Talking to themselves: autoregulation and quorum sensing in fungi

Degradation of bacterial quorum sensing signaling molecules by the microscopic yeast Trichosporon loubieri isolated from tropical wetland waters

Proteobacteria produce N-acylhomoserine lactones as signaling molecules, which will bind to their cognate receptor and activate quorum sensing-mediated phenotypes in a population-dependent manner. Although quorum sensing signaling molecules can be degraded by bacteria or fungi, there is no reported work on the degradation of such molecules by basidiomycetous yeast. By using a minimal growth medium containing N-3-oxohexanoylhomoserine lactone as the sole source of carbon, a wetland water sample from Malaysia was enriched for microbial strains that can degrade N-acylhomoserine lactones, and consequently, a basidiomycetous yeast strain WW1C was isolated. Morphological phenotype and molecular analyses confirmed that WW1C was a strain of Trichosporon loubieri. We showed that WW1C degraded AHLs with N-acyl side chains ranging from 4 to 10 carbons in length, with or without oxo group substitutions at the C3 position. Re-lactonisation bioassays revealed that WW1C degraded AHLs via a lactonase activity. To the best of our knowledge, this is the first report of degradation of N-acyl-homoserine lactones and utilization of N-3-oxohexanoylhomoserine as carbon and nitrogen source for growth by basidiomycetous yeast from tropical wetland water; and the degradation of bacterial quorum sensing molecules by an eukaryotic yeast.

N-acetylglucosamine (GlcNAc) triggers a rapid, temperature-responsive morphogenetic program in thermally dimorphic fungi

The monosaccharide N-acetylglucosamine (GlcNAc) is a major component of microbial cell walls and is ubiquitous in the environment. GlcNAc stimulates developmental pathways in the fungal pathogen Candida albicans, which is a commensal organism that colonizes the mammalian gut and causes disease in the setting of host immunodeficiency. Here we investigate GlcNAc signaling in thermally dimorphic human fungal pathogens, a group of fungi that are highly evolutionarily diverged from C. albicans and cause disease even in healthy individuals. These soil organisms grow as polarized, multicellular hyphal filaments that transition into a unicellular, pathogenic yeast form when inhaled by a human host. Temperature is the primary environmental cue that promotes reversible cellular differentiation into either yeast or filaments; however, a shift to a lower temperature in vitro induces filamentous growth in an inefficient and asynchronous manner. We found GlcNAc to be a potent and specific inducer of the yeast-to-filament transition in two thermally dimorphic fungi, Histoplasma capsulatum and Blastomyces dermatitidis. In addition to increasing the rate of filamentous growth, micromolar concentrations of GlcNAc induced a robust morphological transition of H. capsulatum after temperature shift that was independent of GlcNAc catabolism, indicating that fungal cells sense GlcNAc to promote filamentation. Whole-genome expression profiling to identify candidate genes involved in establishing the filamentous growth program uncovered two genes encoding GlcNAc transporters, NGT1 and NGT2, that were necessary for H. capsulatum cells to robustly filament in response to GlcNAc. Unexpectedly, NGT1 and NGT2 were important for efficient H. capsulatum yeast-to-filament conversion in standard glucose medium, suggesting that Ngt1 and Ngt2 monitor endogenous levels of GlcNAc to control multicellular filamentous growth in response to temperature. Overall, our work indicates that GlcNAc functions as a highly conserved cue of morphogenesis in fungi, which further enhances the significance of this ubiquitous sugar in cellular signaling in eukaryotes.

CO2 mediated interaction in yeast stimulates budding and growth on minimal media

Here we show that carbon dioxide (CO2) stimulates budding and shortens the lag-period of Saccharomyces cerevisiae cultures, grown on specific weak media. CO2 can be both exogenous and secreted by another growing yeast culture. We also show that this effect can be observed only in the lag-period, and demonstrate minimal doses and duration of culture exposition to CO2. Opposite to the effects of CO2 sensitivity, previously shown for pathogens, where increased concentration of CO2 suppressed mitosis and stimulated cell differentiation and invasion, here it stimulates budding and culture growth.

Bioactive molecules in soil ecosystems: masters of the underground

Complex biological and ecological processes occur in the rhizosphere through ecosystem-level interactions between roots, microorganisms and soil fauna. Over the past decade, studies of the rhizosphere have revealed that when roots, microorganisms and soil fauna physically contact one another, bioactive molecular exchanges often mediate these interactions as intercellular signal, which prepare the partners for successful interactions. Despite the importance of bioactive molecules in sustainable agriculture, little is known of their numerous functions, and improving plant health and productivity by altering ecological processes remains difficult. In this review, we describe the major bioactive molecules present in below-ground ecosystems (i.e., flavonoids, exopolysaccharides, antibiotics and quorum-sensing signals), and we discuss how these molecules affect microbial communities, nutrient availability and plant defense responses.

Carot-4-en-9,10-diol, a conidiation-inducing sesquiterpene diol produced by Trichoderma virens PS1-7 upon exposure to chemical stress from highly active iron chelators

To screen biocontrol agents against Burkholderia plantarii, the causative agent of rice seedling blight, we employed catechol, an analog of the virulence factor tropolone, to obtain chemical stress-resistant microorganisms. The fungal isolate PS1-7, identified as a strain of Trichoderma virens, showed the highest resistance to catechol (20 mM) and exhibited efficacy as a biocontrol agent for rice seedling blight. During investigation of metabolic traits of T. virens PS1-7 exposed to catechol, we found a secondary metabolite that was released extracellularly and uniquely accumulated in the culture. The compound induced by chemical stress due to catechol was subsequently isolated and identified as a sesquiterpene diol, carot-4-en-9,10-diol, based on spectroscopic analyses. T. virens PS1-7 produced carot-4-en-9,10-diol as a metabolic response to tropolone at concentrations from 0.05 to 0.2 mM, and the response was enhanced in a dose-dependent manner, similar to its response to catechol at concentrations from 0.1 to 1 mM. Some iron chelators, such as pyrogallol, gallic acid, salicylic acid, and citric acid, at 0.5 mM also showed activation of T. virens PS1-7 production of carot-4-en-9,10-diol. This sesquiterpene diol, formed in response to chemical stress, promoted conidiation of T. virens PS1-7, suggesting that it is involved in an autoregulatory signaling system. In a bioassay of the metabolic and morphological responses of T. virens PS1-7, conidiation in hyphae grown on potato dextrose agar (PDA) plates was either promoted or induced by carot-4-en-9,10-diol. Carot-4-en-9,10-diol can thus be regarded as an autoregulatory signal in T. virens, and our findings demonstrate that intrinsic intracellular signaling regulates conidiation of T. virens.

A novel calcineurin-independent activity of cyclosporin A in Saccharomyces cerevisiae

Fungi rely on regulatory networks to coordinate sensing of environmental stress with initiation of responses crucial for survival. Antifungal drugs are a specific type of environmental stress with broad clinical relevance. Small molecules with antifungal activity are ubiquitous in the environment, and are produced by a myriad of microbes in competitive natural communities. The echinocandins are fungal fermentation products and the most recently developed class of antifungals, with those in clinical use being semisynthetic derivatives that target the fungal cell wall by inhibiting 1,3-β-D-glucan synthase. Recent studies implicate the protein phosphatase calcineurin as a key regulator of cellular stress responses required for fungal survival of echinocandin-induced cell wall stress. Pharmacological inhibition of calcineurin can be achieved using the natural product and immunosuppressive drug cyclosporin A, which inhibits calcineurin by binding to the immunophilin Cpr1. This drug-protein complex inhibits the interaction between the regulatory and catalytic subunits of calcineurin, an interaction necessary for calcineurin function. Here, we report on potent activity of cyclosporin A when combined with the echinocandin micafungin against the model yeast Saccharomyces cerevisiae that is independent of its known mechanism of action of calcineurin inhibition. This calcineurin-independent synergy does not involve any of the 12 immunophilins known in yeast, individually or in combination, and is not mediated by any of the multidrug transporters encoded or controlled by YOR1, SNQ2, PDR5, PDR10, PDR11, YCF1, PDR15, ADP1, VMR1, NFT1, BPT1, YBT1, YNR070w, YOL075c, AUS1, PDR12, PDR1 and/or PDR3. Genome-wide haploinsufficiency profiling (HIP) and homozygous deletion profiling (HOP) strongly implicate the cell wall biosynthesis and integrity pathways as being central to the calcineurin-independent activity of cyclosporin A. Thus, systems level chemical genomic approaches implicate key cellular pathways in a novel mechanism of antifungal drug synergy.

Variation in adhesion and germ tube formation of oral Candida using Egyptian propolis

Adhesion of Candida cells to surfaces is considered the first step in colonization. Some natural products, such as propolis, could be used to block cell adhesion and therefore preventing colonization. In this study, Egyptian propolis ethanol extract concentrations in the range of 25 to 125 ng/μL were used to inhibit the adhesion of oral Candida. The exopolysaccharides showed a 2.5-fold decrease, while the surface-bound exopolysaccharides showed only about 1.15-fold decrease. On the other hand, surface-bound proteins decreased by 7.5-fold upon the addition of 75 ng/μL propolis. The inhibition of adhesion was detected by scanning electron microscopy. The non-slip incubation assay showed a significant decrease in germ tube formation (GTF) compared with an increase upon the addition of fetal bovine serum and corn meal, both of which had a positive effect on GTF compared with a negative GTF effect when using propolis, suggesting that propolis could be considered a quorum-sensing molecule. The use of propolis would help in maintaining the cleanliness of dental fixtures and (or) treating recurrent candidiasis as a complementary and alternative treatment, especially in elders and immunocompromised patients.

Flexible survival strategies of Pseudomonas aeruginosa in biofilms result in increased fitness compared with Candida albicans

The majority of microorganisms persist in nature as surface-attached communities often surrounded by an extracellular matrix, called biofilms. Most natural biofilms are not formed by a single species but by multiple species. Microorganisms not only cooperate as in some multispecies biofilms but also compete for available nutrients. The Gram-negative bacterium Pseudomonas aeruginosa and the polymorphic fungus Candida albicans are two opportunistic pathogens that are often found coexisting in a human host. Several models of mixed biofilms have been reported for these organisms showing antagonistic behavior. To investigate the interaction of P. aeruginosa and C. albicans in more detail, we analyzed the secretome of single and mixed biofilms of both organisms using MALDI-TOF MS/MS at several time points. Overall 247 individual proteins were identified, 170 originated from P. aeruginosa and 77 from C. albicans. Only 39 of the 131 in mixed biofilms identified proteins were assigned to the fungus whereby the remaining 92 proteins belonged to P. aeruginosa. In single-species biofilms, both organisms showed a higher diversity of proteins with 73 being assigned to C. albicans and 154 to P. aeruginosa. Most interestingly, P. aeruginosa in the presence of C. albicans secreted 16 proteins in significantly higher amounts or exclusively among other virulence factors such as exotoxin A and iron acquisition systems. In addition, the high affinity iron-binding siderophore pyoverdine was identified in mixed biofilms but not in bacterial biofilms, indicating that P. aeruginosa increases its capability to sequester iron in competition with C. albicans. In contrast, C. albicans metabolism was significantly reduced, including a reduction in detectable iron acquisition proteins. The results obtained in this study show that microorganisms not only compete with the host for essential nutrients but also strongly with the present microflora in order to gain a competitive advantage.

Candida species: new insights into biofilm formation

Biofilms of Candida albicans, Candida parapsilosis, Candida glabrata and Candida tropicalis are associated with high indices of hospital morbidity and mortality. Major factors involved in the formation and growth of Candida biofilms are the chemical composition of the medical implant and the cell wall adhesins responsible for mediating Candida-Candida, Candida-human host cell and Candida-medical device adhesion. Strategies for elucidating the mechanisms that regulate the formation of Candida biofilms combine tools from biology, chemistry, nanoscience, material science and physics. This review proposes the use of new technologies, such as synchrotron radiation, to study the mechanisms of biofilm formation. In the future, this information is expected to facilitate the design of new materials and antifungal compounds that can eradicate nosocomial Candida infections due to biofilm formation on medical implants. This will reduce dissemination of candidiasis and hopefully improve the quality of life of patients.

Interactions between diatoms and bacteria

Diatoms and bacteria have cooccurred in common habitats for hundreds of millions of years, thus fostering specific associations and interactions with global biogeochemical consequences. Diatoms are responsible for one-fifth of the photosynthesis on Earth, while bacteria remineralize a large portion of this fixed carbon in the oceans. Through their coexistence, diatoms and bacteria cycle nutrients between oxidized and reduced states, impacting bioavailability and ultimately feeding higher trophic levels. Here we present an overview of how diatoms and bacteria interact and the implications of these interactions. We emphasize that heterotrophic bacteria in the oceans that are consistently associated with diatoms are confined to two phyla. These consistent bacterial associations result from encounter mechanisms that occur within a microscale environment surrounding a diatom cell. We review signaling mechanisms that occur in this microenvironment to pave the way for specific interactions. Finally, we discuss known interactions between diatoms and bacteria and exciting new directions and research opportunities in this field. Throughout the review, we emphasize new technological advances that will help in the discovery of new interactions. Deciphering the languages of diatoms and bacteria and how they interact will inform our understanding of the role these organisms have in shaping the ocean and how these interactions may change in future oceans.

The airway microbiota in cystic fibrosis: a complex fungal and bacterial community--implications for therapeutic management

Background

Given the polymicrobial nature of pulmonary infections in patients with cystic fibrosis (CF), it is essential to enhance our knowledge on the composition of the microbial community to improve patient management. In this study, we developed a pyrosequencing approach to extensively explore the diversity and dynamics of fungal and prokaryotic populations in CF lower airways.

Methodology and Principal Findings

Fungi and bacteria diversity in eight sputum samples collected from four adult CF patients was investigated using conventional microbiological culturing and high-throughput pyrosequencing approach targeting the ITS2 locus and the 16S rDNA gene. The unveiled microbial community structure was compared to the clinical profile of the CF patients. Pyrosequencing confirmed recently reported bacterial diversity and observed complex fungal communities, in which more than 60% of the species or genera were not detected by cultures. Strikingly, the diversity and species richness of fungal and bacterial communities was significantly lower in patients with decreased lung function and poor clinical status. Values of Chao1 richness estimator were statistically correlated with values of the Shwachman-Kulczycki score, body mass index, forced vital capacity, and forced expiratory volume in 1 s (p = 0.046, 0.047, 0.004, and 0.001, respectively for fungal Chao1 indices, and p = 0.010, 0.047, 0.002, and 0.0003, respectively for bacterial Chao1 values). Phylogenetic analysis showed high molecular diversities at the sub-species level for the main fungal and bacterial taxa identified in the present study. Anaerobes were isolated with Pseudomonas aeruginosa, which was more likely to be observed in association with Candida albicans than with Aspergillus fumigatus.

Conclusions

In light of the recent concept of CF lung microbiota, we viewed the microbial community as a unique pathogenic entity. We thus interpreted our results to highlight the potential interactions between microorganisms and the role of fungi in the context of improving survival in CF.

Portrait of Candida albicans adherence regulators

Cell-substrate adherence is a fundamental property of microorganisms that enables them to exist in biofilms. Our study focuses on adherence of the fungal pathogen Candida albicans to one substrate, silicone, that is relevant to device-associated infection. We conducted a mutant screen with a quantitative flow-cell assay to identify thirty transcription factors that are required for adherence. We then combined nanoString gene expression profiling with functional analysis to elucidate relationships among these transcription factors, with two major goals: to extend our understanding of transcription factors previously known to govern adherence or biofilm formation, and to gain insight into the many transcription factors we identified that were relatively uncharacterized, particularly in the context of adherence or cell surface biogenesis. With regard to the first goal, we have discovered a role for biofilm regulator Bcr1 in adherence, and found that biofilm regulator Ace2 is a major functional target of chromatin remodeling factor Snf5. In addition, Bcr1 and Ace2 share several target genes, pointing to a new connection between them. With regard to the second goal, our findings reveal existence of a large regulatory network that connects eleven adherence regulators, the zinc-response regulator Zap1, and approximately one quarter of the predicted cell surface protein genes in this organism. This limited yet sensitive glimpse of mutant gene expression changes had thus defined one of the broadest cell surface regulatory networks in C. albicans.

Sub-lethal concentrations of Muscari comosum bulb extract suppress adhesion and induce detachment of sessile yeast cells

The formation of yeast biofilm on food industry equipment can lead to serious hygiene problems and economic losses due to food spoilage and equipment impairment. This study explored the ability of a sub-lethal concentration of the bulb extract of Muscari comosum to modulate adhesion of Candida albicans and subsequent biofilm development by this fungus. The HPLC profile of the ethanolic bulb extract showed phenolic constituents, which were found to undergo Folin-Ciocalteu reagent reduction. Prior to the adhesion tests, it was shown that up to 4000 mg l(-1) of natural extract did not adversely affect fungal growth nor did it act as a carbon energy source for C. albicans. Mathematical models predicted that 4000 mg l(-1) and 700 mg l(-1) of bulb extract would cause more than 98% reduction in fungal coverage on abiotic surfaces, without killing the planktonic cells. When added to C. albicans biofilm, the natural extract was shown to induce the dispersion of sessile cells in a dose-dependent manner.

Modeling of fungal biofilms using a rat central vein catheter

Candida frequently grows as a biofilm, or an adherent community of cells protected from both the host immune system and antimicrobial therapies. Biofilms represent the predominant mode of growth for many clinical infections, including those associated with placement of a medical device. Here, we describe a model for Candida biofilm infection of one important clinical niche, a venous catheter. This animal model system incorporates the anatomical site, immune components, and fluid dynamics of a patient venous catheter infection and can be used for study of biofilm formation, drug resistance, and gene expression.

Theory of the origin, evolution, and nature of life

Life is an inordinately complex unsolved puzzle. Despite significant theoretical progress, experimental anomalies, paradoxes, and enigmas have revealed paradigmatic limitations. Thus, the advancement of scientific understanding requires new models that resolve fundamental problems. Here, I present a theoretical framework that economically fits evidence accumulated from examinations of life. This theory is based upon a straightforward and non-mathematical core model and proposes unique yet empirically consistent explanations for major phenomena including, but not limited to, quantum gravity, phase transitions of water, why living systems are predominantly CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur), homochirality of sugars and amino acids, homeoviscous adaptation, triplet code, and DNA mutations. The theoretical framework unifies the macrocosmic and microcosmic realms, validates predicted laws of nature, and solves the puzzle of the origin and evolution of cellular life in the universe.

Rumor has it...: relay communication of stress cues in plants

Recent evidence demonstrates that plants are able not only to perceive and adaptively respond to external information but also to anticipate forthcoming hazards and stresses. Here, we tested the hypothesis that unstressed plants are able to respond to stress cues emitted from their abiotically-stressed neighbors and in turn induce stress responses in additional unstressed plants located further away from the stressed plants. Pisum sativum plants were subjected to drought while neighboring rows of five unstressed plants on both sides, with which they could exchange different cue combinations. On one side, the stressed plant and its unstressed neighbors did not share their rooting volumes (UNSHARED) and thus were limited to shoot communication. On its other side, the stressed plant shared one of its rooting volumes with its nearest unstressed neighbor and all plants shared their rooting volumes with their immediate neighbors (SHARED), allowing both root and shoot communication. Fifteen minutes following drought induction, significant stomatal closure was observed in both the stressed plants and their nearest unstressed SHARED neighbors, and within one hour, all SHARED neighbors closed their stomata. Stomatal closure was not observed in the UNSHARED neighbors. The results demonstrate that unstressed plants are able to perceive and respond to stress cues emitted by the roots of their drought-stressed neighbors and, via 'relay cuing', elicit stress responses in further unstressed plants. Further work is underway to study the underlying mechanisms of this new mode of plant communication and its possible adaptive implications for the anticipation of forthcoming abiotic stresses by plants.

Autoregulatory properties of (+)-thujopsene and influence of environmental conditions on its production by Penicillium decumbens

A Penicillium decumbens strain was collected from a water-damaged building, and the production of microbial volatile organic compounds (MVOCs) was investigated by means of headspace solid-phase microextraction, followed by GC-MS analysis. The strain was characterized by a high production of (+)-thujopsene. The influence of various temperatures, relative humidity (RH) values, substrates, and inoculum concentrations on fungal growth and (+)-thujopsene production was studied. The optimal temperature and relative humidity for P. decumbens growth were 30°C and 100% RH, respectively. In general, the more favourable the incubation parameters were for growth, the faster maximum (+)-thujopsene production was reached. Moreover, the antifungal activity of thujopsene was tested against 16 fungal strains. The growth of five of these fungal strains was negatively affected both by thujopsene alone and when grown in contact with the MVOCs produced by P. decumbens. Following these results and since growth of P. decumbens itself was also inhibited by thujopsene, an autoregulatory function for this compound was proposed. Few data are present in the literature about chemical communication between fungi. The present research could, therefore, contribute to understanding fungal metabolism and behaviour in indoor environments.

Zap1 control of cell-cell signaling in Candida albicans biofilms

Biofilms of Candida albicans include both yeast cells and hyphae. Prior studies indicated that a zap1Δ/Δ mutant, defective in zinc regulator Zap1, has increased accumulation of yeast cells in biofilms. This altered yeast-hypha balance may arise from internal regulatory alterations or from an effect on the production of diffusible quorum-sensing (QS) molecules. Here, we develop biosensor reporter strains that express yeast-specific YWP1-RFP or hypha-specific HWP1-RFP, along with a constitutive TDH3-GFP normalization standard. Seeding these biosensor strains into biofilms allows a biological activity assay of the surrounding biofilm milieu. A zap1Δ/Δ biofilm induces the yeast-specific YWP1-RFP reporter in a wild-type biosensor strain, as determined by both quantitative reverse transcription-PCR (qRT-PCR) gene expression measurements and confocal microscopy. Remediation of the zap1Δ/Δ zinc uptake defect through zinc transporter gene ZRT2 overexpression reverses induction of the yeast-specific YWP1-RFP reporter. Gas chromatography-mass spectrometry (GC-MS) measurements of known organic QS molecules show that the zap1Δ/Δ mutant accumulates significantly less farnesol than wild-type or complemented strains and that ZRT2 overexpression does not affect farnesol accumulation. Farnesol is a well-characterized inhibitor of hypha formation; hence, a reduction in farnesol levels in zap1Δ/Δ biofilms is unexpected. Our findings argue that a Zap1- and zinc-dependent signal affects the yeast-hypha balance and that it is operative in the low-farnesol environment of the zap1Δ/Δ biofilm. In addition, our results indicate that Zap1 is a positive regulator of farnesol accumulation.

Comparative proteomic profiles of Aspergillus fumigatus and Aspergillus lentulus strains by surface-enhanced laser desorption ionization time-of-flight mass spectrometry (SELDI-TOF-MS)

BACKGROUND

Surface-enhanced laser desorption ionization time-of-flight mass spectrometry (SELDI-TOF-MS) was applied to analyze the protein profiles in both somatic and metabolic extracts of Aspergillus species. The study was carried out on some Aspergillus species within the Fumigati section (Aspergillus fumigatus wild-types and natural abnormally pigmented mutants, and Aspergillus lentulus). The aim was to validate whether mass spectrometry protein profiles can be used as specific signatures to discriminate different Aspergillus species or even mutants within the same species.

RESULTS

The growth conditions and the SELDI-TOF parameters were determined to generate characteristic protein profiles of somatic and metabolic extracts of Aspergillus fumigatus strains using five different ProteinChips®, eight growth conditions combining two temperatures, two media and two oxygenation conditions. Nine strains were investigated: three wild-types and four natural abnormally pigmented mutant strains of A. fumigatus and two strains of A. lentulus. A total of 242 fungal extracts were prepared. The spectra obtained are protein signatures linked to the physiological states of fungal strains depending on culture conditions. The best resolutions were obtained using the chromatographic surfaces CM10, NP20 and H50 with fractions of fungi grown on modified Sabouraud medium at 37 °C in static condition. Under these conditions, the SELDI-TOF analysis allowed A. fumigatus and A. lentulus strains to be grouped into distinct clusters.

CONCLUSIONS

SELDI-TOF analysis distinguishes A. fumigatus from A. lentulus strains and moreover, permits separate clusters of natural abnormally pigmented A. fumigatus strains to be obtained. In addition, this methodology allowed us to point out fungal components specifically produced by a wild-type strain or natural mutants. It offers attractive potential for further studies of the Aspergillus biology or pathogenesis.

The quorum-sensing molecule farnesol is a modulator of drug efflux mediated by ABC multidrug transporters and synergizes with drugs in Candida albicans

Overexpression of the CaCDR1-encoded multidrug efflux pump protein CaCdr1p (Candida drug resistance protein 1), belonging to the ATP binding cassette (ABC) superfamily of transporters, is one of the most prominent contributors of multidrug resistance (MDR) in Candida albicans. Thus, blocking or modulating the function of the drug efflux pumps represents an attractive approach in combating MDR. In the present study, we provide first evidence that the quorum-sensing molecule farnesol (FAR) is a specific modulator of efflux mediated by ABC multidrug transporters, such as CaCdr1p and CaCdr2p of C. albicans and ScPdr5p of Saccharomyces cerevisiae. Interestingly, FAR did not modulate the efflux mediated by the multidrug extrusion pump protein CaMdr1p, belonging to the major facilitator superfamily (MFS). Kinetic data revealed that FAR competitively inhibited rhodamine 6G efflux in CaCdr1p-overexpressing cells, with a simultaneous increase in an apparent K(m) without affecting the V(max) values and the ATPase activity. We also observed that when used in combination, FAR at a nontoxic concentration synergized with the drugs at their respective nonlethal concentrations, as was evident from their <0.5 fractional inhibitory concentration index (FICI) values and from the drop of 14- to 64-fold in the MIC(80) values in the wild-type strain and in azole-resistant clinical isolates of C. albicans. Our biochemical experiments revealed that the synergistic interaction of FAR with the drugs led to reactive oxygen species accumulation, which triggered early apoptosis, and that both could be partly reversed by the addition of an antioxidant. Collectively, FAR modulates drug extrusion mediated exclusively by ABC proteins and is synergistic to fluconazole (FLC), ketoconazole (KTC), miconazole (MCZ), and amphotericin (AMB).

Short term Candida albicans colonization reduces Pseudomonas aeruginosa-related lung injury and bacterial burden in a murine model

Introduction

Pseudomonas aeruginosa is a frequent cause of ventilator-acquired pneumonia (VAP). Candida tracheobronchial colonization is associated with higher rates of VAP related to P. aeruginosa. This study was designed to investigate whether prior short term Candida albicans airway colonization modulates the pathogenicity of P. aeruginosa in a murine model of pneumonia and to evaluate the effect of fungicidal drug caspofungin.

Methods

BALB/c mice received a single or a combined intratracheal administration of C. albicans (1 × 10⁵ CFU/mouse) and P. aeruginosa (1 × 10⁷ CFU/mouse) at time 0 (T0) upon C. albicans colonization, and Day 2. To evaluate the effect of antifungal therapy, mice received caspofungin intraperitoneally daily, either from T0 or from Day 1 post-colonization. After sacrifice at Day 4, lungs were analyzed for histological scoring, measurement of endothelial injury, and quantification of live P. aeruginosa and C. albicans. Blood samples were cultured for dissemination.

Results

A significant decrease in lung endothelial permeability, the amount of P. aeruginosa, and bronchiole inflammation was observed in case of prior C. albicans colonization. Mortality rate and bacterial dissemination were unchanged by prior C. albicans colonization. Caspofungin treatment from T0 (not from Day 1) increased their levels of endothelial permeability and lung P. aeruginosa load similarly to mice receiving P. aeruginosa alone.

Conclusions

P. aeruginosa-induced lung injury is reduced when preceded by short term C. albicans airway colonization. Antifungal drug caspofungin reverses that effect when used from T0 and not from Day 1.

The quorum-sensing molecules farnesol/homoserine lactone and dodecanol operate via distinct modes of action in Candida albicans

Living as a commensal, Candida albicans must adapt and respond to environmental cues generated by the mammalian host and by microbes comprising the natural flora. These signals have opposing effects on C. albicans, with host cues promoting the yeast-to-hyphal transition and bacteria-derived quorum-sensing molecules inhibiting hyphal development. Hyphal development is regulated through modulation of the cyclic AMP (cAMP)/protein kinase A (PKA) signaling pathway, and it has been postulated that quorum-sensing molecules can affect filamentation by inhibiting the cAMP pathway. Here, we show that both farnesol and 3-oxo-C(12)-homoserine lactone, a quorum-sensing molecule secreted by Pseudomonas aeruginosa, block hyphal development by affecting cAMP signaling; they both directly inhibited the activity of the Candida adenylyl cyclase, Cyr1p. In contrast, the 12-carbon alcohol dodecanol appeared to modulate hyphal development and the cAMP signaling pathway without directly affecting the activity of Cyr1p. Instead, we show that dodecanol exerted its effects through a mechanism involving the C. albicans hyphal repressor, Sfl1p. Deletion of SFL1 did not affect the response to farnesol but did interfere with the response to dodecanol. Therefore, quorum sensing in C. albicans is mediated via multiple mechanisms of action. Interestingly, our experiments raise the possibility that the Burkholderia cenocepacia diffusible signal factor, BDSF, also mediates its effects via Sfl1p, suggesting that dodecanol's mode of action, but not farnesol or 3-oxo-C(12)-homoserine lactone, may be used by other quorum-sensing molecules.

Modulation of morphogenesis in Candida albicans by various small molecules

The pathogenic yeast Candida albicans, a member of the mucosal microbiota, is responsible for a large spectrum of infections, ranging from benign thrush and vulvovaginitis in both healthy and immunocompromised individuals to severe, life-threatening infections in immunocompromised patients. A striking feature of C. albicans is its ability to grow as budding yeast and as filamentous forms, including hyphae and pseudohyphae. The yeast-to-hypha transition contributes to the overall virulence of C. albicans and may even constitute a target for the development of antifungal drugs. Indeed, impairing morphogenesis in C. albicans has been shown to be a means to treat candidiasis. Additionally, a large number of small molecules such as farnesol, fatty acids, rapamycin, geldanamycin, histone deacetylase inhibitors, and cell cycle inhibitors have been reported to modulate the yeast-to-hypha transition in C. albicans. In this minireview, we take a look at molecules that modulate morphogenesis in this pathogenic yeast. When possible, we address experimental findings regarding their mechanisms of action and their therapeutic potential. We discuss whether or not modulating morphogenesis constitutes a strategy to treat Candida infections.

LIPOXYGENASE PRODUCTS IN MARINE DIATOMS: A CONCISE ANALYTICAL METHOD TO EXPLORE THE FUNCTIONAL POTENTIAL OF OXYLIPINS(1)

Oxylipins are oxygenated derivatives of polyunsaturated fatty acids (PUFAs) that act as chemical mediators in many ecological and physiological processes in marine and freshwater diatoms. The occurrence and distribution of these molecules are relatively widespread within the lineage with considerable species-specific differences due to the variability of both the fatty acids recognized as substrates and the enzymatic transformations. The present review provides a general introduction to recent studies on diatom oxylipins and describes an analytical method for the detection and assessment of these elusive molecules in laboratory and field samples. This methodology is based on selective enrichment of the oxylipin fraction by solvent extraction, followed by parallel acquisition of full-scan UV and tandem mass spectra on reverse phase liquid chromatography (LC) peaks. The analytical procedure enables identification of potential genetic differences, enzymatic regulation, and ecophysiological conditions that result in different oxylipin signatures, thus providing an effective tool for probing the functional relevance of this class of lipids in plankton communities. Examples of oxylipin measurements in field samples are also provided as a demonstration of the analytical potential of the methodology.

Cell signals, cell contacts, and the organization of yeast communities

Even relatively simple species have evolved mechanisms to organize individual organisms into communities, such that the fitness of the group is greater than the fitness of isolated individuals. Within the fungal kingdom, the ability of many yeast species to organize into communities is crucial for their growth and survival, and this property has important impacts both on the economy and on human health. Over the last few years, studies of Saccharomyces cerevisiae have revealed several fundamental properties of yeast communities. First, strain-to-strain variation in the structures of these groups is attributable in part to variability in the expression and functions of adhesin proteins. Second, the extracellular matrix surrounding these communities can protect them from environmental stress and may also be important in cell signaling. Finally, diffusible signals between cells contribute to community organization so that different regions of a community express different genes and adopt different cell fates. These findings provide an arena in which to view fundamental mechanisms by which contacts and signals between individual organisms allow them to assemble into functional communities.

CO(2) acts as a signalling molecule in populations of the fungal pathogen Candida albicans

When colonising host-niches or non-animated medical devices, individual cells of the fungal pathogen Candida albicans expand into significant biomasses. Here we show that within such biomasses, fungal metabolically generated CO(2) acts as a communication molecule promoting the switch from yeast to filamentous growth essential for C. albicans pathology. We find that CO(2)-mediated intra-colony signalling involves the adenylyl cyclase protein (Cyr1p), a multi-sensor recently found to coordinate fungal responses to serum and bacterial peptidoglycan. We further identify Lys 1373 as essential for CO(2)/bicarbonate regulation of Cyr1p. Disruption of the CO(2)/bicarbonate receptor-site interferes selectively with C. albicans filamentation within fungal biomasses. Comparisons between the Drosophila melanogaster infection model and the mouse model of disseminated candidiasis, suggest that metabolic CO(2) sensing may be important for initial colonisation and epithelial invasion. Our results reveal the existence of a gaseous Candida signalling pathway and its molecular mechanism and provide insights into an evolutionary conserved CO(2)-signalling system.

Streptococcus mutans inhibits Candida albicans hyphal formation by the fatty acid signaling molecule trans-2-decenoic acid (SDSF)

In the human mouth, fungi and several hundred species of bacteria coexist. Here we report a case of interkingdom signaling in the oral cavity: A compound excreted by the caries bacterium Streptococcus mutans inhibits the morphological transition from yeast to hyphae, an important virulence trait, in the opportunistic fungus Candida albicans. The compound excreted by S. mutans was originally studied because it inhibited signaling by the universal bacterial signal autoinducer-2 (AI-2), determined by the luminescence of a Vibrio harveyi sensor strain. The inhibitor was purified from cell-free culture supernatants of S. mutans guided by its activity. Its chemical structure was elucidated by using NMR spectroscopy and GC-MS and proved to be trans-2-decenoic acid. We show that trans-2-decenoic acid does not inhibit AI-2-specific signaling, but rather the luciferase reaction used for its detection. A potential biological role of trans-2-decenoic acid was then discovered. It is able to suppress the transition from yeast to hyphal morphology in the opportunistic human pathogen Candida albicans at concentrations that do not affect growth. The expression of HWP1, a hyphal-specific signature gene of C. albicans, is abolished by trans-2-decenoic acid. trans-2-Decenoic acid is structurally similar to the diffusible signal factor (DSF) family of interkingdom-signaling molecules and is the first member of this family from a Gram-positive organism (Streptococcus DSF, SDSF). SDSF activity was also found in S. mitis, S. oralis, and S. sanguinis, but not in other oral bacteria. SDSF could be relevant in shaping multispecies Candida bacteria biofilms in the human body.

PKC signaling regulates drug resistance of the fungal pathogen Candida albicans via circuitry comprised of Mkc1, calcineurin, and Hsp90

Fungal pathogens exploit diverse mechanisms to survive exposure to antifungal drugs. This poses concern given the limited number of clinically useful antifungals and the growing population of immunocompromised individuals vulnerable to life-threatening fungal infection. To identify molecules that abrogate resistance to the most widely deployed class of antifungals, the azoles, we conducted a screen of 1,280 pharmacologically active compounds. Three out of seven hits that abolished azole resistance of a resistant mutant of the model yeast Saccharomyces cerevisiae and a clinical isolate of the leading human fungal pathogen Candida albicans were inhibitors of protein kinase C (PKC), which regulates cell wall integrity during growth, morphogenesis, and response to cell wall stress. Pharmacological or genetic impairment of Pkc1 conferred hypersensitivity to multiple drugs that target synthesis of the key cell membrane sterol ergosterol, including azoles, allylamines, and morpholines. Pkc1 enabled survival of cell membrane stress at least in part via the mitogen activated protein kinase (MAPK) cascade in both species, though through distinct downstream effectors. Strikingly, inhibition of Pkc1 phenocopied inhibition of the molecular chaperone Hsp90 or its client protein calcineurin. PKC signaling was required for calcineurin activation in response to drug exposure in S. cerevisiae. In contrast, Pkc1 and calcineurin independently regulate drug resistance via a common target in C. albicans. We identified an additional level of regulatory control in the C. albicans circuitry linking PKC signaling, Hsp90, and calcineurin as genetic reduction of Hsp90 led to depletion of the terminal MAPK, Mkc1. Deletion of C. albicans PKC1 rendered fungistatic ergosterol biosynthesis inhibitors fungicidal and attenuated virulence in a murine model of systemic candidiasis. This work establishes a new role for PKC signaling in drug resistance, novel circuitry through which Hsp90 regulates drug resistance, and that targeting stress response signaling provides a promising strategy for treating life-threatening fungal infections.

Treating fungal infections is challenging due to the emergence of drug resistance and the limited number of clinically useful antifungal drugs. We screened a library of 1,280 pharmacologically active compounds to identify those that reverse resistance of the leading human fungal pathogen, Candida albicans, to the most widely used antifungals, the azoles. This revealed a new role for protein kinase C (PKC) signaling in resistance to drugs targeting the cell membrane, including azoles, allylamines, and morpholines. We dissected mechanisms through which PKC regulates resistance in C. albicans and the model yeast Saccharomyces cerevisiae. PKC enabled survival of cell membrane stress at least in part through the mitogen-activated protein kinase (MAPK) cascade in both species. In S. cerevisiae, inhibition of PKC signaling blocked activation of a key regulator of membrane stress responses, calcineurin. In C. albicans, Pkc1 and calcineurin independently regulate resistance via a common target. Deletion of C. albicans PKC1 rendered fungistatic drugs fungicidal and reduced virulence in a mouse model. The molecular chaperone Hsp90, which stabilizes client proteins including calcineurin, also stabilized the terminal C. albicans MAPK, Mkc1. We establish new circuitry connecting PKC with Hsp90 and calcineurin and suggest a promising strategy for treating life-threatening fungal infections.

Pathogenesis, parasitism and mutualism in the trophic space of microbe-plant interactions

Microbe-host interactions can be categorised as pathogenic, parasitic or mutualistic, but in practice few examples exactly fit these descriptions. New molecular methods are providing insights into the dynamics of microbe-host interactions, with most microbes changing their relationship with their host at different life-cycle stages or in response to changing environmental conditions. Microbes can transition between the trophic states of pathogenesis and symbiosis and/or between mutualism and parasitism. In plant-based systems, an understanding of the true ecological niche of organisms and the dynamic state of their trophic interactions with their hosts has important implications for agriculture, including crop rotation, disease control and risk management.

Zoospore density-dependent behaviors of Phytophthora nicotianae are autoregulated by extracellular products

Phytophthora species are destructive fungus-like plant pathogens that use asexual single-celled flagellate zoospores for dispersal and plant infection. Many of the zoospore behaviors are density-dependent although the underlying mechanisms are poorly understood. Here, we use P. nicotianae as a model and demonstrate autoregulation of some zoospore behaviors using signal molecules that zoospores release into the environment. Specifically, zoospore aggregation, plant targeting, and infection required or were enhanced by threshold concentrations of these signal molecules. Below the threshold concentration, zoospores did not aggregate and move toward a cauline leaf of Arabidopsis thaliana (Col-0) and failed to individually attack annual vinca (Catharanthus roseus cv. Little Bright Eye). These processes were reversed when supplemented with zoospore-free fluid (ZFF) prepared from a zoospore suspension above threshold densities but not with calcium chloride at a concentration equivalent to extracellular Ca(2+) in ZFF. These results suggest that Ca(2+) is not a primary signal molecule regulating these communal behaviors. Zoospores coordinated their communal behaviors by releasing, detecting, and responding to signal molecules. This chemical communication mechanism raises the possibility that Phytophthora plant infection may not depend solely on zoospore number in the real world. Single zoospore infection may take place if it is signaled by a common molecule available in the environment which contributes to the destructiveness of these plant pathogens.

Ultra high performance liquid chromatography tandem mass spectrometry analysis of quorum-sensing molecules of Candida albicans

Candida albicans is generally one of the most commonly isolated fungal pathogen from human body. It is a frequent cause of nosocomial infections, bloodstream infections, urinary infections and mucosal infections of oral cavity and vagina C. albicans can grow as hyphae, pseudohyphae, or budding yeast. Morphological conversion of a yeast form to pseudohyphal or hyphal one is often characterized by the change of commensal status to an invasive form. Farnesol and tyrosol can participate in these transformation processes as quorum sensing molecules together with some physical-chemical factors. A new analytical method for identification and quantification of biologically active substances farnesol and tyrosol using ultra high performance liquid chromatography (UHPLC) in connection with tandem mass spectrometry was developed. The analytes were separated on Acquity BEH C18 analytical column using binary mobile phase consisting of acetonitrile and formic acid 0.075% (75:25) at flow-rate 0.20 ml/min. SRM (selected reaction monitoring) mode was applied in order to ensure sufficient selectivity and sensitivity using the first most intensive transition as a quantitative (121>77 and 205>121) and second one for the confirmation purposes (121>93 and 205>109). The method was validated in terms of linearity (>0.9994), precision (0.5-3.8% RSD), accuracy (78.9-106.0%), LOD (limit of detection) and LOQ (limit of quantitation). The method can serve as an analytical tool for the detection and determination of quorum-sensing molecules in biological samples.

Uniform categorization of biocommunication in bacteria, fungi and plants

This article describes a coherent biocommunication categorization for the kingdoms of bacteria, fungi and plants. The investigation further shows that, besides biotic sign use in trans-, inter- and intraorganismic communication processes, a common trait is interpretation of abiotic influences as indicators to generate an appropriate adaptive behaviour. Far from being mechanistic interactions, communication processes within organisms and between organisms are sign-mediated interactions. Sign-mediated interactions are the precondition for every cooperation and coordination between at least two biological agents such as cells, tissues, organs and organisms. Signs of biocommunicative processes are chemical molecules in most cases. The signs that are used in a great variety of signaling processes follow syntactic (combinatorial), pragmatic (context-dependent) and semantic (content-specific) rules. These three levels of semiotic rules are helpful tools to investigate communication processes throughout all organismic kingdoms. It is not the aim to present the latest empirical data concerning communication in these three kingdoms but to present a unifying perspective that is able to interconnect transdisciplinary research on bacteria, fungi and plants.

The Ras/cAMP/PKA signaling pathway and virulence in Candida albicans

Candidiasis is the most common cause of fungal infections, and the majority of these are caused by Candida albicans. The protean pathogenic potential of C. albicans includes the capacity to infect diverse mucosal and epidermal surfaces as well as to disseminate via the bloodstream to internal organs, potentially causing system failure in cases of severe immunosuppression. Many environmental niches in the host may be invaded by C. albicans through modulation of gene expression patterns while changing morphology between yeast and hyphal growth forms. The Ras/cAMP/PKA signaling pathway has attracted particular attention for its role in promoting hyphal growth and because of its importance in virulence. Here, we present an overview of the components of the pathway and their functions, how the pathway may be activated in human hosts and recent updates regarding the role of Ras/cAMP/PKA signaling in virulence.

Quorum sensing and fungal-bacterial interactions in Candida albicans: a communicative network regulating microbial coexistence and virulence

Microorganisms have evolved a complex signature of communication termed quorum sensing (QS), which is based on the exchange and sensing of low molecular- weight signal compounds. The ability to communicate within the microbial population gives the advantage to coordinate a groups behaviour leading to a higher fitness in the environment. The polymorphic fungus Candida albicans is an opportunistic human pathogen able to regulate virulence traits through the production of at least two QS signal molecules: farnesol and tyrosol. The ability to adopt multiple morphotypes and form biofilms on infected surfaces are the most important pathogenic characteristics regulated by QS and are of clinical relevance. In fact, traditional antimicrobial approaches are often ineffective towards these characteristics. Moreover, the intimate association between C. albicans and other pathogens, such as Pseudomonas aeruginosa, increases the complexity of the infection system. This review outlines the current knowledge on fungal QS and fungal-bacterial interactions emphasizing on C. albicans. Further investigations need to concentrate on the molecular mechanisms and the genetic regulation of these phenomena in order to identify putative novel therapeutic options.

Zoosporic plant pathogens produce bacterial autoinducer-2 that affects Vibrio harveyi quorum sensing

The frequent coisolation of bacteria with Phytophthora and Pythium species suggests possible interspecies communication. Zoospore-free fluids (ZFF) from bacteria-free and nutrient-depleted zoospore suspensions were examined to investigate the production of autoinducer-2 (AI-2), a bacterial interspecies signal molecule, by zoosporic oomycetes. ZFF from Phytophthora nicotianae, Phytophthora sojae, and Pythium aphanidermatum triggered luminescence of the Vibrio harve7yi AI-2 reporter, indicating the presence of AI-2 in zoospore extracellular products and the potential of cross-kingdom communication between oomycetes and bacteria. The production of AI-2 by zoospores was confirmed by chemical assays. These results provide a new insight into the physiology and ecology of oomycetes.

The effects of farnesol on Staphylococcus aureus biofilms and osteoblasts. An in vitro study

BACKGROUND

Bacterial biofilms play a major role in chronic orthopaedic infections. Recently, farnesol (an antifungal agent) has been shown to express antimicrobial activities against Staphylococcus aureus and Streptococcus mutans. However, the effects of farnesol on the formation of bacterial biofilms on orthopaedic biomaterials and its effects on osteoblasts have not been investigated, to our knowledge, and are therefore the focus of this study.

METHODS

Biofilms of Staphylococcus aureus (Seattle 1945(GFPuvr)) were grown on titanium alloy discs. The effects of soluble farnesol on biofilm formation with or without gentamicin were examined with fluorescence microscopy and in quantitative cultures. The effect of farnesol coated on titanium alloy discs was also investigated, as was the effect of the agent on MC3T3-E1 pre-osteoblastic cells cultured on titanium alloy discs.

RESULTS

Soluble farnesol at a 30-mM concentration reduced the number of viable bacteria 10(4)-fold and completely inhibited biofilm formation. Low concentrations of soluble farnesol (0.03 to 3 mM) did not inhibit biofilm formation and did not potentiate the effect of a submaximal concentration of gentamicin. Dried farnesol on titanium alloy discs reduced the number of viable bacteria fiftyfold. The effect of farnesol on bacterial biofilm formation lasted for at least three days. Soluble farnesol added after the biofilm had already formed also reduced the final number of viable bacteria, by fifty-six-fold. Soluble farnesol (3-mM and 30-mM concentrations) inhibited spreading of the MC3T3-E1 cells.

CONCLUSIONS

In vitro, a high concentration of farnesol (30 mM) shows antimicrobial properties against bacterial biofilms; however, it also has a negative effect on pre-osteoblasts. Farnesol can also express antimicrobial activity when predried on titanium discs and when added to preformed biofilms.

Interspecies chemical communication in bacterial development

Our view of bacteria, from the earliest observations through the heyday of antibiotic discovery, has shifted dramatically. We recognize communities of bacteria as integral and functionally important components of diverse habitats, ranging from soil collectives to the human microbiome. To function as productive communities, bacteria coordinate metabolic functions, often requiring shifts in growth and development. The hallmark of cellular development, which we characterize as physiological change in response to environmental stimuli, is a defining feature of many bacterial interspecies interactions. Bacterial communities rely on chemical exchanges to provide the cues for developmental change. Traditional methods in microbiology focus on isolation and characterization of bacteria in monoculture, separating the organisms from the surroundings in which interspecies chemical communication has relevance. Developing multispecies experimental systems that incorporate knowledge of bacterial physiology and metabolism with insights from biodiversity and metagenomics shows great promise for understanding interspecies chemical communication in the microbial world.

Can filamentous fungi form biofilms?

The discovery of biofilm formation in bacteria and yeasts has led to a better understanding of microbial ecology and to new insights into the mechanisms of virulence and persistence of pathogenic microorganisms. However, it is generally assumed that filamentous fungi, some of which have a significant impact on our health or our economy, do not form biofilms. In contrast to this assumption, here we discuss recent findings supporting the hypothesis that surface-associated filamentous fungi can form biofilms. Based on these findings and on previous models for bacterial and yeast systems, we propose preliminary criteria and a model for biofilm formation by filamentous fungi.