Quorum sensing in dimorphic fungi: farnesol and beyond

Conserved signaling modules regulate filamentous growth in fungi: a model for eukaryotic cell differentiation

Eukaryotic organisms are composed of different cell types with defined shapes and functions. Specific cell types are produced by the process of cell differentiation, which is regulated by signal transduction pathways. Signaling pathways regulate cell differentiation by sensing cues and controlling the expression of target genes whose products generate cell types with specific attributes. In studying how cells differentiate, fungi have proved valuable models because of their ease of genetic manipulation and striking cell morphologies. Many fungal species undergo filamentous growth-a specialized growth pattern where cells produce elongated tube-like projections. Filamentous growth promotes expansion into new environments, including invasion into plant and animal hosts by fungal pathogens. The same signaling pathways that regulate filamentous growth in fungi also control cell differentiation throughout eukaryotes and include highly conserved mitogen-activated protein kinase (MAPK) pathways, which is the focus of this review. In many fungal species, mucin-type sensors regulate MAPK pathways to control filamentous growth in response to diverse stimuli. Once activated, MAPK pathways reorganize cell polarity, induce changes in cell adhesion, and promote the secretion of degradative enzymes that mediate access to new environments. However, MAPK pathway regulation is complicated because related pathways can share components with each other yet induce unique responses (i.e. signal specificity). In addition, MAPK pathways function in highly integrated networks with other regulatory pathways (i.e. signal integration). Here, we discuss signal specificity and integration in several yeast models (mainly Saccharomyces cerevisiae and Candida albicans) by focusing on the filamentation MAPK pathway. Because of the strong evolutionary ties between species, a deeper understanding of the regulation of filamentous growth in established models and increasingly diverse fungal species can reveal fundamentally new mechanisms underlying eukaryotic cell differentiation.

Absence of farnesol salvage in Candida albicans and probably in other fungi

Farnesol salvage, a two-step pathway converting farnesol to farnesyl pyrophosphate (FPP), occurs in bacteria, plants, and animals. This paper investigates the presence of this pathway in fungi. Through bioinformatics, biochemistry, and physiological analyses, we demonstrate its absence in the yeasts Saccharomyces cerevisiae and Candida albicans, suggesting a likely absence across fungi. We screened 1,053 fungal genomes, including 34 from C. albicans, for potential homologs to four genes (Arabidopsis thaliana AtFOLK, AtVTE5, AtVTE6, and Plasmodium falciparum PfPOLK) known to accomplish farnesol/prenol salvage in other organisms. Additionally, we showed that 3H-farnesol was not converted to FPP or any other phosphorylated prenol, and exogenous farnesol was not metabolized within 90 minutes at any phase of growth and did not rescue cells from the toxic effects of atorvastatin, but it did elevate the levels of intracellular farnesol (Fi). All these experiments were conducted with C. albicans. In sum, we found no evidence for farnesol salvage in fungi.

IMPORTANCE

The absence of farnesol salvage constitutes a major difference in the metabolic capabilities of fungi. In terms of fungal physiology, the lack of farnesol salvage pathways relates to how farnesol acts as a quorum-sensing molecule in Candida albicans and why farnesol should be investigated for use in combination with other known antifungal antibiotics. Its absence is essential for a model (K. W. Nickerson et al., Microbiol Mol Biol Rev 88:e00081-22, 2024), wherein protein farnesylation, protein chaperones, and the unfolded protein response are combined under the unifying umbrella of a cell's intracellular farnesol (Fi). In terms of human health, farnesol should have at least two different modes of action depending on whether those cells have farnesol salvage. Because animals have farnesol salvage, we can now see the importance of dietary prenols as well as the potential importance of farnesol in treating neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, and multiple sclerosis.

Erg251 has complex and pleiotropic effects on sterol composition, azole susceptibility, filamentation, and stress response phenotypes

Ergosterol is essential for fungal cell membrane integrity and growth, and numerous antifungal drugs target ergosterol. Inactivation or modification of ergosterol biosynthetic genes can lead to changes in antifungal drug susceptibility, filamentation and stress response. Here, we found that the ergosterol biosynthesis gene ERG251 is a hotspot for point mutations during adaptation to antifungal drug stress within two distinct genetic backgrounds of Candida albicans. Heterozygous point mutations led to single allele dysfunction of ERG251 and resulted in azole tolerance in both genetic backgrounds. This is the first known example of point mutations causing azole tolerance in C. albicans. Importantly, single allele dysfunction of ERG251 in combination with recurrent chromosome aneuploidies resulted in bona fide azole resistance. Homozygous deletions of ERG251 caused increased fitness in low concentrations of fluconazole and decreased fitness in rich medium, especially at low initial cell density. Homozygous deletions of ERG251 resulted in accumulation of ergosterol intermediates consistent with the fitness defect in rich medium. Dysfunction of ERG251, together with FLC exposure, resulted in decreased accumulation of the toxic sterol (14-ɑ-methylergosta-8,24(28)-dien-3β,6α-diol) and increased accumulation of non-toxic alternative sterols. The altered sterol composition of the ERG251 mutants had pleiotropic effects on transcription, filamentation, and stress responses including cell membrane, osmotic and oxidative stress. Interestingly, while dysfunction of ERG251 resulted in azole tolerance, it also led to transcriptional upregulation of ZRT2, a membrane-bound Zinc transporter, in the presence of FLC, and overexpression of ZRT2 is sufficient to increase azole tolerance in wild-type C. albicans. Finally, in a murine model of systemic infection, homozygous deletion of ERG251 resulted in decreased virulence while the heterozygous deletion mutants maintain their pathogenicity. Overall, this study demonstrates that single allele dysfunction of ERG251 is a recurrent and effective mechanism of acquired azole tolerance. We propose that altered sterol composition resulting from ERG251 dysfunction mediates azole tolerance as well as pleiotropic effects on stress response, filamentation and virulence.

Segmentation of hyphae and yeast in fungi-infected tissue slice images and its application in analyzing antifungal blue light therapy

Candida albicans is a pathogenic fungus that undergoes morphological transitions between hyphal and yeast forms, adapting to diverse environmental stimuli and exhibiting distinct virulence. Existing research works on antifungal blue light (ABL) therapy have either focused solely on hyphae or neglected to differentiate between morphologies, obscuring potential differential effects. To address this gap, we established a novel dataset of 150 C. albicans-infected mouse skin tissue slice images with meticulously annotated hyphae and yeast. Eleven representative convolutional neural networks were trained and evaluated on this dataset using seven metrics to identify the optimal model for segmenting hyphae and yeast in original high pixel size images. Leveraging the segmentation results, we analyzed the differential impact of blue light on the invasion depth and density of both morphologies within the skin tissue. U-Net-BN outperformed other models in segmentation accuracy, achieving the best overall performance. While both hyphae and yeast exhibited significant reductions in invasion depth and density at the highest ABL dose (180 J/cm2), only yeast was significantly inhibited at the lower dose (135 J/cm2). This novel finding emphasizes the importance of developing more effective treatment strategies for both morphologies.

Transdermal administration of farnesol-ethosomes enhances the treatment of cutaneous candidiasis induced by Candida albicans in mice

Cutaneous candidiasis, caused by Candida albicans, is a severe and frustrating condition, and finding effective treatments can be challenging. Therefore, the development of farnesol-loaded nanoparticles is an exciting breakthrough. Ethosomes are a novel transdermal drug delivery carrier that incorporates a certain concentration (10-45%) of alcohols into lipid vesicles, resulting in improved permeability and encapsulation rates compared to conventional liposomes. Farnesol is a quorum-sensing molecule involved in morphogenesis regulation in C. albicans, and these ethosomes offer a promising new approach to treating this common fungal infection. This study develops the formulation of farnesol-loaded ethosomes (farnesol-ethosomes) and assesses applications in treating cutaneous candidiasis induced by C. albicans in vitro and in vivo. Farnesol-ethosomes were successfully developed by ethanol injection method. Therapeutic properties of farnesol-ethosomes, such as particle size, zeta potential, and morphology, were well characterized. According to the results, farnesol-ethosomes demonstrated an increased inhibition effect on cells' growth and biofilm formation in C. albicans. In Animal infection models, treating farnesol-ethosomes by transdermal administration effectively relieved symptoms caused by cutaneous candidiasis and reduced fungal burdens in quantity. We also observed that ethosomes significantly enhanced drug delivery efficacy in vitro and in vivo. These results indicate that farnesol-ethosomes can provide future promising roles in curing cutaneous candidiasis.

IMPORTANCE

Cutaneous candidiasis attributed to Candida infection is a prevalent condition that impacts individuals of all age groups. As a type of microbial community, biofilms confer benefits to host infections and mitigate the clinical effects of antifungal treatments. In C. albicans, the yeast-to-hypha transition and biofilm formation are effectively suppressed by farnesol through its modulation of multiple signaling pathway. However, the characteristics of farnesol such as hydrophobicity, volatility, degradability, and instability in various conditions can impose limitations on its effectiveness. Nanotechnology holds the potential to enhance the efficiency and utilization of this molecule. Treatment of farnesol-ethosomes by transdermal administration demonstrated a very remarkable therapeutic effect against C. albicans in infection model of cutaneous candidiasis in mice. Many patients suffering fungal skin infection will benefit from this study.

Physiological adventures in Candida albicans: farnesol and ubiquinones

SUMMARYFarnesol was first identified as a quorum-sensing molecule, which blocked the yeast to hyphal transition in Candida albicans, 22 years ago. However, its interactions with Candida biology are surprisingly complex. Exogenous (secreted or supplied) farnesol can also act as a virulence factor during pathogenesis and as a fungicidal agent triggering apoptosis in other competing fungi. Farnesol synthesis is turned off both during anaerobic growth and in opaque cells. Distinctly different cellular responses are observed as exogenous farnesol levels are increased from 0.1 to 100 µM. Reported changes include altered morphology, stress response, pathogenicity, antibiotic sensitivity/resistance, and even cell lysis. Throughout, there has been a dearth of mechanisms associated with these observations, in part due to the absence of accurate measurement of intracellular farnesol levels (Fi). This obstacle has recently been overcome, and the above phenomena can now be viewed in terms of changing Fi levels and the percentage of farnesol secreted. Critically, two aspects of isoprenoid metabolism present in higher organisms are absent in C. albicans and likely in other yeasts. These are pathways for farnesol salvage (converting farnesol to farnesyl pyrophosphate) and farnesylcysteine cleavage, a necessary step in the turnover of farnesylated proteins. Together, these developments suggest a unifying model, whereby high, threshold levels of Fi regulate which target proteins are farnesylated or the extent to which they are farnesylated. Thus, we suggest that the diversity of cellular responses to farnesol reflects the diversity of the proteins that are or are not farnesylated.

Erg251 has complex and pleiotropic effects on azole susceptibility, filamentation, and stress response phenotypes

Ergosterol is essential for fungal cell membrane integrity and growth, and numerous antifungal drugs target ergosterol. Inactivation or modification of ergosterol biosynthetic genes can lead to changes in antifungal drug susceptibility, filamentation and stress response. Here, we found that the ergosterol biosynthesis gene ERG251 is a hotspot for point mutations during adaptation to antifungal drug stress within two distinct genetic backgrounds of Candida albicans. Heterozygous point mutations led to single allele dysfunction of ERG251 and resulted in azole tolerance in both genetic backgrounds. This is the first known example of point mutations causing azole tolerance in C. albicans. Importantly, single allele dysfunction of ERG251 in combination with recurrent chromosome aneuploidies resulted in bona fide azole resistance. Homozygous deletions of ERG251 caused increased fitness in low concentrations of fluconazole and decreased fitness in rich medium, especially at low initial cell density. Dysfunction of ERG251 resulted in transcriptional upregulation of the alternate sterol biosynthesis pathway and ZRT2, a Zinc transporter. Notably, we determined that overexpression of ZRT2 is sufficient to increase azole tolerance in C. albicans. Our combined transcriptional and phenotypic analyses revealed the pleiotropic effects of ERG251 on stress responses including cell wall, osmotic and oxidative stress. Interestingly, while loss of either allele of ERG251 resulted in similar antifungal drug responses, we observed functional divergence in filamentation regulation between the two alleles of ERG251 (ERG251-A and ERG251-B) with ERG251-A exhibiting a dominant role in the SC5314 genetic background. Finally, in a murine model of systemic infection, homozygous deletion of ERG251 resulted in decreased virulence while the heterozygous deletion mutants maintain their pathogenicity. Overall, this study provides extensive genetic, transcriptional and phenotypic analysis for the effects of ERG251 on drug susceptibility, fitness, filamentation and stress responses.

Biofilm development of Candida boidinii and the effect of tyrosol on biofilm formation

OBJECTIVES

The applicability of a simple and high-throughput method for quantitative characterization of biofilm formation by Candida boidinii was tested in order to evaluate the effects of exogenous tyrosol on yeast growth and biofilm formation capacity.

RESULTS

Significant concentration-, temperature and time-dependent effect of tyrosol (2-(4-hydroxyphenyl)ethanol) was demonstrated, but it differentially affected the growth and biofilm formation (characterized by crystal violet staining and XTT-reduction assay) of Candida boidinii. Testing biofilm based on metabolic activity displayed sensitively the differences in the intensity of biofilm in terms of temperature, tyrosol concentration, and exposure time. At 22 °C after 24 h none of the tyrosol concentrations had significant effect, while at 30 °C tyrosol-mediated inhibition was observed at 50 mM and 100 mM concentration. After 48 h and 72 h at 22 °C, biofilm formation was stimulated at 6.25-25 mM concentrations, meanwhile at 30 °C tyrosol decreased the biofilm metabolic activity proportionally with the concentration.

CONCLUSIONS

The research concludes that exogenous tyrosol exerts unusual effects on Candida boidinii growth and biofilm formation ability and predicts its potential application as a regulating factor of various fermentations by Candida boidinii.

Volatile Fingerprint Mediates Yeast-to-Mycelial Conversion in Two Strains of Beauveria bassiana Exhibiting Varied Virulence

Beauveria bassiana is a dimorphic and entomopathogenic fungus with different ecological roles in nature. In pathogenic fungi, yeast-to-mycelial conversion, which is controlled by environmental factors, is required for virulence. Here, we studied the effects of different stimuli on the morphology of two B. bassiana strains and compared the toxicities of culture filtrates. In addition, we explored the role of volatiles as quorum sensing-like signals during dimorphic transition. The killing assays in Caenorhabditis elegans (Nematoda: Rhabditidae) showed that strain AI2 isolated from a mycosed insect cadaver had higher toxicity than strain AS5 isolated from soil. Furthermore, AI2 showed earlier yeast-to-mycelial switching than AS5. However, an increase in inoculum size induced faster yeast-to-mycelium conversion in AS5 cells, suggesting a cell-density-dependent phenomenon. Gas chromatography-mass spectrometry (GC-MS) analyses showed that the fingerprint of the volatiles was strain-specific; however, during the morphological switching, an inverse relationship between the abundance of total terpenes and 3-methylbutanol was observed in both strains. Fungal exposure to 3-methylbutanol retarded the yeast-to-mycelium transition. Hence, this study provides evidence that volatile compounds are associated with critical events in the life cycle of B. bassiana.

Farnesol as an antifungal agent: comparisons among MTL and MTLα haploid and diploid Candida albicans and Saccharomyces cerevisiae

Aims: Farnesol was identified 20 years ago in a search for Candida albicans quorum sensing molecules (QSM), but there is still uncertainty regarding many aspects of its mode of action including whether it employs farnesol transport mechanisms other than diffusion. Based on the structural similarity between farnesol and the farnesylated portion of the MTL a pheromone, we explored the effects of ploidy and mating type locus (MTL) on the antifungal activity of exogenous farnesol. Methods and results: We approached this question by examining five MTL a and five MTLα haploid strains with regard to their farnesol sensitivity in comparison to six heterozygous MTL a/ α diploids. We examined the haploid and diploid strains for percent cell death after exposure of exponentially growing cells to 0-200 µM farnesol. The heterozygous (MTL a/α) diploids were tolerant of exogenous farnesol whereas the MTL a and MTLα haploids were on average 2- and 4-times more sensitive, respectively. In the critical range from 10-40 µM farnesol their cell death values were in the ratio of 1:2:4. Very similar results were obtained with two matched sets of MAT a, MATα, and MAT a/α Saccharomyces cerevisiae strains. Conclusion: We propose that the observed MTL dependence of farnesol is based on differentially regulated mechanisms of entry and efflux which determine the actual cellular concentration of farnesol. The mechanisms by which pathogens such as C. albicans tolerate the otherwise lethal effects of farnesol embrace a wide range of physiological functions, including MTL type, ubiquinone type (UQ6-UQ9), energy availability, and aerobic/anaerobic status.

Biological effects of trans, trans-farnesol in Leishmania amazonensis

Introduction

Farnesol, derived from farnesyl pyrophosphate in the sterols biosynthetic pathway, is a molecule with three unsaturations and four possible isomers. Candida albicans predominantly secretes the trans, trans-farnesol (t, t-FOH) isomer, known for its role in regulating the virulence of various fungi species and modulating morphological transition processes. Notably, the evolutionary divergence in sterol biosynthesis between fungi, including Candida albicans, and trypanosomatids resulted in the synthesis of sterols with the ergostane skeleton, distinct from cholesterol. This study aims to assess the impact of exogenously added trans, trans-farnesol on the proliferative ability of Leishmania amazonensis and to identify its presence in the lipid secretome of the parasite.

Methods

The study involved the addition of exogenous trans, trans-farnesol to evaluate its interference with the proliferation of L. amazonensis promastigotes. Proliferation, cell cycle, DNA fragmentation, and mitochondrial functionality were assessed as indicators of the effects of trans, trans-farnesol. Additionally, lipid secretome analysis was conducted, focusing on the detection of trans, trans-farnesol and related products derived from the precursor, farnesyl pyrophosphate. In silico analysis was employed to identify the sequence for the farnesene synthase gene responsible for producing these isoprenoids in the Leishmania genome.

Results

Exogenously added trans, trans-farnesol was found to interfere with the proliferation of L. amazonensis promastigotes, inhibiting the cell cycle without causing DNA fragmentation or loss of mitochondrial functionality. Despite the absence of trans, trans-farnesol in the culture supernatant, other products derived from farnesyl pyrophosphate, specifically α-farnesene and β-farnesene, were detected starting on the fourth day of culture, continuing to increase until the tenth day. Furthermore, the identification of the farnesene synthase gene in the Leishmania genome through in silico analysis provided insights into the enzymatic basis of isoprenoid production.

Discussion

The findings collectively offer the first insights into the mechanism of action of farnesol on L. amazonensis. While trans, trans-farnesol was not detected in the lipid secretome, the presence of α-farnesene and β-farnesene suggests alternative pathways or modifications in the isoprenoid metabolism of the parasite. The inhibitory effects on proliferation and cell cycle without inducing DNA fragmentation or mitochondrial dysfunction raise questions about the specific targets and pathways affected by exogenous trans, trans-farnesol. The identification of the farnesene synthase gene provides a molecular basis for understanding the synthesis of related isoprenoids in Leishmania. Further exploration of these mechanisms may contribute to the development of novel therapeutic strategies against Leishmania infections.

Ecological inducers of the yeast filamentous growth pathway reveal environment-dependent roles for pathway components

Signaling modules, such as mitogen-activated protein kinase (MAPK) pathways, are evolutionarily conserved drivers of cell differentiation and stress responses. In many fungal species including pathogens, MAPK pathways control filamentous growth, where cells differentiate into an elongated cell type. The convenient model budding yeast Saccharomyces cerevisiae undergoes filamentous growth by the filamentous growth (fMAPK) pathway; however, the inducers of the pathway remain unclear, perhaps because pathway activity has been mainly studied in laboratory conditions. To address this knowledge gap, an ecological framework was used, which uncovered new fMAPK pathway inducers, including pectin, a material found in plants, and the metabolic byproduct ethanol. We also show that induction by a known inducer of the pathway, the non-preferred carbon source galactose, required galactose metabolism and induced the pathway differently than glucose limitation or other non-preferred carbon sources. By exploring fMAPK pathway function in fruit, we found that induction of the pathway led to visible digestion of fruit rind through a known target, PGU1, which encodes a pectolytic enzyme. Combinations of inducers (galactose and ethanol) stimulated the pathway to near-maximal levels, which showed dispensability of several fMAPK pathway components (e.g., mucin sensor, p21-activated kinase), but not others (e.g., adaptor, MAPKKK) and required the Ras2-protein kinase A pathway. This included a difference between the transcription factor binding partners for the pathway, as Tec1p, but not Ste12p, was partly dispensable for fMAPK pathway activity. Thus, by exploring ecologically relevant stimuli, new modes of MAPK pathway signaling were uncovered, perhaps revealing how a pathway can respond differently to specific environments. IMPORTANCE Filamentous growth is a cell differentiation response and important aspect of fungal biology. In plant and animal fungal pathogens, filamentous growth contributes to virulence. One signaling pathway that regulates filamentous growth is an evolutionarily conserved MAPK pathway. The yeast Saccharomyces cerevisiae is a convenient model to study MAPK-dependent regulation of filamentous growth, although the inducers of the pathway are not clear. Here, we exposed yeast cells to ecologically relevant compounds (e.g., plant compounds), which identified new inducers of the MAPK pathway. In combination, the inducers activated the pathway to near-maximal levels but did not cause detrimental phenotypes associated with previously identified hyperactive alleles. This context allowed us to identify conditional bypass for multiple pathway components. Thus, near-maximal induction of a MAPK pathway by ecologically relevant inducers provides a powerful tool to assess cellular signaling during a fungal differentiation response.

Lessons on fruiting body morphogenesis from genomes and transcriptomes of Agaricomycetes

Fruiting bodies (sporocarps, sporophores or basidiomata) of mushroom-forming fungi (Agaricomycetes) are among the most complex structures produced by fungi. Unlike vegetative hyphae, fruiting bodies grow determinately and follow a genetically encoded developmental program that orchestrates their growth, tissue differentiation and sexual sporulation. In spite of more than a century of research, our understanding of the molecular details of fruiting body morphogenesis is still limited and a general synthesis on the genetics of this complex process is lacking. In this paper, we aim at a comprehensive identification of conserved genes related to fruiting body morphogenesis and distil novel functional hypotheses for functionally poorly characterised ones. As a result of this analysis, we report 921 conserved developmentally expressed gene families, only a few dozens of which have previously been reported to be involved in fruiting body development. Based on literature data, conserved expression patterns and functional annotations, we provide hypotheses on the potential role of these gene families in fruiting body development, yielding the most complete description of molecular processes in fruiting body morphogenesis to date. We discuss genes related to the initiation of fruiting, differentiation, growth, cell surface and cell wall, defence, transcriptional regulation as well as signal transduction. Based on these data we derive a general model of fruiting body development, which includes an early, proliferative phase that is mostly concerned with laying out the mushroom body plan (via cell division and differentiation), and a second phase of growth via cell expansion as well as meiotic events and sporulation. Altogether, our discussions cover 1 480 genes of Coprinopsis cinerea, and their orthologs in Agaricus bisporus, Cyclocybe aegerita, Armillaria ostoyae, Auriculariopsis ampla, Laccaria bicolor, Lentinula edodes, Lentinus tigrinus, Mycena kentingensis, Phanerochaete chrysosporium, Pleurotus ostreatus, and Schizophyllum commune, providing functional hypotheses for ~10 % of genes in the genomes of these species. Although experimental evidence for the role of these genes will need to be established in the future, our data provide a roadmap for guiding functional analyses of fruiting related genes in the Agaricomycetes. We anticipate that the gene compendium presented here, combined with developments in functional genomics approaches will contribute to uncovering the genetic bases of one of the most spectacular multicellular developmental processes in fungi. Citation: Nagy LG, Vonk PJ, Künzler M, Földi C, Virágh M, Ohm RA, Hennicke F, Bálint B, Csernetics Á, Hegedüs B, Hou Z, Liu XB, Nan S, M. Pareek M, Sahu N, Szathmári B, Varga T, Wu W, Yang X, Merényi Z (2023). Lessons on fruiting body morphogenesis from genomes and transcriptomes of Agaricomycetes. Studies in Mycology 104: 1-85. doi: 10.3114/sim.2022.104.01.

Characterization of an allosteric inhibitor of fungal-specific C-24 sterol methyltransferase to treat Candida albicans infections

Filamentation is an important virulence factor of the pathogenic fungus Candida albicans. The abolition of Candida albicans hyphal formation by disrupting sterol synthesis is an important concept for the development of antifungal drugs with high safety. Here, we conduct a high-throughput screen using a C. albicans strain expressing green fluorescent protein-labeled Dpp3 to identify anti-hypha agents by interfering with ergosterol synthesis. The antipyrine derivative H55 is characterized to have minimal cytotoxicity and potent inhibition of C. albicans hyphal formation in multiple cultural conditions. H55 monotherapy exhibits therapeutic efficacy in mouse models of azole-resistant candidiasis. H55 treatment increases the accumulation of zymosterol, the substrate of C-24 sterol methyltransferase (Erg6). The results of enzyme assays, photoaffinity labeling, molecular simulation, mutagenesis, and cellular thermal shift assays support H55 as an allosteric inhibitor of Erg6. Collectively, H55, an inhibitor of the fungal-specific enzyme Erg6, holds potential to treat C. albicans infections.

Disparity in pseudohyphal morphogenic switching response to the quorum sensing molecule 2-phenylethanol in commercial brewing strains of Saccharomyces cerevisiae

Saccharomyces cerevisiae can undergo filamentous growth in response to specific environmental stressors, particularly nitrogen-limitation, whereby cells undergo pseudohyphal differentiation, a process where cells transition from a singular ellipsoidal appearance to multicellular filamentous chains from the incomplete scission of the mother-daughter cells. Previously, it was demonstrated that filamentous growth in S. cerevisiae is co-regulated by multiple signaling networks, including the glucose-sensing RAS/cAMP-PKA and SNF pathways, the nutrient-sensing TOR pathway, the filamentous growth MAPK pathway, and the Rim101 pathway, and can be induced by quorum-sensing aromatic alcohols, such as 2-phenylethanol. However, the prevalent research on the yeast-pseudohyphal transition and its induction by aromatic alcohols in S. cerevisiae has been primarily limited to the strain Σ1278b. Due to the prospective influence of quorum sensing on commercial fermentation, the native variation of yeast-to-filamentous phenotypic transition and its induction by 2-phenylethanol in commercial brewing strains was investigated. Image analysis software was exploited to enumerate the magnitude of whole colony filamentation in 16 commercial strains cultured on nitrogen-limiting SLAD medium; some supplemented with exogenous 2-phenylethanol. The results demonstrate that phenotypic switching is a generalized, highly varied response occurring only in select brewing strains. Nevertheless, strains exhibiting switching behavior altered their filamentation response to exogenous concentrations of 2-phenylethanol.

Effect of Farnesol in Trichoderma Physiology and in Fungal-Plant Interaction

Farnesol is an isoprenoid intermediate in the mevalonate (MVA) pathway and is produced by the dephosphorylation of farnesyl diphosphate. Farnesol plays a central role in cell growth and differentiation, controls production of ubiquinone and ergosterol, and participates in the regulation of filamentation and biofilm formation. Despite these important functions, studies of farnesol in filamentous fungi are limited, and information on its effects on antifungal and/or biocontrol activity is scarce. In the present article, we identified the Trichoderma harzianum gene dpp1, encoding a diacylglycerol pyrophosphatase that catalyzes production of farnesol from farnesol diphosphate. We analyzed the function of dpp1 to address the importance of farnesol in Trichoderma physiology and ecology. Overexpression of dpp1 in T. harzianum caused an expected increase in farnesol production as well as a marked change in squalene and ergosterol levels, but overexpression did not affect antifungal activity. In interaction with plants, a dpp1-overexpressing transformant acted as a sensitizing agent in that it up-regulated expression of plant defense salicylate-related genes in the presence of a fungal plant pathogen. In addition, toxicity of farnesol on Trichoderma and plants was examined. Finally, a phylogenetic study of dpp1 was performed to understand its evolutionary history as a primary metabolite gene. This article represents a step forward in the acquisition of knowledge on the role of farnesol in fungal physiology and in fungus-environment interactions.

HYPHAEdelity: a quantitative image analysis tool for assessing peripheral whole colony filamentation

The yeast Saccharomyces cerevisiae, also known as brewer's yeast, can undergo a reversible stress-responsive transition from individual ellipsoidal cells to chains of elongated cells in response to nitrogen- or carbon starvation. Whole colony morphology is frequently used to evaluate phenotypic switching response; however, quantifying two-dimensional top-down images requires each pixel to be characterized as belonging to the colony or background. While feasible for a small number of colonies, this labor-intensive assessment process is impracticable for larger datasets. The software tool HYPHAEdelity has been developed to semi-automate the assessment of two-dimensional whole colony images and quantify the magnitude of peripheral whole colony yeast filamentation using image analysis tools intrinsic to the OpenCV Python library. The software application functions by determining the total area of filamentous growth, referred to as the f-measure, by subtracting the area of the inner colony boundary from the outer-boundary area associated with hyphal projections. The HYPHAEdelity application was validated against automated and manually pixel-counted two-dimensional top-down images of S. cerevisiae colonies exhibiting varying degrees of filamentation. HYPHAEdelity's f-measure results were comparable to areas determined through a manual pixel enumeration method and found to be more accurate than other whole colony filamentation software solutions.

Cultivated human intestinal fungus Candida metapsilosis M2006B attenuates colitis by secreting acyclic sesquiterpenoids as FXR agonists

OBJECTIVE

Dysbiosis of the intestinal fungal community has been observed in inflammatory bowel disease (IBD); however, its potential role in IBD development and prevention remains unclear. Here, we explored the biological effects and molecular mechanisms of intestinal fungi isolated from human faeces on colitis in mice.

DESIGN

Intestinal fungal strains with differential abundance in IBD were cultivated in human faeces and their effects on various mouse models of experimental colitis were evaluated. In addition, the bioactive metabolites secreted by the target fungus were accurately identified and their pharmacological effects and potential molecular targets were investigated in vitro and in vivo.

RESULTS

The abundance of Candida spp was significantly higher in patients with IBD. After large-scale human intestinal fungal cultivation and functional analysis, Candida metapsilosis M2006B significantly attenuated various models of experimental colitis in wild-type, antibiotic-treated, germ-free, and IL10^-/- mice by activating farnesoid X receptor (FXR). Among the seven acyclic sesquiterpenoids (F1-F7) identified as major secondary metabolites of M2006B, F4 and F5 attenuated colitis in mice by acting as novel FXR agonists. The therapeutic effects of M2006B and its metabolites on colitis via specific FXR activation were confirmed in Fxr ^-/- mice.

CONCLUSION

This study revealed that C. metapsilosis M2006B significantly attenuated colitis in mice and identified two acyclic sesquiterpenoids (F4 and F5) as major active metabolites of M2006B. Notably, these metabolites were able to effectively treat experimental colitis by selectively activating FXR. Together, this study demonstrates that M2006B could be a beneficial intestinal fungus for treating and preventing IBD.

Quantitative assay for farnesol and the aromatic fusel alcohols from the fungus Candida albicans

Abstract

The dimorphic fungus Candida albicans is a commensal and opportunistic fungal pathogen of humans. It secretes at least four small lipophilic molecules, farnesol and three aromatic fusel alcohols. Farnesol has been identified as both a quorum sensing molecule (QSM) and a virulence factor. Our gas chromatography (GC)-based assay for these molecules exhibits high throughput, prevention of analyte loss by avoiding filtration and rotary evaporation, simultaneous cell lysis and analyte extraction by ethyl acetate, and the ability to compare whole cultures with their cell pellets and supernatants. Farnesol synthesis and secretion were separable phenomena and pellet:supernatant ratios for farnesol were high, up to 12:1. The assay was validated in terms of precision, specificity, ruggedness, accuracy, solution stability, detection limits (DL), quantitation limits (QL), and dynamic range. The DL for farnesol was 0.02 ng/µl (0.09 µM). Measurement quality was assessed by the relative error of the whole culture versus the sum of pellet and supernatant fractions (WPS). C. albicans strain SC5314 grown at 30 °C in complex and defined media (YPD and mRPMI) was assayed in biological triplicate 17 times over 3 days. Farnesol and the three aromatic fusel alcohols can be measured in the same assay. The levels of all four are greatly altered by the growth medium chosen. Significantly, the three fusel alcohols are synthesized during stationary phase, not during growth. They are secreted quickly without being retained in the cell pellet and may accumulate up to mM concentrations.

Key points

• Quantitative analysis of both intra- and extracellular farnesol, and aromatic fusel oils.

• High throughput, whole culture assay with simultaneous lysis and extraction.

• Farnesol secretion and synthesis are distinct and separate events.

Synbiotic Musa acuminata skin extract and Streptococcus salivarius K12 inhibit candida species biofilm formation

This study aimed to determine the effect of synbiotic Musa acuminata skin extract (MASE) and Streptococcus salivarius K12 (K12) on Candida species biofilm formation. Liquid chromatography quadrupole time-of-flight (LC-Q-TOF-MS) was conducted to characterize MASE. To determine the effect of synbiotic on Candida biofilm, 200 µL of RPMI-1640 containing Candida, K12, and MASE were pipetted into the same well and incubated at 37 °C for 72 h. A similar protocol was repeated with K12 or MASE to determine the probiotic and prebiotic effects, respectively. Dimorphism, biofilm biomass, and Candida total cell count (TCC) were determined. A total of 60 compounds were detected in MASE. C. albicans (ALT5) and Candida lusitaniae exhibited the highest reduction in biofilm biomass when co-cultured with prebiotic (77.70 ± 7.67%) and synbiotic (97.73 ± 0.28%), respectively. All Candida spp. had decreased TCC and hyphae when co-cultured with synbiotic. In conclusion, MASE and K12 inhibit Candida biofilm formation.

Surface modification of poly(methyl-methacrylate) with farnesol to prevent Candida biofilm formation

Candida albicans promotes biofilm formation on dentures, which compromises the use of poly(methyl-methacrylate) (PMMA) as a dental material. Farnesol (FAR), a natural compound that prevents C. albicans filamentation and biofilm formation, was incorporated into the PMMA matrix, to obtain antifungal PMMA_FAR materials. The tested concentrations (0·0125% and 0·4%) of FAR, 24 h after incubation on YPD agar, inhibited filamentation of C. albicans. PMMA was modified with different FAR concentrations (3-12%), and physicochemical properties, antifungal activity and cytotoxicity of these modified materials (PMMA_FAR) were tested. The presence of FAR in PMMA_FAR composites was verified by Fourier-transform infrared spectroscopy (FT-IR). Incorporation of FAR into the polymeric matrix significantly decreased hydrophilicity at all tested concentrations and significantly reduced biofilm and planktonic cells metabolic activity in the early stage of biofilm formation at ≥6% FAR in PMMA. PMMA_FAR composites with <9% FAR were non-toxic. Modification of PMMA with FAR is a good strategy for reducing C. albicans biofilm formation on dentures.

The Antibiofilm Role of Biotics Family in Vaginal Fungal Infections

“Unity in strength” is a notion that can be exploited to characterize biofilms as they bestow microbes with protection to live freely, escalate their virulence, confer high resistance to therapeutic agents, and provide active grounds for the production of biofilms after dispersal. Naturally, fungal biofilms are inherently resistant to many conventional antifungals, possibly owing to virulence factors as their ammunitions that persistently express amid planktonic transition to matured biofilm state. These ammunitions include the ability to form polymicrobial biofilms, emergence of persister cells post-antifungal treatment and acquisition of resistance genes. One of the major disorders affecting vaginal health is vulvovaginal candidiasis (VVC) and its reoccurrence is termed recurrent VVC (RVVC). It is caused by the Candida species which include Candida albicans and Candida glabrata. The aforementioned Candida species, notably C. albicans is a biofilm producing pathogen and habitually forms part of the vaginal microbiota of healthy women. Latest research has implicated the role of fungal biofilms in VVC, particularly in the setting of treatment failure and RVVC. Consequently, a plethora of studies have advocated the utilization of probiotics in addressing these infections. Specifically, the excreted or released compounds of probiotics which are also known as postbiotics are being actively researched with vast potential to be used as therapeutic options for the treatment and prevention of VVC and RVVC. These potential sources of postbiotics are harnessed due to their proven antifungal and antibiofilm. Hence, this review discusses the role of Candida biofilm formation in VVC and RVVC. In addition, we discuss the application of pro-, pre-, post-, and synbiotics either individually or in combined regimen to counteract the abovementioned problems. A clear understanding of the role of biofilms in VVC and RVVC will provide proper footing for further research in devising novel remedies for prevention and treatment of vaginal fungal infections.

An Approach to Minimize Tumour Proliferation by Reducing the Formation of Components for Cell Membrane

Isoprenoids are natural compounds essential for a great number of cellular functions. One of them is farnesol (FOH), which can reduce cell proliferation, but its low solubility in aqueous solvents limits its possible clinical use as a pharmacological tool. One alternative is the use of cyclodextrins (CDs) which house hydrophobic molecules forming inclusion complexes. To assess FOH potential application in anticancer treatments, Sulfobutylated β-cyclodextrin Sodium Salt (SBE-β-CD) was selected, due to it has high solubility, approbation by the FDA, and numerous studies that ensure its safety to be administered parenterally or orally without nephrotoxicity associated. The therapeutic action of farnesol and complex were studied in different carcinoma cells, compared with a normal cell line. Farnesol showed selectivity, affecting the viability of colon and liver cancer cells more than in breast cancer cells and fibroblasts. All cells suffered apoptosis after being treated with 150 μM of free FOH, but the complex reduced their cell viability between 50 and 75%. Similar results were obtained for both types of isomers, and the addition of phosphatidylcholine reverses this effect. Finally, cell cycle analysis corroborates the action of FOH as inducer of a G0/G1 phase; when the cells were treated using the complex form, this viability was reduced, reaching 50% in the case of colon and liver, 60% in fibroblasts, and only 75% in breast cancer.

Anti-Candida Properties of Gossypium hirsutum L.: Enhancement of Fungal Growth, Biofilm Production and Antifungal Resistance

(1) Background: Candida is a genus of yeasts with notable pathogenicity and significant ability to develop antimicrobial resistance. Gossypium hirsutum L., a medicinal plant that is traditionally used due to its antimicrobial properties, has demonstrated significant antifungal activity. Therefore, this study investigated the chemical composition and anti-Candida effects of aqueous (AELG) and hydroethanolic (HELG) extracts obtained from the leaves of this plant. (2) Methods: The extracts were chemically characterized by UPLC–QTOF-MS/MS, and their anti-Candida activities were investigated by analyzing cell viability, biofilm production, morphological transition, and enhancement of antifungal resistance. (3) Results: The UPLC–QTOF-MS/MS analysis revealed the presence of twenty-one compounds in both AELG and HELG, highlighting the predominance of flavonoids. The combination of the extracts with fluconazole significantly reduced its IC₅₀ values against Candida albicans INCQS 40006, Candida tropicalis INCQS 40042, and C. tropicalis URM 4262 strains, indicating enhanced antifungal activity. About biofilm production, significant inhibition was observed only for the AELG-treated C. tropicalis URM 4262 strain in comparison with the untreated control. Accordingly, this extract showed more significant inhibitory effects on the morphological transition of the INCQS 40006 and URM 4387 strains of C. albicans (4) Conclusions: Gossypium hirsutum L. presents promising antifungal effects, that may be potentially linked to the combined activity of chemical constituents identified in its extracts.

Development, characterization and in vitro-in vivo evaluation of Farnesol loaded niosomal gel for applications in oral candidiasis treatment

Objectives

The aim of the study was to formulate and characterize the farnesol loaded niosomes comprising gel formulation and perform their in vitro–in vivo evaluation for applications in the treatment of oral candidiasis infections.

Methods

Various gelling systems were evaluated for their rheological and stability properties. The formulation was statistically optimized using experimental design method (Box-Behnken). Transmission electron microscopy (TEM) and Atomic force microscopy (AFM) were used to observe the niosomal surface morphology. Centrifugation method and dialysis method were used to find out the % entrapment efficiency (%EE) and in-vitro release of Farnesol, respectively. In-vitro antifungal effect and cell biocompatibility of the Farnesol loaded niosomal gel was also performed using Candida albicans (C. albicans) as the model organism and epithelial cell line (SW480) by MTT cytotoxicity assay. In-vivo skin irritation test was performed on rabbit skin.

Key findings

Farnesol loaded niosomes were integrated into polymeric gel solution. The optimized formulation demonstrated acceptable % EE (>80%) and an optimum particle size (168.8 nm) along with a sustained release and a long-term storage stability for up to a period of 6 months. TEM and AFM observations displayed a spherical niosome morphology. Farnesol niosomal gel showed a higher antifungal efficacy, ex-vivo skin permeation and deposition in comparison to plain farnesol solution. The niosomal gel also showed negligible cytotoxicity to normal cells citing biocompatibility and was found to be non-toxic and non-irritant to rabbit skin.

Conclusions

This novel niosome loaded gel-based formulation could make the oral candidiasis healing process more efficient while improving patient compliance. With the optimized methodology used in this work, such formulation approaches can become an efficient, industrially scalable, and cost-effective alternatives to the existing conventional formulations.

Fungal diseases: Oral dysbiosis in susceptible hosts

The oral cavity is colonized by a large number of microorganisms that are referred to collectively as the oral microbiota. These indigenous microorganisms have evolved in symbiotic relationships with the oral mucosal immune system and are involved in maintaining homeostasis in the oral cavity. Although Candida species are commonly found in the healthy oral cavity without causing infection, these fungi can become pathogenic. Recents advances indicate that the development of oral candidiasis is driven both by Candida albicans overgrowth in a dysbiotic microbiome and by disturbances in the host's immune system. Perturbation of the oral microbiota triggered by host-extrinsic (ie, medications), host-intrinsic (ie, host genetics), and microbiome-intrinsic (ie, microbial interactions) factors may increase the risk of oral candidiasis. In this review, we provide an overview of the oral mycobiome, with a particular focus on the interactions of Candida albicans with some of the most common oral bacteria and the oral mucosal immune system. Also, we present a summary of our current knowledge of the host-intrinsic and host-extrinsic factors that can predispose to oral candidiasis.

Quorum Sensing in Fungal Species

Quorum sensing (QS) is one of the most studied cell-cell communication mechanisms in fungi. Research in the last 20 years has explored various fungal QS systems that are involved in a wide range of biological processes, especially eukaryote- or fungus-specific behaviors, mirroring the significant contribution of QS regulation to fungal biology and evolution. Based on recent progress, we summarize in this review fungal QS regulation, with an emphasis on its functional role in behaviors unique to fungi or eukaryotes. We suggest that using fungi as genetically amenable eukaryotic model systems to address why and how QS regulation is integrated into eukaryotic reproductive strategies and molecular or cellular processes could be an important direction for QS research. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Volatile organic compounds: from figurants to leading actors in fungal symbiosis

Symbiosis involving two (or more) prokaryotic and/or eukaryotic partners is extremely widespread in nature, and it has performed, and is still performing, a key role in the evolution of several biological systems. The interaction between symbiotic partners is based on the emission and perception of a plethora of molecules, including volatile organic compounds (VOCs), synthesized by both prokaryotic and eukaryotic (micro)organisms. VOCs acquire increasing importance since they spread above and below ground and act as infochemicals regulating a very complex network. In this work we review what is known about the VOCs synthesized by fungi prior to and during the interaction(s) with their partners (either prokaryotic or eukaryotic) and their possible role(s) in establishing and maintaining the symbiosis. Lastly, we also describe the potential applications of fungal VOCs from different biotechnological perspectives, including medicinal, pharmaceutical and agronomical.

Farnesol: An approach on biofilms and nanotechnology

Biofilms are important virulence factor in infections caused by microorganisms because of its complex structure, which provide resistance to conventional antimicrobials. Strategies involving the use of molecules capable of inhibiting their formation and also act synergistically with conventional drugs have been explored. Farnesol is a molecule present in essential oils and produced by Candida albicans as a quorum sensing component. This sesquiterpene presents inhibitory properties in the formation of microbial biofilms and synergism with antimicrobials used in clinical practice, and can be exploited even for eradication of biofilms formed by drug-resistant microorganisms. Despite this, farnesol has physical and chemical characteristics that can limit its use, such as high hydrophobicity and volatility. Therefore, nanotechnology may represent an option to improve the efficiency of this molecule in high complex environments such as biofilms. Nanostructured systems present important results in the improvement of treatment with different commercial drugs and molecules with therapeutic or preventive potential. The formation of nanoparticles offers advantages such as protection of the incorporated drugs against degradation, improved biodistribution and residence time in specific treatment sites. The combination of farnesol with nanotechnology may be promising for the development of more effective antibiofilm therapies, as it can improve its solubility, reduce volatility, and increase bioavailability. This review summarizes existing data about farnesol, its action on biofilms, and discusses its encapsulation in nanostructured systems.

LAY SUMMARY

Farnesol is a natural compound that inhibits the formation of biofilms from different microbial species. The encapsulation of this molecule in nanoparticles is a promising alternative for the development of more effective therapies against biofilms.

Mechanisms of Candida Resistance to Antimycotics and Promising Ways to Overcome It: The Role of Probiotics

Pathogenic Candida and infections caused by those species are now considered as a serious threat to public health. The treatment of candidiasis is significantly complicated by the increasing resistance of pathogenic strains to current treatments and the stagnant development of new antimycotic drugs. Many species, such as Candida auris, have a wide range of resistance mechanisms. Among the currently used synthetic and semi-synthetic antifungal drugs, the most effective are azoles, echinocandins, polyenes, nucleotide analogs, and their combinations. However, the use of probiotic microorganisms and/or the compounds they produce is quite promising, although underestimated by modern pharmacology, to control the spread of pathogenic Candida species.

Dual identities for various alcohols in two different yeasts

Most of the yeast bypasses the developmental stage from simple unicellular yeast to elongated structure like hyphae. Regulation of this transition is governed by various quorum sensing and signalling molecules produced under different conditions of growth, that differ significantly, both physiologically and chemically. The evidence of fungal quorum sensing was uncovered ten years ago after the discovery of farnesol as first eukaryotic quorum sensing molecules in Candida albicans. In addition to farnesol, tyrosol was identified as second quorum sensing molecules in C. albicans controlling physiological activities. After the discovery of farnesol and tyrosol, regulation of morphogenesis through the production of chemical signalling molecules such as isoamyl alcohol, 2-phenylethyl alcohol, 1-dodecanol, E-nerolidol, etc. is reported in C. albicans. Some of the evidence suggests that the budding yeast Saccharomyces cerevisiae exhibits this type of regulation and the signals are regulated by aromatic alcohols which are the end product of amino acid metabolism. The effects of these molecules on morphogenesis are not similar in both yeasts, making comparisons hard. It is hypothesized that these signals works in microorganisms to derive a competitive advantage. Here, we present an example for utilization of competitive strategy by C. albicans and S. cerevisiae over other microorganisms.

Longer Ubiquinone Side Chains Contribute to Enhanced Farnesol Resistance in Yeasts

Ubiquinones (UQ) are intrinsic lipid components of many membranes. Besides their role in electron-transfer reactions there is evidence for them acting as free radical scavengers, yet their other roles in biological systems have received little study. The dimorphic fungal pathogen Candida albicans secretes farnesol as both a virulence factor and a quorum-sensing molecule. Thus, we were intrigued by the presence of UQ9 isoprenologue in farnesol-producing Candida species while other members of this genera harbor UQ7 as their major electron carrier. We examined the effect of UQ side chain length in Saccharomyces cerevisiae and C. albicans with a view towards identifying the mechanisms by which C. albicans protects itself from the high levels of farnesol it secretes, levels that are toxic to many other fungi including S. cerevisiae. In this study, we identify UQ9 as the major UQ isoprenoid in C. albicans, regardless of growth conditions or cell morphology. A S. cerevisiae model yeast engineered to make UQ9 instead of UQ6 was 4-5 times more resistant to exogenous farnesol than the parent yeast and this resistance was accompanied by greatly reduced reactive oxygen species (ROS) production. The resistance provided by UQ9 is specific for farnesol in that it does not increase resistance to high salt (1M NaCl) or other oxidants (5 mM H₂O₂ or 1 mM menadione). Additionally, the protection provided by UQ9 appears to be structural rather than transcriptional; UQ9 does not alter key transcriptional responses to farnesol stress. Here, we propose a model in which the longer UQ side chains are more firmly embedded in the mitochondrial membrane making them harder to pry out, so that in the presence of farnesol they remain functional without producing excess ROS. C. albicans and Candida dubliniensis evolved to use UQ9 rather than UQ7 as in other Candida species or UQ6 as in S. cerevisiae. This adaptive mechanism highlights the significance of UQ side chains in farnesol production and resistance quite apart from being an electron carrier in the respiratory chain.

Potential antibiofilm activity of farnesol-loaded poly(DL-lactide-co-glycolide) (PLGA) nanoparticles against Candida albicans

Candida species are ubiquitous fungal pathogens and are the most common causes of mucosal and invasive fungal infections in humans. Especially Candida albicans commonly resides as a commensal in the mucosal tissues of approximately half of the human population. When the balance of the normal flora is disrupted or the immune defenses are compromised, Candida species can become pathogenic, often causing recurrent disease in susceptible individuals.

The treatments available for Candida infection are commonly drug-based and can involve topical and systemic antifungal agents. However, the use of standard antifungal therapies can be limited because of toxicity, low efficacy rates, and drug resistance. Candida species ability to produce drug-resistant biofilm is an important factor in human infections, because microorganisms within biofilm benefit from various advantages over their planktonic counterparts including protection from antimicrobials and chemicals. These limitations emphasize the need to develop new and more effective antifungal agents. Natural products are attractive alternatives for this purpose due to their broad spectrum of biological activities. Farnesol is produced by many microorganisms and found in some essential oils. It has also a great attention as a quorum-sensing molecule and virulence factor. It has also antimicrobial potential due to its inhibitory effects on various bacteria and fungi. However, as it is a hydrophobic component, its solubility and biofilm inhibiting properties are limited.

To overcome these shortcomings, nanoparticle-based drug delivery systems have been successfully used. For this purpose, especially using biodegradable polymeric nanoparticles has gained increasing attention owing to their biocompatibility and minimal toxicity. Poly (DL-lactide-co-glycolide) (PLGA) is the most widely used polymer in this area. In this study, farnesol is loaded to PLGA nanoparticles (F-PLGA NPs) by emulsion evaporation method and characterized by DLS, TEM, and FT-IR analyses. Our TEM findings indicate that the sizes of F-PLGA NPs are approximately 140 nm. The effects of F-PLGA NPs on planktonic cells and biofilm formation of C. albicans were compared with effects of farnesol alone. Farnesol inhibits the growth at a range of 53% at a concentration of 2.5 μL compared to the control group. This rate is 45% for F-PLGA NPs at the same concentration. However, although farnesol amount in F-PLGA is approximately 22.5% of the total volume, the observed effect is significant. In TEM examinations, planktonic Candida cells treated with farnesol showed relatively regular ultrastructural morphology. Few membrane and wall damage and electron density in the cytoplasm were determined. In F-PLGA NP-treated cells, increased irregular cell morphology, membrane and wall damages, and large vacuoles are observed. Our SEM and XTT data suggest that F-PLGA NPs can reduce the biofilm formation at lower concentrations than farnesol alone 57%, and our results showed that F-PLGA NPs are effective and biocompatible alternatives for inhibiting growth and biofilm formation of C. albicans, but detailed studies are needed.

Effect of cell-free spent media prepared from Aggregatibacter actinomycetemcomitans on the growth of Candida albicans and Streptococcus mutans in co-species biofilms

This study explored the influence of cell-free spent media prepared from Aggregatibacter actinomycetemcomitans LuxS mutant (Aa-LuxS), its wild type strain (Aa-WT), and the laboratory strain (Aa-Y4), on the interaction between Candida albicans and Streptococcus mutans while growing in co-species biofilm for 48 h. By analyzing the results of crystal violet staining, [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide] (MTT) assays, and quantitative real-time polymerase chain reaction (qPCR), we found that the presence of Aa-LuxS in treated biofilms did not affect biofilm development, while added Aa-WT or Aa-Y4 resulted in a significant decrease in both biofilm mass and the number of cells. The inhibitory effect of Aa-WT or Aa-Y4 was not dependent on the protein concentration in the spent media tested (1 and 10%). Gene transcription analyses indicated that Aa-WT/Aa-Y4 exhibits comparable inhibitory effects on the expression of hyphal-associated genes (ALS3 and HWP1), but not on the expression of YWP1, which encodes a yeast form of C. albicans. In contrast, except for gtfD, the expression of S. mutans gtfB/C genes encoding glucosyltransferase was not affected in Aa-WT and Aa-Y4 treated biofilms compared to the levels found in Aa-LuxS treated biofilms. Our results indicate that AI-2-containing spent media derived from Aa can reduce biofilm biomass without significantly inhibiting the survival rate of S. mutans.

Fungal Quorum-Sensing Molecules: A Review of Their Antifungal Effect against Candida Biofilms

The number of effective therapeutic strategies against biofilms is limited; development of novel therapies is urgently needed to treat a variety of biofilm-associated infections. Quorum sensing is a special form of microbial cell-to-cell communication that is responsible for the release of numerous extracellular molecules, whose concentration is proportional with cell density. Candida-secreted quorum-sensing molecules (i.e., farnesol and tyrosol) have a pivotal role in morphogenesis, biofilm formation, and virulence. Farnesol can mediate the hyphae-to-yeast transition, while tyrosol has the opposite effect of inducing transition from the yeast to hyphal form. A number of questions regarding Candida quorum sensing remain to be addressed; nevertheless, the literature shows that farnesol and tyrosol possess remarkable antifungal and anti-biofilm effect at supraphysiological concentration. Furthermore, previous in vitro and in vivo data suggest that they may have a potent adjuvant effect in combination with certain traditional antifungal agents. This review discusses the most promising farnesol- and tyrosol-based in vitro and in vivo results, which may be a foundation for future development of novel therapeutic strategies to combat Candida biofilms.

Can community-based signalling behaviour in Saccharomyces cerevisiae be called quorum sensing? A critical review of the literature

Quorum sensing is a well-described mechanism of intercellular signalling among bacteria, which involves cell-density-dependent chemical signal molecules. The concentration of these quorum-sensing molecules increases in proportion to cell density until a threshold value is exceeded, which triggers a community-wide response. In this review, we propose that intercellular signalling mechanisms can be associated with a corresponding ecological interaction type based on similarities between how the interaction affects the signal receiver and producer. Thus, we do not confine quorum sensing, a specific form of intercellular signalling, to only cooperative behaviours. Instead, we define it as cell-density-dependent responses that occur at a critical concentration of signal molecules and through a specific signalling pathway. For fungal species, the medically important yeast Candida albicans has a well-described quorum sensing system, while this system is not well described in Saccharomyces cerevisiae, which is involved in food and beverage fermentations. The more precise definition for quorum sensing proposed in this review is based on the studies suggesting that S. cerevisiae may undergo intercellular signalling through quorum sensing. Through this lens, we conclude that there is a lack of evidence to support a specific signalling mechanism and a critical signal concentration of these behaviours in S. cerevisiae, and, thus, these features require further investigation.

Intraspecies cell-cell communication in yeast

Although yeasts are unicellular microorganisms that can live independently, they can also communicate with other cells, in order to adapt to the environment. Two yeast species, the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe, engage in various kinds of intraspecies cell-cell communication using peptides and chemical molecules that they produce, constituting a sort of 'language'. Cell-cell communication is a fundamental biological process, and its ultimate purpose is to promote survival by sexual reproduction and acquisition of nutrients from the environment. This review summarizes what is known about intraspecies cell-cell communication mediated by molecules including mating pheromones, volatile gases, aromatic alcohols and oxylipins in laboratory strains of S. cerevisiae and S. pombe.

Unambiguous determination of farnesol and tyrosol in vaginal fluid using fast and sensitive UHPLC-MS/MS method

The new ultra-high performance liquid chromatography method with tandem mass spectrometry detection (UHPLC-MS/MS) has been optimized to allow fast, selective, and high-throughput analysis of two Candida albicans quorum sensing molecules (QSM), farnesol and tyrosol. The problem of the presence of the interference in the samples and system was successfully solved by careful optimization of chromatographic conditions. Charged hybrid stationary phase modified with pentafluorophenyl group and optimized gradient elution provided adequate separation selectivity and peak shapes. The impurity was identified as dibutyl phthalate and had the same m/z ions as farnesol leading to an important interference on selected reaction monitoring channel. Two different types of biological matrices originating from vaginal fluid, supernatant and sediment, were analysed. Micro-solid phase extraction in pipette tips was optimized for the selective isolation of QSM from the supernatant. The insufficient retention of farnesol on the extraction sorbent was improved when 1% of organic solvent was added prior to extraction, while the retention of tyrosol was only possible when using combined C8 and polymer sorbent type. Strong retention of farnesol had to be solved by increasing elution solvent strength and volume up to 600 μL. However, this approach did not allow the pretreatment of sediment samples due to the sorbent clogging. Therefore, our previously developed protein precipitation method was modified and validated to analyse the sediments. New developed UHPLC-MS/MS method provided suitable accuracy and precision for the determination of QSM in vaginal fluid while using only 50 μL sample volume and two different sample preparation methods.

Design and synthesis of new drugs inhibitors of Candida albicans hyphae and biofilm formation by upregulating the expression of TUP1 transcription repressor gene

Candida albicans is a common human fungal pathogen that causes disease ranging from superficial to lethal infections. C. albicans grows as budding yeast which can transform into hyphae in response to various environmental or biological stimuli. Although both forms have been associated with virulence, the hyphae form is responsible for the formation of multi-drug resistance biofilm. Here, new compounds were designed to selectively inhibit C. albicans hyphae formation without affecting human cells to afford sufficient safety. The newly designed 5-[3-substitued-4-(4-substituedbenzyloxy)-benzylidene]-2-thioxo-thiazolidin-4-one derivatives, named SR, showed very specific and effective inhibition activity against C. albicans hyphae formation. SR compounds caused hyphae inhibition activity at concentrations 10-40 fold lower than the concentration required to inhibit Candida yeast and bacterial growths. The anti-hyphae inhibition activities of SR compounds were via activation of the hyphae transcription repressor gene, TUP1. Correlation studies between the expression of TUP1 gene and the activity of SR compounds confirmed that the anti-C. albicans activities of SR compounds were via inhibition of hyphae formation. The newly designed SR compounds showed 10-40% haemolytic activity on human erythrocytes when compared to 100% haemolysis by 0.1% triton employed as positive control. Furthermore, theoretical prediction of absorption, distribution, metabolism, excretion, and toxicity (ADMET) of SR compounds confirmed their safety, efficient metabolism and possible oral bioavailability. With the minimal toxicity and significant activity of the newly-designed SR compounds, a future optimization of pharmaceutical formulation may develop a promising inhibitor of hyphal formation not only for C. albicans but also for other TUP1- dependent dimorphic fungal infections.

Evaluation of tyrosol and farnesol as inducer in pigment production by Monascus purpureus ATCC16365

This study focused on investigating the effect of exogenously applied two quorum sensing molecules (tyrosol and farnesol) on the synthesis of bioactive metabolites (pigments, lactic acid, ethanol, and citric acid) in Monascus purpureus ATCC16365. None of the tested concentrations (62.5, 125, 250, and 500 µl/L) of farnesol affected the synthesis of metabolites as well as cell growth. As with farnesol application, none of the tested concentrations (3.45, 6.9, 13.8, and 27.6 mg/L) of tyrosol caused a significant change in the synthesis of lactic acid and citric acid as well as cell growth. Conversely, all of the tested concentrations of tyrosol increased pigment synthesis but reduced ethanol synthesis, compared with the control. Maximum increases (3.16-, 2.68-, and 2.87-fold increase, respectively) in yellow, orange, and red pigment production were achieved, especially when 6.9-mg/L tyrosol was added to the culture on day 3. On the contrary, 6.9-mg/L tyrosol reduced the content of citrinin by approximately 51.5%. This is the first report on the effect of tyrosol and farnesol on the synthesis of Monascus metabolites. Due to potential properties, such as low price, nonhuman toxicity, promotion of pigment synthesis, and reduction in citrinin synthesis, tyrosol can be used as a novel inducer in the fermentative production of Monascus pigments.

Farnesol-Containing Macromolecular Systems for Antibiofilm Strategies

Farnesol is a natural sesquiterpenoid and an interesting quorum-sensing molecule. Its insolubility in water is the biggest obstacle to its application for bacterial biofilm treatments since it compromises the bioavailability. Recently, an increasing interest in farnesol encapsulation or loading in polymeric materials may be noted due to the prolonged action of the active macromolecular systems. In this short review, we present an overview of methods leading to improved interactions between farnesol and microbial biofilms.

Fluconazole resistance in Candida albicans is induced by Pseudomonas aeruginosa quorum sensing

Microorganisms employ quorum sensing (QS) mechanisms to communicate with each other within microbial ecosystems. Emerging evidence suggests that intraspecies and interspecies QS plays an important role in antimicrobial resistance in microbial communities. However, the relationship between interkingdom QS and antimicrobial resistance is largely unknown. Here, we demonstrate that interkingdom QS interactions between a bacterium, Pseudomonas aeruginosa and a yeast, Candida albicans, induce the resistance of the latter to a widely used antifungal fluconazole. Phenotypic, transcriptomic, and proteomic analyses reveal that P. aeruginosa's main QS molecule, N-(3-Oxododecanoyl)-L-homoserine lactone, induces candidal resistance to fluconazole by reversing the antifungal's effect on the ergosterol biosynthesis pathway. Accessory resistance mechanisms including upregulation of C. albicans drug-efflux, regulation of oxidative stress response, and maintenance of cell membrane integrity, further confirm this phenomenon. These findings demonstrate that P. aeruginosa QS molecules may confer protection to neighboring yeasts against azoles, in turn strengthening their co-existence in hostile polymicrobial infection sites.

Inhibition of Respiration of Candida albicans by Small Molecules Increases Phagocytosis Efficacy by Macrophages

Candida albicans adapts to various conditions in different body niches by regulating gene expression, protein synthesis, and metabolic pathways. These adaptive reactions not only allow survival but also influence the interaction with host cells, which is governed by the composition and structure of the fungal cell wall. Numerous studies had shown linkages between mitochondrial functionality, cell wall integrity and structure, and pathogenicity. Thus, we decided to inhibit single complexes of the respiratory chain of C. albicans and to analyze the resultant interaction with macrophages via their phagocytic activity. Remarkably, inhibition of the fungal bc₁ complex by antimycin A increased phagocytosis, which correlated with an increased accessibility of β-glucans. To contribute to mechanistic insights, we performed metabolic studies, which highlighted significant changes in the abundance of constituents of the plasma membrane. Collectively, our results reinforce the strong linkage between fungal energy metabolism and other components of fungal physiology, which also determine the vulnerability to immune defense reactions.IMPORTANCE The yeast Candida albicans is one of the major fungal human pathogens, for which new therapeutic approaches are required. We aimed at enhancements of the phagocytosis efficacy of macrophages by targeting the cell wall structure of C. albicans, as the coverage of the β-glucan layer by mannans is one of the immune escape mechanisms of the fungus. We unambiguously show that inhibition of the fungal bc₁ complex correlates with increased accessibilities of β-glucans and improved phagocytosis efficiency. Metabolic studies proved not only the known direct effects on reactive oxygen species (ROS) production and fermentative pathways but also the clear downregulation of the ergosterol pathway and upregulation of unsaturated fatty acids. The changed composition of the plasma membrane could also influence the interaction with the overlying cell wall. Thus, our work highlights the far-reaching relevance of energy metabolism, indirectly also for host-pathogen interactions, without affecting viability.

Optimum Inhibition of Amphotericin-B-Resistant Candida albicans Strain in Single- and Mixed-Species Biofilms by Candida and Non-Candida Terpenoids

Candida albicans is one of the most common human fungal pathogens and represents the most important cause of opportunistic mycoses worldwide. Surgical devices including catheters are easily contaminated with C. albicans via its formation of drug-resistant biofilms. In this study, amphotericin-B-resistant C. albicans strains were isolated from surgical devices at an intensive care center. The objective of this study was to develop optimized effective inhibitory treatment of resistant C. albicans by terpenoids, known to be produced naturally as protective signals. Endogenously produced farnesol by C. albicans yeast and plant terpenoids, carvacrol, and cuminaldehyde were tested separately or in combination on amphotericin-B-resistant C. albicans in either single- or mixed-infections. The results showed that farnesol did not inhibit hyphae formation when associated with bacteria. Carvacrol and cuminaldehyde showed variable inhibitory effects on C. albicans yeast compared to hyphae formation. A combination of farnesol with carvacrol showed synergistic inhibitory activities not only on C. albicans yeast and hyphae, but also on biofilms formed from single- and mixed-species and at reduced doses. The combined terpenoids also showed biofilm-penetration capability. The aforementioned terpenoid combination will not only be useful in the treatment of different resistant Candida forms, but also in the safe prevention of biofilm formation.

The Search for Quorum Sensing in Botrytis cinerea: Regulatory Activity of Its Extracts on Its Development

Botrytis cinerea is a phytopathogenic fungus that causes large crop and post-harvest losses. Therefore, new and effective strategies are needed to control the disease and to reduce resistance to fungicides. Modulating pathogenicity and virulence by manipulating microbial communication is a promising strategy. This communication mechanism, called Quorum Sensing (QS), has already been reported in bacteria and yeasts; however, it has not yet been studied in B. cinerea. To establish the existence of this biochemical process in B. cinerea, we prepared extracts at different growth times (D1-D12), which were applied to fresh cultures of the same fungi. The chemical analysis of the extracts obtained from several fermentations showed different compositions and biological activities. We confirmed the presence of several phytotoxins, as well as compounds 1-phenylethanol and 3-phenylpropanol. Day five extract (0.1%) inhibited conidia germination and elongation of germ tubes, day seven extract (1%) produced the greatest phytotoxic effect in tomato leaves, and day nine extract (0.1%) was a sporulation inhibitor. In contrast, the extracts from days 7, 9, and 12 of fermentation (0.1% and 0.01%) promoted pellet and biofilm formation. Sporulation was slightly induced at 0.01%, while at 0.1% there was a great inhibition. At the highest extract concentrations, a biocidal effect was detected, but at the lowest, we observed a QS-like effect, regulating processes such as filamentation, morphogenesis, and pathogenesis. These results of the biological activity and composition of extracts suggest the existence of a QS-like mechanism in B. cinerea, which could lead to new non-biocidal alternatives for its control through interference in the pathogenicity and virulence mechanisms of the fungi.

A novel, quorum sensor-infused liposomal drug delivery system suppresses Candida albicans biofilms

In contrast to the plethora of antibacterial agents, only a handful of antifungals are currently available to treat Candida albicans biofilm-associated infections. Additional novel antibiofilm strategies to eliminate C. albicans biofilm infections are needed. This study aims to improve the efficacy of a widely used azole, fluconazole by co-delivering it with a Pseudomonas aeruginosa quorum sensing molecule (QSM), N-3-oxo-dodecanoyl-L-homoserine lactone (C12AHL) in a liposomal formulation. C12AHL is known to inhibit C. albicans' morphological transition and biofilm formation. Four different formulations of liposomes with fluconazole (L-F), with C12AHL (L-H), with fluconazole and C12AHL (L-HF), and a drug-free control (L-C) were prepared using a thin-film hydration followed by extrusion method, and characterised. The effect of liposomes on colonising (90 min-24 h) and preformed (24 h) C. albicans biofilms were assessed using a standard biofilm assay. Biofilm viability (XTT reduction assay), biomass (Safranin-O staining) and architecture (confocal laser scanning microscopy, CLSM) were determined. Similar efficiencies of fluconazole entrapment were noticed in L-HF and L-F (11.74% vs 10.2%), however, L-HF released greater quantities of fluconazole compared to L-F during 24 h (4.27% vs 0.97%, P < 0.05). The entrapment and release of C12AHL was similar for L-H and L-HF liposomes (33.3% vs 33% and 88.9% vs 92.3% respectively). L-HF treated colonising, and preformed biofilms exhibited >80%, and 60% reduction in their respective viabilities at a fluconazole concentration as low as 5.5 µg/mL compared to 12% and 36%, respective reductions observed in L-F treated biofilms (P < 0.05). CLSM confirmed biofilm disruption, lack of hyphae, and reduction in biomass when treated with L-HF compared to other liposomal preparations. Liposomal co-delivery of C12AHL and fluconazole appears to suppress C. albicans biofilms through efficacious disruption of the biofilm, killing of constituent yeasts, and diminishing their virulence at a significantly lower antifungal dose. Therefore, liposomal co-formulation of C12AHL and fluconazole appears to be a promising approach to improve the efficacy of this common triazole against biofilm-mediated candidal infections.