Candida albicans-Endothelial Cell Interactions: a Key Step in the Pathogenesis of Systemic Candidiasis

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Candida albicans is a normal commensal organism of the oral cavity, gastrointestinal tract, and vagina. Under certain circumstances, C. albicans is capable of causing host damage (or disease) leading to oral, vaginal, cutaneous, or systemic candidiasis. The latter is a serious infection with a high mortality range of 33% to 54% and high morbidity in those who survive. In fact, in recent years, systemic candidal infections have become the fourth most frequent cause of nosocomial bloodstream infections in the United States, giving rise to an enormous associated personal and economic cost. Systemic candidiasis involves the hematogenous spread of C. albicans to multiple organs, including the brain, kidneys, heart, liver, and lungs. Histologically, infection of these organs is characterized by ramifying candidal hyphae and accompanying yeast forms that produce multiple necrotic nodules or abscesses that result in extensive organ damage leading to organ failure. Risk factors for systemic candidiasis include neutropenia, intravascular catheters, hemodialysis, total parenteral nutrition, abdominal surgery, burns, broad-spectrum antibiotics, and corticosteroids. Systemic innate immune responses by phagocytic cells, particularly neutrophils and macrophages, appear to play a critical role in the host defense against systemic Candida infections, and consequently, the majority of candidal infections occur in patients with neutropenia or defects in neutrophil or macrophage function.

MORPHOGENETIC CONVERSIONS AND C. ALBICANS VIRULENCE

C. albicans is a polymorphic organism that is capable of converting between yeast, pseudohyphal, and hyphal forms. The conventional view was that yeast forms were associated with commensal carriage, whereas hyphal forms were associated with disease. This was based on evidence showing that mutant forms of C. albicans that were locked into the yeast form were avirulent. However, this notion was challenged by Braun et al., who found that a tup1-deficient C. albicans strain that was constitutively pseudohyphal was avirulent in a murine model of systemic candidiasis. Although the precise nature of the association between fungal morphogenesis and host invasion is a hotly debated topic, it is now widely accepted that it is the ability to undergo morphogenetic conversion, rather than the morphological form itself, that is the primary determinant of pathogenicity.

The dissemination of fungal organisms in systemic candidiasis starts with their entry into the bloodstream. Given the known risk factors for systemic candidiasis, this is most likely to occur in susceptible individuals by seeding from contaminated intravascular devices, by persorption of C. albicans across the gastrointestinal mucosa, by invasion of epithelially denuded surfaces, or via trauma or surgically related inoculation. Exit from the circulation is thought to occur by adhesion and then penetration into the endothelial lining of blood vessels, except possibly in the kidney, where direct adhesion to exposed extracellular matrix components within glomerular regions may occur. Animal studies suggest that candidal trafficking from the circulation into the tissues occurs rapidly. This review discusses the two critical steps in the migration of C. albicans cells from the circulation into the tissues, which are (i) candidal adhesion to endothelial cells lining the blood vessels and (ii) transmigration of C. albicans across the endothelium into the tissues.

ADHESION OF C. ALBICANS TO ENDOTHELIAL CELLS

During hematogenous dissemination of C. albicans, organisms must first adhere to the endothelial lining of blood vessels before transmigrating across the endothelium to invade the tissues. However, little is known about the mechanisms involved in either process. What is known is complicated further by conflicting evidence concerning the roles played by yeast, pseudohyphal, and hyphal forms of C. albicans and the role of morphogenetic change in the adhesion and transmigration processes.

There are currently two different theories as to how C. albicans adheres to the endothelium. The first theory proposes that cells must first undergo morphogenetic conversion to hyphal forms, which then bind to and damage the endothelial lining of blood vessels before undergoing transmigration from the circulation into the tissues. However, more recent data indicate a second possibility in which morphogenetic change is not necessary for C. albicans invasion of the tissues. In this scenario, yeast cells adhere to the endothelium and then transmigrate into the tissues without undergoing morphogenetic conversion.

The basis of the first theory is morphogenetic conversion of C. albicans to the hyphal form, and there are many lines of evidence to support this hypothesis. These include the observations that germination of C. albicans is necessary for the organism to damage endothelial cells and that substances that inhibit germination block C. albicans-induced endothelial cell damage. Moreover, the time course of candidal germination and germ tube elongation on endothelial cells parallels the time course of endothelial cell damage. Further evidence has come from experiments using genetically engineered forms of C. albicans with filamentation defects. The ability of these organisms to damage and invade endothelial cells is severely impaired compared to that of wild-type parent strains.

Studies showing that germinated C. albicans cells exhibit much greater adherence to epithelial cells than do yeast forms prompted suggestions that C. albicansadherence to endothelial cells might also be hypha dependent. Indeed, there is some evidence to suggest that germinated candidal forms exhibit greater endothelial cell adhesion than do yeast forms of C. albicans. However, it is also possible that yeast forms adhere to the endothelial surface, germinate there, and then penetrate and damage the endothelium during transmigration or that yeast forms adhere and are then endocytosed before germinating within the endothelial cell to cause damage. Taken together, the data suggest that morphogenetic transformation is involved in endothelial cell adhesion but, more particularly, in the subsequent trans-endothelial cell migration.

Conversely, there is also evidence to suggest that morphogenetic change may not be necessary for C. albicans invasion, and this is the basis for the alternative hypothesis. In animal studies in which mice were intravenously inoculated with different mutant strains, Bendel et al. found that cells from a C. albicans mutant strain locked into the yeast form were able to leave the circulation and enter the tissues in greater numbers than those of the wild-type control. However, once cells were in the tissues, the ability of the wild-type strain to undergo hyphal transformation was associated with higher mortality, despite the lower fungal burden in the tissues than that with mutant yeast forms. Further evidence to support this theory has come from in vivo experiments investigating tissue invasion and damage, performed by Saville et al. using a genetically engineered strain of C. albicans (SSY50-B). This study demonstrated that yeast cells are capable of extravasating from blood vessels into the tissues without undergoing morphogenetic change. However, once cells were in the tissues, morphogenetic conversion from yeast to hyphal forms was crucial in causing tissue damage leading to death.

Such observations have led to a hypothesis in which circulating yeast cells bind to the endothelium and then transmigrate into the tissues before undergoing the hyphal transformation that results in tissue damage. In support of this, C. albicans migration from the circulation is very rapid (80 to 90% migration within 5 min), whereas hyphal transformation and endothelial cell damage may take several hours. Furthermore, because of their more compact shape and size, yeast cells may be better adapted for free dissemination within the circulation. In addition, the emergence of C. glabrata and C. parapsilosis as contenders for the second most common cause of disseminated candidiasis, after C. albicans, indicates that the ability to form true hyphae may not be essential for tissue adhesion, invasion, and pathogenesis among Candida species.

CANDIDATE CANDIDA ADHESINS AND THEIR ENDOTHELIAL LIGANDS

The cell wall of C. albicans is composed primarily of an inner structural layer of β1,3- and β1,6-glucans and chitin (a β1,4-linked polymer of N-acetylglucosamine) and a matrix primarily consisting of proteins (mannoproteins) that are heavily glycosylated with mannose-containing polysaccharides, sometimes called mannans. These take the form of short, linear, O-linked mannan side chains that stabilize the protein in the cell wall and large, highly branched N-linked mannans. It is this outermost layer that represents the first point of contact between C. albicans and the endothelium, although not at bud scars, where the components of the inner layers of the cell wall are exposed. Proteins and carbohydrates in these outer layers may have a number of functions, including the ability to act as adhesion molecules, and over recent years several C. albicans cell wall components with the potential to mediate adhesion to the endothelium have been identified. These include proteins with integrin-like properties, Candida agglutinin-like sequence (ALS) gene products, and mannans.

Cell wall protein adhesin candidates. (i) Integrin αMβ2-like adhesin.

Integrin analogues first gained interest in 1991, when Gustafson et al. found that adhesion of yeast forms of C. albicans to cultured monolayers of human endothelial cells was mediated in part by a candidal protein antigenically and structurally related to the leukocyte integrin αMβ2 (Mac-1, CD11b/CD18, CR3, or iC3b receptor) (). They demonstrated the expression of the αMβ2-like molecule on yeast forms of C. albicans and showed that expression was increased by growth in 20 mM d-glucose, as opposed to 20 mM l-glutamine (). Furthermore, the adhesion of yeast forms of C. albicans to endothelial cells was significantly reduced by anti-αMβ2 antibodies or pretreatment of the Candida cells with purified iC3b. Expression of this ligand may be altered at different temperatures and in different morphogenetic forms of C. albicans (), and this may affect the ability of C. albicans to adhere to endothelium (). αMβ2 has many different ligands, including iC3b, fibrinogen, factor X, urokinase receptor, CD14, CD23, CD54 (ICAM-1), CD102 (ICAM-2), CD242 (ICAM-4), heparin, haptoglobin, kininogen, and various microbial proteins (). Of these molecules, only ICAM-1 and -2 are widely expressed on endothelial cells, although CD14 was recently identified on primary, but not passaged, cultures of human umbilical vein endothelial cells (). There are no data on the role of CD14, CD102, or CD242 as a possible endothelial ligand for C. albicans adhesion, but Yokomura et al. () have shown that anti-CD54 monoclonal antibodies can partially inhibit the adhesion of yeast forms of C. albicans to rat pulmonary artery endothelial cells in vitro and significantly prolong the survival of rats injected intravenously with C. albicans. In certain circumstances, it is also possible that αMβ2 ligands such as fibrinogen, heparin and iC3b could in turn bind to endothelial cells and act as an intermediary in Candida-endothelial cell adhesion.

(ii) Integrin αvβ3– and αvβ5-like adhesins.

Two other integrin-like adhesins that may play a role in candidal adhesion to endothelium have been identified. They are homologs of the vitronectin-binding integrins αvβ3 (CD51/CD61) and αvβ5 (). Spreghini et al. () reported the expression of both of these adhesins on yeast forms of C. albicans, while Santoni et al. () showed that transformation to germ tubes was associated with a marked reduction in αvβ5-like integrin expression and an increase in αvβ3-like integrin expression. They also showed that adhesion of C. albicans germ tubes to endothelium was partially inhibited by anti-αvβ3 antibodies or an RGD sequence peptide. Heparin also inhibited germ tube adhesion, and when heparin treatment was combined with either anti-αvβ3 antibody or RGD peptide, the reduction in adhesion was greater still (). More recently, it was shown that a candidal focal adhesion kinase-like protein may be involved in regulating yeast cell adhesion to endothelium via the αvβ3– or αvβ5-like adhesins () or in mediating intracellular signaling following ligand binding, much as focal adhesion kinase proteins are involved in integrin-mediated signaling in mammalian cells (). Like its human counterpart, the candidal αvβ3-like adhesin has been shown to bind to vitronectin (), but other ligands for αvβ3 include CD31 (PECAM-1), fibronectin, fibrinogen, thrombospondin, von Willebrand factor, and RGD sequence peptides (). CD31 is expressed by endothelial cells and could act as a direct ligand for Candida adhesion (), while in certain circumstances it is possible that other ligands could act as a bridge in Candida-endothelial cell binding. Like its human counterpart, the αvβ5-like adhesin on C. albicans also binds vitronectin and RGD peptides (), but αvβ5 lacks a known endothelial cell target ligand and thus may not be involved directly with adherence to the endothelium.

(iii) ALS gene family.

The ALS (agglutinin-like sequence) gene family encodes a group of large glycosylphosphatidylinositol-linked cell surface glycoproteins (). To date, eight ALS genes have been identified, including ALS1 to ALS7 and ALS9, all of which appear to have differing roles in adhesion and transmigration. These genes have gained particular interest recently, and evidence shows that ALS1-transformed Saccharomyces cerevisiae exhibits up to 100-fold greater adherence to endothelial cells (), while Als1-deficient C. albicans hyphae exhibit reduced adhesion to endothelial cells (). Similarly, S. cerevisiae transformed with ALS3 shows increased adhesion (), while Als3-deficient hyphal forms of C. albicans exhibit defective adhesion to endothelial cells (). The loss of Als9 from yeast forms of C. albicans () or the loss of Als4 and decreased expression of Als2 from 1-hour-old germ tubes () also inhibit the adhesion of mutant C. albicans strains to endothelial cells. In contrast, mutational analysis has shown that deletion of ALS5ALS6, or ALS7 results in increased adhesion of yeast forms of C. albicans to endothelial cells, suggesting an antiadhesive role for these proteins (). On the other hand, the protein Als5 has been found to mediate adhesion, along with Als1, when expressed in S. cerevisiae (). To date, the only ligand for the ALSgene products that has been found on endothelial cells is N-cadherin, which binds to Als3 on C. albicans hyphae ().

(iv) C4BP.

The complement protein regulator C4b binding protein (C4BP) is able to bind to both yeast and hyphal forms of C. albicans and is predominantly localized at the tip of the germ tube on hyphae (). This binding is normally regarded as a survival mechanism that inhibits complement activation and the attachment of opsonins to the microbial surface. However, it may also enhance the adhesion of yeast forms of C. albicans to endothelial cells. It is not clear if this enhancement of adhesion by the C4BP coating occurs by activating other Candidaadhesins or by acting as a bridge.

Cell wall carbohydrate adhesin candidates.

The outer cell wall proteins of Candida are heavily glycosylated with N- or O-linked mannosyl residues and have been found to be strongly involved in the recognition of C. albicans by the innate immune system. Indeed, some of these sugar residues provide conserved Candida-associated chemical signatures, known as pathogen-associated molecular patterns, by which the host is able to recognize the presence of the pathogen via host pattern recognition receptors (PRRs). In recent years, it has become apparent that specific host PRRs bind to and recognize specific mannosyl residues on C. albicans. For example, the mannose receptor (MR) recognizes and binds to N-linked mannosyl residues, while Toll-like receptor 4 (TLR-4) binds O-linked mannosyl residues. Similarly, TLR-2 recognizes and binds phospholipomannan, and galectin-3 binds β-mannosides. As these mannosyl residues are part of the structure of the cell wall, they are expressed on all three different morphological forms of C. albicans. However, there is evidence to suggest that there are differences in the recognition of yeast and hyphae by TLR-2 and TLR-4.

Although these PRRs are principally involved in the recognition of C. albicans by components of the host immune response, it is also possible that they are used by C. albicans to adhere to and transmigrate across the endothelial lining of blood vessels. Indeed, several studies have demonstrated the important role of the TLRs in experimental models of disseminated candidiasis. Netea et al. showed that TLR-4-defective C3H/HeJ mice have an increased susceptibility to disseminated candidiasis, and mice deficient in the universal TLR adaptor protein myeloid differentiation factor 88 (MyD88) are extremely susceptible to C. albicans infection. However, it has also been shown that TLR-4-deficient mice are more resistant to disseminated Candida infection. This is also the case for TLR-2-deficient mice, which have also been shown to be more resistant to disseminated candidiasis. However, the majority of the literature on knockout mice and disseminated candidiasis looks at susceptibility to infection and correlates it with the immune response without focusing on receptor expression on endothelial cells. To date, endothelial cells have been shown to express a number of PRRs, including the MR, TLRs, and galectins. The MR was the first receptor on the surfaces of macrophages to be described as a mannan receptor, and it recognizes oligosaccharides that terminate in mannose, fucose, and N-acetylglucosamine. It is also expressed on subtypes of dendritic cells and endothelial cells from certain vascular beds, including human dermal microvascular endothelial cells but not human umbilical vein endothelial cells. So far, 10 TLRs have been found, of which 7 or 8 are expressed on unstimulated endothelial cells. However, upon stimulation with proinflammatory cytokines, all 10 TLRs are expressed. Perhaps most importantly for interactions with C. albicans, endothelial cells express TLR-2 and TLR-4. TLR-4 is expressed constitutively at a higher level than that of TLR-2 by endothelial cells. However, the expression of both is significantly upregulated by stimulation with gamma interferon or bacterial lipopolysaccharide. It is also notable that the expression of TLR-2 on endothelial cells is strongly affected by the effects of flow on the endothelial cells. The galectins are a family of 15 carbohydrate binding proteins with high affinities for β-galactosides, extracellular glycoproteins, and glycolipids. So far, expression of galectin-1, -3, and -9 has been found on cultured endothelial cells, but only galectin-3 has been found to recognize C. albicans. Other PRRs that have been found to be involved in the recognition of C. albicans include DC-SIGN, αMβ2, FcγR, and dectin-1, but so far these receptors have not been found to be expressed on endothelial cells.

With so many different cell wall components having the potential to mediate adhesion of C. albicans to the endothelium, it seems that there could be a number of different mechanisms of adhesion. This may have consequences for the development of therapies aimed at blocking adhesion, because with so many molecules potentially playing a role, blocking only one could simply result in its role being taken up by other molecules. However, to investigate this further, more research is needed on the molecules involved in adhesion of C. albicans to the endothelial lining of blood vessels.

STATIC VERSUS FLOW ADHESION ASSAYS

The majority of the above studies that have directly explored candidal adhesion to endothelium were performed by using static in vitro assays where C. albicans was left in prolonged contact with cultured monolayers of endothelial cells. This is very different from the fleeting interactions C. albicans has with endothelial cells under the conditions of shear stress and flow that occur in blood vessels in vivo. Numerous studies with other cells and microorganisms have shown that static assays do not replicate the dynamic interactions that occur with endothelium under conditions of flow and are poor at elucidating the contributions of specific adhesion molecules. Only a few studies have attempted to study candidal adhesion to synthetic substrata under conditions of flow. These have shown that there are significant differences in the adhesion of Candida to the same substrata when the assays are performed under static and flow conditions. To date, only one study has attempted to examine the adhesion of Candida to endothelium under conditions of flow. Glee et al. found that under shear flow, C. albicans formed rapid, tight adhesions in less than 67 ms. This is much quicker than in static assays and is comparable to the rapid adhesion interactions that occur between leukocytes and endothelial cells. In view of this, it is difficult to fully evaluate the contributions of the mechanisms and adhesion molecules discussed above to the adhesion of C. albicans to endothelium in vivo, as none have been studied under conditions of flow.

TRANSMIGRATION

After adhesion of C. albicans to the endothelial lining of blood vessels, the second step in the migration of C. albicans from the circulation into the tissues is transmigration across the endothelial barrier. This step may involve some of the same molecules used for adhesion but could involve others. Transmigration is hard to research in isolation, which explains why there is little information on specific methods of Candida-endothelial cell transmigration. Even so, there are several proposed mechanisms for Candida transmigration across the endothelium. The first mechanism proposes that endothelial cells endocytose adherent organisms and allow their passage through to the abluminal surface of the endothelial cell layer. It is this mechanism that has gained the most interest and for which a model has evolved to explain how candidal hyphae adhere to and then induce endothelial cells to endocytose them. In this model, C. albicanshyphae bind to N-cadherin and other, as yet unidentified proteins on the endothelial cell surface via the candidal protein Als3. This adhesive interaction induces tyrosine phosphorylation of unidentified intracellular endothelial cell proteins, causing microfilament rearrangement to produce pseudopods, which initiate the endocytosis of adherent hyphal forms of C. albicans. However, endothelial endocytosis of C. albicans is not restricted to hyphal forms, and strains that do not undergo hyphal change and cause little endothelial cell damage are endocytosed to a significant degree. Since the Als3 protein is predominantly expressed on candidal hyphae, this could involve other adhesin-endothelial ligand pairs. There is also evidence that suggests that adherent yeast forms could penetrate endothelial cells, damaging them in the process, without undergoing morphogenetic change allowing them to cross the endothelial barrier. Another proposed mechanism of trans-endothelial cell migration of adherent C. albicans involves the extension of penetrating hyphal processes through the endothelial cells, likely destroying them in the process, much as fungal hyphae ramify through other tissues. Alternatively, a further proposal suggests that adherent C. albicans cells may pass between adjacent endothelial cells as a result of translocation and cyclical switching of adhesion molecules at the junction between endothelial cells, in a manner similar to that of leukocyte and tumor cell trans-endothelial cell migration.

Two alternative methods of transmigration across the endothelium that may not require prior adhesion of C. albicans to the endothelial cell surface have also been proposed. The first mechanism proposes that organisms phagocytosed by leukocytes are transported across the endothelial barrier inside the leukocytes. It is well known that leukocytes are able to cross the endothelium, between adjacent endothelial cells, by diapedesis and cyclical switching of adhesion molecules. Furthermore, there is evidence of C. albicans being found inside circulating leukocytes in systemic candidiasis. However, it is unlikely that this represents the only mechanism for candidal transmigration, since neutropenia is a major risk factor for invasive disease. The second mechanism, which may or may not require prior adhesion, suggests that circulating Candida cells simply pass through endothelial fenestrations between adjacent endothelial cells in vascular beds such as the kidney.

Some of these mechanisms may operate only for the yeast, pseudohyphal, or hyphal form of C. albicans, some may work for all forms, and others may require morphogenetic change for transmigration to occur. As with C. albicans adhesion to endothelial cells, there is clearly much more research required in order to elucidate the precise mechanism by which C. albicans migrates across the endothelium and into the tissues. Additionally, as with leukocyte and tumor cell transmigration, the validity of these mechanisms may become apparent only when transmigration is studied in vivo or in situations where the endothelium is subject to conditions of flow.

CONCLUSION

In summary, the interaction of C. albicans with the endothelial lining of blood vessels and its invasion of the tissues involve a complex series of processes that is further complicated by the role played by the morphogenetic conversion of C. albicans. There is still a large amount of work required to clarify these processes. Furthermore, it is important that this work be performed under conditions that replicate the fleeting contacts of C. albicans with the endothelium and the dynamic conditions of flow that occur in vivo. Nonetheless, understanding these mechanisms may be critical in identifying a means for preventing Candida invasion of the tissues and its lethal sequelae in systemic candidiasis.

Statin Therapy and Decreased Incidence of Positive Candida Cultures Among Patients With Type 2 Diabetes Mellitus Undergoing Gastrointestinal Surgery

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OBJECTIVE

To assess whether statin therapy decreases the incidence of cultures positive for Candida species among high-risk hospitalized patients with type 2 diabetes mellitus (DM).

PATIENTS AND METHODS

We performed a retrospective cohort study analyzing the records of all patients with type 2 DM who were admitted to Massachusetts General Hospital for lower gastrointestinal tract surgery between January 1, 2001, and May 1, 2008. We defined statin exposure as the filling of at least 1 prescription of statins during the 6 months before hospitalization or during the current hospital stay. The primary outcome was a culture positive for Candida species during hospitalization. Clinical information on a wide range of covariates was collected. Logistic regression analysis was used to adjust for possible confounders.

RESULTS

Of the 1019 patients who were eligible for the study, 493 (48%) were receiving statins. A total of 139 patients (14%) had at least 1 culture positive for Candida species during hospitalization. An adjusted multivariate model based on a backward stepwise elimination procedure showed that statin therapy significantly decreased the incidence of cultures positive for Candida species (odds ratio, 0.60; 95% confidence interval [CI], 0.38-0.96; P=.03) with a statistically significant prolonged time to event compared with no statin therapy (adjusted hazard ratio, 0.62; 95% CI, 0.44-0.88; P=.01). The benefit of statins was more prominent in patients with type 2 DM who had greater comorbidities (Charlson Comorbidity Index ≥2) (adjusted odds ratio, 0.47; 95% CI, 0.27-0.79; P=.01).

CONCLUSION

Among patients with type 2 DM who underwent gastrointestinal surgery, use of statins correlated with a decreased incidence of cultures positive for Candida species.

What Are Fungal Fimbriae

BiofilmsAdhesions: Example – Adhesions are bands of scar-like tissue. Normally, internal tissues and organs have slippery surfaces so they can shift easily as the body moves. Adhesions cause tissues and organs to stick together. Example: They might connect the loops of the intestines to each other, to nearby organs, or to the wall of the abdomen.

Fungal fimbriae are surface appendages that were first described on the haploid cells of the smut fungus, Microbotryum violaceum. They are long (1-20 gm), narrow (7 nm) flexuous structures that have been implicated in cellular functions such as mating and pathogenesis. Since the initial description, numerous fungi from all five phyla have been shown to produce fimbriae on their extracellular surfaces.

Cell-to-cell interactions are fundamental to the processes of fungal growth and development. In particular, cell-to-cell adhesions occur during mating and pathogenesis.

Many fungi produce flexible, long (1-20 ,um), narrow (7 nm), unbranched appendages which appear similar to pili or fimbriae found on the surface of prokaryotic cells. These structures, termed fungal fimbriae, were first observed on the surface of haploid yeast-like cells of the anther smut Microbotryum violaceum (= Ustilago violacea) by Poon and Day (1974). Since their original description, fungal fimbriae have been shown to be widespread among the Mycota (Gardiner et al., 1981, 1982; Benhamou and Ouellette, 1987; Castle et al., 1992; Rghei et al., 1992; Celerin et al., 1995).

In M.violaceum, fungal fimbriae appear to be involved in cell-to-cell communication during mating before pathogenesis. Both mechanical and enzymatic removal of fimbriae from the haploid cells delays mating until fimbrial regeneration occurs (Poon and Day, 1975). In addition, mating is almost completely blocked by coating fimbriae with anti-fimbrial protein antiserum (Castle et al., 1996).

Fungal fimbriae have also been implicated as factors involved in pathogenic adhesion. Rghei et al. (1992) suggested that fimbriae are used in the initial interactions between a parasitic fungus and its host.

PDF: Fungal fimbriae are composed of collagen

Read about Filamentous Biofilm.

Fungal Genome Initiative-Aspergillus Genome Projects

PROJECT BACKGROUND

Perhaps no other fungal genus contains species that are so harmful and species that are so beneficial to humans as the genus Aspergillus (1), and a large number of Aspergillus species are of biomedical and industrial significance. For example, A. nidulans is a key fungal model system for genetics and cell biology (2, 3), A. niger is widely exploited by the fermentation industry for the production of citric acid, whereas A. oryzae plays a key role in the fermentation process of several traditional Japanese beverages and sauces (4). In contrast, A. flavus is a plant and animal pathogen that also produces the potent carcinogen aflatoxin (5), whereas several other species (most notably A. fumigatus and A. terreus) are important opportunistic pathogens of individuals with compromised immune systems (6).

The genome sequences of A. nidulans (7), A. fumigatus (6) and A. oryzae (4) represented an enormous advance in the study of Aspergillus, providing the foundation for comparative and functional genomics studies. As part of the Fungal Genome Initiative, we have sequenced and annotated an additional Aspergillus species, A. terreus. Four additional recently sequenced genomes also fall within this phylogenetic group: A. flavusA. nigerA. clavatus, and Neosartorya fischeri. The profoundly different lifestyles exhibited by each of this growing set of Aspergillus species for which genome sequences are available coupled with the varying degrees of evolutionary affinity shared by their genomes make Aspergillus a model clade to address fundamental questions in functional and comparative genomics.

References

1. Volk, T., (1997) Aspergillus in Fungus of the Month, http://botit.botany.wisc.edu/toms_fungi/feb97.html
2. Pontecorvo, G., Roper, J. A., Hemmons, L. M., Macdonald, K. D. and Bufton, A. W. (1953) The genetics of Aspergillus nidulans. Adv. Genet. 5, 141-238.
3. Morris, N. R. and Enos, A. P. (1992) Mitotic gold in a mold: Aspergillus genetics and the biology of mitosis. Trends Genet. 8, 32-37.
4. Machida, M. et al. (2005) Genome sequencing and analysis of Aspergillus oryzae. Nature 438, 1157-1161.
5. Payne, G. A. et al. (2006) Whole genome comparison of Aspergillus flavus and A. oryzae. Med. Mycol. 44 Suppl, 9-11.
6. Nierman, W. C. et al. (2005) Genomic sequence of the pathogenic and allergenic filamentous fungus Aspergillus fumigatus. Nature 438, 1151-1156.
7. Galagan, J. E. et al. (2005) Sequencing of Aspergillus nidulans and comparative analysis with A. fumigatus and A. oryzae. Nature 438, 1105-1115.

Fungi and Human Pathogenesis

~ Content Source – Neal Chamberlain

Humans have a high level of innate immunity to fungi and most of the infections they cause are mild and self-limiting.

This resistance is due to:

  • 1. the fatty acid content of the skin.
  • 2. the pH of the skin, mucosal surfaces and body fluids.
  • 3. Epithelial cell turnover.
  • 4. Normal flora.
  • 5. Transferrin.
  • 6. Cilia of the respiratory tract.

When fungi do pass the resistance barriers of the human body and establish infections, the infections are classified according to the tissue levels initially colonized.

A. Superficial mycoses– infections limited to the outermost layers of the skin and hair.

B. Cutaneous mycoses– infections that extend deeper into the epidermis, as well as invasive hair and nail diseases.

These diseases are restricted to the keritinized layers of the skin, hair, and nails. Unlike the superficial mycoses, host immune responses may be evoked, resulting in pathologic changes expressed in the deeper layers of the skin. The organisms that cause these diseases are called dermatophytes. These diseases are often called ringworm or tinea. All the following diseases are causes by MicrosporumTrichophytonand Epidermophytonwhich comprise 41 species.

C. Subcutaneous mycoses- infections involve the dermis, subcutaneous tissues, muscle, and fascia. These infections are chronic and are initiated by trauma to the skin. These infections are difficult to treat and may require surgical intervention.

D. Systemic mycoses- infections that originate primarily in the lungs and may spread to many organ systems. These organisms are inherently virulent. All but Cryptococcus are dimorphic fungi.

Histoplasma capsulatum– Ohio and Mississippi river valleys, Yeast cells in tissue, Tuberculate macroconidia in mycelial phase.

Blastomyces dermatitidis– Ohio and Mississippi river valleys, Broad Base Budding yeast in tissue, Mycelia= microconidia

Coccidioides immitis– Southwestern US. Spherule in tissue, barrel-shaped Arthroconidia in mycelia phase.

Cryptococcus neoformans– Only yeast phase but unusual in that the cells are encapsulated as demonstrated by an India Ink stain.

E. Opportunistic mycoses– infections of patients with immune deficiencies who would otherwise not be infected. Ex. AIDS, altered normal flora, diabetes mellitus, immunosuppressive therapy, malignancy.

Candidiasis– Candida albicans– Creamy growth on various body surfaces. ex. mouth, skin, vagina. Budding yeast. Form pseudohyphae in tissue. Germ tube when grown in serum.

Aspergillosis– Aspergillus niger.

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Cancer Cell ‘Ferment’ Sugar to Speed Their Metabolism and Spread

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Cancer cells are known to be able to speed up their metabolism, reprogramming it so they can proliferate more quickly. Now, researchers have identified one way that tumors alter their metabolism — fermenting rather than burning up glucose for energy, called the Warburg Hypothesis — a finding that may lead to a new way of treating virtually all cancers.

Indeed, the study, “Addiction to Coupling of the Warburg Effect with Glutamine Catabolism in Cancer Cells,” published in Cell Reports, shows that inhibiting one of the enzymes required for this skewed metabolism was sufficient to slow or halt the growth of colon cancer cells.

The researchers also identified, for the first time, how cancer-causing mutations can alter the way cancer cells metabolize specific nutrient sources in order to replicate.

“Every tissue or cell type in the body has different metabolic needs but as cells become cancerous their metabolism shifts in ways that are very different from normal cells,” Joshua Munger, PhD, an associate professor of Biochemistry and Biophysics, said in a press release. “Being able to identify those differences is critical for developing treatment targets.”

Scientists have known for decades that cancer cells take up the glucose in the blood at alarming rates. But these cells addiction to sugar is only one part of the story, said Hucky Land, PhD, the Robert and Dorothy Markin professor and chair of Biomedical Genetics, and director of research at the URMC’s Wilmot Cancer Institute, who closely collaborated with Munger on this project.

While sugar is the primary source of energy and biosynthesis for healthy cells, in cancer cells it is metabolized differently; instead of burning sugar to produce large amounts of energy, cancer cells ferment sugar, to give them a more continual source of energy for fast cell division and proliferation.

Cancer-causing mutations are responsible for this change in these cells’ metabolism, the researchers discovered. And they found that fermented glucose also allows cancer cells to increase their consumption of glutamine, another nutrient source that is abundant in the bloodstream.

“Our paper demonstrates that cancer cells, but not normal cells, depend on this link between sugar fermentation and glutamine consumption,” Land said. “This suggests a novel way that we might be able to intervene with treatment.”

To test their hypothesis, the researchers used colon cancer cell lines resulting from two specific cancer-causing mutations. They found that the glutamine uptake was critical for the cancer cell malignant transformation, and that inhibiting an enzyme involved in glutamine consumption prevented the growth of these cancer cells.

A number of early clinical trials are now testing ways to target cancer cells’ altered metabolism. This research may have found a key mechanism to apply to this concept, but further studies are required.

“Is it possible to apply this to other cancers? That’s our next question,” Munger said. “We’re testing how this could be broadly applied in the clinic.”

Advances in citric acid fermentation by Aspergillus niger: Biochemical aspects, membrane transport and modeling

Research Review Paper

Maria Papagianni ⁎
Department of Hygiene and Technology of Food of Animal Origin, School of Veterinary Medicine, Aristotle University of Thessaloniki,
54006 Thessaloniki, Greece
Received 8 October 2006; received in revised form 11 January 2007; accepted 11 January 2007
Available online 19 January 2007

Content Source


Citric acid is regarded as a metabolite of energy metabolism, of which the concentration will rise to appreciable amounts only under conditions of substantive metabolic imbalances. Citric acid fermentation conditions were established during the 1930s and 1940s, when the effects of various medium components were evaluated.

The biochemical mechanism by which Aspergillus niger accumulates citric acid has continued to attract interest even though its commercial production by fermentation has been established for decades. Although extensive basic biochemical research has been carried out with A. niger, the understanding of the events relevant for citric acid accumulation is not completely understood. This review is focused on citric acid fermentation by A. niger.

Emphasis is given to aspects of fermentation biochemistry, membrane transport in A. niger and modeling of the production process.

© 2007 Elsevier Inc. All rights reserved.

PDF: Advances in citric acid fermentation by Aspergillus niger: Biochemical aspects, membrane transport and modeling

Additional reading-PDF: Comparative genomics of citric-acid-producing
Aspergillus niger ATCC 1015 versus enzyme-producing
CBS 513.88

Invasive Fungal Infections in Acute Leukemia

Content Source ~ NIH/NCBI – Invasive Fungal Infections in Acute Leukemia

Invasive fungal infection (IFI) is among the leading causes for morbidity, mortality, and economic burden for patients with acute leukemia. In the past few decades, the incidence of IFI has increased dramatically. The certainty of diagnosis of IFI is based on host factors, clinical evidence, and microbiological examination. Advancement in molecular diagnostic modalities (e.g. non-culture-based serum biomarkers such as β-glucan or galactomannan assays) and high-resolution radiological imaging has improved our diagnostic approach. The early use of these diagnostic tests assists in the early initiation of preemptive therapy. Nonetheless, the complexity of IFI in patients with leukemia and the limitations of these diagnostic tools still mandate astute clinical acumen. Its management has been further complicated by the increasing frequency of infection by non-Aspergillus molds (e.g. zygomycosis) and the emergence of drug-resistant fungal pathogens. In addition, even though the antifungal armamentarium has expanded rapidly in the past few decades, the associated mortality remains high. The decision to initiate antifungal treatment and the choice of anti-fungal therapy requires careful consideration of several factors (e.g. risk stratification, local fungal epidemiologic patterns, concomitant comorbidities, drug-drug interactions, prior history of antifungal use, overall cost, and the pharmacologic profile of the antifungal agents). In order to optimize our diagnostic and therapeutic management of IFI in patients with acute leukemia, further basic research and clinical trials are desperately needed.


Invasive fungal infections in acute leukemia-10.1177_2040620711410098

Fungemia and Leukemia – Like or Look-Alike?

Fungemia in patients with leukemia ~ NIH-PubMed ~ A nine-year retrospective study on fungemia in patients with leukemia was conducted. A total of 79 episodes of fungemia in 77 patients with leukemia were documented.

Candidemia in acute leukemia patients ~ NIH-PubMed

Candidemia in patients with acute leukemia ~ Fungal infections are not uncommon in patients with hematological malignancies, but they are rarely micro-biologically documented. A fast and reliable means of diagnosis of invasive fungal infections is urgently needed.