Innovations in host and microbial sialic acid biosynthesis revealed by phylogenomic prediction of nonulosonic acid structure

Virulent properties and genomic diversity of Vibrio vulnificus isolated from environment, human, diseased fish

The incidence of Vibrio vulnificus infections, with high mortality rates in humans and aquatic animals, has escalated, highlighting a significant public health challenge. Currently, reliable markers to identify strains with high virulence potential are lacking, and the understanding of evolutionary drivers behind the emergence of pathogenic strains is limited. In this study, we analyzed the distribution of virulent genotypes and phenotypes to discern the infectious potential of V. vulnificus strains isolated from three distinct sources. Most isolates, traditionally classified as biotype 1, possessed the virulence-correlated gene-C type. Environmental isolates predominantly exhibited YJ-like alleles, while clinical and diseased fish isolates were significantly associated with the nanA gene and pathogenicity region XII. Hemolytic activity was primarily observed in the culture supernatants of clinical and diseased fish isolates. Genetic relationships, as determined by multiple-locus variable-number tandem repeat analysis, suggested that strains originating from the same source tended to cluster together. However, multilocus sequence typing revealed considerable genetic diversity across clusters and sources. A phylogenetic analysis using single nucleotide polymorphisms of diseased fish strains alongside publicly available genomes demonstrated a high degree of evolutionary relatedness within and across different isolation sources. Notably, our findings reveal no direct correlation between phylogenetic patterns, isolation sources, and virulence capabilities. This underscores the necessity for proactive risk management strategies to address pathogenic V. vulnificus strains emerging from environmental reservoirs.IMPORTANCEAs the global incidence of Vibrio vulnificus infections rises, impacting human health and marine aquacultures, understanding the pathogenicity of environmental strains remains critical yet underexplored. This study addresses this gap by evaluating the virulence potential and genetic relatedness of V. vulnificus strains, focusing on environmental origins. We conduct an extensive genotypic analysis and phenotypic assessment, including virulence testing in a wax moth model. Our findings aim to uncover genetic and evolutionary factors that drive pathogenic strain emergence in the environment. This research advances our ability to identify reliable virulence markers and understand the distribution of pathogenic strains, offering significant insights for public health and environmental risk management.

Sialic acids in infection and their potential use in detection and protection against pathogens

In structural terms, the sialic acids are a large family of nine carbon sugars based around an alpha-keto acid core. They are widely spread in nature, where they are often found to be involved in molecular recognition processes, including in development, immunology, health and disease. The prominence of sialic acids in infection is a result of their exposure at the non-reducing terminus of glycans in diverse glycolipids and glycoproteins. Herein, we survey representative aspects of sialic acid structure, recognition and exploitation in relation to infectious diseases, their diagnosis and prevention or treatment. Examples covered span influenza virus and Covid-19, Leishmania and Trypanosoma, algal viruses, Campylobacter, Streptococci and Helicobacter, and commensal Ruminococci.

Anionic extracellular polymeric substances extracted from seawater-adapted aerobic granular sludge

Anionic polymers, such as heparin, have been widely applied in the chemical and medical fields, particularly for binding proteins (e.g., fibroblast growth factor 2 (FGF-2) and histones). However, the current animal-based production of heparin brings great risks, including resource shortages and product contamination. Recently, anionic compounds, nonulosonic acids (NulOs), and sulfated glycoconjugates were discovered in the extracellular polymeric substances (EPS) of aerobic granular sludge (AGS). Given the prevalence of anionic polymers, in marine biofilms, it was hypothesized that the EPS from AGS grown under seawater condition could serve as a raw material for producing the alternatives to heparin. This study aimed to isolate and enrich the anionic fractions of EPS and evaluate their potential application in the chemical and medical fields. The AGS was grown in a lab-scale reactor fed with acetate, under the seawater condition (35 g/L sea salt). The EPS was extracted with an alkaline solution at 80 °C and fractionated by size exclusion chromatography. Its protein binding capacity was evaluated by native gel electrophoresis. It was found that the two highest molecular weight fractions (438- > 14,320 kDa) were enriched with NulO and sulfate-containing glycoconjugates. The enriched fractions can strongly bind the two histones involved in sepsis and a model protein used for purification by heparin-column. These findings demonstrated possibilities for the application of the extracted EPS and open up a novel strategy for resource recovery. KEY POINTS: • High MW EPS from seawater-adapted AGS are dominant with sulfated groups and NulOs • Fifty-eight percent of the EPS is high MW of 68-14,320 kDa • EPS and its fractions can bind histones and fibroblast growth factor 2.

Antimicrobial, anti-inflammatory and anti-arthritic activity of hemolymph lectin (NagLec) isolated from the freshwater crab, Oziotelphusanaga

Lectins are non-immune glycoproteins or proteins having a unique capacity to interact with carbohydrate ligands found on the surface of their host cells. In the present investigation, the lectin was purified from the hemolymph of freshwater crab, Oziotelphusa naga and its antimicrobial, anti-inflammatory and anti-arthritic activity was analysed. The preliminary characterization of the hemagglutinin was carried out to identify the erythrocyte and sugar specificity, optimum pH and temperature and cation dependency. The agglutinin was found to be highly specific to rabbit erythrocyte and inhibited by fetuin and α-lactose. Maximum hemagglutination activity was noted at pH 7.5-8 and temperature 20-40 °C. An O-acetyl sialic acid specific 75 kDa hemolymph lectin, designated as NagLec was isolated from the freshwater crab, Oziotelphusa naga by affinity chromatography on fetuin coupled Sepharose 4 B, with a purification fold of 185. The bacteria Staphylococcus aureus, Proteus mirabilis and fungus Candida albicans had the greatest zone of inhibition when treated with NagLec. The results of the Minimum inhibitory concentration (MIC) and Minimum bactericidal concentration (MBC) assays showed that the purified lectin inhibited the growth of Staphylococcus aureus at 0.031 and 0.065 μg/ml, which proved the bactericidal property of NagLec. NagLec generated alterations on the bacterial cells and led to protein leakage, which was dosage (24 and 48 μg/ml) and time dependent (10-40 min). COX and LOX enzyme was inhibited to 49.43% and 61.81% with 100 μg/ml concentration of NagLec respectively, demonstrating NagLec's ability to reduce inflammation. Furthermore, NagLec (500 μg) suppressed protein denaturation up to 77.12% whereas diclofenac sodium (a standard drug) was inhibited by 89.36%. The results indicate that NagLec, a sialic acid specific lectin isolated from the freshwater crab O. naga could be formulated as a nano drug in future owing to its antimicrobial, anti-inflammatory and anti-arthritic potential that could be targeted to specific pathogenic microbes and treat arthritis.

Metagenomic survey reveals global distribution and evolution of microbial sialic acid catabolism

Sialic acids comprise a varied group of nine-carbon amino sugars found mostly in humans and other higher metazoans, playing major roles in cell interactions with external environments as well as other cells. Microbial sialic acid catabolism (SAC) has long been considered a virulence determinant, and appears to be mainly the purview of pathogenic and commensal bacterial species associated with eukaryotic hosts. Here, we used 2,521 (pre-)assembled metagenomes to evaluate the distribution of SAC in microbial communities from diverse ecosystems and human body parts. Our results demonstrated that microorganisms possessing SAC globally existed in non-host associated environments, although much less frequently than in mammal hosts. We also showed that the ecological significance and taxonomic diversity of microbial SAC have so far been largely underestimated. Phylogenetic analysis revealed a strong signal of horizontal gene transfer among distinct taxa and habitats, and also suggested a specific ecological pressure and a relatively independent evolution history in environmental communities. Our study expanded the known diversity of microbial SAC, and has provided the backbone for further studies on its ecological roles and potential pathogenesis.

In situ evaluation of in vivo sialylation with a dual-color imaging strategy

This work designs a functional dendrimer probe to conveniently identify newly generated sialic acid groups in vivo with a dual-color imaging strategy, which achieves in situ semiquantitative evaluation of the sialylation difference between tumor and normal tissues to reveal sialylation-related biological events and promote clinical tumor diagnosis.

Stereoisomer-specific reprogramming of a bacterial flagellin sialyltransferase

Glycosylation of surface structures diversifies cells chemically and physically. Nucleotide-activated sialic acids commonly serve as glycosyl donors, particularly pseudaminic acid (Pse) and its stereoisomer legionaminic acid (Leg), which decorate eubacterial and archaeal surface layers or protein appendages. FlmG, a recently identified protein sialyltransferase, O-glycosylates flagellins, the subunits of the flagellar filament. We show that flagellin glycosylation and motility in Caulobacter crescentus and Brevundimonas subvibrioides is conferred by functionally insulated Pse and Leg biosynthesis pathways, respectively, and by specialized FlmG orthologs. We established a genetic glyco-profiling platform for the classification of Pse or Leg biosynthesis pathways, discovered a signature determinant of eubacterial and archaeal Leg biosynthesis, and validated it by reconstitution experiments in a heterologous host. Finally, by rewiring FlmG glycosylation using chimeras, we defined two modular determinants that govern flagellin glycosyltransferase specificity: a glycosyltransferase domain that either donates Leg or Pse and a specialized flagellin-binding domain that identifies the acceptor.

Enrichment and application of extracellular nonulosonic acids containing polymers of Accumulibacter

Pseudaminic and legionaminic acids are a subgroup of nonulosonic acids (NulOs) unique to bacterial species. There is a lack of advances in the study of these NulOs due to their complex synthesis and production. Recently, it was seen that "Candidatus Accumulibacter" can produce Pse or Leg analogues as part of its extracellular polymeric substances (EPS). In order to employ a "Ca. Accumulibacter" enrichment as production platform for bacterial sialic acids, it is necessary to determine which fractions of the EPS of "Ca. Accumulibacter" contain NulOs and how to enrich and/or isolate them. We extracted the EPS from granules enriched with "Ca. Accumulibcater" and used size-exclusion chromatography (SEC) to separate them into different molecular weight (MW) fractions. This separation resulted in two high molecular weight (> 5500 kDa) fractions dominated by polysaccharides, with a NulO content up to 4 times higher than the extracted EPS. This suggests that NulOs in "Ca. Accumulibacter" are likely located in high molecular weight polysaccharides. Additionally, it was seen that the extracted EPS and the NulO-rich fractions can bind and neutralize histones. This opens the possibility of EPS and NulO-rich fractions as potential source for sepsis treatment drugs. KEY POINTS: • NulOs in "Ca. Accumulibacter" are likely located in high MW polysaccharides • SEC allows to obtain high MW polysaccharide-rich fractions enriched with NulOs • EPS and the NulOs-rich fractions are a potential source for sepsis treatment drugs.

Tumor identification via in vivo portable Raman detection of sialic acid with a dual gold nanoprobe system

A dual gold nanoprobe system was designed for in vivo portable Raman detection of sialic acid (SA) for tumor identification. The dual gold nanoprobe system contained two gold nanoprobes, Au10-DTTC/PEG-PBA and Au40-PEG-SA. Au10-DTTC/PEG-PBA was constructed on a 10 nm gold nanoparticle modified with 3,3'-diethylthia tricarbocyanine iodide (DTTCI) as the Raman reporter and 3-aminophenylboronic acid (APBA) through a thiol PEG succinimidyl carboxymethyl ester (HS-PEG-NHS) linker for specific recognition of SA. Au40-PEG-SA was constructed on a 40 nm gold nanoparticle modified with SA through HS-PEG-NHS. For in vivo detection of SA, Au10-DTTC/PEG-PBA and Au40-PEG-SA were subsequently injected into tumor xenografted mice with optimal interval and retention times. Through the specific recognition between PBA and SA, the conjugates of Au10-DTTC/PEG-PBA and Au40-PEG-SA formed in the tumor region emitted strong SERS signals of DTTC, which could be detected by a portable Raman detector. This work provides a convenient and portable method to detect SA in tumor xenografted mice, which is useful for family-stay identification and clinical cleavage of tumors.

Preparation and properties of boron affinity molecularly imprinted mesoporous polymers based on Mn-doped ZnS quantum dots

Sialic acid (SA), known as N-acetyl neuraminic acid, is a natural 9-carbomonosaccharide derivative. SA has been widely applied in the early diagnosis of diseases as therapeutic target. However, the abundance of SA is very low in biological samples, which is usually interfered by the similar molecules coexisting at high abundance. Combining the advantages of high selectivity and specificity of molecularly imprinted technology, high specific surface area of mesoporous materials and excellent optical properties of quantum dots, we chose Mn-doped ZnS quantum dots as signal elements, and sialic acid as the template molecule. KH-4-MAPB with recognition ability to SA was synthesized by one-step hydrothermal method using thiolene click reaction as functional monomer. Based on the principle of boron affinity, molecularly imprinted polymers with highly ordered mesoporous structure were prepared, and the structure and fluorescence properties of fluorescent molecularly imprinted polymers were studied. FT-IR, XRD, TEM and nitrogen adsorption-desorption experiments were used to characterize the structure and morphology of the molecularly imprinted polymers. The results showed that the prepared molecularly imprinted polymers had highly ordered mesoporous structure and a large number of imprinted holes, which ensured the specific selectivity of the molecularly imprinted polymers. The fluorescence properties of MIMPs were characterized and analyzed by fluorescence spectra, equilibrium adsorption kinetics experiments were conducted and imprinting properties were recorded under different pH. The above experimental results showed that the fluorescence quenching was successfully achieved when the template molecule SA was captured by the molecularly imprinted polymer. When the concentration of SA was 1.25-100 × 10^-2 g/L, the fluorescence quenching degree of MIMPs showed a fine linear relationship with SA. The correlation coefficient was 0.9946, and the detection equation was F₀/F - 1 = 0.0215 [CSA] + 0.0241. MIMPs had a high recognition ability for SA, and the imprinting factor was 2.44. As a fluorescent sensor for SA, the response time of MIMPs was 20 min. When the buffer solution pH was 7, the imprinting factor was the largest. Under the best conditions, MIMPs revealed good selectivity and specificity for the fluorescence recognition of SA. MIMPS were also applied to the analysis of SA in real human serum samples with satisfactory results.

Glucose Reduces Norovirus Binding to Enterobacter cloacae and Alters Gene Expression of Bacterial Surface Structures in a Growth Phase Dependent Manner

Norovirus is the leading cause of acute viral gastroenteritis. Both human and murine noroviruses attach to commensal bacteria belonging to the mammalian gut flora, and binding levels are influenced by nutrients present in bacterial media. However, it is not known which nutrients are responsible for altering viral binding or why binding is altered. Gene expression of commensal bacteria can be changed by the external environment as well as by interaction with pathogens. For example, growth phase and incubation conditions impact expression levels of specific bacterial genes in Escherichia coli. We have previously shown that binding by both human and murine noroviruses to the commensal bacterium Enterobacter cloacae induces genome-wide changes in gene expression with a large number of differentially expressed genes associated with the surface structure of the bacterial cell. The current study evaluated norovirus binding under nutrient-limited conditions and assessed the expression of a select panel of these genes that are significantly altered by norovirus binding under these conditions. The goal of this work was to determine how norovirus attachment to Enterobacter cloacae affected the expression of these genes under varying nutrient and growth phase conditions. We found that the presence of glucose in minimal media reduced murine norovirus binding to E. cloacae and viral binding in the presence of glucose reduced gene expression for surface structures previously associated with norovirus attachment. Changes in viral binding and gene expression occurred in a growth phase-dependent manner. Collectively, these data demonstrate that both the growth phase and nutrient availability alter viral interactions with commensal bacteria and the subsequent changes in gene expression. Ultimately, this work advances our understanding of norovirus-bacterium interactions and provides a foundation for elucidating the conditions and surface structures that regulate norovirus attachment to bacteria.

Sweet impersonators: Molecular mimicry of host glycans by bacteria

All bacteria display surface-exposed glycans that can play an important role in their interaction with the host and in select cases mimic the glycans found on host cells, an event called molecular or glycan mimicry. In this review, we highlight the key bacteria that display human glycan mimicry and provide an overview of the involved glycan structures. We also discuss the general trends and outstanding questions associated with human glycan mimicry by bacteria. Finally, we provide an overview of several techniques that have emerged from the discipline of chemical glycobiology, which can aid in the study of the composition, variability, interaction and functional role of these mimicking glycans.

Chemical Reporters for Bacterial Glycans: Development and Applications

Bacteria possess an extraordinary repertoire of cell envelope glycans that have critical physiological functions. Pathogenic bacteria have glycans that are essential for growth and virulence but are absent from humans, making them high-priority targets for antibiotic, vaccine, and diagnostic development. The advent of metabolic labeling with bioorthogonal chemical reporters and small-molecule fluorescent reporters has enabled the investigation and targeting of specific bacterial glycans in their native environments. These tools have opened the door to imaging glycan dynamics, assaying and inhibiting glycan biosynthesis, profiling glycoproteins and glycan-binding proteins, and targeting pathogens with diagnostic and therapeutic payload. These capabilities have been wielded in diverse commensal and pathogenic Gram-positive, Gram-negative, and mycobacterial species─including within live host organisms. Here, we review the development and applications of chemical reporters for bacterial glycans, including peptidoglycan, lipopolysaccharide, glycoproteins, teichoic acids, and capsular polysaccharides, as well as mycobacterial glycans, including trehalose glycolipids and arabinan-containing glycoconjugates. We cover in detail how bacteria-targeting chemical reporters are designed, synthesized, and evaluated, how they operate from a mechanistic standpoint, and how this information informs their judicious and innovative application. We also provide a perspective on the current state and future directions of the field, underscoring the need for interdisciplinary teams to create novel tools and extend existing tools to support fundamental and translational research on bacterial glycans.

Known Cellular and Receptor Interactions of Animal and Human Coronaviruses: A Review

This article aims to review all currently known interactions between animal and human coronaviruses and their cellular receptors. Over the past 20 years, three novel coronaviruses have emerged that have caused severe disease in humans, including SARS-CoV-2 (severe acute respiratory syndrome virus 2); therefore, a deeper understanding of coronavirus host-cell interactions is essential. Receptor-binding is the first stage in coronavirus entry prior to replication and can be altered by minor changes within the spike protein-the coronavirus surface glycoprotein responsible for the recognition of cell-surface receptors. The recognition of receptors by coronaviruses is also a major determinant in infection, tropism, and pathogenesis and acts as a key target for host-immune surveillance and other potential intervention strategies. We aim to highlight the need for a continued in-depth understanding of this subject area following on from the SARS-CoV-2 pandemic, with the possibility for more zoonotic transmission events. We also acknowledge the need for more targeted research towards glycan-coronavirus interactions as zoonotic spillover events from animals to humans, following an alteration in glycan-binding capability, have been well-documented for other viruses such as Influenza A.

A general approach to explore prokaryotic protein glycosylation reveals the unique surface layer modulation of an anammox bacterium

The enormous chemical diversity and strain variability of prokaryotic protein glycosylation makes their large-scale exploration exceptionally challenging. Therefore, despite the universal relevance of protein glycosylation across all domains of life, the understanding of their biological significance and the evolutionary forces shaping oligosaccharide structures remains highly limited. Here, we report on a newly established mass binning glycoproteomics approach that establishes the chemical identity of the carbohydrate components and performs untargeted exploration of prokaryotic oligosaccharides from large-scale proteomics data directly. We demonstrate our approach by exploring an enrichment culture of the globally relevant anaerobic ammonium-oxidizing bacterium Ca. Kuenenia stuttgartiensis. By doing so we resolve a remarkable array of oligosaccharides, which are produced by two seemingly unrelated biosynthetic routes, and which modify the same surface-layer protein simultaneously. More intriguingly, the investigated strain also accomplished modulation of highly specialized sugars, supposedly in response to its energy metabolism-the anaerobic oxidation of ammonium-which depends on the acquisition of substrates of opposite charges. Ultimately, we provide a systematic approach for the compositional exploration of prokaryotic protein glycosylation, and reveal a remarkable example for the evolution of complex oligosaccharides in bacteria.

Specificity and modularity of flagellin nonulosonic acid glycosyltransferases

Many bacterial flagella are specifically O-glycosylated with nonulosonic acids, including the sialic acid derivatives, pseudaminic acid or legionaminic acid. Unlike protein glycosyltransferases that are extracytoplasmic, flagellin glycosyltransferases (fGTs) act cytoplasmically with unknown donor or acceptor specificities. The recent reconstitution of fGT-based glycosylation in heterologous hosts enables analyses underpinning such specificity.

Sialic acid O-acetylation: From biosynthesis to roles in health and disease

Sialic acids are nine-carbon sugars that frequently cap glycans at the cell surface in cells of vertebrates as well as cells of certain types of invertebrates and bacteria. The nine-carbon backbone of sialic acids can undergo extensive enzymatic modification in nature and O-acetylation at the C-4/7/8/9 position in particular is widely observed. In recent years, the detection and analysis of O-acetylated sialic acids have advanced, and sialic acid-specific O-acetyltransferases (SOATs) and O-acetylesterases (SIAEs) that add and remove O-acetyl groups, respectively, have been identified and characterized in mammalian cells, invertebrates, bacteria, and viruses. These advances now allow us to draw a more complete picture of the biosynthetic pathway of the diverse O-acetylated sialic acids to drive the generation of genetically and biochemically engineered model cell lines and organisms with altered expression of O-acetylated sialic acids for dissection of their roles in glycoprotein stability, development, and immune recognition, as well as discovery of novel functions. Furthermore, a growing number of studies associate sialic acid O-acetylation with cancer, autoimmunity, and infection, providing rationale for the development of selective probes and inhibitors of SOATs and SIAEs. Here, we discuss the current insights into the biosynthesis and biological functions of O-acetylated sialic acids and review the evidence linking this modification to disease. Furthermore, we discuss emerging strategies for the design, synthesis, and potential application of unnatural O-acetylated sialic acids and inhibitors of SOATs and SIAEs that may enable therapeutic targeting of this versatile sialic acid modification.

Origin and evolution of nonulosonic acid synthases and their relationship with bacterial pathogenicity revealed by a large-scale phylogenetic analysis

Nonulosonic acids (NulOs) are a group of nine-carbon monosaccharides with different functions in nature. N-acetylneuraminic acid (Neu5Ac) is the most common NulO. It covers the membrane surface of all human cells and is a central molecule in the process of self-recognition via SIGLECS receptors. Some pathogenic bacteria escape the immune system by copying the sialylation of the host cell membrane. Neu5Ac production in these bacteria is catalysed by the enzyme NeuB. Some bacteria can also produce other NulOs named pseudaminic and legionaminic acids, through the NeuB homologues PseI and LegI, respectively. In Opisthokonta eukaryotes, the biosynthesis of Neu5Ac is catalysed by the enzyme NanS. In this study, we used publicly available data of sequences of NulOs synthases to investigate its distribution within the three domains of life and its relationship with pathogenic bacteria. We mined the KEGG database and found 425 NeuB sequences. Most NeuB sequences (58.74 %) from the KEGG orthology database were classified as from environmental bacteria; however, sequences from pathogenic bacteria showed higher conservation and prevalence of a specific domain named SAF. Using the HMM profile we identified 13 941 NulO synthase sequences in UniProt. Phylogenetic analysis of these sequences showed that the synthases were divided into three main groups that can be related to the lifestyle of these bacteria: (I) predominantly environmental, (II) intermediate and (III) predominantly pathogenic. NeuB was widely distributed in the groups. However, LegI and PseI were more concentrated in groups II and III, respectively. We also found that PseI appeared later in the evolutionary process, derived from NeuB. We use this same methodology to retrieve sialic acid synthase sequences from Archaea and Eukarya. A large-scale phylogenetic analysis showed that while the Archaea sequences are spread across the tree, the eukaryotic NanS sequences were grouped in a specific branch in group II. None of the bacterial NanS sequences grouped with the eukaryotic branch. The analysis of conserved residues showed that the synthases of Archaea and Eukarya present a mutation in one of the three catalytic residues, an E134D change, related to a Neisseria meningitidis reference sequence. We also found that the conservation profile is higher between NeuB of pathogenic bacteria and NanS of eukaryotes than between NeuB of environmental bacteria and NanS of eukaryotes. Our large-scale analysis brings new perspectives on the evolution of NulOs synthases, suggesting their presence in the last common universal ancestor.

Production of nonulosonic acids in the extracellular polymeric substances of "Candidatus Accumulibacter phosphatis"

Nonulosonic acids (NulOs) are a family of acidic carbohydrates with a nine-carbon backbone, which include different related structures, such as sialic acids. They have mainly been studied for their relevance in animal cells and pathogenic bacteria. Recently, sialic acids have been discovered as an important compound in the extracellular matrix of virtually all microbial life and in "Candidatus Accumulibacter phosphatis", a well-studied polyphosphate-accumulating organism, in particular. Here, bioaggregates highly enriched with these bacteria (approx. 95% based on proteomic data) were used to study the production of NulOs in an enrichment of this microorganism. Fluorescence lectin-binding analysis, enzymatic quantification, and mass spectrometry were used to analyze the different NulOs present, showing a wide distribution and variety of these carbohydrates, such as sialic acids and bacterial NulOs, in the bioaggregates. Phylogenetic analysis confirmed the potential of "Ca. Accumulibacter" to produce different types of NulOs. Proteomic analysis showed the ability of "Ca. Accumulibacter" to reutilize and reincorporate these carbohydrates. This investigation points out the importance of diverse NulOs in non-pathogenic bacteria, which are normally overlooked. Sialic acids and other NulOs should be further investigated for their role in the ecology of "Ca. Accumulibacter" in particular, and biofilms in general. KEY POINTS: •"Ca. Accumulibacter" has the potential to produce a range of nonulosonic acids. •Mass spectrometry and lectin binding can reveal the presence and location of nonulosonic acids. •The role of nonulosonic acid in non-pathogenic bacteria needs to be studied in detail.

Structural and Biosynthetic Diversity of Nonulosonic Acids (NulOs) That Decorate Surface Structures in Bacteria

Nonulosonic acids (NulOs) are a diverse family of 9-carbon α-keto acid sugars that are involved in a wide range of functions across all branches of life. The family of NulOs includes the sialic acids as well as the prokaryote-specific NulOs. Select bacteria biosynthesize the sialic acid N-acetylneuraminic acid (Neu5Ac), and the ability to produce this sugar and its subsequent incorporation into cell-surface structures is implicated in a variety of bacteria-host interactions. Furthermore, scavenging of sialic acid from the environment for energy has been characterized across a diverse group of bacteria, mainly human commensals and pathogens. In addition to sialic acid, bacteria have the ability to biosynthesize prokaryote-specific NulOs, of which there are several known isomers characterized. These prokaryotic NulOs are similar in structure to Neu5Ac but little is known regarding their role in bacterial physiology. Here, we discuss the diversity in structure, the biosynthesis pathways, and the functions of bacteria-specific NulOs. These carbohydrates are phylogenetically widespread among bacteria, with numerous structurally unique modifications recognized. Despite the diversity in structure, the NulOs are involved in similar functions such as motility, biofilm formation, host colonization, and immune evasion.

Exploring the Impact of Ketodeoxynonulosonic Acid in Host-Pathogen Interactions Using Uptake and Surface Display by Nontypeable Haemophilus influenzae

All cells in vertebrates are coated with a dense array of glycans often capped with sugars called sialic acids. Sialic acids have many functions, including serving as a signal for recognition of “self” cells by the immune system, thereby guiding an appropriate immune response against foreign “nonself” and/or damaged cells.

Surface expression of the common vertebrate sialic acid (Sia) N-acetylneuraminic acid (Neu5Ac) by commensal and pathogenic microbes appears structurally to represent “molecular mimicry” of host sialoglycans, facilitating multiple mechanisms of host immune evasion. In contrast, ketodeoxynonulosonic acid (Kdn) is a more ancestral Sia also present in prokaryotic glycoconjugates that are structurally quite distinct from vertebrate sialoglycans. We detected human antibodies against Kdn-terminated glycans, and sialoglycan microarray studies found these anti-Kdn antibodies to be directed against Kdn-sialoglycans structurally similar to those on human cell surface Neu5Ac-sialoglycans. Anti-Kdn-glycan antibodies appear during infancy in a pattern similar to those generated following incorporation of the nonhuman Sia N-glycolylneuraminic acid (Neu5Gc) onto the surface of nontypeable Haemophilus influenzae (NTHi), a human commensal and opportunistic pathogen. NTHi grown in the presence of free Kdn took up and incorporated the Sia into its lipooligosaccharide (LOS). Surface display of the Kdn within NTHi LOS blunted several virulence attributes of the pathogen, including Neu5Ac-mediated resistance to complement and whole blood killing, complement C3 deposition, IgM binding, and engagement of Siglec-9. Upper airway administration of Kdn reduced NTHi infection in human-like Cmah null (Neu5Gc-deficient) mice that express a Neu5Ac-rich sialome. We propose a mechanism for the induction of anti-Kdn antibodies in humans, suggesting that Kdn could be a natural and/or therapeutic “Trojan horse” that impairs colonization and virulence phenotypes of free Neu5Ac-assimilating human pathogens.

An overview and future prospects of sialic acids

Sialic acids (Sias) are negatively charged functional monosaccharides present in a wide variety of natural sources (plants, animals and microorganisms). Sias play an important role in many life processes, which are widely applied in the medical and food industries as intestinal antibacterials, antivirals, anti-oxidative agents, food ingredients, and detoxification agents. Most Sias are composed of N-acetylneuraminic acid (Neu5Ac, >99%), and Sia is its most commonly used name. In this article, we review Sias in terms of their structures, applications, determination methods, metabolism, and production strategies. In particular, we summarise and compare different production strategies, including extraction from natural sources, chemical synthesis, polymer decomposition, enzymatic synthesis, whole-cell catalysis, and de novo biosynthesis via microorganism fermentation. We also discuss research on their physiological functions and applications, barriers to efficient production, and strategies for overcoming these challenges. We focus on efficient de novo biosynthesis strategies for Neu5Ac via microbial fermentation using novel synthetic biology tools and methods that may be applied in future. This work provides a comprehensive overview of recent advances on Sias, and addresses future challenges regarding their functions, applications, and production.

Bacteriophage Tail-Spike Proteins Enable Detection of Pseudaminic-Acid-Coated Pathogenic Bacteria and Guide the Development of Antiglycan Antibodies with Cross-Species Antibacterial Activity

Pseudaminic acid (Pse), a unique carbohydrate in surface-associated glycans of pathogenic bacteria, has pivotal roles in virulence. Owing to its significant antigenicity and absence in mammals, Pse is considered an attractive target for vaccination or antibody-based therapies against bacterial infections. However, a specific and universal probe for Pse, which could also be used in immunotherapy, has not been reported. In a prior study, we used a tail spike protein from a bacteriophage (ΦAB6TSP) that digests Pse-containing exopolysaccharide (EPS) from Acinetobacter baumannii strain 54149 (Ab-54149) to form a glycoconjugate for preparing anti-Ab-54149 EPS serum. We report here that a catalytically inactive ΦAB6TSP (I-ΦAB6TSP) retains binding ability toward Pse. In addition, an I-ΦAB6TSP-DyLight-650 conjugate (Dy-I-ΦAB6TSP) was more sensitive in detecting Ab-54149 than an antibody purified from anti- Ab-54149 EPS serum. Dy-I-ΦAB6TSP also cross-reacted with other pathogenic bacteria containing Pse on their surface polysaccharides (e.g., Helicobacter pylori and Enterobacter cloacae), revealing it to be a promising probe for detecting Pse across bacterial species. We also developed a detection method that employs I-ΦAB6TSP immobilized on microtiter plate. These results suggested that the anti-Ab-54149 EPS serum would exhibit cross-reactivity to Pse on other organisms. When this was tested, this serum facilitated complement-mediated killing of H. pylori and E. cloacae, indicating its potential as a cross-species antibacterial agent. This work opens new avenues for diagnosis and treatment of multidrug resistant (MDR) bacterial infections.

Glycan cross-feeding supports mutualism between Fusobacterium and the vaginal microbiota

Women with bacterial vaginosis (BV), an imbalance of the vaginal microbiome, are more likely to be colonized by potential pathogens such as Fusobacterium nucleatum, a bacterium linked with intrauterine infection and preterm birth. However, the conditions and mechanisms supporting pathogen colonization during vaginal dysbiosis remain obscure. We demonstrate that sialidase activity, a diagnostic feature of BV, promoted F. nucleatum foraging and growth on mammalian sialoglycans, a nutrient resource that was otherwise inaccessible because of the lack of endogenous F. nucleatum sialidase. In mice with sialidase-producing vaginal microbiotas, mutant F. nucleatum unable to consume sialic acids was impaired in vaginal colonization. These experiments in mice also led to the discovery that F. nucleatum may also “give back” to the community by reinforcing sialidase activity, a biochemical feature of human dysbiosis. Using human vaginal bacterial communities, we show that F. nucleatum supported robust outgrowth of Gardnerella vaginalis, a major sialidase producer and one of the most abundant organisms in BV. These results illustrate that mutually beneficial relationships between vaginal bacteria support pathogen colonization and may help maintain features of dysbiosis. These findings challenge the simplistic dogma that the mere absence of “healthy” lactobacilli is the sole mechanism that creates a permissive environment for pathogens during vaginal dysbiosis. Given the ubiquity of F. nucleatum in the human mouth, these studies also suggest a possible mechanism underlying links between vaginal dysbiosis and oral sex.

Bacterial mutualism involving the prominent oral bacterium Fusobacterium nucleatum may drive vaginal dysbiosis in women and could help to explain the clinical correlations between vaginal dysbiosis and oral sex.

The sps Genes Encode an Original Legionaminic Acid Pathway Required for Crust Assembly in Bacillus subtilis

The crust is the outermost spore layer of most Bacillus strains devoid of an exosporium. This outermost layer, composed of both proteins and carbohydrates, plays a major role in the adhesion and spreading of spores into the environment. Recent studies have identified several crust proteins and have provided insights about their organization at the spore surface. However, although carbohydrates are known to participate in adhesion, little is known about their composition, structure, and localization. In this study, we showed that the spore surface of Bacillus subtilis is covered with legionaminic acid (Leg), a nine-carbon backbone nonulosonic acid known to decorate the flagellin of the human pathogens Helicobacter pylori and Campylobacter jejuni We demonstrated that the spsC, spsD, spsE, spsG, and spsM genes of Bacillus subtilis are required for Leg biosynthesis during sporulation, while the spsF gene is required for Leg transfer from the mother cell to the surface of the forespore. We also characterized the activity of SpsM and highlighted an original Leg biosynthesis pathway in B. subtilis Finally, we demonstrated that Leg is required for the assembly of the crust around the spores, and we showed that in the absence of Leg, spores were more adherent to stainless steel probably because of their reduced hydrophilicity and charge.IMPORTANCE Bacillus species are a major economic and food safety concern of the food industry because of their food spoilage-causing capability and persistence. Their persistence is mainly due to their ability to form highly resistant spores adhering to the surfaces of industrial equipment. Spores of the Bacillus subtilis group are surrounded by the crust, a superficial layer which plays a key role in their adhesion properties. However, knowledge of the composition and structure of this layer remains incomplete. Here, for the first time, we identified a nonulosonic acid (Leg) at the surfaces of bacterial spores (B. subtilis). We uncovered a novel Leg biosynthesis pathway, and we demonstrated that Leg is required for proper crust assembly. This work contributes to the description of the structure and composition of Bacillus spores which has been under way for decades, and it provides keys to understanding the importance of carbohydrates in Bacillus adhesion and persistence in the food industry.

Sequence analysis of nonulosonic acid biosynthetic gene clusters in Vibrionaceae and Moritella viscosa

Nonulosonic acid (NulO) biosynthesis in bacteria is directed by nab gene clusters that can lead to neuraminic, legionaminic or pseudaminic acids. Analysis of the gene content from a set mainly composed of Aliivibrio salmonicida and Moritella viscosa strains reveals the existence of several unique nab clusters, for which the NulO products were predicted. This prediction method can be used to guide tandem mass spectrometry studies in order to verify the products of previously undescribed nab clusters and identify new members of the NulOs family.

Attach Me If You Can: Murine Norovirus Binds to Commensal Bacteria and Fungi

The presence of commensal bacteria enhances both acute and persistent infection of murine noroviruses. For several enteric viral pathogens, mechanisms by which these bacteria enhance infection involve direct interactions between the virus and bacteria. While it has been demonstrated that human noroviruses bind to a variety of commensal bacteria, it is not known if this is also true for murine noroviruses. The goal of this study was to characterize interactions between murine noroviruses and commensal bacteria and determine the impact of bacterial growth conditions, incubation temperature and time, on murine norovirus attachment to microbes that comprise the mammalian microbiome. We show that murine noroviruses bind directly to commensal bacteria and show similar patterns of attachment as human norovirus VLPs examined under the same conditions. Furthermore, while binding levels are not impacted by the growth phase of the bacteria, they do change with time and incubation temperature. We also found that murine norovirus can bind to a commensal fungal species, Candidaalbicans.

Decorating the Anammox House: Sialic Acids and Sulfated Glycosaminoglycans in the Extracellular Polymeric Substances of Anammox Granular Sludge

Anammox (anaerobic ammonium oxidation) bacteria are important for the nitrogen cycle in both natural environments and wastewater treatment plants. These bacteria have a strong tendency to grow in aggregates like biofilms and granular sludge. To understand the formation of anammox aggregates, it is required to unravel the composition of the extracellular polymeric substances (EPS), which are produced by the bacteria to develop into aggregates and granules. Here, we investigated anionic polymers in anammox granular sludge, focussing on sialic acids and sulfated glycosaminoglycans. Quantification assays and fluorescent stains indicated that sialic acids and sulfated glycosaminoglycans were present in the anammox EPS (1.6% equivalents of sialic acids and 2.4% equivalents of sulfated glycosaminoglycans). Additionally, the potential genes for the biosynthesis of sialic acids and sulfated glycosaminoglycans were analyzed in the anammox draft genomes. The finding of these components in anammox granular sludge and previously in other nonpathogenic bacteria pointed out that sialic acids and sulfated glycosaminoglycans are worth investigating in the context of a broader function in microbial communities and biofilm systems in general.

Synthetic Approaches for Accessing Pseudaminic Acid (Pse) Bacterial Glycans

Pseudaminic acids (Pses) are a group of non-mammalian nonulosonic acids (nulOs) that have been shown to be an important virulence factor for a number of pathogenic bacteria, including emerging multidrug-resistant ESKAPE pathogens. Despite their discovery over 30 years ago, relatively little is known about the biological significance of Pse glycans compared with their sialic acid analogues, primarily due to a lack of access to the synthetically challenging Pse architecture. Recently, however, the Pse backbone has been subjected to increasing synthetic exploration by carbohydrate (bio)chemists, and the total synthesis of complex Pse glycans achieved with inspiration from the biosynthesis and subsequent detailed study of chemical glycosylation by using Pse donors. Herein, context is provided for these efforts by summarising recent synthetic approaches pioneered for accessing Pse glycans, which are set to open up this underexplored area of glycoscience to the wider scientific community.

Tackling the chemical diversity of microbial nonulosonic acids - a universal large-scale survey approach

Nonulosonic acids, commonly referred to as sialic acids, are a highly important group of nine-carbon sugars common to all domains of life. They all share biosynthetic and structural features, but otherwise display a remarkable chemical diversity. In humans, sialic acids cover all cells which makes them important for processes such as cellular protection, immunity and brain development. On the other hand, sialic acids and other nonulosonic acids have been associated with pathological processes including cancer and viral infections. In prokaryotes, nonulosonic acids are commonly associated with pathogens, which developed through molecular mimicry a strategy to circumvent the host's immune response. However, the remarkably large chemical diversity of prokaryotic nonulosonic acids challenges their discovery, and research on molecular characteristics essential for medical applications are often not feasible. Here, we demonstrate a novel, universal large-scale discovery approach that tackles the unmapped diversity of prokaryotic nonulosonic acids. Thereby, we utilize selective chemical labelling combined with a newly established mass spectrometric all-ion-reaction scanning approach to identify nonulosonic acids and other ulosonic acid-like sugars. In doing so, we provide a first molecular-level comparative study on the frequency and diversity across different phyla. We not only illustrate their surprisingly wide-spread occurrence in non-pathogenic species, but also provide evidence of potential higher carbon variants. Many biomedical studies rely on synthetic routes for sialic acids, which are highly demanding and often of low product yields. Our approach enables large-scale exploration for alternative sources of these highly important compounds.

A novel large-scale survey approach for microbial nonulosonic acids (sialic acids) including a first molecular level comparative study is presented.

Bacterial carbohydrate diversity - a Brave New World

Glycans and glycoconjugates feature on the 'front line' of bacterial cells, playing critical roles in the mechanical and chemical stability of the microorganisms, and orchestrating interactions with the environment and all other living organisms. To negotiate such central tasks, bacterial glycomes incorporate a dizzying array of carbohydrate building blocks and non-carbohydrate modifications, which create opportunities for infinite structural variation. This review highlights some of the challenges and opportunities for the chemical biology community in the field of bacterial glycobiology.

Sialic acids in the extracellular polymeric substances of seawater-adapted aerobic granular sludge

Sialic acids have been discovered in the extracellular polymeric substances (EPS) of seawater-adapted aerobic granular sludge (AGS). Sialic acids are a group of monosaccharides with a nine-carbon backbone, commonly found in mammalian cells and pathogenic bacteria, and frequently described to protect EPS molecules and cells from attack by proteases or glycosidases. In order to further understand the role of these compounds in AGS, lectin staining, genome analysis of the dominant bacterial species, and shielding tests were done. Fluorescence lectin bar-coding (FLBC) analysis showed an overlap with protein staining, indicating presence of sialoglycoproteins in the EPS matrix. Genome analysis gives a positive indication for putative production of sialic acids by the dominant bacteria Candidatus Accumulibacter. FT-IR analysis shows upon selective removal of sialic acids a decrease in carbohydrates, extension of the protein side chain, and exposure of penultimate sugars. Enzymatic removal of sialic acids results in the removal of galactose residues from the EPS upon subsequent treatment with β-galactosidase, indicating a linkage between galactose and sialic acid at the terminus of glycan chains. This work indicates the importance of sialic acids in the protection of penultimate sugar residues of glycoproteins in EPS, and provides basis for future research in the composition of EPS from biofilms and granular sludge.

Nonulosonic acids contribute to the pathogenicity of the oral bacterium Tannerella forsythia

Periodontitis is a polymicrobial, biofilm-caused, inflammatory disease affecting the tooth-supporting tissues. It is not only the leading cause of tooth loss worldwide, but can also impact systemic health. The development of effective treatment strategies is hampered by the complicated disease pathogenesis which is best described by a polymicrobial synergy and dysbiosis model. This model classifies the Gram-negative anaerobe Tannerella forsythia as a periodontal pathogen, making it a prime candidate for interference with the disease. Tannerella forsythia employs a protein O-glycosylation system that enables high-density display of nonulosonic acids via the bacterium's two-dimensional crystalline cell surface layer. Nonulosonic acids are sialic acid-like sugars which are well known for their pivotal biological roles. This review summarizes the current knowledge of T. forsythia's unique cell envelope with a focus on composition, biosynthesis and functional implications of the cell surface O-glycan. We have obtained evidence that glycobiology affects the bacterium's immunogenicity and capability to establish itself in the polymicrobial oral biofilm. Analysis of the genomes of different T. forsythia isolates revealed that complex protein O-glycosylation involving nonulosonic acids is a hallmark of pathogenic T. forsythia strains and, thus, constitutes a valuable target for the design of novel anti-infective strategies to combat periodontitis.

Genetics behind the Biosynthesis of Nonulosonic Acid-Containing Lipooligosaccharides in Campylobacter coli

Campylobacter jejuni and Campylobacter coli are the most common causes of bacterial gastroenteritis in the world. Ganglioside mimicry by C. jejuni lipooligosaccharide (LOS) is the triggering factor of Guillain-Barré syndrome (GBS), an acute polyneuropathy. Sialyltransferases from glycosyltransferase family 42 (GT-42) are essential for the expression of ganglioside mimics in C. jejuni Recently, two novel GT-42 genes, cstIV and cstV, have been identified in C. coli Despite being present in ∼11% of currently available C. coli genomes, the biological role of cstIV and cstV is unknown. In the present investigation, mutation studies with two strains expressing either cstIV or cstV were performed and mass spectrometry was used to investigate differences in the chemical composition of LOS. Attempts were made to identify donor and acceptor molecules using in vitro activity tests with recombinant GT-42 enzymes. Here we show that CstIV and CstV are involved in C. coli LOS biosynthesis. In particular, cstV is associated with LOS sialylation, while cstIV is linked to the addition of a diacetylated nonulosonic acid residue.IMPORTANCE Despite the fact that Campylobacter coli a major foodborne pathogen, its glycobiology has been largely neglected. The genetic makeup of the C. coli lipooligosaccharide biosynthesis locus was largely unknown until recently. C. coli harbors a large set of genes associated with lipooligosaccharide biosynthesis, including genes for several putative glycosyltransferases involved in the synthesis of sialylated lipooligosaccharide in Campylobacter jejuni In the present study, C. coli was found to express lipooligosaccharide structures containing sialic acid and other nonulosonate acids. These findings have a strong impact on our understanding of C. coli ecology, host-pathogen interaction, and pathogenesis.

Structural and functional characterization of a modified legionaminic acid involved in glycosylation of a bacterial lipopolysaccharide

Nonulosonic acids (NulOs) are a diverse family of α-keto acid carbohydrates present across all branches of life. Bacteria biosynthesize NulOs among which are several related prokaryotic-specific isomers and one of which, N-acetylneuraminic acid (sialic acid), is common among all vertebrates. Bacteria display various NulO carbohydrates on lipopolysaccharide (LPS), and the identities of these molecules tune host-pathogen recognition mechanisms. The opportunistic bacterial pathogen Vibrio vulnificus possesses the genes for NulO biosynthesis; however, the structures and functions of the V. vulnificus NulO glycan are unknown. Using genetic and chemical approaches, we show here that the major NulO produced by a clinical V. vulnificus strain CMCP6 is 5-N-acetyl-7-N-acetyl-d-alanyl-legionaminic acid (Leg5Ac7AcAla). The CMCP6 strain could catabolize modified legionaminic acid, whereas V. vulnificus strain YJ016 produced but did not catabolize a NulO without the N-acetyl-d-alanyl modification. In silico analysis suggested that Leg5Ac7AcAla biosynthesis follows a noncanonical pathway but appears to be present in several bacterial species. Leg5Ac7AcAla contributed to bacterial outer-membrane integrity, as mutant strains unable to produce or incorporate Leg5Ac7AcAla into the LPS have increased membrane permeability, sensitivity to bile salts and antimicrobial peptides, and defects in biofilm formation. Using the crustacean model, Artemia franciscana, we demonstrate that Leg5Ac7AcAla-deficient bacteria have decreased virulence potential compared with WT. Our data indicate that different V. vulnificus strains produce multiple NulOs and that the modified legionaminic acid Leg5Ac7AcAla plays a critical role in the physiology, survivability, and pathogenicity of V. vulnificus CMCP6.

Possible Influences of Endogenous and Exogenous Ligands on the Evolution of Human Siglecs

Sialic acids, a group of acidic sugars abundantly expressed in the tissues of deuterostome animals but rarely found in microbes, serve as a "signature of self" for these animals. Cognate sensors for sialic acids include Siglecs, a family of transmembrane lectins of vertebrate immune systems that recognize glycans containing sialic acids. A type of sialic acid called N-glycolylneuraminic acid (Neu5Gc) is abundant in many mammalian lineages including great apes, the closest extant relatives of modern human, but was lost in the lineage leading to modern human via the pseudogenization of the CMAH gene encoding the enzyme that converts N-acetylneuraminic acid (Neu5Ac) to Neu5Gc. Loss of Neu5Gc appears to have influenced the evolution of human Siglecs, such as the adjustment of sialic acid binding preferences and the inactivation of at least one Siglec. In addition, various mechanistic studies using model systems and genetic association studies have revealed that some human Siglecs interact with pathogens and influence the outcome of infections, and these pathogens in turn likely influence the evolution of these Siglecs. By understanding the evolutionary forces affecting Siglecs, we shall achieve a better appreciation of Siglec functions, and by understanding Siglec functions, we can obtain deeper insight into the evolutionary processes driving Siglec evolution.

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Metabolic Labeling of Pseudaminic Acid-Containing Glycans on Bacterial Surfaces

The rise in antibiotic-resistant bacteria is causing worldwide concerns. The urgent need for new antibacterial drugs calls for new thinking and strategies to explore novel, narrow-spectrum, and pathogen-specific antibacterial targets. Legionaminic acid (Leg) and pseudaminic acid (Pse) are nonulosonic acid carbohydrates with structural similarity to eukaryotic sialic acid, and are distributed in numerous pathogenic Gram-negative bacteria as components of cell surface-associated glycans. They are involved in the host interaction, pathogenicity, antiphage defense mechanism, and immune escape mechanism. To further explore their biological significance, we developed a synthesis of 2-acetamido-4-azidoacetamido-2,4,6-trideoxy-l-altrose (Alt-4NAz) and 2-azidoacetamido-4-acetamido-2,4,6-trideoxy-l-altrose (Alt-2NAz), among which Alt-4NAz served as an effective chemical reporter to realize bacterial Pse metabolic labeling. The effectiveness of this chemical reporter has been demonstrated in Pseudomonas aeruginosa, Vibrio vulnificus, and Acinetobacter baumannii strains. Expectedly, this strategy can provide a useful assay to detect phenotypic presence of Pse biosynthesis and screen for agents targeting this pathway.

Sialic Acid-Like Sugars in Archaea: Legionaminic Acid Biosynthesis in the Halophile Halorubrum sp. PV6

N-glycosylation is a post-translational modification that occurs in all three domains. In Archaea, however, N-linked glycans present a degree of compositional diversity not observed in either Eukarya or Bacteria. As such, it is surprising that nonulosonic acids (NulOs), nine-carbon sugars that include sialic acids, pseudaminic acids, and legionaminic acids, are routinely detected as components of protein-linked glycans in Eukarya and Bacteria but not in Archaea. In the following, we report that the N-linked glycan attached to the S-layer glycoprotein of the haloarchaea Halorubrum sp. PV6 includes an N-formylated legionaminic acid. Analysis of the Halorubrum sp. PV6 genome led to the identification of sequences predicted to comprise the legionaminic acid biosynthesis pathway. The transcription of pathway genes was confirmed, as was the co-transcription of several of these genes. In addition, the activities of LegI, which catalyzes the condensation of 2,4-di-N-acetyl-6-deoxymannose and phosphoenolpyruvate to generate legionaminic acid, and LegF, which catalyzes the addition of cytidine monophosphate (CMP) to legionaminic acid, both heterologously expressed in Haloferax volcanii, were demonstrated. Further genome analysis predicts that the genes encoding enzymes of the legionaminic acid biosynthetic pathway are clustered together with sequences seemingly encoding components of the N-glycosylation pathway in this organism. In defining the first example of a legionaminic acid biosynthesis pathway in Archaea, the findings reported here expand our insight into archaeal N-glycosylation, an almost universal post-translational modification in this domain of life.

Chemical Synthesis of Rare, Deoxy-Amino Sugars Containing Bacterial Glycoconjugates as Potential Vaccine Candidates

Bacteria often contain rare deoxy amino sugars which are absent in the host cells. This structural difference can be harnessed for the development of vaccines. Over the last fifteen years, remarkable progress has been made toward the development of novel and efficient protocols for obtaining the rare sugar building blocks and their stereoselective assembly to construct conjugation ready bacterial glycans. In this review, we discuss the total synthesis of a variety of rare sugar containing bacterial glycoconjugates which are potential vaccine candidates.

A pseudaminic acid or a legionaminic acid derivative transferase is strain-specifically implicated in the general protein O-glycosylation system of the periodontal pathogen Tannerella forsythia

The occurrence of nonulosonic acids in bacteria is wide-spread and linked to pathogenicity. However, the knowledge of cognate nonulosonic acid transferases is scarce. In the periodontopathogen Tannerella forsythia, several proposed virulence factors carry strain-specifically either a pseudaminic or a legionaminic acid derivative as terminal sugar on an otherwise structurally identical, protein-bound oligosaccharide. This study aims to shed light on the transfer of either nonulosonic acid derivative on a proximal N-acetylmannosaminuronic acid residue within the O-glycan structure, exemplified with the bacterium's abundant S-layer glycoproteins. Bioinformatic analyses provided the candidate genes Tanf_01245 (strain ATCC 43037) and TFUB4_00887 (strain UB4), encoding a putative pseudaminic and a legionaminic acid derivative transferase, respectively. These transferases have identical C-termini and contain motifs typical of glycosyltransferases (DXD) and bacterial sialyltransferases (D/E-D/E-G and HP). They share homology to type B glycosyltransferases and TagB, an enzyme catalyzing glycerol transfer to an N-acetylmannosamine residue in teichoic acid biosynthesis. Analysis of a cellular pool of nucleotide-activated sugars confirmed the presence of the CMP-activated nonulosonic acid derivatives, which are most likely serving as substrates for the corresponding transferase. Single gene knock-out mutants targeted at either transferase were analyzed for S-layer O-glycan composition by ESI-MS, confirming the loss of the nonulosonic acid derivative. Cross-complementation of the mutants with the nonnative nonulosonic acid transferase was not successful indicating high stringency of the enzymes. This study identified plausible candidates for a pseudaminic and a legionaminic acid derivative transferase; these may serve as valuable tools for engineering of novel sialoglycoconjugates.

A Diazido Mannose Analogue as a Chemoenzymatic Synthon for Synthesizing Di-N-acetyllegionaminic Acid-Containing Glycosides

A chemoenzymatic synthon was designed to expand the scope of the chemoenzymatic synthesis of carbohydrates. The synthon was enzymatically converted into carbohydrate analogues, which were readily derivatized chemically to produce the desired targets. The strategy is demonstrated for the synthesis of glycosides containing 7,9-di-N-acetyllegionaminic acid (Leg5,7Ac₂ ), a bacterial nonulosonic acid (NulO) analogue of sialic acid. A versatile library of α2-3/6-linked Leg5,7Ac₂ -glycosides was built by using chemically synthesized 2,4-diazido-2,4,6-trideoxymannose as a chemoenzymatic synthon for highly efficient one-pot multienzyme (OPME) sialylation followed by downstream chemical conversion of the azido groups into acetamido groups. The syntheses required 10 steps from commercially available d-fucose and had an overall yield of 34-52 %, thus representing a significant improvement over previous methods. Free Leg5,7Ac₂ monosaccharide was also synthesized by a sialic acid aldolase-catalyzed reaction.

Origin, evolution, and distribution of the molecular machinery for biosynthesis of sialylated lipooligosaccharide structures in Campylobacter coli

Campylobacter jejuni and Campylobacter coli are the most common cause of bacterial gastroenteritis worldwide. Additionally, C. jejuni is the most common bacterial etiological agent in the autoimmune Guillain-Barré syndrome (GBS). Ganglioside mimicry by C. jejuni lipooligosaccharide (LOS) is the triggering factor of the disease. LOS-associated genes involved in the synthesis and transfer of sialic acid (glycosyltranferases belonging to family GT-42) are essential in C. jejuni to synthesize ganglioside-like LOS. Despite being isolated from GBS patients, scarce genetic evidence supports C. coli role in the disease. In this study, through data mining and bioinformatics analysis, C. coli is shown to possess a larger GT-42 glycosyltransferase repertoire than C. jejuni. Although GT-42 glycosyltransferases are widely distributed in C. coli population, only a fraction of C. coli strains (1%) are very likely able to express ganglioside mimics. Even though the activity of C. coli specific GT-42 enzymes and their role in shaping the bacterial population are yet to be explored, evidence presented herein suggest that loss of function of some LOS-associated genes occurred during agriculture niche adaptation.

Reconstruction of the sialylation pathway in the ancestor of eukaryotes

The biosynthesis of sialylated molecules of crucial relevance for eukaryotic cell life is achieved by sialyltransferases (ST) of the CAZy family GT29. These enzymes are widespread in the Deuterostoma lineages and more rarely described in Protostoma, Viridiplantae and various protist lineages raising the question of their presence in the Last eukaryotes Common Ancestor (LECA). If so, it is expected that the main enzymes associated with sialic acids metabolism are also present in protists. We conducted phylogenomic and protein sequence analyses to gain insights into the origin and ancient evolution of ST and sialic acid pathway in eukaryotes, Bacteria and Archaea. Our study uncovered the unreported occurrence of bacterial GT29 ST and evidenced the existence of 2 ST groups in the LECA, likely originating from the endosymbiotic event that generated mitochondria. Furthermore, distribution of the major actors of the sialic acid pathway in the different eukaryotic phyla indicated that these were already present in the LECA, which could also access to this essential monosaccharide either endogenously or via a sialin/sialidase uptake mechanism involving vesicles. This pathway was lost in several basal eukaryotic lineages including Archaeplastida despite the presence of two different ST groups likely assigned to other functions.

Tannerella forsythia strains display different cell-surface nonulosonic acids: biosynthetic pathway characterization and first insight into biological implications

Tannerella forsythia is an anaerobic, Gram-negative periodontal pathogen. A unique O-linked oligosaccharide decorates the bacterium's cell surface proteins and was shown to modulate the host immune response. In our study, we investigated the biosynthesis of the nonulosonic acid (NulO) present at the terminal position of this glycan. A bioinformatic analysis of T. forsythia genomes revealed a gene locus for the synthesis of pseudaminic acid (Pse) in the type strain ATCC 43037 while strains FDC 92A2 and UB4 possess a locus for the synthesis of legionaminic acid (Leg) instead. In contrast to the NulO in ATCC 43037, which has been previously identified as a Pse derivative (5-N-acetimidoyl-7-N-glyceroyl-3,5,7,9-tetradeoxy-l-glycero-l-manno-NulO), glycan analysis of strain UB4 performed in this study indicated a 350-Da, possibly N-glycolyl Leg (3,5,7,9-tetradeoxy-d-glycero-d-galacto-NulO) derivative with unknown C5,7 N-acyl moieties. We have expressed, purified and characterized enzymes of both NulO pathways to confirm these genes’ functions. Using capillary electrophoresis (CE), CE–mass spectrometry and NMR spectroscopy, our studies revealed that Pse biosynthesis in ATCC 43037 essentially follows the UDP-sugar route described in Helicobacter pylori, while the pathway in strain FDC 92A2 corresponds to Leg biosynthesis in Campylobacter jejuni involving GDP-sugar intermediates. To demonstrate that the NulO biosynthesis enzymes are functional in vivo, we created knockout mutants resulting in glycans lacking the respective NulO. Compared to the wild-type strains, the mutants exhibited significantly reduced biofilm formation on mucin-coated surfaces, suggestive of their involvement in host-pathogen interactions or host survival. This study contributes to understanding possible biological roles of bacterial NulOs.

A biosynthetic route for polysialylating proteins in Escherichia coli

Polysialic acid (polySia) is a posttranslational modification found on only a handful of proteins in the central nervous and immune systems. The addition of polySia to therapeutic proteins improves pharmacokinetics and reduces immunogenicity. To date, polysialylation of therapeutic proteins has only been achieved in vitro by chemical or chemoenzymatic strategies. In this work, we develop a biosynthetic pathway for site-specific polysialylation of recombinant proteins in the cytoplasm of Escherichia coli. The pathway takes advantage of a bacterial cytoplasmic polypeptide-glycosyltransferase to establish a site-specific primer on the target protein. The glucose primer is extended by glycosyltransferases derived from lipooligosaccharide, lipopolysaccharide and capsular polysaccharide biosynthesis from different bacterial species to synthesize long chain polySia. We demonstrate the new biosynthetic route by modifying green fluorescent proteins and a therapeutic DARPin (designed ankyrin repeat protein).