Microevolution of Candida albicans in macrophages restores filamentation in a nonfilamentous mutant

Commensalism and pathogenesis of Candida albicans at the mucosal interface

Fungi are important and often underestimated human pathogens. Infections with fungi mostly originate from the environment, from soil or airborne spores. By contrast, Candida albicans, one of the most common and clinically important fungal pathogens, permanently exists in the vast majority of healthy individuals as a member of the human mucosal microbiota. Only under certain circumstances will these commensals cause infections. However, although the pathogenic behaviour and disease manifestation of C. albicans have been at the centre of research for many years, its asymptomatic colonization of mucosal surfaces remains surprisingly understudied. In this Review, we discuss the interplay of the fungus, the host and the microbiome on the dualism of commensal and pathogenic life of C. albicans, and how commensal growth is controlled and permitted. We explore hypotheses that could explain how the mucosal environment shapes C. albicans adaptations to its commensal lifestyle, while still maintaining or even increasing its pathogenic potential.

Image-based quantification of Candida albicans filamentation and hyphal length using the open-source visual programming language JIPipe

The formation of hyphae is one of the most crucial virulence traits the human pathogenic fungus Candida albicans possesses. The assessment of hyphal length in response to various stimuli, such as exposure to human serum, provides valuable insights into the adaptation strategies of C. albicans to the host environment. Despite the increasing high-throughput capacity live-cell imaging and data generation, the accurate analysis of hyphal growth has remained a laborious, error-prone, and subjective manual process. We developed an analysis pipeline utilizing the open-source visual programming language Java Image Processing Pipeline (JIPipe) to overcome the limitations associated with manual analysis of hyphal growth. By comparing our automated approach with manual analysis, we refined the strategies to achieve accurate differentiation between yeast cells and hyphae. The automated method enables length measurements of individual hyphae, facilitating a time-efficient, high-throughput, and user-friendly analysis. By utilizing this JIPipe analysis approach, we obtained insights into the filamentation behavior of two C. albicans strains when exposed to human serum albumin (HSA), the most abundant protein in human serum. Our findings indicate that despite the known role of HSA in stimulating fungal growth, it reduces filamentous growth. The implementation of our automated JIPipe analysis approach for hyphal growth represents a long-awaited and time-efficient solution to meet the demand of high-throughput data generation. This tool can benefit different research areas investigating the virulence aspects of C. albicans.

A gain-of-function mutation in zinc cluster transcription factor Rob1 drives Candida albicans adaptive growth in the cystic fibrosis lung environment

Candida albicans chronically colonizes the respiratory tract of patients with Cystic Fibrosis (CF). It competes with CF-associated pathogens (e.g. Pseudomonas aeruginosa) and contributes to disease severity. We hypothesize that C. albicans undergoes specific adaptation mechanisms that explain its persistence in the CF lung environment. To identify the underlying genetic and phenotypic determinants, we serially recovered 146 C. albicans clinical isolates over a period of 30 months from the sputum of 25 antifungal-naive CF patients. Multilocus sequence typing analyses revealed that most patients were individually colonized with genetically close strains, facilitating comparative analyses between serial isolates. We strikingly observed differential ability to filament and form monospecies and dual-species biofilms with P. aeruginosa among 18 serial isolates sharing the same diploid sequence type, recovered within one year from a pediatric patient. Whole genome sequencing revealed that their genomes were highly heterozygous and similar to each other, displaying a highly clonal subpopulation structure. Data mining identified 34 non-synonymous heterozygous SNPs in 19 open reading frames differentiating the hyperfilamentous and strong biofilm-former strains from the remaining isolates. Among these, we detected a glycine-to-glutamate substitution at position 299 (G299E) in the deduced amino acid sequence of the zinc cluster transcription factor ROB1 (ROB1G299E), encoding a major regulator of filamentous growth and biofilm formation. Introduction of the G299E heterozygous mutation in a co-isolated weak biofilm-former CF strain was sufficient to confer hyperfilamentous growth, increased expression of hyphal-specific genes, increased monospecies biofilm formation and increased survival in dual-species biofilms formed with P. aeruginosa, indicating that ROB1G299E is a gain-of-function mutation. Disruption of ROB1 in a hyperfilamentous isolate carrying the ROB1G299E allele abolished hyperfilamentation and biofilm formation. Our study links a single heterozygous mutation to the ability of C. albicans to better survive during the interaction with other CF-associated microbes and illuminates how adaptive traits emerge in microbial pathogens to persistently colonize and/or infect the CF-patient airways.

Candida albicans translocation through the intestinal epithelial barrier is promoted by fungal zinc acquisition and limited by NFκB-mediated barrier protection

The opportunistic fungal pathogen Candida albicans thrives on human mucosal surfaces as a harmless commensal, but frequently causes infections under certain predisposing conditions. Translocation across the intestinal barrier into the bloodstream by intestine-colonizing C. albicans cells serves as the main source of disseminated candidiasis. However, the host and microbial mechanisms behind this process remain unclear. In this study we identified fungal and host factors specifically involved in infection of intestinal epithelial cells (IECs) using dual-RNA sequencing. Our data suggest that host-cell damage mediated by the peptide toxin candidalysin-encoding gene ECE1 facilitates fungal zinc acquisition. This in turn is crucial for the full virulence potential of C. albicans during infection. IECs in turn exhibit a filamentation- and damage-specific response to C. albicans infection, including NFκB, MAPK, and TNF signaling. NFκB activation by IECs limits candidalysin-mediated host-cell damage and mediates maintenance of the intestinal barrier and cell-cell junctions to further restrict fungal translocation. This is the first study to show that candidalysin-mediated damage is necessary for C. albicans nutrient acquisition during infection and to explain how IECs counteract damage and limit fungal translocation via NFκB-mediated maintenance of the intestinal barrier.

DNase improves the efficacy of antimicrobial photodynamic therapy in the treatment of candidiasis induced with Candida albicans

The study evaluated the association of DNase I enzyme with antimicrobial photodynamic therapy (aPDT) in the treatment of oral candidiasis in mice infected with fluconazole-susceptible (CaS) and -resistant (CaR) Candida albicans strains. Mice were inoculated with C. albicans, and after the infection had been established, the tongues were exposed to DNase for 5 min, followed by photosensitizer [Photodithazine®(PDZ)] and light (LED), either singly or combined. The treatments were performed for 5 consecutive days. Treatment efficacy was evaluated by assessing the tongues via fungal viable population, clinical evaluation, histopathological and fluorescence microscopy methods immediately after finishing treatments, and 7 days of follow-up. The combination of DNase with PDZ-aPDT reduced the fungal viability in mice tongues immediately after the treatments by around 4.26 and 2.89 log10 for CaS and CaR, respectively (versus animals only inoculated). In the fluorescence microscopy, the polysaccharides produced by C. albicans and fungal cells were less labeled in animals treated with the combination of DNase with PDZ-aPDT, similar to the healthy animals. After 7 days of the treatment, DNase associated with PDZ-aPDT maintained a lower count, but not as pronounced as immediately after the intervention. For both strains, mice treated with the combination of DNase with PDZ-aPDT showed remission of oral lesions and mild inflammatory infiltrate in both periods assessed, while animals treated only with PDZ-aPDT presented partial remission of oral lesions. DNase I enzyme improved the efficacy of photodynamic treatment.

Proline catabolism is a key factor facilitating Candida albicans pathogenicity

Candida albicans, the primary etiology of human mycoses, is well-adapted to catabolize proline to obtain energy to initiate morphological switching (yeast to hyphal) and for growth. We report that put1-/- and put2-/- strains, carrying defective Proline UTilization genes, display remarkable proline sensitivity with put2-/- mutants being hypersensitive due to the accumulation of the toxic intermediate pyrroline-5-carboxylate (P5C), which inhibits mitochondrial respiration. The put1-/- and put2-/- mutations attenuate virulence in Drosophila and murine candidemia models and decrease survival in human neutrophils and whole blood. Using intravital 2-photon microscopy and label-free non-linear imaging, we visualized the initial stages of C. albicans cells infecting a kidney in real-time, directly deep in the tissue of a living mouse, and observed morphological switching of wildtype but not of put2-/- cells. Multiple members of the Candida species complex, including C. auris, are capable of using proline as a sole energy source. Our results indicate that a tailored proline metabolic network tuned to the mammalian host environment is a key feature of opportunistic fungal pathogens.

The Yeast and Hypha Phases of Candida krusei Induce the Apoptosis of Bovine Mammary Epithelial Cells via Distinct Signaling Pathways

Infection with Candida spp. is a significant cause of bovine mastitis globally. We previously found that C. krusei was the main pathogen causing mycotic mastitis in dairy cows in Yinchuan, Ningxia, China. However, whether the infection of this pathogen could induce apoptosis in BMECs remained unclear. In this report, we explored the apoptosis and underlying mechanism of BMECs induced by C. krusei yeast and hypha phases using a pathogen/host cell co-culture model. Our results revealed that both the yeast and hypha phases of C. krusei could induce BMEC apoptosis; however, the yeast phase induced more cell apoptosis than the hypha phase, as assessed via electronic microscopy and flow cytometry assays. This finding was further corroborated via the measurement of the mitochondrial membrane potential (MMP) and the TUNEL test. Infection by both the yeast and hypha phases of C. krusei greatly induced the expression of proteins associated with cell death pathways and important components of toll-like receptor (TLR) signaling, including TLR2 and TLR4 receptors, as determined via a Western blotting assay. BMECs mainly underwent apoptosis after infection by the C. krusei yeast phase through a mitochondrial pathway. Meanwhile, BMEC apoptosis induced by the C. krusei hypha phase was regulated by a death ligand/receptor pathway. In addition, C. krusei-induced BMEC apoptosis was regulated by both the TLR2/ERK and JNK/ERK signaling pathways. These data suggest that the yeast phase and hypha phase of C. krusei induce BMEC apoptosis through distinct cell signaling pathways. This study represents a unique perspective on the molecular processes underlying BMEC apoptosis in response to C. krusei infection.

Distinct pathways of adaptive evolution in Cryptococcus neoformans reveal a mutation in adenylyl cyclase with trade-offs for pathogenicity

Pathogenic fungi populate a wide range of environments and infect a diversity of host species. Despite this substantial biological flexibility, the impact of interactions between fungi and their hosts on the evolution of pathogenicity remains unclear. We studied how repeated interactions between the fungus Cryptococcus neoformans and relevant environmental and mammalian host cells-amoeba and mouse macrophages-shape the evolution of this model fungal pathogen. First, using a collection of clinical and environmental isolates of C. neoformans, we characterized a range of survival phenotypes for these strains when exposed to host cells of different species. We then performed serial passages of an environmentally isolated C. neoformans strain through either amoeba or macrophages for ∼75 generations to observe how these interactions select for improved replication within hosts. In one adapted population, we identified a single point mutation in the adenylyl cyclase gene, CAC1, that swept to fixation and confers a strong competitive advantage for growth inside macrophages. Strikingly, this growth advantage in macrophages is inversely correlated with disease severity during mouse infections, suggesting that adaptation to specific host niches can markedly reduce the pathogenicity of these fungi. These results raise intriguing questions about the influence of cyclic AMP (cAMP) signaling on pathogenicity and highlight the role of seemingly small adaptive changes in promoting fundamental shifts in the intracellular behavior and virulence of these important human pathogens.

Ncs2* mediates in vivo virulence of pathogenic yeast through sulphur modification of cytoplasmic transfer RNA

Fungal pathogens threaten ecosystems and human health. Understanding the molecular basis of their virulence is key to develop new treatment strategies. Here, we characterize NCS2*, a point mutation identified in a clinical baker's yeast isolate. Ncs2 is essential for 2-thiolation of tRNA and the NCS2* mutation leads to increased thiolation at body temperature. NCS2* yeast exhibits enhanced fitness when grown at elevated temperatures or when exposed to oxidative stress, inhibition of nutrient signalling, and cell-wall stress. Importantly, Ncs2* alters the interaction and stability of the thiolase complex likely mediated by nucleotide binding. The absence of 2-thiolation abrogates the in vivo virulence of pathogenic baker's yeast in infected mice. Finally, hypomodification triggers changes in colony morphology and hyphae formation in the common commensal pathogen Candida albicans resulting in decreased virulence in a human cell culture model. These findings demonstrate that 2-thiolation of tRNA acts as a key mediator of fungal virulence and reveal new mechanistic insights into the function of the highly conserved tRNA-thiolase complex.

Systematic Metabolic Profiling Identifies De Novo Sphingolipid Synthesis as Hypha Associated and Essential for Candida albicans Filamentation

The yeast-to-hypha transition is a key virulence attribute of the opportunistic human fungal pathogen Candida albicans, since it is closely tied to infection-associated processes such as tissue invasion and escape from phagocytes. While the nature of hypha-associated gene expression required for fungal virulence has been thoroughly investigated, potential morphotype-dependent activity of metabolic pathways remained unclear. Here, we combined global transcriptome and metabolome analyses for the wild-type SC5314 and the hypha-defective hgc1Δ and cph1Δefg1Δ strains under three hypha-inducing (human serum, N-acetylglucosamine, and alkaline pH) and two yeast-promoting conditions to identify metabolic adaptions that accompany the filamentation process. We identified morphotype-related activities of distinct pathways and a metabolic core signature of 26 metabolites with consistent depletion or enrichment during the yeast-to-hypha transition. Most strikingly, we found a hypha-associated activation of de novo sphingolipid biosynthesis, indicating a connection of this pathway and filamentous growth. Consequently, pharmacological inhibition of this partially fungus-specific pathway resulted in strongly impaired filamentation, verifying the necessity of de novo sphingolipid biosynthesis for proper hypha formation. IMPORTANCE The reversible switch of Candida albicans between unicellular yeast and multicellular hyphal growth is accompanied by a well-studied hypha-associated gene expression, encoding virulence factors like adhesins, toxins, or nutrient scavengers. The investigation of this gene expression consequently led to fundamental insights into the pathogenesis of this fungus. In this study, we applied this concept to hypha-associated metabolic adaptations and identified morphotype-dependent activities of distinct pathways and a stimulus-independent metabolic signature of hyphae. Most strikingly, we found the induction of de novo sphingolipid biosynthesis as hypha associated and essential for the filamentation of C. albicans. These findings verified the presence of morphotype-specific metabolic traits in the fungus, which appear connected to the fungal virulence. Furthermore, the here-provided comprehensive description of the fungal metabolome will help to foster future research and lead to a better understanding of fungal physiology.

Improved Subcutaneous Delivery of Ketoconazole Using EpiDerm and HSPiP Software-Based Simulations as Assessed by Cell Viability, Cellular Uptake, Permeation, and Hemolysis In Vitro Studies

Ketoconazole (KETO) is the drug of choice to control local, systemic, and resistant types of fungal infections. Subcutaneous (sub-Q) delivery offers several benefits. The present study investigated the sub-Q delivery of KETO using HSPiP software based on optimized concentrations of dimethylacetamide (DMA) in binary solvents (DMA + water), in vitro cellular uptake (J774A.1) assays, cellular toxicity (L929), and in vitro hemolysis studies. Results showed that the estimated permeation coefficient (9.6 × 10^-3 cm/h) and diffusion coefficient (3.9 × 10^-3 cm²/h) of KETO (22.3 mg) in KF3 (300 mg of DMA + water) across EpiDerm were relatively higher as compared to the other formulations [KF1 (11.2 and 150 mg as KETO and DMA, respectively) and KF2 [(22.3 and 300 mg as KETO and DMA, respectively)] due to the increased content of DMA and KETO. HSPiP simulated and predicted the impact of constant and variable diffusion coefficients on the percent drug absorption across EpiDerm and the time needed to achieve equilibrium. The concentration-dependent diffusion coefficient fed into HSPiP predicted that the drug absorption and permeation values were linearly dependent on the square root of time. The HSPiP predicted permeation flux values from KF3, KF2, and KF1 across the EpiDerm were 4.07 × 10^-6, 4.01 × 10^-6, and 1.1 × 10^-6 g/cm²/s, respectively, at respective D range values. The selected K30G (324 mOsm/Kg) showed an optimal pH (6.9) and minimum drug loss (0.01%) over a period of 1 month at room temperature. KG30 was found to be less toxic to normal L292 cells and caused maximum cytotoxicity to candida cells residing within infected macrophage cells (J774A.1 incubated for 24 h), which was attributed to the slow diffusion of K30G compared to DS (the drug solution with an equivalent concentration). KG30 elicited substantial internalization with candida albicans (MTCC 4748) compared to the control group (24 h). Lastly, in vitro hemolysis studies (1 and 5 μg/mL) corroborated the safety of K30G for sub-Q delivery. Therefore, this new formulation and approach for delivering KETO is a promising alternative to conventional products to control fungal infections and, thus, should be further studied in vivo.

The escape of Candida albicans from macrophages is enabled by the fungal toxin candidalysin and two host cell death pathways

The egress of Candida hyphae from macrophages facilitates immune evasion, but it also alerts macrophages to infection and triggers inflammation. To better define the mechanisms, here we develop an imaging assay to directly and dynamically quantify hyphal escape and correlate it to macrophage responses. The assay reveals that Candida escapes by using two pore-forming proteins to permeabilize macrophage membranes: the fungal toxin candidalysin and Nlrp3 inflammasome-activated Gasdermin D. Candidalysin plays a major role in escape, with Nlrp3 and Gasdermin D-dependent and -independent contributions. The inactivation of Nlrp3 does not reduce hyphal escape, and we identify ETosis via macrophage extracellular trap formation as an additional pathway facilitating hyphal escape. Suppressing hyphal escape does not reduce fungal loads, but it does reduce inflammatory activation. Our findings explain how Candida escapes from macrophages by using three strategies: permeabilizing macrophage membranes via candidalysin and engaging two host cell death pathways, Gasdermin D-mediated pyroptosis and ETosis.

Unraveling the anti-virulence potential and antifungal efficacy of 5-aminotetrazoles using the zebrafish model of disseminated candidiasis

Candida albicans remains the main causal agent of candidiasis, the most common fungal infection with disturbingly high mortality rates worldwide. The limited diversity and efficacy of clinical antifungal drugs, exacerbated by emerging drug resistance, have resulted in the failure of current antifungal therapies. This imposes an urgent demand for the development of innovative strategies for effective eradication of candidal infections. While the existing clinical drugs display fungicidal or fungistatic activity, the strategy specifically targeting C. albicans filamentation, as the most important virulence trait, represents an attractive approach for overcoming the drawbacks related to clinical antifungals. The results acquired in this study revealed the significant potential of 5-aminotetrazoles as a new class of effective and safe anti-virulence agents. Moreover, these novel agents were active when applied both alone and in combination with clinically approved polyenes. Complete prevention of C. albicans morphogenetic yeast-to-hyphae transition was achieved at doses as low as 1.3 μM under conditions mimicking various filamentation-responsive stimuli in the human body, while no cardio- or hepatotoxicity was observed at doses as high as 200 μM. The treatment of C. albicans-infected zebrafish embryos with nystatin alone had low efficacy, while the combination of nystatin and selected 5-aminotetrazoles prevented fungal filamentation, successfully eliminating the infection and rescuing the infected embryos from lethal disseminated candidiasis. In addition, the most potent anti-virulence 5-aminotetrazole prevented C. albicans in developing the resistance to nystatin when applied in combination, keeping the fungus sensitive to the antifungal drug.

The Candida albicans Cdk8-dependent phosphoproteome reveals repression of hyphal growth through a Flo8-dependent pathway

Ssn3, also known as Cdk8, is a member of the four protein Cdk8 submodule within the multi-subunit Mediator complex involved in the co-regulation of transcription. In Candida albicans, the loss of Ssn3 kinase activity affects multiple phenotypes including cellular morphology, metabolism, nutrient acquisition, immune cell interactions, and drug resistance. In these studies, we generated a strain in which Ssn3 was replaced with a functional variant of Ssn3 that can be rapidly and selectively inhibited by the ATP analog 3-MB-PP1. Consistent with ssn3 null mutant and kinase dead phenotypes, inhibition of Ssn3 kinase activity promoted hypha formation. Furthermore, the increased expression of hypha-specific genes was the strongest transcriptional signal upon inhibition of Ssn3 in transcriptomics analyses. Rapid inactivation of Ssn3 was used for phosphoproteomic studies performed to identify Ssn3 kinase substrates associated with filamentation potential. Both previously validated and novel Ssn3 targets were identified. Protein phosphorylation sites that were reduced specifically upon Ssn3 inhibition included two sites in Flo8 which is a transcription factor known to positively regulate C. albicans morphology. Mutation of the two Flo8 phosphosites (threonine 589 and serine 620) was sufficient to increase Flo8-HA levels and Flo8 dependent transcriptional and morphological changes, suggesting that Ssn3 kinase activity negatively regulates Flo8.Under embedded conditions, when ssn3Δ/Δ and efg1Δ/Δ mutants were hyperfilamentous, FLO8 was essential for hypha formation. Previous work has also shown that loss of Ssn3 activity leads to increased alkalinization of medium with amino acids. Here, we show that the ssn3Δ/Δ medium alkalinization phenotype, which is dependent on STP2, a transcription factor involved in amino acid utilization, also requires FLO8 and EFG1. Together, these data show that Ssn3 activity can modulate Flo8 and its direct and indirect interactions in different ways, and underscores the potential importance of considering Ssn3 function in the control of transcription factor activities.

In Candida albicans, Ssn3 kinase activity co-regulates the transcription of numerous genes involved in hyphal growth, metabolism and nutrient acquisition, immune cell interactions, and drug resistance. Using a strain in which Ssn3 could be rapidly and selectively inhibited, we identified both known and novel Ssn3 targets. We identified two phosphosites in Flo8, a regulator of morphology and virulence, that were shown to negatively regulate Flo8 levels and activity. The data and tools presented here will enable a better understanding of how Ssn3 impacts transcriptional and post-transcriptional regulation in order to coordinate processes during physiological and morphological transitions as well as during steady state growth.

The macrophage-derived protein PTMA induces filamentation of the human fungal pathogen Candida albicans

Evasion of killing by immune cells is crucial for fungal survival in the host. For the human fungal pathogen Candida albicans, internalization by macrophages induces a transition from yeast to filaments that promotes macrophage death and fungal escape. Nutrient deprivation, alkaline pH, and oxidative stress have been implicated as triggers of intraphagosomal filamentation; however, the impact of other host-derived factors remained unknown. Here, we show that lysates prepared from macrophage-like cell lines and primary macrophages robustly induce C. albicans filamentation. Enzymatic treatment of lysate implicates a phosphorylated protein, and bioactivity-guided fractionation coupled to mass spectrometry identifies the immunomodulatory phosphoprotein PTMA as a candidate trigger of C. albicans filamentation. Immunoneutralization of PTMA within lysate abolishes its activity, strongly supporting PTMA as a filament-inducing component of macrophage lysate. Adding to the known repertoire of physical factors, this work implicates a host protein in the induction of C. albicans filamentation within immune cells.

The human fungal pathogen Candida albicans filaments within host macrophages, enabling its escape. Case et al. demonstrate that lysates prepared from macrophage-like cell lines and primary macrophages induce C. albicans filamentation and implicate the immunomodulatory protein prothymosin alpha (PTMA) as a trigger of filamentation produced by host immune cells.

Integrative multi-omics profiling reveals cAMP-independent mechanisms regulating hyphal morphogenesis in Candida albicans

Microbial pathogens grow in a wide range of different morphologies that provide distinct advantages for virulence. In the fungal pathogen Candida albicans, adenylyl cyclase (Cyr1) is thought to be a master regulator of the switch to invasive hyphal morphogenesis and biofilm formation. However, faster growing cyr1Δ/Δ pseudorevertant (PR) mutants were identified that form hyphae in the absence of cAMP. Isolation of additional PR mutants revealed that their improved growth was due to loss of one copy of BCY1, the negative regulatory subunit of protein kinase A (PKA) from the left arm of chromosome 2. Furthermore, hyphal morphogenesis was improved in some of PR mutants by multigenic haploinsufficiency resulting from loss of large regions of the left arm of chromosome 2, including global transcriptional regulators. Interestingly, hyphal-associated genes were also induced in a manner that was independent of cAMP. This indicates that basal protein kinase A activity is an important prerequisite to induce hyphae, but activation of adenylyl cyclase is not needed. Instead, phosphoproteomic analysis indicated that the Cdc28 cyclin-dependent kinase and the casein kinase 1 family member Yck2 play key roles in promoting polarized growth. In addition, integrating transcriptomic and proteomic data reveals hyphal stimuli induce increased production of key transcription factors that contribute to polarized morphogenesis.

Candidalysin delivery to the invasion pocket is critical for host epithelial damage induced by Candida albicans

The human pathogenic fungus Candida albicans is a frequent cause of mucosal infections. Although the ability to transition from the yeast to the hypha morphology is essential for virulence, hypha formation and host cell invasion per se are not sufficient for the induction of epithelial damage. Rather, the hypha-associated peptide toxin, candidalysin, a product of the Ece1 polyprotein, is the critical damaging factor. While synthetic, exogenously added candidalysin is sufficient to damage epithelial cells, the level of damage does not reach the same level as invading C. albicans hyphae. Therefore, we hypothesized that a combination of fungal attributes is required to deliver candidalysin to the invasion pocket to enable the full damaging potential of C. albicans during infection. Utilising a panel of C. albicans mutants with known virulence defects, we demonstrate that the full damage potential of C. albicans requires the coordinated delivery of candidalysin to the invasion pocket. This process requires appropriate epithelial adhesion, hyphal extension and invasion, high levels of ECE1 transcription, proper Ece1 processing and secretion of candidalysin. To confirm candidalysin delivery, we generated camelid V_H Hs (nanobodies) specific for candidalysin and demonstrate localization and accumulation of the toxin only in C. albicans-induced invasion pockets. In summary, a defined combination of virulence attributes and cellular processes is critical for delivering candidalysin to the invasion pocket to enable the full damage potential of C. albicans during mucosal infection. TAKE AWAYS: Candidalysin is a peptide toxin secreted by C. albicans causing epithelial damage. Candidalysin delivery to host cell membranes requires specific fungal attributes. Candidalysin accumulates in invasion pockets created by invasive hyphae. Camelid nanobodies enabled visualisation of candidalysin in the invasion pocket.

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.

Deletion of the ATP2 Gene in Candida albicans Blocks Its Escape From Macrophage Clearance

Macrophages provide the first-line defense against invasive fungal infections and, therefore, escape from macrophage becomes the basis for the establishment of Candida albicans invasive infection. Here, we found that deletion of ATP2 (atp2Δ/Δ) in C. albicans resulted in a dramatic decrease from 69.2% (WT) to 1.2% in the escape rate in vitro. The effect of ATP2 on macrophage clearance stands out among the genes currently known to affect clearance. In the normal mice, the atp2Δ/Δ cells were undetectable in major organs 72 h after systemic infection, while WT cells persisted in vivo. However, in the macrophage-depleted mice, atp2Δ/Δ could persist for 72 h at an amount comparable to that at 24 h. Regarding the mechanism, WT cells sustained growth and switched to hyphal form, which was more conducive to escape from macrophages, in media that mimic the glucose-deficient environment in macrophages. In contrast, atp2Δ/Δ cells can remained viable but were unable to complete morphogenesis in these media, resulting in them being trapped within macrophages in the yeast form. Meanwhile, atp2Δ/Δ cells were killed by oxidative stress in alternative carbon sources by 2- to 3-fold more than WT cells. Taken together, ATP2 deletion prevents C. albicans from escaping macrophage clearance, and therefore ATP2 has a functional basis as a drug target that interferes with macrophage clearance.

Significant Differences in Host-Pathogen Interactions Between Murine and Human Whole Blood

Murine infection models are widely used to study systemic candidiasis caused by C. albicans. Whole-blood models can help to elucidate host-pathogens interactions and have been used for several Candida species in human blood. We adapted the human whole-blood model to murine blood. Unlike human blood, murine blood was unable to reduce fungal burden and more substantial filamentation of C. albicans was observed. This coincided with less fungal association with leukocytes, especially neutrophils. The lower neutrophil number in murine blood only partially explains insufficient infection and filamentation control, as spiking with murine neutrophils had only limited effects on fungal killing. Furthermore, increased fungal survival is not mediated by enhanced filamentation, as a filament-deficient mutant was likewise not eliminated. We also observed host-dependent differences for interaction of platelets with C. albicans, showing enhanced platelet aggregation, adhesion and activation in murine blood. For human blood, opsonization was shown to decrease platelet interaction suggesting that complement factors interfere with fungus-to-platelet binding. Our results reveal substantial differences between murine and human whole-blood models infected with C. albicans and thereby demonstrate limitations in the translatability of this ex vivo model between hosts.

Experimental Evolution of Candida by Serial Passaging in Host Cells

Experimental evolution is an experiment class of its own; instead of requiring an a priori hypothesis, the genetic adaptation of microbes to defined environments tells us about the underlying pathways and mechanisms. Such experiments are often deceptively simple in their design, based on a single abiotic stressor and what is in essence a long-term continuous culture. However, they generally provide a starting point to thorough follow-up analyses (which are specific for the organism at hand and not part of this method chapter). In this chapter, we describe a method to use a biotic stressor which is frequently encountered by pathogenic fungi-macrophage-like cells-in a serial passaging regime. Experimental evolution under such conditions can reveal new virulence attributes and mechanisms by selecting for adaptive mutations against the host cell-induced stress.It is important to note that every evolution experiment is different, and these techniques should be taken as a general guideline to be adapted to different organisms and questions. Then, it is a powerful tool with many potential applications in pathobiology research.

Candida Pathogenicity and Interplay with the Immune System

Candida species are opportunistic fungal pathogens that are part of the normal skin and mucosal microflora. Overgrowth of Candida can cause infections such as thrush or life-threatening invasive candidiasis in immunocompromised patients. Though Candida albicans is highly prevalent, several non-albicans species are also isolated from nosocomial infections. Candida sp. are over presented in the gut of people with Crohn's disease and certain types of neurological disorders, with hyphal form and biofilms being the most virulent states. In addition, Candida uses several secreted and cell surface molecules such as pH related antigen 1, High affinity glucose transporter, Phosphoglycerate mutase 1 and lipases to establish pathogenicity. A strong innate immune response is elicited against Candida via dendritic cells, neutrophils and macrophages. All three complement pathways are also activated. Production of proinflammatory cytokines IL-10 and IL-12 signal differentiation of CD4⁺ cells into Th1 and Th2 cells, whereas IL-6, IL-17 and IL-23 induce Th17 cells. Importance of T-lymphocytes is reflected in depleted T-cell count patients being more prone to Candidiasis. Anti- Candida antibodies also play a role against candidiasis using various mechanisms such as targeting virulent enzymes and exhibiting direct candidacidal activity. However, the significance of antibody response during infection remains controversial. Furthermore, some of the Candida strains have evolved molecular strategies to evade the sophisticated host attack by proteolysis of components of immune system and interfering with immune signalling pathways. Emergence of several non-albicans species that are resistant to current antifungal agents makes treatment more difficult. Therefore, deeper insight into interactions between Candida and the host immune system is required for discovery of novel therapeutic options.

Survival Strategies of Pathogenic Candida Species in Human Blood Show Independent and Specific Adaptations

To ensure their survival, pathogens have to adapt immediately to new environments in their hosts, for example, during the transition from the gut to the bloodstream. Here, we investigated the basis of this adaptation in a group of fungal species which are among the most common causes of hospital-acquired infections, the Candida species. On the basis of a human whole-blood infection model, we studied which genes and processes are active over the course of an infection in both the host and four different Candida pathogens. Remarkably, we found that, while the human host response during the early phase of infection is predominantly uniform, the pathogens pursue largely individual strategies and each one regulates genes involved in largely disparate processes in the blood. Our results reveal that C. albicans, C. glabrata, C. parapsilosis, and C. tropicalis all have developed individual strategies for survival in the host. This indicates that their pathogenicity in humans has evolved several times independently and that genes which are central for survival in the host for one species may be irrelevant in another.

Only four species, Candida albicans, C. glabrata, C. parapsilosis, and C. tropicalis, together account for about 90% of all Candida bloodstream infections and are among the most common causes of invasive fungal infections of humans. However, virulence potential varies among these species, and the phylogenetic tree reveals that their pathogenicity may have emerged several times independently during evolution. We therefore tested these four species in a human whole-blood infection model to determine, via comprehensive dual-species RNA-sequencing analyses, which fungal infection strategies are conserved and which are recent evolutionary developments. The ex vivo infection progressed from initial immune cell interactions to nearly complete killing of all fungal cells. During the course of infection, we characterized important parameters of pathogen-host interactions, such as fungal survival, types of interacting immune cells, and cytokine release. On the transcriptional level, we obtained a predominantly uniform and species-independent human response governed by a strong upregulation of proinflammatory processes, which was downregulated at later time points after most of the fungal cells were killed. In stark contrast, we observed that the different fungal species pursued predominantly individual strategies and showed significantly different global transcriptome patterns. Among other findings, our functional analyses revealed that the fungal species relied on different metabolic pathways and virulence factors to survive the host-imposed stress. These data show that adaptation of Candida species as a response to the host is not a phylogenetic trait, but rather has likely evolved independently as a prerequisite to cause human infections.

Lysosome Fusion Maintains Phagosome Integrity during Fungal Infection

Phagosomes must maintain membrane integrity to exert their microbicidal function. Some microorganisms, however, survive and grow within phagosomes. In such instances, phagosomes must expand to avoid rupture and microbial escape. We studied whether phagosomes regulate their size to preserve integrity during infection with the fungal pathogen Candida albicans. Phagosomes release calcium as C. albicans hyphae elongate, inducing lysosome recruitment and insertion, thereby increasing the phagosomal surface area. As hyphae grow, the expanding phagosome consumes the majority of free lysosomes. Simultaneously, lysosome biosynthesis is stimulated by activation of TFEB, a transcriptional regulator of lysosomal biogenesis. Preventing lysosomal insertion causes phagosomal rupture, NLRP3 inflammasome activation, IL-1β secretion and host-cell death. Whole-genome transcriptomic analysis demonstrate that stress responses elicited in C. albicans upon engulfment are reversed if phagosome expansion is prevented. Our findings reveal a mechanism whereby phagosomes maintain integrity while expanding, ensuring that growing pathogens remain entrapped within this microbicidal compartment.

Application of Probiotic Yeasts on Candida Species Associated Infection

Superficial and life-threatening invasive Candida infections are a major clinical challenge in hospitalized and immuno-compromised patients. Emerging drug-resistance among Candida species is exacerbated by the limited availability of antifungals and their associated side-effects. In the current review, we discuss the application of probiotic yeasts as a potential alternative/ combination therapy against Candida infections. Preclinical studies have identified several probiotic yeasts that effectively inhibit virulence of Candida species, including Candida albicans, Candida tropicalis, Candida glabrata, Candida parapsilosis, Candida krusei and Candida auris. However, Saccharomyces cerevisiae var. boulardii is the only probiotic yeast commercially available. In addition, clinical studies have further confirmed the in vitro and in vivo activity of the probiotic yeasts against Candida species. Probiotics use a variety of protective mechanisms, including posing a physical barrier, the ability to aggregate pathogens and render them avirulent. Secreted metabolites such as short-chain fatty acids effectively inhibit the adhesion and morphological transition of Candida species. Overall, the probiotic yeasts could be a promising effective alternative or combination therapy for Candida infections. Additional studies would bolster the application of probiotic yeasts.

Glutamate dehydrogenase (Gdh2)-dependent alkalization is dispensable for escape from macrophages and virulence of Candida albicans

Candida albicans cells depend on the energy derived from amino acid catabolism to induce and sustain hyphal growth inside phagosomes of engulfing macrophages. The concomitant deamination of amino acids is thought to neutralize the acidic microenvironment of phagosomes, a presumed requisite for survival and initiation of hyphal growth. Here, in contrast to an existing model, we show that mitochondrial-localized NAD⁺-dependent glutamate dehydrogenase (GDH2) catalyzing the deamination of glutamate to α-ketoglutarate, and not the cytosolic urea amidolyase (DUR1,2), accounts for the observed alkalization of media when amino acids are the sole sources of carbon and nitrogen. C. albicans strains lacking GDH2 (gdh2-/-) are viable and do not extrude ammonia on amino acid-based media. Environmental alkalization does not occur under conditions of high glucose (2%), a finding attributable to glucose-repression of GDH2 expression and mitochondrial function. Consistently, inhibition of oxidative phosphorylation or mitochondrial translation by antimycin A or chloramphenicol, respectively, prevents alkalization. GDH2 expression and mitochondrial function are derepressed as glucose levels are lowered from 2% (~110 mM) to 0.2% (~11 mM), or when glycerol is used as primary carbon source. Using time-lapse microscopy, we document that gdh2-/- cells survive, filament and escape from primary murine macrophages at rates indistinguishable from wildtype. In intact hosts, such as in fly and murine models of systemic candidiasis, gdh2-/- mutants are as virulent as wildtype. Thus, although Gdh2 has a critical role in central nitrogen metabolism, Gdh2-catalyzed deamination of glutamate is surprisingly dispensable for escape from macrophages and virulence. Consistently, using the pH-sensitive dye (pHrodo), we observed no significant difference between wildtype and gdh2-/- mutants in phagosomal pH modulation. Following engulfment of fungal cells, the phagosomal compartment is rapidly acidified and hyphal growth initiates and sustained under consistently acidic conditions within phagosomes. Together, our results demonstrate that amino acid-dependent alkalization is not essential for hyphal growth, survival in macrophages and hosts. An accurate understanding of the microenvironment within macrophage phagosomes and the metabolic events underlying the survival of phagocytized C. albicans cells and their escape are critical to understanding the host-pathogen interactions that ultimately determine the pathogenic outcome.

Candida albicans is a commensal component of the human microflora and the most common fungal pathogen. The incidence of candidiasis is low in healthy populations. Consequently, environmental factors, such as interactions with innate immune cells, play critical roles. Macrophages provide the first line of defense and rapidly internalize C. albicans cells within specialized intracellular compartments called phagosomes. The microenvironment within phagosomes is dynamic and ill defined, but has a low pH, and contains potent hydrolytic enzymes and oxidative stressors. Despite the inhospitable conditions, phagocytized C. albicans cells catabolize amino acids to obtain energy to survive and grow. Here, we have critically examined amino acid catabolism and ammonia extrusion in C. albicans, the latter is thought to neutralize the phagosomal pH and induce the switch of morphologies from round “yeast-like” to elongated hyphal cells that can pierce the phagosomal membrane leading to escape from macrophages. We report that Gdh2, which catalyzes the deamination of glutamate to α-ketoglutarate, is responsible for the observed environmental alkalization when C. albicans catabolize amino acids in vitro. However, the phagosomes formed as macrophages engulf wildtype or gdh2-/- cells rapidly become acidified, indicating that Gdh2 has no apparent role in modulating phagosomal pH. Strikingly, and similar to wildtype cells, gdh2-/- cells initiate and sustain hyphal growth enabling them to escape from macrophages. Also, Gdh2 is dispensable for virulent growth in systemic models of infection. These results provide new insights into host-pathogen interactions that determine the pathogenic outcome of C. albicans infections.

Synthetic Naphthofuranquinone Derivatives Are Effective in Eliminating Drug-Resistant Candida albicans in Hyphal, Biofilm, and Intracellular Forms: An Application for Skin-Infection Treatment

Candida albicans is the most common cause of fungal infection. The emergence of drug resistance leads to the need for novel antifungal agents. We aimed to design naphthofuranquinone analogs to treat drug-resistant C. albicans for topical application on cutaneous candidiasis. The time-killing response, agar diffusion, and live/dead assay of the antifungal activity were estimated against 5-fluorocytosine (5-FC)- or fluconazole-resistant strains. A total of 14 naphthofuranquinones were compared for their antifungal potency. The lead compounds with hydroxyimino (TCH-1140) or O-acetyl oxime (TCH-1142) moieties were the most active agents identified, showing a minimum inhibitory concentration (MIC) of 1.5 and 1.2 μM, respectively. Both compounds were superior to 5-FC and fluconazole for killing planktonic fungi. Naphthofuranquinones efficiently diminished the microbes inside and outside the biofilm. TCH-1140 and TCH-1142 were delivered into C. albicans-infected keratinocytes to eradicate intracellular fungi. The compounds did not reduce the C. albicans burden inside the macrophages, but the naphthofuranquinones promoted the transition of fungi from the virulent hypha form to the yeast form. In the in vivo skin mycosis mouse model, topically applied 5-FC and TCH-1140 reduced the C. albicans load from 1.5 × 10⁶ to 5.4 × 10⁵ and 1.4 × 10⁵ CFU, respectively. The infected abscess diameter was significantly decreased by TCH-1140 (3-4 mm) as compared to the control (8 mm). The disintegrated skin-barrier function induced by the fungi was recovered to the baseline by the compound. The data support the potential of TCH-1140 as a topical agent for treating drug-resistant C. albicans infection without causing skin irritation.

Molecular Elucidation of Riboflavin Production and Regulation in Candida albicans, toward a Novel Antifungal Drug Target

Candida albicans is an important fungal pathogen causing common superficial infections as well as invasive diseases with an extremely high morbidity and mortality. Antifungal therapies are limited in efficiency and availability. In this research, we describe the regulation of riboflavin production in C. albicans. Since riboflavin biosynthesis is essential to this organism, we can appreciate that targeting it would be a promising new strategy to combat these fungal infections. We provide evidence that one particular enzyme in the production process, CaRib1, would be most promising as an antifungal drug target, as it plays a central role in regulation and proves to be essential in a mouse model of systemic infection.

Candida albicans is a major cause of fungal infections, both superficial and invasive. The economic costs as well as consequences for patient welfare are substantial. Only a few treatment options are available due to the high resemblance between fungal targets and host molecules, as both are eukaryotes. Riboflavin is a yellow pigment, also termed vitamin B₂. Unlike animals, fungi can synthesize this essential component themselves, thereby leading us to appreciate that targeting riboflavin production is a promising novel strategy against fungal infections. Here, we report that the GTP cyclohydrolase encoded by C. albicansRIB1 (CaRIB1) is essential and rate-limiting for production of riboflavin in the fungal pathogen. We confirm the high potential of CaRib1 as an antifungal drug target, as its deletion completely impairs in vivo infectibility by C. albicans in model systems. Furthermore, the stimulating effect of iron deprivation and PKA activation on riboflavin production seems to involve CaRib1 and the upstream transcription factor CaSef1. Gathering insights in the synthesis mechanism of riboflavin in pathogenic fungi, like C. albicans, will allow us to design a novel strategy and specifically target this process to combat fungal infections.

IMPORTANCECandida albicans is an important fungal pathogen causing common superficial infections as well as invasive diseases with an extremely high morbidity and mortality. Antifungal therapies are limited in efficiency and availability. In this research, we describe the regulation of riboflavin production in C. albicans. Since riboflavin biosynthesis is essential to this organism, we can appreciate that targeting it would be a promising new strategy to combat these fungal infections. We provide evidence that one particular enzyme in the production process, CaRib1, would be most promising as an antifungal drug target, as it plays a central role in regulation and proves to be essential in a mouse model of systemic infection.

The Dual Function of the Fungal Toxin Candidalysin during Candida albicans-Macrophage Interaction and Virulence

The dimorphic fungus Candida albicans is both a harmless commensal organism on mucosal surfaces and an opportunistic pathogen. Under certain predisposing conditions, the fungus can overgrow the mucosal microbiome and cause both superficial and life-threatening systemic infections after gaining access to the bloodstream. As the first line of defense of the innate immune response, infecting C. albicans cells face macrophages, which mediate the clearance of invading fungi by intracellular killing. However, the fungus has evolved sophisticated strategies to counteract macrophage antimicrobial activities and thus evade immune surveillance. The cytolytic peptide toxin, candidalysin, contributes to this fungal defense machinery by damaging immune cell membranes, providing an escape route from the hostile phagosome environment. Nevertheless, candidalysin also induces NLRP3 inflammasome activation, leading to an increased host-protective pro-inflammatory response in mononuclear phagocytes. Therefore, candidalysin facilitates immune evasion by acting as a classical virulence factor but also contributes to an antifungal immune response, serving as an avirulence factor. In this review, we discuss the role of candidalysin during C. albicans infections, focusing on its implications during C. albicans-macrophage interactions.

Candida albicans necrotizing fasciitis following elective surgery

Necrotizing fasciitis is a potentially fatal soft tissue infection that requires prompt clinical suspicion, pharmacological and surgical interventions. Bacterial pathogens, such as beta-hemolytic streptococcus and Staphylococcus aureus, are the main etiology of necrotizing fasciitis, however, rare cases caused by fungal pathogens, such as Candida albicans, have been reported following trauma. Here, we present the first case of C. albicans necrotizing fasciitis following an elective surgical procedure in an immunocompetent adult.

Looking for New Antifungal Drugs from Flavonoids: Impact of the Genetic Diversity of Candida albicans on the in-vitro Response

BACKGROUND

In an era in which antimicrobial resistance is increasing at an alarming pace, it is very important to find new antimicrobial agents effective against pathogenic microrganisms resistant to traditional treatments. Among the notable breakthroughs in the past years of research in natural-drug discovery, there is the identification and testing of flavonoids, a group of plant-derived substances capable of promoting many beneficial effects on humans. These compounds show different biological activities such as inhibition of neuroinflammation and tumor growth as well as antimicrobial activity against many microbial pathogens.

METHODS

We undertook a review of protocols and standard strains used in studies reporting the inhibitory effects of flavonoids against Candida albicans by focusing our attention on genetic characterization of the strains examined. Moreover, using the C. albicans MLST-database, we performed a phylogenetic analysis showing the genetic variation occurring in this species.

RESULTS

Today, we have enough information to estimate genetic diversity within microbial species and recent data revealed that most of fungal pathogens show complex population structures in which not a single isolate can be designated as representative of the entire taxon. This is especially true for the highly divergent fungal pathogen C. albicans, in which the assumption that one or few "standard strains" can represent the whole species is overly unrealistic and should be laid to rest.

CONCLUSION

The goal of this article is to shed light on the extent of genetic variation in C. albicans and how this phenomenon can largely influence the activity of flavonoids against this species.

Identification and Functional Analysis of the Pheromone Response Factor Gene of Sporisorium scitamineum

The sugarcane smut fungus Sporisorium scitamineum is bipolar and produces sporidia of two different mating types. During infection, haploid cells of opposite mating types can fuse to form dikaryotic hyphae that can colonize plant tissue. Mating and filamentation are therefore essential for S. scitamineum pathogenesis. In this study, we obtained one T-DNA insertion mutant disrupted in the gene encoding the pheromone response factor (Prf1), hereinafter named SsPRF1, of S. scitamineum, via Agrobacterium tumefaciens-mediated transformation (ATMT) mutagenesis. Targeted deletion of SsPRF1 resulted in mutants with phenotypes similar to the T-DNA insertion mutant, including failure to mate with a compatible wild-type partner strain and being non-pathogenic on its host sugarcane. qRT-PCR analyses showed that SsPRF1 was essential for the transcription of pheromone-responsive mating type genes of the a1 locus. These results show that SsPRF1 is involved in mating and pathogenicity and plays a key role in pheromone signaling and filamentous growth in S. scitamineum.

Serum FHR1 binding to necrotic-type cells activates monocytic inflammasome and marks necrotic sites in vasculopathies

Persistent inflammation is a hallmark of many human diseases, including anti-neutrophil cytoplasmic antibody-associated vasculitis (AAV) and atherosclerosis. Here, we describe a dominant trigger of inflammation: human serum factor H-related protein FHR1. In vitro, this protein selectively binds to necrotic cells via its N-terminus; in addition, it binds near necrotic glomerular sites of AAV patients and necrotic areas in atherosclerotic plaques. FHR1, but not factor H, FHR2 or FHR3 strongly induces inflammasome NLRP3 in blood-derived human monocytes, which subsequently secrete IL-1β, TNFα, IL-18 and IL-6. FHR1 triggers the phospholipase C-pathway via the G-protein coupled receptor EMR2 independent of complement. Moreover, FHR1 concentrations of AAV patients negatively correlate with glomerular filtration rates and associate with the levels of inflammation and progressive disease. These data highlight an unexpected role for FHR1 during sterile inflammation, may explain why FHR1-deficiency protects against certain diseases, and identifies potential targets for treatment of auto-inflammatory diseases.

Recent advances in understanding Candida albicans hyphal growth

Morphological changes are critical for the virulence of a range of plant and human fungal pathogens. Candida albicans is a major human fungal pathogen whose ability to switch between different morphological states is associated with its adaptability and pathogenicity. In particular, C. albicans can switch from an oval yeast form to a filamentous hyphal form, which is characteristic of filamentous fungi. What mechanisms underlie hyphal growth and how are they affected by environmental stimuli from the host or resident microbiota? These questions are the focus of intensive research, as understanding C. albicans hyphal growth has broad implications for cell biological and medical research.

Dual-species biofilms of Streptococcus mutans and Candida albicans exhibit more biomass and are mutually beneficial compared with single-species biofilms

Background: Streptococcus mutans (Sm) and Candida albicans (Ca) are found in biofilms of early childhood caries. Objective: To characterize in vitro dual- and single-species biofilms of Sm and Ca formed on saliva-coated hydroxyapatite discs in the presence of sucrose. Design: Evaluation of biofilms included biochemical [biomass, proteins, matrix’s water-soluble (WSP) and alkali-soluble (ASP) polysaccharides, microbiological, 3D structure, gene expression, and stress tolerance analyses. Results: Biomass and proteins were higher for dual-species and lower for Ca (p = 0.001). Comparison of Sm single- and dual-species biofilms revealed no significant difference in Sm numbers or quantity of WSP (p > 0.05). Dual-species biofilms contained a higher population of Ca (p < 0.001). The quantity of ASP was higher in dual-species biofilms (vs Ca single-species biofilms; p = 0.002). The 3D structure showed larger microcolonies and distinct distribution of Sm-derived exopolysaccharides in dual-species biofilms. Compared with dual-species biofilms, expression of gtfB (ASP) and nox1 (oxidative stress) was higher for single-species of Sm whilst expression of BGL2 (matrix), PHR1 (matrix, acid tolerance) and SOD1 (oxidative stress) was higher in single-species of Ca. There was no difference for acid tolerance genes (Sm atpD and Ca PHR2), which was confirmed by acid tolerance challenge. Dual-species biofilms were more tolerant to oxidative and antimicrobial stresses (p < 0.05). Conclusions: Dual-species biofilms present greater 3D complexity, thereby, making them more resistant to stress conditions.

Microevolution of the pathogenic yeasts Candida albicans and Candida glabrata during antifungal therapy and host infection

Infections by the pathogenic yeasts Candida albicans and Candida glabrata are among the most common fungal diseases. The success of these species as human pathogens is contingent on their ability to resist antifungal therapy and thrive within the human host. C. glabrata is especially resilient to azole antifungal treatment, while C. albicans is best known for its wide array of virulence features. The core mechanisms that underlie antifungal resistance and virulence in these pathogens has been continuously addressed, but the investigation on how such mechanisms evolve according to each environment is scarcer. This review aims to explore current knowledge on micro-evolution experiments to several treatment and host-associated conditions in C. albicans and C. glabrata. The analysis of adaptation strategies that evolve over time will allow to better understand the mechanisms by which Candida species are able to achieve stable phenotypes in real-life scenarios, which are the ones that should constitute the most interesting drug targets.

CO2 Signaling through the Ptc2-Ssn3 Axis Governs Sustained Hyphal Development of Candida albicans by Reducing Ume6 Phosphorylation and Degradation

Candida albicans is the most common cause of invasive fungal infections in humans. Its ability to sense and adapt to changing carbon dioxide levels is crucial for its pathogenesis. Carbon dioxide promotes hyphal development. The hypha-specific transcription factor Ume6 is rapidly degraded in air, but is stable under physiological CO₂ and hypoxia to sustain hyphal elongation. Here, we show that Ume6 stability is regulated by two parallel E3 ubiquitin ligases, SCF^Grr1 and Ubr1, in response to CO₂ and O₂, respectively. To uncover the CO₂ signaling pathway that regulates Ume6 stability, we performed genetic screens for mutants unable to respond to CO₂ for sustained filamentation. We find that the type 2C protein phosphatase Ptc2 is specifically required for CO₂-induced stabilization of Ume6 and hyphal elongation. In contrast, the cyclin-dependent kinase Ssn3 is found to be required for Ume6 phosphorylation and degradation in atmospheric CO₂ Furthermore, we find that Ssn3 is dephosphorylated in 5% CO₂ in a Ptc2-dependent manner, whereas deletion of PTC2 has no effect on Ssn3 phosphorylation in air. Our study uncovers the Ptc2-Ssn3 axis as a new CO₂ signaling pathway that controls hyphal elongation by regulating Ume6 stability in C. albicans IMPORTANCE The capacity to sense and adapt to changing carbon dioxide levels is crucial for all organisms. In fungi, CO₂ is a key determinant involved in fundamental biological processes, including growth, morphology, and virulence. In the pathogenic fungus Candida albicans, high CO₂ is directly sensed by adenylyl cyclase to promote hyphal growth. However, little is known about the mechanism by which hyphal development is maintained in response to physiological levels of CO₂ Here we report that a signal transduction system mediated by a phosphatase-kinase pair controls CO₂-responsive Ume6 phosphorylation and stability that in turn dictate hyphal elongation. Our results unravel a new regulatory mechanism of CO₂ signaling in fungi.

Functional divergence of a global regulatory complex governing fungal filamentation

Morphogenetic transitions are prevalent in the fungal kingdom. For a leading human fungal pathogen, Candida albicans, the capacity to transition between yeast and filaments is key for virulence. For the model yeast Saccharomyces cerevisiae, filamentation enables nutrient acquisition. A recent functional genomic screen in S. cerevisiae identified Mfg1 as a regulator of morphogenesis that acts in complex with Flo8 and Mss11 to mediate transcriptional responses crucial for filamentation. In C. albicans, Mfg1 also interacts physically with Flo8 and Mss11 and is critical for filamentation in response to diverse cues, but the mechanisms through which it regulates morphogenesis remained elusive. Here, we explored the consequences of perturbation of Mfg1, Flo8, and Mss11 on C. albicans morphogenesis, and identified functional divergence of complex members. We observed that C. albicans Mss11 was dispensable for filamentation, and that overexpression of FLO8 caused constitutive filamentation even in the absence of Mfg1. Harnessing transcriptional profiling and chromatin immunoprecipitation coupled to microarray analysis, we identified divergence between transcriptional targets of Flo8 and Mfg1 in C. albicans. We also established that Flo8 and Mfg1 cooperatively bind to promoters of key regulators of filamentation, including TEC1, for which overexpression was sufficient to restore filamentation in the absence of Flo8 or Mfg1. To further explore the circuitry through which Mfg1 regulates morphogenesis, we employed a novel strategy to select for mutations that restore filamentation in the absence of Mfg1. Whole genome sequencing of filamentation-competent mutants revealed chromosome 6 amplification as a conserved adaptive mechanism. A key determinant of the chromosome 6 amplification is FLO8, as deletion of one allele blocked morphogenesis, and chromosome 6 was not amplified in evolved lineages for which FLO8 was re-located to a different chromosome. Thus, this work highlights rewiring of key morphogenetic regulators over evolutionary time and aneuploidy as an adaptive mechanism driving fungal morphogenesis.

Fungal infections pose a severe burden to human health worldwide. Candida albicans is a leading cause of systemic fungal infections, with mortality rates approaching 40%. One of the key virulence traits of this fungus is its ability to transition between yeast and filamentous forms in response to diverse host-relevant cues. The model yeast Saccharomyces cerevisiae is also capable of filamentous growth in certain conditions, and previous work has identified a key transcriptional complex required for filamentation in both species. However, here we discover that the circuitry governed by this complex in C. albicans is largely distinct from that in the non-pathogenic S. cerevisiae. We also employ a novel selection strategy to perform experimental evolution, identifying chromosome triplication as a mechanism to restore filamentation in a non-filamentous mutant. This work reveals unique circuitry governing a key virulence trait in a leading fungal pathogen, identifying potential therapeutic targets to combat these life-threatening infections.

Experimental evolution of a fungal pathogen into a gut symbiont

Gut microbes live in symbiosis with their hosts, but how mutualistic animal-microbe interactions emerge is not understood. By adaptively evolving the opportunistic fungal pathogenCandida albicansin the mouse gastrointestinal tract, we selected strains that not only had lost their main virulence program but also protected their new hosts against a variety of systemic infections. This protection was independent of adaptive immunity, arose as early as a single day postpriming, was dependent on increased innate cytokine responses, and was thus reminiscent of “trained immunity.” Because both the microbe and its new host gain some advantages from their interaction, this experimental system might allow direct study of the evolutionary forces that govern the emergence of mutualism between a mammal and a fungus.

The fungal peptide toxin Candidalysin activates the NLRP3 inflammasome and causes cytolysis in mononuclear phagocytes

Clearance of invading microbes requires phagocytes of the innate immune system. However, successful pathogens have evolved sophisticated strategies to evade immune killing. The opportunistic human fungal pathogen Candida albicans is efficiently phagocytosed by macrophages, but causes inflammasome activation, host cytolysis, and escapes after hypha formation. Previous studies suggest that macrophage lysis by C. albicans results from early inflammasome-dependent cell death (pyroptosis), late damage due to glucose depletion and membrane piercing by growing hyphae. Here we show that Candidalysin, a cytolytic peptide toxin encoded by the hypha-associated gene ECE1, is both a central trigger for NLRP3 inflammasome-dependent caspase-1 activation via potassium efflux and a key driver of inflammasome-independent cytolysis of macrophages and dendritic cells upon infection with C. albicans. This suggests that Candidalysin-induced cell damage is a third mechanism of C. albicans-mediated mononuclear phagocyte cell death in addition to damage caused by pyroptosis and the growth of glucose-consuming hyphae.

Yeast and Filaments Have Specialized, Independent Activities in a Zebrafish Model of Candida albicans Infection

Candida albicans dimorphism is a crucial virulence factor during invasive candidiasis infections, which claim the lives of nearly one-half of those afflicted. It has long been believed that filaments drive tissue invasion and yeast mediates bloodstream dissemination, but observation of these activities during infection has been prevented by technical limitations. We used a transparent zebrafish infection model to analyze more comprehensively how C. albicans utilizes shape to disseminate and invade. This model facilitated the use of diverse, complementary strategies to manipulate shape, allowing us to monitor dissemination, invasion, and pathogenesis via intravital imaging of individual fungal cells throughout the host. To control fungal cell shape, we employed three different strategies: gene deletion (efg1Δ/Δ cph1Δ/Δ, eed1Δ/Δ), overexpression of master regulators (NRG1 or UME6), and modulation of the infection temperature (21°C, 28°C, or 33°C). The effects of these orthogonal manipulations were consistent, support the proposed specialized roles of yeast in dissemination and filaments in tissue invasion and pathogenesis, and indicate conserved mechanisms in zebrafish. To test if either morphotype changes the effectiveness of the other, we infected fish with a known mixture of shape-locked strains. Surprisingly, mixed-strain infections were associated with additive, but not synergistic, filament invasion and yeast dissemination. These findings provide the most complete view of morphotype-function relationships for C. albicans to date, revealing independent roles of yeast and filaments during disseminated candidiasis.

Evidence for Mitochondrial Genome Methylation in the Yeast Candida albicans: A Potential Novel Epigenetic Mechanism Affecting Adaptation and Pathogenicity?

The commensal yeast Candida albicans is an opportunistic pathogen. In order to successfully colonize or infect the human body, the fungus must adapt to the host’s environmental conditions, such as low oxygen tension (hypoxia), temperature (37°C), and the different carbon sources available. Previous studies demonstrated the adaptive importance of C. albicans genetic variability for its pathogenicity, although the contributions of epigenetic and the influence of environmental factors are not fully understood. Mitochondria play important roles in fungal energetic metabolism, regulation of nuclear epigenetic mechanisms and pathogenicity. However, the specific impact of inter-strain mitochondrial genome variability and mitochondrial epigenetics in pathogenicity is unclear. Here, we draw attention to this relevant organelle and its potential role in C. albicans pathogenicity and provide preliminary evidence, for the first time, for methylation of the yeast mitochondrial genome. Our results indicate that environmental conditions, such as continuous exposure for 12 weeks to hypoxia and 37°C, decrease the mitochondrial genome methylation in strains SC5314 and L757. However, the methylation decrease is quantitatively different in specific genome positions when strains SC5314 and L757 are compared. We hypothesize that this phenomenon can be promising for future research to understand how physical factors of the host affect the C. albicans mitochondrial genome and its possible impact on adaptation and pathogenicity.

Genome-Wide Screen for Saccharomyces cerevisiae Genes Contributing to Opportunistic Pathogenicity in an Invertebrate Model Host

Environmental opportunistic pathogens can exploit vulnerable hosts through expression of traits selected for in their natural environments. Pathogenicity is itself a complicated trait underpinned by multiple complex traits, such as thermotolerance, morphology, and stress response. The baker's yeast, Saccharomyces cerevisiae, is a species with broad environmental tolerance that has been increasingly reported as an opportunistic pathogen of humans. Here we leveraged the genetic resources available in yeast and a model insect species, the greater waxmoth Galleria mellonella, to provide a genome-wide analysis of pathogenicity factors. Using serial passaging experiments of genetically marked wild-type strains, a hybrid strain was identified as the most fit genotype across all replicates. To dissect the genetic basis for pathogenicity in the hybrid isolate, bulk segregant analysis was performed which revealed eight quantitative trait loci significantly differing between the two bulks with alleles from both parents contributing to pathogenicity. A second passaging experiment with a library of deletion mutants for most yeast genes identified a large number of mutations whose relative fitness differed in vivovs.in vitro, including mutations in genes controlling cell wall integrity, mitochondrial function, and tyrosine metabolism. Yeast is presumably subjected to a massive assault by the innate insect immune system that leads to melanization of the host and to a large bottleneck in yeast population size. Our data support that resistance to the innate immune response of the insect is key to survival in the host and identifies shared genetic mechanisms between S. cerevisiae and other opportunistic fungal pathogens.

When green and red mycology meet: Impressions from an interdisciplinary forum on virulence mechanisms of phyto- and human-pathogenic fungi

Fungal infections pose a constant threat to plants and humans, but detailed knowledge about pathogenesis, immunity, or virulence is rather scarce. Due to the fact that a certain overlap in the armoury of infection exists between plant- and human-pathogenic fungi, an interdisciplinary forum was held in October 2016 at the Institute for Clinical Microbiology, Immunology and Hygiene in Erlangen under the organisational umbrella from two special interest groups of German microbial societies. Scientific exchange and intense discussion of this timely topic was fostered by bringing together renowned experts in their respective fields to present their thoughts and recent findings in the course of a plenary lecture and six themed sessions, accompanied by oral and poster contributions of young researchers. By targeting the topic of fungal virulence mechanisms from various angles and in the context of plant and human hosts, some common grounds and exciting perspectives could be deduced during this vibrant scientific event.

The intraspecies diversity of C. albicans triggers qualitatively and temporally distinct host responses that determine the balance between commensalism and pathogenicity

The host immune status is critical for preventing opportunistic infections with Candida albicans. Whether the natural fungal diversity that exists between C. albicans isolates also influences disease development remains unclear. Here, we used an experimental model of oral infection to probe the host response to diverse C. albicans isolates in vivo and found dramatic differences in their ability to persist in the oral mucosa, which inversely correlated with the degree and kinetics of immune activation in the host. Strikingly, the requirement of interleukin (IL)-17 signaling for fungal control was conserved between isolates, including isolates with delayed induction of IL-17. This underscores the relevance of IL-17 immunity in mucosal defense against C. albicans. In contrast, the accumulation of neutrophils and induction of inflammation in the infected tissue was strictly strain dependent. The dichotomy of the inflammatory neutrophil response was linked to the capacity of fungal strains to cause cellular damage and release of alarmins from the epithelium. The epithelium thus translates differences in the fungus into qualitatively distinct host responses. Altogether, this study provides a comprehensive understanding of the antifungal response in the oral mucosa and demonstrates the relevance of evaluating intraspecies differences for the outcome of fungal-host interactions in vivo.