Identification and characterization of ORF19.1725, a novel gene contributing to the white cell pheromone response and virulence-associated functions in Candida albicans

Candida albicans and Candida glabrata: global priority pathogens

SUMMARYA significant increase in the incidence of Candida-mediated infections has been observed in the last decade, mainly due to rising numbers of susceptible individuals. Recently, the World Health Organization published its first fungal pathogen priority list, with Candida species listed in medium, high, and critical priority categories. This review is a synthesis of information and recent advances in our understanding of two of these species-Candida albicans and Candida glabrata. Of these, C. albicans is the most common cause of candidemia around the world and is categorized as a critical priority pathogen. C. glabrata is considered a high-priority pathogen and has become an increasingly important cause of candidemia in recent years. It is now the second most common causative agent of candidemia in many geographical regions. Despite their differences and phylogenetic divergence, they are successful as pathogens and commensals of humans. Both species can cause a broad variety of infections, ranging from superficial to potentially lethal systemic infections. While they share similarities in certain infection strategies, including tissue adhesion and invasion, they differ significantly in key aspects of their biology, interaction with immune cells, host damage strategies, and metabolic adaptations. Here we provide insights on key aspects of their biology, epidemiology, commensal and pathogenic lifestyles, interactions with the immune system, and antifungal resistance.

Bibliometric analysis and thematic review of Candida pathogenesis: Fundamental omics to applications as potential antifungal drugs and vaccines

Invasive candidiasis caused by the pathogenic Candida yeast species has resulted in elevating global mortality. The pathogenicity of Candida spp. is not only originated from its primary invasive yeast-to-hyphal transition; virulence factors (transcription factors, adhesins, invasins, and enzymes), biofilm, antifungal drug resistance, stress tolerance, and metabolic adaptation have also contributed to a greater clinical burden. However, the current research theme in fungal pathogenicity could hardly be delineated with the increasing research output. Therefore, our study analysed the research trends in Candida pathogenesis over the past 37 years via a bibliometric approach against the Scopus and Web of Science databases. Based on the 3993 unique documents retrieved, significant international collaborations among researchers were observed, especially between Germany (Bernhard Hube) and the UK (Julian Naglik), whose focuses are on Candida proteinases, adhesins, and candidalysin. The prominent researchers (Neils Gow, Alistair Brown, and Frank Odds) at the University of Exeter and the University of Aberdeen (second top performing affiliation) UK contribute significantly to the mechanisms of Candida adaptation, tolerance, and stress response. However, the science mapping of co-citation analysis performed herein could not identify a hub representative of subsequent work since the clusters were semi-redundant. The co-word analysis that was otherwise adopted, revealed three research clusters; the cluster-based thematic analyses indicated the severeness of Candida biofilm and antifungal resistance as well as the elevating trend on molecular mechanism elucidation for drug screening and repurposing. Importantly, the in vivo pathogen adaptation and interactions with hosts are crucial for potential vaccine development.

Enhanced Virulence of Candida albicans by Staphylococcus aureus: Evidence in Clinical Bloodstream Infections and Infected Zebrafish Embryos

Coinfection with Candida and Staphylococcus results in higher mortality in animal studies. However, the pathogenesis and interplay between C. albicans and S. aureus in bloodstream infections (BSIs) is unclear. This study determines the clinical features and outcomes of mixed C. albicans/S. aureus (CA/SA) BSIs and biofilm formation on pathogenesis during coinfection. Demographics and outcomes for mixed BSIs and monomicrobial candidemia were compared. Compared to 115 monomicrobial C. albicans BSIs, 22 patients with mixed CA/SA BSIs exhibited a significantly higher mortality rate and shorter survival time. In vitro and in vivo biofilm analysis showed that C. albicans accounted for the main biofilm architecture, and S. aureus increased its amount. Antibiotic tolerance in S. aureus, which adhered to Candida hyphae observed by scanning electron microscope, was demonstrated by the presence of wild-type C. albicans co-biofilm. Upregulation in exotoxin genes of S. aureus was evidenced by quantitative RT-PCR when a co-biofilm was formed with C. albicans. Mixed CA/SA BSIs result in a higher mortality rate in patients and in vivo surrogate models experiments. This study demonstrates that the virulence enhancement of C. albicans and S. aureus during co-biofilm formation contributes to the high mortality rate.

Candida tropicalis RON1 is required for hyphal formation, biofilm development, and virulence but is dispensable for N-acetylglucosamine catabolism

NDT80-like family genes are highly conserved across a large group of fungi, but the functions of each Ndt80 protein are diverse and have evolved differently among yeasts and pathogens. The unique NDT80 gene in budding yeast is required for sexual reproduction, whereas three NDT80-like genes, namely, NDT80, REP1, and RON1, found in Candida albicans exhibit distinct functions. Notably, it was suggested that REP1, rather than RON1, is required for N-acetylglucosamine (GlcNAc) catabolism. Although Candida tropicalis, a widely dispersed fungal pathogen in tropical and subtropical areas, is closely related to Candida albicans, its phenotypic, pathogenic and environmental adaptation characteristics are remarkably divergent. In this study, we focused on the Ron1 transcription factor in C. tropicalis. Protein alignment showed that C. tropicalis Ron1 (CtRon1) shares 39.7% identity with C. albicans Ron1 (CaRon1). Compared to the wild-type strain, the C. tropicalis ron1Δ strains exhibited normal growth in different carbon sources and had similar expression levels of several GlcNAc catabolic genes during GlcNAc treatment. In contrast, C. tropicalis REP1 is responsible for GlcNAc catabolism and is involved in GlcNAc catabolic gene expressions, similar to C. albicans Rep1. However, REP1 deletion strains in C. tropicalis promote hyphal development in GlcNAc with low glucose content. Interestingly, CtRON1, but not CaRON1, deletion mutants exhibited significantly impaired hyphal growth and biofilm formation. As expected, CtRON1 was required for full virulence. Together, the results of this study showed divergent functions of CtRon1 compared to CaRon1; CtRon1 plays a key role in yeast-hyphal dimorphism, biofilm formation and virulence.

LAY ABSTRACT

In this study, we identified the role of RON1, an NDT80-like gene, in Candida tropicalis. Unlike the gene in Candida albicans, our studies showed that RON1 is a key regulator of hyphal formation, biofilm development and virulence but is dispensable for N-acetylglucosamine catabolism in C. tropicalis.

A Potential Antifungal Effect of Chitosan Against Candida albicans Is Mediated via the Inhibition of SAGA Complex Component Expression and the Subsequent Alteration of Cell Surface Integrity

Due to the high incidence of nosocomial Candida albicans infection, the first-line drugs for C. albicans infection have been heavily used, and the emergence of drug-resistant strains has gradually increased. Thus, a new antifungal drug or therapeutic method is needed. Chitosan, a product of chitin deacetylation, is considered to be potentially therapeutic for fungal infections because of its excellent biocompatibility, biodegradability and low toxicity. The biocidal action of chitosan against C. albicans shows great commercial potential, but the exact mechanisms underlying its antimicrobial activity are unclear. To reveal these mechanisms, mutant library screening was performed. ADA2 gene, which encodes a histone acetylation coactivator in the SAGA complex, was identified. Transmission electronic microscopy images showed that the surface of chitosan-treated ada2Δ cells was substantially disrupted and displayed an irregular morphology. Interestingly, the cell wall of ada2Δ cells was significantly thinner than that of wild-type cells, with a thickness similar to that seen in the chitosan-treated wild-type strain. Although ADA2 is required for chitosan tolerance, expression of ADA2 and several Ada2-mediated cell wall-related genes (ALS2, PGA45, and ACE2) and efflux transporter genes (MDR1 and CDR1) were significantly inhibited by chitosan. Furthermore, GCN5 encoding a SAGA complex catalytic subunit was inhibited by chitosan, and gcn5Δ cells exhibited phenotypes comparable to those of ada2Δ cells in response to chitosan and other cell surface-disrupting agents. This study demonstrated that a potential antifungal mechanism of chitosan against C. albicans operates by inhibiting SAGA complex gene expression, which decreases the protection of the cell surface against chitosan.

The Zebrafish as a Model Host for Invasive Fungal Infections

The zebrafish has become a widely accepted model host for studies of infectious disease, including fungal infections. The species is genetically tractable, and the larvae are transparent and amenable to prolonged in vivo imaging and small molecule screening. The aim of this review is to provide a thorough introduction into the published studies of fungal infection in the zebrafish and the specific ways in which this model has benefited the field. In doing so, we hope to provide potential new zebrafish researchers with a snapshot of the current toolbox and prior results, while illustrating how the model has been used well and where the unfulfilled potential of this model can be found.