for the first time the antifungal activity (in liquid cultures and in biofilms) of ten alkylated aminoglycosides covalently linked to either mono- or bisbenzimidazoles. We also investigate their toxicity against mammalian cells, their hemolytic activity, and their potential mechanism(s) of action (inhibition of fungal ergosterol biosynthetic pathway and/or reactive oxygen species (ROS) production). Overall, many of our hybrids exhibited broad-spectrum antifungal activity. We also found them to be less cytotoxic to mammalian cells and less hemolytic than the FDA-approved antifungal agents amphotericin B and voriconazole, respectively. Finally, we show with our best derivative (8) that the mechanism of action of our compounds is not the inhibition of ergosterol biosynthesis, but that it involves ROS production in yeast cells.

Scolopendin 2 leads to cellular stress response in Candida albicans

Centipedes, a kind of arthropod, have been reported to produce antimicrobial peptides as part of an innate immune response. Scolopendin 2 (AGLQFPVGRIGRLLRK) is a novel antimicrobial peptide derived from the body of the centipede Scolopendra subspinipes mutilans by using RNA sequencing. To investigate the intracellular responses induced by scolopendin 2, reactive oxygen species (ROS) and glutathione accumulation and lipid peroxidation were monitored over sublethal and lethal doses. Intracellular ROS and antioxidant molecule levels were elevated and lipids were peroxidized at sublethal concentrations. Moreover, the Ca(2+) released from the endoplasmic reticulum accumulated in the cytosol and mitochondria. These stress responses were considered to be associated with yeast apoptosis. Candida albicans cells exposed to scolopendin 2 were identified using diagnostic markers of apoptotic response. Various responses such as phosphatidylserine externalization, chromatin condensation, and nuclear fragmentation were exhibited. Scolopendin 2 disrupted the mitochondrial membrane potential and activated metacaspase, which was mediated by cytochrome c release. In conclusion, treatment of C. albicans with scolopendin 2 induced the apoptotic response at sublethal doses, which in turn led to mitochondrial dysfunction, metacaspase activation, and cell death. The cationic antimicrobial peptide scolopendin 2 from the centipede is a potential antifungal peptide, triggering the apoptotic response.

Metabolomic profiling for the identification of potential biomarkers involved in a laboratory azole resistance in Candida albicans

Candida albicans, one of the most common fungal pathogens, is responsible for several yeast infections in human hosts, being resistant to classically used antifungal drugs, such as azole drugs. Multifactorial and multistep alterations are involved in the azole resistance in Candida albicans. In this study, a FCZ-resistant C. albicans strain was obtained by serial cultures of a FCZ-susceptible C. albicans strain in incrementally increasing concentrations of FCZ. We performed an integrated profile of different classes of molecules related to azole resistance in C. albicans by combining several mass-spectrometry based methodologies. The comparative metabolomic study was performed with the sensitive and resistant strains of C.albicans to identify metabolites altered during the development of resistance to fluconazole, while the intervention strains and non-intervention strains of C.albicans to identify metabolites altered involved in cross-resistant to azole drugs. Our analysis of the different metabolites identified molecules mainly involved in metabolic processes such as amino acid metabolism, tricarboxylic acid cycle and phospholipid metabolism. We also compared the phospholipid composition of each group, revealing that the relative content of phospholipids significantly changed during the development of resistance to azole drugs. According with these results, we hypothesized that the metabolism shift might contribute to azole drugs resistance in C.albicans from multifactorial alterations. Our result paves the way to understand processes underlying the resistance to azole drugs in C. albicans, providing the basis for developing new antifungal drugs.

Fluconazole-Resistant Candida auris Is Susceptible to Salivary Histatin 5 Killing and to Intrinsic Host Defenses

Candida auris is a newly identified species causing invasive candidemia and candidiasis. It has broad multidrug resistance (MDR) not observed for other pathogenic Candida species. Histatin 5 (Hst 5) is a well-studied salivary cationic peptide with significant antifungal activity against Candida albicans and is an attractive candidate for treating MDR fungi, since antimicrobial peptides induce minimal drug resistance. We investigated the susceptibility of C. auris to Hst 5 and neutrophils, two first-line innate defenses in the human host. The majority of C. auris clinical isolates, including fluconazole-resistant strains, were highly sensitive to Hst 5: 55 to 90% of cells were killed by use of 7.5 μM Hst 5. Hst 5 was translocated to the cytosol and vacuole in C. auris cells; such translocation is required for the killing of C. albicans by Hst 5. The inverse relationship between fluconazole resistance and Hst 5 killing suggests different cellular targets for Hst 5 than for fluconazole. C. auris showed higher tolerance to oxidative stress than C. albicans, and higher survival within neutrophils, which correlated with resistance to oxidative stress in vitro Thus, resistance to reactive oxygen species (ROS) is likely one, though not the only, important factor in the killing of C. auris by neutrophils. Hst 5 has broad and potent candidacidal activity, enabling it to combat MDR C. auris strains effectively.

Processing of Candida albicans Ece1p Is Critical for Candidalysin Maturation and Fungal Virulence

Candida albicans is an opportunistic fungal pathogen responsible for superficial and life-threatening infections in humans. During mucosal infection, C. albicans undergoes a morphological transition from yeast to invasive filamentous hyphae that secrete candidalysin, a 31-amino-acid peptide toxin required for virulence. Candidalysin damages epithelial cell plasma membranes and stimulates the activating protein 1 (AP-1) transcription factor c-Fos (via p38-mitogen-activated protein kinase [MAPK]), and the MAPK phosphatase MKP1 (via extracellular signal-regulated kinases 1 and 2 [ERK1/2]-MAPK), which trigger and regulate proinflammatory cytokine responses, respectively. The candidalysin toxin resides as a discrete cryptic sequence within a larger 271-amino-acid parental preproprotein, Ece1p. Here, we demonstrate that kexin-like proteinases, but not secreted aspartyl proteinases, initiate a two-step posttranslational processing of Ece1p to produce candidalysin. Kex2p-mediated proteolysis of Ece1p after Arg61 and Arg93, but not after other processing sites within Ece1p, is required to generate immature candidalysin from Ece1p, followed by Kex1p-mediated removal of a carboxyl arginine residue to generate mature candidalysin. C. albicans strains harboring mutations of Arg61 and/or Arg93 did not secrete candidalysin, were unable to induce epithelial damage and inflammatory responses in vitro, and showed attenuated virulence in vivo in a murine model of oropharyngeal candidiasis. These observations identify enzymatic processing of C. albicans Ece1p by kexin-like proteinases as crucial steps required for candidalysin production and fungal pathogenicity.IMPORTANCECandida albicans is an opportunistic fungal pathogen that causes mucosal infection in millions of individuals worldwide. Successful infection requires the secretion of candidalysin, the first cytolytic peptide toxin identified in any human fungal pathogen. Candidalysin is derived from its parent protein Ece1p. Here, we identify two key amino acids within Ece1p vital for processing and production of candidalysin. Mutations of these residues render C. albicans incapable of causing epithelial damage and markedly reduce mucosal infection in vivo Importantly, candidalysin production requires two individual enzymatic events. The first involves processing of Ece1p by Kex2p, yielding immature candidalysin, which is then further processed by Kex1p to produce the mature toxin. These observations identify important steps for C. albicans pathogenicity at mucosal surfaces.

Accessibility and contribution to glucan masking of natural and genetically tagged versions of yeast wall protein 1 of Candida albicans

Yeast wall protein 1 (Ywp1) is an abundant glycoprotein of the cell wall of the yeast form of Candida albicans, the most prevalent fungal pathogen of humans. Antibodies that bind to the polypeptide backbone of isolated Ywp1 show little binding to intact yeast cells, presumably because the Ywp1 epitopes are masked by the polysaccharides of the mannoproteins that form the outer layer of the cell wall. Rare cells do exhibit much greater anti-Ywp1 binding, however, and one of these was isolated and characterized. No differences were seen in its Ywp1, but it exhibited greater adhesiveness, sensitivity to wall perturbing agents, and exposure of its underlying β-1,3-glucan layer to external antibodies. The molecular basis for this greater epitope accessibility has not been determined, but has facilitated exploration of how these properties change as a function of cell growth and morphology. In addition, previously engineered strains with reduced quantities of Ywp1 in their cell walls were also found to have greater β-1,3-glucan exposure, indicating that Ywp1 itself contributes to the masking of wall epitopes, which may be important for understanding the anti-adhesive effect of Ywp1. Ectopic production of Ywp1 by hyphae, which reduces the adhesivity of these filamentous forms of C. albicans, was similarly found to reduce exposure of the β-1,3-glucan in their walls. To monitor Ywp1 in the cell wall irrespective of its accessibility, green fluorescent protein (Gfp) was genetically inserted into wall-anchored Ywp1 using a bifunctional cassette that also allowed production from a single transfection of a soluble, anchor-free version. The wall-anchored Ywp1-Gfp-Ywp1 accumulated in the wall of the yeast forms but not hyphae, and appeared to have properties similar to native Ywp1, including its adhesion-inhibiting effect. Some pseudohyphal walls also detectably accumulated this probe. Strains of C. albicans with tandem hemagglutinin (HA) epitopes inserted into wall-anchored Ywp1 were previously created by others, and were further explored here. As above, rare cells with much greater accessibility of the HA epitopes were isolated, and also found to exhibit greater exposure of Ywp1 and β-1,3-glucan. The placement of the HA cassette inhibited the normal N-glycosylation and propeptide cleavage of Ywp1, but the wall-anchored Ywp1-HA-Ywp1 still accumulated in the cell wall of yeast forms. Bifunctional transformation cassettes were used to additionally tag these molecules with Gfp, generating soluble Ywp1-HA-Gfp and wall-anchored Ywp1-HA-Gfp-Ywp1 molecules. The former revealed unexpected electrophoretic properties caused by the HA insertion, while the latter further highlighted differences between the presence of a tagged Ywp1 molecule (as revealed by Gfp fluorescence) and its accessibility in the cell wall to externally applied antibodies specific for HA, Gfp and Ywp1, with accessibility being greatest in the rapidly expanding walls of budding daughter cells. These strains and results increase our understanding of cell wall properties and how C. albicans masks itself from recognition by the human immune system.

Candida albicans-derived mannoproteins activate NF-κB in reporter cells expressing TLR4, MD2 and CD14

The ability of soluble C. albicans 20A (serotype A) mannoprotein (CMP) to serve as a ligand for toll-like receptor 4 (TLR4) and its co-receptors was examined using commercially available and stably-transfected HEK293 cells that express human TLR4, MD2 and CD14, but not MR. These TLR4 reporter cells also express an NF-κB-dependent, secreted embryonic alkaline phosphatase (SEAP) reporter gene. TLR4-reporter cells exhibited a dose-dependent SEAP response to both LPS and CMP, wherein peak activation was achieved after stimulation with 40–50 μg/mL of CMP. Incubation on polymyxin B resin had no effect on CMP’s ligand activity, but neutralized LPS-spiked controls. HEK293 Null cells lacking TLR4 and possessing the same SEAP reporter failed to respond to LPS or CMP, but produced SEAP when activated with TNFα. Reporter cell NF-κB responses were accompanied by transcription of IL-8, TNFα, and COX-2 genes. Celecoxib inhibited LPS-, CMP-, and TNFα-dependent NF-κB responses; whereas, indomethacin had limited effect on LPS and CMP responses. SEAP production in response to C. albicans A9 mnn4Δ mutant CMP, lacking phosphomannosylations on N-linked glycans, was significantly greater (p ≤ 0.005) than SEAP responses to CMP derived from parental A9 (both serotype B). These data confirm that engineered human cells expressing TLR4, MD2 and CD14 can respond to CMP with NF-κB activation and the response can be influenced by variations in CMP-mannosylation. Future characterizations of CMPs from other sources and their application in this model may provide further insight into variations observed with TLR4 dependent innate immune responses targeting different C. albicans strains.

Pan-Domain Analysis of ZIP Zinc Transporters

The ZIP (Zrt/Irt-like protein) family of zinc transporters is found in all three domains of life. However, little is known about the phylogenetic relationship amongst ZIP transporters, their distribution, or their origin. Here we employed phylogenetic analysis to explore the evolution of ZIP transporters, with a focus on the major human fungal pathogen, Candida albicans. Pan-domain analysis of bacterial, archaeal, fungal, and human proteins revealed a complex relationship amongst the ZIP family members. Here we report (i) a eukaryote-wide group of cellular zinc importers, (ii) a fungal-specific group of zinc importers having genetic association with the fungal zincophore, and, (iii) a pan-kingdom supercluster made up of two distinct subgroups with orthologues in bacterial, archaeal, and eukaryotic phyla.

PBIT: Pipeline Builder for Identification of drug Targets for infectious diseases

Summary

PBIT (Pipeline Builder for Identification of drug Targets) is an online webserver that has been developed for screening of microbial proteomes for critical features of human drug targets such as being non-homologous to human proteome as well as the human gut microbiota, essential for the pathogen's survival, participation in pathogen-specific pathways etc. The tool has been validated by analyzing 57 putative targets of Candida albicans documented in literature. PBIT integrates various in silico approaches known for drug target identification and will facilitate high-throughput prediction of drug targets for infectious diseases, including multi-pathogenic infections.

Availability and Implementation

PBIT is freely accessible at http://www.pbit.bicnirrh.res.in/ .

Contact

thomass@nirrh.res.in.

Supplementary information

Supplementary data are available at Bioinformatics online.

Synergistic in vitro activity of sodium houttuyfonate with fluconazole against clinical Candida albicans strains under planktonic growing conditions

CONTEXT

Fluconazole resistance is an intractable problem of treating Candida albicans, calling for more antifungal agents to enhance the activity of fluconazole.

OBJECTIVE

This work investigates the anti-C. albicans activities of sodium houttuyfonate (SH) and/or fluconazole and the associated mechanism.

MATERIALS AND METHODS

The minimum inhibitory concentrations (MICs) of SH and fluconazole both ranging from 0.5 to 1024 μg/mL were determined by broth microdilution method in 19 C. albicans isolates, and their fractional inhibitory concentration index (FICI) was evaluated by checkerboard assay. After MIC_SH and/or MIC_fluconazole treatments, the expressions of IFD6, PHR1, ZAP1, ADH5, BGL2, XOG1 and FKS1 were analyzed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) in C. albicans 1601.

RESULTS AND CONCLUSION

The MICs of SH alone ranged from 32 to 256 μg/mL and decreased 2-16-fold in combination. SH showed strong synergism with fluconazole with FICI <0.13-0.5. In C. albicans 1601, we observed that (i) the expression of the seven genes increased notably in a range between 3.71- and 12.63-fold (p < 0.05) when SH was used alone, (ii) the combined use of SH and fluconazole slightly inhibited the expression of IFD6 and PHR1 by 1.23- and 1.35-fold (p > 0.05), but promoted evidently the expression of ZAP1, ADH5, XOG1 and FKS1 by 1.98-, 3.56-, 4.10- and 2.86-fold (p < 0.05). The results suggested SH to be a potential synergist to enhance the antifungal activity of fluconazole in C. albicans resistant isolates, and the underlying mechanism may be associated with β-1,3-glucan synthesis and transportation.

Candida albicans FRE8 encodes a member of the NADPH oxidase family that produces a burst of ROS during fungal morphogenesis

Until recently, NADPH oxidase (NOX) enzymes were thought to be a property of multicellularity, where the reactive oxygen species (ROS) produced by NOX acts in signaling processes or in attacking invading microbes through oxidative damage. We demonstrate here that the unicellular yeast and opportunistic fungal pathogen Candida albicans is capable of a ROS burst using a member of the NOX enzyme family, which we identify as Fre8. C. albicans can exist in either a unicellular yeast-like budding form or as filamentous multicellular hyphae or pseudohyphae, and the ROS burst of Fre8 begins as cells transition to the hyphal state. Fre8 is induced during hyphal morphogenesis and specifically produces ROS at the growing tip of the polarized cell. The superoxide dismutase Sod5 is co-induced with Fre8 and our findings are consistent with a model in which extracellular Sod5 acts as partner for Fre8, converting Fre8-derived superoxide to the diffusible H₂O₂ molecule. Mutants of fre8Δ/Δ exhibit a morphogenesis defect in vitro and are specifically impaired in development or maintenance of elongated hyphae, a defect that is rescued by exogenous sources of H₂O₂. A fre8Δ/Δ deficiency in hyphal development was similarly observed in vivo during C. albicans invasion of the kidney in a mouse model for disseminated candidiasis. Moreover C. albicans fre8Δ/Δ mutants showed defects in a rat catheter model for biofilms. Together these studies demonstrate that like multicellular organisms, C. albicans expresses NOX to produce ROS and this ROS helps drive fungal morphogenesis in the animal host.

We demonstrate here that the opportunistic human fungal pathogen Candida albicans uses a NADPH oxidase enzyme (NOX) and reactive oxygen species (ROS) to control morphogenesis in an animal host. C. albicans was not previously known to express NOX enzymes as these were thought to be a property of multicellular organisms, not unicellular yeasts. We describe here the identification of C. albicans Fre8 as the first NOX enzyme that can produce extracellular ROS in a unicellular yeast. C. albicans can exist as either a unicellular yeast or as multicellular elongated hyphae, and Fre8 is specially expressed during transition to the hyphal state where it works to produce ROS at the growing tip of the polarized cell. C. albicans cells lacking Fre8 exhibit a deficiency in elongated hyphae during fungal invasion of the kidney in a mouse model for systemic candidiasis. Moreover, Fre8 is required for fungal survival in a rodent model for catheter biofilms. These findings implicate a role for fungal derived ROS in controlling morphogenesis of this important fungal pathogen for public health.

Light-driven photosensitizer uptake increases Candida albicans photodynamic inactivation

Photodynamic Inactivation (PDI) is based on the use of a photosensitizer (PS) and light that results mainly in the production of reactive oxygen species, aiming to produce microorganism cell death. PS incubation time and light dose are key protocol parameters that influence PDI response; the correct choice of them can increase the efficiency of inactivation. The results of this study show that a minor change in the PDI protocol, namely light-driven incubation leads to a higher photosensitizer and more uniform cell uptake inside the irradiated zone. Furthermore, as the uptake increases, the damage caused by PDI also increases. The proposed light-driven incubation prior to the inactivation illumination dose has advantages when compared to the traditional PDI treatments since it can be more selective and effective. Using a violet light as pre-illumination (light-driven incubation) source and a red-light system as PDI source, it was possible to demonstrate that when compared to the traditional protocol of dark incubation, the pre-illuminated cell culture showed an inactivation increase of 7 log units. These in vitro results performed in Candida albicans cells may result in the introduction of a new protocol for PDI.

Stress-induced nuclear accumulation is dispensable for Hog1-dependent gene expression and virulence in a fungal pathogen

Stress-activated protein kinase (SAPK) pathways are evolutionarily conserved eukaryotic signalling modules that are essential for the virulence of human pathogenic fungi. The Hog1 SAPK in Candida albicans is robustly phosphorylated in response to a number of host-imposed stresses, and is essential for virulence. The current dogma is that stress-induced phosphorylation activates the SAPK, and promotes its nuclear accumulation that is necessary for the expression of SAPK-dependent stress-protective genes. Here we challenge this dogma. C. albicans strains were constructed in which Hog1 was either tethered to the plasma membrane or constitutively nuclear. Strikingly, tethering Hog1 to the plasma membrane did not abrogate stress resistance or stress-induced gene expression. Furthermore, preventing the nuclear accumulation of Hog1 had no impact on C. albicans virulence in two distinct models of systemic infection. However, tethering Hog1 to the plasma membrane did impact on signal fidelity, and on the magnitude and kinetics of the stress-induced phosphorylation of this SAPK. Taken together, these findings challenge the dogma that nuclear accumulation of SAPKs is a pre-requisite for SAPK-dependent gene expression, and reveal that stress-induced nuclear accumulation of Hog1 is dispensable for the virulence of a major human fungal pathogen.

Antifungal Effect of Arabidopsis SGT1 Proteins via Mitochondrial Reactive Oxygen Species

The highly conserved SGT1 (suppressor of the G2 alleles of skp1) proteins from Arabidopsis are known to contribute to plant resistance to pathogens. While SGT1 proteins respond to fungal pathogens, their antifungal activity is not reported and the mechanism for this inhibition is not well understood. Therefore, recombinant Arabidopsis SGT1 proteins were cloned, expressed, and purified to evaluate their antifungal activity, resulting in their potent inhibition of pathogen growth. Dye-labeled proteins are localized to the cytosol of Candida albicans cells without the disruption of the cell membrane. Moreover, we showed that entry of the proteins into C. albicans cells resulted in the accumulation of reactive oxygen species (ROS) and cell death via altered mitochondrial potential. Morphological changes of C. albicans cells in the presence of proteins were visualized by scanning electron microscopy. Our data suggest that AtSGT1 proteins play a critical role in plant resistance to pathogenic fungal infection and they can be classified to a new plant antifungal protein.

An acquired mechanism of antifungal drug resistance simultaneously enables Candida albicans to escape from intrinsic host defenses

The opportunistic fungal pathogen Candida albicans frequently produces genetically altered variants to adapt to environmental changes and new host niches in the course of its life-long association with the human host. Gain-of-function mutations in zinc cluster transcription factors, which result in the constitutive upregulation of their target genes, are a common cause of acquired resistance to the widely used antifungal drug fluconazole, especially during long-term therapy of oropharyngeal candidiasis. In this study, we investigated if C. albicans also can develop resistance to the antimicrobial peptide histatin 5, which is secreted in the saliva of humans to protect the oral mucosa from pathogenic microbes. As histatin 5 has been shown to be transported out of C. albicans cells by the Flu1 efflux pump, we screened a library of C. albicans strains that contain artificially activated forms of all zinc cluster transcription factors of this fungus for increased FLU1 expression. We found that a hyperactive Mrr1, which confers fluconazole resistance by upregulating the multidrug efflux pump MDR1 and other genes, also causes FLU1 overexpression. Similarly to the artificially activated Mrr1, naturally occurring gain-of-function mutations in this transcription factor also caused FLU1 upregulation and increased histatin 5 resistance. Surprisingly, however, Mrr1-mediated histatin 5 resistance was mainly caused by the upregulation of MDR1 instead of FLU1, revealing a previously unrecognized function of the Mdr1 efflux pump. Fluconazole-resistant clinical C. albicans isolates with different Mrr1 gain-of-function mutations were less efficiently killed by histatin 5, and this phenotype was reverted when MRR1 was deleted. Therefore, antimycotic therapy can promote the evolution of strains that, as a consequence of drug resistance mutations, simultaneously have acquired increased resistance against an innate host defense mechanism and are thereby better adapted to certain host niches.

The yeast Candida albicans is part of the normal microflora of most healthy persons, but it can also cause symptomatic infections when host defenses are compromised. C. albicans frequently generates genetically altered variants that are better adapted to changes in its environment during colonization and infection. We investigated if C. albicans can evolve resistance to histatin 5 (Hst 5), an antimicrobial peptide that is produced in the saliva of humans and protects the oral cavity against this pathogen. We found that activated forms of the transcription factor Mrr1 reduce the susceptibility of C. albicans to killing by Hst 5, a phenotype that was partially caused by Mrr1-mediated overexpression of the multidrug efflux pump MDR1. Gain-of-function (GOF) mutations in Mrr1 are a frequent cause of resistance to the antifungal drug fluconazole, especially during long-term treatment of oropharyngeal candidiasis in AIDS patients, but they may also reduce the fitness of the fungus in the absence of the drug. Fluconazole-resistant clinical C. albicans isolates containing GOF mutations in Mrr1 displayed enhanced Hst 5 resistance, demonstrating that antimycotic therapy can promote the evolution of strains that simultaneously have acquired increased resistance against an innate host defense mechanism and are thereby better adapted to specific host niches.

Biomimetically engineered Amphotericin B nano-aggregates circumvent toxicity constraints and treat systemic fungal infection in experimental animals

Biomimetic synthesis of nanoparticles offers a convenient and bio friendly approach to fabricate complex structures with sub-nanometer precision from simple precursor components. In the present study, we have synthesized nanoparticles of Amphotericin B (AmB), a potent antifungal agent, using Aloe vera leaf extract. The synthesis of AmB nano-assemblies (AmB-NAs) was established employing spectro-photometric and electron microscopic studies, while their crystalline nature was established by X-ray diffraction. AmB-nano-formulation showed much higher stability in both phosphate buffer saline and serum and exhibit sustained release of parent drug over an extended time period. The as-synthesized AmB-NA possessed significantly less haemolysis as well as nephrotoxicity in the host at par with Ambisome®, a liposomized AmB formulation. Interestingly, the AmB-NAs were more effective in killing various fungal pathogens including Candida spp. and evoked less drug related toxic manifestations in the host as compared to free form of the drug. The data of the present study suggest that biomimetically synthesized AmB-NA circumvent toxicity issues and offer a promising approach to eliminate systemic fungal infections in Balb/C mice.

The Candida albicans transcription factor Cas5 couples stress responses, drug resistance and cell cycle regulation

The capacity to coordinate environmental sensing with initiation of cellular responses underpins microbial survival and is crucial for virulence and stress responses in microbial pathogens. Here we define circuitry that enables the fungal pathogen Candida albicans to couple cell cycle dynamics with responses to cell wall stress induced by echinocandins, a front-line class of antifungal drugs. We discover that the C. albicans transcription factor Cas5 is crucial for proper cell cycle dynamics and responses to echinocandins, which inhibit β-1,3-glucan synthesis. Cas5 has distinct transcriptional targets under basal and stress conditions, is activated by the phosphatase Glc7, and can regulate the expression of target genes in concert with the transcriptional regulators Swi4 and Swi6. Thus, we illuminate a mechanism of transcriptional control that couples cell wall integrity with cell cycle regulation, and uncover circuitry governing antifungal drug resistance.Cas5 is a transcriptional regulator of responses to cell wall stress in the fungal pathogen Candida albicans. Here, Xie et al. show that Cas5 also modulates cell cycle dynamics and responses to antifungal drugs.

Negative regulation of filamentous growth in Candida albicans by Dig1p

Transcriptional regulation involves both positive and negative regulatory elements. The Dig1 negative regulators are part of a fungal-specific module that includes a transcription factor (a Ste12 family member) and a Dig1 family member. In Saccharomyces cerevisiae, the post-genome-duplication Dig1/Dig2 proteins regulate MAP kinase controlled signalling pathways involved in mating and filamentous growth. We have identified the single Dig1 orthologue in the fungal pathogen Candida albicans. Genetic studies and transcriptional profiling experiments show that this single protein is implicated in the regulation of MAP kinase-controlled processes involved in mating, filamentous growth and biofilm formation, and also influences cAMP-regulated processes. This suggests that the multiple cellular roles of the Dig1 protein are ancestral and predate the sub-functionalization apparent in S. cerevisiae after the genome duplication. Intriguingly, even though loss of Dig1 function in C. albicans enhances filamentous growth and biofilm formation, colonization of the murine gastrointestinal tract is reduced in the mutant. The complexity of the processes influenced by Dig1 in C. albicans, and the observation that Dig1 is one of the few regulatory proteins that were retained in the duplicated state after the whole genome duplication event in yeast, emphasizes the important role of these negative regulators in fungal transcriptional control.

Honokiol induces superoxide production by targeting mitochondrial respiratory chain complex I in Candida albicans

Background

Honokiol, a compound extracted from Magnolia officinalis, has antifungal activities by inducing mitochondrial dysfunction and triggering apoptosis in Candida albicans. However, the mechanism of honokiol-induced oxidative stress is poorly understood. The present investigation was designed to determine the specific mitochondrial reactive oxygen species (ROS)-generation component.

Methods/results

We found that honokiol induced mitochondrial ROS accumulation, mainly superoxide anions (O₂^•−) measured by fluorescent staining method. The mitochondrial respiratory chain complex I (C I) inhibitor rotenone completely blocked O₂^•− production and provided the protection from the killing action of honokiol. Moreover, respiratory activity and the C I enzyme activity was significantly reduced after honokiol treatment. The differential gene-expression profile also showed that genes involved in oxidoreductase activity, electron transport, and oxidative phosphorylation were upregulated.

Conclusions

The present work shows that honokiol may bind to mitochondrial respiratory chain C I, leading to mitochondrial dysfunction, accompanied by increased cellular superoxide anion and oxidative stress.

General significance

This work not only provides insights on the mechanism by which honokiol interferes with fungal cell, demonstrating previously unknown effects on mitochondrial physiology, but also raises a note of caution on the use of M. officinalis as a Chinese medicine due to the toxic for mitochondria and suggests the possibility of using honokiol as chemosensitizer.

Sfp1 and Rtg3 reciprocally modulate carbon source-conditional stress adaptation in the pathogenic yeast Candida albicans

The pathogenicity of the clinically important yeast, Candida albicans, is dependent on robust responses to host-imposed stresses. These stress responses have generally been dissected in vitro at 30°C on artificial growth media that do not mimic host niches. Yet host inputs, such as changes in carbon source or temperature, are known to affect C. albicans stress adaptation. Therefore, we performed screens to identify novel regulators that promote stress resistance during growth on a physiologically relevant carboxylic acid and at elevated temperatures. These screens revealed that, under these 'non-standard' growth conditions, numerous uncharacterised regulators are required for stress resistance in addition to the classical Hog1, Cap1 and Cta4 stress pathways. In particular, two transcription factors (Sfp1 and Rtg3) promote stress resistance in a reciprocal, carbon source-conditional manner. SFP1 is induced in stressed glucose-grown cells, whereas RTG3 is upregulated in stressed lactate-grown cells. Rtg3 and Sfp1 regulate the expression of key stress genes such as CTA4, CAP1 and HOG1 in a carbon source-dependent manner. These mechanisms underlie the stress sensitivity of C. albicans sfp1 cells during growth on glucose, and rtg3 cells on lactate. The data suggest that C. albicans exploits environmentally contingent regulatory mechanisms to retain stress resistance during host colonisation.

N-glycan mediated adhesion strengthening during pathogen-receptor binding revealed by cell-cell force spectroscopy

Glycan-protein lateral interactions have gained increased attention as important modulators of receptor function, by regulating surface residence time and endocytosis of membrane glycoproteins. The pathogen-recognition receptor DC-SIGN is highly expressed at the membrane of antigen-presenting dendritic cells, where it is organized in nanoclusters and binds to different viruses, bacteria and fungi. We recently demonstrated that DC-SIGN N-glycans spatially restrict receptor diffusion within the plasma membrane, favoring its internalization through clathrin-coated pits. Here, we investigated the involvement of the N-glycans of DC-SIGN expressing cells on pathogen binding strengthening when interacting with Candida fungal cells by using atomic force microscope (AFM)-assisted single cell-pathogen adhesion measurements. The use of DC-SIGN mutants lacking the N-glycans as well as blocking glycan-mediated lateral interactions strongly impaired cell stiffening during pathogen binding. Our findings demonstrate for the first time the direct involvement of the cell membrane glycans in strengthening cell-pathogen interactions. This study, therefore, puts forward a possible role for the glycocalyx as extracellular cytoskeleton contributing, possibly in connection with the intracellular actin cytoskeleton, to optimize strengthening of cell-pathogen interactions in the presence of mechanical forces.

Potent in vitro and in vivo antifungal activity of a small molecule host defense peptide mimic through a membrane-active mechanism

Lethal systemic fungal infections of Candida species are increasingly common, especially in immune compromised patients. By in vitro screening of small molecule mimics of naturally occurring host defense peptides (HDP), we have identified several active antifungal molecules, which also exhibited potent activity in two mouse models of oral candidiasis. Here we show that one such compound, C4, exhibits a mechanism of action that is similar to the parent HDP upon which it was designed. Specifically, its initial interaction with the anionic microbial membrane is electrostatic, as its fungicidal activity is inhibited by cations. We observed rapid membrane permeabilization to propidium iodide and ATP efflux in response to C4. Unlike the antifungal peptide histatin 5, it did not require energy-dependent transport across the membrane. Rapid membrane disruption was observed by both fluorescence and electron microscopy. The compound was highly active in vitro against numerous fluconazole-resistant clinical isolates of C. albicans and non-albicans species, and it exhibited potent, dose-dependent activity in a mouse model of invasive candidiasis, reducing kidney burden by three logs after 24 hours, and preventing mortality for up to 17 days. Together the results support the development of this class of antifungal drug to treat invasive candidiasis.

Candida albicans Impairments Induced by Peppermint and Clove Oils at Sub-Inhibitory Concentrations

Members of Candida species cause significant health problems, inducing various types of superficial and deep-seated mycoses in humans. In order to prevent from Candida sp. development, essential oils are more and more frequently applied, due to their antifungal activity, low toxicity if used appropriately, and biodegrability. The aim of the study was to characterize the early alterations in Candida albicans metabolic properties in relation to proteins and chromosomal DNA profiles, after treatment with peppermint and clove oils at sub-inhibitory concentrations. The yeasts were affected by the oils even at a concentration of 0.0075% v/v, which resulted in changes in colony morphotypes and metabolic activities. Peppermint and clove oils at concentrations ranging from 0.015× MIC (minimal inhibitory concentration) to 0.5× MIC values substantially affected the enzymatic abilities of C. albicans, and these changes were primarily associated with the loss or decrease of activity of all 9 enzymes detected in the untreated yeast. Moreover, 29% isolates showed additional activity of N-acetyl-β-glucosaminidase and 14% isolates—α-fucosidase in comparison to the yeast grown without essential oils addition. In response to essential oils at 0.25–0.5× MIC, extensive changes in C. albicans whole-cell protein profiles were noted. However, the yeast biochemical profiles were intact with the sole exception of the isolate treated with clove oil at 0.5× MIC. The alterations were not attributed to gross chromosomal rearrangements in C. albicans karyotype. The predominantly observed decrease in protein fractions and the yeast enzymatic activity after treatment with the oils should be considered as a phenotypic response of C. albicans to the essential oils at their sub-inhibitory concentrations and may lead to the reduction of this yeast pathogenicity.

Candida albicans mannans mediate Streptococcus mutans exoenzyme GtfB binding to modulate cross-kingdom biofilm development in vivo

Candida albicans is frequently detected with heavy infection by Streptococcus mutans in plaque-biofilms from children with early-childhood caries (ECC). This cross-kingdom biofilm contains an extensive matrix of extracellular α-glucans that is produced by an exoenzyme (GtfB) secreted by S. mutans. Here, we report that mannans located on the outer surface of C. albicans cell-wall mediates GtfB binding, enhancing glucan-matrix production and modulating bacterial-fungal association within biofilms formed in vivo. Using single-molecule atomic force microscopy, we determined that GtfB binds with remarkable affinity to mannans and to the C. albicans surface, forming a highly stable and strong bond (1–2 nN). However, GtfB binding properties to C. albicans was compromised in strains defective in O-mannan (pmt4ΔΔ) or N-mannan outer chain (och1ΔΔ). In particular, the binding strength of GtfB on och1ΔΔ strain was severely disrupted (>3-fold reduction vs. parental strain). In turn, the GtfB amount on the fungal surface was significantly reduced, and the ability of C. albicans mutant strains to develop mixed-species biofilms with S. mutans was impaired. This phenotype was independent of hyphae or established fungal-biofilm regulators (EFG1, BCR1). Notably, the mechanical stability of the defective biofilms was weakened, resulting in near complete biomass removal by shear forces. In addition, these in vitro findings were confirmed in vivo using a rodent biofilm model. Specifically, we observed that C. albicans och1ΔΔ was unable to form cross-kingdom biofilms on the tooth surface of rats co-infected with S. mutans. Likewise, co-infection with S. mutans defective in GtfB was also incapable of forming mixed-species biofilms. Taken together, the data support a mechanism whereby S. mutans-secreted GtfB binds to the mannan layer of C. albicans to promote extracellular matrix formation and their co-existence within biofilms. Enhanced understanding of GtfB-Candida interactions may provide new perspectives for devising effective therapies to disrupt this cross-kingdom relationship associated with an important childhood oral disease.

Early childhood caries is a severe form of tooth decay that affects toddlers and is clinically distinct from other forms of the disease. It is characterized by the frequent isolation of the commensal yeast Candida albicans and the bacterial pathogen Streptococcus mutans within caries-associated biofilms. This mixed-kingdom biofilm contains an extensive matrix of glue-like polymer of glucan that is generated by an enzyme (GtfB) secreted by S. mutans. The production of the glucan component of the biofilm matrix is promoted by the presence of C. albicans, creating a sticky accumulation of pathogenic biofilms. Here, we report that GtfB binds to mannoproteins on the outer surface of the C. albicans cell wall with remarkable strength. C. albicans strains with defects in mannoprotein biosynthesis have reduced levels of extracellular glucan polymer in the presence of S. mutans, and are unable to co-exist within biofilms. Accordingly, C. albicans strains lacking mannoproteins do not support cross-kingdom biofilm formation in an animal model of the disease. Detailed molecular studies of cross-kingdom biofilms can provide important information that may be useful in the development of new approaches for the prevention and treatment of this prevalent and costly oral disease of childhood.

Solasodine-3-O-β-d-glucopyranoside kills Candida albicans by disrupting the intracellular vacuole

The increasing incidence of fungal infections and emergence of drug resistance underlie the constant search for new antifungal agents and exploration of their modes of action. The present study aimed to investigate the antifungal mechanisms of solasodine-3-O-β-d-glucopyranoside (SG) isolated from the medicinal plant Solanum nigrum L. In vitro, SG displayed potent fungicidal activity against both azole-sensitive and azole-resistant Candida albicans strains in Spider medium with its MICs of 32 μg/ml. Analysis of structure and bioactivity revealed that both the glucosyl residue and NH group were required for SG activity. Quantum dot (QD) assays demonstrated that the glucosyl moiety was critical for SG uptake into Candida cells, as further confirmed by glucose rescue experiments. Measurement of the fluorescence intensity of 2',7'-dichlorofluorescin diacetate (DCFHDA) by flow cytometry indicated that SG even at 64 μg/ml just caused a moderate increase of reactive oxygen species (ROS) generation by 58% in C. albicans cells. Observation of vacuole staining by confocal microscopy demonstrated that SG alkalized the intracellular vacuole of C. albicans and caused hyper-permeability of the vacuole membrane, resulting in cell death. These results support the potential application of SG in fighting fungal infections and reveal a novel fungicidal mechanism.

Elevated catalase expression in a fungal pathogen is a double-edged sword of iron

Most fungal pathogens of humans display robust protective oxidative stress responses that contribute to their pathogenicity. The induction of enzymes that detoxify reactive oxygen species (ROS) is an essential component of these responses. We showed previously that ectopic expression of the heme-containing catalase enzyme in Candida albicans enhances resistance to oxidative stress, combinatorial oxidative plus cationic stress, and phagocytic killing. Clearly ectopic catalase expression confers fitness advantages in the presence of stress, and therefore in this study we tested whether it enhances fitness in the absence of stress. We addressed this using a set of congenic barcoded C. albicans strains that include doxycycline-conditional tetON-CAT1 expressors. We show that high basal catalase levels, rather than CAT1 induction following stress imposition, reduce ROS accumulation and cell death, thereby promoting resistance to acute peroxide or combinatorial stress. This conclusion is reinforced by our analyses of phenotypically diverse clinical isolates and the impact of stochastic variation in catalase expression upon stress resistance in genetically homogeneous C. albicans populations. Accordingly, cat1Δ cells are more sensitive to neutrophil killing. However, we find that catalase inactivation does not attenuate C. albicans virulence in mouse or invertebrate models of systemic candidiasis. Furthermore, our direct comparisons of fitness in vitro using isogenic barcoded CAT1, cat1Δ and tetON-CAT1 strains show that, while ectopic catalase expression confers a fitness advantage during peroxide stress, it confers a fitness defect in the absence of stress. This fitness defect is suppressed by iron supplementation. Also high basal catalase levels induce key iron assimilatory functions (CFL5, FET3, FRP1, FTR1). We conclude that while high basal catalase levels enhance peroxide stress resistance, they place pressure on iron homeostasis through an elevated cellular demand for iron, thereby reducing the fitness of C. albicans in iron-limiting tissues within the host.

The pathogenic yeast Candida albicans faces multiple challenges within its human host. These include the need to protect itself against the toxic oxidants used by the host to kill invading microbes, and the need to scavenge iron, an essential micronutrient that is limiting in certain tissues. The iron-containing enzyme, catalase, detoxifies hydrogen peroxide, thereby playing a major role in protecting C. albicans against reactive oxygen species and neutrophil killing. Indeed, we show that high basal catalase expression increases the resistance of this yeast to oxidative and combinatorial (oxidative plus cationic) stresses. Yet, rather than enhancing the virulence of C. albicans as had been predicted, high basal catalase expression decreases fungal colonisation in certain iron-limiting tissues. Furthermore, we demonstrate that catalase inactivation does not significantly perturb the virulence of C. albicans in models of systemic infection. We also show that ectopic catalase expression increases the demand for iron in C. albicans, thereby reducing the fitness of this pathogen in the absence of stress under iron-limiting conditions. Therefore, high basal catalase expression is a double-edged sword: it enhances the fitness of C. albicans in the presence of stress, but reduces fitness in the absence of stress. This explains why catalase overexpression reduces rather than enhances virulence.

The antibacterial agent, moxifloxacin inhibits virulence factors of Candida albicans through multitargeting

Fluoroquinolines are broad spectrum fourth generation antibiotics. Some of the Fluoroquinolines exhibit antifungal activity. We are reporting the potential mechanism of action of a fluoroquinoline antibiotic, moxifloxacin on the growth, morphogenesis and biofilm formation of the human pathogen Candida albicans. Moxifloxacin was found to be Candidacidal in nature. Moxifloxacin seems to inhibit the yeast to Hyphal morphogenesis by affecting signaling pathways. It arrested the cell cycle of C. albicans at S phase. Docking of moxifloxacin with predicted structure of C. albicans DNA Topoisomerase II suggests that moxifloxacin may bind and inhibit the activity of DNA Topoisomerase II in C. albicans. Moxifloxacin could be used as a dual purpose antibiotic for treating mixed infections caused by bacteria as well as C. albicans. In addition chances of developing moxifloxacin resistance in C. albicans are less considering the fact that moxifloxacin may target multiple steps in yeast to hyphal transition in C. albicans.

From Lipid Homeostasis to Differentiation: Old and New Functions of the Zinc Cluster Proteins Ecm22, Upc2, Sut1 and Sut2

Zinc cluster proteins are a large family of transcriptional regulators with a wide range of biological functions. The zinc cluster proteins Ecm22, Upc2, Sut1 and Sut2 have initially been identified as regulators of sterol import in the budding yeast Saccharomyces cerevisiae. These proteins also control adaptations to anaerobic growth, sterol biosynthesis as well as filamentation and mating. Orthologs of these zinc cluster proteins have been identified in several species of Candida. Upc2 plays a critical role in antifungal resistance in these important human fungal pathogens. Upc2 is therefore an interesting potential target for novel antifungals. In this review we discuss the functions, mode of actions and regulation of Ecm22, Upc2, Sut1 and Sut2 in budding yeast and Candida.

Crystal structures of Hsp104 N-terminal domains from Saccharomyces cerevisiae and Candida albicans suggest the mechanism for the function of Hsp104 in dissolving prions

Hsp104 is a yeast member of the Hsp100 family which functions as a molecular chaperone to disaggregate misfolded polypeptides. To understand the mechanism by which the Hsp104 N-terminal domain (NTD) interacts with its peptide substrates, crystal structures of the Hsp104 NTDs from Saccharomyces cerevisiae (ScHsp104NTD) and Candida albicans (CaHsp104NTD) have been determined at high resolution. The structures of ScHsp104NTD and CaHsp104NTD reveal that the yeast Hsp104 NTD may utilize a conserved putative peptide-binding groove to interact with misfolded polypeptides. In the crystal structures ScHsp104NTD forms a homodimer, while CaHsp104NTD exists as a monomer. The consecutive residues Gln105, Gln106 and Lys107, and Lys141 around the putative peptide-binding groove mediate the monomer-monomer interactions within the ScHsp104NTD homodimer. Dimer formation by ScHsp104NTD suggests that the Hsp104 NTD may specifically interact with polyQ regions of prion-prone proteins. The data may reveal the mechanism by which Hsp104 NTD functions to suppress and/or dissolve prions.

Micafungin alters the amino acid, nucleic acid and central carbon metabolism of Candida albicans at subinhibitory concentrations: novel insights into mechanisms of action

Background

Echinocandins are an important class of antifungal agents in the treatment of invasive candidiasis. However, little is known about the metabolomic effects of echinocandins on Candida . We therefore performed LC-high-resolution MS (LC-HRMS)-based metabolomics profiling of the response of Candida albicans cells to increasing concentrations of micafungin to determine the metabolic response of Candida to micafungin subinhibitory injury.

Methods

Isolates of C. albicans were cultured on nitrocellulose filters to mid-logarithmic phase of growth and micafungin (0-0.25 mg/L) was added. At mid-logarithmic phase, replicates were metabolically quenched. Intracellular metabolites were analysed by LC-HRMS. Changes in pool sizes of individual metabolites were analysed by Student's t -test adjusted for multiple hypothesis testing by Benjamini-Hochberg correction. Metabolites were ascribed by the Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic pathways database.

Results

Among 3446 detected metabolites, 204 were identified by comparison against pure standard or comparison against a library of mass-retention-time pairs. Fifty had significantly altered abundances in response to increasing micafungin concentrations. Pool sizes of amino acids, nucleic acids and polyamine metabolism were significantly increased at subinhibitory concentrations, while exposure to inhibitory concentrations resulted in a precipitous decrease consistent with fungicidal activity.

Conclusions

Micafungin induces a re-routing of metabolic pathways inhibiting protein synthesis and cell replication. These results shed light on new mechanisms of action of echinocandins.

Silymarin exerts antifungal effects via membrane-targeted mode of action by increasing permeability and inducing oxidative stress

Silymarin, which is derived from the seeds of Silybum marianum, has been widely used to prevent and treat liver disorders. It is also consumed as a dietary supplement to improve liver function, as it does not exhibit any toxic effects in humans. Recently, silymarin has been reported to show antimicrobial effects against various pathogenic microorganisms, but the mode of action remains unknown. Thus, we investigated the antifungal activity of silymarin and aimed to determine the underlying mechanism. Initially, a propidium iodide assay was carried out; the results indicated that silymarin induced injury to the fungal plasma membrane. Subsequently, large unilamellar vesicles encapsulating calcein and fluorescein isothiocyanate-labeled dextrans (FDs) 4, 10, and 20 were prepared to analyze whether silymarin affects an artificial membrane model. The results indicated that silymarin increased membrane permeability by disturbing the membrane structure, thereby allowing free access to molecules smaller than FD20 (approximately 3.3nm). The accumulation of reactive oxygen species (ROS) results in deleterious effects to various cellular components. In particular, ROS easily react with the membrane lipids and induce lipid peroxidation, which increases membrane permeability and disturbs hydrophobic phospholipids. Using 2',7'-dichlorodihydrofluorescein diacetate and thiobarbituric acid, we confirmed that silymarin induced harmful effects on the plasma membrane. Membrane depolarization and K+ leakage, which were associated with an increase in membrane permeability, were also observed in Candida albicans cells. An assay using 1,6-diphenyl-1,3,5-hexatriene showed that silymarin decreased membrane fluidity. Taken together, we suggest that silymarin exerts its antifungal activity by targeting the C. albicans plasma membrane.

Evaluation of Jatrophane Esters from Euphorbia spp. as Modulators of Candida albicans Multidrug Transporters

Twenty-nine jatrophane esters (1-10, 12-30) and one lathyrane (11) diterpenoid ester isolated from Euphorbia species were evaluated for their capacity to inhibit drug-efflux activities of the primary ABC transporter CaCdr1p and the secondary MFS transporter CaMdr1p of Candida albicans, in yeast strains overexpressing the corresponding transporter. These diterpenoid esters were obtained from Euphorbia semiperfoliata (1-10), E. insularis (11), and E. dendroides (12-30) and included five new compounds, euphodendroidins P-T (26-30). The jatrophane esters 12 and 23 were found to inhibit the efflux of Nile Red (NR) mediated by the two multidrug transporters, at 85-64% for CaCdr1p and 79-65% for CaMdr1p. In contrast, compound 21 was selective for CaCdr1p and induced a strong inhibition (92%), whereas compound 8 was selective for CaMdr1p, with a 74% inhibition. It was demonstrated further that potency and selectivity are sensitive to the substitution pattern on the jatrophane skeleton. However, these compounds were not transported and showed no synergism with fluconazole cytotoxicity.

Honokiol induces reactive oxygen species-mediated apoptosis in Candida albicans through mitochondrial dysfunction

Objective

To investigate the effects of honokiol on induction of reactive oxygen species (ROS), antioxidant defense systems, mitochondrial dysfunction, and apoptosis in Candida albicans.

Methods

To measure ROS accumulation, 2′,7′-dichlorofluorescein diacetate fluorescence was used. Lipid peroxidation was assessed using both fluorescence staining and a thiobarbituric acid reactive substances (TBARS) assay. Protein oxidation was determined using dinitrophenylhydrazine derivatization. Antioxidant enzymatic activities were measured using commercially available detection kits. Superoxide dismutase (SOD) genes expression was measured using real time RT-PCR. To assess its antifungal abilities and effectiveness on ROS accumulation, honokiol and the SOD inhibitor N,N′-diethyldithiocarbamate (DDC) were used simultaneously. Mitochondrial dysfunction was assessed by measuring the mitochondrial membrane potential (mtΔψ). Honokiol-induced apoptosis was assessed using an Annexin V-FITC apoptosis detection kit.

Results

ROS, lipid peroxidation, and protein oxidation occurred in a dose-dependent manner in C. albicans after honokiol treatment. Honokiol caused an increase in antioxidant enzymatic activity. In addition, honokiol treatment induced SOD genes expression in C. albicans cells. Moreover, addition of DDC resulted in increased endogenous ROS levels and potentiated the antifungal activity of honokiol. Mitochondrial dysfunction was confirmed by measured changes to mtΔψ. The level of apoptosis increased in a dose-dependent manner after honokiol treatment.

Conclusions

Collectively, these results indicate that honokiol acts as a pro-oxidant in C. albicans. Furthermore, the SOD inhibitor DDC can be used to potentiate the activity of honokiol against C. albicans.

The Iron-Dependent Regulation of the Candida albicans Oxidative Stress Response by the CCAAT-Binding Factor

Candida albicans is the most frequently encountered fungal pathogen in humans, capable of causing mucocutaneous and systemic infections in immunocompromised individuals. C. albicans virulence is influenced by multiple factors. Importantly, iron acquisition and avoidance of the immune oxidative burst are two critical barriers for survival in the host. Prior studies using whole genome microarray expression data indicated that the CCAAT-binding factor is involved in the regulation of iron uptake/utilization and the oxidative stress response. This study examines directly the role of the CCAAT-binding factor in regulating the expression of oxidative stress genes in response to iron availability. The CCAAT-binding factor is a heterooligomeric transcription factor previously shown to regulate genes involved in respiration and iron uptake/utilization in C. albicans. Since these pathways directly influence the level of free radicals, it seemed plausible the CCAAT-binding factor regulates genes necessary for the oxidative stress response. In this study, we show the CCAAT-binding factor is involved in regulating some oxidative stress genes in response to iron availability, including CAT1, SOD4, GRX5, and TRX1. We also show that CAT1 expression and catalase activity correlate with the survival of C. albicans to oxidative stress, providing a connection between iron obtainability and the oxidative stress response. We further explore the role of the various CCAAT-binding factor subunits in the formation of distinct protein complexes that modulate the transcription of CAT1 in response to iron. We find that Hap31 and Hap32 can compensate for each other in the formation of an active transcriptional complex; however, they play distinct roles in the oxidative stress response during iron limitation. Moreover, Hap43 was found to be solely responsible for the repression observed under iron deprivation.

Expression of Major Efflux Pumps in Fluconazole-Resistant Candida albicans

INTRODUCTION

Resistance to azole antifungals is considered as a significant problem in Candida albicans infections. Several molecular mechanisms of fluconazole resistance including alterations in the gene encoding the target enzyme ERG11 or overexpression of efflux pump genes including CDR1, CDR2, MDR1, MDR2 and FLU1 have been reported. The aim of this study was to investigate overexpression of efflux pump genes including CDR1, CDR2, MDR1, MDR2 and FLU1 in fluconazole- resistant C. albicans.

MATERIAL AND METHODS

In this study, a total of 97 clinical isolates of C. albicans were isolated from hospitalized children in Children medical center, an Iranian referral hospital. Fluconazole susceptibility testing of C. albicans was performed using the broth microdilution method according to the CLSI guideline. Expression of CDR1, CDR2, MDR1, MDR2 and FLU1 genes was measured using quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) and 18SrRNA gene was used as a housekeeping gene.

RESULTS

Among 97 C. albicans isolates, 5 strains were categorized as fluconazoleresistant. Overexpression of CDR1, CDR2 and MDR2 genes was found in all isolates. MDR1 overexpression was observed in four resistant isolates. None of the resistant strains displayed increases in FLU1 transcript levels.

CONCLUSION

Overexpression of the CDR1, CDR2, MDR1 and MDR2 genes might play an important role in fluconazole-resistant C. albicans. No link between expression of FLU1 and fluconazole resistance was found.

β-(1,3)-Glucan Unmasking in Some Candida albicans Mutants Correlates with Increases in Cell Wall Surface Roughness and Decreases in Cell Wall Elasticity

Candida albicans is among the most common human fungal pathogens, causing a broad range of infections, including life-threatening systemic infections. The cell wall of C. albicans is the interface between the fungus and the innate immune system. The cell wall is composed of an outer layer enriched in mannosylated glycoproteins (mannan) and an inner layer enriched in β-(1,3)-glucan and chitin. Detection of C. albicans by Dectin-1, a C-type signaling lectin specific for β-(1,3)-glucan, is important for the innate immune system to recognize systemic fungal infections. Increased exposure of β-(1,3)-glucan to the immune system occurs when the mannan layer is altered or removed in a process called unmasking. Nanoscale changes to the cell wall during unmasking were explored in live cells with atomic force microscopy (AFM). Two mutants, the cho1Δ/Δ and kre5Δ/Δ mutants, were selected as representatives that exhibit modest and strong unmasking, respectively. Comparisons of the cho1Δ/Δ and kre5Δ/Δ mutants to the wild type reveal morphological changes in their cell walls that correlate with decreases in cell wall elasticity. In addition, AFM tips functionalized with Dectin-1 revealed that the forces of binding of Dectin-1 to all of the strains were similar, but the frequency of binding was highest for the kre5Δ/Δ mutant, decreased for the cho1Δ/Δ mutant, and rare for the wild type. These data show that nanoscale changes in surface topology are correlated with increased Dectin-1 adhesion and decreased cell wall elasticity. AFM, using tips functionalized with immunologically relevant molecules, can map epitopes of the cell wall and increase our understanding of pathogen recognition by the immune system.

Effects of hemin, CO2, and pH on the branching of Candida albicans filamentous forms

Morphological transitions of wild-type and oxidative stress-tolerant Candida albicans strains were followed in the RPMI-FBS culture medium at pH values and CO₂ levels characteristic for the anatomical niches inhabited by this opportunistic human pathogen fungus, including the oral cavity as well as the intestinal and vaginal lumens. Selected cultures were also supplemented with hemin modeling bleedings. Germination as well as elongation and branching of hyphae were monitored in the cultures using time-lapse video microscopy. Unexpectedly, branching time, which is defined as the time taken until the first branch of hypha emerges for the first time after germination, correlated well with alterations in the environmental conditions meanwhile no such correlations were found for germination time (time lasted until the appearance of the germination tube). Based on these observations, hypotheses were set up to estimate the significance of branching time in the pathogenesis of both superficial and systemic candidiases.

Robust Extracellular pH Modulation by Candida albicans during Growth in Carboxylic Acids

The opportunistic fungal pathogen Candida albicans thrives within diverse niches in the mammalian host. Among the adaptations that underlie this fitness is an ability to utilize a wide array of nutrients, especially sources of carbon that are disfavored by many other fungi; this contributes to its ability to survive interactions with the phagocytes that serve as key barriers against disseminated infections. We have reported that C. albicans generates ammonia as a byproduct of amino acid catabolism to neutralize the acidic phagolysosome and promote hyphal morphogenesis in a manner dependent on the Stp2 transcription factor. Here, we report that this species rapidly neutralizes acidic environments when utilizing carboxylic acids like pyruvate, α-ketoglutarate (αKG), or lactate as the primary carbon source. Unlike in cells growing in amino acid-rich medium, this does not result in ammonia release, does not induce hyphal differentiation, and is genetically distinct. While transcript profiling revealed significant similarities in gene expression in cells grown on either carboxylic or amino acids, genetic screens for mutants that fail to neutralize αKG medium identified a nonoverlapping set of genes, including CWT1, encoding a transcription factor responsive to cell wall and nitrosative stresses. Strains lacking CWT1 exhibit retarded αKG-mediated neutralization in vitro, exist in a more acidic phagolysosome, and are more susceptible to macrophage killing, while double cwt1Δ stp2Δ mutants are more impaired than either single mutant. Together, our observations indicate that C. albicans has evolved multiple ways to modulate the pH of host-relevant environments to promote its fitness as a pathogen.

IMPORTANCE

The fungal pathogen Candida albicans is a ubiquitous and usually benign constituent of the human microbial ecosystem. In individuals with weakened immune systems, this organism can cause potentially life-threatening infections and is one of the most common causes of hospital-acquired infections. Understanding the interactions between C. albicans and immune phagocytic cells, such as macrophages and neutrophils, will define the mechanisms of pathogenesis in this species. One such adaptation is an ability to make use of nonstandard nutrients that we predict are plentiful in certain niches within the host, including within these phagocytic cells. We show here that the metabolism of certain organic acids enables C. albicans to neutralize acidic environments, such as those within macrophages. This phenomenon is distinct in several significant ways from previous reports of similar processes, indicating that C. albicans has evolved multiple mechanisms to combat the harmful acidity of phagocytic cells.

Silibinin triggers yeast apoptosis related to mitochondrial Ca2+ influx in Candida albicans

Candida albicans is a common yeast that resides in the human body, but can occasionally cause systemic fungal infection, namely candidiasis. As this infection rate is gradually increasing, it is becoming a major problem to public health. Accordingly, we for the first time investigated the antifungal activity and mode of action of silibinin, a natural product extracted from Silybum marianum (milk thistle), against C. albicans. On treatment with 100μM silibinin, generation of reactive oxygen species (ROS) from mitochondria, which can cause yeast apoptosis via oxidative stress, was increased by 24.17% compared to that in untreated cells. Subsequently, we found disturbances in ion homeostasis such as release of intracellular K+ and accumulation of cytoplasmic and mitochondrial Ca2+. Among these phenomena, mitochondrial Ca2+ overload particularly plays a crucial role in the process of apoptosis, promoting the activation of pro-apoptotic factors. Therefore, we investigated the significance of mitochondrial Ca2+ in apoptosis by employing 20mM ruthenium red (RR). Additional apoptosis hallmarks such as mitochondrial membrane depolarization, cytochrome c release, caspase activation, phosphatidylserine (PS) exposure, and DNA damage were observed in response to silibinin treatment, whereas RR pre-treatment seemed to block these responses. In summary, our results suggest that silibinin induces yeast apoptosis mediated by mitochondrial Ca2+ signaling in C. albicans.

Candida albicans glutathione reductase downregulates Efg1-mediated cyclic AMP/protein kinase A pathway and leads to defective hyphal growth and virulence upon decreased cellular methylglyoxal content accompanied by activating alcohol dehydrogenase and glycolytic enzymes

BACKGROUND

Glutathione reductase maintains the glutathione level in a reduced state. As previously demonstrated, glutathione is required for cell growth/division and its biosynthesizing-enzyme deficiency causes methylglyoxal accumulation. However, experimental evidences for reciprocal relationships between Cph1-/Efg1-mediated signaling pathway regulation and methylglyoxal production exerted by glutathione reductase on yeast morphology remain unclear.

METHODS

Glutathione reductase (GLR1) disruption/overexpression were performed to investigate aspects of pathological/morphological alterations in Candida albicans. These assumptions were proved by observations of cellular susceptibility to oxidants and thiols, and measurements of methylglyoxal and glutathione content in hyphal-inducing conditions mainly through the activity of GLR1-overexpressing cells. Additionally, the transcriptional/translational levels of bioenergetic enzymes and dimorphism-regulating protein kinases were examined in the strain.

RESULTS

The GLR1-deficient strain was non-viable when GLR1 expression under the control of a CaMAL2 promoter was conditionally repressed, despite partial rescue of growth by exogenous thiols. During filamentation, non-growing hyphal GLR1-overexpressing cells exhibited resistance against oxidants and cellular methylglyoxal was significantly decreased, which concomitantly increased expressions of genes encoding energy-generating enzymes, including fructose-1,6-bisphosphate aldolase, glyceraldehyde-3-phosphate dehydrogenase, and alcohol dehydrogenase (ADH1), with remarkable repression of Efg1-signaling cascades.

CONCLUSIONS

This is the first report that GLR1-triggered Efg1-mediated signal transduction repression strictly reduces dimorphic switching and virulence by maintaining the basal level of methylglyoxal following the enhanced gene expressions of glycolytic enzymes and ADH1.

GENERAL SIGNIFICANCE

The Efg1 downregulatory mechanism by GLR1 expression has possibilities to involve in other complex network of signal pathways. Understanding how GLR1 overexpression affects multiple signaling pathways can help identify attractive targets for antifungal drugs.

Anti-biofilm mechanisms of 3,5-di-tert-butylphenol against clinically relevant fungal pathogens

The methanolic extract (PFME) of Pleurotus florida was assessed for anti-biofilm activity against Candida species. 3,5-Di-tert-butylphenol (3,5-DTB) was identified as the major antifungal constituent in PFME. In its pure form 3,5-DTB inhibits, disrupts, and reduces the viability of biofilm cells as seen from scanning electron and confocal microscopy studies. Microscopic studies and propidium iodide uptake assays confirmed that 3,5-DTB damages the cell membrane of Candida cells. In addition, 3,5-DTB induces accumulation of reactive oxygen species (ROS) which contribute to its pronounced anti-biofilm activity. The results of the present study show that 3,5-DTB exhibits combined anti-biofilm and conventional fungicidal activity against Candida species and elucidate the underlying mechanisms.

Endoplasmic reticulum-derived reactive oxygen species (ROS) is involved in toxicity of cell wall stress to Candida albicans

The cell wall is an important cell structure in both fungi and bacteria, and hence becomes a common antimicrobial target. The cell wall-perturbing agents disrupt synthesis and function of cell wall components, leading to cell wall stress and consequent cell death. However, little is known about the detailed mechanisms by which cell wall stress renders fungal cell death. In this study, we found that ROS scavengers drastically attenuated the antifungal effect of cell wall-perturbing agents to the model fungal pathogen Candida albicans, and these agents caused remarkable ROS accumulation and activation of oxidative stress response (OSR) in this fungus. Interestingly, cell wall stress did not cause mitochondrial dysfunction and elevation of mitochondrial superoxide levels. Furthermore, the iron chelator 2,2'-bipyridyl (BIP) and the hydroxyl radical scavengers could not attenuate cell wall stress-caused growth inhibition and ROS accumulation. However, cell wall stress up-regulated expression of unfold protein response (UPR) genes, enhanced protein secretion and promoted protein folding-related oxidation of Ero1, an important source of ROS production. These results indicated that oxidation of Ero1 in the endoplasmic reticulum (ER), rather than mitochondrial electron transport and Fenton reaction, contributed to cell wall stress-related ROS accumulation and consequent growth inhibition. Our findings uncover a novel link between cell wall integrity (CWI), ER function and ROS production in fungal cells, and shed novel light on development of strategies promoting the antifungal efficacy of cell wall-perturbing agents against fungal infections.