Anin vitroassay to study the transcriptional response during adherence ofCandida albicansto different human epithelia

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.

Host Cell Geometry and Cytoskeletal Organization Governs Candida-Host Cell Interactions at the Nanoscale

Candida is one of the most common opportunistic fungal pathogens in humans. Its adhesion to the host cell is required in parasitic states and is important for pathogenesis. Many studies have shown that there is an increased risk of developing candidiasis when normal tissue barriers are weakened or when immune defenses are compromised, for example, during cancer treatment that induces immunosuppression. The mechanical properties of malignant cells, such as adhesiveness and viscoelasticity, which contribute to cellular invasion and migration are different from those of noncancerous cells. To understand host invasion and its relationship with host cell health, we probed the interaction of Candida spp. with cancerous and noncancerous human cell lines using atomic force microscopy in the single-cell force spectroscopy mode. There was significant adhesion between Candida and human cells, with more adhesion to cancerous versus noncancerous cell lines. This increase in adhesion is related to the mechanobiological properties of cancer cells, which have a disorganized cytoskeleton and lower rigidity. Altered geometry and cytoskeletal disruption of the human cells impacted adhesion parameters, underscoring the role of cytoskeletal organization in Candida-human cell adhesion and implicating the manipulation of cell properties as a potential future therapeutic strategy.

Different expression levels of ALS and SAP genes contribute to recurrent vulvovaginal candidiasis by Candida albicans

Aim: To study the behavior of Candida albicans in women with vulvovaginal candidiasis (VVC), recurrent VVC (RVVC) and asymptomatic (AS), regarding adhesion on HeLa cells and their ability to express secreted aspartic proteinases (SAP) genes, agglutinin-like sequence (ALS) genes and HWP1. Materials & methods: The adhesion of Candida albicans to HeLa cells was evaluated by colony-forming units, and the expressed genes were evaluated by qRT-PCR. Results: AS and VVC isolates showed greater ability to adhere HeLa cells when compared with RVVC isolate. Nevertheless, RVVC isolate exhibited upregulation of a large number of genes of ALS and SAP gene families and HWP1 gene. Conclusion: The results demonstrated that RVVC isolate expressed significantly important genes for invasion and yeast-host interactions.

Changes in Adhesion of Candida tropicalis Clinical Isolates Exhibiting Switch Phenotypes to Polystyrene and HeLa Cells

BACKGROUND

Candida tropicalis is an important human pathogen that can undergo multiple forms of phenotypic switching.

AIM

We aimed to evaluate the effect of phenotypic switching on the adhesion ability of C. tropicalis.

METHODS

C. tropicalis morphotypes included parental phenotypes (clinical isolates) and switch phenotypes (crepe, revertant of crepe-CR, rough, revertant of rough-RR, irregular center and revertant of irregular center-ICR). Adhesion to polystyrene and HeLa cells was determined by crystal violet assay. The percentage of HeLa cells with adhered yeasts and the number of adhered yeasts per HeLa cell were determined by light microscopy. Filamentation among adhered cells was assessed by direct counting.

RESULTS

On polystyrene, 60% of the switch strains showed difference (p < 0.05) on adhesion ability compared to their parental counterpart strains, and altered thickness of adhered cells layers. Filamentation was increased among adhered cells of the switched strains compared to parental strains. A positive correlation was observed between adhesion on polystyrene and filamentation for morphotypes of the system 49.07. The majority of the switched strains showed higher adhesion capability to HeLa cells in comparison to the adherence of the clinical strains. All revertant strains showed a higher number of yeast cells per HeLa cell compared to their variant counterparts (p < 0.05), with exception of the ICR.

CONCLUSIONS

Our findings indicate that switching events in C. tropicalis affect adhesion and filamentation of adhered cells on polystyrene and HeLa cells. The rise of switch strains with increased adhesion ability may contribute to the success of infection associated with C. tropicalis.

Adhesion of Candida tropicalis to polystyrene and epithelial cell lines: Insights of correlation of the extent of adherent yeast cells among distinct surfaces

Candida tropicalis is an emerging fungal pathogen associated with high mortality. We aimed to compare adherence capability of C. tropicalis to polystyrene and epithelial cell lines (HeLa and Vero), and determine whether adherent blastoconidia is cell-type specific. Blastoconidia adhesion to epithelial cells and polystyrene were determined by crystal violet assay. The percentage of epithelial cells with adhered blastoconidia and the number of adhered blastoconidia per cell line were determined by light microscopy. The correlation between adhesion surfaces was assessed by Pearson's correlation coefficient. The adhesiveness of C. tropicalis to polystyrene was greater than that observed for ephitelial cells. High correlation values (r² 0.9999222, p 0.007941) were found for the adhesion capability between biotic and polystyrene surface for isolates 100.10 (obtained from blood) and 335.07 (obtained from tracheal secretion). The number of adherent blastoconidia per HeLa cell was greater in comparison to that observed for Vero cells (P<0.05). Further, high correlation (r² 1, p 0.0001) was found for the adhesion ability between HeLa cells and Vero cells. The results suggest a correlation of C. tropicalis adhesion capability among different surfaces, and that the adhesion to epithelial cells is specific to the cell type.

"Candida Albicans Interactions With The Host: Crossing The Intestinal Epithelial Barrier"

Formerly a commensal organism of the mucosal surfaces of most healthy individuals, Candida albicans is an opportunistic pathogen that causes infections ranging from superficial to the more life-threatening disseminated infections, especially in the ever-growing population of vulnerable patients in the hospital setting. In these situations, the fungus takes advantage of its host following a disturbance in the host defense system and/or the mucosal microbiota. Overwhelming evidence suggests that the gastrointestinal tract is the main source of disseminated C. albicans infections. Major risk factors for disseminated candidiasis include damage to the mucosal intestinal barrier, immune dysfunction, and dysbiosis of the resident microbiota. A better understanding of C. albicans' interaction with the intestinal epithelial barrier will be useful for designing future therapies to avoid systemic candidiasis. In this review, we provide an overview of the current knowledge regarding the mechanisms of pathogenicity that allow the fungus to reach and translocate the gut barrier.

Anti-biofilm activity of antibody directed against surface antigen complement receptor 3-related protein-comparison of Candida albicans and Candida dubliniensis

Candida albicans and C. dubliniensis are related yeasts that differ in the expression of virulence-associated proteins involved in adherence and biofilm development. CR3-RP (complement receptor 3-related protein) is one of the surface antigens expressed by Candida species. The main objective of this research was to elucidate the effect of the polyclonal anti-CR3-RP antibody (Ab) on adherence and the biofilm formed by C. albicans SC5314 and C. dubliniensis CBS 7987 and two clinical isolates in vitro, ex vivo and in vivo. A comparison of species, and of treated vs. non-treated with the anti-CR3-RP Ab showed a reduction in adherence (22%-41%) that was dependent on the time point of evaluation (60, 90 or 120 min), but did not prove to be species-dependent. Confocal microscopy revealed a decreased thickness in biofilms formed by both species after pre-treatment with the anti-CR3-RP Ab. This observation was confirmed ex vivo by immunohistochemistry analysis of biofilms formed on mouse tongues. Moreover, anti-CR3-RP Ab administration, 1 h post-infection, has been shown to promote larval survival compared to the control group in a Galleria mellonella infection model. Our data suggest a potential activity of the anti-CR3-RP Ab relevant to immunotherapy or vaccine development against biofilm-associated Candida infections.

Microarray Technologies in Fungal Diagnostics

Microarray technologies have been a major research tool in the last decades. In addition they have been introduced into several fields of diagnostics including diagnostics of infectious diseases. Microarrays are highly parallelized assay systems that initially were developed for multiparametric nucleic acid detection. From there on they rapidly developed towards a tool for the detection of all kind of biological compounds (DNA, RNA, proteins, cells, nucleic acids, carbohydrates, etc.) or their modifications (methylation, phosphorylation, etc.). The combination of closed-tube systems and lab on chip devices with microarrays further enabled a higher automation degree with a reduced contamination risk. Microarray-based diagnostic applications currently complement and may in the future replace classical methods in clinical microbiology like blood cultures, resistance determination, microscopic and metabolic analyses as well as biochemical or immunohistochemical assays. In addition, novel diagnostic markers appear, like noncoding RNAs and miRNAs providing additional room for novel nucleic acid based biomarkers. Here I focus an microarray technologies in diagnostics and as research tools, based on nucleic acid-based arrays.

An Update on Candida tropicalis Based on Basic and Clinical Approaches

Candida tropicalis has emerged as one of the most important Candida species. It has been widely considered the second most virulent Candida species, only preceded by C. albicans. Besides, this species has been recognized as a very strong biofilm producer, surpassing C. albicans in most of the studies. In addition, it produces a wide range of other virulence factors, including: adhesion to buccal epithelial and endothelial cells; the secretion of lytic enzymes, such as proteinases, phospholipases, and hemolysins, bud-to-hyphae transition (also called morphogenesis) and the phenomenon called phenotypic switching. This is a species very closely related to C. albicans and has been easily identified with both phenotypic and molecular methods. In addition, no cryptic sibling species were yet described in the literature, what is contradictory to some other medically important Candida species. C. tropicalis is a clinically relevant species and may be the second or third etiological agent of candidemia, specifically in Latin American countries and Asia. Antifungal resistance to the azoles, polyenes, and echinocandins has already been described. Apart from all these characteristics, C. tropicalis has been considered an osmotolerant microorganism and this ability to survive to high salt concentration may be important for fungal persistence in saline environments. This physiological characteristic makes this species suitable for use in biotechnology processes. Here we describe an update of C. tropicalis, focusing on all these previously mentioned subjects.

Interaction of Candida Species with the Skin

The human skin is commonly colonized by diverse fungal species. Some Candida species, especially C. albicans, do not only reside on the skin surface as commensals, but also cause infections by growing into the colonized tissue. However, defense mechanisms at the skin barrier level are very efficient, involving residential non-immune and immune cells as well as immune cells specifically recruited to the site of infection. Therefore, the skin is an effective barrier against fungal infection. While most studies about commensal and pathogenic interaction of Candida species with host epithelia focus on the interaction with mucosal surfaces such as the vaginal and gastrointestinal epithelia, less is known about the mechanisms underlying Candida interaction with the skin. In this review, we focus on the ecology and molecular pathogenesis of Candida species on the skin and give an overview of defense mechanisms against C. albicans in this context. We also discuss new research avenues in dermal infection, including the involvement of neurons, fibroblasts, and commensal bacteria in both mouse and human model systems.

Decreased expression of 14-3-3 in Paracoccidioides brasiliensis confirms its involvement in fungal pathogenesis

The interaction between the fungal pathogen Paracoccidioides brasiliensis and host cells is usually mediated by specific binding events between adhesins on the fungal surface and receptors on the host extracellular matrix or cell surface. One molecule implicated in the P. brasiliensis-host interaction is the 14-3-3 protein. The 14-3-3 protein belongs to a family of conserved regulatory molecules that are expressed in all eukaryotic cells and are involved in diverse cellular functions. Here, we investigated the relevance of the 14-3-3 protein to the virulence of P. brasiliensis. Using antisense RNA technology and Agrobacterium tumefaciens-mediated transformation, we generated a 14-3-3-silenced strain (expression reduced by ˜55%). This strain allowed us to investigate the interaction between 14-3-3 and the host and to correlate the functions of P. brasiliensis 14-3-3 with cellular features, such as morphological characteristics and virulence, that are important for pathogenesis.

Adhesion and biofilm formation in artificial saliva and susceptibility of yeasts isolated from chronic kidney patients undergoing haemodialysis

Yeasts of the genera Candida and Saccharomyces are opportunist pathogens and cause oral lesions, especially in immunocompromised patients. This study assessed yeasts isolated from chronic kidney patients undergoing haemodialysis for their adhesion capacity, biofilm formation and susceptibility to antifungal agents. Ten isolates of Candida spp. and one isolate of Saccharomyces cerevisiae were tested for adhesion to buccal epithelial cells (BECs), adhesion and formation of biofilm in artificial saliva and their susceptibility profile to antifungal agents. Adhesion and biofilm formation were undertaken in polystyrene plates with artificial saliva, whilst susceptibility to antifungal agents was evaluated by broth microdilution. Candida parapsilosis had the highest adhesion index in BECs (154.55 ± 22.13) and Candida rugosa was the species with the highest adhesion capacity (18 398  Abs cm(-2)) in abiotic surface with artificial saliva. Candida albicans provided the greatest biofilm formation (2035  Abs cm(-2) ± 0.09) but was revealed to be susceptible to the five antifungal agents under analysis. However, some non-albicans Candida isolates showed a lower susceptibility for the antifungal agents itraconazole, fluconazole and voriconazole. All of the species were sensitive to amphotericin B and nystatin. The current analysis showed that yeasts isolated from the mouth of chronic kidney patients undergoing haemodialysis varied significantly with regard to their capacity for adherence, biofilm formation and susceptibility to antifungal agents, underscoring the high virulence of non-albicans Candida species.

Control of morphogenesis, protease secretion and gene expression in Candida albicans by the preferred nitrogen source ammonium

Micro-organisms sense the availability of nutrients in their environment to control cellular behaviour and the expression of transporters and enzymes that are required for the utilization of these nutrients. In the pathogenic yeast Candida albicans, the preferred nitrogen source ammonium suppresses the switch from yeast to filamentous growth in response to certain stimuli, and it also represses the secretion of proteases, which are required for the utilization of proteins as an alternative nitrogen source. To investigate whether C. albicans senses the availability of ammonium in the extracellular environment or if ammonium uptake into the cell is required to regulate morphogenesis and gene expression, we compared the behaviour of wild-type cells and ammonium uptake-deficient mutants in the presence and absence of extracellular ammonium. Arginine-induced filamentous growth was suppressed by ammonium in the wild-type, but not in mutants lacking the ammonium permeases Mep1 and Mep2. Similarly, ammonium suppressed protease secretion and extracellular protein degradation in the wild-type, but not in mutants lacking the ammonium transporters. By comparing the gene expression profiles of C. albicans grown in the presence of low or high ammonium concentrations, we identified a set of genes whose expression is controlled by nitrogen availability. The repression of genes involved in the utilization of alternative nitrogen sources, which occurred under ammonium-replete conditions in the wild-type, was abrogated in mep1Δ mep2Δ mutants. These results demonstrate that C. albicans does not respond to the presence of sufficient amounts of the preferred nitrogen source ammonium by sensing its availability in the environment. Instead, ammonium has to be taken up into the cell to control morphogenesis, protease secretion and gene expression.

Paracoccidoides brasiliensis 30 kDa adhesin: identification as a 14-3-3 protein, cloning and subcellular localization in infection models

Paracoccidoides brasiliensis adhesion to lung epithelial cells is considered an essential event for the establishment of infection and different proteins participate in this process. One of these proteins is a 30 kDa adhesin, pI 4.9 that was described as a laminin ligand in previous studies, and it was more highly expressed in more virulent P. brasiliensis isolates. This protein may contribute to the virulence of this important fungal pathogen. Using Edman degradation and mass spectrometry analysis, this 30 kDa adhesin was identified as a 14-3-3 protein. These proteins are a conserved group of small acidic proteins involved in a variety of processes in eukaryotic organisms. However, the exact function of these proteins in some processes remains unknown. Thus, the goal of the present study was to characterize the role of this protein during the interaction between the fungus and its host. To achieve this goal, we cloned, expressed the 14-3-3 protein in a heterologous system and determined its subcellular localization in in vitro and in vivo infection models. Immunocytochemical analysis revealed the ubiquitous distribution of this protein in the yeast form of P. brasiliensis, with some concentration in the cytoplasm. Additionally, this 14-3-3 protein was also present in P. brasiliensis cells at the sites of infection in C57BL/6 mice intratracheally infected with P. brasiliensis yeast cells for 72 h (acute infections) and 30 days (chronic infection). An apparent increase in the levels of the 14-3-3 protein in the cell wall of the fungus was also noted during the interaction between P. brasiliensis and A549 cells, suggesting that this protein may be involved in host-parasite interactions, since inhibition assays with the protein and this antibody decreased P. brasiliensis adhesion to A549 epithelial cells. Our data may lead to a better understanding of P. brasiliensis interactions with host tissues and paracoccidioidomycosis pathogenesis.

A family of secreted pathogenesis-related proteins in Candida albicans

Analysing culture supernatants of yeast and hyphal cells of Candida albicans, we found two close homologues of pathogenesis-related (PR-) 1 proteins, Rbe1p and Rbt4p, in the secretome. Due to sequence homology, three additional, yet not characterized open reading frames, ORF19.6200, ORF19.2787 and ORF19.2336, together with RBE1 and RBT4 were assigned to a novel family of CaPRY proteins. In a Δrbe1/Δrbt4 deletion strain, genome-wide transcriptional analysis revealed differential transcription of only a limited set of genes implicated in virulence and oxidative stress response. Single deletion of RBE1 or RBT4 in a clinical C. albicans isolate resulted in a moderate but significant attenuation in virulence in a mouse model for disseminated candidiasis. However, a synergistic effect was observed in a Δrbe1/Δrbt4 double deletion strain, where virulence was strongly affected. Remarkably, transcription of RBT4 and RBE1 was each upregulated in blastospores of Δrbe1 or hyphae of Δrbt4 deletion strains respectively, indicating functional complementation thereby compensating a potential virulence defect in the single deletion strains. Furthermore, the double deletion strain showed increased sensitivity to attack by polymorphonuclear leucocytes. Therefore, the crucial contribution of both C. albicans pathogenesis-related proteins to virulence might be vested in protection against phagocyte attack.

Candida tropicalis biofilms: effect on urinary epithelial cells

Candida tropicalis infection is strongly associated with the presence of biofilms in urinary catheters. Thus, the aim of this work was to study the behaviour of C. tropicalis in biofilms of different ages (24-120 h) formed in artificial urine (AU) and their effect in human urinary bladder cells (TCC-SUP). Reference strain ATCC 750 and two isolates from patients with candiduria (U69 and U75) were used in this study. The adhesion to human cells was evaluated after 2 h of contact with Candida biofilms, using the Crystal violet staining method, and the human cells response was evaluated in terms of activity inhibition and cell damage. Candida tropicalis aspartyl proteinase (SAPT) gene expression was determined by real-time PCR. Candida tropicalis biofilm cells were able to adhere to TCC-SUP cells. The highest extent of yeast attachment was obtained for the 72 h old biofilm cells. Yeasts affected TCC-SUP cells, with 120 h-biofilm cells causing the highest levels of cell injury. Generally, SAPT3 was highly expressed and SAPT4 was only detected in the reference strain. Overall, it is important to highlight that C. tropicalis cells detached from biofilms are able to colonize human cells and cause some injury and reduction of metabolic activity.

R432 is a key residue for the multiple functions of Ndt80p in Candida albicans

Ndt80p is an important transcription modulator to various stress-response genes in Candida albicans, the most common human fungal pathogen in systemic infections. We found that Ndt80p directly regulated its target genes, such as YHB1, via the mid-sporulation element (MSE). Furthermore, the ndt80(R432A) allele, with a reduced capability to bind MSE, failed to complement the defects caused by null mutations of NDT80. Thus, the R432 residue in the Ndt80p DNA-binding domain is involved in all tested functions, including cell separation, drug resistance, nitric oxide inactivation, germ tube formation, hyphal growth, and virulence. Hence, the importance of the R432 residue suggests a novel approach for designing new antifungal drugs by blocking the interaction between Ndt80p and its targets.

Comparative transcriptome analysis of Paracoccidioides brasiliensis during in vitro adhesion to type I collagen and fibronectin: identification of potential adhesins

Paracoccidioidomycosis is caused by the dimorphic fungus Paracoccidioides brasiliensis. The extracellular matrix (ECM) plays an important role in regulation of cell adhesion, differentiation, migration and proliferation of cells. An in vitro binding assay of P. brasiliensis yeast cells adhering to type I collagen and fibronectin was performed in order to identify novel adhesins. Representational difference analysis (RDA) was employed to identify genes upregulated under adhesion-inducing conditions. Expressed sequence tags (ESTs) from cDNA libraries generated by the RDA technique were analyzed. Genes related to functional categories, such as metabolism, transcription, energy, protein synthesis and fate, cellular transport and biogenesis of cellular components were upregulated. Transcripts encoding the P. brasiliensis protein enolase (PbEno) and the high-affinity cooper transporter (PbCtr3) were identified and further characterized. The recombinant enolase (rPbEno) and a synthetic peptide designed for PbCtr3 were obtained and demonstrated to be able to bind ECM components. Immunofluorescence assays demonstrated that rPbEno specifically binds to the macrophage surface, reinforcing the role of this molecule in the P. brasiliensis interaction with host cells. In addition, upregulation of selected genes was demonstrated by qRT-PCR. In synthesis, the strategy can be useful in characterization of potential P. brasiliensis adhesins.

Biofilm formation and adhesive/invasive properties of Candida dubliniensis in comparison with Candida albicans

Candida dubliniensis and Candida albicans are closely related spp. exhibiting differences in their virulence potency. This study compared clinical isolates of C. dubliniensis with C. albicans from HIV patients with oropharyngeal candidiasis (OPC) and standard strains in power to form biofilm and their adhesive and invasive properties. Members of both spp. were able to form strong biofilms. However, SEM microscopy confirmed that C. albicans undergoes the more effective yeast-to-hyphae transition than C. dubliniensis with prevalent yeast form and limited ability to form filaments. Kinetic patterns indicated that while the first 30 min are critical for sufficient attachment to a polystyrene surface, adhesion to human carcinoma cell lines (Caco-2 and TR 146) needs additional time with maximal saturation observed at 240 min for both spp. The invasion process was tested on 3D RHE (reconstituted human epithelium) with Caco-2 or TR 146 on the collagen surface. C. albicans rapidly produced hyphae that penetrated the tissue layer, demonstrating substantive invasion within 21 h. In contrast, C. dubliniensis attached to the tissue surface and proliferated, suggesting the formation of a biofilm-like structure. After 21 h, C. dubliniensis was able to penetrate the RHE layer and invade unusually, with a cluster of the yeast cells.

Insights into Candida tropicalis nosocomial infections and virulence factors

Candida tropicalis is considered the first or the second non-Candida albicans Candida (NCAC) species most frequently isolated from candidosis, mainly in patients admitted in intensive care units (ICUs), especially with cancer, requiring prolonged catheterization, or receiving broad-spectrum antibiotics. The proportion of candiduria and candidemia caused by C. tropicalis varies widely with geographical area and patient group. Actually, in certain countries, C. tropicalis is more prevalent, even compared with C. albicans or other NCAC species. Although prophylactic treatments with fluconazole cause a decrease in the frequency of candidosis caused by C. tropicalis, it is increasingly showing a moderate level of fluconazole resistance. The propensity of C. tropicalis for dissemination and the high mortality associated with its infections might be strongly related to the potential of virulence factors exhibited by this species, such as adhesion to different host surfaces, biofilm formation, infection and dissemination, and enzymes secretion. Therefore, the aim of this review is to outline the present knowledge on all the above-mentioned C. tropicalis virulence traits.

Immune escape of the human facultative pathogenic yeast Candida albicans: the many faces of the Candida Pra1 protein

Infectious diseases caused by human pathogenic fungi represent a major and global health problem. Based on the limited efficacy of existing drugs and the increasing resistance to antifungal compounds, new strategies are urgently needed to fight such fungal infections. The medically important pathogen Candida albicans can exist as an opportunistic yeast, but can also cause severe diseases, septicaemia, and death. In order to establish new strategies to fight Candida infections and to interfere with survival of the pathogen, it is highly relevant to understand the molecular and immunology interactions between the pathogen C. albicans and the human host. This immune cross talk has moved into the focus of infectious disease research. In this review, we summarize the diverse and multiple levels of the immune cross talk between the fungal pathogen C. albicans and the human host. In particular, we define how one single fungal protein Pra1 (pH-regulated antigen 1) interferes and controls host immune attack at multiple levels and thus contributes to fungal immune escape. Candida Pra1 represents a promising candidate for immune interference.

Experimental superficial candidiasis on tissue models

Candida species are common pathogens causing superficial mycoses primarily affecting the mucosa and the skin in humans. Crucial steps during pathogenesis of superficial candidiasis comprise fungal adhesion, colonisation and subsequent penetration of the respective tissues. Exploring these pathological events and perhaps fungal and tissue responses towards drug treatment is imperative in the management of this infection. Unfortunately, pathological biopsies of superficial candidiasis do not exhibit the early changes in the host-pathogen interaction as the tissues are already invaded by the fungi. In vivo experimental assessments of pathological processes of superficial candidiasis are also limited because of the difficulties in providing reproducible and comparable conditions in the host environment. Conversely, in vitro models have helped studying fungal-host interactions under more defined and controlled conditions. Some common in vitro models used to simulate superficial candidiasis are chick chorioallantoic membrane, mucosal explants and single layer or multiple layer cell cultures. Interestingly, these experimental approaches share advantages as well as disadvantages when compared with in vivo conditions. Hence, this review intends to discuss about the experimental superficial candidiasis produced in various tissue models and their advantages as well as disadvantages with a particular reference to further improvement of validity and reliability of such experiments.

Massive induction of innate immune response to Candida albicans in the kidney in a murine intravenous challenge model

In the experimental Candida albicans intravenous challenge model, the kidney is one of the main organs involved in disease. In progressive infection, fungal burdens are found to increase over time, with rapid increases occurring from 24 h postinfection. Renal transcriptional responses were analyzed at this time in the kidneys of mice infected by either a virulent or an attenuated C. albicans strain, allowing comparison of host responses in progressive and nonprogressive infection. The results of this study demonstrate that both infections share a common transcriptional response, consisting of functions associated with the acute-phase reaction. In addition, challenge with the virulent strain led to a massively increased expression of cytokine genes, other innate response genes and genes suggestive of initiation of the adaptive immune response. This immune response to C. albicans infection, which occurs only in progressive infection, may contribute to development of sepsis and, ultimately, host death.

Cellular interactions of Candida albicans with human oral epithelial cells and enterocytes

The human pathogenic fungus Candida albicans can cause systemic infections by invading epithelial barriers to gain access to the bloodstream. One of the main reservoirs of C. albicans is the gastrointestinal tract and systemic infections predominantly originate from this niche. In this study, we used scanning electron and fluorescence microscopy, adhesion, invasion and damage assays, fungal mutants and a set of fungal and host cell inhibitors to investigate the interactions of C. albicans with oral epithelial cells and enterocytes. Our data demonstrate that adhesion, invasion and damage by C. albicans depend not only on fungal morphology and activity, but also on the epithelial cell type and the differentiation stage of the epithelial cells, indicating that epithelial cells differ in their susceptibility to the fungus. C. albicans can invade epithelial cells by induced endocytosis and/or active penetration. However, depending on the host cell faced by the fungus, these routes are exploited to a different extent. While invasion into oral cells occurs via both routes, invasion into intestinal cells occurs only via active penetration.

Examination of potential virulence factors of Candida tropicalis clinical isolates from hospitalized patients

Candida tropicalis has been reported to be one of the Candida species which is most likely to cause bloodstream and urinary tract infections in hospitalized patients. Accordingly, the aim of this study was to characterize the virulence of C. tropicalis by assessing antifungal susceptibility and comparing the expression of several virulence factors. This study was conducted with seven isolates of C. tropicalis from urine and blood cultures and from central venous catheter. C. tropicalis ATCC 750 was used as reference strain. Yeasts adhered (2 h) to epithelial cells and silicone and 24 h biofilm biomass were determined by crystal violet staining. Pseudohyphae formation ability was determined after growth in fetal bovine serum. Enzymes production (hemolysins, proteases, phospholipases) was assessed by halo formation on agar plates. Susceptibility to antifungal agents was determined by E-test. Regarding adhesion, it can be highlighted that C. tropicalis strains adhered significantly more to epithelium than to silicone. Furthermore, all C. tropicalis strains were able to form biofilms and to express total hemolytic activity. However, protease was only produced by two isolates from urine and by the isolates from catheter and blood. Moreover, only one C. tropicalis (from catheter) was phospholipase positive. All isolates were susceptible to voriconazole, fluconazole and amphotericin B. Four strains were susceptible-dose dependent to itraconazole and one clinical isolate was found to be resistant.

Transcriptional responses of candida albicans to epithelial and endothelial cells

Candida albicans interacts with oral epithelial cells during oropharyngeal candidiasis and with vascular endothelial cells when it disseminates hematogenously. We set out to identify C. albicans genes that govern interactions with these host cells in vitro. The transcriptional response of C. albicans to the FaDu oral epithelial cell line and primary endothelial cells was determined by microarray analysis. Contact with epithelial cells caused a decrease in transcript levels of genes related to protein synthesis and adhesion, whereas contact with endothelial cells did not significantly influence any specific functional category of genes. Many genes whose transcripts were increased in response to either host cell had not been previously characterized. We constructed mutants with homozygous insertions in 22 of these uncharacterized genes to investigate their function during host-pathogen interaction. By this approach, we found that YCK2, VPS51, and UEC1 are required for C. albicans to cause normal damage to epithelial cells and resist antimicrobial peptides. YCK2 is also necessary for maintenance of cell polarity. VPS51 is necessary for normal vacuole formation, resistance to multiple stressors, and induction of maximal endothelial cell damage. UEC1 encodes a unique protein that is required for resistance to cell membrane stress. Therefore, some C. albicans genes whose transcripts are increased upon contact with epithelial or endothelial cells are required for the organism to damage these cells and withstand the stresses that it likely encounters during growth in the oropharynx and bloodstream.

Human epithelial model systems for the study of Candida infections in vitro: part I. Adhesion to epithelial models

Adhesion to host tissue represents one of the first steps during the early phase of fungal infections. In order to mediate pathogenesis in the infected host, this process is crucial for colonization and subsequent penetration of the respective tissue. In vivo analyses of the adhesion process in whole organisms are limited because of difficulties in providing reproducible and comparable conditions in the host environment. Therefore, in vitro assays provide the opportunity to study such processes under more defined conditions thus allowing for the analysis of events that are involved in more detail. Here we describe an in vitro adhesion assay making use of human epithelial cell lines to study fungal associations with host epithelia. This assay not only is suited to determine the rate of adhesion in a time-dependent manner but also facilitates global transcriptional profiling in order to determine the fungal response during adhesion at the molecular level.

Hypoxic conditions and iron restriction affect the cell-wall proteome of Candida albicans grown under vagina-simulative conditions

Proteins that are covalently linked to the skeletal polysaccharides of the cell wall of Candida albicans play a major role in the colonization of the vaginal mucosal surface, which may result in vaginitis. Here we report on the variability of the cell-wall proteome of C. albicans as a function of the ambient O(2) concentration and iron availability. For these studies, cells were cultured at 37 degrees C in vagina-simulative medium and aerated with a gas mixture consisting of 6 % (v/v) CO(2), 0.01-7 % (v/v) O(2) and N(2), reflecting the gas composition in the vaginal environment. Under these conditions, the cells grew exclusively in the non-hyphal form, with the relative growth rate being halved at approximately 0.02 % (v/v) O(2). Using tandem MS and immunoblot analysis, we identified 15 covalently linked glycosylphosphatidylinositol (GPI) proteins in isolated walls (Als1, Als3, Cht2, Crh11, Ecm33, Hwp1, Pga4, Pga10, Phr2, Rbt5, Rhd3, Sod4, Ssr1, Ywp1, Utr2) and 4 covalently linked non-GPI proteins (MP65, Pir1, Sim1/Sun42, Tos1). Five of them (Als3, Hwp1, Sim1, Tos1, Utr2) were absent in cells grown in rich medium. Immunoblot analysis revealed that restricted O(2) availability resulted in higher levels of the non-GPI protein Pir1, a putative beta-1,3-glucan cross-linking protein, and of the GPI-proteins Hwp1, an adhesion protein, and Pga10 and Rbt5, which are involved in iron acquisition. Addition of the iron chelator ferrozine at saturating levels of O(2) resulted in higher cell wall levels of Hwp1 and Rbt5, suggesting that the responses to hypoxic conditions and iron restriction are related.

Deletion of the high-affinity cAMP phosphodiesterase encoded by PDE2 affects stress responses and virulence in Candida albicans

Previously, we have shown that PDE2 is required for hyphal development and cell wall integrity in Candida albicans. In the present study, we have investigated the effects of its deletion by genome-wide transcriptome profiling. Changes in expression levels of genes involved in metabolism, transcription, protein and nucleic acids synthesis, as well as stress responses, cell wall and membrane biogenesis, adherence and virulence have been observed. By comparing these changes with previously reported transcriptome profiles of pde2Delta mutants of Saccharomyces cerevisiae, as well as cdc35Delta, ras1Delta and efg1Delta mutants of C. albicans, conserved and species-specific cAMP-regulated genes have been identified. The genes whose transcription is altered upon deletion of PDE2 in C. albicans has also allowed us to predict that the pde2Delta mutant would have a defective ability to adhere to, and invade host cells, and an impaired virulence as well as response to different stresses. Using appropriate assays, we have tested these predictions and compared the roles of the high- and low-affinity cAMP phosphodiesterases, Pde2p and Pde1p in stress, adhesion and virulence. We suggest that phosphodiesterases, and in particular the high-affinity cAMP phosphodiesterase encoded by PDE2, have real potential as targets for antifungal chemotherapy.

Interactions of the fungal pathogen Candida albicans with the host

Fungal infections represent a serious health problem in industrialized countries. In particular, multimorbid patients are highly susceptible to life-threatening infections by opportunistic fungi, most often Candida or Aspergillus species. In Europe, fungal infections account for 17% of intensive care unit infections. In addition, common non-life-threatening superficial infections impose significant restrictions on patients, resulting in a reduced quality of life. One of the first steps of pathogens during infection of the host is to attach to the surface of host tissues. This step in host–pathogen interaction is crucial for colonization by the pathogen and for the persistance of the pathogen in the host. Commensal organisms, such as Candida albicans, are able to persistently colonize the host without causing symptoms. However, the balance between commensalism and pathogenicity is delicate. How these two states are modulated during C. albicans colonization is a major area of research in medical mycology, with the aim of utilizing the knowledge gained for the benefit of the patient.