N-acetylglucosamine Regulates Virulence Properties in Microbial Pathogens

The pathobiology of human fungal infections

Human fungal infections are a historically neglected area of disease research, yet they cause more than 1.5 million deaths every year. Our understanding of the pathophysiology of these infections has increased considerably over the past decade, through major insights into both the host and pathogen factors that contribute to the phenotype and severity of these diseases. Recent studies are revealing multiple mechanisms by which fungi modify and manipulate the host, escape immune surveillance and generate complex comorbidities. Although the emergence of fungal strains that are less susceptible to antifungal drugs or that rapidly evolve drug resistance is posing new threats, greater understanding of immune mechanisms and host susceptibility factors is beginning to offer novel immunotherapeutic options for the future. In this Review, we provide a broad and comprehensive overview of the pathobiology of human fungal infections, focusing specifically on pathogens that can cause invasive life-threatening infections, highlighting recent discoveries from the pathogen, host and clinical perspectives. We conclude by discussing key future challenges including antifungal drug resistance, the emergence of new pathogens and new developments in modern medicine that are promoting susceptibility to infection.

Engineering E. coli strains using antibiotic-resistance-gene-free plasmids

We created a generalizable pipeline for antibiotic-resistance-gene-free plasmid (ARGFP)-based cloning using a dual auxotrophic- and essential-gene-based selection strategy. We use auxotrophic selection to construct plasmids in engineered E. coli DH10B cloning strains and both auxotrophic- and essential-gene-based selection to (1) select for recombinant strains and (2) maintain a plasmid in E. coli Nissle 1917, a common chassis for engineered probiotic applications, and E. coli MG1655, the laboratory "wild-type" E. coli strain. We show that our approach has comparable efficiency to that of antibiotic-resistance-gene-based cloning. We also show that the double-knockout Nissle and MG1655 strains are simple to transform with plasmids of interest. Notably, we show that the engineered Nissle strains are amenable to long-term plasmid maintenance in repeated culturing as well as in the mouse gut, demonstrating the potential for broad applications while minimizing the risk of antibiotic resistance spread via horizontal gene transfer.

Establishment of the colloidal chitin enzymatic hydrolysis conditions to obtain N-acetyl glucosamine

The wastes generated by the shrimp industry are approximately between 50 and 60% of the catch volume. These residues such as head, viscera, and shell are potential pollutants if they are not treated for proper disposal. One way to solve this problem is to use the residues as functional food ingredients. In this regard, shrimp residues are rich in chitin, the second most abundant biopolymer on the planet after cellulose. Chitin is composed of N-acetyl glucosamine, a molecule used as a sweetener in the food industry and as an aid in the treatment of coronary diseases and gonarthrosis. N-acetyl glucosamine can be obtained by the hydrolysis of colloidal chitin using chemical or enzymatic methods; however, chemical methods are associated with pollution. In this study, we determined the hydrolysis conditions of shrimp colloidal chitin for obtaining N-acetyl glucosamine, using the extracellular enzymes produced by a marine bacterium isolated in the coastal zone of Progreso, Yucatan, Mexico. The best N-acetyl glucosamine yield obtained was 2.65%, using 10 mg/mL colloidal chitin, at 60°C, and pH 8.9 with 3.5% NaCl.

Bacterial virulence regulation through soluble peptidoglycan fragments sensing and response: knowledge gaps and therapeutic potential

Given the growing clinical-epidemiological threat posed by the phenomenon of antibiotic resistance, new therapeutic options are urgently needed, especially against top nosocomial pathogens such as those within the ESKAPE group. In this scenario, research is pushed to explore therapeutic alternatives and, among these, those oriented toward reducing bacterial pathogenic power could pose encouraging options. However, the first step in developing these antivirulence weapons is to find weak points in the bacterial biology to be attacked with the goal of dampening pathogenesis. In this regard, during the last decades some studies have directly/indirectly suggested that certain soluble peptidoglycan-derived fragments display virulence-regulatory capacities, likely through similar mechanisms to those followed to regulate the production of several β-lactamases: binding to specific transcriptional regulators and/or sensing/activation of two-component systems. These data suggest the existence of intra- and also intercellular peptidoglycan-derived signaling capable of impacting bacterial behavior, and hence likely exploitable from the therapeutic perspective. Using the well-known phenomenon of peptidoglycan metabolism-linked β-lactamase regulation as a starting point, we gather and integrate the studies connecting soluble peptidoglycan sensing with fitness/virulence regulation in Gram-negatives, dissecting the gaps in current knowledge that need filling to enable potential therapeutic strategy development, a topic which is also finally discussed.

Human tear metabolites associated with nucleoside-signalling pathways in bacterial keratitis

OBJECTIVE

The study aimed to profile and quantify tear metabolites associated with bacterial keratitis using both untargeted and targeted metabolomic platforms.

METHODS

Untargeted metabolomic analysis using liquid-chromatography-Q Exactive-HF mass-spectrometry explored tear metabolites significantly associated with bacterial keratitis (n = 6) compared to healthy participants (n = 6). Differential statistics and principal component analysis determined meaningful metabolite differences between cases and controls. Purines and nucleosides were further quantified and compared between 15 cases and 15 controls in the targeted metabolomic platform using TSQ quantum access triple quadrupole mass spectrometry. Compound quantification was done by plotting the calibration curves and the difference in the compound levels was evaluated using the Wilcoxon rank-sum test.

RESULTS

In the untargeted analysis, 49 tear metabolites (27 upregulated and 22 downregulated) were differentially expressed between cases and controls. The untargeted analysis indicated that the purine metabolism pathway was the most affected by bacterial keratitis. Metabolite quantification in the targeted analysis further confirmed the upregulation of xanthine (P = 0.02) and downregulation of adenine (P < 0.0001), adenosine (P < 0.0001) and cytidine (P < 0.0001) in the tears of participants with bacterial keratitis compared to that of healthy participants.

CONCLUSIONS

Bacterial keratitis significantly changes the tear metabolite profile, including five major compound classes such as indoles, amino acids, nucleosides, carbohydrates, and steroids. This study also indicates that tear fluids can be used to map the metabolic pathways and uncover metabolic markers associated with bacterial keratitis. Conceivably, the inhibition of nucleoside synthesis may contribute to the pathophysiology of bacterial keratitis because nucleosides are required for maintaining cellular energy homeostasis and immune adaptability.

Functional Characterization of the GlcNAc Catabolic Pathway in Cryptococcus deneoformans

The amino sugar N-acetyl-d-glucosamine (GlcNAc) is the key constituent of cell wall components and plays an important role in pathogenesis in a wide range of fungi. However, catabolism of GlcNAc has not been studied in basidiomycete fungi. In this study, we identified and characterized a gene cluster essential for GlcNAc utilization in Cryptococcus deneoformans, an environmental human fungal pathogen. The C. deneoformans genome contains a GlcNAc transporter (Ngt1), a GlcNAc kinase (Hxk3), a GlcNAc-6-phosphate deacetylase (Dac1), and a glucosamine-6-phosphate deaminase (Nag1). Their expression levels were highly induced in cultures containing GlcNAc as the sole carbon source, and the corresponding mutants showed severe growth defects in the presence of GlcNAc. Functional and biochemical analyses revealed that HXK3 encodes a novel GlcNAc kinase. Site-directed mutations of conserved residues of Hxk3 indicated that ATP binding and GlcNAc binding are essential for GlcNAc kinase activities. Taken together, the results from this study provide crucial insights into basidiomycete GlcNAc catabolism. IMPORTANCEN-Acetylglucosamine (GlcNAc) is recognized as not only the building block of chitin but also an important signaling molecule in fungi. The catabolic pathway of GlcNAc also plays an important role in vital biological processes in fungi. However, the utilization pathway of GlcNAc in the phylum Basidiomycota, which contains more than 41,000 species, remains unknown. Cryptococcus deneoformans is a representative basidiomycetous pathogen that causes life-threatening meningitis. In this study, we characterized a gene cluster essential for GlcNAc utilization in C. deneoformans and identified a novel GlcNAc kinase. The results of this study provide important insights into basidiomycete GlcNAc catabolism and offer a starting point for revealing its role in pathogenesis.

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.

N-Acetylglucosamine Sensing and Metabolic Engineering for Attenuating Human and Plant Pathogens

During evolution, both human and plant pathogens have evolved to utilize a diverse range of carbon sources. N-acetylglucosamine (GlcNAc), an amino sugar, is one of the major carbon sources utilized by several human and phytopathogens. GlcNAc regulates the expression of many virulence genes of pathogens. In fact, GlcNAc catabolism is also involved in the regulation of virulence and pathogenesis of various human pathogens, including Candida albicans, Vibrio cholerae, Leishmania donovani, Mycobacterium, and phytopathogens such as Magnaporthe oryzae. Moreover, GlcNAc is also a well-known structural component of many bacterial and fungal pathogen cell walls, suggesting its possible role in cell signaling. Over the last few decades, many studies have been performed to study GlcNAc sensing, signaling, and metabolism to better understand the GlcNAc roles in pathogenesis in order to identify new drug targets. In this review, we provide recent insights into GlcNAc-mediated cell signaling and pathogenesis. Further, we describe how the GlcNAc metabolic pathway can be targeted to reduce the pathogens’ virulence in order to control the disease prevalence and crop productivity.

Multifunctional transcription factor CytR of Vibrio cholerae is important for pathogenesis

Vibrio cholerae, the Gram-negative facultative pathogen, resides in the aquatic environment and infects humans and causes diarrhoeagenic cholera. Although the environment differs drastically, V. cholerae thrives in both of these conditions aptly and chitinases play a vital role in their persistence and nutrient acquisition. Chitinases also play a role in V. cholerae pathogenesis. Chitinases and its downstream chitin utilization genes are regulated by sensor histidine kinase ChiS, which also plays a significant role in pathogenesis. Recent exploration suggests that CytR, a transcription factor of the LacI family in V. cholerae, also regulates chitinase secretion in environmental conditions. Since chitinases and chitinase regulator ChiS is involved in pathogenesis, CytR might also play a significant role in pathogenicity. However, the role of CytR in pathogenesis is yet to be known. This study explores the regulation of CytR on the activation of ChiS in the presence of mucin and its role in pathogenesis. Therefore, we created a CytR isogenic mutant strain of V. cholerae (CytR¯) and found considerably less β-hexosaminidase enzyme production, which is an indicator of ChiS activity. The CytR¯ strain greatly reduced the expression of chitinases chiA1 and chiA2 in mucin-supplemented media. Electron microscopy showed that the CytR¯ strain was aflagellate. The expression of flagellar-synthesis regulatory genes flrB, flrC and class III flagellar-synthesis genes were reduced in the CytR¯ strain. The isogenic CytR mutant showed less growth compared to the wild-type in mucin-supplemented media as well as demonstrated highly retarded motility and reduced mucin-layer penetration. The CytR mutant revealed decreased adherence to the HT-29 cell line. In animal models, reduced fluid accumulation and colonization were observed during infection with the CytR¯ strain due to reduced expression of ctxB, toxT and tcpA. Collectively these data suggest that CytR plays an important role in V. cholerae pathogenesis.

From Jekyll to Hyde: The Yeast-Hyphal Transition of Candida albicans

Candida albicans is a major fungal pathogen of humans, accounting for 15% of nosocomial infections with an estimated attributable mortality of 47%. C. albicans is usually a benign member of the human microbiome in healthy people. Under constant exposure to highly dynamic environmental cues in diverse host niches, C. albicans has successfully evolved to adapt to both commensal and pathogenic lifestyles. The ability of C. albicans to undergo a reversible morphological transition from yeast to filamentous forms is a well-established virulent trait. Over the past few decades, a significant amount of research has been carried out to understand the underlying regulatory mechanisms, signaling pathways, and transcription factors that govern the C. albicans yeast-to-hyphal transition. This review will summarize our current understanding of well-elucidated signal transduction pathways that activate C. albicans hyphal morphogenesis in response to various environmental cues and the cell cycle machinery involved in the subsequent regulation and maintenance of hyphal morphogenesis.

Lactobacillus acidophilus LA14 Alleviates Liver Injury

Although the probiotic Lactobacillus acidophilus LA14 is used worldwide, its effect on liver diseases remains unelucidated. Here, 32 rats were divided into four groups, gavaged with L. acidophilus LA14 (3 × 10⁹ CFU) or phosphate-buffered saline for 7 days, and then intraperitoneally injected with d-galactosamine or saline. After 24 h, blood, liver, ileum, and feces samples were collected for liver injury, inflammation, intestinal barrier, gut microbiota, metabolome, and transcriptome analyses. Pretreatment with L. acidophilus LA14 alleviated the d-galactosamine-induced elevation of serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), and bile acids; mitigated the histological injury to the liver and gut; and suppressed the inflammatory cytokines macrophage inflammatory protein 1α (MIP-1α), MIP-3α, and MCP-1. L. acidophilus LA14 also ameliorated the d-galactosamine-induced dysbiosis of the gut microbiota and metabolism, such as the enrichment of Bacteroides sp. strain dnLKV3 and the depletion of Streptococcus, butanoic acid, and N-acetyl-d-glucosamine. The underlying mechanism of L. acidophilus LA14 included prevention of not only the d-galactosamine-induced upregulation of infection- and tumor-related pathways but also the d-galactosamine-induced downregulation of antioxidation-related pathways during this process, as reflected by the liver transcriptome and proteome analyses. Furthermore, the administration of L. acidophilus LA14 to healthy rats did not alter the tested liver indicators but significantly enriched the beneficial Lactobacillus and Bifidobacterium species, promoted metabolism and regulated pathways to improve immunity. The ability of L. acidophilus LA14 to alleviate liver injury was further confirmed with an acetaminophen-induced mouse model. These results might provide a reference for future studies on the application of L. acidophilus LA14 for the prevention of liver injury.

IMPORTANCE The probiotic Lactobacillus acidophilus LA14 is widely used, but its effect on liver diseases has not been elucidated. We explored the protective effect of L. acidophilus LA14 on the liver using rats with d-galactosamine-induced liver injury. Pretreatment with L. acidophilus LA14 alleviated the d-galactosamine-induced elevation of serum ALT, AST, ALP, and bile acids, mitigated the histological injury to the liver and gut, and suppressed the inflammatory cytokines MIP-1α, MIP-3α, and MCP-1. These effects were correlated with the modulations of the gut microbiome, metabolome, and hepatic gene expression induced by L. acidophilus LA14. Moreover, the ability of L. acidophilus LA14 to alleviate liver injury was further confirmed with an acetaminophen-induced mouse model. These results might provide a reference for future studies on the application of L. acidophilus LA14 for the prevention of liver injury.

Plant-associated fungal biofilms-knowns and unknowns

Nearly all microbes, including fungi, grow firmly attached to surfaces as a biofilm. Yet, attention toward fungal interactions with plants and the environment is dedicated to free-floating (planktonic) cells. Fungal biofilms are generally thought to configure interactions across and among plant populations. Despite this, plant fungal biofilm research lags far behind the research on biofilms of medically important fungi. The deficit in noticing and exploring this research avenue could limit disease management and plant improvement programs. Here, we provide the current state of knowledge of fungal biofilms and the different pivotal ecological roles they impart in the context of disease, through leveraging evidence across medically important fungi, secondary metabolite production, plant beneficial functions and climate change. We also provide views on several important information gaps potentially hampering plant fungal biofilm research, and propose a way forward to address these gaps.

Potential prebiotic substrates modulate composition, metabolism, virulence and inflammatory potential of an in vitro multi-species oral biofilm

Background: Modulation of the commensal oral microbiota constitutes a promising preventive/therapeutic approach in oral healthcare. The use of prebiotics for maintaining/restoring the health-associated homeostasis of the oral microbiota has become an important research topic.

Aims: This study hypothesised that in vitro 14-species oral biofilms can be modulated by (in)direct stimulation of beneficial/commensal bacteria with new potential prebiotic substrates tested at 1 M and 1%_(w/v), resulting in more host-compatible biofilms with fewer pathogens, decreased virulence and less inflammatory potential.

Methods: Established biofilms were repeatedly rinsed with N-acetyl-D-glucosamine, α-D-lactose, D-(+)-trehalose or D-(+)-raffinose at 1 M or 1%_(w/v). Biofilm composition, metabolic profile, virulence and inflammatory potential were eventually determined.

Results: Repeated rinsing caused a shift towards a more health-associated microbiological composition, an altered metabolic profile, often downregulated virulence gene expression and decreased the inflammatory potential on oral keratinocytes. At 1 M, the substrates had pronounced effects on all biofilm aspects, whereas at 1%_(w/v) they had a pronounced effect on virulence gene expression and a limited effect on inflammatory potential.

Conclusion: Overall, this study identified four new potential prebiotic substrates that exhibit different modulatory effects at two different concentrations that cause in vitro multi-species oral biofilms to become more host-compatible.

Analyses of transcriptomics and metabolomics reveal pathway of vacuolar Sur7 contributed to biocontrol potential of entomopathogenic Beauveria bassiana

Beauveria bassiana is a critical entomopathogenic fungus for pest biocontrol, whose efficiency depends on fungal development and stress resistance. Unlike its revealed location in plasma membrane patches in other organisms, B. bassiana Sur7 specifically localized in vacuoles. This vacuolar Sur7 was previously demonstrated to affect stress tolerance, hyphal development and virulence. There, however, remain more mechanistic details to be explored. In this study, transcriptomics and metabolomics were applied to investigate the mechanism of vacuolar Sur7. Analyses of transcriptomics and metabolomics displayed many differentially expressed genes and abundant metabolites in response to Sur7 loss, respectively. Together with genes associated with vacuolar biofunction (including transportation and hydrolysis), the altered metabolites contributed to cell wall construction and stress resistance. Particularly, an N-acetylglucosamine-associated Brg1/Nrg1 pathway was enriched and partially affected by Sur7. Absence of Sur7 changed the expression level of Brg1/Nrg1 pathway-related transcript factors, which interfered with downstream phenotype of sporulation. In addition, Sur7 was involved in the accumulation of sphingoid bases, which may affect sphingolipid-related signaling pathway. Although experimental evidence is further required, our studies provide a preliminary framework for future exploring the regulatory mechanism of Sur7, and give a new version of metabolic agency connecting Sur7 and downstream signaling pathway.

Calf Diarrhea Caused by Prolonged Expansion of Autochthonous Gut Enterobacteriaceae and Their Lytic Bacteriophages

Neonatal calf diarrhea is a common disease leading to a major economic loss for cattle producers worldwide. Several infectious and noninfectious factors are implicated in calf diarrhea, but disease control remains problematic because of the multifactorial etiology of the disease. Here, we conducted diagnostic multiplex PCR assay and meta-omics analysis (16S rRNA gene-based metataxonomics and untargeted transcriptional profiling) of rectal content of normal and diarrheic beef calves (n = 111). In the diarrheic calf gut, we detected both microbial compositional dysbiosis (i.e., increased abundances of the family Enterobacteriaceae members and their lytic bacteriophages) and functional dysbiosis (i.e., elevated levels of aerobic respiration and virulence potential). The calf diarrheic transcriptome mirrored the gene expression of the bovine host and was enriched in cellular pathways of sulfur metabolism, innate immunity, and gut motility. We then isolated 12 nontoxigenic Enterobacteriaceae strains from the gut of diarrheic calves. Feeding a strain mixture to preweaning mice resulted in a significantly higher level of fecal moisture content, with decreased body weight gain and shortened colon length. The presented findings suggest that gut inflammation followed by a prolonged expansion of nontoxigenic autochthonous Enterobacteriaceae contributes to the onset of diarrhea in preweaning animals.IMPORTANCE Calf diarrhea is the leading cause of death of neonatal calves worldwide. Several infectious and noninfectious factors are implicated in calf diarrhea, but disease control remains problematic because of the multifactorial etiology of the disease. The major finding of the current study centers around the observation of microbial compositional and functional dysbiosis in rectal samples from diarrheic calves. These results highlight the notion that gut inflammation followed by a prolonged expansion of autochthonous Enterobacteriaceae contributes to the onset of calf diarrhea. Moreover, this condition possibly potentiates the risk of invasion of notorious enteric pathogens, including Salmonella spp., and the emergence of inflammation-resistant (or antibiotic-resistant) microbiota via active horizontal gene transfer mediated by lytic bacteriophages.

Dietary Glutamic Acid Modulates Immune Responses and Gut Health of Weaned Pigs

Simple Summary

Weaning stress can lead to intestinal barrier dysfunction, immune system destruction, and intestinal microbiota disruption, thereby reducing the absorption of nutrients and causing intestinal diseases. Glutamic acid is a non-essential amino acid that is abundantly present in the body and plays an essential function in cellular metabolism and immune responses. In this study, the effects of dietary glutamic acid on the growth performance, nutrient digestibility, immune responses, and intestinal health of weaned pigs were evaluated. Based on the results, dietary glutamic acid increased growth performance, nutrient digestibility, intestinal morphology, and ileal gene expression of tight junction proteins of weaned pigs and modified immune responses and gut microbiota. This study provides information to understand the functional use of dietary glutamic acid as a feed additive for improving the growth performance and intestinal health of weaned pigs.

Abstract

Dietary glutamic acid (GLU) is used as a feed additive because of its functional characteristics that may affect the growth performance and health of pigs. This study was carried out to determine the effects of dietary GLU on growth performance, nutrient digestibility, immune responses, and intestinal health of weaned pigs. A total of ninety-six weaned pigs (8.07 ± 1.17 kg of body weight; 28 days of age) were assigned to two dietary treatments (8 pigs/pen; 6 replicates/treatment) in a randomized complete block design (block: body weight): (1) a typical weaner diet (CON) and (2) CON supplemented with 0.5% GLU. The experimental period was for 4 weeks. All data and sample collections were performed at the specific time points during the experimental period. Pigs fed GLU had higher average daily gain and average daily feed intake for the first two weeks and nutrient digestibility than pigs fed CON. In addition, dietary GLU increased villus height to crypt depth ratio, number of goblet cells, and ileal gene expression of claudin family and occludin compared with CON, but decreased serum TNF-α and IL-6 and ileal gene expression of TNF-α. Moreover, pigs fed GLU had increased relative composition of bacterial communities of genus Prevotella and Anaerovibrio and decreased genus Clostridium and Terrisporobacter compared with those fed CON. This study suggests that dietary GLU influences growth performance and health of weaned pigs by modulating nutrient digestibility, intestinal morphology, ileal gene expression of tight junction proteins and cytokines, immune responses, and microbial community in the gut.

NGT1 Is Essential for N-Acetylglucosamine-Mediated Filamentous Growth Inhibition and HXK1 Functions as a Positive Regulator of Filamentous Growth in Candida tropicalis

Candida tropicalis is a pathogenic fungus that can cause opportunistic infections in humans. The ability of Candida species to transition between yeast and filamentous growth forms is essential to their ability to undergo environmental adaptation and to maintain virulence. In other fungal species, such as Candida albicans, N-acetylglucosamine (GlcNAc) can induce filamentous growth, whereas it suppresses such growth in C. tropicalis. In the present study, we found that knocking out the GlcNA-specific transporter gene NGT1 was sufficient to enhance C. tropicalis filamentous growth on Lee’s plus GlcNAc medium. This suggests that GlcNAc uptake into C. tropicalis cells is essential to the disruption of mycelial growth. As such, we further studied how GlcNAc catabolism-related genes were able to influence C. tropicalis filamentation. We found that HXK1 overexpression drove filamentous growth on Lee’s media containing glucose and GlcNAc, whereas the deletion of the same gene disrupted this filamentous growth. Interestingly, the deletion of the DAC1 or NAG1 genes impaired C. tropicalis growth on Lee’s plus GlcNAc plates. Overall, these results indicate that HXK1 can serve as a positive regulator of filamentous growth, with excess GlcNAc-6-PO₄ accumulation being toxic to C. tropicalis. These findings may highlight novel therapeutic targets worthy of future investigation.

What Are the Functions of Chitin Deacetylases in Aspergillus fumigatus?

Deacetylation of chitin by chitin deacetylases (Cda) results in the formation of chitosan. Chitosan, a polymer of β1,4 linked glucosamine, plays multiple roles in the function of the fungal cell wall, including virulence and evasion of host immune responses. In this study, the roles of chitosan and putative CDAs in cell wall structure and virulence of Aspergillus fumigatus were investigated. Low levels of chitosan were found in the conidial and cell wall of A. fumigatus. Seven putative CDA genes were identified, disrupted and the phenotype of the single mutants and the septuple mutants were investigated. No alterations in fungal cell wall chitosan levels, changes in fungal growth or alterations in virulence were detected in the single or septuple Δcda1-7 mutant strains. Collectively, these results suggest that chitosan is a minority component of the A. fumigatus cell wall, and that the seven candidate Cda proteins do not play major roles in fungal cell wall synthesis or virulence. However, Cda2 is involved in conidiation, suggesting that this enzyme may play a role in N-acetyl-glucosamine metabolism.

Multiple Alternative Carbon Pathways Combine To Promote Candida albicans Stress Resistance, Immune Interactions, and Virulence

The phagocytic cells of the innate immune system are an essential first line of antimicrobial defense, and yet Candida albicans, one of the most problematic fungal pathogens, is capable of resisting the stresses imposed by the macrophage phagosome, eventually resulting in the destruction of the phagocyte. C. albicans rapidly adapts to the phagosome by upregulating multiple alternative carbon utilization pathways, particularly those for amino acids, carboxylic acids, and N-acetylglucosamine (GlcNAc). Here, we report that C. albicans recognizes these carbon sources both as crucial nutrients and as independent signals in its environment. Even in the presence of glucose, each carbon source promotes increased resistance to a unique profile of stressors; lactate promotes increased resistance to osmotic and cell wall stresses, amino acids increased resistance to oxidative and nitrosative stresses, and GlcNAc increased resistance to oxidative stress and caspofungin, while all three alternative carbon sources have been shown to induce resistance to fluconazole. Moreover, we show mutants incapable of utilizing these carbon sources, in particular, strains engineered to be defective in all three pathways, are significantly attenuated in both macrophage and mouse models, with additive effects observed as multiple carbon pathways are eliminated, suggesting that C. albicans simultaneously utilizes multiple carbon sources within the macrophage phagosome and during disseminated candidiasis. Taking the data together, we propose that, in addition to providing energy to the pathogen within host environments, alternative carbon sources serve as niche-specific priming signals that allow C. albicans to recognize microenvironments within the host and to prepare for stresses associated with that niche, thus promoting host adaptation and virulence.IMPORTANCECandida albicans is a fungal pathogen and a significant cause of morbidity and mortality, particularly in people with defects, sometimes minor ones, in innate immunity. The phagocytes of the innate immune system, particularly macrophages and neutrophils, generally restrict this organism to its normal commensal niches, but C. albicans shows a robust and multifaceted response to these cell types. Inside macrophages, a key component of this response is the activation of multiple pathways for the utilization of alternative carbon sources, particularly amino acids, carboxylic acids, and N-acetylglucosamine. These carbon sources are key sources of energy and biomass but also independently promote stress resistance, induce cell wall alterations, and affect C. albicans interactions with macrophages. Engineered strains incapable of utilizing these alternative carbon pathways are attenuated in infection models. These data suggest that C. albicans recognizes nutrient composition as an indicator of specific host environments and tailors its responses accordingly.

N-Acetylglucosamine Regulates Morphogenesis and Virulence Pathways in Fungi

N-acetylglucosamine (GlcNAc) is being increasingly recognized for its ability to stimulate cell signaling. This amino sugar is best known as a component of cell wall peptidoglycan in bacteria, cell wall chitin in fungi and parasites, exoskeletons of arthropods, and the extracellular matrix of animal cells. In addition to these structural roles, GlcNAc is now known to stimulate morphological and stress responses in a wide range of organisms. In fungi, the model organisms Saccharomyces cerevisiae and Schizosaccharomyces pombe lack the ability to respond to GlcNAc or catabolize it, so studies with the human pathogen Candida albicans have been providing new insights into the ability of GlcNAc to stimulate cellular responses. GlcNAc potently induces C. albicans to transition from budding to filamentous hyphal growth. It also promotes an epigenetic switch from White to Opaque cells, which differ in morphology, metabolism, and virulence properties. These studies have led to new discoveries, such as the identification of the first eukaryotic GlcNAc transporter. Other results have shown that GlcNAc can induce signaling in C. albicans in two ways. One is to act as a signaling molecule independent of its catabolism, and the other is that its catabolism can cause the alkalinization of the extracellular environment, which provides an additional stimulus to form hyphae. GlcNAc also induces the expression of virulence genes in the C. albicans, indicating it can influence pathogenesis. Therefore, this review will describe the recent advances in understanding the role of GlcNAc signaling pathways in regulating C. albicans morphogenesis and virulence.

Candida albicans Morphogenesis Programs Control the Balance between Gut Commensalism and Invasive Infection

Candida albicans is a gut commensal and opportunistic pathogen. The transition between yeast and invasive hyphae is central to virulence but has unknown functions during commensal growth. In a mouse model of colonization, yeast and hyphae co-occur throughout the gastrointestinal tract. However, competitive infections of C. albicans homozygous gene disruption mutants revealed an unanticipated, inhibitory role for the yeast-to-hypha morphogenesis program on commensalism. We show that the transcription factor Ume6, a master regulator of filamentation, inhibits gut colonization, not by effects on cell shape, but by activating the expression of a hypha-specific pro-inflammatory secreted protease, Sap6, and a hyphal cell surface adhesin, Hyr1. Like a ume6 mutant, strains lacking SAP6 exhibit enhanced colonization fitness, whereas SAP6-overexpression strains are attenuated in the gut. These results reveal a tradeoff between fungal programs supporting commensalism and virulence in which selection against hypha-specific markers limits the disease-causing potential of this ubiquitous commensal-pathogen.

Mucin Cross-Feeding of Infant Bifidobacteria and Eubacterium hallii

Mucus production is initiated before birth and provides mucin glycans to the infant gut microbiota. Bifidobacteria are the major bacterial group in the feces of vaginally delivered and breast milk-fed infants. Among the bifidobacteria, only Bifidobacterium bifidum is able to degrade mucin and to release monosaccharides which can be used by other gut microbes colonizing the infant gut. Eubacterium hallii is an early occurring commensal that produces butyrate and propionate from fermentation metabolites but that cannot degrade complex oligo- and polysaccharides. We aimed to demonstrate that mucin cross-feeding initiated by B. bifidum enables growth and metabolite formation of E. hallii leading to short-chain fatty acid (SCFA) formation. Growth and metabolite formation of co-cultures of B. bifidum, of Bifidobacterium breve or Bifidobacterium infantis, which use mucin-derived hexoses and fucose, and of E. hallii were determined. Growth of E. hallii in the presence of lactose and mucin monosaccharides was tested. In co-culture fermentations, the presence of B. bifidum enabled growth of the other strains. B. bifidum/B. infantis co-cultures yielded acetate, formate, and lactate while co-cultures of B. bifidum and E. hallii formed acetate, formate, and butyrate. In three-strain co-cultures, B. bifidum, E. hallii, and B. breve or B. infantis produced up to 16 mM acetate, 5 mM formate, and 4 mM butyrate. The formation of propionate (approximately 1 mM) indicated cross-feeding on fucose. Lactose, galactose, and GlcNAc were identified as substrates of E. hallii. This study shows that trophic interactions of bifidobacteria and E. hallii lead to the formation of acetate, butyrate, propionate, and formate, potentially contributing to intestinal SCFA formation with potential benefits for the host and for microbial colonization of the infant gut. The ratios of SCFA formed differed depending on the microbial species involved in mucin cross-feeding.

N-Acetylglucosamine Metabolism Promotes Survival of Candida albicans in the Phagosome

Candida albicans is the most important medically relevant fungal pathogen, with disseminated candidiasis being the fourth most common hospital-associated bloodstream infection. Macrophages and neutrophils are innate immune cells that play a key role in host defense by phagocytosing and destroying C. albicans cells. To survive this attack by macrophages, C. albicans generates energy by utilizing alternative carbon sources that are available in the phagosome. Interestingly, metabolism of amino acids and carboxylic acids by C. albicans raises the pH of the phagosome and thereby blocks the acidification of the phagosome, which is needed to initiate antimicrobial attack. In this work, we demonstrate that metabolism of a third type of carbon source, the amino sugar GlcNAc, also induces pH neutralization and survival of C. albicans upon phagocytosis. This mechanism is genetically and physiologically distinct from the previously described mechanisms of pH neutralization, indicating that the robust metabolic plasticity of C. albicans ensures survival upon macrophage phagocytosis.

Phagocytosis by innate immune cells is one of the most effective barriers against the multiplication and dissemination of microbes within the mammalian host. Candida albicans, a pathogenic yeast, has robust mechanisms that allow survival upon macrophage phagocytosis. C. albicans survives in part because it can utilize the alternative carbon sources available in the phagosome, including carboxylic acids and amino acids. Furthermore, metabolism of these compounds raises the pH of the extracellular environment, which combats the acidification and maturation of the phagolysosome. In this study, we demonstrate that metabolism by C. albicans of an additional carbon source, N-acetylglucosamine (GlcNAc), facilitates neutralization of the phagosome by a novel mechanism. Catabolism of GlcNAc raised the ambient pH through release of ammonia, which is distinct from growth on carboxylic acids but similar to growth on amino acids. However, the effect of GlcNAc metabolism on pH was genetically distinct from the neutralization induced by catabolism of amino acids, as mutation of STP2 or ATO5 did not impair the effects of GlcNAc. In contrast, mutants lacking the dedicated GlcNAc transporter gene NGT1 or the enzymes responsible for catabolism of GlcNAc were defective in altering the pH of the phagosome. This correlated with reduced survival following phagocytosis and decreased ability to damage macrophages. Thus, GlcNAc metabolism represents the third genetically independent mechanism that C. albicans utilizes to combat the rapid acidification of the phagolysosome, allowing for cells to escape and propagate infection.

IMPORTANCECandida albicans is the most important medically relevant fungal pathogen, with disseminated candidiasis being the fourth most common hospital-associated bloodstream infection. Macrophages and neutrophils are innate immune cells that play a key role in host defense by phagocytosing and destroying C. albicans cells. To survive this attack by macrophages, C. albicans generates energy by utilizing alternative carbon sources that are available in the phagosome. Interestingly, metabolism of amino acids and carboxylic acids by C. albicans raises the pH of the phagosome and thereby blocks the acidification of the phagosome, which is needed to initiate antimicrobial attack. In this work, we demonstrate that metabolism of a third type of carbon source, the amino sugar GlcNAc, also induces pH neutralization and survival of C. albicans upon phagocytosis. This mechanism is genetically and physiologically distinct from the previously described mechanisms of pH neutralization, indicating that the robust metabolic plasticity of C. albicans ensures survival upon macrophage phagocytosis.

An N-acetylglucosamine transporter required for arbuscular mycorrhizal symbioses in rice and maize

Most terrestrial plants, including crops, engage in beneficial interactions with arbuscular mycorrhizal fungi. Vital to the association is mutual recognition involving the release of diffusible signals into the rhizosphere. Previously, we identified the maize no perception 1 (nope1) mutant to be defective in early signalling. Here, we report cloning of ZmNope1 on the basis of synteny with rice. NOPE1 encodes a functional homologue of the Candida albicans N-acetylglucosamine (GlcNAc) transporter NGT1, and represents the first plasma membrane GlcNAc transporter identified from plants. In C. albicans, exposure to GlcNAc activates cell signalling and virulence. Similarly, in Rhizophagus irregularis treatment with rice wild-type but not nope1 root exudates induced transcriptome changes associated with signalling function, suggesting a requirement of NOPE1 function for presymbiotic fungal reprogramming.

Regulation of Hyphal Growth and N-Acetylglucosamine Catabolism by Two Transcription Factors in Candida albicans

The amino sugar N-acetylglucosamine (GlcNAc) is increasingly recognized as an important signaling molecule in addition to its well-known structural roles at the cell surface. In the human fungal pathogen Candida albicans, GlcNAc stimulates several responses including the induction of the genes needed for its catabolism and a switch from budding to filamentous hyphal growth. We identified two genes needed for growth on GlcNAc (RON1 and NGS1) and found that mutants lacking these genes fail to induce the genes needed for GlcNAc catabolism. NGS1 was also important for growth on other sugars, such as maltose, but RON1 appeared to be specific for GlcNAc. Both mutants could grow on nonfermentable carbon sources indicating that they do not affect mitochondrial function, which we show is important for growth on GlcNAc but not for GlcNAc induction of hyphal morphogenesis. Interestingly, both the ron1Δ and ngs1Δ mutants were defective in forming hyphae in response to GlcNAc, even though GlcNAc catabolism is not required for induction of hyphal morphogenesis. The ron1Δ mutant showed a partial defect in forming hyphae, which was surprising since it displayed an elevated level of filamentous cells under noninducing conditions. The ron1Δ mutant also displayed an elevated basal level of expression of genes that are normally upregulated during hyphal growth. Consistent with this, Ron1 contains an Ndt80-like DNA-binding domain, indicating that it regulates gene expression. Thus, Ron1 is a key new component of the GlcNAc response pathway that acts as both an activator and a repressor of hyphal morphogenesis.

cAMP-independent signal pathways stimulate hyphal morphogenesis in Candida albicans

The fungal pathogen Candida albicans can transition from budding to hyphal growth, which promotes biofilm formation and invasive growth into tissues. Stimulation of adenylyl cyclase to form cAMP induces hyphal morphogenesis. The failure of cells lacking adenylyl cyclase (cyr1Δ) to form hyphae has suggested that cAMP signaling is essential for hyphal growth. However, cyr1Δ mutants also grow slowly and have defects in morphogenesis, making it unclear whether hyphal inducers must stimulate cAMP, or if normal basal levels of cAMP are required to maintain cellular health needed for hyphal growth. Interestingly, supplementation of cyr1Δ cells with low levels of cAMP enabled them to form hyphae in response to the inducer N-acetylglucosamine (GlcNAc), suggesting that a basal level of cAMP is sufficient for stimulation. Furthermore, we isolated faster-growing cyr1Δ pseudorevertant strains that can be induced to form hyphae even though they lack cAMP. The pseudorevertant strains were not induced by CO₂ , consistent with reports that CO₂ directly stimulates adenylyl cyclase. Mutational analysis showed that induction of hyphae in a pseudorevertant strain was independent of RAS1, but was dependent on the EFG1 transcription factor that acts downstream of protein kinase A. Thus, cAMP-independent signals contribute to the induction of hyphal responses.

Systematic protein interactome analysis of glycosaminoglycans revealed YcbS as a novel bacterial virulence factor

Microbial pathogens have evolved several strategies for interacting with host cell components, such as glycosaminoglycans (GAGs). Some microbial proteins involved in host-GAG binding have been described; however, a systematic study on microbial proteome-mammalian GAG interactions has not been conducted. Here, we used Escherichia coli proteome chips to probe four typical mammalian GAGs, heparin, heparan sulphate (HS), chondroitin sulphate B (CSB), and chondroitin sulphate C (CSC), and identified 185 heparin-, 62 HS-, 98 CSB-, and 101 CSC-interacting proteins. Bioinformatics analyses revealed the unique functions of heparin- and HS-specific interacting proteins in glycine, serine, and threonine metabolism. Among all the GAG-interacting proteins, three were outer membrane proteins (MbhA, YcbS, and YmgH). Invasion assays confirmed that mutant E. coli lacking ycbS could not invade the epithelial cells. Introducing plasmid carrying ycbS complemented the invading defects at ycbS lacking E. coli mutant, that can be further improved by overexpressing ycbS. Preblocking epithelial cells with YcbS reduced the percentage of E. coli invasions. Moreover, we observed that whole components of the ycb operon were crucial for invasion. The displacement assay revealed that YcbS binds to the laminin-binding site of heparin and might affect the host extracellular matrix structure by displacing heparin from laminin.

Adaptations of the Secretome of Candida albicans in Response to Host-Related Environmental Conditions

The wall proteome and the secretome of the fungal pathogen Candida albicans help it to thrive in multiple niches of the human body. Mass spectrometry has allowed researchers to study the dynamics of both subproteomes. Here, we discuss some major responses of the secretome to host-related environmental conditions. Three β-1,3-glucan-modifying enzymes, Mp65, Sun41, and Tos1, are consistently found in large amounts in culture supernatants, suggesting that they are needed for construction and expansion of the cell wall β-1,3-glucan layer and thus correlate with growth and might serve as diagnostic biomarkers. The genes ENG1, CHT3, and SCW11, which encode an endoglucanase, the major chitinase, and a β-1,3-glucan-modifying enzyme, respectively, are periodically expressed and peak in M/G₁. The corresponding protein abundances in the medium correlate with the degree of cell separation during single-yeast-cell, pseudohyphal, and hyphal growth. We also discuss the observation that cells treated with fluconazole, or other agents causing cell surface stress, form pseudohyphal aggregates. Fluconazole-treated cells secrete abundant amounts of the transglucosylase Phr1, which is involved in the accumulation of β-1,3-glucan in biofilms, raising the question whether this is a general response to cell surface stress. Other abundant secretome proteins also contribute to biofilm formation, emphasizing the important role of secretome proteins in this mode of growth. Finally, we discuss the relevance of these observations to therapeutic intervention. Together, these data illustrate that C. albicans actively adapts its secretome to environmental conditions, thus promoting its survival in widely divergent niches of the human body.