Insight Into the Properties and Immunoregulatory Effect of Extracellular Vesicles Produced by Candida glabrata, Candida parapsilosis, and Candida tropicalis Biofilms

Extracellular vesicle production: A bidirectional effect in the interplay between host and Candida fungi

Candida fungi exploit various virulence strategies to invade the human host, while host cells employ diverse mechanisms to maintain homeostasis and respond to infection. Extracellular vesicles (EVs) are integral components of the multifaceted landscape of host-pathogen interactions, with their abundant production by all contributors involved in these complex and dynamic relations. Herein, we present the current state of knowledge regarding the host response by releasing EVs in reaction to Candida, as well as the influence of fungal EVs on the functionality of the confronted host cells. Fungal vesicles contribute to enhanced adhesion of pathogens to human cells as evidenced for C. auris, and may modulate the production of several cytokines, including IL-6, IL-8, IL-10, IL-12p40, TGF-β and TNF-α, thereby exerting pro-infective and pro-inflammatory effects, as described for C. albicans and other Candida species. Whereas the biosynthesis of EVs by host cells can dynamically modulate the proliferation and viability of fungal cells and affect the candidacidal functionality of other effector cells. The reciprocal influence of EVs from host cells and Candida pathogens is a key focus, explaining their significant role in cell signaling and interkingdom communication.

Isolation and Characteristics of Extracellular Vesicles Produced by Probiotics: Yeast Saccharomyces boulardii CNCM I-745 and Bacterium Streptococcus salivarius K12

Numerous probiotic microorganisms have repeatedly been shown to produce nanometer-sized structures named extracellular vesicles (EVs). Recently, it has been suggested that similarly to whole microbial cells, EVs produced by probiotics may also demonstrate health benefits to the host, while their application does not involve the risk of infection caused by live microorganisms. In this work, we isolated EVs from two probiotic species originating from different taxonomic domains - yeast Saccharomyces boulardii CNCM I-745 and bacterium Streptococcus salivarius K12. The diameters of S. boulardii EVs were about 142 nm and for S. salivarius EVs about 123 nm. For S. boulardii EVs, 1641 proteins and for S. salivarius EVs, 466 proteins were identified with a liquid chromatography-coupled tandem mass spectrometry and then functionally classified. In both microbial species, metabolic proteins significantly contributed to the cargo of EVs comprising 25% and 26% of all identified vesicular proteins for fungi and bacteria, respectively. Moreover, enzymes associated with cell wall rearrangement, including enzymatically active glucanases, were also identified in EVs. Furthermore, probiotic EVs were shown to influence host cells and stimulate the production of IL-1β and IL-8 by the human monocytic cell line THP-1, and, at the same time, did not cause any remarkable reduction in the survival rate of Galleria mellonella larvae in this invertebrate model commonly used to evaluate microbial EV toxicity. These observations suggest that the EVs produced by the investigated probiotic microorganisms may be promising structures for future use in pro-health applications.

Microbial extracellular vesicles contribute to antimicrobial resistance

With the escalating global antimicrobial resistance crisis, there is an urgent need for innovative strategies against drug-resistant microbes. Accumulating evidence indicates microbial extracellular vesicles (EVs) contribute to antimicrobial resistance. Therefore, comprehensively elucidating the roles and mechanisms of microbial EVs in conferring resistance could provide new perspectives and avenues for novel antimicrobial approaches. In this review, we systematically examine current research on antimicrobial resistance involving bacterial, fungal, and parasitic EVs, delineating the mechanisms whereby microbial EVs promote resistance. Finally, we discuss the application of bacterial EVs in antimicrobial therapy.

Secreted Aspartic Proteinases: Key Factors in Candida Infections and Host-Pathogen Interactions

Extracellular proteases are key factors contributing to the virulence of pathogenic fungi from the genus Candida. Their proteolytic activities are crucial for extracting nutrients from the external environment, degrading host defenses, and destabilizing the internal balance of the human organism. Currently, the enzymes most frequently described in this context are secreted aspartic proteases (Saps). This review comprehensively explores the multifaceted roles of Saps, highlighting their importance in biofilm formation, tissue invasion through the degradation of extracellular matrix proteins and components of the coagulation cascade, modulation of host immune responses via impairment of neutrophil and monocyte/macrophage functions, and their contribution to antifungal resistance. Additionally, the diagnostic challenges associated with Candida infections and the potential of Saps as biomarkers were discussed. Furthermore, we examined the prospects of developing vaccines based on Saps and the use of protease inhibitors as adjunctive therapies for candidiasis. Given the complex biology of Saps and their central role in Candida pathogenicity, a multidisciplinary approach may pave the way for innovative diagnostic strategies and open new opportunities for innovative clinical interventions against candidiasis.

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.

Extracellular Vesicles from a Biofilm of a Clinical Isolate of Candida albicans Negatively Impact on Klebsiella pneumoniae Adherence and Biofilm Formation

The opportunistic human fungal pathogen Candida albicans produces and releases into the surrounding medium extracellular vesicles (EVs), which are involved in some processes as communication between fungal cells and host-pathogen interactions during infection. Here, we have conducted the isolation of EVs produced by a clinical isolate of C. albicans during biofilm formation and proved their effect towards the ability of the Gram-negative bacterial pathogen Klebsiella pneumoniae to adhere to HaCaT cells and form a biofilm in vitro. The results represent the first evidence of an antagonistic action of fungal EVs against bacteria.

Glyceraldehyde 3-Phosphate Dehydrogenase on the Surface of Candida albicans and Nakaseomyces glabratus Cells-A Moonlighting Protein That Binds Human Vitronectin and Plasminogen and Can Adsorb to Pathogenic Fungal Cells via Major Adhesins Als3 and Epa6

Candida albicans and other closely related pathogenic yeast-like fungi carry on their surface numerous loosely adsorbed "moonlighting proteins"-proteins that play evolutionarily conserved intracellular functions but also appear on the cell surface and exhibit additional functions, e.g., contributing to attachment to host tissues. In the current work, we characterized this "moonlighting" role for glyceraldehyde 3-phosphate dehydrogenase (GAPDH, EC 1.2.1.12) of C. albicans and Nakaseomyces glabratus. GAPDH was directly visualized on the cell surface of both species and shown to play a significant part in the total capacity of fungal cells to bind two selected human host proteins-vitronectin and plasminogen. Using purified proteins, both host proteins were found to tightly interact with GAPDH, with dissociation constants in an order of 10-8 M, as determined by bio-layer interferometry and surface plasmon resonance measurements. It was also shown that exogenous GAPDH tightly adheres to the surface of candidal cells, suggesting that the cell surface location of this moonlighting protein may partly result from the readsorption of its soluble form, which may be present at an infection site (e.g., due to release from dying fungal cells). The major dedicated adhesins, covalently bound to the cell wall-agglutinin-like sequence protein 3 (Als3) and epithelial adhesin 6 (Epa6)-were suggested to serve as the docking platforms for GAPDH in C. albicans and N. glabratus, respectively.

Candida parapsilosis complex in the clinical setting

Representatives of the Candida parapsilosis complex are important yeast species causing human infections, including candidaemia as one of the leading diseases. This complex comprises C. parapsilosis, Candida orthopsilosis and Candida metapsilosis, and causes a wide range of clinical presentations from colonization to superficial and disseminated infections with a high prevalence in preterm-born infants and the potential to cause outbreaks in hospital settings. Compared with other Candida species, the C. parapsilosis complex shows high minimal inhibitory concentrations for echinocandin drugs due to a naturally occurring FKS1 polymorphism. The emergence of clonal outbreaks of strains with resistance to commonly used antifungals, such as fluconazole, is causing concern. In this Review, we present the latest medical data covering epidemiology, diagnosis, resistance and current treatment approaches for the C. parapsilosis complex. We describe its main clinical manifestations in adults and children and highlight new treatment options. We compare the three sister species, examining key elements of microbiology and clinical characteristics, including the population at risk, disease manifestation and colonization status. Finally, we provide a comprehensive resource for clinicians and researchers focusing on Candida species infections and the C. parapsilosis complex, aiming to bridge the emerging translational knowledge and future therapeutic challenges associated with this human pathogen.

Stress Conditions Affect the Immunomodulatory Potential of Candida albicans Extracellular Vesicles and Their Impact on Cytokine Release by THP-1 Human Macrophages

Human immune cells possess the ability to react complexly and effectively after contact with microbial virulence factors, including those transported in cell-derived structures of nanometer sizes termed extracellular vesicles (EVs). EVs are produced by organisms of all kingdoms, including fungi pathogenic to humans. In this work, the immunomodulatory properties of EVs produced under oxidative stress conditions or at host concentrations of CO2 by the fungal pathogen Candida albicans were investigated. The interaction of EVs with human pro-monocytes of the U-937 cell line was established, and the most notable effect was attributed to oxidative stress-related EVs. The immunomodulatory potential of tested EVs against human THP-1 macrophages was verified using cytotoxicity assay, ROS-production assay, and the measurement of cytokine production. All fungal EVs tested did not show a significant cytotoxic effect on THP-1 cells, although a slight pro-oxidative impact was indicated for EVs released by C. albicans cells grown under oxidative stress. Furthermore, for all tested types of EVs, the pro-inflammatory properties related to increased IL-8 and TNF-α production and decreased IL-10 secretion were demonstrated, with the most significant effect observed for EVs released under oxidative stress conditions.

Candida albicans Biofilm-Derived Extracellular Vesicles Are Involved in the Tolerance to Caspofungin, Biofilm Detachment, and Fungal Proteolytic Activity

It has been repeatedly reported that the cells of organisms in all kingdoms of life produce nanometer-sized lipid membrane-enveloped extracellular vesicles (EVs), transporting and protecting various substances of cellular origin. While the composition of EVs produced by human pathogenic fungi has been studied in recent decades, another important challenge is the analysis of their functionality. Thus far, fungal EVs have been shown to play significant roles in intercellular communication, biofilm production, and modulation of host immune cell responses. In this study, we verified the involvement of biofilm-derived EVs produced by two different strains of Candida albicans-C. albicans SC5314 and 3147 (ATCC 10231)-in various aspects of biofilm function by examining its thickness, stability, metabolic activity, and cell viability in the presence of EVs and the antifungal drug caspofungin. Furthermore, the proteolytic activity against the kininogen-derived antimicrobial peptide NAT26 was confirmed by HPLC analysis for C. albicans EVs that are known to carry, among others, particular members of the secreted aspartic proteinases (Saps) family. In conclusion, EVs derived from C. albicans biofilms were shown to be involved in biofilm tolerance to caspofungin, biofilm detachment, and fungal proteolytic activity.

Fungal extracellular vesicle-mediated regulation: from virulence factor to clinical application

Invasive fungal disease (IFD) poses a significant threat to immunocompromised patients and remains a global challenge due to limited treatment options, high mortality and morbidity rates, and the emergence of drug-resistant strains. Despite advancements in antifungal agents and diagnostic techniques, the lack of effective vaccines, standardized diagnostic tools, and efficient antifungal drugs contributes to the ongoing impact of invasive fungal infections (IFI). Recent studies have highlighted the presence of extracellular vesicles (EVs) released by fungi carrying various components such as enzymes, lipids, nucleic acids, and virulence proteins, which play roles in both physiological and pathological processes. These fungal EVs have been shown to interact with the host immune system during the development of fungal infections whereas their functional role and potential application in patients are not yet fully understood. This review summarizes the current understanding of the biologically relevant findings regarding EV in host-pathogen interaction, and aim to describe our knowledge of the roles of EV as diagnostic tools and vaccine vehicles, offering promising prospects for the treatment of IFI patients.

The emerging role of extracellular vesicles in fungi: a double-edged sword

Fungi are eukaryotic microorganisms found in nature, which can invade the human body and cause tissue damage, inflammatory reactions, organ dysfunctions, and diseases. These diseases can severely damage the patient's body systems and functions, leading to a range of clinical symptoms that can be life-threatening. As the incidence of invasive fungal infections has progressively increased in the recent years, a wealth of evidence has confirmed the "double-edged sword" role of fungal extracellular vesicles (EVs) in intercellular communication and pathogen-host interactions. Fungal EVs act as mediators of cellular communication, affecting fungal-host cell interactions, delivering virulence factors, and promoting infection. Fungal EVs can also have an induced protective effect, affecting fungal growth and stimulating adaptive immune responses. By integrating recent studies, we discuss the role of EVs in fungi, providing strong theoretical support for the early prevention and treatment of invasive fungal infections. Finally, we highlight the feasibility of using fungal EVs as drug carriers and in vaccine development.

Exploring Extracellular Vesicles of Probiotic Yeast as Carriers of Biologically Active Molecules Transferred to Human Intestinal Cells

Extracellular vesicles (EVs) are nanoparticles containing various bioactive cargos-e.g., proteins, RNAs, and lipids-that are released into the environment by all cell types. They are involved in, amongst other functions, intercellular communication. This article presents studies on EVs produced by the probiotic yeast Saccharomyces boulardii CNCM I-745. The size distribution and concentration of EVs in the liquid culture of yeast were estimated. Moreover, the vesicles of S. boulardii were tested for their cytotoxicity against three model human intestinal cell lines. This study did not show any significant negative effect of yeast EVs on these cells under tested conditions. In addition, EVs of S. boulardii were verified for their ability to internalize in vitro with human cells and transfer their cargo. The yeast vesicles were loaded with doxorubicin, an anticancer agent, and added to the cellular cultures. Subsequently, microscopic observations revealed that these EVs transferred the compound to human intestinal cell lines. A cytotoxicity test confirmed the activity of the transferred doxorubicin. Detailed information about the proteins present in EVs might be important in terms of exploring yeast EVs as carriers of active molecules. Thus, proteomic analysis of the EV content was also conducted within the present study, and it allowed the identification of 541 proteins after matching them to the Saccharomyces Genome Database (SGD). Altogether, this study provides strong evidence that the EVs of the probiotic CNCM I-745 strain could be considered a drug delivery system.

Cecropin D-derived synthetic peptides in the fight against Candida albicans cell filamentation and biofilm formation

The recent progressive increase in the incidence of invasive fungal infections, especially in immunocompromised patients, makes the search for new therapies crucial in the face of the growing drug resistance of prevalent nosocomial yeast strains. The latest research focuses on the active compounds of natural origin, inhibiting fungal growth, and preventing the formation of fungal biofilms. Antimicrobial peptides are currently the subject of numerous studies concerning effective antifungal therapy. In the present study, the antifungal properties of two synthetic peptides (ΔM3, ΔM4) derived from an insect antimicrobial peptide - cecropin D - were investigated. The fungicidal activity of both compounds was demonstrated against the yeast forms of Candida albicans, Candida tropicalis, and Candida parapsilosis, reaching a MFC_99.9 in the micromolar range, while Candida glabrata showed greater resistance to these peptides. The scanning electron microscopy revealed a destabilization of the yeast cell walls upon treatment with both peptides; however, their effectiveness was strongly modified by the presence of salt or plasma in the yeast environment. The transition of C. albicans cells from yeast to filamentous form, as well as the formation of biofilms, was effectively reduced by ΔM4. Mature biofilm viability was inhibited by a higher concentration of this peptide and was accompanied by increased ROS production, activation of the GPX3 and SOD5 genes, and finally, increased membrane permeability. Furthermore, both peptides showed a synergistic effect with caspofungin in inhibiting the metabolic activity of C. albicans cells, and an additive effect was also observed for the mixtures of peptides with amphotericin B. The results indicate the possible potential of the tested peptides in the prevention and treatment of candidiasis.

Caspofungin Affects Extracellular Vesicle Production and Cargo in Candida auris

Antifungal resistance has become more frequent, either due to the emergence of naturally resistant species or the development of mechanisms that lead to resistance in previously susceptible species. Among these fungal species of global threat, Candida auris stands out for commonly being highly resistant to antifungal drugs, and some isolates are pan-resistant. The rate of mortality linked to C. auris infections varies from 28% to 78%. In this study, we characterized C. auris extracellular vesicles (EVs) in the presence of caspofungin, an echinocandin, which is the recommended first line antifungal for the treatment of infections due to this emerging pathogen. Furthermore, we also analyzed the protein and RNA content of EVs generated by C. auris cultivated with or without treatment with caspofungin. We observed that caspofungin led to the increased production of EVs, and treatment also altered the type and quantity of RNA molecules and proteins enclosed in the EVs. There were distinct classes of RNAs in the EVs with ncRNAs being the most identified molecules, and tRNA-fragments (tRFs) were abundant in each of the strains studied. We also identified anti-sense RNAs, varying from 21 to 55 nt in length. The differentially abundant mRNAs detected in EVs isolated from yeast subjected to caspofungin treatment were related to translation, nucleosome core and cell wall. The differentially regulated proteins identified in the EVs produced during caspofungin treatment were consistent with the results observed with the RNAs, with the enriched terms being related to translation and cell wall. Our study adds new information on how an echinocandin can affect the EV pathway, which is associated with the yeast cell being able to evade treatment and persist in the host. The ability of C. auris to efficiently alter the composition of EVs may represent a mechanism for the fungus to mitigate the effects of antifungal agents.

A common vesicle proteome drives fungal biofilm development

Candida species are commensal organisms commonly interacting in the same host niche. In the pathogenic state, they frequently grow as a biofilm, often in mixed infections. The present studies observe a reliance upon common extracellular vesicle cargo for biofilm structure and function supporting interactions among species. The results reveal a vesicle cargo-driven coordination among Candida species during biofilm formation.

Extracellular vesicles mediate community interactions among cells ranging from unicellular microbes to complex vertebrates. Extracellular vesicles of the fungal pathogen Candida albicans are vital for biofilm communities to produce matrix, which confers environmental protection and modulates community dispersion. Infections are increasingly due to diverse Candida species, such as the emerging pathogen Candida auris, as well as mixed Candida communities. Here, we define the composition and function of biofilm-associated vesicles among five species across the Candida genus. We find similarities in vesicle size and release over the biofilm lifespan. Whereas overall cargo proteomes differ dramatically among species, a group of 36 common proteins is enriched for orthologs of C. albicans biofilm mediators. To understand the function of this set of proteins, we asked whether mutants in select components were important for key biofilm processes, including drug tolerance and dispersion. We found that the majority of these cargo components impact one or both biofilm processes across all five species. Exogenous delivery of wild-type vesicle cargo returned mutant phenotypes toward wild type. To assess the impact of vesicle cargo on interspecies interactions, we performed cross-species vesicle addition and observed functional complementation for both biofilm phenotypes. We explored the biologic relevance of this cross-species biofilm interaction in mixed species and mutant studies examining the drug-resistance phenotype. We found a majority of biofilm interactions among species restored the community’s wild-type behavior. Our studies indicate that vesicles influence the development of protective monomicrobial and mixed microbial biofilm communities.

Evaluation of the biofilm life cycle between Candida albicans and Candida tropicalis

Candida tropicalis is an emergent pathogen with a high rate of mortality associated with its biofilm formation. Biofilm formation has important repercussions on the public health system. However, little is still known about its biofilm life cycle. The present study analyzed the biofilm life cycle of Candida albicans and C. tropicalis during various timepoints (24, 48, 72, and 96 h) through biomass assays, colony-forming unit (CFU) counting, and epifluorescence and scanning electron microscopies. Our results showed a significant difference between C. albicans and C. tropicalis biofilms in each biomass and viability assay. All-time samples in the biomass and viability assays confirmed statistical differences between the Candida species through pairwise Wilcoxon tests (p < 0.05). C. albicans demonstrated a lower biomass growth but reached nearly the same level of C. tropicalis biomass at 96 h, while the CFU counting assays exhibited a superior number of viable cells within the C. tropicalis biofilm. Statistical differences were also found between C. albicans and C. tropicalis biofilms from 48- and 72-h microscopies, demonstrating C. tropicalis with a higher number of total cells within biofilms and C. albicans cells with a superior cell area and higher matrix production. Therefore, the present study proved the higher biofilm production of C. tropicalis.