Candida albicans cellular internalization: a new pathogenic factor?

Trans-cellular tunnels induced by the fungal pathogen Candida albicans facilitate invasion through successive epithelial cells without host damage

The opportunistic fungal pathogen Candida albicans is normally commensal, residing in the mucosa of most healthy individuals. In susceptible hosts, its filamentous hyphal form can invade epithelial layers leading to superficial or severe systemic infection. Although invasion is mainly intracellular, it causes no apparent damage to host cells at early stages of infection. Here, we investigate C. albicans invasion in vitro using live-cell imaging and the damage-sensitive reporter galectin-3. Quantitative single cell analysis shows that invasion can result in host membrane breaching at different stages and host cell death, or in traversal of host cells without membrane breaching. Membrane labelling and three-dimensional 'volume' electron microscopy reveal that hyphae can traverse several host cells within trans-cellular tunnels that are progressively remodelled and may undergo 'inflations' linked to host glycogen stores. Thus, C. albicans early invasion of epithelial tissues can lead to either host membrane breaching or trans-cellular tunnelling.

Candida albicans Virulence Factors and Pathogenicity for Endodontic Infections

Candida albicans (C. albicans) is the fungus most frequently isolated from endodontic root canal infections. Although recognized by dental pulp and periradicular tissue cells that elicit immune responses, it eludes host defenses and elicits cell death. Then, C. albicans binds tooth dentin, forms biofilms, and invades dentinal tubules to resist intracanal disinfectants and endodontic treatments. Insensitive to most common medicaments, it survives sequestered within biofilms and intratubular dentin. Thus, C. albicans has been associated with cases of persistent or refractory root canal infections. Its treatment strategies may require alternative intracanal irrigants, intracanal medicaments such as chlorhexidine gel or human beta defensin-3 (HBD3), Ca-Si-based obturating materials, and microsurgical procedures.

Cell culture models of oral mucosal barriers: A review with a focus on applications, culture conditions and barrier properties

Understanding the function of oral mucosal epithelial barriers is essential for a plethora of research fields such as tumor biology, inflammation and infection diseases, microbiomics, pharmacology, drug delivery, dental and biomarker research. The barrier properties are comprised by a physical, a transport and a metabolic barrier, and all these barrier components play pivotal roles in the communication between saliva and blood. The sum of all epithelia of the oral cavity and salivary glands is defined as the blood-saliva barrier. The functionality of the barrier is regulated by its microenvironment and often altered during diseases. A huge array of cell culture models have been developed to mimic specific parts of the blood-saliva barrier, but no ultimate standard in vitro models have been established. This review provides a comprehensive overview about developed in vitro models of oral mucosal barriers, their applications, various cultivation protocols and corresponding barrier properties.

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.

Candida-Epithelial Interactions

A plethora of intricate and dynamic molecular interactions occur between microbes and the epithelial cells that form the mucosal surfaces of the human body. Fungi, particularly species of Candida, are commensal members of our microbiota, continuously interacting with epithelial cells. Transient and localised perturbations to the mucosal environment can facilitate the overgrowth of fungi, causing infection. This minireview will examine the direct and indirect mechanisms by which Candida species and epithelial cells interact with each other, and explore the factors involved in the central processes of adhesion, invasion, and destruction of host mucosal surfaces.

Biomedical applications of nisin

Nisin is a bacteriocin produced by a group of Gram-positive bacteria that belongs to Lactococcus and Streptococcus species. Nisin is classified as a Type A (I) lantibiotic that is synthesized from mRNA and the translated peptide contains several unusual amino acids due to post-translational modifications. Over the past few decades, nisin has been used widely as a food biopreservative. Since then, many natural and genetically modified variants of nisin have been identified and studied for their unique antimicrobial properties. Nisin is FDA approved and generally regarded as a safe peptide with recognized potential for clinical use. Over the past two decades the application of nisin has been extended to biomedical fields. Studies have reported that nisin can prevent the growth of drug-resistant bacterial strains, such as methicillin-resistant Staphylococcus aureus, Streptococcus pneumoniae, Enterococci and Clostridium difficile. Nisin has now been shown to have antimicrobial activity against both Gram-positive and Gram-negative disease-associated pathogens. Nisin has been reported to have anti-biofilm properties and can work synergistically in combination with conventional therapeutic drugs. In addition, like host-defence peptides, nisin may activate the adaptive immune response and have an immunomodulatory role. Increasing evidence indicates that nisin can influence the growth of tumours and exhibit selective cytotoxicity towards cancer cells. Collectively, the application of nisin has advanced beyond its role as a food biopreservative. Thus, this review will describe and compare studies on nisin and provide insight into its future biomedical applications.

Inhibition of Candida albicans biofilm formation and modulation of gene expression by probiotic cells and supernatant

Oral candidiasis is a disease caused by opportunistic species of Candida that normally reside on human mucosal surfaces. The transition of Candida from budding yeast to filamentous hyphae allows for covalent attachment to oral epithelial cells, followed by biofilm formation, invasion and tissue damage. In this study, combinations of Lactobacillus plantarum SD5870, Lactobacillus helveticus CBS N116411 and Streptococcus salivarius DSM 14685 were assessed for their ability to inhibit the formation of and disrupt Candida albicans biofilms. Co-incubation with probiotic supernatants under hyphae-inducing conditions reduced C. albicans biofilm formation by >75 % in all treatment groups. Likewise, combinations of live probiotics reduced biofilm formation of C. albicans by >67 %. When live probiotics or their supernatants were overlaid on preformed C. albicans biofilms, biofilm size was reduced by >63 and >65 % respectively. Quantitative real-time PCR results indicated that the combined supernatants of SD5870 and CBS N116411 significantly reduced the expression of several C. albicans genes involved in the yeast-hyphae transition: ALS3 (adhesin/invasin) by 70 % (P < 0.0001), EFG1 (hyphae-specific gene activator) by 47 % (P = 0.0061), SAP5 (secreted protease) by 49 % (P < 0.0001) and HWP1 (hyphal wall protein critical to biofilm formation) by >99 % (P < 0.0001). These findings suggest the combination of L. plantarum SD5870, L. helveticus CBS N116411 and S. salivarius DSM 14685 is effective at both preventing the formation of and removing preformed C. albicans biofilms. Our novel results point to the downregulation of several Candida genes critical to the yeast-hyphae transition, biofilm formation, tissue invasion and cellular damage.

Coriandrum sativum L. (Coriander) essential oil: antifungal activity and mode of action on Candida spp., and molecular targets affected in human whole-genome expression

Oral candidiasis is an opportunistic fungal infection of the oral cavity with increasingly worldwide prevalence and incidence rates. Novel specifically-targeted strategies to manage this ailment have been proposed using essential oils (EO) known to have antifungal properties. In this study, we aim to investigate the antifungal activity and mode of action of the EO from Coriandrum sativum L. (coriander) leaves on Candida spp. In addition, we detected the molecular targets affected in whole-genome expression in human cells. The EO phytochemical profile indicates monoterpenes and sesquiterpenes as major components, which are likely to negatively impact the viability of yeast cells. There seems to be a synergistic activity of the EO chemical compounds as their isolation into fractions led to a decreased antimicrobial effect. C. sativum EO may bind to membrane ergosterol, increasing ionic permeability and causing membrane damage leading to cell death, but it does not act on cell wall biosynthesis-related pathways. This mode of action is illustrated by photomicrographs showing disruption in biofilm integrity caused by the EO at varied concentrations. The EO also inhibited Candida biofilm adherence to a polystyrene substrate at low concentrations, and decreased the proteolytic activity of Candida albicans at minimum inhibitory concentration. Finally, the EO and its selected active fraction had low cytotoxicity on human cells, with putative mechanisms affecting gene expression in pathways involving chemokines and MAP-kinase (proliferation/apoptosis), as well as adhesion proteins. These findings highlight the potential antifungal activity of the EO from C. sativum leaves and suggest avenues for future translational toxicological research.

Candida albicans-epithelial interactions: dissecting the roles of active penetration, induced endocytosis and host factors on the infection process

Candida albicans frequently causes superficial infections by invading and damaging epithelial cells, but may also cause systemic infections by penetrating through epithelial barriers. C. albicans is a remarkable pathogen because it can invade epithelial cells via two distinct mechanisms: induced endocytosis, analogous to facultative intracellular enteropathogenic bacteria, and active penetration, similar to plant pathogenic fungi. Here we investigated the contributions of the two invasion routes of C. albicans to epithelial invasion. Using selective cellular inhibition approaches and differential fluorescence microscopy, we demonstrate that induced endocytosis contributes considerably to the early time points of invasion, while active penetration represents the dominant epithelial invasion route. Although induced endocytosis depends mainly on Als3-E–cadherin interactions, we observed E–cadherin independent induced endocytosis. Finally, we provide evidence of a protective role for serum factors in oral infection: human serum strongly inhibited C. albicans adhesion to, invasion and damage of oral epithelial cells.

Trafficking of Candida albicans through oral epithelial endocytic compartments

Oral epithelial cells are the first cells that interact with C. albicans during the establishment of oropharyngeal candidiasis. Following initial adhesion, C. albicans invades oral epithelial cells by inducing its own endocytosis and gains access to epithelial vacuolar compartments. Epithelial endocytic pathways are key innate immune mechanisms in host defense. We examined the trafficking of C. albicans through oral epithelial endocytic compartments. We present evidence that C. albicans is internalized by oral epithelial cells through actin-dependent clathrin-mediated endocytosis and is taken into vacuolar compartments immediately following its internalization. C. albicans-containing endosomes transiently acquired early endosomal marker EEA1, but showed marked defects in acquisition of late endosomal marker LAMP1 and lysosomal marker cathepsin D. Defective endolysosomal maturation may partially explain the inability of oral epithelial cells to kill C. albicans.

A review of the ultrastructural features of superficial candidiasis

Commensal yeast Candida causes opportunistic infections ranging from superficial lesions to disseminated mycoses in compromised patients. Superficial candidiasis, the commonest form of candidal infections, primarily affects the mucosa and the skin where Candida lives as a commensal. Conversion of candidal commensalism into opportunism at the fungal-epithelial interface is still ill-defined. Nevertheless, fungal virulence mechanisms such as adhesion to epithelia, morphogenesis, production of secretory hydrolytic enzymes, and phenotypic switching are thought to contribute in the process of pathogenesis. On the other hand, host responses in terms of immunity and local epithelial responses are actively involved in resisting the fungal challenge at the advancing front of the infection. Ultrastructural investigations using electron microscopy along with immunohistochemistry, cytochemistry, etc. have helped better viewing of Candida-host interactions. Thus, studies on the ultrastructure of superficial candidiasis have revealed a number of fungal behaviors and associated host responses such as adhesion, morphogenesis (hyphae and appresoria formation), thigmotropism, production and distribution of extracellular enzymes, phagocytosis, and epithelial changes. The purpose of this review is to sum up most of the ultrastructural findings of Candida-host interactions and to delineate the important pathological processes underlying superficial candidiasis.

Ultrastructural imaging of Candida albicans adhesion to rat genital epithelium through scanning and transmission electron microscopy

The adhesion of Candida albicans to the genital epithelium has not been fully investigated in vivo. The objective of this study was to evaluate the ultrastructural aspects of C. albicans adhesion in the lower genital system of female Wistar rats through scanning and transmission electron microscopy. The genital infection persisted until the end of the experiment, and all rats showed the same adhesion aspects. Various associated yeast/hyphae were observed in the lumen and adhered both at the vaginal and endocervical levels where the fungal filamentation process occurred. In the vaginal epithelium, closely adhered yeasts were observed as stretched strands bridging between yeasts and the epithelium surface. Different stages of the adhesion, where yeasts internalized into the epithelial cell inside a cytoplasmic vacuole, resembling endocytosis, and a wide fibrillar-floccular, glycocalyx-like layer on the yeasts were observed. On the endocervix, the adhesion occurred between the cilia. In the uterine body, only a yeast-like form was observed with superficial contact. This study reached the initial goal of demonstrating an experimental model for in vivo studies. Continuation of this line of research is important for studies of vulvovaginal candidiasis.

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.

Interactions of Candida albicans with epithelial cells

The fungus, Candida albicans, interacts with epithelial cells in the human host both as a normal commensal and as an invasive pathogen. It has evolved multiple complementary mechanisms to adhere to epithelial cells. Adherent C. albicans cells can invade epithelial surfaces both by penetrating into individual epithelial cells, and by degrading interepithelial cell junctions and passing between epithelial cells. Invasion into epithelial cells is mediated by both induced endocytosis and active penetration, whereas degradation of epithelial cell junction proteins, such as E-cadherin, occurs mainly via proteolysis by secreted aspartyl proteinases. C. albicans invasion of epithelial cells results in significant epithelial cell damage, which is probably induced by lytic enzymes, such as proteases and phospholipase secreted by the organism. Future challenges include identifying the epithelial cell targets of adhesins and invasins, and determining the mechanisms by which C. albicans actively penetrates epithelial cells and induces epithelial cell damage.

Candida albicans internalization by host cells is mediated by a clathrin-dependent mechanism

Candida albicans is a major cause of oropharyngeal, vulvovaginal and haematogenously disseminated candidiasis. Endocytosis of C. albicans hyphae by host cells is a prerequisite for tissue invasion. This internalization involves interactions between the fungal invasin Als3 and host E- or N-cadherin. Als3 shares some structural similarity with InlA, a major invasion protein of the bacterium Listeria monocytogenes. InlA mediates entry of L. monocytogenes into host cells through binding to E-cadherin. A role in internalization, for a non-classical stimulation of the clathrin-dependent endocytosis machinery, was recently highlighted. Based on the similarities between the C. albicans and L. monocytogenes invasion proteins, we studied the role of clathrin in the internalization of C. albicans. Using live-cell imaging and indirect immunofluorescence of epithelial cells infected with C. albicans, we observed that host E-cadherin, clathrin, dynamin and cortactin accumulated at sites of C. albicans internalization. Similarly, in endothelial cells, host N-cadherin, clathrin and cortactin accumulated at sites of fungal endocytosis. Furthermore, clathrin, dynamin or cortactin depletion strongly inhibited C. albicans internalization by epithelial cells. Finally, beads coated with Als3 were internalized in a clathrin-dependent manner. These data indicate that C. albicans, like L. monocytogenes, hijacks the clathrin-dependent endocytic machinery to invade host cells.

Association of small Rho GTPases and actin ring formation in epithelial cells during the invasion by Candida albicans

Invasion of epithelial cells is a major virulence determinant of Candida albicans; however, the molecular events that occur during invasion are not discerned. This study is aimed to elucidate the role of the host's actin remodeling and involvement of small GTPases during invasion. Actin filaments formed a rigid ring-like structure in the rabbit corneal epithelial cell line SIRC after C. albicans invasion. During invasion, an increase in the mRNA content of Cdc42, Rac1 and RhoA GTPase was observed in SIRC cells. Immunochemical staining and expression of chimeric green fluorescent protein (GFP)-GTPases showed that all three GTPases colocalize at invasion and actin polymerization sites. This colocalization was not seen in SIRC cells expressing a GFP-tagged dominant-negative mutant of GTPases. Inhibition of invasion was observed in SIRC cells expressing dominant-negative mutants of Rac1 and RhoA GTPases. Involvement of zonula occludens-1 (ZO-1) was observed in the process of actin-mediated endocytosis of C. albicans. Actin, GTPases and ZO-1 were colocalized in epithelial cells during uptake of polymethylmethacrylate beads coated with spent medium from a C. albicans culture. The results indicate that host actin remodeling and recruitment of small GTPases occur during invasion and molecules that are shed or secreted by C. albicans are probably responsible for cytoskeletal reorganization.

Candida albicans transcription factor Rim101 mediates pathogenic interactions through cell wall functions

pH-responsive transcription factors of the Rim101/PacC family govern virulence in many fungal pathogens. These family members control expression of target genes with diverse functions in growth, morphology and environmental adaptation, so the mechanistic relationship between Rim101/PacC and infection is unclear. We have focused on Rim101 from Candida albicans, which we find to be required for virulence in an oropharyngeal candidiasis model. Rim101 affects the yeast-hypha morphological transition, a major virulence requirement in disseminated infection models. However, virulence in the oropharyngeal candidiasis model is independent of the yeast-hypha transition because it is unaffected by an nrg1 mutation, which prevents formation of yeast cells. Here we have identified Rim101 target genes in an nrg1Delta/Delta mutant background and surveyed function using an overexpression-rescue approach. Increased expression of Rim101 target genes ALS3, CHT2, PGA7/RBT6, SKN1 or ZRT1 can partially restore pathogenic interaction of a rim101Delta/Delta mutant with oral epithelial cells. Four of these five genes govern cell wall structure. Our results indicate that Rim101-dependent cell wall alteration contributes to C. albicans pathogenic interactions with oral epithelial cells, independently of cell morphology.

Als3 is a Candida albicans invasin that binds to cadherins and induces endocytosis by host cells

Candida albicans is the most common cause of hematogenously disseminated and oropharyngeal candidiasis. Both of these diseases are characterized by fungal invasion of host cells. Previously, we have found that C. albicans hyphae invade endothelial cells and oral epithelial cells in vitro by inducing their own endocytosis. Therefore, we set out to identify the fungal surface protein and host cell receptors that mediate this process. We found that the C. albicans Als3 is required for the organism to be endocytosed by human umbilical vein endothelial cells and two different human oral epithelial lines. Affinity purification experiments with wild-type and an als3Δ/als3Δ mutant strain of C. albicans demonstrated that Als3 was required for C. albicans to bind to multiple host cell surface proteins, including N-cadherin on endothelial cells and E-cadherin on oral epithelial cells. Furthermore, latex beads coated with the recombinant N-terminal portion of Als3 were endocytosed by Chinese hamster ovary cells expressing human N-cadherin or E-cadherin, whereas control beads coated with bovine serum albumin were not. Molecular modeling of the interactions of the N-terminal region of Als3 with the ectodomains of N-cadherin and E-cadherin indicated that the binding parameters of Als3 to either cadherin are similar to those of cadherin–cadherin binding. Therefore, Als3 is a fungal invasin that mimics host cell cadherins and induces endocytosis by binding to N-cadherin on endothelial cells and E-cadherin on oral epithelial cells. These results uncover the first known fungal invasin and provide evidence that C. albicans Als3 is a molecular mimic of human cadherins.

The fungus Candida albicans is usually a harmless colonizer of human mucosal surfaces. In the mouth, it can cause oropharyngeal candidiasis, also called thrush. In hospitalized and immunocompromised patients, C. albicans can enter the blood stream and be carried throughout the body to cause a disseminated infection, which is associated with a mortality rate of up to 40%. The organism invades the epithelial cell lining of the mouth during oropharyngeal candidiasis and invades the endothelial cell lining of the blood vessels during disseminated candidiasis. We discovered that Als3, a protein expressed on the surface of C. albicans, is required for this invasion process. Cadherins on the surface of human cells normally bind other cadherins for adhesion and signaling; however, we found that Als3 also binds to cadherins on endothelial cells and oral epithelial cells, and this binding induces these host cells to take up the fungus. The structure of Als3 is predicted to be quite similar to that of the two cadherins studied, and the parameters of the binding of Als3 to either cadherin are similar to those of cadherin–cadherin binding. These results suggest that Als3 is a functional and structural mimic of human cadherins, and provide new insights into how C. albicans invades host cells.

Als3 aids the invasion of the fungal pathogenCandida albicans into human host cells by mimicking human cadherins to induce endocytosis.

Fungal invasion of normally non-phagocytic host cells

Many fungi that cause invasive disease invade host epithelial cells during mucosal and respiratory infection, and subsequently invade endothelial cells during hematogenous infection. Most fungi invade these normally non-phagocytic host cells by inducing their own uptake. Candida albicans hyphae interact with endothelial cells in vitro by binding to N-cadherin on the endothelial cell surface. This binding induces rearrangement of endothelial cell microfilaments, which results in the endocytosis of the organism. The capsule of Cryptococcus neoformans is composed of glucuronoxylomannan, which binds specifically to brain endothelial cells, and appears to mediate both adherence and induction of endocytosis. The mechanisms by which other fungal pathogens induce their own uptake are largely unknown. Some angioinvasive fungi, such as Aspergillus species and the Zygomycetes, invade endothelial cells from the abluminal surface during the initiation of invasive disease, and subsequently invade the luminal surface of endothelial cells during hematogenous dissemination. Invasion of normally non-phagocytic host cells has different consequences, depending on the type of invading fungus. Aspergillus fumigatus blocks apoptosis of pulmonary epithelial cells, whereas Paracoccidioides brasiliensis induces apoptosis of epithelial cells. This review summarizes the mechanisms by which diverse fungal pathogens invade normally non-phagocytic host cells and discusses gaps in our knowledge that provide opportunities for future research.

Antifungal activity of Lavandula angustifolia essential oil against Candida albicans yeast and mycelial form

The antifungal activity of the essential oil of Lavandula angustifolia Mill. (lavender oil) and its main components, linalool and linalyl acetate, was investigated against 50 clinical isolates of Candida albicans (28 oropharyngeal strains, 22 vaginal strains) and C. albicans ATCC 3153. Growth inhibition, killing time and inhibition of germ tube formation were evaluated. The chemical composition of the essential oil was determined by gas chromatography and mass spectrometry. Lavender oil inhibited C. albicans growth: mean minimum inhibitory concentration (MIC) of 0.69% (vol./vol.) (vaginal strains) and 1.04% (oropharyngeal strains); mean MFC of 1.1% (vaginal strains) and 1.8% (oropharyngeal strains). Linalool was more effective than essential oil: mean MIC of 0.09% (vaginal strains) and 0.29% (oropharyngeal strains); mean MFC of 0.1% (vaginal strains) and 0.3% (oropharyngeal strains). Linalyl acetate was almost ineffective. Lavender oil (2%) killed 100% of the C. albicans ATCC 3153 cells within 15 min; linalool (0.5%) killed 100% of the cells within 30 s. The essential oil inhibited germ tube formation (mean MIC of 0.09%), as did the main components (MIC of 0.11% for linalool and 0.08% for linalyl acetate). Both the essential oil and its main components inhibited hyphal elongation of C. albicans ATCC 3153 (about 50% inhibition at 0.016% with each substance). Lavender oil shows both fungistatic and fungicidal activity against C. albicans strains. At lower concentrations, it inhibits germ tube formation and hyphal elongation, indicating that it is effective against C. albicans dimorphism and may thus reduce fungal progression and the spread of infection in host tissues.

Role of the fungal Ras-protein kinase A pathway in governing epithelial cell interactions during oropharyngeal candidiasis

Tpk1p, Tpk2p and Efg1p are members of the Ras-protein kinase A pathway that governs the yeast-to-hyphal transition in Candida albicans. We used tpk1Delta/tpk1Delta, tpk2Delta/tpk2Delta and efg1Delta/efg1Delta mutants to investigate the role of these proteins in regulating the interactions of C. albicans with oral epithelial cell lines in vitro and virulence in murine models of oropharyngeal candidiasis (OPC) and haematogenously disseminated candidiasis (HDC). The tpk1Delta/tpk1Delta strain adhered to, invaded and damaged oral epithelial cells in vitro similarly to the wild-type strain. In contrast, both the tpk2Delta/tpk2Delta and efg1Delta/efg1Delta strains had reduced capacity to invade and damage oral epithelial cells, and the efg1Delta/efg1Delta strain also exhibited decreased adherence to these cells. Consistent with these in vitro findings, the tpk2Delta/tpk2Delta and efg1Delta/efg1Delta strains also had significantly attenuated virulence during OPC. Therefore, Tpk2p and Efg1p both govern factors that enable C. albicans to invade and damage oral epithelial cells in vitro and cause OPC. These results also suggest that hyphal formation mediated by the Ras-protein kinase A pathway is a key virulence mechanism during OPC. Interestingly, the efg1Delta/efg1Delta strain, but not the tpk2Delta/tpk2Delta had reduced virulence during HDC. Thus, Tpk2p may be more important for governing virulence during OPC than HDC.

An ultrastructural and a cytochemical study of candidal invasion of reconstituted human oral epithelium

BACKGROUND

Opportunistic yeast, Candida albicans causes superficial and systemic mycoses in compromised patients. Adhesion to host tissues, morphogenesis and extracellular phospholipases (PL) are thought to contribute to its virulence. The nature of numerous host-parasite interactions at the invasive phase of oral candidiasis is not fully understood. Hence in this study, we explore the ultrastructural features of oral candidiasis using a tissue culture model based on reconstituted human oral epithelium (RHOE).

METHODS

Reconstituted human oral epithelium (Skinethic Laboratory, Nice, France) was inoculated with C. albicans SC5314 and incubated up to 48 h. The infected tissue was harvested at 12, 24 and 48 h and examined using light, scanning (SEM) and transmission electron microscopy (TEM). Localized activity of PLs of C. albicans during tissue invasion was also examined using a cytochemical method.

RESULTS

Over a period of 48 h C. albicans invaded the RHOE, and histological examination revealed characteristic hallmarks of pathological tissue invasion. Hyphal penetration into the superficial epithelium, particularly at cell junctions, together with features of cellular internalization of yeasts was noted. Phospholipase activity was visible at the tips of hyphae and initial sites of bud formation. Further, SEM studies revealed cavitations on the surface epithelial cells particularly pronounced at the sites of hyphal invasion. Hyphal invasion was seen both at cell surfaces and intercellular cell junctions of the epithelium, the latter resembling thigmotropic behaviour.

CONCLUSIONS

Our findings confirm that multiple cellular interactions such as internalization, thigmotropism and extracellular PLs contribute to invasive candidiasis. The RHOE model, described here, appears to be a satisfactory model for the investigation of ultrastructural and histochemical features of invasive candidiasis in humans.

Garlic (Allium sativum) as an anti-Candida agent: a comparison of the efficacy of fresh garlic and freeze-dried extracts

AIMS

To determine the effects of fresh and freeze-dried extracts of Allium sativum on the physiology and morphology of Candida albicans.

METHODS AND RESULTS

Inhibition of growth in glucose-yeast extract-peptone was measured using a multiwell plate reader. Scanning and transmission electron microscopy investigations indicated loss of structural integrity. Gas chromatography-mass spectrometry of extracts was employed to separate and quantify putative inhibitory sulphur-containing components.

CONCLUSIONS

Fresh garlic extract has a greater efficacy than garlic powder extract as indicated both by its effects on morphology and inhibition of growth.

SIGNIFICANCE AND IMPACT OF THE STUDY

The ubiquitous opportunistic pathogen C. albicans is sensitive to garlic; resistance to the broad spectrum of active principles present is unlikely so that its anticandidal effects may provide an important alternative route to chemotherapy.

Effect of Candida albicans metabolite(s) on cellular actin

The present study describes experiments of the effect of Candida albicans metabolite(s) (arcsf) on cellular actin, and constitutes a continuation of our previous investigations regarding the effect of the fungus on actin. Western blot analysis of HEp2 cells exposed to arcsf or subfraction of arcsf (<100 kDa) revealed that the amount of soluble actin was decreased, and total actin increased, as compared to untreated cells. Transmission electron microscopy observations of HEp2 cells interacted with C. albicans or arcsf, and gold-labeled specifically for actin, revealed more intensive labeling, with labeled particles clustered in groups. Confocal laser scanning microscopy analysis has shown, as well, that exposure of HEp2 cells to the <100-kDa subfraction resulted in actin rearrangement, as did the unfractionated arcsf. Our results point to: (1) C. albicans metabolite(s) affects cellular actin by increasing the transition of soluble actin to the insoluble form; (2) the effect is associated with the subfraction of arcsf which is smaller than <100 kDa.

Potential role for a carbohydrate moiety in anti-Candida activity of human oral epithelial cells

Candida albicans is both a commensal and a pathogen at the oral mucosa. Although an intricate network of host defense mechanisms are expected for protection against oropharyngeal candidiasis, anti-Candida host defense mechanisms at the oral mucosa are poorly understood. Our laboratory recently showed that primary epithelial cells from human oral mucosa, as well as an oral epithelial cell line, inhibit the growth of blastoconidia and/or hyphal phases of several Candida species in vitro with a requirement for cell contact and with no demonstrable role for soluble factors. In the present study, we show that oral epithelial cell-mediated anti-Candida activity is resistant to gamma-irradiation and is not mediated by phagocytosis, nitric oxide, hydrogen peroxide, and superoxide oxidative inhibitory pathways or by nonoxidative components such as soluble defensin and calprotectin peptides. In contrast, epithelial cell-mediated anti-Candida activity was sensitive to heat, paraformaldehyde fixation, and detergents, but these treatments were accompanied by a significant loss in epithelial cell viability. Treatments that removed existing membrane protein or lipid moieties in the presence or absence of protein synthesis inhibitors had no effect on epithelial cell inhibitory activity. In contrast, the epithelial cell-mediated anti-Candida activity was abrogated after treatment of the epithelial cells with periodic acid, suggesting a role for carbohydrates. Adherence of C. albicans to oral epithelial cells was unaffected, indicating that the carbohydrate moiety is exclusively associated with the growth inhibition activity. Subsequent studies that evaluated specific membrane carbohydrate moieties, however, showed no role for sulfated polysaccharides, sialic acid residues, or glucose- and mannose-containing carbohydrates. These results suggest that oral epithelial cell-mediated anti-Candida activity occurs exclusively with viable epithelial cells through contact with C. albicans by an as-yet-undefined carbohydrate moiety.