The regulation of hyphae growth in Candida albicans

The Importance of Vacuolar Ion Homeostasis and Trafficking in Hyphal Development and Virulence in Candida albicans

The vacuole of Candida albicans plays a significant role in many processes including homeostasis control, cellular trafficking, dimorphic switching, and stress tolerance. Thus, understanding the factors affecting vacuole function is important for the identification of new drug targets needed in response to the world’s increasing levels of invasive infections and the growing issue of fungal drug resistance. Past studies have shown that vacuolar proton-translocating ATPases (V-ATPases) play a central role in pH homeostasis and filamentation. Vacuolar protein sorting components (VPS) regulate V-ATPases assembly and at the same time affect hyphal development. As well, vacuolar calcium exchange systems like Yvc1 and Pmc1 maintain cytosolic calcium levels while being affected by V-ATPases function. All these proteins play a role in the virulence and pathogenesis of C. albicans. This review highlights the relationships among V-ATPases, VPS, and vacuolar calcium exchange proteins while summarizing their importance in C. albicans infections.

Inhibition of morphological transition and hyphae extension in Candida spp. by occidiofungin

AIMS

To assess the efficacy of the antifungal, occidiofungin, against Candida albicans and Candida tropicalis morphological transformation.

METHODS AND RESULTS

Susceptibility assays and morphological data were used to demonstrate that occidiofungin effectively targets C. albicans and C. tropicalis undergoing morphological transformation. Susceptibility assays found that cell sensitivity to occidiofungin varied with the media conditions used for morphological switching. Microscopy data showed that occidiofungin inhibited hyphae formation when added at the time of morphological induction and hyphal extension when added within the first hour following hyphae induction. Immunoblot analysis demonstrated that occidiofungin addition prevented activation of Cek1p MAPK signalling.

CONCLUSIONS

The data indicated that the antimicrobial compound, occidiofungin, effectively targets hyphae elongation in Candida spp. and suggests the biological target of occidiofungin is necessary for the morphological changes associated with yeast-to-hyphae switching.

SIGNIFICANCE AND IMPACT OF THE STUDY

Findings from this study demonstrated that occidiofungin effectively targets the invasive growth of dimorphic Candida which suggests this compound may also inhibit the heterogenous population of cells present in a clinical setting. This presents occidiofungin as a promising candidate for the treatment of Candida associated infections.

The two-component signal transduction system and its regulation in Candida albicans

Candida albicans, which can cause superficial and life-threatening systemic infections, is the most common opportunistic fungal pathogen in the human microbiome. The two-component system is one of the most important C. albicans signal transduction pathways, regulating the response to oxidative and osmotic stresses, adhesion, morphogenesis, cell wall synthesis, virulence, drug resistance, and the host-pathogen interactions. Notably, some components of this signaling pathway have not been found in the human genome, indicating that the two-component system of C. albicans can be a potential target for new antifungal agents. Here, we summarize the composition, signal transduction, and regulation of the two-component system of C. albicans to emphasize its essential roles in the pathogenesis of C. albicans and the new therapeutic target for antifungal drugs.

Adenylyl Cyclase and Protein Kinase A Play Redundant and Distinct Roles in Growth, Differentiation, Antifungal Drug Resistance, and Pathogenicity of Candida auris

Candida auris is a globally emerging multidrug-resistant fungal pathogen. Its pathogenicity-related signaling networks are largely unknown. Here, we characterized the pathobiological functions of the cyclic AMP (cAMP)/protein kinase A (PKA) signaling pathway in C. auris. We focused on adenylyl cyclase (CYR1), the PKA regulatory subunit (BCY1), and the PKA catalytic subunits (TPK1 and TPK2). We concluded that PKA acts both dependently and independently of Cyr1 in C. auris. Tpk1 and Tpk2 have major and minor roles, respectively, in PKA activity and functions. Both Cyr1 and PKA promote growth, thermotolerance, filamentous growth, and resistance to stress and antifungal drugs by regulating expression of multiple effector genes. In addition, Cyr1 and PKA subunits were involved in disinfectant resistance of C. auris. However, deletion of both TPK1 and TPK2 generally resulted in more severe defects than CYR1 deletion, indicating that Cyr1 and PKA play redundant and distinct roles. Notably, Tpk1 and Tpk2 have redundant but Cyr1-independent roles in haploid-to-diploid cell transition, which increases virulence of C. auris. However, Tpk1 and Tpk2 often play opposing roles in formation of biofilms and the cell wall components chitin and chitosan. Surprisingly, deletion of CYR1 or TPK1/TPK2, which resulted in severe in vitro growth defects at 37°C, did not attenuate virulence, and BCY1 deletion reduced virulence of C. auris in a systemic murine infection model. In conclusion, this study provides comprehensive insights into the role of the cAMP/PKA pathway in drug resistance and pathogenicity of C. auris and suggests a potential therapeutic option for treatment of C. auris-mediated candidemia.

Antifungal Activity and Potential Mechanism of 6,7, 4'-O-Triacetylscutellarein Combined With Fluconazole Against Drug-Resistant C. albicans

The increased resistance of Candida albicans to conventional antifungal drugs poses a huge challenge to the clinical treatment of this infection. In recent years, combination therapy, a potential treatment method to overcome C. albicans resistance, has gained traction. This study assessed the effect of 6,7,4′-O-triacetylscutellarein (TA) combined with fluconazole (FLC) on C. albicans in vitro and in vivo. TA combined with FLC showed good synergistic antifungal activity against drug-resistant C. albicans in vitro, with a partial inhibitory concentration index (FICI) of 0.0188–0.1800. In addition, the time-kill curve confirmed the synergistic effect of TA and FLC. TA combined with FLC showed a strong synergistic inhibitory effect on the biofilm formation of resistant C. albicans. The combined antifungal efficacy of TA and FLC was evaluated in vivo in a mouse systemic fungal infection model. TA combined with FLC prolonged the survival rate of mice infected with drug-resistant C. albicans and reduced tissue invasion. TA combined with FLC also significantly inhibited the yeast-hypha conversion of C. albicans and significantly reduced the expression of RAS-cAMP-PKA signaling pathway-related genes (RAS1 and EFG1) and hyphal-related genes (HWP1 and ECE1). Furthermore, the mycelium growth on TA combined with the FLC group recovered after adding exogenous db-cAMP. Collectively, these results show that TA combined with FLC inhibits the formation of hyphae and biofilms through the RAS-cAMP-PKA signaling pathway, resulting in reduced infectivity and resistance of C. albicans. Therefore, this study provides a basis for the treatment of drug-resistant C. albicans infections.

Staphylococcus aureus Synergized with Candida albicans to Increase the Pathogenesis and Drug Resistance in Cutaneous Abscess and Peritonitis Murine Models

The mixed species of Staphylococcus aureus and Candida albicans can cause infections on skin, mucosa or bloodstream; however, mechanisms of their cross-kingdom interactions related to pathogenesis and drug resistance are still not clear. Here an increase of S. aureus proliferation and biofilm formation was observed in S. aureus and C. albicans dual-species culture, and the synergistic pathogenic effect was then confirmed in both local (cutaneous abscess) and systemic infection (peritonitis) murine models. According to the transcriptome analysis of the dual-species culture, virulence factors of S. aureus were significantly upregulated. Surprisingly, the beta-lactams and vancomycin-resistant genes in S. aureus as well as azole-resistant genes in C. albicans were also significantly increased. The synergistic effects on drug resistance to both antibacterial and antifungal agents were further proved both in vitro and in cutaneous abscess and peritonitis murine models treated by methicillin, vancomycin and fluconazole. The synergistic interactions between S. aureus and C. albicans on pathogenesis and drug resistance highlight the importance of targeting the microbial interactions in polyspecies-associated infections.

Histatin 5 Metallopeptides and Their Potential against Candida albicans Pathogenicity and Drug Resistance

Usually caused by Candida albicans, buccal candidiasis begins with the morphological transition between yeast and hyphal cells. Over time and without the correct treatment, it can be disseminated through the bloodstream becoming a systemic infection with high mortality rates. C. albicans already shows resistance against antifungals commonly used in treatments. Therefore, the search for new drugs capable of overcoming antifungal resistance is essential. Histatin 5 (Hst5) is an antimicrobial peptide of the Histatin family, that can be found naturally in human saliva. This peptide presents high antifungal activity against C. albicans. However, Hst5 action can be decreased for interaction with enzymes and metal ions present in the oral cavity. The current work aims to bring a brief review of relevant aspects of the pathogenesis and resistance mechanisms already reported for C. albicans. In addition, are also reported here the main immune responses of the human body and the most common antifungal drugs. Finally, the most important aspects regarding Histatin 5 and the benefits of its interaction with metals are highlighted. The intention of this review is to show the promising use of Hst5 metallopeptides in the development of effective drugs.

From Jekyll to Hyde: The Yeast-Hyphal Transition of Candida albicans

Candida albicans is a major fungal pathogen of humans, accounting for 15% of nosocomial infections with an estimated attributable mortality of 47%. C. albicans is usually a benign member of the human microbiome in healthy people. Under constant exposure to highly dynamic environmental cues in diverse host niches, C. albicans has successfully evolved to adapt to both commensal and pathogenic lifestyles. The ability of C. albicans to undergo a reversible morphological transition from yeast to filamentous forms is a well-established virulent trait. Over the past few decades, a significant amount of research has been carried out to understand the underlying regulatory mechanisms, signaling pathways, and transcription factors that govern the C. albicans yeast-to-hyphal transition. This review will summarize our current understanding of well-elucidated signal transduction pathways that activate C. albicans hyphal morphogenesis in response to various environmental cues and the cell cycle machinery involved in the subsequent regulation and maintenance of hyphal morphogenesis.

Metabolic flexibility and extensive adaptability governing multiple drug resistance and enhanced virulence in Candida albicans

Commensal fungus-Candida albicans turn pathogenic during the compromised immunity of the host, causing infections ranging from superficial mucosal to dreadful systemic ones. C. albicans has evolved various adaptive measures which collectively contribute towards its enhanced virulence. Among fitness attributes, metabolic flexibility and vigorous stress response are essential for its pathogenicity and virulence. Metabolic flexibility provides a means for nutrient assimilation and growth in diverse host microenvironments and reduces the vulnerability of the pathogen to various antifungals besides evading host immune response(s). Inside the host micro-environments, C. albicans efficiently utilizes the multiple fermentable and non-fermentable carbon sources to sustain and proliferate in glucose deficit conditions. The utilization of alternative carbon sources further highlights the importance of understanding these pathways as the attractive and potential therapeutic target. A thorough understanding of metabolic flexibility and adaptation to environmental stresses is warranted to decipher in-depth insights into virulence and molecular mechanisms of fungal pathogenicity. In this review, we have attempted to provide a detailed and recent understanding of some key aspects of fungal biology. Particular focus will be placed on processes like nutrient assimilation and utilization, metabolic adaptability, virulence factors, and host immune response in C. albicans leading to its enhanced pathogenicity.

Suppression of hyphal formation and virulence of Candida albicans by natural and synthetic compounds

Candida albicans undergoes a morphological yeast-to-hyphal transition during infection, which plays a significant role in its pathogenesis. The filamentous morphology of the hyphal form has been identified as a virulence factor as it facilitates surface adherence, intertwining with biofilm, invasion, and damage to host tissues and organs. Hence, inhibition of filamentation in addition to biofilm formation is considered a viable strategy against C. albicans infections. Furthermore, a good understanding of the signaling pathways involved in response to environmental cues driving hyphal growth is also critical to an understanding of C. albicans pathogenicity and to develop novel therapies. In this review, first the clinical significance and transcriptional control of C. albicans hyphal morphogenesis are addressed. Then, various strategies employed to suppress filamentation, prevent biofilm formation, and reduce virulence are discussed. These strategies include the inhibition of C. albicans filament formation using natural or synthetic compounds, and their combination with other agents or nanoformulations.

The Multiple Functions of Rho GTPases in Fission Yeasts

The Rho family of GTPases represents highly conserved molecular switches involved in a plethora of physiological processes. Fission yeast Schizosaccharomyces pombe has become a fundamental model organism to study the functions of Rho GTPases over the past few decades. In recent years, another fission yeast species, Schizosaccharomyces japonicus, has come into focus offering insight into evolutionary changes within the genus. Both fission yeasts contain only six Rho-type GTPases that are spatiotemporally controlled by multiple guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), and whose intricate regulation in response to external cues is starting to be uncovered. In the present review, we will outline and discuss the current knowledge and recent advances on how the fission yeasts Rho family GTPases regulate essential physiological processes such as morphogenesis and polarity, cellular integrity, cytokinesis and cellular differentiation.

Disinfection mechanism of the photocatalytic activity of SnO2 thin films against Candida albicans, proposed from experimental and simulated perspectives

Nosocomial infections are an important health problem and cause of complications and death in hospitalized patients. This problem should be solved from the preventive angle, avoiding the spread of infections by designing disinfection methods based on the photocatalytic activity of semiconductor materials such as tin oxide (SnO₂). The antimicrobial activity of UV light was tested by using inoculation with Candida albicans ATCC10231 on SnO₂ thin films and counting colony forming units (CFU). The interaction of UV light with SnO₂ was analyzed by density functional theory (DFT) and the extension to the Hubbard model (DFT+U) schemes to predict the electron behavior at the subatomic level. After exposure to UV light, C. albicans showed a reduction of 36.5% in viable cells, and when SnO₂ was included, cell viability was reduced by 60.2%. Measurements of the electronic structure obtained by the first-principle calculations under the DFT and DFT+U schemes showed that the O-p orbitals mediate the oxidation process in the bulk semiconductor. By including the surface effects when cleaving the (1 0 0) plane, the three orbitals O-p, Sn-p, and Sn-s are the mediators. SnO₂ films are promising antimicrobial coatings because UV light has a synergic activity with thin films, resulting in faster disinfection.

Escherichia coli enhances the virulence factors of Candida albicans, the cause of vulvovaginal candidiasis, in a dual bacterial/fungal biofilm

Co-infection with other microorganisms can promote the Candida albicans to be invasive. In this study, Escherichia coli and C. albicans were co-isolated from the women with candidiasis symptoms. The in vitro effects of E. coli on C. albicans hypha development, biofilm formation, antibiotic susceptibility, dispersion from the biofilm, expression of Als3, Hwp1, and Tup1 genes, and pathogenesis in Galleria mellonella were investigated. Electron microscopic images revealed that hypha induction was markedly increased in the bacteria-fungi co-culture. Biofilm formation was increased 2.2 fold in the presence of E. coli. The minimum inhibitory concentration of nystatin against Candida was increased from (μg mL^-1) 25 to 50 in the dual biofilm. Candida dissemination was increased up to 2.7 fold from the mixed fungi/bacteria biofilm. The expression of ALS3 and HWP1 genes was increased (5.9 and 2.0 fold, respectively) while the TUP1 gene expression was decreased (0.4 fold) when C. albicans was incubated with E. coli. The simultaneous injection of C. albicans and E. coli to the insect larvae increased Galleria mortality up to 40%. This study demonstrated the effects of E. coli to promote fungi virulence factors, which suggest polymicrobial interaction should be considered during treatment of fungal infections.

The impact of the Fungus-Host-Microbiota interplay upon Candida albicans infections: current knowledge and new perspectives

Candida albicans is a major fungal pathogen of humans. It exists as a commensal in the oral cavity, gut or genital tract of most individuals, constrained by the local microbiota, epithelial barriers and immune defences. Their perturbation can lead to fungal outgrowth and the development of mucosal infections such as oropharyngeal or vulvovaginal candidiasis, and patients with compromised immunity are susceptible to life-threatening systemic infections. The importance of the interplay between fungus, host and microbiota in driving the transition from C. albicans commensalism to pathogenicity is widely appreciated. However, the complexity of these interactions, and the significant impact of fungal, host and microbiota variability upon disease severity and outcome, are less well understood. Therefore, we summarise the features of the fungus that promote infection, and how genetic variation between clinical isolates influences pathogenicity. We discuss antifungal immunity, how this differs between mucosae, and how individual variation influences a person's susceptibility to infection. Also, we describe factors that influence the composition of gut, oral and vaginal microbiotas, and how these affect fungal colonisation and antifungal immunity. We argue that a detailed understanding of these variables, which underlie fungal-host-microbiota interactions, will present opportunities for directed antifungal therapies that benefit vulnerable patients.

A modified culture medium and hyphae isolation method can increase quality of the RNA extracted from mycelia of a dimorphic fungal species

The capability of RNA isolation with good efficiency and high quality is essential for a downstream application such as RNA sequencing. It requires successful cell culturing and an effective RNA isolation method. Although effective methods are available, production of the homogenous mycelia and extraction of good-quality mycelial RNA from true invasive hyphae, which penetrated into the agar plates, are difficult. To overcome these problems, the aim of this study was to develop technical modifications which allow production of homogenous mycelial biomass without extra stimuli agents and improve quality of the RNA extracted from the fungal hyphae. Our alternative culture medium was suitable for production both yeast-phase cells and hyphae of the Schizosaccharomyces japonicus and other dimorphic species, such as the Candida albicans, Saccharomyces cerevisiae, and Jaminaea angkorensis. To improve quality of the mycelial RNA, we developed an isolation procedure of the hyphal tip, which eliminated the unnecessary vacuoles-containing parts of the hyphae. To increase RNA quantity, we used glass beads in the RNA extraction protocol to achieve stronger breaking of the mycelial walls. All these modifications can also be useful for researchers working with other dimorphic fungi and can contribute to the higher comparability of the transcriptional data coming from yeast-phase cells and hyphae or even from different species.

Dental Restorative Materials for Elderly Populations

The incidence of dental caries, especially root caries, has risen in elderly populations in recent years. Specialized restorative materials are needed due to the specific site of root caries and the age-related changes in general and oral health in the elderly. Unfortunately, the restorative materials commonly used clinically cannot fully meet the requirements in this population. Specifically, the antibacterial, adhesive, remineralization, mechanical, and anti-aging properties of the materials need to be significantly improved for dental caries in the elderly. This review mainly discusses the strengths and weaknesses of currently available materials, including amalgam, glass ionomer cement, and light-cured composite resin, for root caries. It also reviews the studies on novel anti-caries materials divided into three groups, antimicrobial, remineralization, and self-healing materials, and explores their potential in the clinical use for caries in the elderly. Therefore, specific restorative materials for caries in the elderly, especially for root caries, need to be further developed and applied in clinical practice.

Candida albicans CHK1 gene from two-component system is essential for its pathogenicity in oral candidiasis

The roles of Candida albicans CHK1, a key gene from two-component system, in oral mucosal infection are not clear. This study evaluated the key roles of CHK1 gene in vitro and in vivo. The expression of CHK1 and its regulated virulence factors were tested during the oral epithelial cell infection. The production of lactate dehydrogenase, ROS, and IL-1α combined with the confocal and scanning electron microscope observation was employed to identify the capability of CHK1 in damaging the epithelial cells. Both immunocompetent and immunodeficient mice oropharyngeal infection models were involved to confirm the roles of CHK1 gene in vivo. The expression of CHK1 gene was significantly increased during the oral epithelial cell infection. The chk1Δ/Δ mutant failed to damage the epithelial cells or induce IL-α and ROS production. Interestingly, chk1Δ/Δ can also form the similar hyphae with WT and complementary strains. Accordingly, chk1Δ/Δ did not affect the adhesion and invasion rates of C. albicans to oral epithelial cells. However, chk1Δ/Δ significantly decreased the expression levels of the virulence factors, including ALS2, SAP6, and YWP1. The chk1Δ/Δ also failed to cause oral candidiasis in both immunocompetent and immunodeficient mice indicating that CHK1 gene from the two-component system is essential for the pathogenicity of C. albicans. KEY POINTS: • CHK1gene is essential for C. albicans in oral candidiasis • C. albicans without CHK1 gene can form "non-pathogenic" hyphae. • CHK1 gene regulates the virulence of C. albicans.

Involvement of amyloid proteins in the formation of biofilms in the pathogenic yeast Candida albicans

Candida species represent a major fungal threat for human health. Within the Candida genus, the yeast Candida albicans is the most frequently incriminated species during episodes of candidiasis or candidemia. Biofilm formation is used by C. albicans to produce a microbial community that is important in an infectious context. The cell wall, the most superficial cellular compartment, is of paramount importance regarding the establishment of biofilms. C. albicans cell wall contains proteins with amyloid properties that are necessary for biofilm formation due to their adhesion properties. This review focuses on these amyloid proteins during biofilm formation in the yeast C. albicans.

Candida albicans-The Virulence Factors and Clinical Manifestations of Infection

Candida albicans is a common commensal fungus that colonizes the oropharyngeal cavity, gastrointestinal and vaginal tract, and healthy individuals' skin. In 50% of the population, C. albicans is part of the normal flora of the microbiota. The various clinical manifestations of Candida species range from localized, superficial mucocutaneous disorders to invasive diseases that involve multiple organ systems and are life-threatening. From systemic and local to hereditary and environmental, diverse factors lead to disturbances in Candida's normal homeostasis, resulting in a transition from normal flora to pathogenic and opportunistic infections. The transition in the pathophysiology of the onset and progression of infection is also influenced by Candida's virulence traits that lead to the development of candidiasis. Oral candidiasis has a wide range of clinical manifestations, divided into primary and secondary candidiasis. The main supply of C. albicans in the body is located in the gastrointestinal tract, and the development of infections occurs due to dysbiosis of the residential microbiota, immune dysfunction, and damage to the muco-intestinal barrier. The presence of C. albicans in the blood is associated with candidemia-invasive Candida infections. The commensal relationship exists as long as there is a balance between the host immune system and the virulence factors of C. albicans. This paper presents the virulence traits of Candida albicans and clinical manifestations of specific candidiasis.

Antifungal In Vitro Activity of Pilosulin- and Ponericin-Like Peptides from the Giant Ant Dinoponera quadriceps and Synergistic Effects with Antimycotic Drugs

Venoms from ants comprise a rich source of bioactive peptides, including antimicrobial peptides. From the proteome and peptidome of the giant ant Dinoponera quadriceps venom, members of five known classes of antimicrobial peptides were disclosed (e.g., dermaseptin-, defensin-, ICK-, pilosulin- and ponericin-like types). Based on comparative analysis, these family members have structural determinants that indicate they could display antimicrobial activities. In previous works, pilosulin- and ponericin-like peptides were demonstrated to be active against bacteria, fungi, and parasites. Herein, the antifungal activity of ponericin- and pilosulin-like peptides were assessed, aiming at the expansion of the knowledge about AMPs in predatory ants and the development of new microbicide strategies to deal with difficult-to-treat fungal infections. Synthetic pilosulin- (Dq-2562, Dq-1503, and Dq-1319) and ponericin-like (Dq-3162) peptides were evaluated for their fungicide and fungistatic activities against different species of Candida, including a drug-resistant clinical strain. The MICs and MLCs were determined for all peptides individually and in combination with general antifungal drugs by the microdilution method. The time-kill kinetic curves were set up by means of a luminescent reagent, of which the light signal is proportional to the number of viable cells. The candicidal synergism observed by the combination of subinhibitory concentrations of peptides and general antimycotic drugs were quantified by the checkerboard test and fluorescent dye permeation assay. The influence of ergosterol on the antifungal activity was verified by supplementation of culture medium. The pilosulin- (Dq-2562 and Dq-1503) and ponericin-like (Dq-3162) were the most active peptides, displaying a broad spectrum of antifungal activity in vitro, with MICs in the range of 0.625 to 10 µM. The combination of peptides and conventional antimycotic drugs displayed a synergistic reduction in the MIC values of individual peptides and drugs, while soluble ergosterol in the culture medium increased the MICs. The fungicide and fungistatic activity of the individual peptides and peptides in combination with antimycotics were time-dependent with a rapid onset of action and long-lasting effect, which involved membrane disruption as an underlying mechanism of their action. Altogether, pilosulin- and ponericin-like peptides from the giant ant D. quadriceps venom display a broad-spectrum of candicidal activity, what allows their inclusion in the row of the antifungal peptides and gives support for further studies on the development of strategies to fight candidiasis.