The capsule of Cryptococcus neoformans

Impact of biofilm formation and azoles' susceptibility in Scedosporium/Lomentospora species using an in vitro model that mimics the cystic fibrosis patients' airway environment

BACKGROUND

Scedosporium species are the second most isolated filamentous fungi from cystic fibrosis (CF) patients; however, little is known about their virulence aspects in a CF environment. In this context, the current study aimed to evaluate the (i) antifungal susceptibility profiles, (ii) ability to form biofilm and (iii) impact of biofilm formation on the susceptibility to azoles in 21 clinical isolates of Scedosporium recovered from CF patients.

METHODS

Scedosporium apiospermum (n=6), S. aurantiacum (n=6), S. minutisporum (n=3) and Lomentospora prolificans (n=6) were firstly used to compare the antifungal susceptibility profile using a standard culture broth (RPMI-1640) and a mucin (M)-containing synthetic CF sputum medium (SCFM). The ability to form biofilms was investigated in polystyrene microtiter plates containing Sabouraud-dextrose (a classical medium), SCFM and SCFM+M. Mature biofilms were tested for their susceptibility to azoles by microdilution assay.

RESULTS

Our results showed that the minimum inhibitory concentrations (MICs) for planktonic conidia ranged from 0.25 to >16.0 mg/L for voriconazole and 1.0 to >16.0 mg/L for posaconazole. Overall, the MICs for azoles increased from 2- to 8-folds when the susceptibility tests were performed using SCFM+M compared to RPMI-1640. All fungi formed robust biofilms on polystyrene surface at 72 h, with a significant increase in the MICs (ranging from 128- to 1024-times) against both azoles compared to the planktonic cells.

CONCLUSION

These findings confirm the challenge of antifungal treatment of CF patients infected with Scedosporium/Lomentospora and also demonstrated a strong biofilm formation, with extensive increase in antifungal resistance, triggered underconditions mimicking the CF patient airway.

Avoiding the trap: Mechanisms developed by pathogens to escape neutrophil extracellular traps

Neutrophils are the first cells of the innate immune system that respond to infection by arriving at sites when pathogens have exceeded physical barriers. Among their response mechanisms against pathogens is the release of neutrophil extracellular traps (NETs), which are composed of deoxyribonucleic acid and antimicrobial proteins such as neutrophil elastase, myeloperoxidase, antimicrobial peptides, and other proteins in neutrophil granules. The formation of extracellular traps is considered an effective strategy to capture and, in some cases, neutralize pathogenic bacteria, fungi, parasites, or viruses. However, it is also known that pathogens can respond to NETs by expressing some virulence factors, thus evading the antimicrobial effect of these structures. These include the secretion of proteins to degrade the deoxyribonucleic acid scaffold, the formation of biofilms that impede the effect of NETs, or the modification of its membrane structure to avoid interaction with NETs. In this review, we discuss these mechanisms and summarize the different pathogens that employ one or more mechanisms to evade the NET-mediated neutrophil response.

A synthetic glycan array containing Cryptococcus neoformans glucuronoxylomannan capsular polysaccharide fragments allows the mapping of protective epitopes

A convergent synthetic strategy to Cryptococcus neoformans glucuronoxylomannan (GXM) capsular polysaccharide part structures was developed based on di-, tri-, tetra-, penta- and hexasaccharide thioglycoside building blocks. The approach permitted the synthesis of a library of spacer-containing serotype A and D related GXM oligosaccharide structures, ranging from di- to octadecasaccharides. Ten deprotected GXM compounds (mono- to decasaccharide) were printed onto microarray plates and screened with seventeen mouse monoclonal antibodies (mAbs) to GXM. For the first time a GXM oligosaccharide structure (a serotype A decasaccharide), capable of being recognized by neutralizing forms of these GXM-specific mAbs, has been identified, offering insight into the binding epitopes of a range of protective monoclonal antibodies and furthering our efforts to develop semi-synthetic conjugate vaccine candidates against C. neoformans.

A Cytoplasmic Heme Sensor Illuminates the Impacts of Mitochondrial and Vacuolar Functions and Oxidative Stress on Heme-Iron Homeostasis in Cryptococcus neoformans

Invasive fungal diseases are increasing in frequency, and new drug targets and antifungal drugs are needed to bolster therapy. The mechanisms by which pathogens obtain critical nutrients such as iron from heme during host colonization represent a promising target for therapy. In this study, we employed a fluorescent heme sensor to investigate heme homeostasis in Cryptococcus neoformans. We demonstrated that endocytosis is a key aspect of heme acquisition and that vacuolar and mitochondrial functions are important in regulating the pool of available heme in cells. Stress generated by oxidative conditions impacts the heme pool, as do the drugs artemisinin and metformin; these drugs have heme-related activities and are in clinical use for malaria and diabetes, respectively. Overall, our study provides insights into mechanisms of fungal heme acquisition and demonstrates the utility of the heme sensor for drug characterization in support of new therapies for fungal diseases.

Pathogens must compete with hosts to acquire sufficient iron for proliferation during pathogenesis. The pathogenic fungus Cryptococcus neoformans is capable of acquiring iron from heme, the most abundant source in vertebrate hosts, although the mechanisms of heme sensing and acquisition are not entirely understood. In this study, we adopted a chromosomally encoded heme sensor developed for Saccharomyces cerevisiae to examine cytosolic heme levels in C. neoformans using fluorescence microscopy, fluorimetry, and flow cytometry. We validated the responsiveness of the sensor upon treatment with exogenous hemin, during proliferation in macrophages, and in strains defective for endocytosis. We then used the sensor to show that vacuolar and mitochondrial dysregulation and oxidative stress reduced the labile heme pool in the cytosol. Importantly, the sensor provided a tool to further demonstrate that the drugs artemisinin and metformin have heme-related activities and the potential to be repurposed for antifungal therapy. Overall, this study provides insights into heme sensing by C. neoformans and establishes a powerful tool to further investigate mechanisms of heme-iron acquisition in the context of fungal pathogenesis.

Antifungal activity of Allamanda polyantha seed extract and its iridoids promote morphological alterations in Cryptococcus spp

Cryptococcosis, caused by Cryptococcus spp., is an invasive fungal infection of the central nervous system, associated with high mortality, affecting mainly immunocompromised patients. Due to the development of resistance to the current therapy, there is an urgent need for less toxic and more effective antifungal agents. In this study, we describe the antifungal activity against Cryptococcus spp. of an aqueous seed extract from Allamanda polyantha (ASEAP) and two iridoids, plumieride and plumieridine, isolated from this extract with an antifungal activity. The capsule formation and the morphological alterations were evaluated using fluorescent microscopy. The cytotoxic activity was also investigated. The minimal inhibitory concentration (MIC) values of ASEAP for Cryptococcus gattii were 70 and 36 µg/ml (for the R265 and R272 strains, respectively) and 563 µg/ml for Cryptococcus neoformans H99. ASEAP inhibited C. neoformans H99 capsule formation, an important virulence factor, and decreased the cell body size for both the C. gattii strains. H99 cells also presented morphological alterations, with defects in bud detachment and nuclear fragmentation. Plumieride and plumieridine presented higher MIC values than ASEAP, indicating that other compounds might contribute to antifungal activity and/or that combination of the compounds results in a higher antifungal activity.

Epitope Shaving Promotes Fungal Immune Evasion

The immune system plays a critical role in protecting us against potentially fatal fungal infections. However, some fungal pathogens have evolved evasion strategies that reduce the efficacy of our immune defenses. Previously, we reported that the fungal pathogen Candida albicans exploits specific host-derived signals (such as lactate and hypoxia) to trigger an immune evasion strategy that involves reducing the exposure of β-glucan at its cell surface. Here, we show that this phenomenon is mediated by the induction of a major secreted exoglucanase (Xog1) by the fungus in response to these host signals. Inactivating XOG1-mediated “shaving” of cell surface-exposed β-glucan enhances immune responses against the fungus. Furthermore, inhibiting exoglucanase activity pharmacologically attenuates C. albicans virulence. In addition to revealing the mechanism underlying a key immune evasion strategy in a major fungal pathogen of humans, our work highlights the potential therapeutic value of drugs that block fungal immune evasion.

The cell wall provides a major physical interface between fungal pathogens and their mammalian host. This extracellular armor is critical for fungal cell homeostasis and survival. Fungus-specific cell wall moieties, such as β-1,3-glucan, are recognized as pathogen-associated molecular patterns (PAMPs) that activate immune-mediated clearance mechanisms. We have reported that the opportunistic human fungal pathogen Candida albicans masks β-1,3-glucan following exposure to lactate, hypoxia, or iron depletion. However, the precise mechanism(s) by which C. albicans masks β-1,3-glucan has remained obscure. Here, we identify a secreted exoglucanase, Xog1, that is induced in response to lactate or hypoxia. Xog1 functions downstream of the lactate-induced β-glucan “masking” pathway to promote β-1,3-glucan “shaving.” Inactivation of XOG1 blocks most but not all β-1,3-glucan masking in response to lactate, suggesting that other activities contribute to this phenomenon. Nevertheless, XOG1 deletion attenuates the lactate-induced reductions in phagocytosis and cytokine stimulation normally observed for wild-type cells. We also demonstrate that the pharmacological inhibition of exoglucanases undermines β-glucan shaving, enhances the immune visibility of the fungus, and attenuates its virulence. Our study establishes a new mechanism underlying environmentally induced PAMP remodeling that can be manipulated pharmacologically to influence immune recognition and infection outcomes.

Innate Immune Receptors and Defense Against Primary Pathogenic Fungi

The innate immune system is critical for natural resistance to all pathogenic microorganisms, including fungi. The innate response plays a vital role in resistance to infections before the antigen-specific immune response and also influences antigen-specific adaptive immunity. There are many different receptors for the innate immune response to fungi, and some receptors have been found to play a significant role in the response to human infections with opportunistic fungi. Most human infections are caused by opportunistic fungi, but a small number of organisms are capable of causing infections in normal hosts. The primary pathogenic fungi that cause invasive infections include Blastomyces spp., Cryptococcus gattii, Coccidioides spp., Histoplasma spp., and Paracoccidioides spp. In this review of innate immune receptors that play a role in infections caused by these organisms, we find that innate immunity differs between organisms.

Cryptococcus neoformans Capsular GXM Conformation and Epitope Presentation: A Molecular Modelling Study

The pathogenic encapsulated Cryptococcus neoformans fungus causes serious disease in immunosuppressed hosts. The capsule, a key virulence factor, consists primarily of the glucuronoxylomannan polysaccharide (GXM) that varies in composition according to serotype. While GXM is a potential vaccine target, vaccine development has been confounded by the existence of epitopes that elicit non-protective antibodies. Although there is evidence for protective antibodies binding conformational epitopes, the secondary structure of GXM remains an unsolved problem. Here an array of molecular dynamics simulations reveal that the GXM mannan backbone is consistently extended and relatively inflexible in both C. neoformans serotypes A and D. Backbone substitution does not alter the secondary structure, but rather adds structural motifs: β DGlcA and β DXyl side chains decorate the mannan backbone in two hydrophillic fringes, with mannose-6-O-acetylation forming a hydrophobic ridge between them. This work provides mechanistic rationales for clinical observations-the importance of O-acetylation for antibody binding; the lack of binding of protective antibodies to short GXM fragments; the existence of epitopes that elicit non-protective antibodies; and the self-aggregation of GXM chains-indicating that molecular modelling can play a role in the rational design of conjugate vaccines.

Fenbendazole Controls In Vitro Growth, Virulence Potential, and Animal Infection in the Cryptococcus Model

The human diseases caused by the fungal pathogens Cryptococcus neoformans and Cryptococcus gattii are associated with high indices of mortality and toxic and/or cost-prohibitive therapeutic protocols. The need for affordable antifungals to combat cryptococcal disease is unquestionable. Previous studies suggested benzimidazoles as promising anticryptococcal agents combining low cost and high antifungal efficacy, but their therapeutic potential has not been demonstrated so far. In this study, we investigated the antifungal potential of fenbendazole, the most effective anticryptococcal benzimidazole. Fenbendazole was inhibitory against 17 different isolates of C. neoformans and C. gattii at a low concentration. The mechanism of anticryptococcal activity of fenbendazole involved microtubule disorganization, as previously described for human parasites. In combination with fenbendazole, the concentrations of the standard antifungal amphotericin B required to control cryptococcal growth were lower than those required when this antifungal was used alone. Fenbendazole was not toxic to mammalian cells. During macrophage infection, the anticryptococcal effects of fenbendazole included inhibition of intracellular proliferation rates and reduced phagocytic escape through vomocytosis. Fenbendazole deeply affected the cryptococcal capsule. In a mouse model of cryptococcosis, the efficacy of fenbendazole to control animal mortality was similar to that observed for amphotericin B. These results indicate that fenbendazole is a promising candidate for the future development of an efficient and affordable therapeutic tool to combat cryptococcosis.

Fenbendazole Controls In Vitro Growth, Virulence Potential, and Animal Infection in the Cryptococcus Model

The human diseases caused by the fungal pathogens Cryptococcus neoformans and Cryptococcus gattii are associated with high indices of mortality and toxic and/or cost-prohibitive therapeutic protocols. The need for affordable antifungals to combat cryptococcal disease is unquestionable. Previous studies suggested benzimidazoles as promising anticryptococcal agents combining low cost and high antifungal efficacy, but their therapeutic potential has not been demonstrated so far. In this study, we investigated the antifungal potential of fenbendazole, the most effective anticryptococcal benzimidazole. Fenbendazole was inhibitory against 17 different isolates of C. neoformans and C. gattii at a low concentration. The mechanism of anticryptococcal activity of fenbendazole involved microtubule disorganization, as previously described for human parasites. In combination with fenbendazole, the concentrations of the standard antifungal amphotericin B required to control cryptococcal growth were lower than those required when this antifungal was used alone. Fenbendazole was not toxic to mammalian cells. During macrophage infection, the anticryptococcal effects of fenbendazole included inhibition of intracellular proliferation rates and reduced phagocytic escape through vomocytosis. Fenbendazole deeply affected the cryptococcal capsule. In a mouse model of cryptococcosis, the efficacy of fenbendazole to control animal mortality was similar to that observed for amphotericin B. These results indicate that fenbendazole is a promising candidate for the future development of an efficient and affordable therapeutic tool to combat cryptococcosis.

Transposon mobilization in the human fungal pathogen Cryptococcus is mutagenic during infection and promotes drug resistance in vitro

When transitioning from the environment, pathogenic microorganisms must adapt rapidly to survive in hostile host conditions. This is especially true for environmental fungi that cause opportunistic infections in immunocompromised patients since these microbes are not well adapted human pathogens. Cryptococcus species are yeastlike fungi that cause lethal infections, especially in HIV-infected patients. Using Cryptococcus deneoformans in a murine model of infection, we examined contributors to drug resistance and demonstrated that transposon mutagenesis drives the development of 5-fluoroorotic acid (5FOA) resistance. Inactivation of target genes URA3 or URA5 primarily reflected the insertion of two transposable elements (TEs): the T1 DNA transposon and the TCN12 retrotransposon. Consistent with in vivo results, increased rates of mutagenesis and resistance to 5FOA and the antifungal drugs rapamycin/FK506 (rap/FK506) and 5-fluorocytosine (5FC) were found when Cryptococcus was incubated at 37° compared to 30° in vitro, a condition that mimics the temperature shift that occurs during the environment-to-host transition. Inactivation of the RNA interference (RNAi) pathway, which suppresses TE movement in many organisms, was not sufficient to elevate TE movement at 30° to the level observed at 37°. We propose that temperature-dependent TE mobilization in Cryptococcus is an important mechanism that enhances microbial adaptation and promotes pathogenesis and drug resistance in the human host.

Exploring Cryptococcus neoformans capsule structure and assembly with a hydroxylamine-armed fluorescent probe

Chemical biology is an emerging field that enables the study and manipulation of biological systems with probes whose reactivities provide structural insights. The opportunistic fungal pathogen Cryptococcus neoformans possesses a polysaccharide capsule that is a major virulence factor, but is challenging to study. We report here the synthesis of a hydroxylamine-armed fluorescent probe that reacts with reducing glycans and its application to study the architecture of the C. neoformans capsule under a variety of conditions. The probe signal localized intracellularly and at the cell wall-membrane interface, implying the presence of reducing-end glycans at this location where the capsule is attached to the cell body. In contrast, no fluorescence signal was detected in the capsule body. We observed vesicle-like structures containing the reducing-end probe, both intra- and extracellularly, consistent with the importance of vesicles in capsular assembly. Disrupting the capsule with DMSO, ultrasound, or mechanical shear stress resulted in capsule alterations that affected the binding of the probe, as reducing ends were exposed and cell membrane integrity was compromised. Unlike the polysaccharides in the assembled capsule, isolated exopolysaccharides contained reducing ends. The reactivity of the hydroxylamine-armed fluorescent probe suggests a model for capsule assembly whereby reducing ends localize to the cell wall surface, supporting previous findings suggesting that this is an initiation point for capsular assembly. We propose that chemical biology is a promising approach for studying the C. neoformans capsule and its associated polysaccharides to unravel their roles in fungal virulence.

Basic principles of the virulence of Cryptococcus

Among fungal pathogens, Cryptococcus neoformans has gained great importance among the scientific community of several reasons. This fungus is the causative agent of cryptococcosis, a disease mainly associated to HIV immunosuppression and characterized by the appearance of meningoencephalitis. Cryptococcal meningitis is responsible for hundreds of thousands of deaths every year. Research of the pathogenesis and virulence mechanisms of this pathogen has focused on three main different areas: Adaptation to the host environment (nutrients, pH, and free radicals), mechanism of immune evasion (which include phenotypic variations and the ability to behave as a facultative intracellular pathogen), and production of virulence factors. Cryptococcus neoformans has two phenotypic characteristics, the capsule and synthesis of melanin that have a profound effect in the virulence of the yeast because they both have protective effects and induce host damage as virulence factors. Finally, the mechanisms that result in dissemination and brain invasion are also of key importance to understand cryptococcal disease. In this review, I will provide a brief overview of the main mechanisms that makes C. neoformans a pathogen in susceptible patients.

Abbreviations: RNS: reactive nitrogen species; BBB: brain blood barrier; GXM: glucuronoxylomannan; GXMGal: glucuronoxylomannogalactan

Host Carbon Dioxide Concentration Is an Independent Stress for Cryptococcus neoformans That Affects Virulence and Antifungal Susceptibility

The ability of Cryptococcus neoformans to cause disease in humans varies significantly among strains with highly related genotypes. In general, environmental isolates of pathogenic species such as Cryptococcus neoformans var. grubii have reduced virulence relative to clinical isolates, despite having no differences in the expression of the canonical virulence traits (high-temperature growth, melanization, and capsule formation). In this observation, we report that environmental isolates of C. neoformans tolerate host CO₂ concentrations poorly compared to clinical isolates and that CO₂ tolerance correlates well with the ability of the isolates to cause disease in mammals. Initial experiments also suggest that CO₂ tolerance is particularly important for dissemination of C. neoformans from the lung to the brain. Furthermore, CO₂ concentrations affect the susceptibility of both clinical and environmental C. neoformans isolates to the azole class of antifungal drugs, suggesting that antifungal testing in the presence of CO₂ may improve the correlation between in vitro azole activity and patient outcome.IMPORTANCE A number of studies comparing either patient outcomes or model system virulence across large collections of Cryptococcus isolates have found significant heterogeneity in virulence even among strains with highly related genotypes. Because this heterogeneity cannot be explained by variations in the three well-characterized virulence traits (growth at host body temperature, melanization, and polysaccharide capsule formation), it has been widely proposed that additional C. neoformans virulence traits must exist. The natural niche of C. neoformans is in the environment, where the carbon dioxide concentration is very low (∼0.04%); in contrast, mammalian host tissue carbon dioxide concentrations are 125-fold higher (5%). We have found that the ability to grow in the presence of 5% carbon dioxide distinguishes low-virulence strains from high-virulence strains, even those with a similar genotype. Our findings suggest that carbon dioxide tolerance is a previously unrecognized virulence trait for C. neoformans.

The Spectrum of Interactions between Cryptococcus neoformans and Bacteria

Cryptococcus neoformans is a major fungal pathogen that infects immunocompromised people and causes life-threatening meningoencephalitis. C. neoformans does not occur in isolation either in the environment or in the human host, but is surrounded by other microorganisms. Bacteria are ubiquitously distributed in nature, including soil, and make up the dominant part of the human microbiota. Pioneering studies in the 1950s demonstrated antifungal activity of environmental bacteria against C. neoformans. However, the mechanisms and implications of these interactions remain largely unknown. Recently, interest in polymicrobial interaction studies has been reignited by the development of improved sequencing methodologies, and by the realization that such interactions may have a huge impact on ecology and human health. In this review, we summarize our current understanding of the interaction of bacteria with C. neoformans.

A Glucuronoxylomannan Epitope Exhibits Serotype-Specific Accessibility and Redistributes towards the Capsule Surface during Titanization of the Fungal Pathogen Cryptococcus neoformans

Disseminated infections with the fungal species Cryptococcus neoformans or, less frequently, Cryptococcus gattii are an important cause of mortality in immunocompromised individuals. Central to the virulence of both species is an elaborate polysaccharide capsule that consists predominantly of glucuronoxylomannan (GXM). Due to its abundance, GXM is an ideal target for host antibodies, and several monoclonal antibodies (mAbs) have previously been derived using purified GXM or whole capsular preparations as antigens. In addition to their application in the diagnosis of cryptococcosis, anti-GXM mAbs are invaluable tools for studying capsule structure. In this study, we report the production and characterization of a novel anti-GXM mAb, Crp127, that unexpectedly reveals a role for GXM remodeling during the process of fungal titanization. We show that Crp127 recognizes a GXM epitope in an O-acetylation-dependent, but xylosylation-independent, manner. The epitope is differentially expressed by the four main serotypes of Cryptococcus neoformans and C. gattii, is heterogeneously expressed within clonal populations of C. gattii serotype B strains, and is typically confined to the central region of the enlarged capsule. Uniquely, however, this epitope redistributes to the capsular surface in titan cells, a recently characterized morphotype where haploid 5-μm cells convert to highly polyploid cells of >10 μm with distinct but poorly understood capsular characteristics. Titan cells are produced in the host lung and critical for successful infection. Crp127 therefore advances our understanding of cryptococcal morphological change and may hold significant potential as a tool to differentially identify cryptococcal strains and subtypes.

The Buoyancy of Cryptococcus neoformans Is Affected by Capsule Size

Cryptococcus neoformans is an environmental pathogenic fungus with a worldwide geographical distribution that is responsible for hundreds of thousands of human cryptococcosis cases each year. During infection, the yeast undergoes a morphological transformation involving capsular enlargement that increases microbial volume. To understand the factors that play a role in environmental dispersal of C. neoformans and C. gattii, we evaluated the cell density of Cryptococcus using Percoll isopycnic gradients. We found differences in the cell densities of strains belonging to C. neoformans and C. gattii species complexes. The buoyancy of C. neoformans strains varied depending on growth medium. In minimal medium, the cryptococcal capsule made a major contribution to the cell density such that cells with larger capsules had lower density than those with smaller capsules. Removing the capsule, by chemical or mechanical methods, increased the C. neoformans cell density and reduced buoyancy. Melanization of the C. neoformans cell wall, which also contributes to virulence, produced a small but consistent increase in cell density. Encapsulated C. neoformans sedimented much more slowly in seawater as its density approached the density of water. Our results suggest a new function for the capsule whereby it can function as a flotation device to facilitate transport and dispersion in aqueous fluids.IMPORTANCE The buoyancy of a microbial cell is an important physical characteristic that may affect its transportability in fluids and interactions with tissues during infection. The polysaccharide capsule surrounding C. neoformans is required for infection and dissemination in the host. Our results indicate that the capsule has a significant effect on reducing cryptococcal cell density, altering its sedimentation in seawater. Modulation of microbial cell density via encapsulation may facilitate dispersal for other important encapsulated pathogens.

A chemical genetic screen reveals a role for proteostasis in capsule and biofilm formation by Cryptococcus neoformans

Pathogenic microorganisms employ specialized virulence factors to cause disease. Biofilm formation and the production of a polysaccharide capsule are two important virulence factors in Cryptococcus neoformans, the fungal pathogen that causes meningoencephalitis. Here, we show that the bipolar disorder drug lithium inhibits formation of both virulence factors by a mechanism involving dysregulation of the ubiquitin/proteasome system. By using a chemical genetics approach and bioinformatic analyses, we describe the cellular landscape affected by lithium treatment. We demonstrate that lithium affects many different pathways in C. neoformans, including the cAMP/protein kinase A, inositol biosynthesis, and ubiquitin/proteasome pathways. By analyzing mutants with defects in the ubiquitin/proteasome system, we uncover a role for proteostasis in both capsule and biofilm formation. Moreover, we demonstrate an additive influence of lithium and the proteasome inhibitor bortezomib in inhibiting capsule production, thus establishing a link between lithium activity and the proteasome system. Finally, we show that the lithium-mimetic drug ebselen potently blocks capsule and biofilm formation, and has additive activity with lithium or bortezomib. In summary, our results illuminate the impact of lithium on C. neoformans, and link dysregulation of the proteasome to capsule and biofilm inhibition in this important fungal pathogen.

Cryptococcal Traits Mediating Adherence to Biotic and Abiotic Surfaces

Several species in the genus Cryptococcus are facultative intracellular pathogens capable of causing disease associated with high mortality and morbidity in humans. These fungi interact with other organisms in the soil, and these interactions may contribute to the development of adaptation mechanisms that function in virulence by promoting fungal survival in animal hosts. Fungal adhesion molecules, also known as adhesins, have been classically considered as cell-surface or secreted proteins that play critical roles in microbial pathogenesis or in biofilm formation as structural components. Pathogenic Cryptococcus spp. differ from other pathogenic yeasts in having a polysaccharide capsule that covers the cell wall surface and precludes interactions of those structures with host cell receptors. Hence, pathogenic Cryptococcus spp. use unconventional tools for surface attachment. In this essay, we review the unique traits and mechanisms favoring adhesion of Cryptococcus spp. to biotic and abiotic surfaces. Knowledge of the traits that mediate adherence could be exploited in the development of therapeutic, biomedical, and/or industrial products.