Cell Wall Remodeling Enzymes Modulate Fungal Cell Wall Elasticity and Osmotic Stress Resistance

Isolation, characterization and antimicrobial properties of hepatopancreas lectin of the freshwater crab Oziotelphusanaga

Lectins are versatile proteins that specifically recognize and interact with sugar moieties expressed on the cell surface. The potential of lectin in drug targeting and delivery has instigated interest to identify natural lectins. Crabs have been identified as a rich source of lectin because the innate immune system is activated on encounter of pathogens and helps in the production of lectin. Although the presence of lectins in crab's hemolymph is well documented, little information about lectin in hepatopancreas, a vital organ for immunity and digestion in crustaceans, is currently available. A calcium dependent lectin (75 kDa) was purified from the hepatopancreas of the freshwater crab Oziotelphusa naga by bioadsorption and fetuin linked Sepharose 4B affinity chromatography technique. The isolated hepatopancreas lectin is calcium dependent and maximum agglutination was observed with rabbit erythrocytes. The hemagglutinating activity of the hepatopancreas lectin was effectively inhibited by sugars, such as α-lactose, GlcNAc, trehalose and NeuAc. Compared to sialylated N-glycosylated proteins including transferrin and apo transferrin, sialylated O-glycosylated proteins like fetuin exhibited stronger inhibitory effect. The ability of erythrocytes to bind hepatopancreas lectin has been diminished by desialylation of the potent inhibitor, indicating the significance of sialic acid in lectin-ligand interactions. The purified hepatopancreas lectin showed a broad spectrum of antimicrobial activity against bacteria Staphylococcus aureus, Klebsiella pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, E. coli and fungi Candida albicans and Aspergillus niger. The findings of this study demonstrate the significance of hepatopancreas lectin as a multifunctional defense protein that inhibits the growth of bacteria and fungi.

The sunflower jacalin Helja: biological and structural insights of its antifungal activity against Candida albicans

The limited availability of efficient treatments for Candida infections and the increased emergence of antifungal-resistant strains stimulates the search for new antifungal agents. We have previously isolated a sunflower mannose-binding lectin (Helja) with antifungal activity against Candida albicans, capable of binding mannose-bearing oligosaccharides exposed on the cell surface. This work aimed to investigate the biological and biophysical basis of Helja's binding to C. albicans cell wall mannans and its influence on the fungicidal activity of the lectin. We evaluated the interaction of Helja with the cell wall mannans extracted from the isogenic parental strain (WT) and a glycosylation-defective C. albicans with altered cell wall phosphomannosylation (mnn4∆ null mutants) and investigated its antifungal effect. Helja exhibited stronger antifungal activity on the mutant strain, showing greater inhibition of fungal growth, loss of cell viability, morphological alteration, and formation of clusters with agglutinated cells. This differential biological activity of Helja was correlated with the biophysical parameters determined by solid phase assays and isothermal titration calorimetry, which demonstrated that the lectin established stronger interactions with the cell wall mannans of the mnn4∆ null mutant than with the WT strain. In conclusion, our results provide new evidence on the nature of the Helja molecular interactions with cell wall components, i.e. phosphomannan, and its impact on the antifungal activity. This study highlights the relevance of plant lectins in the design of effective antifungal therapies.

Screening of Candida spp. in wastewater in Brazil during COVID-19 pandemic: workflow for monitoring fungal pathogens

Fungal diseases are often linked to poverty, which is associated with poor hygiene and sanitation conditions that have been severely worsened by the COVID-19 pandemic. Moreover, COVID-19 patients are treated with Dexamethasone, a corticosteroid that promotes an immunosuppressive profile, making patients more susceptible to opportunistic fungal infections, such as those caused by Candida species. In this study, we analyzed the prevalence of Candida yeasts in wastewater samples collected to track viral genetic material during the COVID-19 pandemic and identified the yeasts using polyphasic taxonomy. Furthermore, we investigated the production of biofilm and hydrolytic enzymes, which are known virulence factors. Our findings revealed that all Candida species could form biofilms and exhibited moderate hydrolytic enzyme activity. We also proposed a workflow for monitoring wastewater using Colony PCR instead of conventional PCR, as this technique is fast, cost-effective, and reliable. This approach enhances the accurate taxonomic identification of yeasts in environmental samples, contributing to environmental monitoring as part of the One Health approach, which preconizes the monitoring of possible emergent pathogenic microorganisms, including fungi.

Comparing the Mechanical Properties of Rice Cells and Protoplasts under PEG6000 Drought Stress Using Double Resonator Piezoelectric Cytometry

Plant cells' ability to withstand abiotic stress is strongly linked to modifications in their mechanical characteristics. Nevertheless, the lack of a workable method for consistently tracking plant cells' mechanical properties severely restricts our comprehension of the mechanical alterations in plant cells under stress. In this study, we used the Double Resonator Piezoelectric Cytometry (DRPC) method to dynamically and non-invasively track changes in the surface stress (ΔS) generated and viscoelasticity (storage modulus G' and loss modulus G″) of protoplasts and suspension cells of rice under a drought stress of 5-25% PEG6000. The findings demonstrate that rice suspension cells and protoplasts react mechanically differently to 5-15% PEG6000 stress, implying distinct resistance mechanisms. However, neither of them can withstand 25% PEG6000 stress; they respond mechanically similarly to 25% PEG6000 stress. The results of DRPC are further corroborated by the morphological alterations of rice cells and protoplasts observed under an optical microscope. To sum up, the DRPC technique functions as a precise cellular mechanical sensor and offers novel research tools for the evaluation of plant cell adversity and differentiating between the mechanical reactions of cells and protoplasts under abiotic stress.

Functional analysis of basidiomycete specific chitin synthase genes in the agaricomycete fungus Pleurotus ostreatus

Chitin is an essential structural component of fungal cell walls composed of transmembrane proteins called chitin synthases (CHSs), which have a large range of reported effects in ascomycetes; however, are poorly understood in agaricomycetes. In this study, evolutionary and molecular genetic analyses of chs genes were conducted using genomic information from nine ascomycete and six basidiomycete species. The results support the existence of seven previously classified chs clades and the discovery of three novel basidiomycete-specific clades (BI-BIII). The agaricomycete fungus Pleurotus ostreatus was observed to have nine putative chs genes, four of which were basidiomycete-specific. Three of these basidiomycete specific genes were disrupted in the P. ostreatus 20b strain (ku80 disruptant) through homologous recombination and transformants were obtained (Δchsb2, Δchsb3, and Δchsb4). Despite numerous transformations Δchsb1 was unobtainable, suggesting disruption of this gene causes a crucial negative effect in P. ostreatus. Disruption of these chsb2-4 genes caused sparser mycelia with rougher surfaces and shorter aerial hyphae. They also caused increased sensitivity to cell wall and membrane stress, thinner cell walls, and overexpression of other chitin and glucan synthases. These genes have distinct roles in the structural formation of aerial hyphae and cell walls, which are important for understanding basidiomycete evolution in filamentous fungi.

Structural diversification of fungal cell wall in response to the stress signaling and remodeling during fungal pathogenesis

Fungi are one of the most diverse organisms found in our surroundings. The heterotrophic lifestyle of fungi and the ever-changing external environmental factors pose numerous challenges for their survival. Despite all adversities, fungi continuously develop new survival strategies to secure nutrition and space from their host. During host-pathogen interaction, filamentous phytopathogens in particular, effectively infect their hosts by maintaining polarised growth at the tips of hyphae. The fungal cell wall, being the prime component of host contact, plays a crucial role in fortifying the intracellular environment against the harsh external environment. Structurally, the fungal cell wall is a highly dynamic yet rigid component, responsible for maintaining cellular morphology. Filamentous pathogens actively maintain their dynamic cell wall to compensate rapid growth on the host. Additionally, they secrete effectors to dampen the sophisticated mechanisms of plant defense and initiate various downstream signaling cascades to repair the damage inflicted by the host. Thus, the fungal cell wall serves as a key modulator of fungal pathogenicity. The fungal cell wall with their associated signaling mechanisms emerge as intriguing targets for host immunity. This review comprehensively examines and summarizes the multifaceted findings of various research groups regarding the dynamics of the cell wall in filamentous fungal pathogens during host invasion.

Temporal dynamics of stress response in Halomonas elongata to NaCl shock: physiological, metabolomic, and transcriptomic insights

BACKGROUND

The halophilic bacterium Halomonas elongata is an industrially important strain for ectoine production, with high value and intense research focus. While existing studies primarily delve into the adaptive mechanisms of this bacterium under fixed salt concentrations, there is a notable dearth of attention regarding its response to fluctuating saline environments. Consequently, the stress response of H. elongata to salt shock remains inadequately understood.

RESULTS

This study investigated the stress response mechanism of H. elongata when exposed to NaCl shock at short- and long-time scales. Results showed that NaCl shock induced two major stresses, namely osmotic stress and oxidative stress. In response to the former, within the cell's tolerable range (1-8% NaCl shock), H. elongata urgently balanced the surging osmotic pressure by uptaking sodium and potassium ions and augmenting intracellular amino acid pools, particularly glutamate and glutamine. However, ectoine content started to increase until 20 min post-shock, rapidly becoming the dominant osmoprotectant, and reaching the maximum productivity (1450 ± 99 mg/L/h). Transcriptomic data also confirmed the delayed response in ectoine biosynthesis, and we speculate that this might be attributed to an intracellular energy crisis caused by NaCl shock. In response to oxidative stress, transcription factor cysB was significantly upregulated, positively regulating the sulfur metabolism and cysteine biosynthesis. Furthermore, the upregulation of the crucial peroxidase gene (HELO_RS18165) and the simultaneous enhancement of peroxidase (POD) and catalase (CAT) activities collectively constitute the antioxidant defense in H. elongata following shock. When exceeding the tolerance threshold of H. elongata (1-13% NaCl shock), the sustained compromised energy status, resulting from the pronounced inhibition of the respiratory chain and ATP synthase, may be a crucial factor leading to the stagnation of both cell growth and ectoine biosynthesis.

CONCLUSIONS

This study conducted a comprehensive analysis of H. elongata's stress response to NaCl shock at multiple scales. It extends the understanding of stress response of halophilic bacteria to NaCl shock and provides promising theoretical insights to guide future improvements in optimizing industrial ectoine production.

"You Are What You Eat": How Fungal Adaptation Can Be Leveraged toward Myco-Material Properties

Fungi adapt to their surroundings, modifying their behaviors and composition under different conditions like nutrient availability and environmental stress. This perspective examines how a basic understanding of fungal genetics and the different ways that fungi can be influenced by their surroundings can be leveraged toward the production of functional mycelium materials. Simply put, within the constraints of a given genetic script, both the quality and quantity of fungal mycelium are shaped by what they eat and where they grow. These two levers, encompassing their global growth environment, can be turned toward different materials outcomes. The final properties of myco-materials are thus intimately shaped by the conditions of their growth, enabling the design of new biobased and biodegradable material constructions for applications that have traditionally relied on petroleum-based chemicals.This perspective highlights aspects of fungal genetics and environmental adaptation that have potential materials science implications, along the way touching on key studies, both to situate the state of the art within the field and to punctuate the viewpoints of the authors. Finally, this work ends with future perspectives, reinforcing key topics deemed important to consider in emerging myco-materials research.

Fungal Cell Wall-Associated Effectors: Sensing, Integration, Suppression, and Protection

The cell wall (CW) of plant-interacting fungi, as the direct interface with host plants, plays a crucial role in fungal development. A number of secreted proteins are directly associated with the fungal CW, either through covalent or non-covalent interactions, and serve a range of important functions. In the context of plant-fungal interactions many are important for fungal development in the host environment and may therefore be considered fungal CW-associated effectors (CWAEs). Key CWAE functions include integrating chemical/physical signals to direct hyphal growth, interfering with plant immunity, and providing protection against plant defenses. In recent years, a diverse range of mechanisms have been reported that underpin their roles, with some CWAEs harboring conserved motifs or functional domains, while others are reported to have novel features. As such, the current understanding regarding fungal CWAEs is systematically presented here from the perspective of their biological functions in plant-fungal interactions. An overview of the fungal CW architecture and the mechanisms by which proteins are secreted, modified, and incorporated into the CW is first presented to provide context for their biological roles. Some CWAE functions are reported across a broad range of pathosystems or symbiotic/mutualistic associations. Prominent are the chitin interacting-effectors that facilitate fungal CW modification, protection, or suppression of host immune responses. However, several alternative functions are now reported and are presented and discussed. CWAEs can play diverse roles, some possibly unique to fungal lineages and others conserved across a broad range of plant-interacting fungi. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Antioxidant Dipeptides Enhance Osmotic Stress Tolerance by Regulating the Yeast Cell Wall and Membrane

This study aimed to investigate the role of the yeast cell wall and membrane in enhancing osmotic tolerance by antioxidant dipeptides (ADs) including Ala-His (AH), Thr-Tyr (TY), and Phe-Cys (FC). Results revealed that ADs could improve the integrity of the cell wall by restructuring polysaccharide structures. Specifically, FC significantly (p < 0.05) reduced the leakage of nucleic acid and protein by 2.86% and 5.36%, respectively, compared to the control. In addition, membrane lipid composition played a crucial role in enhancing yeast tolerance by ADs, including the increase of cell membrane integrity and the decrease of permeability by regulating the ratio of unsaturated fatty acids. The up-regulation of gene expression associated with the cell wall integrity pathway (RLM1, SLT2, MNN9, FKS1, and CHS3) and fatty acid biosynthesis (ACC1, HFA1, OLE1, ERG1, and FAA1) further confirmed the positive impact of ADs on yeast tolerance against osmotic stress.

Importance of Non-Covalent Interactions in Yeast Cell Wall Molecular Organization

This review covers a group of non-covalently associated molecules, particularly proteins (NCAp), incorporated in the yeast cell wall (CW) with neither disulfide bridges with proteins covalently attached to polysaccharides nor other covalent bonds. Most NCAp, particularly Bgl2, are polysaccharide-remodeling enzymes. Either directly contacting their substrate or appearing as CW lipid-associated molecules, such as in vesicles, they represent the most movable enzymes and may play a central role in CW biogenesis. The absence of the covalent anchoring of NCAp allows them to be there where and when it is necessary. Another group of non-covalently attached to CW molecules are polyphosphates (polyP), the universal regulators of the activity of many enzymes. These anionic polymers are able to form complexes with metal ions and increase the diversity of non-covalent interactions through charged functional groups with both proteins and polysaccharides. The mechanism of regulation of polysaccharide-remodeling enzyme activity in the CW is unknown. We hypothesize that polyP content in the CW is regulated by another NCAp of the CW-acid phosphatase-which, along with post-translational modifications, may thus affect the activity, conformation and compartmentalization of Bgl2 and, possibly, some other polysaccharide-remodeling enzymes.

The role of edaphic variables and management practices in regulating soil microbial resilience to drought - A meta-analysis

Environmental disturbances such as drought can impact soil health and the resistance (ability to withstand environmental stress) and resilience (ability to recover functional and structural integrity after stress) of soil microbial functional activities. A paucity of information exists on the impact of drought on soil microbiome and how soil biological systems respond to and demonstrate resilience to drought stress. To address this, we conducted a systematic review and meta-analysis (using only laboratory studies) to assess the response of soil microbial biomass and respiration to drought stress across agriculture, forest, and grassland ecosystems. The meta-analysis revealed an overall negative response of microbial biomass in resistance (-31.6 %) and resilience (-0.3 %) to drought, suggesting a decrease in soil microbial biomass content. Soil microbial respiration also showed a negative response in resistance to drought stress indicating a decrease in soil microbial respiration in agriculture (-17.5 %), forest (-64.0 %), and grassland (-65.5 %) ecosystems. However, it showed a positive response in resilience to drought, suggesting an effective recovery in microbial respiration post-drought. Soil organic carbon (SOC), clay content, and pH were the main regulating factors of the responses of soil microbial biomass and respiration to drought. In agriculture ecosystem, soil pH was primarily correlated with soil microbial respiration resistance and resilience to drought, potentially influenced by frequent land preparation and fertilizer applications, while in forest ecosystem SOC, clay content, and pH significantly impacted microbial biomass and respiration resistance and resilience. In grassland ecosystem, SOC was strongly associated with biomass resilience to drought. The impact of drought stress on soil microbiome showed different patterns in natural and agriculture ecosystems, and the magnitude of microbial functional responses regulated by soil intrinsic properties. This study highlighted the importance of understanding the role of soil properties in shaping microbial responses to drought stress for better ecosystem management.

Physiological and transcriptomic insights into sugar stress resistance in osmophilic yeast Zygosaccharomyces rouxii

The osmophilic yeast Zygosaccharomyces rouxii has attracted increasing attention for its ability to survive and grow in extremely high sugar environments. This trait determines its role in fermentation process and results in contamination in the food industry. However, the behavior of Z. rouxii in regulating cell metabolism to combat high sugar stress and the corresponding mechanism have not been completely elucidated. Here, the resistance strategies of Z. rouxii against high glucose stress were explored by physiological analysis at cell membrane level and transcriptomic analysis. Physiological analysis showed that under high glucose stress, colony transparency increased, cell volume decreased, which was accompanied by reduction in permeability and integrity of cell membrane and subsequent gradual recovering. Additionally, the proportion of ergosterol and unsaturated fatty acids in cell membrane significantly increased under high glucose stress. A comparison of transcriptome data showed that most of the obtained differentially expressed genes (DEGs) involved in ergosterol and linoleic acid synthesis pathways as well as cell wall integrity (CWI) and high osmolarity glycerol mitogen-activated protein kinase (HOG-MAPK) pathways, which was in line with the results of physiological data. Our results provided a theoretic basis to develop the process control for the production of high sugar foods.

How do the carbon and nitrogen sources affect the synthesis of β-(1,3/1,6)-glucan, its structure and the susceptibility of Candida utilis yeast cells to immunolabelling with β-(1,3)-glucan monoclonal antibodies?

BACKGROUND

The need to limit antibiotic therapy due to the spreading resistance of pathogenic microorganisms to these medicinal substances stimulates research on new therapeutic agents, including the treatment and prevention of animal diseases. This is one of the goals of the European Green Deal and the Farm-To-Fork strategy. Yeast biomass with an appropriate composition and exposure of cell wall polysaccharides could constitute a functional feed additive in precision animal nutrition, naturally stimulating the immune system to fight infections.

RESULTS

The results of the research carried out in this study showed that the composition of Candida utilis ATCC 9950 yeast biomass differed depending on growth medium, considering especially the content of β-(1,3/1,6)-glucan, α-glucan, and trehalose. The highest β-(1,3/1,6)-glucan content was observed after cultivation in deproteinated potato juice water (DPJW) as a nitrogen source and glycerol as a carbon source. Isolation of the polysaccharide from yeast biomass confirmed the highest yield of β-(1,3/1,6)-glucan after cultivation in indicated medium. The differences in the susceptibility of β-(1,3)-glucan localized in cells to interaction with specific β-(1,3)-glucan antibody was noted depending on the culture conditions. The polymer in cells from the DPJW supplemented with glycerol and galactose were labelled with monoclonal antibodies with highest intensity, interestingly being less susceptible to such an interaction after cell multiplication in medium with glycerol as carbon source and yeast extract plus peptone as a nitrogen source.

CONCLUSIONS

Obtained results confirmed differences in the structure of the β-(1,3/1,6)-glucan polymers considering side-chain length and branching frequency, as well as in quantity of β-(1,3)- and β-(1,6)-chains, however, no visible relationship was observed between the structural characteristics of the isolated polymers and its susceptibility to immunolabeling in whole cells. Presumably, other outer surface components and molecules can mask, shield, protect, or hide epitopes from antibodies. β-(1,3)-Glucan was more intensely recognized by monoclonal antibody in cells with lower trehalose and glycogen content. This suggests the need to cultivate yeast biomass under appropriate conditions to fulfil possible therapeutic functions. However, our in vitro findings should be confirmed in further studies using tissue or animal models.

Helium Cold Atmospheric Plasma Causes Morphological and Biochemical Alterations in Candida albicans Cells

(1) Background: Previous studies reported the promising inhibitory effect of cold atmospheric plasma (CAP) on Candida albicans. However, the exact mechanisms of CAP's action on the fungal cell are still poorly understood. This study aims to elucidate the CAP effect on C. albicans cell wall, by evaluating the alterations on its structure and biochemical composition; (2) Methods: C. albicans cells treated with Helium-CAP were analyzed by atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR) in order to detect morphological, topographic and biochemical changes in the fungal cell wall. Cells treated with caspofungin were also analyzed for comparative purposes; (3) Results: Expressive morphological and topographic changes, such as increased roughness and shape modification, were observed in the cells after CAP exposure. The alterations detected were similar to those observed after the treatment with caspofungin. The main biochemical changes occurred in polysaccharides content, and an overall decrease in glucans and an increase in chitin synthesis were detected; (4) Conclusions: Helium-CAP caused morphological and topographic alterations in C. albicans cells and affected the cell wall polysaccharide content.

Subinhibitory effects of 2,4-diacetylphloroglucinol on filamentous fungus Aspergillus fumigatus

AIMS

Aspergillus fungi are common members of the soil microbiota. Some physiological and structural characteristics of Aspergillus species make them important participants in soil ecological processes. In this study, we aimed to evaluate the impact of 2,4-diacetylphloroglucinol (2,4-DAPG), a common metabolite of soil and rhizosphere bacteria, on the physiology of Aspergillus fumigatus.

METHODS AND RESULTS

Integrated analysis using microscopy, spectrophotometry, and liquid chromatography showed the following effects of 2,4-DAPG on Aspergillus physiology. It was found that A. fumigatus in the biofilm state is resistant to high concentrations of 2,4-DAPG. However, experimental exposure led to a depletion of the extracellular polymeric substance, changes in the structure of the cell wall of the mycelium (increase in the content of α- and β-glucans, chitin, and ergosterol), and conidia (decrease in the content of DHN-melanin). 2,4-DAPG significantly reduced the production of mycotoxins (gliotoxin and fumagillin) but increased the secretion of proteases and galactosaminogalactan.

CONCLUSIONS

Overall, the data obtained suggest that 2,4-DAPG-producing Pseudomonas bacteria are unlikely to directly eliminate A. fumigatus fungi, as they exhibit a high level of resistance when in the biofilm state. However, at low concentrations, 2,4-DAPG significantly alters the physiology of aspergilli, potentially reducing the adaptive and competitive capabilities of these fungi.

Osmotic Stress Responses, Cell Wall Integrity, and Conidiation Are Regulated by a Histidine Kinase Sensor in Trichoderma atroviride

Trichoderma atroviride responds to various environmental stressors through the mitogen-activated protein kinase (MAPK) Tmk3 and MAPK-kinase Pbs2 signaling pathways. In fungi, orthologues to Tmk3 are regulated by a histidine kinase (HK) sensor. However, the role of T. atroviride HKs remains unknown. In this regard, the function of the T. atroviride HK Nik1 was analyzed in response to stressors regulated by Tmk3. The growth of the Δnik1 mutant strains was compromised under hyperosmotic stress; mycelia were less resistant to lysing enzymes than the WT strain, while conidia of Δnik1 were more sensitive to Congo red; however, ∆pbs2 and ∆tmk3 strains showed a more drastic defect in cell wall stability. Light-regulated blu1 and grg2 gene expression was induced upon an osmotic shock through Pbs2-Tmk3 but was independent of Nik1. The encoding chitin synthases chs1 and chs2 genes were downregulated after an osmotic shock in the WT, but chs1 and chs3 expression were enhanced in ∆nik1, ∆pbs2, and ∆tmk3. The vegetative growth and conidiation by light decreased in ∆nik1, although Nik1 was unrequired to activate the light-responsive genes by Tmk3. Altogether, Nik1 regulates responses related to the Pbs2-Tmk3 pathway and suggests the participation of additional HKs to respond to stress.

Understanding Fungi in Glacial and Hypersaline Environments

Hypersaline waters and glacial ice are inhospitable environments that have low water activity and high concentrations of osmolytes. They are inhabited by diverse microbial communities, of which extremotolerant and extremophilic fungi are essential components. Some fungi are specialized in only one of these two environments and can thrive in conditions that are lethal to most other life-forms. Others are generalists, highly adaptable species that occur in both environments and tolerate a wide range of extremes. Both groups efficiently balance cellular osmotic pressure and ion concentration, stabilize cell membranes, remodel cell walls, and neutralize intracellular oxidative stress. Some species use unusual reproductive strategies. Further investigation of these adaptations with new methods and carefully designed experiments under ecologically relevant conditions will help predict the role of fungi in hypersaline and glacial environments affected by climate change, decipher their stress resistance mechanisms and exploit their biotechnological potential.

Genomic characterization and radiation tolerance of Naganishia kalamii sp. nov. and Cystobasidium onofrii sp. nov. from Mars 2020 mission assembly facilities

During the construction and assembly of the Mars 2020 mission components at two different NASA cleanrooms, several fungal strains were isolated. Based on their colony morphology, two strains that showed yeast-like appearance were further characterized for their phylogenetic position. The species-level classification of these two novel strains, using traditional colony and cell morphology methods combined with the phylogenetic reconstructions using multi-locus sequence analysis (MLSA) based on several gene loci (ITS, LSU, SSU, RPB1, RPB2, CYTB and TEF1), and whole genome sequencing (WGS) was carried out. This polyphasic taxonomic approach supported the conclusion that the two basidiomycetous yeasts belong to hitherto undescribed species. The strain FJI-L2-BK-P3T, isolated from the Jet Propulsion Laboratory Spacecraft Assembly Facility, was placed in the Naganishia albida clade (Filobasidiales, Tremellomycetes), but is genetically and physiologically different from other members of the clade. Another yeast strain FKI-L6-BK-PAB1T, isolated from the Kennedy Space Center Payload Hazardous and Servicing Facility, was placed in the genus Cystobasidium (Cystobasidiales, Cystobasidiomycetes) and is distantly related to C. benthicum. Here we propose two novel species with the type strains, Naganishia kalamii sp. nov. (FJI-L2-BK-P3T = NRRL 64466 = DSM 115730) and Cystobasidium onofrii sp. nov. (FKI-L6-BK-PAB1T = NRRL 64426 = DSM 114625). The phylogenetic analyses revealed that single gene phylogenies (ITS or LSU) were not conclusive, and MLSA and WGS-based phylogenies were more advantageous for species discrimination in the two genera. The genomic analysis predicted proteins associated with dehydration and desiccation stress-response and the presence of genes that are directly related to osmotolerance and psychrotolerance in both novel yeasts described. Cells of these two newly-described yeasts were exposed to UV-C radiation and compared with N. onofrii, an extremophilic UV-C resistant cold-adapted Alpine yeast. Both novel species were UV resistant, emphasizing the need for collecting and characterizing extremotolerant microbes, including yeasts, to improve microbial reduction techniques used in NASA planetary protection programs.

The Cerato-Platanin EPL2 from Trichoderma reesei Is Not Directly Involved in Cellulase Formation but in Cell Wall Remodeling

Trichoderma reesei is a saprophytic fungus that produces large amounts of cellulases and is widely used for biotechnological applications. Cerato-platanins (CPs) are a family of proteins universally distributed among Dikarya fungi and have been implicated in various functions related to fungal physiology and interaction with the environment. In T. reesei, three CPs are encoded in the genome: Trire2_111449, Trire2_123955, and Trire2_82662. However, their function is not fully elucidated. In this study, we deleted the Trire2_123955 gene (named here as epl2) in the wild-type QM6aΔtmus53Δpyr4 (WT) strain and examined the behavior of the Δepl2 strain compared with WT grown for 72 h in 1% cellulose using RNA sequencing. Of the 9143 genes in the T. reesei genome, 760 were differentially expressed, including 260 only in WT, 214 only in Δepl2, and 286 in both. Genes involved in oxidative stress, oxidoreductase activity, antioxidant activity, and transport were upregulated in the Δepl2 mutant. Genes encoding cell wall synthesis were upregulated in the mutant strain during the late growth stage. The Δepl2 mutant accumulated chitin and glucan at higher levels than the parental strain and was more resistant to cell wall stressors. These results suggest a compensatory effect in cell wall remodeling due to the absence of EPL2 in T. reesei. This study is expected to contribute to a better understanding of the role of the EPL2 protein in T. reesei and improve its application in biotechnological fields.

Candida palmioleophila: A New Emerging Threat in Brazil?

Human activity directly or indirectly causes climate change, promoting changes in the composition of the atmosphere. This change is beyond the variation of the natural climate. In this manner, climate change could create an environmental pressure which is enough to trigger new fungal diseases. In addition to climate alterations, the onset of the COVID-19 pandemic has also been associated with the emergence of fungal pathogens. Fungi showed that an inability to grow at high temperatures limits the capacity of fungi to infect mammals. However, fungi can develop thermotolerance, gradually adapting to rising temperatures due to climate change, and generating a greater number of disease-causing organisms. In the present study, we reported the detection and identification of Candida palmioleophila isolates recovered from raw sewage samples in Niteroi city, Rio de Janeiro State, Brazil, during a monitoring program for measuring SARS-CoV-2 presence and concentration. Using polyphasic taxonomy to identify the species and evaluating some virulence aspects of this species, such as biofilm formation and extracellular enzyme production, our data highlight this species as a possible emerging pathogen in Brazil, especially in the pandemic context.

Insights into the Antifungal Properties of Myxobacteria Outer Membrane β-1,6-Glucanase

Fungal cell wall decomposition enzymes exhibit great potential for the development of efficient antifungal agents. However, their practical application is restricted due to incomplete understanding of the action mode. In our previous study, we identified that a novel outer membrane (OM) β-1,6-glucanase GluM is deployed by predatory myxobacteria to feed on fungi. In this work, we provide deep insights into the antifungal mechanism of β-1,6-glucanase and its potential in improving plant disease resistance. The fungal cell wall decomposition ability of GluM resulted in irregular hyphae morphology, changed chitin distribution, increased membrane permeability, and leakage of cell constituents in Magnaporthe oryzae Guy11. Under the attack pattern, the cell wall integrity pathway was activated by strain Guy11 for self-protection. GluM exhibited a distinct endo-model toward fungal cell wall; the favorite substrate of GluM toward fungal β-1,6-glucan may give reason for its efficient antifungal activity compared with Trichoderma β-1,6-glucanase. Moreover, released glucans from GluM hydrolysis of fungal cell wall functioned as an elicitor and induced rice immunity by means of jasmonic acid pathway. Based on the dual roles of antifungal properties, gluM transgenic plants conferred enhanced resistance against fungal infection.

A C. albicans TRAPP Complex-Associated Gene Contributes to Cell Wall Integrity, Hyphal and Biofilm Formation, and Tissue Invasion

While endocytic and secretory pathways are well-studied cellular processes in the model yeast Saccharomyces cerevisiae, they remain understudied in the opportunistic fungal pathogen Candida albicans. We previously found that null mutants of C. albicans homologs of the S. cerevisiae early endocytosis genes ENT2 and END3 not only exhibited delayed endocytosis but also had defects in cell wall integrity, filamentation, biofilm formation, extracellular protease activity, and tissue invasion in an in vitro model. In this study, we focused on a potential C. albicans homolog to S. cerevisiae TCA17, which was discovered in our whole-genome bioinformatics approach aimed at identifying genes involved in endocytosis. In S. cerevisiae, TCA17 encodes a transport protein particle (TRAPP) complex-associated protein. Using a reverse genetics approach with CRISPR-Cas9-mediated gene deletion, we analyzed the function of the TCA17 homolog in C. albicans. Although the C. albicans tca17Δ/Δ null mutant did not have defects in endocytosis, it displayed an enlarged cell and vacuole morphology, impaired filamentation, and reduced biofilm formation. Moreover, the mutant exhibited altered sensitivity to cell wall stressors and antifungal agents. When assayed using an in vitro keratinocyte infection model, virulence properties were also diminished. Our findings indicate that C. albicans TCA17 may be involved in secretion-related vesicle transport and plays a role in cell wall and vacuolar integrity, hyphal and biofilm formation, and virulence. IMPORTANCE The fungal pathogen Candida albicans causes serious opportunistic infections in immunocompromised patients and has become a major cause of hospital-acquired bloodstream infections, catheter-associated infections, and invasive disease. However, due to a limited understanding of Candida molecular pathogenesis, clinical approaches for the prevention, diagnosis, and treatment of invasive candidiasis need significant improvement. In this study, we focus on identifying and characterizing a gene potentially involved in the C. albicans secretory pathway, as intracellular transport is critical for C. albicans virulence. We specifically investigated the role of this gene in filamentation, biofilm formation, and tissue invasion. Ultimately, these findings advance our current understanding of C. albicans biology and may have implications for the diagnosis and treatment of candidiasis.

Architecture of the dynamic fungal cell wall

The fungal cell wall is essential for growth and survival, and is a key target for antifungal drugs and the immune system. The cell wall must be robust but flexible, protective and shielding yet porous to nutrients and membrane vesicles and receptive to exogenous signals. Most fungi have a common inner wall skeleton of chitin and β-glucans that functions as a flexible viscoelastic frame to which a more diverse set of outer cell wall polymers and glycosylated proteins are attached. Whereas the inner wall largely determines shape and strength, the outer wall confers properties of hydrophobicity, adhesiveness, and chemical and immunological heterogeneity. The spatial organization and dynamic regulation of the wall in response to prevailing growth conditions enable fungi to thrive within changing, diverse and often hostile environments. Understanding this architecture provides opportunities to develop diagnostics and drugs to combat life-threatening fungal infections.

Cryptococcus neoformans Mar1 function links mitochondrial metabolism, oxidative stress, and antifungal tolerance

Introduction: Microbial pathogens undergo significant physiological changes during interactions with the infected host, including alterations in metabolism and cell architecture. The Cryptococcus neoformans Mar1 protein is required for the proper ordering of the fungal cell wall in response to host-relevant stresses. However, the precise mechanism by which this Cryptococcus-specific protein regulates cell wall homeostasis was not defined. Methods: Here, we use comparative transcriptomics, protein localization, and phenotypic analysis of a mar1D loss-of-function mutant strain to further define the role of C. neoformans Mar1 in stress response and antifungal resistance. Results: We demonstrate that C. neoformans Mar1 is highly enriched in mitochondria. Furthermore, a mar1Δ mutant strain is impaired in growth in the presence of select electron transport chain inhibitors, has altered ATP homeostasis, and promotes proper mitochondrial morphogenesis. Pharmacological inhibition of complex IV of the electron transport chain in wild-type cells promotes similar cell wall changes as the mar1Δ mutant strain, supporting prior associations between mitochondrial function and cell wall homeostasis. Although Mar1 is not required for general susceptibility to the azole antifungals, the mar1Δ mutant strain displays increased tolerance to fluconazole that correlates with repressed mitochondrial metabolic activity. Discussion: Together, these studies support an emerging model in which the metabolic activity of microbial cells directs cell physiological changes to allow persistence in the face of antimicrobial and host stress.

Melaninization Reduces Cryptococcus neoformans Susceptibility to Mechanical Stress

Melanin is a complex pigment that is found in various fungal species and is associated with a multitude of protective functions against environmental stresses. In Cryptococcus neoformans, melanin is synthesized from exogenous substrate and deposited in the cell wall. Although melanin is often cited as a protector against mechanical stress, there is a paucity of direct experimental data supporting this claim. To probe whether melanin enhances cellular strength, we used ultrasonic cavitation and French cell press pressure to stress cryptococcal cells and then measured changes in cellular morphology and fragmentation for melanized and nonmelanized C. neoformans cells. Melanized yeast cells exhibited lower rates of fragmentation and greater cell areas than did nonmelanized yeast cells after sonication or French press passage. When subjected to French press passage, both melanized and nonmelanized cells exhibited responses that were dependent on their culture age. Our results indicate that melanization protects against some of the morphological changes, such as fragmentation and cellular shrinkage, that are initiated by mechanical energy derived from either sonic cavitation or French press passage, thus supporting the notion that this pigment provides mechanical strength for fungal cell walls. IMPORTANCE Melanin was shown in prior microbiological experiments to be associated with protection against environmental stressors, and it has often been cited as being associated with mechanical stress protection. However, there is a lack of direct experimentation to confirm this claim. We examined the responses of melanized and nonmelanized C. neoformans cells to sonication and French press passage, and we report differences in outcomes depending not only on melanization status but also on culture age. Such findings have important implications for the design and interpretation of laboratory experiments involving C. neoformans. In addition, the elucidation of some of the mechanical properties of melanin promotes further research into fungal melanin applications in health care and industry.

Fungal resilience and host-pathogen interactions: Future perspectives and opportunities

We are constantly exposed to the threat of fungal infection. The outcome-clearance, commensalism or infection-depends largely on the ability of our innate immune defences to clear infecting fungal cells versus the success of the fungus in mounting compensatory adaptive responses. As each seeks to gain advantage during these skirmishes, the interactions between host and fungal pathogen are complex and dynamic. Nevertheless, simply compromising the physiological robustness of fungal pathogens reduces their ability to evade antifungal immunity, their virulence, and their tolerance against antifungal therapy. In this article I argue that this physiological robustness is based on a 'Resilience Network' which mechanistically links and controls fungal growth, metabolism, stress resistance and drug tolerance. The elasticity of this network probably underlies the phenotypic variability of fungal isolates and the heterogeneity of individual cells within clonal populations. Consequently, I suggest that the definition of the fungal Resilience Network represents an important goal for the future which offers the clear potential to reveal drug targets that compromise drug tolerance and synergise with current antifungal therapies.

Associations of Rap1 with Cell Wall Integrity, Biofilm Formation, and Virulence in Candida albicans

Rap1 (repressor activator protein 1) is a multifunctional protein, playing important roles in telomeric and nontelomeric functions in many eukaryotes. Candida albicans Rap1 has been previously shown to be involved in telomeric regulation, but its other functions are still mostly unknown. In this study, we found that the deletion of the RAP1 gene altered cell wall properties, composition, and gene expression. In addition, deletion of RAP1 affected C. albicans biofilm formation and modulated phagocytosis and cytokine release by host immune cells. Finally, the RAP1 gene deletion mutant showed attenuation of C. albicans virulence in a Galleria mellonella infection model. Therefore, these findings provide new insights into Rap1 functions that are particularly relevant to pathogenesis and virulence of C. albicans. IMPORTANCE C. albicans is an important fungal pathogen of humans. The cell wall is the outermost layer of C. albicans and is important for commensalism and infection by this pathogen. Moreover, the cell wall is also an important target for antifungals. Studies of how C. albicans maintains its cell wall integrity are critical for a better understanding of fungal pathogenesis and virulence. This work focuses on exploring unknown functions of C. albicans Rap1 and reveals its contribution to cell wall integrity, biofilm formation, and virulence. Notably, these findings will also improve our general understanding of complex machinery to control pathogenesis and virulence of fungal pathogens.

Fusarium graminearum Ste3 G-Protein Coupled Receptor: A Mediator of Hyphal Chemotropism and Pathogenesis

Fungal hyphal chemotropism has been shown to be a major contributor to host-pathogen interactions. Previous studies on Fusarium species have highlighted the involvement of the Ste2 G-protein-coupled receptor (GPCR) in mediating polarized hyphal growth toward host-released peroxidase. Here, the role of the opposite mating type GPCR, Ste3, is characterized with respect to Fusarium graminearum chemotropism and pathogenicity. Fgste3Δ deletion strains were found to be compromised in the chemotropic response toward peroxidase, development of lesions on germinating wheat, and infection of Arabidopsis thaliana leaves. In the absence of FgSte3 or FgSte2, F. graminearum cells exposed to peroxidase showed no phosphorylation of the cell-wall integrity, mitogen-activated protein kinase pathway component Mgv1. In addition, transcriptomic gene expression profiling yielded a list of genes involved in cellular reorganization, cell wall remodeling, and infection-mediated responses that were differentially modulated by peroxidase when FgSte3 was present. Deletion of FgSte3 yielded the downregulation of genes associated with mycotoxin biosynthesis and appressorium development, compared to the wild-type strain, both in the presence of peroxidase. Together, these findings contribute to our understanding of the mechanism underlying fungal chemotropism and pathogenesis while raising the novel hypothesis that FgSte2 and FgSte3 are interdependent on each other for the mediation of the redirection of hyphal growth in response to host-derived peroxidase. IMPORTANCE Fusarium head blight of wheat, caused by the filamentous fungus Fusarium graminearum, leads to devastating global food shortages and economic losses. Fungal hyphal chemotropism has been shown to be a major contributor to host-pathogen interactions. Here, the role of the opposite mating type GPCR, Ste3, is characterized with respect to F. graminearum chemotropism and pathogenicity. These findings contribute to our understanding of the mechanisms underlying fungal chemotropism and pathogenesis.

Nature of β-1,3-Glucan-Exposing Features on Candida albicans Cell Wall and Their Modulation

Candida albicans exists as a commensal of mucosal surfaces and the gastrointestinal tract without causing pathology. However, this fungus is also a common cause of mucosal and systemic infections when antifungal immune defenses become compromised. The activation of antifungal host defenses depends on the recognition of fungal pathogen-associated molecular patterns (PAMPs), such as β-1,3-glucan. In C. albicans, most β-1,3-glucan is present in the inner cell wall, concealed by the outer mannan layer, but some β-1,3-glucan becomes exposed at the cell surface. In response to host signals, such as lactate, C. albicans induces the Xog1 exoglucanase, which shaves exposed β-1,3-glucan from the cell surface, thereby reducing phagocytic recognition. We show here that β-1,3-glucan is exposed at bud scars and punctate foci on the lateral wall of yeast cells, that this exposed β-1,3-glucan is targeted during phagocytic attack, and that lactate-induced masking reduces β-1,3-glucan exposure at bud scars and at punctate foci. β-1,3-Glucan masking depends upon protein kinase A (PKA) signaling. We reveal that inactivating PKA, or its conserved downstream effectors, Sin3 and Mig1/Mig2, affects the amounts of the Xog1 and Eng1 glucanases in the C. albicans secretome and modulates β-1,3-glucan exposure. Furthermore, perturbing PKA, Sin3, or Mig1/Mig2 attenuates the virulence of lactate-exposed C. albicans cells in Galleria. Taken together, the data are consistent with the idea that β-1,3-glucan masking contributes to Candida pathogenicity. IMPORTANCE Microbes that coexist with humans have evolved ways of avoiding or evading our immunological defenses. These include the masking by these microbes of their "pathogen-associated molecular patterns" (PAMPs), which are recognized as "foreign" and used to activate protective immunity. The commensal fungus Candida albicans masks the proinflammatory PAMP β-1,3-glucan, which is an essential component of its cell wall. Most of this β-1,3-glucan is hidden beneath an outer layer of the cell wall on these microbes, but some can become exposed at the fungal cell surface. Using high-resolution confocal microscopy, we examine the nature of the exposed β-1,3-glucan at C. albicans bud scars and at punctate foci on the lateral cell wall, and we show that these features are targeted by innate immune cells. We also reveal that downstream effectors of protein kinase A (Mig1/Mig2, Sin3) regulate the secretion of major glucanases, modulate the levels of β-1,3-glucan exposure, and influence the virulence of C. albicans in an invertebrate model of systemic infection. Our data support the view that β-1,3-glucan masking contributes to immune evasion and the virulence of a major fungal pathogen of humans.

Molecular sensors reveal the mechano-chemical response of Phytophthora infestans walls and membranes to mechanical and chemical stress

Phytophthora infestans, causal agent of late blight in potato and tomato, remains challenging to control. Unravelling its biomechanics of host invasion, and its response to mechanical and chemical stress, could provide new handles to combat this devastating pathogen. Here we introduce two fluorescent molecular sensors, CWP-BDP and NR12S, that reveal the micromechanical response of the cell wall-plasma membrane continuum in P. infestans during invasive growth and upon chemical treatment. When visualized by live-cell imaging, CWP-BDP reports changes in cell wall (CW) porosity while NR12S reports variations in chemical polarity and lipid order in the plasma membrane (PM). During invasive growth, mechanical interactions between the pathogen and a surface reveal clear and localized changes in the structure of the CW. Moreover, the molecular sensors can reveal the effect of chemical treatment to CW and/or PM, thereby revealing the site-of-action of crop protection agents. This mechano-chemical imaging strategy resolves, non-invasively and with high spatio-temporal resolution, how the CW-PM continuum adapts and responds to abiotic stress, and provides information on the dynamics and location of cellular stress responses for which, to date, no other methods are available.

The Role of Sfp1 in Candida albicans Cell Wall Maintenance

The cell wall is the first interface for Candida albicans interaction with the surrounding environment and the host cells. Therefore, maintenance of cell wall integrity (CWI) is crucial for C. albicans survival and host-pathogen interaction. In response to environmental stresses, C. albicans undergoes cell wall remodeling controlled by multiple signaling pathways and transcription regulators. Here, we explored the role of the transcription factor Sfp1 in CWI. A deletion of the SFP1 gene not only caused changes in cell wall properties, cell wall composition and structure but also modulated expression of cell wall biosynthesis and remodeling genes. In addition, Cas5 is a known transcription regulator for C. albicans CWI and cell wall stress response. Interestingly, our results indicated that Sfp1 negatively controls the CAS5 gene expression by binding to its promoter element. Together, this study provides new insights into the regulation of C. albicans CWI and stress response.

Ending the (Cell) wall metaphor in microbiology

•The outer surfaces of fungi and bacteria are highly dynamic, flexible, regulated and adaptable structures.•The term "cell wall" is conceptually limiting and insufficient to adequately describe the outer surface of bacteria and fungi.•The Greek term plégma or in Latin word "reticulum" better capture the nature of the cell wall.

Mechanobiology of the cell wall – insights from tip-growing plant and fungal cells

The cell wall (CW) is a thin and rigid layer encasing the membrane of all plant and fungal cells. It ensures mechanical integrity by bearing mechanical stresses derived from large cytoplasmic turgor pressure, contacts with growing neighbors or growth within restricted spaces. The CW is made of polysaccharides and proteins, but is dynamic in nature, changing composition and geometry during growth, reproduction or infection. Such continuous and often rapid remodeling entails risks of enhanced stress and consequent damages or fractures, raising the question of how the CW detects and measures surface mechanical stress and how it strengthens to ensure surface integrity? Although early studies in model fungal and plant cells have identified homeostatic pathways required for CW integrity, recent methodologies are now allowing the measurement of pressure and local mechanical properties of CWs in live cells, as well as addressing how forces and stresses can be detected at the CW surface, fostering the emergence of the field of CW mechanobiology. Here, using tip-growing cells of plants and fungi as case study models, we review recent progress on CW mechanosensation and mechanical regulation, and their implications for the control of cell growth, morphogenesis and survival.

Bending stiffness of Candida albicans hyphae as a proxy of cell wall properties

The cell wall is a key component of fungi. It constitutes a highly regulated viscoelastic shell which counteracts internal cell turgor pressure. Its mechanical properties thus contribute to define cell morphology. Measurements of the elastic moduli of the fungal cell wall have been carried out in many species including Candida albicans, a major human opportunistic pathogen. They mainly relied on atomic force microscopy, and mostly considered the yeast form. We developed a parallelized pressure-actuated microfluidic device to measure the bending stiffness of hyphae. We found that the cell wall stiffness lies in the MPa range. We then used three different ways to disrupt cell wall physiology: inhibition of beta-glucan synthesis, a key component of the inner cell wall; application of a hyperosmotic shock triggering a sudden decrease of the hyphal diameter; deletion of two genes encoding GPI-modified cell wall proteins resulting in reduced cell wall thickness. The bending stiffness values were affected to different extents by these environmental stresses or genetic modifications. Overall, our results support the elastic nature of the cell wall and its ability to remodel at the scale of the entire hypha over minutes.

Protective effects of peptides on the cell wall structure of yeast under osmotic stress

Three peptides (LL, LML, and LLL) were used to examine their influences on the osmotic stress tolerance and cell wall properties of brewer's yeast. Results suggested that peptide supplementation improved the osmotic stress tolerance of yeast through enhancing the integrity and stability of the cell wall. Transmission electron micrographs showed that the thickness of yeast cell wall was increased by peptide addition under osmotic stress. Additionally, quantitative analysis of cell wall polysaccharide components in the LL and LLL groups revealed that they had 27.34% and 24.41% higher chitin levels, 25.73% and 22.59% higher mannan levels, and 17.86% and 21.35% higher β-1,3-glucan levels, respectively, than the control. Furthermore, peptide supplementation could positively modulate the cell wall integrity pathway and up-regulate the expressions of cell wall remodeling-related genes, including FKS1, FKS2, KRE6, MNN9, and CRH1. Thus, these results demonstrated that peptides improved the osmotic stress tolerance of yeast via remodeling the yeast cell wall and reinforcing the structure of the cell wall. KEY POINTS: • Peptide supplementation improved yeast osmotic stress tolerance via cell wall remodeling. • Peptide supplementation enhanced cell wall thickness and stability under osmotic stress. • Peptide supplementation positively modulated the CWI pathway under osmotic stress.

Human gut bifidobacteria inhibit the growth of the opportunistic fungal pathogen Candida albicans

The human gut microbiota protects the host from invading pathogens and the overgrowth of indigenous opportunistic species via a process called colonization resistance. Here, we investigated the antagonistic activity of human gut bacteria towards Candida albicans, an opportunistic fungal pathogen that can cause severe infections in susceptible individuals. Coculture batch incubations of C. albicans in the presence of faecal microbiota from six healthy individuals revealed varying levels of inhibitory activity against C. albicans. 16S rRNA gene amplicon profiling of these faecal coculture bacterial communities showed that the Bifidobacteriaceae family, and Bifidobacterium adolescentis in particular, were most correlated with antagonistic activity against C. albicans. Follow-up mechanistic studies performed under anaerobic conditions confirmed that culture supernatants of Bifidobacterium species, particularly B. adolescentis, inhibited C. albicans in vitro. Fermentation acids (FA), including acetate and lactate, present in the bifidobacterial supernatants were important contributors to inhibitory activity. However, increasing the pH of both bacterial supernatants and mixtures of FA reduced their anti-Candida effects, indicating a combinatorial effect of prevailing pH and FA. This work, therefore, demonstrates potential mechanisms underpinning gut microbiome-mediated colonization resistance against C. albicans, and identifies particularly inhibitory components such as bifidobacteria and FA as targets for further study.

Differential Expression of Cell Wall Remodeling Genes Is Part of the Dynamic Phase-Specific Transcriptional Program of Conidial Germination of Trichoderma asperelloides

The nature of saprophytic and mycoparasitic hyphal growth of Trichoderma spp. has been studied extensively, yet its initiation via conidial germination in this genus is less well understood. Using near-synchronous germinating cultures of Trichoderma asperelloides, we followed the morphological progression from dormant conidia to initial polar growth to germling formation and to evidence for first branching. We found that the stage-specific transcriptional profile of T. asperelloides is one of the most dynamic described to date: transcript abundance of over 5000 genes-comprising approximately half of the annotated genome-was unremittingly reduced in the transition from dormancy to polar growth. Conversely, after the onset of germination, the transcript abundance of approximately a quarter of the genome was unremittingly elevated during the transition from elongation to initial branching. These changes are a testimony to the substantial developmental events that accompany germination. Bayesian network analysis identified several chitinase- and glucanase-encoding genes as active transcriptional hubs during germination. Furthermore, the expression of specific members of the chitin synthase and glucan elongase families was significantly increased during germination in the presence of Rhizoctonia solani-a known host of the mycoparasite-indicating that host recognition can occur during the early stages of mycoparasite development.

The cell wall and the response and tolerance to stresses of biotechnological relevance in yeasts

In industrial settings and processes, yeasts may face multiple adverse environmental conditions. These include exposure to non-optimal temperatures or pH, osmotic stress, and deleterious concentrations of diverse inhibitory compounds. These toxic chemicals may result from the desired accumulation of added-value bio-products, yeast metabolism, or be present or derive from the pre-treatment of feedstocks, as in lignocellulosic biomass hydrolysates. Adaptation and tolerance to industrially relevant stress factors involve highly complex and coordinated molecular mechanisms occurring in the yeast cell with repercussions on the performance and economy of bioprocesses, or on the microbiological stability and conservation of foods, beverages, and other goods. To sense, survive, and adapt to different stresses, yeasts rely on a network of signaling pathways to modulate the global transcriptional response and elicit coordinated changes in the cell. These pathways cooperate and tightly regulate the composition, organization and biophysical properties of the cell wall. The intricacy of the underlying regulatory networks reflects the major role of the cell wall as the first line of defense against a wide range of environmental stresses. However, the involvement of cell wall in the adaptation and tolerance of yeasts to multiple stresses of biotechnological relevance has not received the deserved attention. This article provides an overview of the molecular mechanisms involved in fine-tuning cell wall physicochemical properties during the stress response of Saccharomyces cerevisiae and their implication in stress tolerance. The available information for non-conventional yeast species is also included. These non-Saccharomyces species have recently been on the focus of very active research to better explore or control their biotechnological potential envisaging the transition to a sustainable circular bioeconomy.

In vitro and in vivo efficacies of Dectin-1-Fc(IgG)(s) fusion proteins against invasive fungal infections

Fungal infections have increased in the last years, particularly associated to an increment in the number of immunocompromised individuals and the emergence of known or new resistant species, despite the difficulties in the often time-consuming diagnosis. The controversial efficacy of the currently available strategies for their clinical management, apart from their high toxicity and severe side effects, have renewed the interest in the research and development of new broad antifungal alternatives. These encompass vaccines and passive immunization strategies with monoclonal antibodies (mAbs), recognizing ubiquitous fungal targets, such as fungal cell wall β-1,3-glucan polysaccharides, which could be used in early therapeutic intervention without the need for the diagnosis at species-level. As additional alternatives, based on the Dectin-1 great affinity to β-1,3-glucan, our group developed broad antibody-like Dectin1-Fc(IgG)(s) from distinct subclasses (IgG2a and IgG2b) and compared their antifungal in vitro and passive immunizations in vivo performances. Dectin1-Fc(IgG2a) and Dectin1-Fc(IgG2b) demonstrated high affinity to laminarin and the fungal cell wall by ELISA, flow cytometry and microscopy. Both Dectin-1-Fc(IgG)(s) inhibited H. capsulatum and C. neoformans growth in a dose-dependent fashion. For C. albicans, such inhibitory effect was observed with concentrations as low as 0.098 and 0.049 µg/mL, respectively, which correlated with the impairment of the kinetics and lengths of germ tubes in comparison to controls. Previous opsonization with Dectin-1-Fc(IgG)(s) enhanced considerably the macrophage antifungal effector functions, increasing the fungi macrophages-interactions and significantly reducing the intraphagosome fungal survival, as lower CFUs were observed. The administration of both Dectin1-Fc(IgG)(s) reduced the fungal burden and mortality in murine histoplasmosis and candidiasis models, in accordance with previous evaluations in aspergillosis model. These results altogether strongly suggested that therapeutic interventions with Dectin-1-Fc(IgG)(s) fusion proteins could directly impact the innate immunity and disease outcome in favor of the host, by direct neutralization, opsonization, phagocytosis, and fungal elimination, providing interesting information on the potential of these new strategies for the control of invasive fungal infections.

Molecular Mapping of Antifungal Mechanisms Accessing Biomaterials and New Agents to Target Oral Candidiasis

Oral candidiasis has a high rate of development, especially in immunocompromised patients. Immunosuppressive and cytotoxic therapies in hospitalized HIV and cancer patients are known to induce the poor management of adverse reactions, where local and systemic candidiasis become highly resistant to conventional antifungal therapy. The development of oral candidiasis is triggered by several mechanisms that determine oral epithelium imbalances, resulting in poor local defense and a delayed immune system response. As a result, pathogenic fungi colonies disseminate and form resistant biofilms, promoting serious challenges in initiating a proper therapeutic protocol. Hence, this study of the literature aimed to discuss possibilities and new trends through antifungal therapy for buccal drug administration. A large number of studies explored the antifungal activity of new agents or synergic components that may enhance the effect of classic drugs. It was of significant interest to find connections between smart biomaterials and their activity, to find molecular responses and mechanisms that can conquer the multidrug resistance of fungi strains, and to transpose them into a molecular map. Overall, attention is focused on the nanocolloids domain, nanoparticles, nanocomposite synthesis, and the design of polymeric platforms to satisfy sustained antifungal activity and high biocompatibility with the oral mucosa.

Antifungal activity and potential mechanism of berberine hydrochloride against fluconazole-resistant Candida albicans

Introduction. The emergence of resistance to fluconazole in Candida albicans has made the clinical treatment of this microbe difficult. A potential strategy to address this problem involves diminishing fungal resistance to antimicrobial drugs.Hypothesis. Berberine hydrochloride (BH), the primary active ingredient of the traditional Chinese medicine (TCM) Coptis, inhibits the growth of fluconazole-resistant C. albicans through its action on the high-osmolarity glycerol mitogen-activated protein kinase (HOG-MAPK) pathway.Aim. To examine the effect of BH on the HOG-MAPK pathway to assess the potential molecular mechanism by which BH inhibits fluconazole-resistant C. albicans.Methodology. The minimum inhibitory concentration (MIC) of BH to fluconazole-resistant C. albicans was measured using the broth microdilution approach to determine the concentration of effective drug intervention. Changes in physiological functions regulated by the HOG-MAPK pathway in response to BH treatment were measured, as well as the expression of central signalling pathway genes and key downstream factors by qRT-PCR and Western blotting, respectively.Results. BH inhibited fluconazole-resistant C. albicans and the sensitivity to fluconazole increased after BH treatment. At a concentration of 256 and 64 μg ml^-1 BH may affect key downstream factors that regulate several physiological functions of C. albicans by upregulating the core genes expression of SLN1, SSK2, HOG1, and PBS2 in the HOG-MAPK pathway. Upregulation of GPD1, the key gene for glycerol synthesis, increased cell osmotic pressure. BH treatment increased the accumulation of reactive oxygen species by upregulating the expression of the key respiratory metabolism gene ATP11 and downregulating the expression of the superoxide dismutase gene SOD2. Furthermore, downregulation of mycelial-specific HWP1 hindered the morphological transformation of C. albicans and inhibition of the chitin synthase gene CHS3 and the β-(1,3) glucan synthase gene GSC1 impaired cytoderm integrity.Conclusion. BH affects multiple target genes in diminishing the resistance of C. albicans strains to fluconazole. This effect may be related to the action of BH on the HOG-MAPK pathway.

Genetic Screening of Candida albicans Inactivation Mutants Identifies New Genes Involved in Macrophage-Fungal Cell Interactions

Candida albicans, an important fungal pathogen of humans, displays different morphologies, such as yeast, pseudo-hyphae and hyphae, which are recognized unequally by phagocytic cells of the innate immune response. Once C. albicans cells invade host tissues, immune cells such as macrophages are attracted to the site of infection and activated to recognize, engulf and kill the pathogen. We have investigated this fungal cell-macrophage interface by using high-throughput screening of the C. albicans GRACE library to identify genes that can influence this interaction and modify the kinetics of engulfment. Compared with the wild-type (WT) strain, we identified generally faster rates of engulfment for those fungal strains with constitutive pseudo-hyphal and hyphal phenotypes, whereas yeast-form-locked strains showed a reduced and delayed recognition and internalization by macrophages. We identified a number of GRACE strains that showed normal morphological development but exhibited different recognition and engulfment kinetics by cultured macrophages and characterized two mutants that modified interactions with the murine and human-derived macrophages. One mutant inactivated an uncharacterized C. albicans open reading frame that is the ortholog of S. cerevisiae OPY1, the other inactivated CaKRE1. The modified interaction was monitored during a 4 h co-culture. Early in the interaction, both opy1 and kre1 mutant strains showed reduced recognition and engulfment rates by macrophages when compared with WT cells. At fungal germ tube initiation, the engulfment kinetics increased for both mutants and WT cells, however the WT cells still showed a higher internalization by macrophages up to 2 h of interaction. Subsequently, between 2 and 4 h of the interaction, when most macrophages contain engulfed fungal cells, the engulfment kinetics increased for the opy1 mutant and further decreased for the kre1 mutant compared with Ca-WT. It appears that fungal morphology influences macrophage association with C. albicans cells and that both OPY1 and KRE1 play roles in the interaction of the fungal cells with phagocytes.

Ultra-centrifugation force in adaptive evolution changes the cell structure of oleaginous yeast Trichosporon cutaneum into a favorable space for lipid accumulation

Microbial lipid production from lignocellulose biomass provides an essential option for sustainable and carbon neutral supply of future aviation fuels, biodiesel, as well as various food and nutrition products. Oleaginous yeast is the major microbial cell factory but its lipid producing performance is far below the requirements of industrial application. Here we show an ultra-centrifugation fractionation in adaptive evolution (UCF) of Trichosporon cutaneum based on the minor cell density difference. The lightest cells with the maximum intracellular lipid content were isolated by ultra-centrifugation fractionation in the long-term adaptive evolution. Significant changes occurred in the cell morphology with a fragile cell wall wrapping and enlarged intracellular space (two orders of magnitude increase in cell size). Complete and coordinate assimilations of all non-glucose sugars derived from lignocellulose were triggered and fluxed into lipid synthesis. Genome mutations and significant transcriptional regulations of the genes responsible for cell structure were identified and experimentally confirmed. The obtained Trichosporon cutaneum MP11 cells achieved a high lipid production of wheat straw, approximately five-folds greater than that of the parental cells. The study provided an effective method for screening the high lipid containing oleaginous yeast cells as well as the intracellular products accumulating cells in general. This article is protected by copyright. All rights reserved.

Surface Architecture Influences the Rigidity of Candida albicans Cells

Atomic force microscopy (AFM) was used to investigate the morphology and rigidity of the opportunistic pathogenic yeast, Candida albicans ATCC 10231, during its attachment to surfaces of three levels of nanoscale surface roughness. Non-polished titanium (npTi), polished titanium (pTi), and glass with respective average surface roughness (Sa) values of 389 nm, 14 nm, and 2 nm, kurtosis (Skur) values of 4, 16, and 4, and skewness (Sskw) values of 1, 4, and 1 were used as representative examples of each type of nanoarchitecture. Thus, npTi and glass surfaces exhibited similar Sskw and Skur values but highly disparate Sa. C. albicans cells that had attached to the pTi surfaces exhibited a twofold increase in rigidity of 364 kPa compared to those yeast cells attached to the surfaces of npTi (164 kPa) and glass (185 kPa). The increased rigidity of the C. albicans cells on pTi was accompanied by a distinct round morphology, condensed F-actin distribution, lack of cortical actin patches, and the negligible production of cell-associated polymeric substances; however, an elevated production of loose extracellular polymeric substances (EPS) was observed. The differences in the physical response of C. albicans cells attached to the three surfaces suggested that the surface nanoarchitecture (characterized by skewness and kurtosis), rather than average surface roughness, could directly influence the rigidity of the C. albicans cells. This work contributes to the next-generation design of antifungal surfaces by exploiting surface architecture to control the extent of biofilm formation undertaken by yeast pathogens and highlights the importance of performing a detailed surface roughness characterization in order to identify and discriminate between the surface characteristics that may influence the extent of cell attachment and the subsequent behavior of the attached cells.

In Silico Predictions of Ecological Plasticity Mediated by Protein Family Expansions in Early-Diverging Fungi

Early-diverging fungi (EDF) are ubiquitous and versatile. Their diversity is reflected in their genome sizes and complexity. For instance, multiple protein families have been reported to expand or disappear either in particular genomes or even whole lineages. The most commonly mentioned are CAZymes (carbohydrate-active enzymes), peptidases and transporters that serve multiple biological roles connected to, e.g., metabolism and nutrients intake. In order to study the link between ecology and its genomic underpinnings in a more comprehensive manner, we carried out a systematic in silico survey of protein family expansions and losses among EDF with diverse lifestyles. We found that 86 protein families are represented differently according to EDF ecological features (assessed by median count differences). Among these there are 19 families of proteases, 43 CAZymes and 24 transporters. Some of these protein families have been recognized before as serine and metallopeptidases, cellulases and other nutrition-related enzymes. Other clearly pronounced differences refer to cell wall remodelling and glycosylation. We hypothesize that these protein families altogether define the preliminary fungal adaptasome. However, our findings need experimental validation. Many of the protein families have never been characterized in fungi and are discussed in the light of fungal ecology for the first time.

Cells under pressure: how yeast cells respond to mechanical forces

In their natural habitats, unicellular fungal microbes are exposed to a myriad of mechanical cues such as shear forces from fluid flow, osmotic changes, and contact forces arising from microbial expansion in confined niches. While the rigidity of the cell wall is critical to withstand such external forces and balance high internal turgor pressure, it poses mechanical challenges during physiological processes such as cell growth, division, and mating that require cell wall remodeling. Thus, even organisms as simple as yeast have evolved complex signaling networks to sense and respond to intrinsic and extrinsic mechanical forces. In this review, we summarize the type and origin of mechanical forces experienced by unicellular yeast and discuss how these forces reorganize cell polarity and how pathogenic fungi exploit polarized assemblies to track weak spots in host tissues for successful penetration. We then describe mechanisms of force-sensing by conserved sets of mechanosensors. Finally, we elaborate downstream mechanotransduction mechanisms that orchestrate appropriate cellular responses, leading to improved mechanical fitness.

Crosstalk between the calcineurin and cell wall integrity pathways prevents chitin overexpression in Candida albicans

Echinocandins such as caspofungin are frontline antifungal drugs that compromise β-1,3 glucan synthesis in the cell wall. Recent reports have shown that fungal cells can resist killing by caspofungin by upregulation of chitin synthesis, thereby sustaining cell wall integrity (CWI). When echinocandins are removed, the chitin content of cells quickly returns to basal levels, suggesting that there is a fitness cost associated with having elevated levels of chitin in the cell wall. We show here that simultaneous activation of the calcineurin and CWI pathways generates a subpopulation of Candida albicans yeast cells that have supra-normal chitin levels interspersed throughout the inner and outer cell wall, and that these cells are non-viable, perhaps due to loss of wall elasticity required for cell expansion and growth. Mutations in the Ca2+-calcineurin pathway prevented the formation of these non-viable supra-high chitin cells by negatively regulating chitin synthesis driven by the CWI pathway. The Ca2+-calcineurin pathway may therefore act as an attenuator that prevents the overproduction of chitin by coordinating both chitin upregulation and negative regulation of the CWI signaling pathway. This article has an associated First Person interview with the first author of the paper.

A Novel Role for Histatin 5 in Combination with Zinc to Promote Commensalism in C. albicans Survivor Cells

Candida albicans is maintained as a commensal by immune mechanisms at the oral epithelia. Oral antifungal peptide Histatin 5 (Hst 5) may function in innate immunity, but the specific role Hst 5 plays in C. albicans commensalism is unclear. Since Zn-binding potentiates the candidacidal activity of Hst 5, we hypothesized that Hst 5+Zn would elicit a unique fungal stress response to shape interactions between C. albicans and oral epithelial cells (OECs). We found that Hst 5+Zn but not Hst 5 alone resulted in the activation of cell wall integrity (CWI) signaling, and deletion mutants were then used to determine that CWI-mediated chitin synthesis was protective against killing. Using flow cytometry, we confirmed that Hst 5+Zn-treated cells had significantly elevated levels of cell-wall chitin, mannan and β-1,3 glucan compared to Hst 5-treated cells. We then tested the activation of host signaling components involved in C. albicans cell-wall recognition. The immunoblot assay of C. albicans-exposed oral epithelial cells showed increased activation of EphA2 and NF-κB but not EGFR. Interestingly, C. albicans treated with Hst 5+Zn induced the global suppression of pro-inflammatory cytokine release from OECs, but an increase in negative regulator IL-10. Hst 5+Zn-treated cells were more adherent but ultimately less invasive to OECs than control cells, thus indicating lowered virulence. Therefore, Hst 5+Zn-treated C. albicans cells are discerned by epithelial monolayers, but are less virulent and promote anti-inflammatory signaling, suggesting that Hst 5+Zn in combination could play a role in regulating commensalism of oral C. albicans through cell wall reorganization.