A novel class of Candida glabrata cell wall proteins with β-helix fold mediates adhesion in clinical isolates

Induction of Antifungal Tolerance Reveals Genetic and Phenotypic Changes in Candida glabrata

Candida glabrata is an opportunistic, pathogenic fungus that is increasingly isolated from hospitalized patients. The incidence of drug tolerance, heteroresistance, and resistance is on the rise due to an overuse of antifungal drugs. The aim of this study was to expose a sensitive C. glabrata strain to sequentially increasing concentrations of two antifungal drugs, fluconazole, an azole that targets ergosterol biosynthesis, or caspofungin, an echinocandin that targets cell wall glucan synthesis. Analysis of the drug-exposed isolates showed development of antifungal tolerance, chromosomal abnormalities, decreased adhesion, attenuated virulence, and an increase in efflux pump activity. Furthermore, whole genome sequencing of all isolates exposed to different concentrations of fluconazole or caspofungin was performed to determine mutations in key genes that could correlate with the observed phenotypes. Mutations were found in genes implicated in adhesion, such as in the AWP, PWP, and EPA family of genes. Isolates exposed to higher drug concentrations displayed more mutations than those at lower concentrations.

Proteomic characterization of clinical Candida glabrata isolates with varying degrees of virulence and resistance to fluconazole

Candida glabrata, an opportunistic fungal pathogen, is a significant contributor to mortality among individuals with weakened immune systems. Antifungal drugs such as azoles work by inhibiting the Erg11 enzyme, altering the conversion of lanosterol to ergosterol. Resistance to azoles is increasing among Candida species worldwide, and in Lebanon. This study aims to determine the identity of cell wall proteins that could be involved in resistance and virulence in Candida glabrata Lebanese hospital isolates. Four isolates with varying degrees of resistance and virulence to fluconazole were subjected to proteomic analysis. Cell wall proteins of each isolate were extracted and analyzed using MALDI TOF TOF mass spectrometry to identify proteins responsible for virulence and resistance under exposure to fluconazole. Results showed the exclusive presence of efflux pumps such as Cdr1 and Pdr1 after exposure to fluconazole, in addition to other resistance mechanisms such as activation of multidrug transporter proteins and specific response pathways such as the RIM 101 pathway that could be involved in drug resistance and adhesion. Proteomic profiling exhibited proteins differentially detected in the virulent isolates such as the autophagy related proteins Atg 11 and Atg16, and stress response proteins Sgf11 and Alg2. In conclusion, our study suggests several mechanisms that contribute to resistance and virulence in C. glabrata.

Thymol inhibits ergosterol biosynthesis in Nakaseomyces glabratus, but differently from azole antifungals

INTRODUCTION

Nakaseomyces glabratus is considered a high priority of attention according to WHO, and also is an important yeast species due to its high rate of intrinsic/acquired resistance against fluconazole. This study aimed at the possible mechanisms of action of thymol, as the promising new antifungal agent, in N. glabratus.

METHODS

Thirty previously identified N. glabratus isolates were selected for investigation of the thymol susceptibility pattern. The antifungal susceptibility test was performed according to the Clinical and Laboratory Standards protocol published as M27-A2 document. Likely changes in the expression pattern of genes involved in the ergosterol biosynthesis pathway were assessed by Real-time PCR assay. The ultrastructure characteristics of thymol-treated yeasts and also the possible interactive proteins, as targets for thymol binding, were performed by transmission electron microscopy (TEM) and reverse molecular docking, respectively.

RESULTS

Minimum inhibitory concentrations ranged between 32-128 µg/mL which were statistically significant between the fluconazole-susceptible and fluconazole-resistant yeast group (P<0. 05). TEM observation results showed that thymol led to peripheral vacuole formation which refers to plasma membrane damage and cell membrane separation from the cell wall. Thymol exhibits antifungal activity against N. glabratus by regulating multiple signaling pathways including ergosterol biosynthesis (ERG1) and HOG (high-osmolarity glycerol) MAPK (mitogen-activated protein kinase) pathways. In consistence with the yielded gene expression patterns, docking evaluation findings also revealed the high affinity of thymol with proteins related to the ERG1 gene. Accordingly, thymol's high affinity to chitin synthase and calcineurin subunit B was noteworthy.

CONCLUSION

Thymol might employ its antifungal effect by involving different pathways comprising ergosterol biosynthesis inhibition but not identical to the azole drugs. It is highly suggested that thymol ruins cell membrane function by decreasing the ergosterol/or chitin content. However, studying more ergosterol biosynthesis-related genes and also the yeast apoptotic responses is highly recommended for future investigations.

Evaluation of Fifteen 5,6-Dihydrotetrazolo[1,5-c]quinazolines Against Nakaseomyces glabrata: Integrating In Vitro Studies, Molecular Docking, QSAR, and In Silico Toxicity Assessments

Nakaseomyces glabrata (Candida glabrata), the second most prevalent Candida pathogen globally, has emerged as a major clinical threat due to its ability to develop high-level azole resistance. In this study, two new 5,6-dihydrotetrazolo[1,5-c]quinazoline derivatives (c11 and c12) were synthesized and characterized using IR, LC-MS, 1H, and 13C NMR spectra. Along with 13 previously reported analogues, these compounds underwent in vitro antifungal testing against clinical N. glabrata isolates using a serial dilution method (0.125-64 mg/L). Remarkably, compounds c5 and c1 exhibited potent antifungal activity, with minimum inhibitory concentrations of 0.37 μM and 0.47 μM, respectively-about a 20-fold improvement in μM concentration over standard drugs like amphotericin B, caspofungin, and micafungin. A detailed structure-activity relationship analysis revealed crucial molecular features enhancing antifungal potency. Extensive molecular docking studies across 18 protein targets explored potential binding pockets and affinities of the lead compounds. A robust 3D-QSAR model, incorporating molecular descriptors Mor26m and Mor29e, displayed good predictive ability for antifungal activity. In silico predictions indicated an absence of herbicidal effect, negligible environmental toxicity (to honeybees, avian species, and aquatic organisms), and mild human toxicity concerns for these compounds. This comprehensive approach aims to develop novel and effective antifungal compounds against the clinically relevant pathogen N. glabrata.

Chromosome level assemblies of Nakaseomyces (Candida) bracarensis uncover two distinct clades and define its adhesin repertoire

BACKGROUND

The Nakaseomyces clade is formed by at least nine described species among which three can be pathogenic to humans, namely Nakaseomyces glabratus (Candida glabrata), the second most-common cause of candidiasis worldwide, and two rarer emerging pathogens: Nakaseomyces (Candida) nivarensis and Nakaseomyces (Candida) bracarensis. Early comparative genomics analyses identified parallel expansions of subtelomeric adhesin genes in N. glabratus and N. nivarensis/bracarensis, and suggested possible links with the emergence of the virulence potential in these species. However, as shown for N. glabratus, the proper assessment of subtelomeric genes is hindered by the use of incomplete assemblies and reliance on a single isolate.

RESULTS

Here we sequenced seven N. bracarensis isolates and reconstructed chromosome level assemblies of two divergent strains. We show that N. bracarensis isolates belong to two diverging clades that have slightly different genomic structures. We identified the set of encoded adhesins in the two complete assemblies, and uncovered the presence of a novel adhesin motif, found mainly in N. bracarensis. Our analysis revealed a larger adhesin content in N. bracarensis than previously reported, and similar in size to that of N. glabratus. We confirm the independent adhesin expansion in these two species, which could relate to their different levels of virulence.

CONCLUSION

N. bracarensis clinical isolates belong to at least two differentiated clades. We describe a novel repeat motif found in N. bracarensis adhesins, which helps in their identification. Adhesins underwent independent expansions in N. glabratus and N. bracarensis, leading to repertoires that are qualitatively different but quantitatively similar. Given that adhesins are considered virulence factors, some of the observed differences could contribute to variations in virulence capabilities between N. glabratus and N. bracarensis.

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.

The good, the bad, and the hazardous: comparative genomic analysis unveils cell wall features in the pathogen Candidozyma auris typical for both baker's yeast and Candida

The drug-resistant pathogenic yeast Candidozyma auris (formerly named Candida auris) is considered a critical health problem of global importance. As the cell wall plays a crucial role in pathobiology, here we performed a detailed bioinformatic analysis of its biosynthesis in C. auris and related Candidozyma haemuli complex species using Candida albicans and Saccharomyces cerevisiae as references. Our data indicate that the cell wall architecture described for these reference yeasts is largely conserved in Candidozyma spp.; however, expansions or reductions in gene families point to subtle alterations, particularly with respect to β--1,3--glucan synthesis and remodeling, phosphomannosylation, β-mannosylation, and glycosylphosphatidylinositol (GPI) proteins. In several aspects, C. auris holds a position in between C. albicans and S. cerevisiae, consistent with being classified in a separate genus. Strikingly, among the identified putative GPI proteins in C. auris are adhesins typical for both Candida (Als and Hyr/Iff) and Saccharomyces (Flo11 and Flo5-like flocculins). Further, 26 putative C. auris GPI proteins lack homologs in Candida genus species. Phenotypic analysis of one such gene, QG37_05701, showed mild phenotypes implicating a role associated with cell wall β-1,3-glucan. Altogether, our study uncovered a wealth of information relevant for the pathogenicity of C. auris as well as targets for follow-up studies.

Integrated post-genomic cell wall analysis reveals floating biofilm formation associated with high expression of flocculins in the pathogen Pichia kudriavzevii

The pathogenic yeast Pichia kudriavzevii, previously known as Candida krusei, is more distantly related to Candida albicans than clinically relevant CTG-clade Candida species. Its cell wall, a dynamic organelle that is the first point of interaction between pathogen and host, is relatively understudied, and its wall proteome remains unidentified to date. Here, we present an integrated study of the cell wall in P. kudriavzevii. Our comparative genomic studies and experimental data indicate that the general structure of the cell wall in P. kudriavzevii is similar to Saccharomyces cerevisiae and C. albicans and is comprised of β-1,3-glucan, β-1,6-glucan, chitin, and mannoproteins. However, some pronounced differences with C. albicans walls were observed, for instance, higher mannan and protein levels and altered protein mannosylation patterns. Further, despite absence of proteins with high sequence similarity to Candida adhesins, protein structure modeling identified eleven proteins related to flocculins/adhesins in S. cerevisiae or C. albicans. To obtain a proteomic comparison of biofilm and planktonic cells, P. kudriavzevii cells were grown to exponential phase and in static 24-h cultures. Interestingly, the 24-h static cultures of P. kudriavzevii yielded formation of floating biofilm (flor) rather than adherence to polystyrene at the bottom. The proteomic analysis of both conditions identified a total of 33 cell wall proteins. In line with a possible role in flor formation, increased abundance of flocculins, in particular Flo110, was observed in the floating biofilm compared to exponential cells. This study is the first to provide a detailed description of the cell wall in P. kudriavzevii including its cell wall proteome, and paves the way for further investigations on the importance of flor formation and flocculins in the pathogenesis of P. kudriavzevii.

Parallel expansion and divergence of an adhesin family in pathogenic yeasts

Opportunistic yeast pathogens arose multiple times in the Saccharomycetes class, including the recently emerged, multidrug-resistant (MDR) Candida auris. We show that homologs of a known yeast adhesin family in Candida albicans, the Hyr/Iff-like (Hil) family, are enriched in distinct clades of Candida species as a result of multiple, independent expansions. Following gene duplication, the tandem repeat-rich region in these proteins diverged extremely rapidly and generated large variations in length and β-aggregation potential, both of which are known to directly affect adhesion. The conserved N-terminal effector domain was predicted to adopt a β-helical fold followed by an α-crystallin domain, making it structurally similar to a group of unrelated bacterial adhesins. Evolutionary analyses of the effector domain in C. auris revealed relaxed selective constraint combined with signatures of positive selection, suggesting functional diversification after gene duplication. Lastly, we found the Hil family genes to be enriched at chromosomal ends, which likely contributed to their expansion via ectopic recombination and break-induced replication. Combined, these results suggest that the expansion and diversification of adhesin families generate variation in adhesion and virulence within and between species and are a key step toward the emergence of fungal pathogens.

Sticking to the Subject: Multifunctionality in Microbial Adhesins

Bacterial and fungal adhesins mediate microbial aggregation, biofilm formation, and adhesion to host. We divide these proteins into two major classes: professional adhesins and moonlighting adhesins that have a non-adhesive activity that is evolutionarily conserved. A fundamental difference between the two classes is the dissociation rate. Whereas moonlighters, including cytoplasmic enzymes and chaperones, can bind with high affinity, they usually dissociate quickly. Professional adhesins often have unusually long dissociation rates: minutes or hours. Each adhesin has at least three activities: cell surface association, binding to a ligand or adhesive partner protein, and as a microbial surface pattern for host recognition. We briefly discuss Bacillus subtilis TasA, pilin adhesins, gram positive MSCRAMMs, and yeast mating adhesins, lectins and flocculins, and Candida Awp and Als families. For these professional adhesins, multiple activities include binding to diverse ligands and binding partners, assembly into molecular complexes, maintenance of cell wall integrity, signaling for cellular differentiation in biofilms and in mating, surface amyloid formation, and anchorage of moonlighting adhesins. We summarize the structural features that lead to these diverse activities. We conclude that adhesins resemble other proteins with multiple activities, but they have unique structural features to facilitate multifunctionality.

Protective Efficacy of Anti-Hyr1p Monoclonal Antibody against Systemic Candidiasis Due to Multi-Drug-Resistant Candida auris

Candida auris is a multi-drug-resistant fungal pathogen that can survive outside the host and can easily spread and colonize the healthcare environment, medical devices, and human skin. C. auris causes serious life-threatening infections (up to 60% mortality) in immunosuppressed patients staying in such contaminated healthcare facilities. Some isolates of C. auris are resistant to virtually all clinically available antifungal drugs. Therefore, alternative therapeutic approaches are urgently needed. Using in silico protein modeling and analysis, we identified a highly immunogenic and surface-exposed epitope that is conserved between C. albicans hyphal-regulated protein (Cal-Hyr1p) and Hyr1p/Iff-like proteins in C. auris (Cau-HILp). We generated monoclonal antibodies (MAb) against this Cal-Hyr1p epitope, which recognized several clinical isolates of C. auris representing all four clades. An anti-Hyr1p MAb prevented biofilm formation and enhanced opsonophagocytic killing of C. auris by macrophages. When tested for in vivo efficacy, anti-Hyr1p MAb protected 55% of mice against lethal systemic C. auris infection and showed significantly less fungal burden. Our study is highly clinically relevant and provides an effective alternative therapeutic option to treat infections due to MDR C. auris.

CBD resistant Salmonella strains are susceptible to epsilon 34 phage tailspike protein

The rise of antimicrobial resistance is a global public health crisis that threatens the effective control and prevention of infections. Due to the emergence of pandrug-resistant bacteria, most antibiotics have lost their efficacy. Bacteriophages or their components are known to target bacterial cell walls, cell membranes, and lipopolysaccharides (LPS) and hydrolyze them. Bacteriophages being the natural predators of pathogenic bacteria, are inevitably categorized as "human friends", thus fulfilling the adage that "the enemy of my enemy is my friend". Leveraging on their lethal capabilities against pathogenic bacteria, researchers are searching for more ways to overcome the current antibiotic resistance challenge. In this study, we expressed and purified epsilon 34 phage tailspike protein (E34 TSP) from the E34 TSP gene, then assessed the ability of this bacteriophage protein in the killing of two CBD-resistant strains of Salmonella spp. We also assessed the ability of the tailspike protein to cause bacteria membrane disruption, and dehydrogenase depletion. We observed that the combined treatment of CBD-resistant strains of Salmonella with CBD and E34 TSP showed poor killing ability whereas the monotreatment with E34 TSP showed considerably higher killing efficiency. This study demonstrates that the inhibition of the bacteria by E34 TSP was due in part to membrane disruption, and dehydrogenase inactivation by the protein. The results of this work provides an interesting background to highlight the crucial role phage protein such as E34 TSP could play in pathogenic bacterial control.

Chromosome-level assemblies from diverse clades reveal limited structural and gene content variation in the genome of Candida glabrata

Background

Candida glabrata is an opportunistic yeast pathogen thought to have a large genetic and phenotypic diversity and a highly plastic genome. However, the lack of chromosome-level genome assemblies representing this diversity limits our ability to accurately establish how chromosomal structure and gene content vary across strains.

Results

Here, we expanded publicly available assemblies by using long-read sequencing technologies in twelve diverse strains, obtaining a final set of twenty-one chromosome-level genomes spanning the known C. glabrata diversity. Using comparative approaches, we inferred variation in chromosome structure and determined the pan-genome, including an analysis of the adhesin gene repertoire. Our analysis uncovered four new adhesin orthogroups and inferred a rich ancestral adhesion repertoire, which was subsequently shaped through a still ongoing process of gene loss, gene duplication, and gene conversion.

Conclusions

C. glabrata has a largely stable pan-genome except for a highly variable subset of genes encoding cell wall-associated functions. Adhesin repertoire was established for each strain and showed variability among clades.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01412-1.

Structure of the Yeast Cell Wall Integrity Sensor Wsc1 Reveals an Essential Role of Surface-Exposed Aromatic Clusters

In the yeast Saccharomyces cerevisiae and other ascomycetes, the maintenance of cell wall integrity is governed by a family of plasma-membrane spanning sensors that include the Wsc-type proteins. These cell wall proteins apparently sense stress-induced mechanical forces at the cell surface and target the cell wall integrity (CWI) signaling pathway, but the structural base for their sensor function is yet unknown. Here, we solved a high-resolution crystal structure of the extracellular cysteine-rich domain (CRD) of yeast Wsc1, which shows the characteristic PAN/Apple domain fold with two of the four Wsc1 disulfide bridges being conserved in other PAN domain cores. Given the general function of PAN domains in mediating protein–protein and protein–carbohydrate interactions, this finding underpins the importance of Wsc domains in conferring sensing and localization functions. Our Wsc1 CRD structure reveals an unusually high number of surface-exposed aromatic residues that are conserved in other fungal CRDs, and can be arranged into three solvent-exposed clusters. Mutational analysis demonstrates that two of the aromatic clusters are required for conferring S. cerevisiae Wsc1-dependent resistance to the glucan synthase inhibitor caspofungin, and the chitin-binding agents Congo red and Calcofluor white. These findings suggest an essential role of surface-exposed aromatic clusters in fungal Wsc-type sensors that might include an involvement in stress-induced sensor-clustering required to elicit appropriate cellular responses via the downstream CWI pathway.