Adaptation of Candida albicans to environmental pH induces cell wall remodelling and enhances innate immune recognition

Innate immune responses against the fungal pathogen Candida auris

Candida auris is a multidrug-resistant human fungal pathogen responsible for nosocomial outbreaks worldwide. Although considerable progress has increased our understanding of the biological and clinical aspects of C. auris, its interaction with the host immune system is only now beginning to be investigated in-depth. Here, we compare the innate immune responses induced by C. auris BJCA001 and Candida albicans SC5314 in vitro and in vivo. Our results indicate that C. auris BJCA001 appears to be less immunoinflammatory than C. albicans SC5314, and this differential response correlates with structural features of the cell wall.

Candida auris is a multidrug-resistant human fungal pathogen responsible for nosocomial outbreaks worldwide. Here, the authors identify differential innate immune responses induced by C. auris and Candida albicans in vitro and in vivo, which correlate with structural features of the cell wall.

The transcription factor Ron1 is required for chitin metabolism, asexual development and pathogenicity in Beauveria bassiana, an entomopathogenic fungus

Ndt80-like transcription factor Ron1 is best known for its essential role in the regulation of N-acetylglucosamine (GlcNAc) catabolism. Ron1 was again found to be essential for sensing GlcNAc in Beauveria bassiana. Importantly, our study revealed that Ron1 is involved in the metabolic processes of chitin and asexual development. To further investigate the novel functions of Ron1 in B. bassiana, extracellular chitinase activity in the ΔRon1 mutant was found to decrease by 84.73% compared with wild type. The deletion of Ron1 made it difficult for the fungus to accumulate intracellular GlcNAc. Furthermore, transcriptomic analysis revealed that Ron1 exerted a significant effect on global transcription and positively regulated genes encoding chitin metabolism in respond to chitin nutrition. Yeast one-hybrid assay confirmed that Ron1 could bind to specific cis-acting elements in the promoters of chitinase and hexokinase. In addition, ΔRon1 displayed an impaired chitin component of the cell wall, with a chitin synthetase (ChsVII) predicted to function downstream of Ron1. Finally, the virulence of ΔRon1 mutant was significantly reduced in the Galleria mellonella insect model through cuticle infection or cuticle bypassing infection. These data functionally characterize Ron1 in B. bassiana and expand our understanding of how the transcription factor Ron1 works in pathogens.

When Is It Appropriate to Take Off the Mask? Signaling Pathways That Regulate ß(1,3)-Glucan Exposure in Candida albicans

Candida spp. are an important source of systemic and mucosal infections in immune compromised populations. However, drug resistance or toxicity has put limits on the efficacy of current antifungals. The C. albicans cell wall is considered a good therapeutic target due to its roles in viability and fungal pathogenicity. One potential method for improving antifungal strategies could be to enhance the detection of fungal cell wall antigens by host immune cells. ß(1,3)-glucan, which is an important component of fungal cell walls, is a highly immunogenic epitope. Consequently, multiple host pattern recognition receptors, such as dectin-1, complement receptor 3 (CR3), and the ephrin type A receptor A (EphA2) are capable of recognizing exposed (unmasked) ß(1,3)-glucan moieties on the cell surface to initiate an anti-fungal immune response. However, ß(1,3)-glucan is normally covered (masked) by a layer of glycosylated proteins on the outer surface of the cell wall, hiding it from immune detection. In order to better understand possible mechanisms of unmasking ß(1,3)-glucan, we must develop a deeper comprehension of the pathways driving this phenotype. In this review, we describe the medical importance of ß(1,3)-glucan exposure in anti-fungal immunity, and highlight environmental stimuli and stressors encountered within the host that are capable of inducing changes in the levels of surface exposed ß(1,3)-glucan. Furthermore, particular focus is placed on how signal transduction cascades regulate changes in ß(1,3)-glucan exposure, as understanding the role that these pathways have in mediating this phenotype will be critical for future therapeutic development.

From environmental adaptation to host survival: Attributes that mediate pathogenicity of Candida auris

Candida species are a major cause of invasive fungal infections. While Candida albicans, C. glabrata, C. parapsilosis, and C. tropicalis are the most dominant species causing life-threatening candidiasis, C. auris recently emerged as a new species causing invasive infections with high rates of clinical treatment failures. To mimic initial phases of systemic Candida infections with dissemination via the bloodstream and to elucidate the pathogenic potential of C. auris, we used an ex vivo whole blood infection model. Similar to other clinically relevant Candida spp., C. auris is efficiently killed in human blood, but showed characteristic patterns of immune cell association, survival rates, and cytokine induction. Dual-species transcriptional profiling of C. auris-infected blood revealed a unique C. auris gene expression program during infection, while the host response proofed similar and conserved compared to other Candida species. C. auris-specific responses included adaptation and survival strategies, such as counteracting oxidative burst of immune cells, but also expression of potential virulence factors, (drug) transporters, and cell surface-associated genes. Despite comparable pathogenicity to other Candida species in our model, C. auris-specific transcriptional adaptations as well as its increased stress resistance and long-term environmental survival, likely contribute to the high risk of contamination and distribution in a nosocomial setting. Moreover, infections of neutrophils with pre-starved C. auris cells suggest that environmental preconditioning can have modulatory effects on the early host interaction. In summary, we present novel insights into C. auris pathogenicity, revealing adaptations to human blood and environmental niches distinctive from other Candida species.

Estrogen promotes innate immune evasion of Candida albicans through inactivation of the alternative complement system

Candida albicans is a commensal of the urogenital tract and the predominant cause of vulvovaginal candidiasis (VVC). Factors that increase circulatory estrogen levels such as pregnancy, the use of oral contraceptives, and hormone replacement therapy predispose women to VVC, but the reasons for this are largely unknown. Here, we investigate how adaptation of C. albicans to estrogen impacts the fungal host-pathogen interaction. Estrogen promotes fungal virulence by enabling C. albicans to avoid the actions of the innate immune system. Estrogen-induced innate immune evasion is mediated via inhibition of opsonophagocytosis through enhanced acquisition of the human complement regulatory protein, Factor H, on the fungal cell surface. Estrogen-induced accumulation of Factor H is dependent on the fungal cell surface protein Gpd2. The discovery of this hormone-sensing pathway might pave the way in explaining gender biases associated with fungal infections and may provide an alternative approach to improving women's health.

Molecular and Microscopic Methods of Quantifying Candida albicans Cell Wall PAMP Exposure

The cell wall of Candida albicans is a multilayered structure consisting of polysaccharides and proteins. The inner cell wall layer is comprised of chitin and β1-3 and β1-6-glucan which contribute to the overall shape and structure of the cell, while the outer layer of highly glycosylated mannoproteins provides key functional traits such as cell adhesion required for virulence. However, the cell wall is not a static structure but is constantly being remodeled in response to the external environment. Given that all of the cell wall components act as pathogen-associated molecular patterns (PAMPs) that are recognized by a variety of receptors on the surface of innate immune cells, remodeling of the cell wall can have a dramatic impact on the host-pathogen interaction. For example, during growth in standard media, C. albicans shields its major cell wall PAMPs from the innate immune system, but during growth in acidic environments as encountered during colonization of the female reproductive tract, key PAMPs become exposed on the fungal cell surface initiating a strong pro-inflammatory innate immune response. The impact of environmental adaptation on fungal cell wall remodeling, and the subsequent impact this has on the host-pathogen interaction, has been the subject of much research. In this chapter, we outline techniques to assess cell wall components in both resting and environmentally adapted C. albicans cells.

Labelling of Candida auris Cell Walls to Examine Levels of PAMP Exposure by Flow Cytometry and Fluorescence Microscopy

Pathogen-associated molecular patterns (PAMPs) of the fungal cell wall are primary targets for the innate immune system of animals. Therefore, characterizing PAMP exposure of fungal pathogens helps to elucidate how they interact with their hosts at a molecular level. Fluorescent labelling can be used to monitor exposure of multiple fungal cell wall PAMPs in a single experiment. Here, we describe a protocol to simultaneously label chitin, mannan, and β-1,3-glucan in Candida auris to study these PAMPs by fluorescence microscopy and allow high-throughput examination of their exposure by flow cytometry.

Control of β-glucan exposure by the endo-1,3-glucanase Eng1 in Candida albicans modulates virulence

Candida albicans is a major opportunistic pathogen of humans. It can grow as morphologically distinct yeast, pseudohyphae and hyphae, and the ability to switch reversibly among different forms is critical for its virulence. The relationship between morphogenesis and innate immune recognition is not quite clear. Dectin-1 is a major C-type lectin receptor that recognizes β-glucan in the fungal cell wall. C. albicans β-glucan is usually masked by the outer mannan layer of the cell wall. Whether and how β-glucan masking is differentially regulated during hyphal morphogenesis is not fully understood. Here we show that the endo-1,3-glucanase Eng1 is differentially expressed in yeast, and together with Yeast Wall Protein 1 (Ywp1), regulates β-glucan exposure and Dectin-1-dependent immune activation of macrophage by yeast cells. ENG1 deletion results in enhanced Dectin-1 binding at the septa of yeast cells; while eng1 ywp1 yeast cells show strong overall Dectin-1 binding similar to hyphae of wild-type and eng1 mutants. Correlatively, hyphae of wild-type and eng1 induced similar levels of cytokines in macrophage. ENG1 expression and Eng1-mediated β-glucan trimming are also regulated by antifungal drugs, lactate and N-acetylglucosamine. Deletion of ENG1 modulates virulence in the mouse model of hematogenously disseminated candidiasis in a Dectin-1-dependent manner. The eng1 mutant exhibited attenuated lethality in male mice, but enhanced lethality in female mice, which was associated with a stronger renal immune response and lower fungal burden. Thus, Eng1-regulated β-glucan exposure in yeast cells modulates the balance between immune protection and immunopathogenesis during disseminated candidiasis.

The Vaginal Microbiome: III. The Vaginal Microbiome in Various Urogenital Disorders

OBJECTIVE

This series of articles, titled The Vaginal Microbiome (VMB), written on behalf of the International Society for the Study of Vulvovaginal Disease, aims to summarize the recent findings and understanding of the vaginal bacterial microbiota, mainly regarding areas relevant to clinicians specializing in vulvovaginal disorders.

MATERIALS AND METHODS

A search of PubMed database was performed, using the search terms "vaginal microbiome" with "Candida," "vaginitis," "urinary microbiome," "recurrent urinary tract infections," "sexually transmitted infections," "human immunodeficiency virus," "human papillomavirus," "nonspecific vaginitis," "vulvodynia," and "vulvovaginal symptoms." Full article texts were reviewed. Reference lists were screened for additional articles. The third article in this series describes VMB in various urogenital disorders.

RESULTS

Variable patterns of the VMB are found in patients with vulvovaginal candidiasis, challenging the idea of a protective role of lactobacilli. Highly similar strains of health-associated commensal bacteria are shared in both the bladder and vagina of the same individual and may provide protection against urinary tract infections. Dysbiotic VMB increases the risk of urinary tract infection. Loss of vaginal lactic acid-producing bacteria combined with elevated pH, increase the risk for sexually transmitted infections, although the exact protective mechanisms of the VMB against sexually transmitted infections are still unknown.

CONCLUSIONS

The VMB may constitute a biological barrier to pathogenic microorganisms. When the predominance of lactobacilli community is disrupted, there is an increased risk for the acquisition of various vaginal pathogents. Longitudinal studies are needed to describe the association between the host, bacterial, and fungal components of the VMB.

Ketoconazole resistant Candida albicans is sensitive to a wireless electroceutical wound care dressing

Wireless electroceutical dressing (WED) fabric kills bacteria and disrupts bacterial biofilm. This work tested, comparing with standard of care topical antibiotic ketoconazole, whether the weak electric field generated by WED is effective to manage infection caused by ketoconazole-resistant yeast Candida albicans. WED inhibited Candida albicans biofilm formation and planktonic growth. Unlike ketoconazole, WED inhibited yeast to hyphal transition and downregulated EAP1 curbing cell attachment. In response to WED-dependent down-regulation of biofilm-forming BRG1 and ROB1, BCR1 expression was markedly induced in what seems to be a futile compensatory response. WED induced NRG1 and TUP1, negative regulators of filamentation; it down-regulated EFG1, a positive regulator of hyphal pathway. Consistent with the anti-hyphal properties of WED, the expression of ALS3 and HWP1 were diminished. Ketoconazole failed to reproduce the effects of WED on NRG1, TUP1 and EFG1. WED blunted efflux pump activity; this effect was in direct contrast to that of ketoconazole. WED exposure compromised cellular metabolism. In the presence of ketoconazole, the effect was synergistic. Unlike ketoconazole, WED caused membrane depolarization, changes in cell wall composition and loss of membrane integrity. This work presents first evidence that weak electric field is useful in managing pathogens which are otherwise known to be antibiotic resistant.

Monoclonal Antibodies against Cell Wall Chitooligomers as Accessory Tools for the Control of Cryptococcosis

Therapeutic strategies against systemic mycoses can involve antifungal resistance and significant toxicity. Thus, novel therapeutic approaches to fight fungal infections are urgent. Monoclonal antibodies (MAbs) are promising tools to fight systemic mycoses. In this study, MAbs of the IgM isotype were developed against chitin oligomers. Chitooligomers derive from chitin, an essential component of the fungal cell wall and a promising therapeutic target, as it is not synthesized by humans or animals. Surface plasmon resonance (SPR) assays and cell-binding tests showed that the MAbs recognizing chitooligomers have high affinity and specificity for the chitin derivatives. In vitro tests showed that the chitooligomer MAbs increased the fungicidal capacity of amphotericin B against Cryptococcus neoformans. The chitooligomer-binding MAbs interfered with two essential properties related to cryptococcal pathogenesis: biofilm formation and melanin production. In a murine model of C. neoformans infection, the combined administration of the chitooligomer-binding MAb and subinhibitory doses of amphotericin B promoted disease control. The data obtained in this study support the hypothesis that chitooligomer antibodies have great potential as accessory tools in the control of cryptococcosis.

Raman Imaging of Pathogenic Candida auris: Visualization of Structural Characteristics and Machine-Learning Identification

Invasive fungal infections caused by yeasts of the genus Candida carry high morbidity and cause systemic infections with high mortality rate in both immunocompetent and immunosuppressed patients. Resistance rates against antifungal drugs vary among Candida species, the most concerning specie being Candida auris, which exhibits resistance to all major classes of available antifungal drugs. The presently available identification methods for Candida species face a severe trade-off between testing speed and accuracy. Here, we propose and validate a machine-learning approach adapted to Raman spectroscopy as a rapid, precise, and labor-efficient method of clinical microbiology for C. auris identification and drug efficacy assessments. This paper demonstrates that the combination of Raman spectroscopy and machine learning analyses can provide an insightful and flexible mycology diagnostic tool, easily applicable on-site in the clinical environment.

In Vitro Antifungal Activity of Silver Nanoparticles Biosynthesized with Beech Bark Extract

Biosynthesis is a green method for the synthesis of silver nanoparticles (AgNPs). This study aimed to assess the antifungal activity of two silver nanoparticle solutions, synthesized using beech bark extract (BBE) and acetate and nitrate silver salts (AgNP Acetate BBE and AgNP Nitrate BBE), their influence on biofilm production, their potential synergistic effects with fluconazole, on different Candida spp., and their influence on virulence factors of C. albicans (germ tube production, gene expression for ALS3, SAP2, HSP70). Both the AgNP BBEs presented different minimum inhibitory concentrations for all the studied Candida spp., but biofilm production was inhibited only for C. albicans and C. guilliermondii. The growth rates of all the studied Candida spp. were inhibited in the presence of both AgNP BBEs, except for C. auris. Synergistic activity was observed for C. parapsilosis and C. guilliermondii, for different combinations of fluconazole with both the AgNP BBEs. The germ tube production of C. albicans was slightly inhibited by the AgNP BBEs. Only AgNP Acetate BBE was able to down-regulate the expression of SAP2. Overall, we can conclude that, even if more studies are necessary, AgNPs synthesized with beech bark extract might be an interesting alternative to classic antifungal treatments.

The Auto-Brewery Syndrome: A Perfect Metabolic "Storm" with Clinical and Forensic Implications

Auto-brewery syndrome (ABS) is a rare, unstudied, unknown, and underreported phenomenon in modern medicine. Patients with this syndrome become inebriated and may suffer the medical and social implications of alcoholism, including arrest for inebriated driving. The pathophysiology of ABS is reportedly due to a fungal type dysbiosis of the gut that ferments some carbohydrates into ethanol and may mimic a food allergy or intolerance. This syndrome should be considered in patients with chronic obstruction or hypomotility presenting with elevated breath and blood alcohol concentrations, especially after a high carbohydrate intake. A glucose challenge test should be performed as the confirmatory test. Treatment typically includes antifungal drugs combined with changes in lifestyle and nutrition. Additional studies are particularly needed on the human microbiome to shed light on how imbalances of commensal bacteria in the gut allow yeast to colonize on a pathological level.

Fungal Cell Wall Proteins and Signaling Pathways Form a Cytoprotective Network to Combat Stresses

Candida species are part of the normal flora of humans, but once the immune system of the host is impaired and they escape from commensal niches, they shift from commensal to pathogen causing candidiasis. Candida albicans remains the primary cause of candidiasis, accounting for about 60% of the global candidiasis burden. The cell wall of C. albicans and related fungal pathogens forms the interface with the host, gives fungal cells their shape, and also provides protection against stresses. The cell wall is a dynamic organelle with great adaptive flexibility that allows remodeling, morphogenesis, and changes in its components in response to the environment. It is mainly composed of the inner polysaccharide rich layer (chitin, and β-glucan) and the outer protein coat (mannoproteins). The highly glycosylated protein coat mediates interactions between C. albicans cells and their environment, including reprograming of wall architecture in response to several conditions, such as carbon source, pH, high temperature, and morphogenesis. The mannoproteins are also associated with C. albicans adherence, drug resistance, and virulence. Vitally, the mannoproteins contribute to cell wall construction and especially cell wall remodeling when cells encounter physical and chemical stresses. This review describes the interconnected cell wall integrity (CWI) and stress-activated pathways (e.g., Hog1, Cek1, and Mkc1 mediated pathways) that regulates cell wall remodeling and the expression of some of the mannoproteins in C. albicans and other species. The mannoproteins of the surface coat is of great importance to pathogen survival, growth, and virulence, thus understanding their structure and function as well as regulatory mechanisms can pave the way for better management of candidiasis.

Profuse diversity and acidogenicity of the candida-biome of deep carious lesions of Severe Early Childhood Caries (S-ECC)

Introduction: The retentive niches of deep caries lesions have a distinct biome. Methods: We evaluated the site-specific (occlusal and proximal) Candida-biome of Severe-Early Childhood Caries (S-ECC) in 66- children (132 lesions). Asymptomatic primary molars fitting the definition of the International Caries Detection and Assessment-(ICDAS)-caries-code 5/6 were analyzed. Deep-dentinal sampling and simultaneous assessment of pH were performed. Clinical isolates were speciated using multiplex-PCR and evaluated for their acidogenic and aciduric potential.Results: Surprisingly, a high prevalence of Candida species (72.7%), either singly or in combination, was noted from both the proximal and occlusal cavities. C. tropicalis was the most prevalent species (47%; 34/72), followed by C. krusei (43.1%; 31/72) and C. albicans (40.3%; 29/72), with C. glabrata being the least (9.7%; 7/72). Over 45% low-pH niches (pH <7) of both sites yielded either dual or triple species of Candida. Genotyping revealed three distinct C. albicans genotypes (A, B, and C) with (14/29; 48.3%) of strains belonging to Genotype A. All four evaluated Candida species exhibited acidogenic and aciduric potential, C. tropicalis being the most potent.Conclusion: This, the first report of the high-density, multispecies, yeast colonization of deep-dentinal lesions in S-ECC, suggests that the Candida-biome plays a significant etiologic role in the condition, possibly due to their profound acidogenicity in milieus rich in dietary carbohydrates.

Activation of Cph1 causes ß(1,3)-glucan unmasking in Candida albicans and attenuates virulence in mice in a neutrophil-dependent manner

Masking the immunogenic cell wall epitope ß(1,3)-glucan under an outer layer of mannosylated glycoproteins is an important virulence factor deployed by Candida albicans during infection. Consequently, increased ß(1,3)-glucan exposure (unmasking) reveals C. albicans to the host's immune system and attenuates its virulence. We have previously shown that activation of the Cek1 MAPK pathway via expression of a hyperactive allele of an upstream kinase (STE11ΔN467) induced unmasking. It also increased survival of mice in a murine disseminated candidiasis model and attenuated kidney fungal burden by ≥33 fold. In this communication, we utilized cyclophosphamide-induced immunosuppression to test if the clearance of the unmasked STE11ΔN467 mutant was dependent on the host immune system. Suppression of the immune response by cyclophosphamide reduced the attenuation in fungal burden caused by the STE11ΔN467 allele. Moreover, specific depletion of neutrophils via 1A8 antibody treatment also reduced STE11ΔN467-dependent fungal burden attenuation, but to a lesser extent than cyclophosphamide, demonstrating an important role for neutrophils in mediating fungal clearance of unmasked STE11ΔN467 cells. In an effort to understand the mechanism by which Ste11ΔN467 causes unmasking, transcriptomics were used to reveal that several components in the Cek1 MAPK pathway were upregulated, including the transcription factor CPH1 and the cell wall sensor DFI1. In this report we show that a cph1ΔΔ mutation restored ß(1,3)-glucan exposure to wild-type levels in the STE11ΔN467 strain, confirming that Cph1 is the transcription factor mediating Ste11ΔN467-induced unmasking. Furthermore, Cph1 is shown to induce a positive feedback loop that increases Cek1 activation. In addition, full unmasking by STE11ΔN467 is dependent on the upstream cell wall sensor DFI1. However, while deletion of DFI1 significantly reduced Ste11ΔN467-induced unmasking, it did not impact activation of the downstream kinase Cek1. Thus, it appears that once stimulated by Ste11ΔN467, Dfi1 activates a parallel signaling pathway that is involved in Ste11ΔN467-induced unmasking.

Revisiting the Vital Drivers and Mechanisms of β-Glucan Masking in Human Fungal Pathogen, Candida albicans

Among the several human fungal pathogens, Candida genus represents one of the most implicated in the clinical scenario. There exist several distinctive features that govern the establishment of Candida infections in addition to their capacity to adapt to multiple stress conditions inside humans which also include evasion of host immune responses. The complex fungal cell wall of the prevalent pathogen, Candida albicans, is one of the main targets of antifungal drugs and recognized by host immune cells. The wall consists of tiered arrangement of an outer thin but dense covering of mannan and inner buried layers of β-glucan and chitin. However, the pathogenic fungi adopt strategies to evade immune recognition by masking these molecules. This capacity to camouflage the immunogenic polysaccharide β-glucan from the host is a key virulence factor of C. albicans. The present review is an attempt to collate various underlying factors and mechanisms involved in Candida β-glucan masking from the available pool of knowledge and provide a comprehensive understanding. This will further improve therapeutic approaches to candidiasis by identifying new antifungal targets that blocks fungal immune evasion.

Metabolic flexibility and extensive adaptability governing multiple drug resistance and enhanced virulence in Candida albicans

Commensal fungus-Candida albicans turn pathogenic during the compromised immunity of the host, causing infections ranging from superficial mucosal to dreadful systemic ones. C. albicans has evolved various adaptive measures which collectively contribute towards its enhanced virulence. Among fitness attributes, metabolic flexibility and vigorous stress response are essential for its pathogenicity and virulence. Metabolic flexibility provides a means for nutrient assimilation and growth in diverse host microenvironments and reduces the vulnerability of the pathogen to various antifungals besides evading host immune response(s). Inside the host micro-environments, C. albicans efficiently utilizes the multiple fermentable and non-fermentable carbon sources to sustain and proliferate in glucose deficit conditions. The utilization of alternative carbon sources further highlights the importance of understanding these pathways as the attractive and potential therapeutic target. A thorough understanding of metabolic flexibility and adaptation to environmental stresses is warranted to decipher in-depth insights into virulence and molecular mechanisms of fungal pathogenicity. In this review, we have attempted to provide a detailed and recent understanding of some key aspects of fungal biology. Particular focus will be placed on processes like nutrient assimilation and utilization, metabolic adaptability, virulence factors, and host immune response in C. albicans leading to its enhanced pathogenicity.

pH Changes Have a Profound Effect on Gene Expression, Hydrolytic Enzyme Production, and Dimorphism in Saccharomycopsis fibuligera

Saccharomycopsis fibuligera is an amylolytic yeast that plays an important role within nuruk (a traditional Korean fermentation starter) used for the production of makgeolli (Korean rice wine), which is characterized by high acidity. However, the effect of pH change (neutral to acidic) on the yeast cell to hyphal transition and carbohydrate-hydrolyzing enzyme activities for S. fibuligera has not been investigated yet. In this study, S. fibuligera strains were cultured under the different pH conditions, and the effect on the enzyme production and gene expression were investigated. An acidic pH induced a hyphal transition from yeast cell of S. fibuligera KPH12 and the hybrid strain KJJ81. In addition, both strains showed a gradual decrease in the ability to degrade starch and cellulose as the pH went down. Furthermore, a transcriptome analysis demonstrated that the pH decline caused global expression changes in genes, which were classified into five clusters. Among the differentially expressed genes (DEGs) under acidic pH, the downregulated genes were involved in protein synthesis, carbon metabolism, and RIM101 and cAMP-PKA signaling transduction pathways for the yeast-hyphal transition. A decrease in pH induced a dimorphic lifestyle switch from yeast cell formation to hyphal growth in S. fibuligera and caused a decrease in carbohydrate hydrolyzing enzyme production, as well as marked changes in the expression of genes related to enzyme production and pH adaptation. This study will help to elucidate the mechanism of adaptation of S. fibuligera to acidification that occur during the fermentation process of makgeolli using nuruk.

Candida auris Cell Wall Mannosylation Contributes to Neutrophil Evasion through Pathways Divergent from Candida albicans and Candida glabrata

Candida auris, a recently emergent fungal pathogen, has caused invasive infections in health care settings worldwide. Mortality rates approach 60% and hospital spread poses a public health threat. Compared to other Candida spp., C. auris avoids triggering the antifungal activity of neutrophils, innate immune cells that are critical for responding to many invasive fungal infections, including candidiasis. However, the mechanism underpinning this immune evasion has been largely unknown. Here, we show that C. auris cell wall mannosylation contributes to the evasion of neutrophils ex vivo and in a zebrafish infection model. Genetic disruption of mannosylation pathways (PMR1 and VAN1) diminishes the outer cell wall mannan, unmasks immunostimulatory components, and promotes neutrophil engagement, phagocytosis, and killing. Upon examination of these pathways in other Candida spp. (Candida albicans and Candida glabrata), we did not find an impact on neutrophil interactions. These studies show how C. auris mannosylation contributes to neutrophil evasion though pathways distinct from other common Candida spp. The findings shed light on innate immune evasion for this emerging pathogen.

IMPORTANCE The emerging fungal pathogen Candida auris presents a global public health threat. Therapeutic options are often limited for this frequently drug-resistant pathogen, and mortality rates for invasive disease are high. Previous study has demonstrated that neutrophils, leukocytes critical for the antifungal host defense, do not efficiently recognize and kill C. auris. Here, we show how the outer cell wall of C. auris promotes immune evasion. Disruption of this mannan polysaccharide layer renders C. auris susceptible to neutrophil killing ex vivo and in a zebrafish model of invasive candidiasis. The role of these mannosylation pathways for neutrophil evasion appears divergent from other common Candida species.

The impact of the Fungus-Host-Microbiota interplay upon Candida albicans infections: current knowledge and new perspectives

Candida albicans is a major fungal pathogen of humans. It exists as a commensal in the oral cavity, gut or genital tract of most individuals, constrained by the local microbiota, epithelial barriers and immune defences. Their perturbation can lead to fungal outgrowth and the development of mucosal infections such as oropharyngeal or vulvovaginal candidiasis, and patients with compromised immunity are susceptible to life-threatening systemic infections. The importance of the interplay between fungus, host and microbiota in driving the transition from C. albicans commensalism to pathogenicity is widely appreciated. However, the complexity of these interactions, and the significant impact of fungal, host and microbiota variability upon disease severity and outcome, are less well understood. Therefore, we summarise the features of the fungus that promote infection, and how genetic variation between clinical isolates influences pathogenicity. We discuss antifungal immunity, how this differs between mucosae, and how individual variation influences a person's susceptibility to infection. Also, we describe factors that influence the composition of gut, oral and vaginal microbiotas, and how these affect fungal colonisation and antifungal immunity. We argue that a detailed understanding of these variables, which underlie fungal-host-microbiota interactions, will present opportunities for directed antifungal therapies that benefit vulnerable patients.

Anticipatory Stress Responses and Immune Evasion in Fungal Pathogens

In certain niches, microbes encounter environmental challenges that are temporally linked. In such cases, microbial fitness is enhanced by the evolution of anticipatory responses where the initial challenge simultaneously activates pre-emptive protection against the second impending challenge. The accumulation of anticipatory responses in domesticated yeasts, which have been termed 'adaptive prediction', has led to the emergence of 'core stress responses' that provide stress cross-protection. Protective anticipatory responses also seem to be common in fungal pathogens of humans. These responses reflect the selective pressures that these fungi have faced relatively recently in their evolutionary history. Consequently, some pathogens have evolved 'core environmental responses' which exploit host signals to trigger immune evasion strategies that protect them against imminent immune attack.

A buffered media system for yeast batch culture growth

Background

Fungi are premier hosts for the high-yield secretion of proteins for biomedical and industrial applications. The stability and activity of these secreted proteins is often dependent on the culture pH. As yeast acidifies the commonly used synthetic complete drop-out (SD) media that contains ammonium sulfate, the pH of the media needs to be buffered in order to maintain a desired extracellular pH during biomass production. At the same time, many buffering agents affect growth at the concentrations needed to support a stable pH. Although the standard for biotechnological research and development is shaken batch cultures or microtiter plate cultures that cannot be easily automatically pH-adjusted during growth, there is no comparative study that evaluates the buffering capacity and growth effects of different media types across pH-values in order to develop a pH-stable batch culture system.

Results

We systematically test the buffering capacity and growth effects of a citrate-phosphate buffer (CPB) from acidic to neutral pH across different media types. These media types differ in their nitrogen source (ammonium sulfate, urea or both). We find that the widely used synthetic drop-out media that uses ammonium sulfate as nitrogen source can only be effectively buffered at buffer concentrations that also affect growth. At lower concentrations, yeast biomass production still acidifies the media. When replacing the ammonium sulfate with urea, the media alkalizes. We then develop a medium combining ammonium sulfate and urea which can be buffered at low CPB concentrations that do not affect growth. In addition, we show that a buffer based on Tris/HCl is not effective in maintaining any of our media types at neutral pH even at relatively high concentrations.

Conclusion

Here we show that the buffering of yeast batch cultures is not straight-forward and addition of a buffering agent to set a desired starting pH does not guarantee pH-maintenance during growth. In response, we present a buffered media system based on an ammonium sulfate/urea medium that enables relatively stable pH-maintenance across a wide pH-range without affecting growth. This buffering system is useful for protein-secretion-screenings, antifungal activity assays, as well as for other pH-dependent basic biology or biotechnology projects.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02191-5.

Sodium houttuyfonate attenuates dextran sulfate sodium associated colitis precolonized with Candida albicans through inducing β-glucan exposure

Inflammatory bowel disease (IBD) including Crohn's disease and ulcerative colitis is a chronic intestinal disease most likely associated with gut dysbiosis. Candida related mycobiota has been demonstrated to play a role in IBD progression. Traditional Chinese herbal medicines (TCHMs) with antifungal activity have a potential in prevention and treatment of fungi-related IBD. Sodium houttuyfonate (SH) is a promising anti-Candida TCHMs. In this study, a dextran sulfate sodium induced colitis model with Candida albicans precolonization is established. SH gavage can significantly decrease the fungal burdens in feces and colon tissues, reduce disease activity index score, elongate colon length, and attenuate colonic damages. Moreover, SH markedly inhibits the levels of anti-Saccharomyces cerevisiae antibodies, β-glucan, and proinflammatory cytokine (IL-1β, IL-6, IL-8, TNF-α), and increases anti-inflammatory factor IL-10 level in serum and colon tissue. Further experiments demonstrate that SH could induce β-glucan exposure, priming intestinal macrophages to get rid of colonized C. albicans through the collaboration of Dectin-1 and TLR2/4. With the decreased fungal burden, the protein levels of Dectin-1, TLR2, TLR4, and NF-κBp65 are fallen back, indicating the primed macrophages calm down and the colitis is alleviated. Collectively, these results manifest that SH can attenuate C. albicans associated colitis via β-glucan exposure, deepening our understanding of TCHMs in the prevention and treatment of fungi associated IBD.

Replicative Aging Remodels the Cell Wall and Is Associated with Increased Intracellular Trafficking in Human Pathogenic Yeasts

Replicative aging is an underexplored field of research in medical mycology. Cryptococcus neoformans (Cn) and Candida glabrata (Cg) are dreaded fungal pathogens that cause fatal invasive infections. The fungal cell wall is essential for yeast viability and pathogenesis. In this study, we provide data characterizing age-associated modifications to the cell wall of Cn and Cg. Here, we report that old yeast cells upregulate genes of cell wall biosynthesis, leading to cell wall reorganization and increased levels of all major components, including glucan, chitin, and its derivatives, as well as mannan. This results in a significant thickening of the cell wall in aged cells. Old-generation yeast cells exhibited drastic ultrastructural changes, including the presence of abundant vesicle-like particles in the cytoplasm, and enlarged vacuoles with altered pH homeostasis. Our findings suggest that the cell wall modifications could be enabled by augmented intracellular trafficking. This work furthers our understanding of the cell phenotype that emerges during aging. It highlights differences in these two fungal pathogens and elucidates mechanisms that explain the enhanced resistance of old cells to antifungals and phagocytic attacks. IMPORTANCE Cryptococcus neoformans and Candida glabrata are two opportunistic human fungal pathogens that cause life-threatening diseases. During infection, both microorganisms have the ability to persist for long periods, and treatment failure can occur even if standard testing identifies the yeasts to be sensitive to antifungals. Replicative life span is a trait that is measured by the number of divisions a cell undergoes before death. Aging in fungi is associated with enhanced tolerance to antifungals and resistance to phagocytosis, and characterization of old cells may help identify novel antifungal targets. The cell wall remains an attractive target for new therapies because it is essential for fungi and is not present in humans. This study shows that the organization of the fungal cell wall changes remarkably during aging and becomes thicker and is associated with increased intracellular trafficking as well as the alteration of vacuole morphology and pH homeostasis.

Immune Sensing of Candida albicans

Candida albicans infections range from superficial to systemic and are one of the leading causes of fungus-associated nosocomial infections. The innate immune responses during these various infection types differ, suggesting that the host environment plays a key role in modulating the host–pathogen interaction. In addition, C. albicans is able to remodel its cell wall in response to environmental conditions to evade host clearance mechanisms and establish infection in niches, such as the oral and vaginal mucosa. Phagocytes play a key role in clearing C. albicans, which is primarily mediated by Pathogen Associated Molecular Pattern (PAMP)–Pattern Recognition Receptor (PRR) interactions. PRRs such as Dectin-1, DC-SIGN, and TLR2 and TLR4 interact with PAMPs such as β-glucans, N-mannan and O-mannan, respectively, to trigger the activation of innate immune cells. Innate immune cells exhibit distinct yet overlapping repertoires of PAMPs, resulting in the preferential recognition of particular Candida morphotypes by them. The role of phagocytes in the context of individual infection types also differs, with neutrophils playing a prominent role in kidney infections, and dendritic cells playing a prominent role in skin infections. In this review, we provide an overview of the key receptors involved in the detection of C. albicans and discuss the differential innate immune responses to C. albicans seen in different infection types such as vulvovaginal candidiasis (VVC) and oral candidiasis.

Association between gastric Candida colonization and surgical site infections after high-level hepatobiliary pancreatic surgeries: the results of prospective observational study

AIM

High-level hepatobiliary pancreatic (HBP) surgeries are highly associated with surgical site infections (SSIs), in which microorganisms have a significant role. In the present study, we investigated whether gastric Candida colonization had a significant role in SSIs after high-level HBP surgeries.

METHODS

Between May 2016 and February 2017, the 66 patients who underwent high-level HBP surgeries were enrolled in the present study. The gastric juice was prospectively collected through nasogastric tube after general anesthesia induction and was incubated onto the CHROMagar Candida plate for the cultivation of various Candida species. First of all, we compared the incidence of SSIs according to the presence or absence of Candida species in gastric juice. Secondly, we evaluated the variables contributing to the development of SSIs by multivariate analysis. The protocol was approved by the medical ethics committee of Mie University Hospital (No.2987).

RESULTS

Gastric Candida colonization was identified in 21 patients (group GC) and was not identified in the other 45 patients (group NGC). There were no differences in preoperative variables including compromised status, such as age, nutritional markers, complications of diabetes mellitus, and types of primary disease between the two groups. SSIs occurred in 57.1% (12/21) of group GC and in 17.8% (8/45) of group NGC, showing a significant difference (p = 0.001). Multivariate analysis revealed gastric Candida colonization as a significant risk factor of SSIs (OR 6.17, p = 0.002).

CONCLUSION

Gastric Candida colonization, which is not a result of immunocompromised status, is highly associated with SSIs after high-level HBP surgeries.

TRIAL REGISTRATION

Japan Primary Registries Network; UMIN-CTR ID: UMIN000040486 (retrospectively registered on 22nd May, 2020).

Carbon flow conversion induces alkali resistance and lipid accumulation under alkaline conditions based on transcriptome analysis in Chlorella sp. BLD

Alkaline environments are abundant globally and cause damage to most organisms, while some microalgae can grow well and accumulate lipids under those conditions. Here the mechanisms of alkali resistance and lipid accumulation in the alkaliphilic microalgae Chlorella sp. BLD were explored using physiological-biochemical and transcriptome analysis. When cultivated at alkaline pH, Chlorella sp. BLD exhibited good alkali-resistance ability and increased biomass (0.97 g L^-1). The biochemical composition of Chlorella sp. BLD changed significantly (lipid content increased 39% and protein content decreased 19.5%) compared with pH 7.5. Through transcriptome analysis, we found that pathways related to carbon metabolism such as photosynthesis, glycolysis, and the TCA cycle were significantly regulated under alkaline conditions. Genes that encoding the key enzyme in carbon-related metabolism such as Rubisco, AMY, PK, ME, CS, ACAT, KAS, and DGAT were identified. Transcriptional regulation of these genes results in carbon flow switching from starch and protein to cell wall metabolism, organic acid synthetic and lipid accumulation in response to alkaline conditions. These results reveal the alkali resistance mechanism of Chlorella sp. BLD and provide a theoretical basis for microalgae oil production under alkaline conditions.

A clinical pilot study on the effect of the probiotic Lacticaseibacillus rhamnosus TOM 22.8 strain in women with vaginal dysbiosis

Lactobacilli with probiotic features play an essential role in maintaining a balanced vaginal microbiota and their administration has been suggested for the treatment and prevention of vaginal dysbiosis. The present study was aimed to in vitro and in vivo investigate the probiotic potential of the Lacticaseibacillus rhamnosus TOM 22.8 strain, isolated from the vaginal ecosystem of a healthy woman. For this purpose, safety and functional properties were in depth evaluated. The strain exhibited a broad spectrum of antagonistic activity against vaginal pathogens; adhesion capacity to both the vaginal VK2/E6E7 and the intestinal Caco-2 cells; anti-inflammatory and antioxidant activities, suggesting its promising probiotic features. In addition, an in vivo pilot-study was planned. Based on both clinical and microbiological parameters, the oral or vaginal strain administration, determined a significant pathogens reduction after 10 days of administration and a maintenance of eubiosis up to 30 days after the end of the treatment. Therefore, the L. rhamnosus TOM 22.8 strain can be proposed as valuable oral and/or vaginal treatment for vaginal dysbiosis.

Exogenous Reproductive Hormones nor Candida albicans Colonization Alter the Near Neutral Mouse Vaginal pH

While human vaginal pH in childbearing-age women is conclusively acidic, the mouse vaginal pH is reported as being near neutral. However, this information appears to be somewhat anecdotal with respect to vulvovaginal candidiasis, as such claims in the literature frequently lack citations of studies that specifically address this physiological factor. Given the disparate pH between mice and humans, the role of exogenous hormones and colonization by the fungal pathogen Candida albicans in shaping vaginal pH was assessed. Use of a convenient modified vaginal lavage technique with the pH indicator dye phenol red demonstrated that indeed vaginal pH was near neutral (7.2 ± 0.24) and was not altered by delivery of progesterone or estrogen in C57BL/6 mice. These trends were conserved in DBA/2 and CD-1 mouse backgrounds, commonly used in the mouse model of vaginitis. It was also determined that vaginal colonization with C. albicans did not alter the globally neutral vaginal pH over the course of one week. Construction and validation of a C. albicans reporter strain expressing GFPy, driven by the pH-responsive PHR1 promoter, confirmed the murine vaginal pH to be at least ≥6.0. Collectively, our data convincingly demonstrate a stable and conserved near neutrality of the mouse vaginal pH during vulvovaginal candidiasis and should serve as a definitive source for future reference. Implications and rationale for disparate pH in this model system are also discussed.

Recognition of Fungal Components by the Host Immune System

By being the first point of contact of the fungus with the host, the cell wall plays an important role in the pathogenesis, having many molecules that participate as antigens that are recognized by immune cells, and also that help the fungus to establish infection. The main molecules reported to trigger an immune response are chitin, glucans, oligosaccharides, proteins, melanin, phospholipids, and others, being present in the principal pathogenic fungi with clinical importance worldwide, such as Histoplasma capsulatum, Paracoccidioides brasiliensis, Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, Blastomyces dermatitidis, and Sporothrix schenckii. Knowledge and understanding of how the immune system recognizes and responds to fungal antigens are relevant for the future research and development of new diagnostic tools and treatments for the control of mycosis caused by these fungi.

Yeasts in Liquid Swine Diets: Identification Methods, Growth Temperatures and Gas-Formation Potential

Liquid feed is susceptible to microbiological growth. Yeasts are said to cause sudden death in swine due to intestinal gas formation. As not all animals given high yeast content feed fall ill, growth and gas formation potential at body temperature were investigated as possible causally required properties. The best identification method for these environmental yeasts should be tested beforehand. Yeasts derived from liquid diets without (LD - S) and liquid diets with maize silage (LD + S) were examined biochemically (ID32C-test) and with MALDI-TOF with direct smear (DS) and an extraction method (EX). Growth temperature and gas-forming potential were measured. With MALDI-EX, most yeast isolates were identified: Candida krusei most often in LD - S, and C. lambica most often in LD + S, significantly more than in LD - S. Larger colonies, 58.75% of all yeast isolates, were formed at 25 °C rather than at 37 °C; 17.5% of all isolates did not grow at 37 °C at all. Most C. krusei isolates formed high gas amounts within 24 h, whereas none of the C. lambica, C. holmii and most other isolates did. The gas pressure formed by yeast isolates varied more than tenfold. Only a minority of the yeasts were able to produce gas at temperatures common in the pig gut.

Scalar nanostructure of the Candida albicans cell wall; a molecular, cellular and ultrastructural analysis and interpretation

Despite the importance of fungal cell walls as the principle determinant of fungal morphology and the defining element determining fungal interactions with other cells, few scalar models have been developed that reconcile chemical and microscopic attributes of its structure. The cell wall of the fungal pathogen Candida albicans is comprised of an amorphous inner skeletal layer of β(1,3)- and β(1,6)-glucan and chitin and an outer fibrillar layer thought to be dominated by highly mannosylated cell wall proteins. The architecture of these two layers can be resolved at the electron microscopy level, but the visualised structure of the wall has not yet been defined precisely in chemical terms. We have therefore examined the precise structure, location and molecular sizes of the cell wall components using transmission electron microscopy and tomography and tested predictions of the cell wall models using mutants and agents that perturb the normal cell wall structure. We demonstrate that the fibrils are comprised of a frond of N-linked outer chain mannans linked to a basal layer of GPI-proteins concentrated in the mid-wall region and that the non-elastic chitin microfibrils are cantilevered with sufficient lengths of non-fibrillar chitin and/or β-glucan to enable the chitin-glucan cage to flex, e.g. during morphogenesis and osmotic swelling. We present the first three-dimensional nano-scalar model of the C. albicans cell wall which can be used to test hypotheses relating to the structure-function relationships that underpin the pathobiology of this fungal pathogen.

Mechanism of Candida pathogenesis: revisiting the vital drivers

Candida is the most implicated fungal pathogen in the clinical setting. Several factors play important roles in the pathogenesis of Candida spp. Multiple transcriptional circuits, morphological and phenotypic switching, biofilm formation, tissue damaging extracellular hydrolytic enzymes, metabolic flexibility, genome plasticity, adaptation to environmental pH fluctuation, robust nutrient acquisition system, adherence and invasions (mediated by adhesins and invasins), heat shock proteins (HSPs), cytolytic proteins, escape from phagocytosis, evasion from host immune system, synergistic coaggregation with resident microbiota, resistance to antifungal agents, and the ability to efficiently respond to multiple stresses are some of the major pathogenic determinants of Candida species. The existence of multiple connections, in addition to the interactions and associations among all of these factors, are distinctive features that play important roles in the establishment of Candida infections. This review describes all the underlying factors and mechanisms involved in Candida pathogenesis by evaluating pathogenic determinants of Candida species. It reinforces the already available pool of data on the pathogenesis of Candida species by providing a clear and simplified understanding of the most important factors implicated in the pathogenesis of Candida species. The Candida pathogenesis network, an illustration linking all the major determinants of Candida pathogenesis, is also presented. Taken together, they will further improve our current understanding of how these factors modulate virulence and consequent infection(s). Development of new antifungal drugs and better therapeutic approaches to candidiasis can be achieved in the near future with continuing progress in the understanding of the mechanisms of Candida pathogenesis.

Understanding How Microorganisms Respond to Acid pH Is Central to Their Control and Successful Exploitation

Microbes from the three domains of life, Bacteria, Archaea, and Eukarya, share the need to sense and respond to changes in the external and internal concentrations of protons. When the proton concentration is high, acidic conditions prevail and cells must respond appropriately to ensure that macromolecules and metabolic processes are sufficiently protected to sustain life. While, we have learned much in recent decades about the mechanisms that microbes use to cope with acid, including the unique challenges presented by organic acids, there is still much to be gained from developing a deeper understanding of the effects and responses to acid in microbes. In this perspective article, we survey the key molecular mechanisms known to be important for microbial survival during acid stress and discuss how this knowledge might be relevant to microbe-based applications and processes that are consequential for humans. We discuss the research approaches that have been taken to investigate the problem and highlight promising new avenues. We discuss the influence of acid on pathogens during the course of infections and highlight the potential of using organic acids in treatments for some types of infection. We explore the influence of acid stress on photosynthetic microbes, and on biotechnological and industrial processes, including those needed to produce organic acids. We highlight the importance of understanding acid stress in controlling spoilage and pathogenic microbes in the food chain. Finally, we invite colleagues with an interest in microbial responses to low pH to participate in the EU-funded COST Action network called EuroMicropH and contribute to a comprehensive database of literature on this topic that we are making publicly available.

Transcriptional and functional insights into the host immune response against the emerging fungal pathogen Candida auris

Candida auris is among the most important emerging fungal pathogens, yet mechanistic insights into its immune recognition and control are lacking. Here, we integrate transcriptional and functional immune-cell profiling to uncover innate defence mechanisms against C. auris. C. auris induces a specific transcriptome in human mononuclear cells, a stronger cytokine response compared with Candida albicans, but a lower macrophage lysis capacity. C. auris-induced innate immune activation is mediated through the recognition of C-type lectin receptors, mainly elicited by structurally unique C. auris mannoproteins. In in vivo experimental models of disseminated candidiasis, C. auris was less virulent than C. albicans. Collectively, these results demonstrate that C. auris is a strong inducer of innate host defence, and identify possible targets for adjuvant immunotherapy.

[Study on growth characteristics of Candida auris under different conditions in vitro and its in vivo toxicity]

OBJECTIVE

To investigate the characteristics of growth and metabolism and the in vivo toxicity of Candida auris under different conditions.

METHODS

We observed the growth of Candida auris and Candida albicans under routine culture conditions and in different pH and salt concentrations, and compared their activities of sugar fermentation using microbiochemical reaction tubes. Four-week-old nude mice were randomized into Candida auris infection group (n=5), Candida albicans infection group (n=5) and control group (n=5) for intragastric administration of 0.3 mL suspension the two Candida species (5×10⁹ cfu/mL) or 0.3 mL normal saline. Samples of the liver, kidney, intestine, feces and blood were taken for analysis of the in vivo distribution and toxicity of Candida albicans by fungal culture and histopathological examination.

RESULTS

Candida auris exhibited logarithmic growth at 8-24 h after inoculation and showed stable growth after 24 h. Candida auris showed optimal growth within the pH value range of 5-7 with a growth pattern identical to that of Candida albicans. Candida auris grew better than Candida albicans in media containing 5% and 10% NaCl, and could ferment glucose, sucrose, trehalose and sorbitol. Candida auris could be isolated from the feces, blood, liver and kidney of infected nude mice, and the liver had the highest fungal load (5.7 log₁₀ cfu/g). Candida auris could cause pathological changes in the liver and intestine of the mice, but with a lesser severity as compared with Candida albicans.

CONCLUSIONS

Candida auris exhibits optimal growth in mildly acidic or neutral conditions with a high salt tolerance, and can potentially penetrate the intestinal barrier into blood and lead to tissue injuries in hosts with immunosuppression.

Fungal plasma membrane domains

The plasma membrane (PM) performs a plethora of physiological processes, the coordination of which requires spatial and temporal organization into specialized domains of different sizes, stability, protein/lipid composition and overall architecture. Compartmentalization of the PM has been particularly well studied in the yeast Saccharomyces cerevisiae, where five non-overlapping domains have been described: The Membrane Compartments containing the arginine permease Can1 (MCC), the H+-ATPase Pma1 (MCP), the TORC2 kinase (MCT), the sterol transporters Ltc3/4 (MCL), and the cell wall stress mechanosensor Wsc1 (MCW). Additional cortical foci at the fungal PM are the sites where clathrin-dependent endocytosis occurs, the sites where the external pH sensing complex PAL/Rim localizes, and sterol-rich domains found in apically grown regions of fungal membranes. In this review, we summarize knowledge from several fungal species regarding the organization of the lateral PM segregation. We discuss the mechanisms of formation of these domains, and the mechanisms of partitioning of proteins there. Finally, we discuss the physiological roles of the best-known membrane compartments, including the regulation of membrane and cell wall homeostasis, apical growth of fungal cells and the newly emerging role of MCCs as starvation-protective membrane domains.

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.

How Fungal Glycans Modulate Platelet Activation via Toll-Like Receptors Contributing to the Escape of Candida albicans from the Immune Response

Platelets are essential for vascular repair and for the maintenance of blood homeostasis. They contribute to the immune defence of the host against many infections caused by bacteria, viruses and fungi. Following infection, platelet function is modified, and these cells form aggregates with microorganisms leading, to a decrease in the level of circulating platelets. During candidaemia, mannans, β-glucans and chitin, exposed on the cell wall of Candida albicans, an opportunistic pathogenic yeast of humans, play an important role in modulation of the host response. These fungal polysaccharides are released into the circulation during infection and their detection allows the early diagnosis of invasive fungal infections. However, their role in the modulation of the immune response and, in particular, that of platelets, is not well understood. The structure and solubility of glycans play an important role in the orientation of the immune response of the host. This short review focuses on the effect of fungal β-glucans and chitin on platelet activation and how these glycans modulate platelet activity via Toll-like receptors, contributing to the escape of C. albicans from the immune response.

The Impact of EBM-Manufactured Ti6Al4V ELI Alloy Surface Modifications on Cytotoxicity toward Eukaryotic Cells and Microbial Biofilm Formation

Electron beam melting (EBM) is an additive manufacturing technique, which allows forming customized implants that perfectly fit the loss of the anatomical structure of bone. Implantation efficiency depends not only on the implant's functional or mechanical properties but also on its surface properties, which are of great importance with regard to such biological processes as bone regeneration or microbial contamination. This work presents the impact of surface modifications (mechanical polishing, sandblasting, and acid-polishing) of EBM-produced Ti6Al4V ELI implants on essential biological parameters. These include wettability, cytotoxicity toward fibroblast and osteoblast cell line, and ability to form biofilm by Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. Obtained results indicated that all prepared surfaces exhibited hydrophilic character and the highest changes of wettability were obtained by chemical modification. All implants displayed no cytotoxicity against osteoblast and fibroblast cell lines regardless of the modification type. In turn, the quantitative microbiological tests and visualization of microbial biofilm by means of electron microscopy showed that type of implant's modification correlated with the species-specific ability of microbes to form biofilm on it. Thus, the results of the presented study confirm the relationship between such technological aspects as surface modification and biological properties. The provided data are useful with regard to applications of the EBM technology and present a significant step towards personalized, customized implantology practice.

In Vitro Incorporation of Helicobacter pylori into Candida albicans Caused by Acidic pH Stress

Yeasts can adapt to a wide range of pH fluctuations (2 to 10), while Helicobacter pylori, a facultative intracellular bacterium, can adapt to a range from pH 6 to 8. This work analyzed if H. pylori J99 can protect itself from acidic pH by entering into Candida albicans ATCC 90028. Growth curves were determined for H. pylori and C. albicans at pH 3, 4, and 7. Both microorganisms were co-incubated at the same pH values, and the presence of intra-yeast bacteria was evaluated. Intra-yeast bacteria-like bodies were detected using wet mounting, and intra-yeast binding of anti-H. pylori antibodies was detected using immunofluorescence. The presence of the H. pylori rDNA 16S gene in total DNA from yeasts was demonstrated after PCR amplification. H. pylori showed larger death percentages at pH 3 and 4 than at pH 7. On the contrary, the viability of the yeast was not affected by any of the pHs evaluated. H. pylori entered into C. albicans at all the pH values assayed but to a greater extent at unfavorable pH values (pH 3 or 4, p = 0.014 and p = 0.001, respectively). In conclusion, it is possible to suggest that H. pylori can shelter itself within C. albicans under unfavorable pH conditions.

Human microbiome: an academic update on human body site specific surveillance and its possible role

Human body is inhabited by vast number of microorganisms which form a complex ecological community and influence the human physiology, in the aspect of both health and diseases. These microbes show a relationship with the human immune system based on coevolution and, therefore, have a tremendous potential to contribute to the metabolic function, protection against the pathogen and in providing nutrients and energy. However, of these microbes, many carry out some functions that play a crucial role in the host physiology and may even cause diseases. The introduction of new molecular technologies such as transcriptomics, metagenomics and metabolomics has contributed to the upliftment on the findings of the microbiome linked to the humans in the recent past. These rapidly developing technologies are boosting our capacity to understand about the human body-associated microbiome and its association with the human health. The highlights of this review are inclusion of how to derive microbiome data and the interaction between human and associated microbiome to provide an insight on the role played by the microbiome in biological processes of the human body as well as the development of major human diseases.

Iron alters the cell wall composition and intracellular lactate to affect Candida albicans susceptibility to antifungals and host immune response

Fungal pathogen Candida albicans has a complex cell wall consisting of an outer layer of mannans and an inner layer of β-glucans and chitin. The fungal cell wall is the primary target for antifungals and is recognized by host immune cells. Environmental conditions such as carbon sources, pH, temperature, and oxygen tension can modulate the fungal cell wall architecture. Cellular signaling pathways, including the mitogen-activated protein kinase (MAPK) pathways, are responsible for sensing environmental cues and mediating cell wall alterations. Although iron has recently been shown to affect β-1,3-glucan exposure on the cell wall, we report here that iron changes the composition of all major C. albicans cell wall components. Specifically, high iron decreased the levels of mannans (including phosphomannans) and chitin; and increased β-1,3-glucan levels. These changes increased the resistance of C. albicans to cell wall-perturbing antifungals. Moreover, high iron cells exhibited adequate mitochondrial functioning; leading to a reduction in accumulation of lactate that signals through the transcription factor Crz1 to induce β-1,3-glucan masking in C. albicans We show here that iron-induced changes in β-1,3-glucan exposure are lactate-dependent; and high iron causes β-1,3-glucan exposure by preventing lactate-induced, Crz1-mediated inhibition of activation of the fungal MAPK Cek1. Furthermore, despite exhibiting enhanced antifungal resistance, high iron C. albicans cells had reduced survival upon phagocytosis by macrophages. Our results underscore the role of iron as an environmental signal in multiple signaling pathways that alter cell wall architecture in C. albicans, thereby affecting its survival upon exposure to antifungals and host immune response.

Adapting to survive: How Candida overcomes host-imposed constraints during human colonization

Successful human colonizers such as Candida pathogens have evolved distinct strategies to survive and proliferate within the human host. These include sophisticated mechanisms to evade immune surveillance and adapt to constantly changing host microenvironments where nutrient limitation, pH fluctuations, oxygen deprivation, changes in temperature, or exposure to oxidative, nitrosative, and cationic stresses may occur. Here, we review the current knowledge and recent findings highlighting the remarkable ability of medically important Candida species to overcome a broad range of host-imposed constraints and how this directly affects their physiology and pathogenicity. We also consider the impact of these adaptation mechanisms on immune recognition, biofilm formation, and antifungal drug resistance, as these pathogens often exploit specific host constraints to establish a successful infection. Recent studies of adaptive responses to physiological niches have improved our understanding of the mechanisms established by fungal pathogens to evade the immune system and colonize the host, which may facilitate the design of innovative diagnostic tests and therapeutic approaches for Candida infections.

Inhibition of Respiration of Candida albicans by Small Molecules Increases Phagocytosis Efficacy by Macrophages

Candida albicans adapts to various conditions in different body niches by regulating gene expression, protein synthesis, and metabolic pathways. These adaptive reactions not only allow survival but also influence the interaction with host cells, which is governed by the composition and structure of the fungal cell wall. Numerous studies had shown linkages between mitochondrial functionality, cell wall integrity and structure, and pathogenicity. Thus, we decided to inhibit single complexes of the respiratory chain of C. albicans and to analyze the resultant interaction with macrophages via their phagocytic activity. Remarkably, inhibition of the fungal bc₁ complex by antimycin A increased phagocytosis, which correlated with an increased accessibility of β-glucans. To contribute to mechanistic insights, we performed metabolic studies, which highlighted significant changes in the abundance of constituents of the plasma membrane. Collectively, our results reinforce the strong linkage between fungal energy metabolism and other components of fungal physiology, which also determine the vulnerability to immune defense reactions.IMPORTANCE The yeast Candida albicans is one of the major fungal human pathogens, for which new therapeutic approaches are required. We aimed at enhancements of the phagocytosis efficacy of macrophages by targeting the cell wall structure of C. albicans, as the coverage of the β-glucan layer by mannans is one of the immune escape mechanisms of the fungus. We unambiguously show that inhibition of the fungal bc₁ complex correlates with increased accessibilities of β-glucans and improved phagocytosis efficiency. Metabolic studies proved not only the known direct effects on reactive oxygen species (ROS) production and fermentative pathways but also the clear downregulation of the ergosterol pathway and upregulation of unsaturated fatty acids. The changed composition of the plasma membrane could also influence the interaction with the overlying cell wall. Thus, our work highlights the far-reaching relevance of energy metabolism, indirectly also for host-pathogen interactions, without affecting viability.

Fluconazole and Lipopeptide Surfactin Interplay During Candida albicans Plasma Membrane and Cell Wall Remodeling Increases Fungal Immune System Exposure

Recognizing the β-glucan component of the Candida albicans cell wall is a necessary step involved in host immune system recognition. Compounds that result in exposed β-glucan recognizable to the immune system could be valuable antifungal drugs. Antifungal development is especially important because fungi are becoming increasingly drug resistant. This study demonstrates that lipopeptide, surfactin, unmasks β-glucan when the C. albicans cells lack ergosterol. This observation also holds when ergosterol is depleted by fluconazole. Surfactin does not enhance the effects of local chitin accumulation in the presence of fluconazole. Expression of the CHS3 gene, encoding a gene product resulting in 80% of cellular chitin, is downregulated. C. albicans exposure to fluconazole changes the composition and structure of the fungal plasma membrane. At the same time, the fungal cell wall is altered and remodeled in a way that makes the fungi susceptible to surfactin. In silico studies show that surfactin can form a complex with β-glucan. Surfactin forms a less stable complex with chitin, which in combination with lowering chitin synthesis, could be a second anti-fungal mechanism of action of this lipopeptide.