Biosynthesis and production of polysialic acids in bacteria

Sialylation in the gut: From mucosal protection to disease pathogenesis

Sialylation, a crucial post-translational modification of glycoconjugates, entails the attachment of sialic acid (SA) to the terminal glycans of glycoproteins and glycolipids through a tightly regulated enzymatic process involving various enzymes. This review offers a comprehensive exploration of sialylation within the gut, encompassing its involvement in mucosal protection and its impact on disease progression. The sialylation of mucins and epithelial glycoproteins contributes to the integrity of the intestinal mucosal barrier. Furthermore, sialylation regulates immune responses in the gut, shaping interactions among immune cells, as well as their activation and tolerance. Additionally, the gut microbiota and gut-brain axis communication are involved in the role of sialylation in intestinal health. Altered sialylation patterns have been implicated in various intestinal diseases, including inflammatory bowel disease (IBD), colorectal cancer (CRC), and other intestinal disorders. Emerging research underscores sialylation as a promising avenue for diagnostic, prognostic, and therapeutic interventions in intestinal diseases. Potential strategies such as sialic acid supplementation, inhibition of sialidases, immunotherapy targeting sialylated antigens, and modulation of sialyltransferases have been utilized in the treatment of intestinal diseases. Future research directions will focus on elucidating the molecular mechanisms underlying sialylation alterations, identifying sialylation-based biomarkers, and developing targeted interventions for precision medicine approaches.

Galactosamine and mannosamine are integral parts of bacterial and fungal extracellular polymeric substances

Extracellular polymeric substances (EPS) are produced by microorganisms and interact to form a complex matrix called biofilm. In soils, EPS are important contributors to the microbial necromass and, thus, to soil organic carbon (SOC). Amino sugars (AS) are used as indicators for microbial necromass in soil, although the origin of galactosamine and mannosamine is largely unknown. However, indications exist that they are part of EPS. In this study, two bacteria and two fungi were grown in starch medium either with or without a quartz matrix to induce EPS production. Each culture was separated in two fractions: one that directly underwent AS extraction (containing AS from both biomass and EPS), and another that first had EPS extracted, followed then by AS determination (exclusively containing AS from EPS). We did not observe a general effect of the quartz matrix neither of microbial type on AS production. The quantified amounts of galactosamine and mannosamine in the EPS fraction represented on average 100% of the total amounts of these two AS quantified in cell cultures, revealing they are integral parts of the biofilm. In contrast, muramic acid and glucosamine were also quantified in the EPS, but with much lower contribution rates to total AS production, of 18% and 33%, respectively, indicating they are not necessarily part of EPS. Our results allow a meaningful ecological interpretation of mannosamine and galactosamine data in the future as indicators of microbial EPS, and also attract interest of future studies to investigate the role of EPS to SOC and its dynamics.

Insights into the central role of N-acetyl-glucosamine-1-phosphate uridyltransferase (GlmU) in peptidoglycan metabolism and its potential as a therapeutic target

Several decades after the discovery of the first antibiotic (penicillin) microbes have evolved novel mechanisms of resistance; endangering not only our abilities to combat future bacterial pandemics but many other clinical challenges such as acquired infections during surgeries. Antimicrobial resistance (AMR) is attributed to the mismanagement and overuse of these medications and is complicated by a slower rate of the discovery of novel drugs and targets. Bacterial peptidoglycan (PG), a three-dimensional mesh of glycan units, is the foundation of the cell wall that protects bacteria against environmental insults. A significant percentage of drugs target PG, however, these have been rendered ineffective due to growing drug resistance. Identifying novel druggable targets is, therefore, imperative. Uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) is one of the key building blocks in PG production, biosynthesized by the bifunctional enzyme N-acetyl-glucosamine-1-phosphate uridyltransferase (GlmU). UDP-GlcNAc metabolism has been studied in many organisms, but it holds some distinctive features in bacteria, especially regarding the bacterial GlmU enzyme. In this review, we provide an overview of different steps in PG biogenesis, discuss the biochemistry of GlmU, and summarize the characteristic structural elements of bacterial GlmU vital to its catalytic function. Finally, we will discuss various studies on the development of GlmU inhibitors and their significance in aiding future drug discoveries.

Comparative Genomics Revealed a Potential Threat of Aeromonas rivipollensis G87 Strain and Its Antibiotic Resistance

Aeromonas rivipollensis is an emerging pathogen linked to a broad range of infections in humans. Due to the inability to accurately differentiate Aeromonas species using conventional techniques, in-depth comparative genomics analysis is imperative to identify them. This study characterized 4 A. rivipollensis strains that were isolated from river water in Johannesburg, South Africa, by whole-genome sequencing (WGS). WGS was carried out, and taxonomic classification was employed to profile virulence and antibiotic resistance (AR). The AR profiles of the A. rivipollensis genomes consisted of betalactams and cephalosporin-resistance genes, while the tetracycline-resistance gene (tetE) was only determined to be in the G87 strain. A mobile genetic element (MGE), transposons TnC, was determined to be in this strain that mediates tetracycline resistance MFS efflux tetE. A pangenomic investigation revealed the G87 strain's unique characteristic, which included immunoglobulin A-binding proteins, extracellular polysialic acid, and exogenous sialic acid as virulence factors. The identified polysialic acid and sialic acid genes can be associated with antiphagocytic and antibactericidal properties, respectively. MGEs such as transposases introduce virulence and AR genes in the A. rivipollensis G87 genome. This study showed that A. rivipollensis is generally resistant to a class of beta-lactams and cephalosporins. MGEs pose a challenge in some of the Aeromonas species strains and are subjected to antibiotics resistance and the acquisition of virulence genes in the ecosystem.

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.

Accelerated production of α2,8- and α2,9-linked polysialic acid in recombinant Escherichia coli using high cell density cultivation

Polysialic acid (polySia) are α2,8- and/or α2,9-linked homopolymers with interesting properties for meningococcal vaccine development or the cure of human neurodegenerative disorders. With the goal to avoid large scale production of pathogenic bacteria, we compare in the current study the efficacy of conventional polySia production to recombinant approaches using the engineered laboratory safety strain E. coli BL21. High cell density cultivation (HCDC) experiments were performed in two different bioreactor systems. Increased cell densities of up to 11.3 (±0.4) g/L and polySia concentrations of up to 774 (±18) mg/L were reached in E. coli K1. However, cultivation of engineered E. coli BL21 strains delivered comparable cell densities but a maximum of only 133 mg/L polySia. Using established downstream procedures, host cell DNA and proteins were removed. All recombinant polySia products showed an identical degree of polymerization >90. Polymers with different glycosidic linkages could be successfully differentiated by nuclear magnetic resonance spectroscopy.

Metagenomic insights into the profile of antibiotic resistomes in a large drinking water reservoir

Reservoirs play a vital role in the control and management of surface water resources. However, the long water residence time in the reservoir potentially increases the storage and accumulation of antibiotic resistant genes (ARGs). The full profiles and potential health risks of antibiotic resistomes in reservoirs are largely unknown. In this study, we investigated the antibiotic resistomes of water and sediment during different seasons in the Danjiangkou Reservoir, which is one of the largest reservoirs in China, using a metagenomic sequencing approach. A total of 436 ARG subtypes belonging to 20 ARG types were detected from 24 water and 18 sediment samples, with an average abundance of 0.138 copies/cell. The overall ARG abundance in the sediment was higher than that in the water, and bacitracin and vancomycin resistance genes were the predominant ARG types in the water and sediment, respectively. The overall ARG abundance in the dry season was higher than that in the wet season, and a significant difference in ARG subtype compositions was observed in water, but not in the sediment, between the different seasons. The potential horizontal gene transfer frequency in the water was higher than that in the sediment, and the ARGs in water mainly came from the sediment upstream of the reservoir. The metagenomic assembly identified 14 contigs as ARG-carrying pathogens including Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa, and 3 of 14 carried virulence factors. Overall, the potential public health risks posed by resistomes in the water of the Danjiangkou Reservoir were higher in the dry season than in the wet season. Based on these results, strategies including sediment control and pathogen monitoring are suggested for water safety management in drinking water reservoirs.

Metabolic engineering of capsular polysaccharides

With rising concerns about sustainable practices, environmental complications, and declining resources, metabolic engineers are transforming microorganisms into cellular factories for producing capsular polysaccharides (CPSs). This review provides an overview of strategies employed for the metabolic engineering of heparosan, chondroitin, hyaluronan, and polysialic acid — four CPSs that are of interest for manufacturing a variety of biomedical applications. Methods described include the exploitation of wild-type and engineered native CPS producers, as well as genetically engineered heterologous hosts developed through the improvement of naturally existing pathways or newly (de novo) designed ones. The implementation of methodologies like gene knockout, promoter engineering, and gene expression level control has resulted in multiple-fold improvements in CPS fermentation titers compared with wild-type strains, and substantial increases in productivity, reaching as high as 100% in some cases. Optimization of these biotechnological processes can permit the adoption of industrially competitive engineered microorganisms to replace traditional sources that are generally toxic, unreliable, and inconsistent in product quality.

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.

Dipotassium phosphate improves the molecular weight stability of polysialic acid in Escherichia coli K235 culture broth

This work elucidated the intrinsic mechanism underlying the influence of K₂HPO₄ on PSA production and molecular weight (MW) stability. Among the different potassium salts mixed with K₂HPO₄ in the initial medium, those with buffering capacity were favorable for PSA production. In the bioreactor culture with pH control, adding an appropriate concentration of K₂HPO₄ could enhance PSA production. A dual-phase pH control strategy with ammonia water and KOH could also increase the yield and maintain the MW stability of PSA. Zeta potential test, UV/circular dichroism spectra, and transmission electric microscopy were utilized to explore the configuration of K₂HPO₄-PSA complex. The results from this study can serve a good basis for the industrial-scale production of PSA with stable MW.

Transcriptional control of RfaH on polysialic and colanic acid synthesis by Escherichia coli K92

The transcriptional antiterminator RfaH promotes transcription of long operons encoding surface cell components important for the virulence of Escherichiacoli pathogens. In this paper, we show that RfaH enhanced kps expression for the synthesis of group 2 polysialic acid capsule in E. coli K92. In addition, we demonstrate for the first time that RfaH promotes cps expression for the synthesis of colanic acid, a cell wall component with apparently no role on pathogenicity. Finally, we show a novel RfaH requirement for growth at low temperatures.

Biochemical characterization of a polysialyltransferase from Mannheimia haemolytica A2 and comparison to other bacterial polysialyltransferases

Polysialic acids are bioactive carbohydrates found in eukaryotes and some bacterial pathogens. The bacterial polysialyltransferases (PSTs), which catalyze the synthesis of polysialic acid capsules, have previously been identified in select strains of Escherichia coli and Neisseria meningitidis and are classified in the Carbohydrate-Active enZYmes Database as glycosyltransferase family GT-38. In this study using DNA sequence analysis and functional characterization we have identified a novel polysialyltransferase from the bovine/ovine pathogen Mannheimia haemolytica A2 (PST_Mh). The enzyme was expressed in recombinant form as a soluble maltose-binding-protein fusion in parallel with the related PSTs from E. coli K1 and N. meningitidis group B in order to perform a side-by-side comparison. Biochemical properties including solubility, acceptor preference, reaction pH optima, thermostability, kinetics, and product chain length for the enzymes were compared using a synthetic fluorescent acceptor molecule. PST_Mh exhibited biochemical properties that make it an attractive candidate for chemi-enzymatic synthesis applications of polysialic acid. The activity of PST_Mh was examined on a model glycoprotein and the surface of a neuroprogenitor cell line where the results supported its development for use in applications to therapeutic protein modification and cell surface glycan remodelling to enable cell migration at implantation sites to promote wound healing. The three PSTs examined here demonstrated different properties that would each be useful to therapeutic applications.

Analysis of the SDS-PAGE patterns of outer membrane proteins from Escherichia coli strains that have lost the ability to form K1 antigen and varied in the susceptibility to normal human serum

We used SDS-polyacrylamide gel electrophoresis to investigate the outer membrane proteins (OMPs) band composition of 19 Escherichia coli K1 strains that have spontaneously lost the ability to form K1 polysaccharide capsule (E. coli K1-) and demonstrated different degrees of susceptibility to the bactericidal action of normal human serum. Presented results showed that there were differences between E. coli K1- strains in OMPs expressing capacity. The analysis performed on OMPs has not revealed a direct association between the different OMPs band composition and the susceptibility of these strains to the serum.

Polysialic and colanic acids metabolism in Escherichia coli K92 is regulated by RcsA and RcsB

We have shown previously that Escherichia coli K92 produces two different capsular polymers known as CA (colanic acid) and PA (polysialic acid) in a thermoregulated manner. The complex Rcs phosphorelay is largely related to the regulation of CA synthesis. Through deletion of rscA and rscB genes, we show that the Rcs system is involved in the regulation of both CA and PA synthesis in E. coli K92. Deletion of either rcsA or rcsB genes resulted in decreased expression of cps (CA biosynthesis cluster) at 19°C and 37°C, but only CA production was reduced at 19°C. Concerning PA, both deletions enhanced its synthesis at 37°C, which does not correlate with the reduced kps (PA biosynthesis cluster) expression observed in the rcsB mutant. Under this condition, expression of the nan operon responsible for PA catabolism was greatly reduced. Although RcsA and RcsB acted as negative regulators of PA synthesis at 37°C, their absence did not reestablish PA expression at low temperatures, despite the deletion of rcsB resulting in enhanced kps expression. Finally, our results revealed that RcsB controlled the expression of several genes (dsrA, rfaH, h-ns and slyA) involved in the thermoregulation of CA and PA synthesis, indicating that RcsB is part of a complex regulatory mechanism governing the surface appearance in E. coli.

Construction and expression of a polycistronic plasmid encoding N-acetylglucosamine 2-epimerase and N-acetylneuraminic acid lyase simultaneously for production of N-acetylneuraminic acid

Synthesis of N-acetylneuraminic acid (Neu5Ac) from N-acetylglucosamine (GlcNAc) and pyruvate was carried out by constructing and expressing a polycistronic plasmid encoding an N-acetylglucosamine 2-epimerase (AGE) gene and an N-acetylneuraminic acid lyase (Nal) gene simultaneously. Nal from Escherichia coli K12 and AGEs from Synechocystis sp. PCC 6803 (snAGE) and Anabaena sp. CH1 (anAGE) were used. And four polycistronic plasmids were constructed in which the positions of AGE gene differed with respect to Nal gene. Among these plasmids, pET-28a-Nal-anAGE with anAGE gene located next to Nal gene caused the production of the highest amount of Neu5Ac, generating 61.3g/L in 60h by whole-cell catalysis without the addition of ATP as AGE activator. And pET-28a-Nal-anAGE lowered anAGE's expression level, allowing it to fold properly. Thus, an inclusion-body-free E. coli strain capable of producing Neu5Ac by whole-cell catalysis with high yield and low cost was constructed in the present study.

Sialic acid metabolism and sialyltransferases: natural functions and applications

Sialic acids are a family of negatively charged monosaccharides which are commonly presented as the terminal residues in glycans of the glycoconjugates on eukaryotic cell surface or as components of capsular polysaccharides or lipooligosaccharides of some pathogenic bacteria. Due to their important biological and pathological functions, the biosynthesis, activation, transfer, breaking down, and recycle of sialic acids are attracting increasing attention. The understanding of the sialic acid metabolism in eukaryotes and bacteria leads to the development of metabolic engineering approaches for elucidating the important functions of sialic acid in mammalian systems and for large-scale production of sialosides using engineered bacterial cells. As the key enzymes in biosynthesis of sialylated structures, sialyltransferases have been continuously identified from various sources and characterized. Protein crystal structures of seven sialyltransferases have been reported. Wild-type sialyltransferases and their mutants have been applied with or without other sialoside biosynthetic enzymes for producing complex sialic acid-containing oligosaccharides and glycoconjugates. This mini-review focuses on current understanding and applications of sialic acid metabolism and sialyltransferases.

Abrogation of neuraminidase reduces biofilm formation, capsule biosynthesis, and virulence of Porphyromonas gingivalis

The oral bacterium Porphyromonas gingivalis is a key etiological agent of human periodontitis, a prevalent chronic disease that affects up to 80% of the adult population worldwide. P. gingivalis exhibits neuraminidase activity. However, the enzyme responsible for this activity, its biochemical features, and its role in the physiology and virulence of P. gingivalis remain elusive. In this report, we found that P. gingivalis encodes a neuraminidase, PG0352 (SiaPg). Transcriptional analysis showed that PG0352 is monocistronic and is regulated by a sigma70-like promoter. Biochemical analyses demonstrated that SiaPg is an exo-α-neuraminidase that cleaves glycosidic-linked sialic acids. Cryoelectron microscopy and tomography analyses revealed that the PG0352 deletion mutant (ΔPG352) failed to produce an intact capsule layer. Compared to the wild type, in vitro studies showed that ΔPG352 formed less biofilm and was less resistant to killing by the host complement. In vivo studies showed that while the wild type caused a spreading type of infection that affected multiple organs and all infected mice were killed, ΔPG352 only caused localized infection and all animals survived. Taken together, these results demonstrate that SiaPg is an important virulence factor that contributes to the biofilm formation, capsule biosynthesis, and pathogenicity of P. gingivalis, and it can potentially serve as a new target for developing therapeutic agents against P. gingivalis infection.

General consideration on sialic acid chemistry

Sialic acids, also known as neuraminic acids, are a family of negatively charged α-keto acids with a nine-carbon backbone. These unique sugars have been found at the termini of many glycan chains of vertebrate cell surface, which play pivotal roles in mediating or modulating a variety of physiological and pathological processes. This brief review covers general approaches for synthesizing sialic acid containing structures. Recently developed synthetic methods along with structural diversities and biological functions of sialic acid are discussed.

Pasteurella multocida CMP-sialic acid synthetase and mutants of Neisseria meningitidis CMP-sialic acid synthetase with improved substrate promiscuity

Cytidine 5'-monophosphate (CMP)-sialic acid synthetases (CSSs) catalyze the formation of CMP-sialic acid from CTP and sialic acid, a key step for sialyltransferase-catalyzed biosynthesis of sialic acid-containing oligosaccharides and glycoconjugates. More than 50 different sialic acid forms have been identified in nature. To facilitate the enzymatic synthesis of sialosides with diverse naturally occurring sialic acid forms and their non-natural derivatives, CMP-sialic acid synthetases with promiscuous substrate specificity are needed. Herein we report the cloning, characterization, and substrate specificity studies of a new CSS from Pasteurella multocida strain P-1059 (PmCSS) and a CSS from Haemophillus ducreyi (HdCSS). Based on protein sequence alignment and substrate specificity studies of these two CSSs and a Neisseria meningitidis CSS (NmCSS), as well as crystal structure modeling and analysis of NmCSS, NmCSS mutants (NmCSS_S81R and NmCSS_Q163A) with improved substrate promiscuity were generated. The strategy of combining substrate specificity studies of enzymes from different sources and protein crystal structure studies can be a general approach for designing enzyme mutants with improved activity and substrate promiscuity.

Growth temperature regulation of some genes that define the superficial capsular carbohydrate composition of Escherichia coli K92

We studied growth temperature as a factor controlling the expression of genes involved in capsular polymers of Escherichia coli K92. These genes are shown to be regulated by growth temperature. Expression levels of genes belonging to the kps cluster, responsible for polysialic acid (PA) biosynthesis, were significantly increased at 37 °C compared with at 19 °C, being up to 500-fold increased for neuE and neuS genes. Similarly, the genes for the nan operon, responsible for PA catabolism, also reached higher expression levels at 37 °C, although with slightly lower values (39-141-fold). In contrast, genes of the cps operon, which are implicated in colanic acid (CA) metabolism, were upregulated when the bacteria were grown at 19 °C, albeit to a much lesser extent (around twofold). This different regulation of genes involved in the biosynthesis of polysialic and CAs correlates with the reported maximal production temperatures for the two polymers. The results suggest that the metabolism of PA is predominantly regulated by changes in gene expression, while CA production may be regulated mainly by post-transcriptional processes such as phosphorylation-dephosphorylation reactions.

Eliciting carbohydrate-specific immune response against sialosides: success and challenges

Chemoenzymatic synthesis has been shown to be a powerful adjunct to carbohydrate chemistry since it allows chemists to prepare a wide range of complex carbohydrate analogs in large amounts and with unparalleled efficiency. Here we summarize investigations conducted in our group into the synthesis and immunochemistry of tumor-associated gangliosides and the development of a chemoenzymatic route to α(2→8)-linked oligosialosides by probing substrate binding to the enzyme, α(2→8)-sialyltransferase.