Relationship between polyene resistance and sterol compositions in Cryptococcus neoformans

Methods for Antifungal Susceptibility Testing of the Cryptococcus neoformans/C. gattii Complex: Strengths and Limitations

When method-dependent categorical endpoints are available, namely either BPs or ECVs, MICs could aid in selecting the best treatment agent(s). BPs can categorize an isolate as either susceptible or resistant while the ECVs/ECOFFs can distinguish the wild type (WT, no known resistance mechanisms) from the Non-WT (NWT, harboring resistant mechanisms). Our literature review focused on the Cryptococcus species complex (SC) and the available methods and categorization endpoints. We also covered the incidence of these infections as well as the numerous Cryptococcus neoformans SC and C. gattii SC genotypes. The most important agents to treat cryptococcal infections are fluconazole (widely used), amphotericin B, and flucytosine. We provide data from the collaborative study that defined CLSI fluconazole ECVs for the most common cryptococcal species or genotypes and modes. EUCAST ECVs/ECOFFs are not yet available for fluconazole. We have summarized the incidence of cryptococccal infections (2000-2015) where fluconazole MICs were obtained by reference and commercial antifungal susceptibility tests. This occurrence is documented all over the world and those fluconazole MICs are mostly categorized by available CLSI ECVs/BPs as "resistant" instead of non-susceptible strains, including those by the commercial methods. As expected, the agreement between the CLSI and commercial methods is variable because SYO and Etest data could yield low/variable agreement (<90%) versus the CLSI method. Therefore, since BPs/ECVs are species and method dependent, why not gather sufficient MICs by commercial methods and define the required ECVs for these species?

Antifungal Peptide SP1 Damages Polysaccharide Capsule of Cryptococcus neoformans and Enhances Phagocytosis of Macrophages

Cryptococcus neoformans is a fungal pathogen which causes nearly half a million deaths worldwide each year. Under host-relevant conditions, it produces a characteristic polysaccharide capsule. The polysaccharide capsule is one of the main virulence factors of C. neoformans, which involves antiphagocytosis and immune responses of the host to cause a lack of an immune. Meanwhile, the polysaccharide capsule is a promising drug target because of the absence of analogs in the host. Here, we demonstrate that antifungal peptide SP1, which is derived from the N terminus of Saccharomyces cerevisiae GAPDH (glyceraldehyde-3-phosphate dehydrogenase), disrupts the polysaccharide capsule of C. neoformans H99. The mechanism is possibly due to the interaction of SP1 with glucuronoxylomannan (GXM). Disruption of the polysaccharide capsule enhances the adhesion and phagocytosis of C. neoformans H99 by macrophages and reduces the replication of C. neoformans H99 within macrophages. Additionally, SP1 exhibits antifungal activity against cryptococcal biofilms associated with the capsular polysaccharides. These findings suggest the potential of SP1 as a drug candidate for the treatment of cryptococcosis. IMPORTANCE C. neoformans is an opportunistic pathogen that causes invasive infections with a high mortality rate. Currently, the clinical drugs available for the treatment of cryptococcosis are limited to amphotericin B, azoles, and flucytosine. Amphotericin is nephrotoxic, and the widespread use of azoles and 5-flucytosine has led to a rapid development of drug resistance in C. neoformans. There is an urgent need to develop new and effective anticryptococcal drugs. Targeting virulence factors is a novel strategy for developing antifungal drugs. The antifungal peptide SP1 is capable of disrupting the polysaccharide capsule, which is a principal virulence factor of C. neoformans. Studying the mechanism by which SP1 damages the polysaccharide capsule and investigating the potential benefits of SP1 in removing C. neoformans from the host provides baseline data to develop a therapeutic strategy against refractory cryptococcal infections. This strategy would involve both inhibiting virulence factors and directly killing C. neoformans cells.

OUP accepted manuscript

The increasing incidence and changing epidemiology of invasive fungal infections continue to present many challenges to their effective management. The repertoire of antifungal drugs available for treatment is still limited although there are new antifungals on the horizon. Successful treatment of invasive mycoses is dependent on a mix of pathogen-, host- and antifungal drug-related factors. Laboratories need to be adept at detection of fungal pathogens in clinical samples in order to effectively guide treatment by identifying isolates with acquired drug resistance. While there are international guidelines on how to conduct in vitro antifungal susceptibility testing, these are not performed as widely as for bacterial pathogens. Furthermore, fungi generally are recovered in cultures more slowly than bacteria, and often cannot be cultured in the laboratory. Therefore, non-culture-based methods, including molecular tests, to detect fungi in clinical specimens are increasingly important in patient management and are becoming more reliable as technology improves. Molecular methods can also be used for detection of target gene mutations or other mechanisms that predict antifungal drug resistance. This review addresses acquired antifungal drug resistance in the principal human fungal pathogens and describes known resistance mechanisms and what in-house and commercial tools are available for their detection. It is emphasized that this approach should be complementary to culture-based susceptibility testing, given the range of mutations, resistance mechanisms and target genes that may be present in clinical isolates, but may not be included in current molecular assays.

Bis(N-picolinamido)cobalt(II) Complexes Display Antifungal Activity toward Candida albicans and Aspergillus fumigatus

This report highlights the synthesis and characterization of ten new bis(N-picolinamido)cobalt(II) complexes of the type [(L)2 CoX2 ]0/2+ , whereby L=N-picolinamide ligand and X=diisothiocyanato (-NCS), dichlorido (-Cl) or diaqua (-OH2 ) ligands. Single crystal X-ray (SC-XRD) analysis for nine of the structures are reported and confirm the picolinamide ligand is bound to the Co(II) center through a neutral N,O binding mode. With the addition of powder X-ray diffraction (PXRD), we have confirmed the cis and trans ligand arrangements of each complex. All complexes were screened against several fungal species and show increased antifungal activity. Notably, these complexes had significant activity against strains of Candida albicans and Aspergillus fumigatus, with several compounds exhibiting growth inhibition of >80 %, and onecompound inhibiting Aspergillus fumigatus hyphal growth by >90 %. Conversely, no antifungal activity was exhibited toward Cryptococcus neoformans and no cytotoxicity towards mammalian cell lines.

Solid-state NMR spectroscopy identifies three classes of lipids in Cryptococcus neoformans melanized cell walls and whole fungal cells

A primary virulence-associated trait of the opportunistic fungal pathogen Cryptococcus neoformans is the production of melanin pigments that are deposited into the cell wall and interfere with the host immune response. Previously, our solid-state NMR studies of isolated melanized cell walls (melanin "ghosts") revealed that the pigments are strongly associated with lipids, but their identities, origins, and potential roles were undetermined. Herein, we exploited spectral editing techniques to identify and quantify the lipid molecules associated with pigments in melanin ghosts. The lipid profiles were remarkably similar in whole C. neoformans cells, grown under either melanizing or nonmelanizing conditions; triglycerides (TGs), sterol esters (SEs), and polyisoprenoids (PPs) were the major constituents. Although no quantitative differences were found between melanized and nonmelanized cells, melanin ghosts were relatively enriched in SEs and PPs. In contrast to lipid structures reported during early stages of fungal growth in nutrient-rich media, variants found herein could be linked to nutrient stress, cell aging, and subsequent production of substances that promote chronic fungal infections. The fact that TGs and SEs are the typical cargo of lipid droplets suggests that these organelles could be connected to C. neoformans melanin synthesis. Moreover, the discovery of PPs is intriguing because dolichol is a well-established constituent of human neuromelanin. The presence of these lipid species even in nonmelanized cells suggests that they could be produced constitutively under stress conditions in anticipation of melanin synthesis. These findings demonstrate that C. neoformans lipids are more varied compositionally and functionally than previously recognized.

Erg6 affects membrane composition and virulence of the human fungal pathogen Cryptococcus neoformans

Ergosterol is the most important membrane sterol in fungal cells and a component not found in the membranes of human cells. We identified the ERG6 gene in the AIDS-associated fungal pathogen, Cryptococcus neoformans, encoding the sterol C-24 methyltransferase of fungal ergosterol biosynthesis. In this work, we have explored its relationship with high-temperature growth and virulence of C. neoformans by the construction of a loss-of-function mutant. In contrast to other genes involved in ergosterol biosynthesis, C. neoformans ERG6 is not essential for growth under permissive conditions in vitro. However, the erg6 mutant displayed impaired thermotolerance and increased susceptibility to osmotic and oxidative stress, as well as to different antifungal drugs. Total lipid analysis demonstrated a decrease in the erg6Δ strain membrane ergosterol content. In addition, this mutant strain was avirulent in an invertebrate model of C. neoformans infection. C. neoformans Erg6 was cyto-localized in the endoplasmic reticulum and Golgi complex. Our results demonstrate that Erg6 is crucial for growth at high temperature and virulence, likely due to its effects on C. neoformans membrane integrity and dynamics. These pathogen-focused investigations into ergosterol biosynthetic pathway components reinforce the multiple roles of ergosterol in the response of diverse fungal species to alterations in the environment, especially that of the infected host. These studies open perspectives to understand the participation of ergosterol in mechanism of resistance to azole and polyene drugs. Observed synergistic growth defects with co-inhibition of Erg6 and other components of the ergosterol biosynthesis pathway suggests novel approaches to treatment in human fungal infections.

The capsule of Cryptococcus neoformans

The capsule of Cryptococcus neoformans is its dominant virulence factor and plays a key role in the biology of this fungus. In this essay, we focus on the capsule as a cellular structure and note the limitations inherent in the current methodologies available for its study. Given that no single method can provide the structure of the capsule, our notions of what is the cryptococcal capsule must be arrived at by synthesizing information gathered from very different methodological approaches including microscopy, polysaccharide chemistry and physical chemistry of macromolecules. The emerging picture is one of a carefully regulated dynamic structure that is constantly rearranged as a response to environmental stimulation and cellular replication. In the environment, the capsule protects the fungus against desiccation and phagocytic predators. In animal hosts the capsule functions in both offensive and defensive modes, such that it interferes with immune responses while providing the fungal cell with a defensive shield that is both antiphagocytic and capable of absorbing microbicidal oxidative bursts from phagocytic cells. Finally, we delineate a set of unsolved problems in the cryptococcal capsule field that could provide fertile ground for future investigations.

ATP leakage from yeast cells treated by extracellular glycolipids of Pseudozyma fusiformata

The ustilaginaceous yeast Pseudozyma fusiformata secreted glycolipids which were lethal to many yeasts and fungi more active at pH of about 4.0, and in the temperature range of 20-30 degrees C. Purified glycolipids enhanced non-specific permeability of the cytoplasmic membrane in sensitive cells, which resulted in ATP leakage and susceptibility of the cells to staining with bromocresol purple. Cells of Saccharomyces cerevisiae lost the ability to acidify the medium. Basidiomycetous yeasts were more sensitive to the glycolipids than ascomycetous ones. The minimal effective glycolipid concentration was 0.13 and 0.26 mg ml(-1) for Cryptococcus terreus and Filobasidiella neoformans, while for Candida albicans and Saccharomyces cerevisiae it was 1.0 and 1.6 mg ml(-1).

[Amphotericin B deoxycholate (Fungizone): old drug, new versions]

Amphotericin B-deoxycholate (Fungizone) remains the main treatment of systemic mycoses. However, its toxicity, especially renal impairment, limits its use. The chemical properties of this molecule led to its association with lipidic structures. Among the three so-called liposomal formulations of amphotericin B, only one (AmBisome) is a true liposome. Its tolerance is good, along with high blood concentrations. The two others formulations, either in disk or ribbon form, are not true liposomes and these formulations are not as well tolerated as the former. These three forms of amphotericin are very expensive, thus limiting their use. The association of amphotericin B with other lipidic structures is of great interest. The direct solubilization of Fungizone in an emulsion (Intralipid 20%) is inexpensive and easily prepared extemporaneously; this preparation of Fungizone leads to a strong reduction of side effects and its efficacy is at least equivalent to conventional Fungizone. In the future, the association with triglycerides or lecithins is probable: possibly providing promising formulations.

Sterol compositions and susceptibilities to amphotericin B of environmental Cryptococcus neoformans isolates are changed by murine passage

Previous studies have shown that sequential isolates from patients with persistent Cryptococcus neoformans meningoencephalitis can vary in sterol composition and susceptibility to antifungal drugs. To investigate the potential of host factors as mediators of this phenomenon, we compared fungal susceptibilities of environmental and clinical isolates from a limited geographic area. Clinical isolates were less susceptible to amphotericin B than environmental isolates. Five environmental isolates were passaged through BALB/c murine hosts; the passaged isolates had changes in sterol composition and reduced amphotericin B susceptibilities relative to those of the parent isolates. In contrast, murine passage of these isolates did not alter their susceptibilities to fluconazole. The results confirm that changes in sterol composition and antifungal susceptibility can occur in vivo as a result of host factors and suggest that human infection can result in selection of variants with reduced susceptibilities to amphotericin B.

Effect of amphotericin B on the lipids of five different strains of Cryptococcus neoformans

Cells of five strains of Cryptococcus neoformans were obtained for partial analysis of lipid composition. Quantitative analysis of lipids and sterols were completed, as well as qualitative analysis of sterols by thin-layer chromatography and by the ultraviolet spectra. Such determinations were made on cells cultured in the absence and presence of amphotericin B at sub-MIC (minimal inhibitory concentration) levels. Marked alterations of the lipid and sterol contents were observed in the amphotericin B-treated cells. Moreover, ergosterol disappeared in these antibiotic-exposed cells. It is concluded that amphotericin B altered the lipid profiles, especially sterols of C. neoformans.

Sterol composition of Cryptococcus neoformans in the presence and absence of fluconazole

Analysis of the sterol compositions of 13 clinical isolates of the pathogenic yeast Cryptococcus neoformans obtained from five patients with recurring cryptococcal meningitis showed that, unlike Candida albicans, the major sterols synthesized by this yeast were obtusifoliol (range, 21.1 to 68.2%) and ergosterol (range, 0.0 to 46.5%). There was considerable variation in the sterol contents among the 13 isolates, with total sterol contents ranging from 0.31 to 5.9% of dry weight. The isolates from the five patients who had relapses had different total sterol contents and compositions in comparison with those of the pretreatment isolates, indicating either that the sterols had been changed by therapy or that the patients were infected with new isolates with different sterol compositions. Growth of the cryptococcal isolates in the presence of subinhibitory concentrations of fluconazole (0.25x the MIC) significantly altered the sterol content and pattern. The total sterol content decreased in nine isolates and increased in four isolates in response to pretreatment with fluconazole. Fluconazole had no consistent effect on ergosterol levels. In contrast, fluconazole caused a decrease in obtusifoliol levels and an increase in 4,14-dimethylzymosterol levels in all isolates. These results indicate extensive diversity in sterol content, sterol composition, and sterol synthesis in response to subinhibitory concentrations of fluconazole in C. neoformans strains. We propose that fluconazole inhibits the sterol synthesis of C. neoformans by interfering with both 14 alpha-demethylase-dependent and -independent pathways. No correlation between the sterol compositions of C. neoformans isolates and their susceptibilities to fluconazole was found.

Emerging role of lipids of Candida albicans, a pathogenic dimorphic yeast

It is clear that C. albicans lipids have gained tremendous importance in recent years. In addition to being a barrier for entrance of various metabolites, it also provides the site of action for the synthesis of enzyme(s) involved in cell wall morphogenesis and antifungal action. While alterations in lipid composition during a yeast to mycelia transition have been observed, in most of the studies, lipid fluctuations reported could have been due to various environmental factors involved in the induction of morphogenesis [4,5]. A clear understanding of lipid biosynthesis and metabolic blocks due to antifungal action is likely to shed further light on selective interactions of antifungals. Despite the multifacet role of lipids in various functions of this pathogenic yeast, their exact involvement is poorly understood. The situation is little better with regard to ergosterol and its metabolism. Ergosterol is, indeed, important for anti-candidal activity and appears to be involved in the morphogenesis of C. albicans. The fluctuation in phospholipid composition have led to altered properties of plasma membrane namely, membrane fluidity, transport activities and drug sensitivity, which suggest that-a critical level of individual phospholipid is important for proper functioning of the plasma membrane. What the exact role is of individual phospholipid is far from clear. Many unanswered questions relating to the role of PI and sphingomyelin in signal transduction, involvement of phospholipases in the maintenance of phospholipid composition, and role of lipid transfer proteins in assembly and asymmetry of lipids are some aspects which merit further work.

The genetics of medically important fungi

The present review is concerned with recent progress in basic genetic investigations with a variety of fungi which are pathogenic for man and animals. The principles and strategies involved in undertaking genetic investigations of sexual species and of asexual species are discussed. Progress in genetic analysis of Cryptococcus neoformans made possible by the discovery of its sexual phase is described in detail, as is progress in development of parasexual methods of analysis in Candida albicans. The genetic bases of virulence and drug resistance are discussed for those few species in which these phenotypes have been investigated. Suggestions for future research, including the application of recent advances in molecular biology to the study of pathogenic fungi, are presented.

How do the polyene macrolide antibiotics affect the cellular membrane properties?

In the 1970's great strides were made in understanding the mechanism of action of amphotericin B and nystatin: the formation of transmembrane pores was clearly demonstrated in planar lipid monolayers, in multilamellar phospholipid vesicles and in Acholeplasma laidlawii cells and the importance of the presence and of the nature of the membrane sterol was analyzed. For polyene antibiotics with shorter chains, a mechanism of membrane disruption was proposed. However, recently obtained data on unilamellar vesicles have complicated the situation. It has been shown that: membranes in the gel state (which is not common in cells), even if they do not contain sterols may be made permeable by polyene antibiotics, several mechanisms may operate, simultaneously or sequentially, depending on the antibiotic/lipid ratio, the time elapsed after mixing and the mode of addition of the antibiotic, there is a rapid exchange of the antibiotic molecules between the vesicles. Although pore formation is apparently involved in the toxicity of amphotericin B and nystatin, it is not the sole factor which contributes to cell death, since K+ leakage induced by these antibiotics is separate from their lethal action. The peroxidation of membrane lipids, which has been demonstrated for erythrocytes and Candida albicans cells in the presence of amphotericin B, may play a determining role in toxicity concurrently with colloid osmotic effect. On the other hand, it has been shown that the action of polyene antibiotics on cells is not always detrimental: at sub-lethal concentrations these drugs stimulate either the activity of some membrane enzymes or cellular metabolism. In particular, some cells of the immune system are stimulated. Furthermore, polyene antibiotics may act synergistically with other drugs, such as antitumor or antifungal compounds. This may occur either by an increased incorporation of the drug, under the influence of a polyene antibiotic-induced change of membrane potential, for example, or by a direct interaction of both drugs. That fungal membranes contain ergosterol while mammalian cell membranes contain cholesterol, has generally been considered the basis for the selective toxicity of amphotericin B and nystatin for fungi. Actually, in vitro studies have not always borne out this assumption, thereby casting doubt on the use of polyene antibiotics as antifungal agents in mammalian cell culture media.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of ketoconazole on the fungicidal action of amphotericin B in Candida albicans

Amphotericin B-susceptible Candida albicans became resistant to the drug after growth in the presence of ketoconazole. Chromatographic analysis of cellular sterols showed that the organisms became depleted of ergosterol in parallel with the development of amphotericin B resistance. The implications of these findings are discussed in relation to combination chemotherapy with these two important antifungal agents.

Incidence of polyene-resistant yeasts recovered from clinical specimens

The development of resistance to amphotericin B and nystatin in yeast isolates was determined. Organisms recovered from patients on the oncology service, undergoing extensive chemotherapy for acute leukemia and bone marrow transplantation, were compared with yeasts recovered from patients on other services in the same hospital over a 7-month period. An agar dilution method was used to assay the susceptibility for each antibiotic; resistance was defined as a minimal inhibitory concentration of greater than or equal to 2 micrograms/ml for amphotericin B and greater than or equal to 16 micrograms/ml for nystatin. None of 625 isolates from 238 patients on non-oncology services demonstrated polyene resistance. Resistance only occurred in a subpopulation of oncology patients, in which 55 isolates (7.4%) from six patients (8.6%) exhibited polyene resistance. Resistance yeasts included Candida albicans (three strains), Candida tropicalis (one strain), and Torulopsis glabrata (two strains). All of the patients from whom resistant yeasts were recovered had experienced extensive chemotherapy with cytotoxic agents, granulocytopenia, and long-term treatment with both antibacterial and polyene antibiotics. Resistance to 2 micrograms of amphotericin B per ml and to 16 micrograms of nystatin per ml was associated with loss or marked depression of ergosterol in the cell membrane as measured by ultraviolet spectra. A significant incidence of polyene resistance in an oncology subpopulation was documented, suggesting a need for susceptibility testing in patients who are at high risk for development of drug-resistant fungal pathogens.