Linkage of adhesion, filamentous growth, and virulence in Candida albicans to a single gene, INT1

Recent gene selection and drug resistance underscore clinical adaptation across Candida species

Understanding how microbial pathogens adapt to treatments, humans and clinical environments is key to infer mechanisms of virulence, transmission and drug resistance. This may help improve therapies and diagnostics for infections with a poor prognosis, such as those caused by fungal pathogens, including Candida. Here we analysed genomic variants across approximately 2,000 isolates from six Candida species (C. glabrata, C. auris, C. albicans, C. tropicalis, C. parapsilosis and C. orthopsilosis) and identified genes under recent selection, suggesting a highly complex clinical adaptation. These involve species-specific and convergently affected adaptive mechanisms, such as adhesion. Using convergence-based genome-wide association studies we identified known drivers of drug resistance alongside potentially novel players. Finally, our analyses reveal an important role of structural variants and suggest an unexpected involvement of (para)sexual recombination in the spread of resistance. Our results provide insights on how opportunistic pathogens adapt to human-related environments and unearth candidate genes that deserve future attention.

Activity of propolis from Mexico on the proliferation and virulence factors of Candida albicans

BACKGROUND

This research evaluated the anti-Candida albicans effect of Mexican propolis from Chihuahua. Chemical composition of the ethanolic extract of propolis was determined by GC-MS, HPLC-DAD, and HPLC-MS. The presence of anthraquinone, aromatic acid, fatty acids, flavonoids, and carbohydrates was revealed.

RESULTS

The anti-Candida activity of propolis was determined. The inhibitions halos were between 10.0 to 11.8 mm; 25% minimum inhibitory concentration (0.5 mg/ml) was fungistatic, and 50% minimum inhibitory concentration (1.0 mg/ml) was fungicidal. The effect of propolis on the capability of C. albicans to change its morphology was evaluated. 25% minimum inhibitory concentration inhibited to 50% of germ tube formation. Staining with calcofluor-white and propidium iodide was performed, showing that the propolis affected the integrity of the cell membrane. INT1 gene expression was evaluated by qRT-PCR. Propolis significantly inhibited the expression of the INT1 gene encodes an adhesin (Int1p). Chihuahua propolis extract inhibited the proliferation of Candida albicans, the development of the germ tube, and the synthesis of adhesin INT1.

CONCLUSIONS

Given the properties demonstrated for Chihuahua propolis, we propose that it is a candidate to be considered as an ideal antifungal agent to help treat this infection since it would not have the toxic effects of conventional antifungals.

Sticking to the Subject: Multifunctionality in Microbial Adhesins

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

Oral Cavity and Candida albicans: Colonisation to the Development of Infection

Candida colonisation of the oral cavity increases in immunocompromised individuals which leads to the development of oral candidiasis. In addition, host factors such as xerostomia, smoking, oral prostheses, dental caries, diabetes and cancer treatment accelerate the disease process. Candida albicans is the primary causative agent of this infection, owing to its ability to form biofilm and hyphae and to produce hydrolytic enzymes and candialysin. Although mucosal immunity is activated, from the time hyphae-associated toxin is formed by the colonising C. albicans cells, an increased number and virulence of this pathogenic organism collectively leads to infection. Prevention of the development of infection can be achieved by addressing the host physiological factors and habits. For maintenance of oral health, conventional oral hygiene products containing antimicrobial compounds, essential oils and phytochemicals can be considered, these products can maintain the low number of Candida in the oral cavity and reduce their virulence. Vulnerable patients should be educated in order to increase compliance.

Genotyping, drug resistance and virulence factors of Candida species isolated from patients using long-term inhaled steroids

AIMS

In this study, it was aimed to determine the isolation frequency and species distribution of Candida species isolated from asthmatic patients using long-term inhaled steroids. It was also aimed to determine the drug resistance patterns and the frequency of erg11, HWP1, ALS1, INT1, SAP1 PLB1 genes in isolates.

METHODS

Genotyping of Candida strains isolated from patients and healthy control group was performed by PCR-RFLP method. Drug resistance was investigated phenotypically, and the presence of erg11 resistance genes and HWP1, ALS1, INT1, SAP1 PLB1 virulence genes were investigated by PCR method.

RESULTS

C albicans was the most isolated species in steroid-using patients and healthy control groups (patients: 44.2%; control group: 30.8%). C tropicalis and C glabrata were found to have the highest rates of non-albicans Candida in patients with 17.4% and 13.77%, respectively. Azole resistance was found to be significantly higher in isolates isolated from patients compared to the control group. Similarly, the presence of erg11 resistance gene was highest in C albicans (17.65%), C glabrata (12.5%) and C tropicalis (8.3%) strains in the control group, while C parapsilosis was highest in patients. (57.1%) and C glabrata (54.2%) strains. Compared to the control group, the virulence of Candida strains isolated from the patients was found to be higher. Presence of HWP1, ALS1, INT1, SAP1 and PLB1 genes in patients were determined as 72.1%, 63.9%, 68.9%, 57.38% and 54.5%, respectively. These rates were 29.4%, 35.3%, 25.5%, 17.7% and 23.5% in the healthy control group, respectively.

CONCLUSIONS

In asthma patients using long-term inhaled steroids, both Isolation rates of Candida species, drug resistance rates, presence of virulence genes were found to be significantly higher in patients than in the control group. We think that this may be due to the suppression of cellular immunity by long-term steroid use.

Effects of oestrogen on vulvovaginal candidosis

As a frequently occurring infectious disease mainly caused by Candida albicans, vulvovaginal candidosis (VVC) affects more than 100 million women worldwide every year. Multiple factors that influence C. albicans colonisation have been linked to the incidence of VVC, including high levels of circulating oestrogen due to pregnancy, the use of oral contraceptives, and hormone replacement therapy. This review provides an overview of the current understanding of the mechanism(s) by which oestrogen contributes to VVC, which might provide meaningful guidance to the prevention and treatment of this disease.

Catechol thwarts virulent dimorphism in Candida albicans and potentiates the antifungal efficacy of azoles and polyenes

The present study was deliberately focused to explore the antivirulence efficacy of a plant allelochemical-catechol against Candida albicans, and attempts were made to elucidate the underlying mechanisms as well. Catechol at its sub-MIC concentrations (2-256 μg/mL) exhibited a dose dependent biofilm as well as hyphal inhibitory efficacies, which were ascertained through both light and fluorescence microscopic analyses. Further, sub-MICs of catechol displayed remarkable antivirulence efficacy, as it substantially inhibited C. albicans' virulence enzymes i.e. secreted hydrolases. Notably, FTIR analysis divulged the potency of catechol in effective loosening of C. albicans' exopolymeric matrix, which was further reinforced using EPS quantification assay. Although, catechol at BIC (256 μg/mL) did not disrupt the mature biofilms of C. albicans, their initial adherence was significantly impeded by reducing their hydrophobic nature. Besides, FTIR analysis also unveiled the ability of catechol in enhancing the production of farnesol-a metabolite of C. albicans, whose accumulation naturally blocks yeast-hyphal transition. The qPCR data showed significant down-regulation of candidate genes viz., RAS1, HWP1 and ALS3 which are the key targets of Ras-cAMP-PKA pathway -the pathway that contribute for C. albicans' pathogenesis. Interestingly, the up-regulation of TUP1 (a gene responsible for farnesol-mediated hyphal inhibition) during catechol exposure strengthen the speculation of catechol triggered farnesol-mediated hyphal inhibition. Furthermore, catechol profusely enhanced the fungicidal efficacy of certain known antifungal agent's viz., azoles (ketoconazole and miconazole) and polyenes (amphotericin-B and nystatin).

Actin Cytoskeleton Regulation by the Yeast NADPH Oxidase Yno1p Impacts Processes Controlled by MAPK Pathways

Reactive oxygen species (ROS) that exceed the antioxidative capacity of the cell can be harmful and are termed oxidative stress. Increasing evidence suggests that ROS are not exclusively detrimental, but can fulfill important signaling functions. Recently, we have been able to demonstrate that a NADPH oxidase-like enzyme (termed Yno1p) exists in the single-celled organism Saccharomyces cerevisiae. This enzyme resides in the peripheral and perinuclear endoplasmic reticulum and functions in close proximity to the plasma membrane. Its product, hydrogen peroxide, which is also produced by the action of the superoxide dismutase, Sod1p, influences signaling of key regulatory proteins Ras2p and Yck1p/2p. In the present work, we demonstrate that Yno1p-derived H2O2 regulates outputs controlled by three MAP kinase pathways that can share components: the filamentous growth (filamentous growth MAPK (fMAPK)), pheromone response, and osmotic stress response (hyperosmolarity glycerol response, HOG) pathways. A key structural component and regulator in this process is the actin cytoskeleton. The nucleation and stabilization of actin are regulated by Yno1p. Cells lacking YNO1 showed reduced invasive growth, which could be reversed by stimulation of actin nucleation. Additionally, under osmotic stress, the vacuoles of a ∆yno1 strain show an enhanced fragmentation. During pheromone response induced by the addition of alpha-factor, Yno1p is responsible for a burst of ROS. Collectively, these results broaden the roles of ROS to encompass microbial differentiation responses and stress responses controlled by MAPK pathways.

Antibiotic-induced gut metabolome and microbiome alterations increase the susceptibility to Candida albicans colonization in the gastrointestinal tract

Antibiotic-induced alterations in the gut ecosystem increases the susceptibility to Candida albicans, yet the mechanisms involved remains poorly understood. Here we show that mice treated with the broad-spectrum antibiotic cefoperazone promoted the growth, morphogenesis and gastrointestinal (GI) colonization of C. albicans. Using metabolomics, we revealed that the cecal metabolic environment of the mice treated with cefoperazone showed a significant alteration in intestinal metabolites. Levels of carbohydrates, sugar alcohols and primary bile acids increased, whereas carboxylic acids and secondary bile acids decreased in antibiotic treated mice susceptible to C. albicans. Furthermore, using in-vitro assays, we confirmed that carbohydrates, sugar alcohols and primary bile acids promote, whereas carboxylic acids and secondary bile acids inhibit the growth and morphogenesis of C. albicans. In addition, in this study we report changes in the levels of gut metabolites correlated with shifts in the gut microbiota. Taken together, our in-vivo and in-vitro results indicate that cefoperazone-induced metabolome and microbiome alterations favor the growth and morphogenesis of C. albicans, and potentially play an important role in the GI colonization of C. albicans.

Current Aspects in the Biology, Pathogeny, and Treatment of Candida krusei, a Neglected Fungal Pathogen

Fungal infections represent a constant and growing menace to human health, because of the emergence of new species as causative agents of diseases and the increment of antifungal drug resistance. Candidiasis is one of the most common fungal infections in humans and is associated with a high mortality rate when the fungi infect deep-seated organs. Candida krusei belongs to the group of candidiasis etiological agents, and although it is not isolated as frequently as other Candida species, the infections caused by this organism are of special relevance in the clinical setting because of its intrinsic resistance to fluconazole. Here, we offer a thorough revision of the current literature dealing with this organism and the caused disease, focusing on its biological aspects, the host-fungus interaction, the diagnosis, and the infection treatment. Of particular relevance, we provide the most recent genomic information, including the gene prediction of some putative virulence factors, like proteases, adhesins, regulators of biofilm formation and dimorphism. Moreover, C. krusei veterinary aspects and the exploration of natural products with anti-C. krusei activity are also included.

The Role of Secretory Pathways in Candida albicans Pathogenesis

Candida albicans is a fungus that is a commensal organism and a member of the normal human microbiota. It has the ability to transition into an opportunistic invasive pathogen. Attributes that support pathogenesis include secretion of virulence-associated proteins, hyphal formation, and biofilm formation. These processes are supported by secretion, as defined in the broad context of membrane trafficking. In this review, we examine the role of secretory pathways in Candida virulence, with a focus on the model opportunistic fungal pathogen, Candida albicans.

Antibiotic-induced alterations in taurocholic acid levels promote gastrointestinal colonization of Candida albicans

Candida albicans is a fungal pathogen that poses a significant public health risk due to high incidence and mortality rates among immunocompromised patients. Candida albicans infections begin with successful gastrointestinal (GI) colonization; however, the mechanisms behind this colonization remain to be elucidated. In this study, we investigated the role of taurocholic acid (TCA) on growth and GI colonization of C. albicans. Our results indicate that cefoperazone-treated mice susceptible to C. albicans infection had significantly increased levels of TCA in the gut contents. In addition, an increase in TCA levels directly correlates with higher C. albicans load in the fecal and gut contents of antibiotic-treated infected mice. Using in vitro assays, we also demonstrated that TCA enhances the growth of C. albicans and its ability to develop filamentous hyphae. Furthermore, TCA significantly increased the ability of C. albicans to attach to mammalian cells. These results demonstrate that antibiotic treatment alters TCA levels in the gut and potentially enhances GI colonization of C. albicans.

Antibiotic-induced decreases in the levels of microbial-derived short-chain fatty acids correlate with increased gastrointestinal colonization of Candida albicans

Candida albicans is the fourth most common cause of systemic nosocomial infections, posing a significant risk in immunocompromised individuals. As the majority of systemic C. albicans infections stem from endogenous gastrointestinal (GI) colonization, understanding the mechanisms associated with GI colonization is essential in the development of novel methods to prevent C. albicans-related mortality. In this study, we investigated the role of microbial-derived short-chain fatty acids (SCFAs) including acetate, butyrate, and propionate on growth, morphogenesis, and GI colonization of C. albicans. Our results indicate that cefoperazone-treated mice susceptible to C. albicans infection had significantly decreased levels of SCFAs in the cecal contents that correlate with a higher fungal load in the feces. Further, using in vivo concentration of SCFAs, we demonstrated that SCFAs inhibit the growth, germ tube, hyphae and biofilm development of C. albicans in vitro. Collectively, results from this study suggest that antibiotic-induced decreases in the levels of SCFAs in the cecum enhances the growth and GI colonization of C. albicans.

Aggregate Filamentous Growth Responses in Yeast

Filamentous growth is a fungal morphogenetic response that is critical for virulence in some fungal species. Many aspects of filamentous growth remain poorly understood. We have identified an aspect of filamentous growth in the budding yeast Saccharomyces cerevisiae and the human pathogen Candida albicans where cells behave collectively to invade surfaces in aggregates. These responses may reflect an extension of normal filamentous growth, as they share the same signaling pathways and effector processes. Aggregate responses may involve cooperation among individual cells, because aggregation was stimulated by cell adhesion molecules, secreted enzymes, and diffusible molecules that promote quorum sensing. Our study may provide insights into the genetic basis of collective cellular responses in fungi. The study may have ramifications in fungal pathogenesis, in situations where collective responses occur to promote virulence.

Many fungal species, including pathogens, undergo a morphogenetic response called filamentous growth, where cells differentiate into a specialized cell type to promote nutrient foraging and surface colonization. Despite the fact that filamentous growth is required for virulence in some plant and animal pathogens, certain aspects of this behavior remain poorly understood. By examining filamentous growth in the budding yeast Saccharomyces cerevisiae and the opportunistic pathogen Candida albicans, we identify responses where cells undergo filamentous growth in groups of cells or aggregates. In S. cerevisiae, aggregate invasive growth was regulated by signaling pathways that control normal filamentous growth. These pathways promoted aggregation in part by fostering aspects of microbial cooperation. For example, aggregate invasive growth required cellular contacts mediated by the flocculin Flo11p, which was produced at higher levels in aggregates than cells undergoing regular invasive growth. Aggregate invasive growth was also stimulated by secreted enzymes, like invertase, which produce metabolites that are shared among cells. Aggregate invasive growth was also induced by alcohols that promote density-dependent filamentous growth in yeast. Aggregate invasive growth also required highly polarized cell morphologies, which may affect the packing or organization of cells. A directed selection experiment for aggregating phenotypes uncovered roles for the fMAPK and RAS pathways, which indicates that these pathways play a general role in regulating aggregate-based responses in yeast. Our study extends the range of responses controlled by filamentation regulatory pathways and has implications in understanding aspects of fungal biology that may be relevant to fungal pathogenesis.

IMPORTANCE Filamentous growth is a fungal morphogenetic response that is critical for virulence in some fungal species. Many aspects of filamentous growth remain poorly understood. We have identified an aspect of filamentous growth in the budding yeast Saccharomyces cerevisiae and the human pathogen Candida albicans where cells behave collectively to invade surfaces in aggregates. These responses may reflect an extension of normal filamentous growth, as they share the same signaling pathways and effector processes. Aggregate responses may involve cooperation among individual cells, because aggregation was stimulated by cell adhesion molecules, secreted enzymes, and diffusible molecules that promote quorum sensing. Our study may provide insights into the genetic basis of collective cellular responses in fungi. The study may have ramifications in fungal pathogenesis, in situations where collective responses occur to promote virulence.

Candida vaginitis: virulence, host response and vaccine prospects

Vulvovaginal candidiasis is a common mucosal infection affecting a large proportion of women with some of them affected by recurrent often intractable forms of the disease. Thus, there is an increasing interest in understanding the pathogenesis of this disease. The aim of our work was to characterize, in animal models of vaginal candidiasis, the components of the host-fungus interaction at the mucosal level.The evidence of an immune response in the vaginal compartment was very encouraging to identify the proper targets for new strategies for vaccination or immunotherapy of vaginal candidiasis. Aspartyl-proteinase (Sap2), which is an important immunodominant antigens and virulence factors of C.albicans acting in mucosal infections, was assembled with virosomes and a vaccine PEV7 was obtained. The results obtained in the mouse model and in the clinical trial conducted by Pevion on women have evidenced that the vaccine PEV7, intravaginally administered, has an encouraging therapeutic potential for the treatment of recurrent vulvovaginal candidiasis. This opens the way to a modality for anti-Candida protection at mucosal level.

Genetic Diversity Among Candida albicans Isolated from Humans and Cattle with Respiratory Distress in Egypt

As human populaces develop, they are progressively squeezed into higher living densities. The same is true for horticulture and animals expected to bolster these communities. Despite the high potential for zoonotic transmission, connections among humans and cattle have been understudied; however, Candida albicans remains the most important medical mycosis. The genesis of the mycobiome can vary, and interactions between humans and cattle are progressively being perceived as a key interface for disease transmission. αINT1 is a unique gene from Candida albicans; hence, it has been used for detection as well as intraspecific and interspecific phylogenetic analysis of C. albicans collected from human patients and cattle with pulmonary distress in urban-rural populations. A total of 1,921 specimens were examined by direct microscopy and culture to recover yeast associated with human infection. Identification was performed by micromorphology using an API 20C AUX system. The fungal species identified in bovine nasal specimens were Alternaria species (15%), Penicillium species, and C. albicans (6.7%). Other fungal species, such as Aspergillus niger, Torulopsis species, Mucor species (5%), Aspergillus flavus, Fusarium species, Trichosporon species (3.3%), C. rugosa, C. tropical, and Saccharomyces species (1.7%), were also isolated. In human sputum specimens, C. albicans (20%) and C. parapsilosis (2.7%) were the only reported yeast species in our samples. The four identified C. albicans species (two human and two cattle) were subjected to αINT1 gene sequence analysis, which confirmed major phylogenetic relationships among human and cattle isolates. This finding highlights the public health importance of bovines as a potential source for C. albicans zoonotic transmission to humans in an urban-rural community. Additionally, the close relationship between circulating C. albicans strains recorded in Egypt and the United States indicates the possible cross-species transmission of C. albicans between imported foreign and native cattle breeds.

Secondary bile acids inhibit Candida albicans growth and morphogenesis

Candida albicans is one of the most common causes of fungal infections in humans with a significant mortality rate. However, the factors involved in C. albicans gastrointestinal (GI) colonization remain unclear. We hypothesize that secondary bile acids have direct antifungal activity against C. albicans and may play a critical role in maintaining GI colonization resistance against C. albicans. In this study, we investigated the effect of secondary bile acids including lithocholic acid (LCA) and deoxycholic acid (DCA) on C. albicans growth and morphogenesis. Results indicate that LCA and DCA at in vivo cecal micelle concentrations inhibit C. albicans growth in vitro. Interestingly, LCA and DCA also significantly inhibited the germ tube, hyphae and biofilm formation in C. albicans. In addition, pre-treatment of C. albicans with LCA and DCA significantly reduced the percentage of C. albicans cells attached to a colon cancer cell line. Collectively, our results demonstrate that secondary bile acids play an important role in controlling the growth and morphological switching of C. albicans. Results from this study demonstrate that secondary bile acid possess direct antifungal activity against C. albicans, explaining a potential mechanism for gastrointestinal colonization resistance against C. albicans.

Hyphal induction under the condition without inoculation in Candida albicans is triggered by Brg1-mediated removal of NRG1 inhibition

Candida albicans can switch between yeast and hyphae growth forms, which is critical for its pathogenesis. Diluting from saturated cells into fresh medium at 37°C is routinely used to induce hyphae, which depends on the cAMP-PKA pathway-activated transcriptional down-regulation of NRG1 and degradation of Nrg1 protein triggered by inoculation. It is reported that N-acetylglucosamine (GlcNAc), serum or neutral pH could stimulate filamentation in log phase cells, whereas how C. albicans develops hyphae without inoculation remains unknown. Here, we show that NRG1 down-regulation is necessary for hyphal growth under this condition. Instead of cAMP-PKA pathway, GlcNAc sensor Ngs1 is responsible for the down-regulation of NRG1 upon GlcNAc induction in log phase cells through its N-acetyltransferase activity. From a genetic screen, Brg1 is found to be essential for hyphal development without inoculation. Ngs1 binds to BRG1 promoter to induce its expression in GlcNAc. Importantly, constitutively expressed BRG1 induces NRG1 down-regulation even in the absence of GlcNAc or Ngs1. Serum or neutral pH-induced filamentation in log phase cells is also through Brg1-mediated NRG1 down-regulation. Our study provides a molecular mechanism for how C. albicans forms hyphae in different cell states. This flexibility may facilitate C. albicans to adapt varied host environment during infection.

Proteomic analysis of a Candida albicans pir32 null strain reveals proteins involved in adhesion, filamentation and virulence

We have previously characterized Pir32, a Candia albicans cell wall protein that we found to be involved in filamentation, virulence, chitin deposition, and resistance to oxidative stress. Other than defining the cell shape, the cell wall is critical for the interaction with the surrounding environment and the point of contact and interaction with the host surface. In this study, we applied tandem mass spectrometry combined with bioinformatics to investigate cell wall proteome changes in a pir32 null strain. A total of 16 and 25 proteins were identified exclusively in the null mutant strains grown under non-filamentous and filamentous conditions. These proteins included members of the PGA family with various functions, lipase and the protease involved in virulence, superoxide dismutases required for resisting oxidative stress, alongside proteins required for cell wall remodeling and synthesis such as Ssr1, Xog1, Dfg5 and Dcw1. In addition proteins needed for filamentation like Cdc42, Ssu81 and Ucf1, and other virulence proteins such as Als3, Rbt5, and Csa2 were also detected. The detection of these proteins in the mutant and their lack of detection in the wild type can explain the differential phenotypes previously observed.

Candida innate immunity at the mucosa

The tremendous diversity in microbial species that colonise the mucosal surfaces of the human body is only now beginning to be fully appreciated. Distinguishing between the behaviour of commensal microbes and harmful pathogens that reside at mucosal sites in the body is a complex, and exquisitely fine-tuned process central to mucosal health. The fungal pathobiont Candida albicans is frequently isolated from mucosal surfaces with an asymptomatic carriage rate of approximately 60% in the human population. While normally a benign member of the microbiota, overgrowth of C. albicans often results in localised mucosal infection causing morbidity in otherwise healthy individuals, and invasive infection that often causes death in the absence of effective immune defence. C. albicans triggers numerous innate immune responses at mucosal surfaces, and detection of C. albicans hyphae in particular, stimulates the production of antimicrobial peptides, danger-associated molecular patterns and cytokines that function to reduce fungal burdens during infection. This review will summarise our current understanding of innate immune responses to C. albicans at mucosal surfaces.

The anillin-related Int1 protein and the Sep7 septin collaborate to maintain cellular ploidy in Candida albicans

Variation in cell ploidy is a common feature of Candida albicans clinical isolates that are resistant to the antifungal drug fluconazole. Here, we report that the anillin-related protein Int1 interacts with septins for coupling cytokinesis with nuclear segregation. Loss of Int1 results in a rapid disassembly of duplicated septin rings from the bud neck at the onset of actomyosin ring contraction. Strikingly, this has no major impact on cytokinesis and septum formation. However, Int1 genetically interacts with the Sep7 septin, maintaining the diffusion barrier at the bud neck and guarantying a faithful nuclear segregation. Indeed, int1ΔΔ sep7ΔΔ mutant cells, in contrast to int1ΔΔ cdc10ΔΔ, undergo a premature activation of mitotic exit prior to the alignment of the mitotic spindle with the division axis, producing large multinucleated cells. Some of these multinucleated cells arise from trimeras similar to those observed upon fluconazole exposure. Finally, the defects in nuclear segregation could be in part due to the inability to maintain the Lte1 mitotic exit activator at the cortex of the daughter cell. These results suggest that Int1 and Sep7 play a role in maintaining genome stability by acting as a diffusion barrier for Lte1.

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.

Anti-Candida Activity of Bursera morelensis Ramirez Essential Oil and Two Compounds, α-Pinene and γ-Terpinene-An In Vitro Study

The candidiasis caused by C. albicans is a public health problem. The abuse of antifungals has contributed to the development of resistance. B. morelensis has demonstrated antibacterial and antifungal activities. In this work the activity of the essential oil of B. morelensis was evaluated and for its two pure compounds with analysis of the different mechanisms of pathogenesis important for C. albicans. The essential oil was obtained by the hydro-distillation method and analyzed using GC–MS. The anti-Candida activity was compared between to essential oil, α-Pinene and γ-Terpinene. GC–MS of the essential oil demonstrated the presence of 13 compounds. The essential oil showed antifungal activity against four C. albicans strains. The most sensitive strain was C. albicans 14065 (MFC 2.0 mg/mL and MIC₅₀ 0.125 mg/mL) with α-Pinene and γ-Terpinene having MFCs of 4.0 and 16.0 mg/mL respectively. The essential oil inhibited the growth of the germ tube in 87.94% (8.0 mg/mL). Furthermore, it was observed that the essential oil diminishes the transcription of the gene INT1. This work provides evidence that confirms the anti-Candida activity of the B. morelensis essential oil and its effect on the growth of the germ tube and transcription of the gene INT1.

Candida albicans-epithelial interactions and induction of mucosal innate immunity

Candida albicans is a human fungal pathogen that causes millions of mucosal and life-threatening infections annually. C. albicans initially interacts with epithelial cells, resulting in fungal recognition and the formation of hyphae. Hypha formation is critical for host cell damage and immune activation, which are both driven by the secretion of Candidalysin, a recently discovered peptide toxin. Epithelial activation leads to the production of inflammatory mediators that recruit innate immune cells including neutrophils, macrophages and innate Type 17 cells, which together work with epithelial cells to clear the fungal infection. This review will focus on the recent discoveries that have advanced our understanding of C. albicans-epithelial interactions and the induction of mucosal innate immunity.

Overview of selected virulence attributes in Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, Trichophyton rubrum, and Exophiala dermatitidis

The incidence of fungal diseases has been increasing since 1980, and is associated with excessive morbidity and mortality, particularly among immunosuppressed patients. Of the known 625 pathogenic fungal species, infections caused by the genera Aspergillus, Candida, Cryptococcus, and Trichophyton are responsible for more than 300 million estimated episodes of acute or chronic infections worldwide. In addition, a rather neglected group of opportunistic fungi known as black yeasts and their filamentous relatives cause a wide variety of recalcitrant infections in both immunocompetent and immunosuppressed hosts. This article provides an overview of selected virulence factors that are known to suppress host immunity and enhance the infectivity of these fungi.

ESCRT genes and regulation of developmental signaling

ESCRT (Endosomal Sorting Complex Required for Transport) proteins have been shown to control an increasing number of membrane-associated processes. Some of these, and prominently regulation of receptor trafficking, profoundly shape signal transduction. Evidence in fungi, plants and multiple animal models support the emerging concept that ESCRTs are main actors in coordination of signaling with the changes in cells and tissues occurring during development and homeostasis. Consistent with their pleiotropic function, ESCRTs are regulated in multiple ways to tailor signaling to developmental and homeostatic needs. ESCRT activity is crucial to correct execution of developmental programs, especially at key transitions, allowing eukaryotes to thrive and preventing appearance of congenital defects.

Structural and In Vivo Studies on Trehalose-6-Phosphate Synthase from Pathogenic Fungi Provide Insights into Its Catalytic Mechanism, Biological Necessity, and Potential for Novel Antifungal Drug Design

The disaccharide trehalose is critical to the survival of pathogenic fungi in their human host. Trehalose-6-phosphate synthase (Tps1) catalyzes the first step of trehalose biosynthesis in fungi. Here, we report the first structures of eukaryotic Tps1s in complex with substrates or substrate analogues. The overall structures of Tps1 from Candida albicans and Aspergillus fumigatus are essentially identical and reveal N- and C-terminal Rossmann fold domains that form the glucose-6-phosphate and UDP-glucose substrate binding sites, respectively. These Tps1 structures with substrates or substrate analogues reveal key residues involved in recognition and catalysis. Disruption of these key residues severely impaired Tps1 enzymatic activity. Subsequent cellular analyses also highlight the enzymatic function of Tps1 in thermotolerance, yeast-hypha transition, and biofilm development. These results suggest that Tps1 enzymatic functionality is essential for the fungal stress response and virulence. Furthermore, structures of Tps1 in complex with the nonhydrolyzable inhibitor, validoxylamine A, visualize the transition state and support an internal return-like catalytic mechanism that is generalizable to other GT-B-fold retaining glycosyltransferases. Collectively, our results depict key Tps1-substrate interactions, unveil the enzymatic mechanism of these fungal proteins, and pave the way for high-throughput inhibitor screening buttressed and guided by the current structures and those of high-affinity ligand-Tps1 complexes.

Invasive fungal diseases have emerged as major threats, resulting in more than 1.5 million deaths annually worldwide. This epidemic has been further complicated by increasing resistance to all major classes of antifungal drugs in the clinic. Trehalose biosynthesis is essential for the fungal stress response and virulence. Critically, this biosynthetic pathway is absent in mammals, and thus, the two enzymes that carry out trehalose biosynthesis, namely, trehalose-6-phosphate synthase (Tps1) and trehalose-6-phosphate phosphatase (Tps2), are prominent targets for antifungal intervention. Here, we report the first eukaryotic Tps1 structures from the pathogenic fungi Candida albicans and Aspergillus fumigatus in complex with substrates, substrate analogues, and inhibitors. These structures reveal key protein-substrate interactions, providing atomic-level scaffolds for structure-guided drug design of novel antifungals that target Tps1.

Candida albicans Sap6 amyloid regions function in cellular aggregation and zinc binding, and contribute to zinc acquisition

Candida albicans is an opportunistic fungal pathogen colonizing the oral cavity. C. albicans secreted aspartic protease Sap6 is important for virulence during oral candidiasis since it degrades host tissues to release nutrients and essential transition metals. We found that zinc specifically increased C. albicans autoaggregation induced by Sap6; and that Sap6 itself bound zinc ions. In silico analysis of Sap6 predicted four amyloidogenic regions that were synthesized as peptides (P1-P4). All peptides, as well as full length Sap6, demonstrated amyloid properties, and addition of zinc further increased amyloid formation. Disruption of amyloid regions by Congo red significantly reduced auotoaggregation. Deletion of C. albicans genes that control zinc acquisition in the ZAP1 regulon, including zinc transporters (Pra1 and Zrt1) and other zinc-regulated surface proteins, resulted in lower autoaggregation and reduction of surface binding of Sap6. Cells with high expression of PRA1 and ZRT1 also showed increased Sap6-mediated autoaggregation. C. albicans ∆sap6 deletion mutants failed to accumulate intracellular zinc comparable to ∆zap1, ∆zrt1, and ∆pra1 cells. Thus Sap6 is a multi-functional molecule containing amyloid regions that promotes autoaggregation and zinc uptake, and may serve as an additional system for the community acquisition of zinc.

The conidial mucilage, natural film coatings, is involved in environmental adaptability and pathogenicity of Hirsutella satumaensis Aoki

The Hirsutella genus is very special asexually-reproducing pathogens of insects by reduced sporulation, host specificity and spores covered by a thick mucilage layer. However, the ecological function of conidial mucilage remains elusive. In this study, the possible ecological role of conidial mucilage from the entomopathogenic fungus Hirsutella satumaensis was functionally investigated through tolerance, adherence and insect bioassays involving aerial conidia (AC) and mucilage-free conidia (MFC). Measurements of hydrophobicity using microbial adhesion to hydrocarbons (MATH) indicated that mucilage is main contributor to the surface hydrophobicity of AC. When subjected in tolerance assays to extreme temperatures, high chemical pressure, extended exposure to ultraviolet radiation and cold stress, AC produced more colonies, exhibited higher conidiation and germination percentages than those of MFC. In adhesion assays, MFC displayed an approximately 40% reduction in adherence to locust, dragonfly cuticle and onion epidermis when washed with 0.05% Tween 20. Similarly, Galleria mellonella and Plutella xylostella larvae infected with mucilage-producing AC experienced a relatively higher mortality rate. Our findings suggest that mucilage is critical to the ecological adaptability of H. satumaensis, where it plays positive roles on maintenance of spore surface hydrophobicity, enhancement of spore resistance to extreme environments and strengthening of spore adhesion and host pathogenicity.

Thigmo Responses: The Fungal Sense of Touch

The growth and development of most fungi take place on a two-dimensional surface or within a three-dimensional matrix. The fungal sense of touch is therefore critical for fungi in the interpretation of their environment and often signals the switch to a new developmental state. Contact sensing, or thigmo-based responses, include thigmo differentiation, such as the induction of invasion structures by plant pathogens in response to topography; thigmonasty, where contact with a motile prey rapidly triggers its capture; and thigmotropism, where the direction of hyphal growth is guided by physical features in the environment. Like plants and some bacteria, fungi grow as walled cells. Despite the well-demonstrated importance of thigmo responses in numerous stages of fungal growth and development, it is not known how fungal cells sense contact through the relatively rigid structure of the cell wall. However, while sensing mechanisms at the molecular level are not entirely understood, the downstream signaling pathways that are activated by contact sensing are being elucidated. In the majority of cases, the response to contact is complemented by chemical cues and both are required, either sequentially or simultaneously, to elicit normal developmental responses. The importance of a sense of touch in the lifestyles and development of diverse fungi is highlighted in this review, and the candidate molecular mechanisms that may be involved in fungal contact sensing are discussed.

Antivirulence Properties of Probiotics in Combating Microbial Pathogenesis

Probiotics are nonpathogenic microorganisms that confer a health benefit on the host when administered in adequate amounts. Ample evidence is documented to support the potential application of probiotics for the prevention and treatment of infections. Health benefits of probiotics include prevention of diarrhea, including antibiotic-associated diarrhea and traveler's diarrhea, atopic eczema, dental carries, colorectal cancers, and treatment of inflammatory bowel disease. The cumulative body of scientific evidence that demonstrates the beneficial effects of probiotics on health and disease prevention has made probiotics increasingly important as a part of human nutrition and led to a surge in the demand for probiotics in clinical applications and as functional foods. The ability of probiotics to promote health is attributed to the various beneficial effects exerted by these microorganisms on the host. These include lactose metabolism and food digestion, production of antimicrobial peptides and control of enteric infections, anticarcinogenic properties, immunologic enhancement, enhancement of short-chain fatty acid production, antiatherogenic and cholesterol-lowering attributes, regulatory role in allergy, protection against vaginal or urinary tract infections, increased nutritional value, maintenance of epithelial integrity and barrier, stimulation of repair mechanism in cells, and maintenance and reestablishment of well-balanced indigenous intestinal and respiratory microbial communities. Most of these attributes primarily focus on the effect of probiotic supplementation on the host. Hence, in most cases, it can be concluded that the ability of a probiotic to protect the host from infection is an indirect result of promoting overall health and well-being. However, probiotics also exert a direct effect on invading microorganisms. The direct modes of action resulting in the elimination of pathogens include inhibition of pathogen replication by producing antimicrobial substances like bacteriocins, competition for limiting resources in the host, antitoxin effect, inhibition of virulence, antiadhesive and antiinvasive effects, and competitive exclusion by competition for binding sites or stimulation of epithelial barrier function. Although much has been documented about the ability of probiotics to promote host health, there is limited discussion on the above mentioned effects of probiotics on pathogens. Being in an era of antibiotic resistance, a better understanding of this complex probiotic-pathogen interaction is critical for development of effective strategies to control infections. Therefore, this chapter will focus on the ability of probiotics to directly modulate the infectious nature of pathogens and the underlying mechanisms that mediate these effects.

Anti-Immune Strategies of Pathogenic Fungi

Pathogenic fungi have developed many strategies to evade the host immune system. Multiple escape mechanisms appear to function together to inhibit attack by the various stages of both the adaptive and the innate immune response. Thus, after entering the host, such pathogens fight to overcome the immune system to allow their survival, colonization and spread to different sites of infection. Consequently, the establishment of a successful infectious process is closely related to the ability of the pathogen to modulate attack by the immune system. Most strategies employed to subvert or exploit the immune system are shared among different species of fungi. In this review, we summarize the main strategies employed for immune evasion by some of the major pathogenic fungi.

Candida albicans glutathione reductase downregulates Efg1-mediated cyclic AMP/protein kinase A pathway and leads to defective hyphal growth and virulence upon decreased cellular methylglyoxal content accompanied by activating alcohol dehydrogenase and glycolytic enzymes

BACKGROUND

Glutathione reductase maintains the glutathione level in a reduced state. As previously demonstrated, glutathione is required for cell growth/division and its biosynthesizing-enzyme deficiency causes methylglyoxal accumulation. However, experimental evidences for reciprocal relationships between Cph1-/Efg1-mediated signaling pathway regulation and methylglyoxal production exerted by glutathione reductase on yeast morphology remain unclear.

METHODS

Glutathione reductase (GLR1) disruption/overexpression were performed to investigate aspects of pathological/morphological alterations in Candida albicans. These assumptions were proved by observations of cellular susceptibility to oxidants and thiols, and measurements of methylglyoxal and glutathione content in hyphal-inducing conditions mainly through the activity of GLR1-overexpressing cells. Additionally, the transcriptional/translational levels of bioenergetic enzymes and dimorphism-regulating protein kinases were examined in the strain.

RESULTS

The GLR1-deficient strain was non-viable when GLR1 expression under the control of a CaMAL2 promoter was conditionally repressed, despite partial rescue of growth by exogenous thiols. During filamentation, non-growing hyphal GLR1-overexpressing cells exhibited resistance against oxidants and cellular methylglyoxal was significantly decreased, which concomitantly increased expressions of genes encoding energy-generating enzymes, including fructose-1,6-bisphosphate aldolase, glyceraldehyde-3-phosphate dehydrogenase, and alcohol dehydrogenase (ADH1), with remarkable repression of Efg1-signaling cascades.

CONCLUSIONS

This is the first report that GLR1-triggered Efg1-mediated signal transduction repression strictly reduces dimorphic switching and virulence by maintaining the basal level of methylglyoxal following the enhanced gene expressions of glycolytic enzymes and ADH1.

GENERAL SIGNIFICANCE

The Efg1 downregulatory mechanism by GLR1 expression has possibilities to involve in other complex network of signal pathways. Understanding how GLR1 overexpression affects multiple signaling pathways can help identify attractive targets for antifungal drugs.

Draft Genome of the Wheat Rust Pathogen (Puccinia triticina) Unravels Genome-Wide Structural Variations during Evolution

Leaf rust is one of the most important diseases of wheat and is caused by Puccinia triticina, a highly variable rust pathogen prevalent worldwide. Decoding the genome of this pathogen will help in unraveling the molecular basis of its evolution and in the identification of genes responsible for its various biological functions. We generated high quality draft genome sequences (approximately 100- 106 Mb) of two races of P. triticina; the variable and virulent Race77 and the old, avirulent Race106. The genomes of races 77 and 106 had 33X and 27X coverage, respectively. We predicted 27678 and 26384 genes, with average lengths of 1,129 and 1,086 bases in races 77 and 106, respectively and found that the genomes consisted of 37.49% and 39.99% repetitive sequences. Genome wide comparative analysis revealed that Race77 differs substantially from Race106 with regard to segmental duplication (SD), repeat element, and SNP/InDel characteristics. Comparative analyses showed that Race 77 is a recent, highly variable and adapted Race compared with Race106. Further sequence analyses of 13 additional pathotypes of Race77 clearly differentiated the recent, active and virulent, from the older pathotypes. Average densities of 2.4 SNPs and 0.32 InDels per kb were obtained for all P. triticina pathotypes. Secretome analysis demonstrated that Race77 has more virulence factors than Race 106, which may be responsible for the greater degree of adaptation of this pathogen. We also found that genes under greater selection pressure were conserved in the genomes of both races, and may affect functions crucial for the higher levels of virulence factors in Race77. This study provides insights into the genome structure, genome organization, molecular basis of variation, and pathogenicity of P. triticina The genome sequence data generated in this study have been submitted to public domain databases and will be an important resource for comparative genomics studies of the more than 4000 existing Puccinia species.

Plasticity of Candida albicans Biofilms

Candida albicans, the most pervasive fungal pathogen that colonizes humans, forms biofilms that are architecturally complex. They consist of a basal yeast cell polylayer and an upper region of hyphae encapsulated in extracellular matrix. However, biofilms formed in vitro vary as a result of the different conditions employed in models, the methods used to assess biofilm formation, strain differences, and, in a most dramatic fashion, the configuration of the mating type locus (MTL). Therefore, integrating data from different studies can lead to problems of interpretation if such variability is not taken into account. Here we review the conditions and factors that cause biofilm variation, with the goal of engendering awareness that more attention must be paid to the strains employed, the methods used to assess biofilm development, every aspect of the model employed, and the configuration of the MTL locus. We end by posing a set of questions that may be asked in comparing the results of different studies and developing protocols for new ones. This review should engender the notion that not all biofilms are created equal.

The Evolving View of IL-17-Mediated Immunity in Defense Against Mucocutaneous Candidiasis in Humans

The discovery of interleukin (IL)-17-mediated immunity has provided a robust framework upon which our current understanding of the mechanism involved in host defense against mucocutaneous candidiasis (CMC) has been built. Studies have shed light on how pattern recognition receptors expressed by innate immune cells recognize various components of Candida cell wall. Inborn errors of immunity affecting IL-17+ T cell differentiation have recently been defined, such as deficiencies of signal transducer and activator of transcription (STAT)3, STAT1, IL-12Rβ1 and IL-12p40, and caspase recruitment domain 9. Impaired receptor-ligand coupling was identified in patients with IL-17F and IL-17 receptor A (IL17RA) deficiency and autoimmune polyendocrine syndrome (APS) type 1. Mutation in the nuclear factor kappa B activator (ACT) 1 was described as a cause of impaired IL-17R-mediated signaling. CMC may be part of a complex clinical phenotype like in patients with deficiencies of STAT3, IL-12Rβ1/IL-12p40 and APS-1 or may be the only or dominant phenotypic manifestation of disease which is referred to as CMC disease. CMCD may result from deficiencies of STAT1, IL-17F, IL-17RA and ACT1. In this review we discuss how recent research on IL-17-mediated immunity shed light on host defense against mucocutaneous infection by Candida and how the discovery of various germ-line mutations and the characterization of associated clinical phenotypes have provided insights into the role of CD4+IL-17+ lymphocytes in the regulation of anticandidal defense of body surfaces.

Effect of long-term exposure to mobile phone radiation on alpha-Int1 gene sequence of Candida albicans

Over the last decade, communication industries have witnessed a tremendous expansion, while, the biological effects of electromagnetic waves have not been fully elucidated. Current study aimed at evaluating the mutagenic effect of long-term exposure to 900-MHz radiation on alpha-Int1 gene sequences of Candida albicans. A standard 900 MHz radiation generator was used for radiation. 10 ml volumes from a stock suspension of C. albicans were transferred into 10 polystyrene tubes. Five tubes were exposed at 4 °C to a fixed magnitude of radiation with different time periods of 10, 70, 210, 350 and 490 h. The other 5 tubes were kept far enough from radiation. The samples underwent genomic DNA extraction. PCR amplification of alpha-Int1 gene sequence was done using one set of primers. PCR products were resolved using agarose gel electrophoresis and the nucleotide sequences were determined. All samples showed a clear electrophoretic band around 441 bp and further sequencing revealed the amplified DNA segments are related to alpha-Int1 gene of the yeast. No mutations in the gene were seen in radiation exposed samples. Long-term exposure of the yeast to mobile phone radiation under the above mentioned conditions had no mutagenic effect on alpha-Int1 gene sequence.

Novel Aggregation Properties of Candida albicans Secreted Aspartyl Proteinase Sap6 Mediate Virulence in Oral Candidiasis

Candida albicans, a commensal fungus of the oral microbiome, causes oral candidiasis in humans with localized or systemic immune deficiencies. Secreted aspartic proteinases (Saps) are a family of 10 related proteases and are virulence factors due to their proteolytic activity, as well as their roles in adherence and colonization of host tissues. We found that mice infected sublingually with C. albicans cells overexpressing Sap6 (SAP6 OE and a Δsap8 strain) had thicker fungal plaques and more severe oral infection, while infection with the Δsap6 strain was attenuated. These hypervirulent strains had highly aggregative colony structure in vitro and higher secreted proteinase activity; however, the levels of proteinase activity of C. albicans Saps did not uniformly match their abilities to damage cultured oral epithelial cells (SCC-15 cells). Hyphal induction in cells overexpressing Sap6 (SAP6 OE and Δsap8 cells) resulted in formation of large cell-cell aggregates. These aggregates could be produced in germinated wild-type cells by addition of native or heat-inactivated Sap6. Sap6 bound only to germinated cells and increased C. albicans adhesion to oral epithelial cells. The adhesion properties of Sap6 were lost upon deletion of its integrin-binding motif (RGD) and could be inhibited by addition of RGD peptide or anti-integrin antibodies. Thus, Sap6 (but not Sap5) has an alternative novel function in cell-cell aggregation, independent of its proteinase activity, to promote infection and virulence in oral candidiasis.

Evidence for a Pneumocystis carinii Flo8-like transcription factor: insights into organism adhesion

Pneumocystis carinii (Pc) adhesion to alveolar epithelial cells is well established and is thought to be a prerequisite for the initiation of Pneumocystis pneumonia. Pc binding events occur in part through the major Pc surface glycoprotein Msg, as well as an integrin-like molecule termed PcInt1. Recent data from the Pc sequencing project also demonstrate DNA sequences homologous to other genes important in Candida spp. binding to mammalian host cells, as well as organism binding to polystyrene surfaces and in biofilm formation. One of these genes, flo8, a transcription factor needed for downstream cAMP/PKA-pathway-mediated activation of the major adhesion/flocculin Flo11 in yeast, was cloned from a Pc cDNA library utilizing a partial sequence available in the Pc genome database. A CHEF blot of Pc genomic DNA yielded a single band providing evidence this gene is present in the organism. BLASTP analysis of the predicted protein demonstrated 41 % homology to the Saccharomyces cerevisiae Flo8. Northern blotting demonstrated greatest expression at pH 6.0-8.0, pH comparable to reported fungal biofilm milieu. Western blot and immunoprecipitation assays of PcFlo8 protein in isolated cyst and tropic life forms confirmed the presence of the cognate protein in these Pc life forms. Heterologous expression of Pcflo8 cDNA in flo8Δ-deficient yeast strains demonstrated that the Pcflo8 was able to restore yeast binding to polystyrene and invasive growth of yeast flo8Δ cells. Furthermore, Pcflo8 promoted yeast binding to HEK293 human epithelial cells, strengthening its functional classification as a Flo8 transcription factor. Taken together, these data suggest that PcFlo8 is expressed by Pc and may exert activity in organism adhesion and biofilm formation.

Analysis of the Candida albicans Phosphoproteome

Candida albicans is an important human fungal pathogen in both immunocompetent and immunocompromised individuals. C. albicans regulation has been studied in many contexts, including morphological transitions, mating competence, biofilm formation, stress resistance, and cell wall synthesis. Analysis of kinase- and phosphatase-deficient mutants has made it clear that protein phosphorylation plays an important role in the regulation of these pathways. In this study, to further our understanding of phosphorylation in C. albicans regulation, we performed a deep analysis of the phosphoproteome in C. albicans. We identified 19,590 unique peptides that corresponded to 15,906 unique phosphosites on 2,896 proteins. The ratios of serine, threonine, and tyrosine phosphosites were 80.01%, 18.11%, and 1.81%, respectively. The majority of proteins (2,111) contained at least two detected phosphorylation sites. Consistent with findings in other fungi, cytoskeletal proteins were among the most highly phosphorylated proteins, and there were differences in Gene Ontology (GO) terms for proteins with serine and threonine versus tyrosine phosphorylation sites. This large-scale analysis identified phosphosites in protein components of Mediator, an important transcriptional coregulatory protein complex. A targeted analysis of the phosphosites in Mediator complex proteins confirmed the large-scale studies, and further in vitro assays identified a subset of these phosphorylations that were catalyzed by Cdk8 (Ssn3), a kinase within the Mediator complex. These data represent the deepest single analysis of a fungal phosphoproteome and lay the groundwork for future analyses of the C. albicans phosphoproteome and specific phosphoproteins.

Mannan-binding lectin inhibits Candida albicans-induced cellular responses in PMA-activated THP-1 cells through Toll-like receptor 2 and Toll-like receptor 4

Background

Candida albicans (C. albicans), the most common human fungal pathogen, can cause fatal systemic infections under certain circumstances. Mannan-binding lectin (MBL),a member of the collectin family in the C-type lectin superfamily, is an important serum component associated with innate immunity. Toll-like receptors (TLRs) are expressed extensively, and have been shown to be involved in C. albicans-induced cellular responses. We first examined whether MBL modulated heat-killed (HK) C. albicans-induced cellular responses in phorbol 12-myristate 13-acetate (PMA)-activated human THP-1 macrophages. We then investigated the possible mechanisms of its inhibitory effect.

Methodology/Principal Finding

Enzyme-linked immunosorbent assay (ELISA) and reverse transcriptasepolymerase chain reaction (RT-PCR) analysis showed that MBL at higher concentrations (10–20 µg/ml) significantly attenuated C. albicans-induced chemokine (e.g., IL-8) and proinflammatory cytokine (e.g., TNF-α) production from PMA-activated THP-1 cells at both protein and mRNA levels. Electrophoretic mobility shift assay (EMSA) and Western blot (WB) analysis showed that MBL could inhibit C. albicans-induced nuclear factor-κB (NF-κB) DNA binding and its translocation in PMA-activated THP-1 cells. MBL could directly bind to PMA-activated THP-1 cells in the presence of Ca²⁺, and this binding decreased TLR2 and TLR4 expressions in C. albicans-induced THP-1 macrophages. Furthermore, the binding could be partially inhibited by both anti-TLR2 monoclonal antibody (clone TL2.1) and anti-TLR4 monoclonal antibody (clone HTA125). In addition, co-immunoprecipitation experiments and microtiter wells assay showed that MBL could directly bind to the recombinant soluble form of extracellular TLR2 domain (sTLR2) and sTLR4.

Conclusions/Significance

Our study demonstrates that MBL can affect proinflammatory cytokine and chemokine expressions by modifying C. albicans-/TLR-signaling pathways. This study supports an important role for MBL on the regulation of C. albicans-induced cellular responses.

Heparin-binding motifs and biofilm formation by Candida albicans

Candida albicans is a leading pathogen in infections of central venous catheters, which are frequently infused with heparin. Binding of C. albicans to medically relevant concentrations of soluble and plate-bound heparin was demonstrable by confocal microscopy and enzyme-linked immunosorbent assay (ELISA). A sequence-based search identified 34 C. albicans surface proteins containing ≥1 match to linear heparin-binding motifs. The virulence factor Int1 contained the most putative heparin-binding motifs (n = 5); peptides encompassing 2 of 5 motifs bound to heparin-Sepharose. Alanine substitution of lysine residues K805/K806 in 804QKKHQIHK811 (motif 1 of Int1) markedly attenuated biofilm formation in central venous catheters in rats, whereas alanine substitution of K1595/R1596 in 1593FKKRFFKL1600 (motif 4 of Int1) did not impair biofilm formation. Affinity-purified immunoglobulin G (IgG) recognizing motif 1 abolished biofilm formation in central venous catheters; preimmune IgG had no effect. After heparin treatment of C. albicans, soluble peptides from multiple C. albicans surface proteins were detected, such as Eno1, Pgk1, Tdh3, and Ssa1/2 but not Int1, suggesting that heparin changes candidal surface structures and may modify some antigens critical for immune recognition. These studies define a new mechanism of biofilm formation for C. albicans and a novel strategy for inhibiting catheter-associated biofilms.

Structure and function of a fungal adhesin that binds heparin and mimics thrombospondin-1 by blocking T cell activation and effector function

Blastomyces adhesin-1 (BAD-1) is a 120-kD surface protein on B. dermatitidis yeast. We show here that BAD-1 contains 41 tandem repeats and that deleting even half of them impairs fungal pathogenicity. According to NMR, the repeats form tightly folded 17-amino acid loops constrained by a disulfide bond linking conserved cysteines. Each loop contains a highly conserved WxxWxxW motif found in thrombospondin-1 (TSP-1) type 1 heparin-binding repeats. BAD-1 binds heparin specifically and saturably, and is competitively inhibited by soluble heparin, but not related glycosaminoglycans. According to SPR analysis, the affinity of BAD-1 for heparin is 33 nM±14 nM. Putative heparin-binding motifs are found both at the N-terminus and within each tandem repeat loop. Like TSP-1, BAD-1 blocks activation of T cells in a manner requiring the heparan sulfate-modified surface molecule CD47, and impairs effector functions. The tandem repeats of BAD-1 thus confer pathogenicity, harbor motifs that bind heparin, and suppress T-cell activation via a CD47-dependent mechanism, mimicking mammalian TSP-1.

Differential virulence of Candida glabrata glycosylation mutants

The fungus Candida glabrata is an important and increasingly common pathogen of humans, particularly in immunocompromised hosts. Despite this, little is known about the attributes that allow this organism to cause disease or its interaction with the host immune system. However, in common with other fungi, the cell wall of C. glabrata is the initial point of contact between the host and pathogen, and as such, it is likely to play an important role in mediating interactions and hence virulence. Here, we show both through genetic complementation and polysaccharide structural analyses that C. glabrata ANP1, MNN2, and MNN11 encode functional orthologues of the respective Saccharomyces cerevisiae mannosyltransferases. Furthermore, we show that deletion of the C. glabrata Anp1, Mnn2, and Mnn11 mannosyltransferases directly affects the structure of the fungal N-linked mannan, in line with their predicted functions, and this has implications for cell wall integrity and consequently virulence. C. glabrata anp1 and mnn2 mutants showed increased virulence, compared with wild-type (and mnn11) cells. This is in contrast to Candida albicans where inactivation of genes involved in mannan biosynthesis has usually been linked to an attenuation of virulence. In the long term, a better understanding of the attributes that allow C. glabrata to cause disease will provide insights that can be adopted for the development of novel therapeutic and diagnostic approaches.