Candida glabrata's Genome Plasticity Confers a Unique Pattern of Expressed Cell Wall Proteins

Microevolution of Candida glabrata (Nakaseomyces glabrata) during an infection

Candida glabrata (Nakaseomyces glabrata) is an emergent and opportunistic fungal pathogen that colonizes and persists in different niches within its human host. In this work, we studied five clinical isolates from one patient (P7), that have a clonal origin, and all of which come from blood cultures except one, P7-3, obtained from a urine culture. We found phenotypic variation such as sensitivity to high temperature, oxidative stress, susceptibility to two classes of antifungal agents, and cell wall porosity. Only isolate P7-3 is highly resistant to the echinocandin caspofungin while the other four isolates from P7 are sensitive. However, this same isolate P7-3, is the only one that displays susceptibility to fluconazole (FLC), while the rest of the isolates are resistant to this antifungal. We sequenced the PDR1 gene which encodes a transcription factor required to induce the expression of several genes involved in the resistance to FLC and found that all the isolates encode for the same Pdr1 amino acid sequence except for the last isolate P7-5, which contains a single amino acid change, G1099C in the putative Pdr1 transactivation domain. Consistent with the resistance to FLC, we found that the CDR1 gene, encoding the main drug efflux pump in C. glabrata, is highly overexpressed in the FLC-resistant isolates, but not in the FLC-sensitive P7-3. In addition, the resistance to FLC observed in these isolates is dependent on the PDR1 gene. Additionally, we found that all P7 isolates have a different proportion of cell wall carbohydrates compared to our standard strains CBS138 and BG14. In P7 isolates, mannan is the most abundant cell wall component, whereas β-glucan is the most abundant component in our standard strains. Consistently, all P7 isolates have a relatively low cell wall porosity compared to our standard strains. These data show phenotypic and genotypic variability between clonal isolates from different niches within a single host, suggesting microevolution of C. glabrata during an infection.

The Hog1 MAPK substrate governs Candida glabrata-epithelial cell adhesion via the histone H2A variant

CgHog1, terminal kinase of the high-osmolarity glycerol signalling pathway, orchestrates cellular response to multiple external stimuli including surplus-environmental iron in the human fungal pathogen Candida glabrata (Cg). However, CgHog1 substrates remain unidentified. Here, we show that CgHog1 adversely affects Cg adherence to host stomach and kidney epithelial cells in vitro, but promotes Cg survival in the iron-rich gastrointestinal tract niche. Further, CgHog1 interactome and in vitro phosphorylation analysis revealed CgSub2 (putative RNA helicase) to be a CgHog1 substrate, with CgSub2 also governing iron homeostasis and host adhesion. CgSub2 positively regulated EPA1 (encodes a major adhesin) expression and host adherence via its interactor CgHtz1 (histone H2A variant). Notably, both CgHog1 and surplus environmental iron had a negative impact on CgSub2-CgHtz1 interaction, with CgHTZ1 or CgSUB2 deletion reversing the elevated adherence of Cghog1Δ to epithelial cells. Finally, the surplus-extracellular iron led to CgHog1 activation, increased CgSub2 phosphorylation, elevated CgSub2-CgHta (canonical histone H2A) interaction, and EPA1 transcriptional activation, thereby underscoring the iron-responsive, CgHog1-induced exchange of histone partners of CgSub2. Altogether, our work mechanistically defines how CgHog1 couples Epa1 adhesin expression with iron abundance, and point towards specific chromatin composition modification programs that probably aid fungal pathogens align their adherence to iron-rich (gut) and iron-poor (blood) host niches.

Molecular Epidemiology and Antifungal Susceptibility Profile in Nakaseomyces glabrata Species Complex: A 5-Year Countrywide Study

BACKGROUND

The current study aimed to identify Iranian Nakaseomyces (Candida) glabrata complex species in the clinical isolates and determine their antifungal susceptibility profile.

METHODS

In total, 320 N. glabrata clinical isolates were collected from patients hospitalized in different geographical regions of Iran. The initial screening was performed by morphological characteristics on CHROMagar Candida. Each isolate was identified by targeting the D1/D2 rDNA using a multiplex-PCR method. To validate the mPCR method and determine genetic diversity, the ITS-rDNA region was randomly sequenced in 40 isolates. Additionally, antifungal susceptibility was evaluated against nine antifungal agents following the CLSI M27-A4 guidelines.

RESULTS

All clinical isolates from Iran were identified as N. glabrata. The analysis of ITS-rDNA sequence data revealed the presence of eight distinct ITS clades and 10 haplotypes among the 40 isolates of N. glabrata. The predominant clades identified were Clades VII, V, and IV, which respectively accounted for 22.5%, 17.5%, and 17.5% isolates. The widest MIC ranges were observed for voriconazole (0.016-8 μg/mL) and isavuconazole (0.016-2 μg/mL), whereas the narrowest ranges were seen with itraconazole and amphotericin B (0.25-2 μg/mL).

CONCLUSION

Haplotype diversity can be a valuable approach for studying the genetic diversity, transmission patterns, and epidemiology of the N. glabrata complex.

SWI/SNF complex-mediated chromatin remodeling in Candida glabrata promotes immune evasion

Immune evasion is critical for fungal virulence. However, how the human opportunistic pathogen Candida glabrata (Cg) accomplishes this is unknown. Here, we present the first genome-wide nucleosome map of the macrophage-internalized Cg consisting of ∼12,000 dynamic and 70,000 total nucleosomes. We demonstrate that CgSnf2 (SWI/SNF chromatin remodeling complex-ATPase subunit)-mediated chromatin reorganization in macrophage-internalized Cg upregulates and downregulates the immunosuppressive seven-gene mannosyltransferase-cluster (CgMT-C) and immunostimulatory cell surface adhesin-encoding EPA1 gene, respectively. Consistently, EPA1 overexpression and CgMT-C deletion elevated IL-1β (pro-inflammatory cytokine) production and diminished Cg proliferation in macrophages. Further, Cgsnf2Δ had higher Epa1 surface expression, and evoked increased IL-1β secretion, and was killed in macrophages. Akt-, p38-, NF-κB- or NLRP3 inflammasome-inhibition partially reversed increased IL-1β secretion in Cgsnf2Δ-infected macrophages. Importantly, macrophages responded to multiple Candida pathogens via NF-κB-dependent IL-1β production, underscoring NF-κB signaling's role in fungal diseases. Altogether, our findings directly link the nucleosome positioning-based chromatin remodeling to fungal immunomodulatory molecule expression.

Unwrap RAP1's Mystery at Kinetoplastid Telomeres

Although located at the chromosome end, telomeres are an essential chromosome component that helps maintain genome integrity and chromosome stability from protozoa to mammals. The role of telomere proteins in chromosome end protection is conserved, where they suppress various DNA damage response machineries and block nucleolytic degradation of the natural chromosome ends, although the detailed underlying mechanisms are not identical. In addition, the specialized telomere structure exerts a repressive epigenetic effect on expression of genes located at subtelomeres in a number of eukaryotic organisms. This so-called telomeric silencing also affects virulence of a number of microbial pathogens that undergo antigenic variation/phenotypic switching. Telomere proteins, particularly the RAP1 homologs, have been shown to be a key player for telomeric silencing. RAP1 homologs also suppress the expression of Telomere Repeat-containing RNA (TERRA), which is linked to their roles in telomere stability maintenance. The functions of RAP1s in suppressing telomere recombination are largely conserved from kinetoplastids to mammals. However, the underlying mechanisms of RAP1-mediated telomeric silencing have many species-specific features. In this review, I will focus on Trypanosoma brucei RAP1's functions in suppressing telomeric/subtelomeric DNA recombination and in the regulation of monoallelic expression of subtelomere-located major surface antigen genes. Common and unique mechanisms will be compared among RAP1 homologs, and their implications will be discussed.

Telomere maintenance in African trypanosomes

Telomere maintenance is essential for genome integrity and chromosome stability in eukaryotic cells harboring linear chromosomes, as telomere forms a specialized structure to mask the natural chromosome ends from DNA damage repair machineries and to prevent nucleolytic degradation of the telomeric DNA. In Trypanosoma brucei and several other microbial pathogens, virulence genes involved in antigenic variation, a key pathogenesis mechanism essential for host immune evasion and long-term infections, are located at subtelomeres, and expression and switching of these major surface antigens are regulated by telomere proteins and the telomere structure. Therefore, understanding telomere maintenance mechanisms and how these pathogens achieve a balance between stability and plasticity at telomere/subtelomere will help develop better means to eradicate human diseases caused by these pathogens. Telomere replication faces several challenges, and the "end replication problem" is a key obstacle that can cause progressive telomere shortening in proliferating cells. To overcome this challenge, most eukaryotes use telomerase to extend the G-rich telomere strand. In addition, a number of telomere proteins use sophisticated mechanisms to coordinate the telomerase-mediated de novo telomere G-strand synthesis and the telomere C-strand fill-in, which has been extensively studied in mammalian cells. However, we recently discovered that trypanosomes lack many telomere proteins identified in its mammalian host that are critical for telomere end processing. Rather, T. brucei uses a unique DNA polymerase, PolIE that belongs to the DNA polymerase A family (E. coli DNA PolI family), to coordinate the telomere G- and C-strand syntheses. In this review, I will first briefly summarize current understanding of telomere end processing in mammals. Subsequently, I will describe PolIE-mediated coordination of telomere G- and C-strand synthesis in T. brucei and implication of this recent discovery.

Whole genome analysis of echinocandin non-susceptible Candida Glabrata clinical isolates: a multi-center study in China

BACKGROUND

Candida glabrata is an important cause of invasive candidiasis. Echinocandins are the first-line treatment of invasive candidiasis caused by C. glabrata. The epidemiological echinocandin sensitivity requires long-term surveillance and the understanding about whole genome characteristics of echinocandin non-susceptible isolates was limited.

RESULTS

The present study investigated the echinocandin susceptibility of 1650 C. glabrata clinical isolates in China from August 2014 to July 2019. The in vitro activity of micafungin was significantly better than those of caspofungin and anidulafungin (P < 0.001), assessed by MIC50/90 values. Whole genome sequencing was conducted on non-susceptible isolates and geography-matched susceptible isolates. Thirteen isolates (0.79%) were resistant to at least one echinocandin. Six isolates (0.36%) were solely intermediate to caspofungin. Common evolutionary analysis of echinocandin-resistant and echinocandin-intermediate isolates revealed genes related with reduced caspofungin sensitivity, including previously identified sphinganine hydroxylase encoding gene SUR2. Genome-wide association study identified SNPs at subtelometric regions that were associated with echinocandin non-susceptibility. In-host evolution of echinocandin resistance of serial isolates revealed an enrichment for non-synonymous mutations in adhesins genes and loss of subtelometric regions containing adhesin genes.

CONCLUSIONS

The echinocandins are highly active against C. glabrata in China with a resistant rate of 0.79%. Echinocandin non-susceptible isolates carried common evolved genes which are related with reduced caspofungin sensitivity. In-host evolution of C. glabrata accompanied intensive changing of adhesins profile.

A novel Candida glabrata doxycycline-inducible system for in vitro/in vivo use

Candida glabrata is an important pathogen causing superficial to invasive disease in human. Conditional expression systems are helpful in addressing the function of genes and especially when they can be applied to in vivo studies. Tetracycline-dependent regulation systems have been used in diverse fungi to turn-on (Tet-on) or turn-off (Tet-off) gene expression either in vitro but also in vivo in animal models. Up to now, only a Tet-off expression has been constructed for gene expression in C. glabrata. Here, we report a Tet-on gene expression system which can be used in vitro and in vivo in any C. glabrata genetic background. This system was used in a mice model of systemic infection to demonstrate that the general amino acid permease Gap1 is important for C. glabrata virulence.

Niacin Limitation Promotes Candida glabrata Adhesion to Abiotic Surfaces

Candida glabrata is a prevalent fungal pathogen in humans, which is able to adhere to host cells and abiotic surfaces. Nicotinic acid (NA) limitation has been shown to promote the adherence of C. glabrata to human epithelial cells. Clinically, the elderly and hospitalized patients who are prone to C. glabrata–related denture stomatitis often suffer from vitamin deficiency. This study aimed to investigate C. glabrata adhesion to abiotic surfaces, including acrylic resin (a denture material) surfaces, cell surface hydrophobicity and adhesion gene expression. C. glabrata CBS138 was grown in media containing decreasing NA concentrations (40, 0.4, 0.04 and 0.004 µM). Adherence of C. glabrata to glass coverslips and acrylic resin was analyzed. C. glabrata adhesion to both surfaces generally increased with decreasing NA concentrations. The highest adhesion was found for the cells grown with 0.004 µM NA. The cell surface hydrophobicity test indicated that NA limitation enhanced hydrophobicity of C. glabrata cells. Quantitative PCR showed that of all adhesion genes tested, EPA1, EPA3 and EPA7 were significantly up-regulated in both 0.004 µM NA and 0.04 µM NA groups compared to those in the 40 µM NA group. No significant up- or down-regulation under NA limitation was observed for the other tested adhesion genes, namely AWP3, AWP4, AWP6 and EPA6. NA limitation resulted in increased expression of some adhesion genes, higher surface hydrophobicity of C. glabrata and enhanced adhesion to abiotic surfaces. NA deficiency is likely a risk factor for C. glabrata–related denture stomatitis in the elderly.

Candida glabrata Antifungal Resistance and Virulence Factors, a Perfect Pathogenic Combination

In recent years, a progressive increase in the incidence of invasive fungal infections (IFIs) caused by Candida glabrata has been observed. The objective of this literature review was to study the epidemiology, drug resistance, and virulence factors associated with the C. glabrata complex. For this purpose, a systematic review (January 2001-February 2021) was conducted on the PubMed, Scielo, and Cochrane search engines with the following terms: "C. glabrata complex (C. glabrata sensu stricto, C. nivariensis, C. bracarensis)" associated with "pathogenicity" or "epidemiology" or "antibiotics resistance" or "virulence factors" with language restrictions of English and Spanish. One hundred and ninety-nine articles were found during the search. Various mechanisms of drug resistance to azoles, polyenes, and echinocandins were found for the C. glabrata complex, depending on the geographical region. Among the mechanisms found are the overexpression of drug transporters, gene mutations that alter thermotolerance, the generation of hypervirulence due to increased adhesion factors, and modifications in vital enzymes that produce cell wall proteins that prevent the activity of drugs designed for its inhibition. In addition, it was observed that the C. glabrata complex has virulence factors such as the production of proteases, phospholipases, and hemolysins, and the formation of biofilms that allows the complex to evade the host immune response and generate fungal resistance. Because of this, the C. glabrata complex possesses a perfect pathogenetic combination for the invasion of the immunocompromised host.

Genome and transcriptome of a pathogenic yeast, Candida nivariensis

We present a highly contiguous genome and transcriptome of the pathogenic yeast, Candida nivariensis. We sequenced both the DNA and RNA of this species using both the Oxford Nanopore Technologies and Illumina platforms. We assembled the genome into an 11.8 Mb draft composed of 16 contigs with an N50 of 886 Kb, including a circular mitochondrial sequence of 28 Kb. Using direct RNA nanopore sequencing and Illumina cDNA sequencing, we constructed an annotation of our new assembly, supplemented by lifting over genes from Saccharomyces cerevisiae and Candida glabrata.

Epitope-Based Peptide Vaccine Design against Fructose Bisphosphate Aldolase of Candida glabrata: An Immunoinformatics Approach

Background

Candida glabrata is a human opportunistic pathogen that can cause life-threatening systemic infections. Although there are multiple effective vaccines against fungal infections and some of these vaccines are engaged in different stages of clinical trials, none of them have yet been approved by the FDA.

Aim

Using immunoinformatics approach to predict the most conserved and immunogenic B- and T-cell epitopes from the fructose bisphosphate aldolase (Fba1) protein of C. glabrata. Material and Method. 13 C. glabrata fructose bisphosphate aldolase protein sequences (361 amino acids) were retrieved from NCBI and presented in several tools on the IEDB server for prediction of the most promising epitopes. Homology modeling and molecular docking were performed.

Result

The promising B-cell epitopes were AYFKEH, VDKESLYTK, and HVDKESLYTK, while the promising peptides which have high affinity to MHC I binding were AVHEALAPI, KYFKRMAAM, QTSNGGAAY, RMAAMNQWL, and YFKEHGEPL. Two peptides, LFSSHMLDL and YIRSIAPAY, were noted to have the highest affinity to MHC class II that interact with 9 alleles. The molecular docking revealed that the epitopes QTSNGGAAY and LFSSHMLDL have the lowest binding energy to MHC molecules.

Conclusion

The epitope-based vaccines predicted by using immunoinformatics tools have remarkable advantages over the conventional vaccines in that they are more specific, less time consuming, safe, less allergic, and more antigenic. Further in vivo and in vitro experiments are needed to prove the effectiveness of the best candidate's epitopes (QTSNGGAAY and LFSSHMLDL). To the best of our knowledge, this is the first study that has predicted B- and T-cell epitopes from the Fba1 protein by using in silico tools in order to design an effective epitope-based vaccine against C. glabrata.

DNA damage response of major fungal pathogen Candida glabrata offers clues to explain its genetic diversity

How cells respond to DNA damage is key to maintaining genome integrity or facilitating genetic change. In fungi, DNA damage responses have been extensively characterized in the model budding yeast Saccharomyces cerevisiae, which is generally not pathogenic. However, it is not clear how closely these responses resemble those in fungal pathogens, in which genetic change plays an important role in the evolutionary arms race between pathogen and host and the evolution of antifungal drug resistance. A close relative of S. cerevisiae, Candida glabrata, is an opportunistic pathogen that displays high variability in chromosome structure among clinical isolates and rapidly evolves antifungal drug resistance. The mechanisms facilitating such genomic flexibility and evolvability in this organism are unknown. Recently we characterized the DNA damage response of C. glabrata and identified several features that distinguish it from the well characterized DNA damage response of S. cerevisiae. First, we discovered that, in contrast to the established paradigm, C. glabrata effector kinase Rad53 is not hyperphosphorylated upon DNA damage. We also uncovered evidence of an attenuated DNA damage checkpoint response, wherein in the presence of DNA damage C. glabrata cells did not accumulate in S-phase and proceeded with cell division, leading to aberrant mitoses and cell death. Finally, we identified evidence of transcriptional rewiring of the DNA damage response of C. glabrata relative to S. cerevisiae, including an upregulation of genes involved in mating and meiosis-processes that have not been reported in C. glabrata. Together, these results open new possibilities and raise tantalizing questions of how this major fungal pathogen facilitates genetic change.

A Noncanonical DNA Damage Checkpoint Response in a Major Fungal Pathogen

In order to preserve genome integrity, all cells must mount appropriate responses to DNA damage, including slowing down or arresting the cell cycle to give the cells time to repair the damage and changing gene expression, for example to induce genes involved in DNA repair. The Rad53 protein kinase is a conserved central mediator of these responses in eukaryotic cells, and its extensive phosphorylation upon DNA damage is necessary for its activation and subsequent activity.

DNA damage checkpoints are key guardians of genome integrity. Eukaryotic cells respond to DNA damage by triggering extensive phosphorylation of Rad53/CHK2 effector kinase, whereupon activated Rad53/CHK2 mediates further aspects of checkpoint activation, including cell cycle arrest and transcriptional changes. Budding yeast Candida glabrata, closely related to model eukaryote Saccharomyces cerevisiae, is an opportunistic pathogen characterized by high genetic diversity and rapid emergence of drug-resistant mutants. However, the mechanisms underlying this genetic variability are unclear. We used Western blotting and mass spectrometry to show that, unlike S. cerevisiae, C. glabrata cells exposed to DNA damage did not induce C. glabrata Rad53 (CgRad53) phosphorylation. Furthermore, flow cytometry analysis showed that, unlike S. cerevisiae, C. glabrata cells did not accumulate in S phase upon DNA damage. Consistent with these observations, time-lapse microscopy showed C. glabrata cells continuing to divide in the presence of DNA damage, resulting in mitotic errors and cell death. Finally, transcriptome sequencing (RNAseq) analysis revealed transcriptional rewiring of the DNA damage response in C. glabrata and identified several key protectors of genome stability upregulated by DNA damage in S. cerevisiae but downregulated in C. glabrata, including proliferating cell nuclear antigen (PCNA). Together, our results reveal a noncanonical fungal DNA damage response in C. glabrata, which may contribute to rapidly generating genetic change and drug resistance.

The Epithelial adhesin 1 tandem repeat region mediates protein display through multiple mechanisms

The pathogenic yeast Candida glabrata is reliant on a suite of cell surface adhesins that play a variety of roles necessary for transmission, establishment and proliferation during infection. One particular adhesin, Epithelial Adhesin 1 [Epa1p], is responsible for binding to host tissue, a process which is essential for fungal propagation. Epa1p structure consists of three domains: an N-terminal intercellular binding domain responsible for epithelial cell binding, a C-terminal GPI anchor for cell wall linkage and a serine/threonine-rich linker domain connecting these terminal domains. The linker domain contains a 40-amino acid tandem repeat region, which we have found to be variable in repeat copy number between isolates from clinical sources. We hypothesized that natural variation in Epa1p repeat copy may modulate protein function. To test this, we recombinantly expressed Epa1p with various repeat copy numbers in S. cerevisiae to determine how differences in repeat copy number affect Epa1p expression, surface display and binding to human epithelial cells. Our data suggest that repeat copy number variation has pleiotropic effects, influencing gene expression, protein surface display and shedding from the cell surface of the Epa1p adhesin. This study serves to demonstrate repeat copy number variation can modulate protein function through a number of mechanisms in order to contribute to pathogenicity of C. glabrata.

Low Level of Antifungal Resistance in Iranian Isolates of Candida glabrata Recovered from Blood Samples in a Multicenter Study from 2015 to 2018 and Potential Prognostic Values of Genotyping and Sequencing of PDR1

Establishing an effective empirical antifungal therapy requires that national surveillance studies be conducted. Herein, we report the clinical outcome of infections with and the microbiological features of Iranian isolates of Candida glabrata derived from patients suffering from candidemia. C. glabrata isolates were retrospectively collected from four major cities in Iran; identified by a 21-plex PCR, matrix-assisted laser desorption ionization-time of flight mass spectrometry, and large subunit of ribosomal DNA sequencing; and genotyped by amplified fragment length polymorphism (AFLP). Mutations in PDR1, ERG11, and hot spot 1 (HS1) of FKS1 and FKS2 were investigated, and antifungal susceptibility testing (AFST) was performed (by the CLSI M27-A3 and M27-S4 methods). Seventy isolates of C. glabrata were collected from 65 patients with a median age of 58 years. Fluconazole was the most widely used (29.23%) and least effective antifungal agent. The overall crude mortality rate was 35.4%. Only one strain was resistant to fluconazole, and 57.7% and 37.5% of the isolates were non-wild type (non-WT) for susceptibility to caspofungin and voriconazole, respectively. All isolates showed the WT phenotype for amphotericin B, posaconazole, and itraconazole. HS1 of FKS1 and FKS2 did not harbor any mutations, while numerous missense mutations were observed in PDR1 and ERG11 AFLP clustered our isolates into nine genotypes; among them, genotypes 1 and 2 were significantly associated with a higher mortality rate (P = 0.034 and P = 0.022, α < 0.05). Moreover, 83.3% of patients infected with strains harboring a single new mutation in PDR1, T745A, died despite treatment with fluconazole or caspofungin. Overall, Iranian isolates of C. glabrata were susceptible to the major antifungal drugs. Application of genotyping techniques and sequencing of a specific gene (PDR1) might have prognostic implications.

Divergent Approaches to Virulence in C. albicans and C. glabrata: Two Sides of the Same Coin

Candida albicans and Candida glabrata are the two most prevalent etiologic agents of candidiasis worldwide. Although both are recognized as pathogenic, their choice of virulence traits is highly divergent. Indeed, it appears that these different approaches to fungal virulence may be equally successful in causing human candidiasis. In this review, the virulence mechanisms employed by C. albicans and C. glabrata are analyzed, with emphasis on the differences between the two systems. Pathogenesis features considered in this paper include dimorphic growth, secreted enzymes and signaling molecules, and stress resistance mechanisms. The consequences of these traits in tissue invasion, biofilm formation, immune system evasion, and macrophage escape, in a species dependent manner, are discussed. This review highlights the observation that C. albicans and C. glabrata follow different paths leading to a similar outcome. It also highlights the lack of knowledge on some of the specific mechanisms underlying C. glabrata pathogenesis, which deserve future scrutiny.

The regulation of iron homeostasis in the fungal human pathogen Candida glabrata

Iron is an essential element to most microorganisms, yet an excess of iron is toxic. Hence, living cells have to maintain a tight balance between iron uptake and iron consumption and storage. The control of intracellular iron concentrations is particularly challenging for pathogens because mammalian organisms have evolved sophisticated high-affinity systems to sequester iron from microbes and because iron availability fluctuates among the different host niches. In this review, we present the current understanding of iron homeostasis and its regulation in the fungal pathogen Candida glabrata. This yeast is an emerging pathogen which has become the second leading cause of candidemia, a life-threatening invasive mycosis. C. glabrata is relatively poorly studied compared to the closely related model yeast Saccharomyces cerevisiae or to the pathogenic yeast Candida albicans. Still, several research groups have started to identify the actors of C. glabrata iron homeostasis and its transcriptional and post-transcriptional regulation. These studies have revealed interesting particularities of C. glabrata and have shed new light on the evolution of fungal iron homeostasis.

Atomic Force Microscopy Demonstrates that Candida glabrata Uses Three Epa Proteins To Mediate Adhesion to Abiotic Surfaces

Candida glabrata cell wall proteins mediate the attachment of C. glabrata to abiotic surfaces through molecular interactions that are poorly understood. Here, we study the forces engaged in Epa-dependent adhesion using single-cell techniques. Fungal adhesion to hydrophilic and hydrophobic substrates involves mainly three Epa proteins, suggesting a broad role for the Epa adhesins in mediating adherence. These proteins might represent a potential target for the development of innovative antifungal drugs.

The fungal pathogen Candida glabrata can cause both mucosal and disseminated infections. Cell adhesion, a key step in colonization and infection, depends in C. glabrata primarily on the Epa family of cell adhesion proteins. While Epa proteins have been documented to mediate specific adhesion to host glycans, some of them also promote nonspecific adhesion to abiotic surfaces, though this is incompletely understood. Here we address this issue using a combination of genetics and single-cell force measurements. By quantifying the forces driving the attachment of single C. glabrata cells to hydrophobic and hydrophilic substrates, we show that cell adhesion is strongly increased by loss of Sir-mediated silencing. Using a series of mutant strains lacking specific EPA genes, we demonstrate unexpectedly that three major Epa proteins, Epa1, Epa6, and Epa7, primarily contribute to both hydrophilic and hydrophobic interactions, suggesting a broad role for the Epa adhesins in mediating specific and nonspecific adherence and implicating Epa genes in biofilm formation on abiotic surfaces.

IMPORTANCECandida glabrata cell wall proteins mediate the attachment of C. glabrata to abiotic surfaces through molecular interactions that are poorly understood. Here, we study the forces engaged in Epa-dependent adhesion using single-cell techniques. Fungal adhesion to hydrophilic and hydrophobic substrates involves mainly three Epa proteins, suggesting a broad role for the Epa adhesins in mediating adherence. These proteins might represent a potential target for the development of innovative antifungal drugs.

Candida glabrata: A Lot More Than Meets the Eye

Candida glabrata is an opportunistic human fungal pathogen that causes superficial mucosal and life-threatening bloodstream infections in individuals with a compromised immune system. Evolutionarily, it is closer to the non-pathogenic yeast Saccharomyces cerevisiae than to the most prevalent Candida bloodstream pathogen, C. albicans. C. glabrata is a haploid budding yeast that predominantly reproduces clonally. In this review, we summarize interactions of C. glabrata with the host immune, epithelial and endothelial cells, and the ingenious strategies it deploys to acquire iron and phosphate from the external environment. We outline various attributes including cell surface-associated adhesins and aspartyl proteases, biofilm formation and stress response mechanisms, that contribute to the virulence of C. glabrata. We further discuss how, C. glabrata, despite lacking morphological switching and secreted proteolytic activity, is able to disarm macrophage, dampen the host inflammatory immune response and replicate intracellularly.

Chromatin architecture and virulence-related gene expression in eukaryotic microbial pathogens

A fundamental question in biology is to understand how appropriate transcriptional regulation and dense packaging of the genetic material within the eukaryotic nucleus are achieved. The exquisite gene expression control and other metabolic processes of DNA require a highly complex, multilayered, three-dimensional architecture of the chromatin and its specific compartmentalization within the nucleus. Some of these architectural and sub-nuclear positioning mechanisms have been extensively co-opted by eukaryotic pathogens to keep fine expression control and expansion of virulence-related gene families in Plasmodium falciparum, Trypanosoma brucei and Candida glabrata. For example non-linear interactions between distant cis-acting regions and the formation of chromatin loops are required for appropriate regulation of the expression of virulence-related multi-gene families encoding cell surface proteins. These gene families are located near the chromosome ends and tethered to the nuclear periphery. Consequently, only one or very few genes of the family are expressed at a time. These genes are involved in antigenic variation in parasites and the generation of subpopulations of cells with diverse antigenic proteins at the surface in some pathogenic fungi, making them highly efficient pathogens.

Expansion of a Telomeric FLO/ALS-Like Sequence Gene Family in Saccharomycopsis fermentans

Non-Saccharomyces species have been recognized for their beneficial contribution to fermented food and beverages based on their volatile compound formation and their ability to ferment glucose into ethanol. At the end of fermentation brewer's yeast flocculate which provides an easy means of separation of yeasts from green beer. Flocculation in Saccharomyces cerevisiae requires a set of flocculation genes. These FLO-genes, FLO1, FLO5, FLO9, FLO10, and FLO11, are located at telomeres and transcription of these adhesins is regulated by Flo8 and Mss11. Here, we show that Saccharomycopsis fermentans, an ascomycete yeast distantly related to S. cerevisiae, possesses a very large FLO/ALS-like Sequence (FAS) family encompassing 34 genes. Fas proteins are variable in size and divergent in sequence and show similarity to the Flo1/5/9 family. Fas proteins show the general build with a signal peptide, an N-terminal carbohydrate binding PA14 domain, a central region differing by the number of repeats and a C-terminus with a consensus sequence for GPI-anchor attachment. Like FLO genes in S. cerevisiae, FAS genes are mostly telomeric with several paralogs at each telomere. We term such genes that share evolutionary conserved telomere localization "telologs" and provide several other examples. Adhesin expression in S. cerevisiae and filamentation in Candida albicans is regulated by Flo8 and Mss11. In Saccharomycopsis we identified only a single protein with similarity to Flo8 based on sequence similarity and the presence of a LisH domain.

Fungal Resistance to Echinocandins and the MDR Phenomenon in Candida glabrata

Candida glabrata has thoroughly adapted to successfully colonize human mucosal membranes and survive in vivo pressures. prior to and during antifungal treatment. Out of all the medically relevant Candida species, C. glabrata has emerged as a leading cause of azole, echinocandin, and multidrug (MDR: azole + echinocandin) adaptive resistance. Neither mechanism of resistance is intrinsic to C. glabrata, since stable genetic resistance depends on mutation of drug target genes, FKS1 and FKS2 (echinocandin resistance), and a transcription factor, PDR1, which controls expression of major drug transporters, such as CDR1 (azole resistance). However, another hallmark of C. glabrata is the ability to withstand drug pressure both in vitro and in vivo prior to stable "genetic escape". Additionally, these resistance events can arise within individual patients, which underscores the importance of understanding how this fungus is adapting to its environment and to drug exposure in vivo. Here, we explore the evolution of echinocandin resistance as a multistep model that includes general cell stress, drug adaptation (tolerance), and genetic escape. The extensive genetic diversity reported in C. glabrata is highlighted.