Exploiting the Campylobacter jejuni protein glycosylation system for glycoengineering vaccines and diagnostic tools directed against brucellosis

A bioconjugate vaccine against Brucella abortus produced by engineered Escherichia coli

Brucellosis, mainly caused by Brucella, is a widespread zoonotic disease worldwide, with no available effective vaccine for human use. Recently, bioconjugate vaccines against Brucella have been prepared in Yersinia enterocolitica O:9 (YeO9), whose O-antigen structure is similar to that of Brucella abortus. However, the pathogenicity of YeO9 still hinders the large-scale production of these bioconjugate vaccines. Here, an attractive system for the preparation of bioconjugate vaccines against Brucella was established in engineered E. coli. Briefly, the OPS gene cluster of YeO9 was modularized into five individual fragments and reassembled using synthetic biological methods through standardized interfaces, then introduced into E. coli. After confirming the synthesis of targeted antigenic polysaccharides, the exogenous protein glycosylation system (PglL system) was used to prepare the bioconjugate vaccines. A series of experiments were conducted to demonstrate that the bioconjugate vaccine could effectively evoke humoral immune responses and induce the production of specific antibodies against B. abortus A19 lipopolysaccharide. Furthermore, the bioconjugate vaccines provide protective roles in both lethal and non-lethal challenge of B. abortus A19 strain. Using the engineered E. coli as a safer chassis to prepare bioconjugate vaccines against B. abortus paves the way for future industrial applications.

Development of a novel glycoprotein-based immunochromatographic test for the rapid serodiagnosis of bovine brucellosis

AIMS

Bovine brucellosis is a worldwide zoonotic disease that causes important economic losses and public health concerns. Because control of the disease depends on vaccination, serodiagnosis and isolation of the infected animals; affordable, rapid and accurate point of care (POC) tests are needed.

METHODS AND RESULTS

We developed and evaluated a novel glycoprotein-based immunochromatographic test for the detection of IgG antibodies against the O-polysaccharide of Brucella in bovine serum samples. Brucella GlycoStrip combines the power of immunochromatographic and bacterial glycoengineering technologies for the diagnosis of bovine brucellosis. The analysis of positive and negative reference samples indicated that the test has a diagnostic sensitivity and specificity of 96.9% (95% CI: 92.7-100.0%) and 100%, respectively.

CONCLUSIONS

Due to the recombinant glycoprotein-based antigen OAg-AcrA, which consists of the O-side chain of Brucella smooth lipopolysaccharide (sLPS) covalently linked to the carrier protein AcrA, the test is highly accurate, allows the differentiation of infected animals from those vaccinated with a rough strain or with a single dose of a smooth strain and fulfill the minimum diagnostic requirements established by the national and international regulations.

SIGNIFICANCE AND IMPACT OF STUDY

This strip test could provide a rapid (10 min) and accurate diagnosis of bovine brucellosis in the field contributing to the control of the disease.

Improving Chicken Responses to Glycoconjugate Vaccination Against Campylobacter jejuni

Campylobacter jejuni is a common cause of diarrheal disease worldwide. Human infection typically occurs through the ingestion of contaminated poultry products. We previously demonstrated that an attenuated Escherichia coli live vaccine strain expressing the C. jejuni N-glycan on its surface reduced the Campylobacter load in more than 50% of vaccinated leghorn and broiler birds to undetectable levels (responder birds), whereas the remainder of the animals was still colonized (non-responders). To understand the underlying mechanism, we conducted three vaccination and challenge studies using 135 broiler birds and found a similar responder/non-responder effect. Subsequent genome-wide association studies (GWAS), analyses of bird sex and levels of vaccine-induced IgY responses did not correlate with the responder versus non-responder phenotype. In contrast, antibodies isolated from responder birds displayed a higher Campylobacter-opsonophagocytic activity when compared to antisera from non-responder birds. No differences in the N-glycome of the sera could be detected, although minor changes in IgY glycosylation warrant further investigation. As reported before, the composition of the microbiota, particularly levels of OTU classified as Clostridium spp., Ruminococcaceae and Lachnospiraceae are associated with the response. Transplantation of the cecal microbiota of responder birds into new birds in combination with vaccination resulted in further increases in vaccine-induced antigen-specific IgY responses when compared to birds that did not receive microbiota transplants. Our work suggests that the IgY effector function and microbiota contribute to the efficacy of the E. coli live vaccine, information that could form the basis for the development of improved vaccines targeted at the elimination of C. jejuni from poultry.

Genetic and process engineering strategies for enhanced recombinant N-glycoprotein production in bacteria

Background

The production of N-linked glycoproteins in genetically amenable bacterial hosts offers great potential for reduced cost, faster/simpler bioprocesses, greater customisation, and utility for distributed manufacturing of glycoconjugate vaccines and glycoprotein therapeutics. Efforts to optimize production hosts have included heterologous expression of glycosylation enzymes, metabolic engineering, use of alternative secretion pathways, and attenuation of gene expression. However, a major bottleneck to enhance glycosylation efficiency, which limits the utility of the other improvements, is the impact of target protein sequon accessibility during glycosylation.

Results

Here, we explore a series of genetic and process engineering strategies to increase recombinant N-linked glycosylation, mediated by the Campylobacter-derived PglB oligosaccharyltransferase in Escherichia coli. Strategies include increasing membrane residency time of the target protein by modifying the cleavage site of its secretion signal, and modulating protein folding in the periplasm by use of oxygen limitation or strains with compromised oxidoreductase or disulphide-bond isomerase activity. These approaches achieve up to twofold improvement in glycosylation efficiency. Furthermore, we also demonstrate that supplementation with the chemical oxidant cystine enhances the titre of glycoprotein in an oxidoreductase knockout strain by improving total protein production and cell fitness, while at the same time maintaining higher levels of glycosylation efficiency.

Conclusions

In this study, we demonstrate that improved protein glycosylation in the heterologous host could be achieved by mimicking the coordination between protein translocation, folding and glycosylation observed in native host such as Campylobacter jejuni and mammalian cells. Furthermore, it provides insight into strain engineering and bioprocess strategies, to improve glycoprotein yield and titre, and to avoid physiological burden of unfolded protein stress upon cell growth. The process and genetic strategies identified herein will inform further optimisation and scale-up of heterologous recombinant N-glycoprotein production.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01689-x.

Multivalent poultry vaccine development using Protein Glycan Coupling Technology

BACKGROUND

Poultry is the world's most popular animal-based food and global production has tripled in the past 20 years alone. Low-cost vaccines that can be combined to protect poultry against multiple infections are a current global imperative. Glycoconjugate vaccines, which consist of an immunogenic protein covalently coupled to glycan antigens of the targeted pathogen, have a proven track record in human vaccinology, but have yet to be used for livestock due to prohibitively high manufacturing costs. To overcome this, we use Protein Glycan Coupling Technology (PGCT), which enables the production of glycoconjugates in bacterial cells at considerably reduced costs, to generate a candidate glycan-based live vaccine intended to simultaneously protect against Campylobacter jejuni, avian pathogenic Escherichia coli (APEC) and Clostridium perfringens. Campylobacter is the most common cause of food poisoning, whereas colibacillosis and necrotic enteritis are widespread and devastating infectious diseases in poultry.

RESULTS

We demonstrate the functional transfer of C. jejuni protein glycosylation (pgl) locus into the genome of APEC χ7122 serotype O78:H9. The integration caused mild attenuation of the χ7122 strain following oral inoculation of chickens without impairing its ability to colonise the respiratory tract. We exploit the χ7122 pgl integrant as bacterial vectors delivering a glycoprotein decorated with the C. jejuni heptasaccharide glycan antigen. To this end we engineered χ7122 pgl to express glycosylated NetB toxoid from C. perfringens and tested its ability to reduce caecal colonisation of chickens by C. jejuni and protect against intra-air sac challenge with the homologous APEC strain.

CONCLUSIONS

We generated a candidate glycan-based multivalent live vaccine with the potential to induce protection against key avian and zoonotic pathogens (C. jejuni, APEC, C. perfringens). The live vaccine failed to significantly reduce Campylobacter colonisation under the conditions tested but was protective against homologous APEC challenge. Nevertheless, we present a strategy towards the production of low-cost "live-attenuated multivalent vaccine factories" with the ability to express glycoconjugates in poultry.

Arming the troops: Post-translational modification of extracellular bacterial proteins

Protein secretion is almost universally employed by bacteria. Some proteins are retained on the cell surface, whereas others are released into the extracellular milieu, often playing a key role in virulence. In this review, we discuss the diverse types and potential functions of post-translational modifications (PTMs) occurring to extracellular bacterial proteins.

Synthetic Glycobiology: Parts, Systems, and Applications

Protein glycosylation, the attachment of sugars to amino acid side chains, can endow proteins with a wide variety of properties of great interest to the engineering biology community. However, natural glycosylation systems are limited in the diversity of glycoproteins they can synthesize, the scale at which they can be harnessed for biotechnology, and the homogeneity of glycoprotein structures they can produce. Here we provide an overview of the emerging field of synthetic glycobiology, the application of synthetic biology tools and design principles to better understand and engineer glycosylation. Specifically, we focus on how the biosynthetic and analytical tools of synthetic biology have been used to redesign glycosylation systems to obtain defined glycosylation structures on proteins for diverse applications in medicine, materials, and diagnostics. We review the key biological parts available to synthetic biologists interested in engineering glycoproteins to solve compelling problems in glycoscience, describe recent efforts to construct synthetic glycoprotein synthesis systems, and outline exemplary applications as well as new opportunities in this emerging space.

Application of an O-Linked Glycosylation System in Yersinia enterocolitica Serotype O:9 to Generate a New Candidate Vaccine against Brucella abortus

Brucellosis is a major zoonotic public health threat worldwide, causing veterinary morbidity and major economic losses in endemic regions. However, no efficacious brucellosis vaccine is yet available, and live attenuated vaccines commonly used in animals can cause human infection. N- and O-linked glycosylation systems have been successfully developed and exploited for the production of successful bioconjugate vaccines. Here, we applied an O-linked glycosylation system to a low-pathogenicity bacterium, Yersinia enterocolitica serotype O:9 (Y. enterocolitica O:9), which has repeating units of O-antigen polysaccharide (OPS) identical to that of Brucella abortus (B. abortus), to develop a bioconjugate vaccine against Brucella. The glycoprotein we produced was recognized by both anti-B. abortus and anti-Y. enterocolitica O:9 monoclonal antibodies. Three doses of bioconjugate vaccine-elicited B. abortus OPS-specific serum IgG in mice, significantly reducing bacterial loads in the spleen following infection with the B. abortus hypovirulent smooth strain A19. This candidate vaccine mitigated B. abortus infection and prevented severe tissue damage, thereby protecting against lethal challenge with A19. Overall, the results indicated that the bioconjugate vaccine elicited a strong immune response and provided significant protection against brucellosis. The described vaccine preparation strategy is safe and avoids large-scale culture of the highly pathogenic B. abortus.

Glycoengineering bioconjugate vaccines, therapeutics, and diagnostics in E. coli

The first, general glycosylation pathway in bacteria, the N-linked glycosylation system of Campylobacter jejuni, was discovered two decades ago. Since then, many diverse prokaryotic glycosylation systems have been characterized, including O-linked glycosylation systems that have no homologous counterparts in eukaryotic organisms. Shortly after these discoveries, glycosylation pathways were recombinantly introduced into E. coli creating the field of bacterial glycoengineering. Bacterial glycoengineering is an emerging biotechnological tool that harnesses prokaryotic glycosylation systems for the generation of recombinantly glycosylated proteins using E. coli as a host. Over the last decade, as our understanding of prokaryotic glycosylation systems has advanced, so too has the glycoengineering toolbox. Currently, glycoengineering utilizes two broad approaches to recombinantly glycosylate proteins, both of which can generate N- or O-linkages: oligosaccharyltransferase (OTase)-dependent and OTase-independent. This review discusses the applications of these bacterial glycoengineering techniques as they relate to the development of glycoconjugate vaccines, therapeutic proteins, and diagnostics.

Clostridium perfringens epsilon toxin vaccine candidate lacking toxicity to cells expressing myelin and lymphocyte protein

A variant form of Clostridium perfringens epsilon toxin (Y30A-Y196A) with mutations, which shows reduced binding to Madin–Darby canine kidney (MDCK) cells and reduced toxicity in mice, has been proposed as the next-generation enterotoxaemia vaccine. Here we show that, unexpectedly, the Y30A-Y196A variant does not show a reduction in toxicity towards Chinese hamster ovary (CHO) cells engineered to express the putative receptor for the toxin (myelin and lymphocyte protein; MAL). The further addition of mutations to residues in a second putative receptor binding site of the Y30A-Y196A variant further reduces toxicity, and we selected Y30A-Y196A-A168F for further study. Compared to Y30A-Y196A, Y30A-Y196A-A168F showed more than a 3-fold reduction in toxicity towards MDCK cells, more than a 4-fold reduction in toxicity towards mice and at least 200-fold reduction in toxicity towards CHO cells expressing sheep MAL. The immunisation of rabbits or sheep with Y30A-Y196A-A168F induced high levels of neutralising antibodies against epsilon toxin, which persisted for at least 1 year. Y30A-Y196A-A168F is a candidate for development as a next-generation enterotoxaemia vaccine.

Cells expressing myelin and lymphocyte protein (MAL), the putative receptor for Clostridium perfringens’ epsilon toxin, can be sensitive to otherwise attenuated mutants of the toxin. Here, the team led by Richard Titball at United Kingdom’s University of Exeter found that a previous variant exhibits differential toxic effects when cells express sheep or human MAL. To circumvent this, Titball’s team applied site-directed mutagenesis of the receptor binding site to develop a new variant with enhanced reduction in toxicity towards MAL-expressing cells and able to induce high levels of neutralising antibodies upon immunisation of sheep. These findings suggests that testing genetic toxoids in cells expressing MAL from the target species might be relevant for enterotoxaemia vaccine development and warrant further studies into the role of MAL in epsilon toxin-mediated pathogenesis.

A platform for glycoengineering a polyvalent pneumococcal bioconjugate vaccine using E. coli as a host

Chemical synthesis of conjugate vaccines, consisting of a polysaccharide linked to a protein, can be technically challenging, and in vivo bacterial conjugations (bioconjugations) have emerged as manufacturing alternatives. Bioconjugation relies upon an oligosaccharyltransferase to attach polysaccharides to proteins, but currently employed enzymes are not suitable for the generation of conjugate vaccines when the polysaccharides contain glucose at the reducing end, which is the case for ~75% of Streptococcus pneumoniae capsules. Here, we use an O-linking oligosaccharyltransferase to generate a polyvalent pneumococcal bioconjugate vaccine with polysaccharides containing glucose at their reducing end. In addition, we show that different vaccine carrier proteins can be glycosylated using this system. Pneumococcal bioconjugates are immunogenic, protective and rapidly produced within E. coli using recombinant techniques. These proof-of-principle experiments establish a platform to overcome limitations of other conjugating enzymes enabling the development of bioconjugate vaccines for many important human and animal pathogens.

Bioconjugation is a promising process to manufacture conjugate vaccines, but currently employed enzymes cannot generate the full spectrum of bacterial glycoproteins. Here, the authors use an O-linking oligosaccharyltransferase to generate a polyvalent pneumococcal bioconjugate vaccine with polysaccharides containing glucose at their reducing end.

Protein glycosylation: Sweet or bitter for bacterial pathogens?

Protein glycosylation systems in many bacteria are often associated with crucial biological processes like pathogenicity, immune evasion and host-pathogen interactions, implying the significance of protein-glycan linkage. Similarly, host protein glycosylation has been implicated in antimicrobial activity as well as in promoting growth of beneficial strains. In fact, few pathogens notably modulate host glycosylation machineries to facilitate their survival. To date, diverse chemical and biological strategies have been developed for conjugate vaccine production for disease control. Bioconjugate vaccines, largely being produced by glycoengineering using PglB (the N-oligosaccharyltransferase from Campylobacter jejuni) in suitable bacterial hosts, have been highly promising with respect to their effectiveness in providing protective immunity and ease of production. Recently, a novel method of glycoconjugate vaccine production involving an O-oligosaccharyltransferase, PglL from Neisseria meningitidis, has been optimized. Nevertheless, many questions on defining antigenic determinants, glycosylation markers, species-specific differences in glycosylation machineries, etc. still remain unanswered, necessitating further exploration of the glycosylation systems of important pathogens. Hence, in this review, we will discuss the impact of bacterial protein glycosylation on its pathogenesis and the interaction of pathogens with host protein glycosylation, followed by a discussion on strategies used for bioconjugate vaccine development.

Functional analysis of the Helicobacter pullorum N-linked protein glycosylation system

N-linked protein glycosylation systems operate in species from all three domains of life. The model bacterial N-linked glycosylation system from Campylobacter jejuni is encoded by pgl genes present at a single chromosomal locus. This gene cluster includes the pglB oligosaccharyltransferase responsible for transfer of glycan from lipid carrier to protein. Although all genomes from species of the Campylobacter genus contain a pgl locus, among the related Helicobacter genus only three evolutionarily related species (H. pullorum, H. canadensis and H. winghamensis) potentially encode N-linked protein glycosylation systems. Helicobacter putative pgl genes are scattered in five chromosomal loci and include two putative oligosaccharyltransferase-encoding pglB genes per genome. We have previously demonstrated the in vitro N-linked glycosylation activity of H. pullorum resulting in transfer of a pentasaccharide to a peptide at asparagine within the sequon (D/E)XNXS/T. In this study, we identified the first H. pullorum N-linked glycoprotein, termed HgpA. Production of histidine-tagged HgpA in the background of insertional knockout mutants of H. pullorum pgl/wbp genes followed by analysis of HgpA glycan structures demonstrated the role of individual gene products in the PglB1-dependent N-linked protein glycosylation pathway. Glycopeptide purification by zwitterionic-hydrophilic interaction liquid chromatography coupled with tandem mass spectrometry identified six glycosites from five H. pullorum proteins, which was consistent with proteins reactive with a polyclonal antiserum generated against glycosylated HgpA. This study demonstrates functioning of a H. pullorum N-linked general protein glycosylation system.

Metabolic engineering of glycoprotein biosynthesis in bacteria

The demonstration more than a decade ago that glycoproteins could be produced in Escherichia coli cells equipped with the N-linked protein glycosylation machinery from Campylobacter jejuni opened the door to using simple bacteria for the expression and engineering of complex glycoproteins. Since that time, metabolic engineering has played an increasingly important role in developing and optimizing microbial cell glyco-factories for the production of diverse glycoproteins and other glycoconjugates. It is becoming clear that future progress in creating efficient glycoprotein expression platforms in bacteria will depend on the adoption of advanced strain engineering strategies such as rational design and assembly of orthogonal glycosylation pathways, genome-wide identification of metabolic engineering targets, and evolutionary engineering of pathway performance. Here, we highlight recent advances in the deployment of metabolic engineering tools and strategies to develop microbial cell glyco-factories for the production of high-value glycoprotein targets with applications in research and medicine.

A General Protein O-Glycosylation Gene Cluster Encodes the Species-Specific Glycan of the Oral Pathogen Tannerella forsythia: O-Glycan Biosynthesis and Immunological Implications

The cell surface of the oral pathogen Tannerella forsythia is heavily glycosylated with a unique, complex decasaccharide that is O-glycosidically linked to the bacterium’s abundant surface (S-) layer, as well as other proteins. The S-layer glycoproteins are virulence factors of T. forsythia and there is evidence that protein O-glycosylation underpins the bacterium’s pathogenicity. To elucidate the protein O-glycosylation pathway, genes suspected of encoding pathway components were first identified in the genome sequence of the ATCC 43037 type strain, revealing a 27-kb gene cluster that was shown to be polycistronic. Using a gene deletion approach targeted at predicted glycosyltransferases (Gtfs) and methyltransferases encoded in this gene cluster, in combination with mass spectrometry of the protein-released O-glycans, we show that the gene cluster encodes the species-specific part of the T. forsythia ATCC 43037 decasaccharide and that this is assembled step-wise on a pentasaccharide core. The core was previously proposed to be conserved within the Bacteroidetes phylum, to which T. forsythia is affiliated, and its biosynthesis is encoded elsewhere on the bacterial genome. Next, to assess the prevalence of protein O-glycosylation among Tannerella sp., the publicly available genome sequences of six T. forsythia strains were compared, revealing gene clusters of similar size and organization as found in the ATCC 43037 type strain. The corresponding region in the genome of a periodontal health-associated Tannerella isolate showed a different gene composition lacking most of the genes commonly found in the pathogenic strains. Finally, we investigated whether differential cell surface glycosylation impacts T. forsythia’s overall immunogenicity. Release of proinflammatory cytokines by dendritic cells (DCs) upon stimulation with defined Gtf-deficient mutants of the type strain was measured and their T cell-priming potential post-stimulation was explored. This revealed that the O-glycan is pivotal to modulating DC effector functions, with the T. forsythia-specific glycan portion suppressing and the pentasaccharide core activating a Th17 response. We conclude that complex protein O-glycosylation is a hallmark of pathogenic T. forsythia strains and propose it as a valuable target for the design of novel antimicrobials against periodontitis.

The Interaction of the Gut Microbiota with the Mucus Barrier in Health and Disease in Human

Glycoproteins are major players in the mucus protective barrier in the gastrointestinal and other mucosal surfaces. In particular the mucus glycoproteins, or mucins, are responsible for the protective gel barrier. They are characterized by their high carbohydrate content, present in their variable number, tandem repeat domains. Throughout evolution the mucins have been maintained as integral components of the mucosal barrier, emphasizing their essential biological status. The glycosylation of the mucins is achieved through a series of biosynthetic pathways processes, which generate the wide range of glycans found in these molecules. Thus mucins are decorated with molecules having information in the form of a glycocode. The enteric microbiota interacts with the mucosal mucus barrier in a variety of ways in order to fulfill its many normal processes. How bacteria read the glycocode and link to normal and pathological processes is outlined in the review.

Production and efficacy of a low-cost recombinant pneumococcal protein polysaccharide conjugate vaccine

Streptococcus pneumoniae is the leading cause of bacterial pneumonia. Although this is a vaccine preventable disease, S. pneumoniae still causes over 1 million deaths per year, mainly in children under the age of five. The biggest disease burden is in the developing world, which is mainly due to unavailability of vaccines due to their high costs. Protein polysaccharide conjugate vaccines are given routinely in the developed world to children to induce a protective antibody response against S. pneumoniae. One of these vaccines is Prevnar13, which targets 13 of the 95 known capsular types. Current vaccine production requires growth of large amounts of the 13 serotypes, and isolation of the capsular polysaccharide that is then chemically coupled to a protein, such as the diphtheria toxoid CRM197, in a multistep expensive procedure. In this study, we design, purify and produce novel recombinant pneumococcal protein polysaccharide conjugate vaccines in Escherichia coli, which act as mini factories for the low-cost production of conjugate vaccines. Recombinant vaccine efficacy was tested in a murine model of pneumococcal pneumonia; ability to protect against invasive disease was compared to that of Prevnar13. This study provides the first proof of principle that protein polysaccharide conjugate vaccines produced in E. coli can be used to prevent pneumococcal infection. Vaccines produced in this manner may provide a low-cost alternative to the current vaccine production methodology.

Peptide adjacent to glycosylation sites impacts immunogenicity of glycoconjugate vaccine

Glycoconjugate vaccine is composed of polysaccharides (PSs) covalently linked with carrier protein. Glycosylation site selection, as a significant factor leading to heterogeneities of glycoconjugate structure, draws more and more attentions for its impact on the immunogenicity of glycoconjugate vaccine. To elucidate the relationship between glycosylation connectivity and immunogenicity of glycoconjugate vaccine, in this study, anti-E. coli O157:H7 glycoconjugate O-PS-MBP with defined connectivity, and three selected peptide segments GS1, GS2, GS3 derived from O-PS-MBP was synthesized. Immunogenicity results showed that only peptides adjacent to the glycosylation sites (GS1 and GS2) promoted the generation of PS-specific IgG antibodies and contributed to PS-specific IgG subclass distribution. Furthermore, GS1 and GS2 had significant priming effect for eliciting PS-specific IgG antibodies. These results indicated that different locations of glycosylation sites could lead to diverse presentation of peptides and glycopeptides to APCs and influence the immunogenicity of glycoconjugate vaccine, which extend the current understanding of mechanism for adaptive immune system activation by glycoconjugate vaccine, and have implications for rational glycoconjugate vaccine design.

Bacterial Glycoengineering as a Biosynthetic Route to Customized Glycomolecules

Bacteria have garnered increased interest in recent years as a platform for the biosynthesis of a variety of glycomolecules such as soluble oligosaccharides, surface-exposed carbohydrates, and glycoproteins. The ability to engineer commonly used laboratory species such as Escherichia coli to efficiently synthesize non-native sugar structures by recombinant expression of enzymes from various carbohydrate biosynthesis pathways has allowed for the facile generation of important products such as conjugate vaccines, glycosylated outer membrane vesicles, and a variety of other research reagents for studying and understanding the role of glycans in living systems. This chapter highlights some of the key discoveries and technologies for equipping bacteria with the requisite biosynthetic machinery to generate such products. As the bacterial glyco-toolbox continues to grow, these technologies are expected to expand the range of glycomolecules produced recombinantly in bacterial systems, thereby opening up this platform to an even larger number of applications.

A recombinant O-polysaccharide-protein conjugate approach to develop highly specific monoclonal antibodies to Shiga toxin-producing Escherichia coli O157 and O145 serogroups

Shiga toxin-producing Escherichia coli (STEC) is the major etiologic agent of hemolytic-uremic syndrome (HUS). The high rate of HUS emphasizes the urgency for the implementation of primary prevention strategies to reduce its public health impact. Argentina shows the highest rate of HUS worldwide, being E. coli O157 the predominant STEC-associated HUS serogroup (>70%), followed by E. coli O145 (>9%). To specifically detect these serogroups we aimed at developing highly specific monoclonal antibodies (mAbs) against the O-polysaccharide (O-PS) section of the lipopolysaccharide (LPS) of the dominant STEC-associated HUS serogroups in Argentina. The development of hybridomas secreting mAbs against O157 or O145 was carried out through a combined immunization strategy, involving adjuvated-bacterial immunizations followed by immunizations with recombinant O-PS-protein conjugates. We selected hybridoma clones that specifically recognized the engineered O-PS-protein conjugates of O157 or O145 serogroups. Indirect ELISA of heat-killed bacteria showed specific binding to O157 or O145 serogroups, respectively, while no cross-reactivity with other epidemiological important STEC strains, Brucella abortus, Salmonella group N or Yersinia enterocolitica O9 was observed. Western blot analysis showed specific recognition of the sought O-PS section of the LPS by all mAbs. Finally, the ability of the developed mAbs to bind the surface of whole bacteria cells was confirmed by flow cytometry, confocal microscopy and agglutination assays, indicating that these mAbs present an exceptional degree of specificity and relative affinity in the detection and identification of E. coli O157 and O145 serogroups. These mAbs may be of significant value for clinical diagnosis and food quality control applications. Thus, engineered O-PS specific moieties contained in the recombinant glycoconjugates used for combined immunization and hybridoma selection are an invaluable resource for the development of highly specific mAbs.

Engineering and Dissecting the Glycosylation Pathway of a Streptococcal Serine-rich Repeat Adhesin

Serine-rich repeat glycoproteins (SRRPs) are conserved in Gram-positive bacteria. They are crucial for modulating biofilm formation and bacterial-host interactions. Glycosylation of SRRPs plays a pivotal role in the process; thus understanding the glycosyltransferases involved is key to identifying new therapeutic drug targets. The glycosylation of Fap1, an SRRP of Streptococcus parasanguinis, is mediated by a gene cluster consisting of six genes: gtf1, gtf2, gly, gtf3, dGT1, and galT2. Mature Fap1 glycan possesses the sequence of Rha1–3Glc1-(Glc1–3GlcNAc1)-2,6-Glc1–6GlcNAc. Gtf12, Gtf3, and dGT1 are responsible for the first four steps of the Fap1 glycosylation, catalyzing the transfer of GlcNAc, Glc, Glc, and GlcNAc residues to the protein backbone sequentially. The role of GalT2 and Gly in the Fap1 glycosylation is unknown. In the present study, we synthesized the fully modified Fap1 glycan in Escherichia coli by incorporating all six genes from the cluster. This study represents the first reconstitution of an exogenous stepwise O-glycosylation synthetic pathway in E. coli. In addition, we have determined that GalT2 mediates the fifth step of the Fap1 glycosylation by adding a rhamnose residue, and Gly mediates the final glycosylation step by transferring glucosyl residues. Furthermore, inactivation of each glycosyltransferase gene resulted in differentially impaired biofilms of S. parasanguinis, demonstrating the importance of Fap1 glycosylation in the biofilm formation. The Fap1 glycosylation system offers an excellent model to engineer glycans using different permutations of glycosyltransferases and to investigate biosynthetic pathways of SRRPs because SRRP genetic loci are highly conserved.

Heterologous Pseudomonas aeruginosa O-antigen delivery using a Salmonella enterica serovar Typhimurium wecA mutant strain

There is a broad interest in adapting live vaccine strains (LVS) for use as recombinant vaccines that can deliver heterologous antigens. The Salmonella enterica serovar Typhimurium SL1344 ΔwecA LVS contains a mutation in wecA that abrogates production of Enterobacterial common antigen. This ΔwecA strain is attenuated in vivo, persistently colonizes the host, and protects against both wild type and cross-Salmonella serovar lethal challenge in a murine model of salmonellosis. Given these characteristics, we hypothesized that the SL1344 ΔwecA strain could be used as a carrier for heterologous antigen expression. To test this hypothesis, SL1344 ΔwecA was engineered to express the Pseudomonas aeruginosa O11 O-antigen gene cluster. Intraperitoneal (IP) but not oral immunization of BALB/c mice with the heterologous expression strain protected against lethal P. aeruginosa intranasal (IN) challenge. Furthermore, IP immunization resulted in P. aeruginosa O11-specific Ig and IgG antibody production. Functional analysis of sera collected from the IP immunized mice showed antibody-mediated agglutination and opsonophagocytic activity against P. aeruginosa. En masse, these results indicate that the S. Typhimurium SL1344 ΔwecA strain expressing the P. aeruginosa O11 O-antigen gene cluster is able to induce a humoral immune response and to protect against lethal P. aeruginosa challenge. As such, the S. Typhimurium SL1344 ΔwecA LVS can likely serve as a vehicle for expression of a wide variety of heterologous antigens as a means to create recombinant vaccines.

A Bacterial Glycoengineered Antigen for Improved Serodiagnosis of Porcine Brucellosis

Brucellosis is a highly zoonotic disease that affects animals and human beings. Brucella suis is the etiological agent of porcine brucellosis and one of the major human brucellosis pathogens. Laboratory diagnosis of porcine brucellosis mainly relies on serological tests, and it has been widely demonstrated that serological assays based on the detection of anti O-polysaccharide antibodies are the most sensitive tests. Here, we validate a recombinant glycoprotein antigen, an N-formylperosamine O-polysaccharide-protein conjugate (OAg-AcrA), for diagnosis of porcine brucellosis. An indirect immunoassay based on the detection of anti-O-polysaccharide IgG antibodies was developed coupling OAg-AcrA to enzyme-linked immunosorbent assay plates (glyco-iELISA). To validate the assay, 563 serum samples obtained from experimentally infected and immunized pigs, as well as animals naturally infected with B. suis biovar 1 or 2, were tested. A receiver operating characteristic (ROC) analysis was performed, and based on this analysis, the optimum cutoff value was 0.56 (relative reactivity), which resulted in a diagnostic sensitivity and specificity of 100% and 99.7%, respectively. A cutoff value of 0.78 resulted in a test sensitivity of 98.4% and a test specificity of 100%. Overall, our results demonstrate that the glyco-iELISA is highly accurate for diagnosis of porcine brucellosis, improving the diagnostic performance of current serological tests. The recombinant glycoprotein OAg-AcrA can be produced in large homogeneous batches in a standardized way, making it an ideal candidate for further validation as a universal antigen for diagnosis of "smooth" brucellosis in animals and humans.

Delta-pgm, a new live-attenuated vaccine against Brucella suis

Brucellosis is one of the most widespread zoonosis in the world affecting many domestic and wild animals including bovines, goats, pigs and dogs. Each species of the Brucella genus has a particular tropism toward different mammals being the most relevant for human health Brucella abortus, Brucella melitensis and Brucella suis that infect bovines, goats/camelids and swine respectively. Although for B. abortus and B. melitensis there are vaccines available, there is no efficient vaccine to protect swine from B. suis infection so far. We describe here the construction of a novel vaccine strain that confers excellent protection against B. suis in a mouse model of infection. This strain is a clean deletion of the phosphoglucomutase (pgm) gene that codes for a protein that catalyzes the conversion of glucose-6-P to glucose-1-P, which is used as a precursor for the biosynthesis of many polysaccharides. The Delta-pgm strain lacks a complete lipopolysaccharide, is unable to synthesize cyclic beta glucans and is sensitive to several detergents and Polymyxin B. We show that this strain replicates in cultured cells, is completely avirulent in the mouse model of infection but protects against a challenge of the virulent strain inducing the production of pro-inflammatory cytokines. This novel strain could be an excellent candidate for the control of swine brucellosis, a disease of emerging concern in many parts of the world.

Increased efficiency of Campylobacter jejuni N-oligosaccharyltransferase PglB by structure-guided engineering

Conjugate vaccines belong to the most efficient preventive measures against life-threatening bacterial infections. Functional expression of N-oligosaccharyltransferase (N-OST) PglB of Campylobacter jejuni in Escherichia coli enables a simplified production of glycoconjugate vaccines in prokaryotic cells. Polysaccharide antigens of pathogenic bacteria can be covalently coupled to immunogenic acceptor proteins bearing engineered glycosylation sites. Transfer efficiency of PglB_Cj is low for certain heterologous polysaccharide substrates. In this study, we increased glycosylation rates for Salmonella enterica sv. Typhimurium LT2 O antigen (which lacks N-acetyl sugars) and Staphylococcus aureus CP5 polysaccharides by structure-guided engineering of PglB. A three-dimensional homology model of membrane-associated PglB_Cj, docked to the natural C. jejuni N-glycan attached to the acceptor peptide, was used to identify potential sugar-interacting residues as targets for mutagenesis. Saturation mutagenesis of an active site residue yielded the enhancing mutation N311V, which facilitated fivefold to 11-fold increased in vivo glycosylation rates as determined by glycoprotein-specific ELISA. Further rounds of in vitro evolution led to a triple mutant S80R-Q287P-N311V enabling a yield improvement of S. enterica LT2 glycoconjugates by a factor of 16. Our results demonstrate that bacterial N-OST can be tailored to specific polysaccharide substrates by structure-guided protein engineering.

Electrochemical magnetic microbeads-based biosensor for point-of-care serodiagnosis of infectious diseases

Access to appropriate diagnostic tools is an essential component in the evaluation and improvement of global health. Additionally, timely detection of infectious agents is critical in early diagnosis and treatment of infectious diseases. Conventional pathogen detection methods such as culturing, enzyme linked immunosorbent assay (ELISA) or polymerase chain reaction (PCR) require long assay times, and complex and expensive instruments making them not adaptable to point-of-care (PoC) needs at resource-constrained places and primary care settings. Therefore, there is an unmet need to develop portable, simple, rapid, and accurate methods for PoC detection of infections. Here, we present the development and validation of a portable, robust and inexpensive electrochemical magnetic microbeads-based biosensor (EMBIA) platform for PoC serodiagnosis of infectious diseases caused by different types of microorganisms (parasitic protozoa, bacteria and viruses). We demonstrate the potential use of the EMBIA platform for in situ diagnosis of human (Chagas disease and human brucellosis) and animal (bovine brucellosis and foot-and-mouth disease) infections clearly differentiating infected from non-infected individuals or animals. For Chagas disease, a more extensive validation of the test was performed showing that the EMBIA platform displayed an excellent diagnostic performance almost indistinguishable, in terms of specificity and sensitivity, from a fluorescent immunomagnetic assay and the conventional ELISA using the same combination of antigens. This platform technology could potentially be applicable to diagnose other infectious and non-infectious diseases as well as detection and/or quantification of biomarkers at the POC and primary care settings.

The sweet tooth of bacteria: common themes in bacterial glycoconjugates

Humans have been increasingly recognized as being superorganisms, living in close contact with a microbiota on all their mucosal surfaces. However, most studies on the human microbiota have focused on gaining comprehensive insights into the composition of the microbiota under different health conditions (e.g., enterotypes), while there is also a need for detailed knowledge of the different molecules that mediate interactions with the host. Glycoconjugates are an interesting class of molecules for detailed studies, as they form a strain-specific barcode on the surface of bacteria, mediating specific interactions with the host. Strikingly, most glycoconjugates are synthesized by similar biosynthesis mechanisms. Bacteria can produce their major glycoconjugates by using a sequential or an en bloc mechanism, with both mechanistic options coexisting in many species for different macromolecules. In this review, these common themes are conceptualized and illustrated for all major classes of known bacterial glycoconjugates, with a special focus on the rather recently emergent field of glycosylated proteins. We describe the biosynthesis and importance of glycoconjugates in both pathogenic and beneficial bacteria and in both Gram-positive and -negative organisms. The focus lies on microorganisms important for human physiology. In addition, the potential for a better knowledge of bacterial glycoconjugates in the emerging field of glycoengineering and other perspectives is discussed.

In vivo production of a novel glycoconjugate vaccine against Shigella flexneri 2a in recombinant Escherichia coli: identification of stimulating factors for in vivo glycosylation

Background

Glycoconjugated vaccines composed of polysaccharide antigens covalently linked to immunogenic carrier proteins have proved to belong to the most effective and safest vaccines for combating bacterial pathogens. The functional transfer of the N-glycosylation machinery from Campylobacter jejuni to the standard prokaryotic host Escherichia coli established a novel bioconjugation methodology termed bacterial glycoengineering.

Results

In this study, we report on the production of a new recombinant glycoconjugate vaccine against Shigella flexneri 2a representing the major serotype for global outbreaks of shigellosis. We demonstrate that S. flexneri 2a O-polysaccharides can be transferred to a detoxified variant of Pseudomonas aeruginosa carrier protein exotoxin A (EPA) by the C. jejuni oligosaccharyltransferase PglB, resulting in glycosylated EPA-2a. Moreover, we optimized the in vivo production of this novel vaccine by identification and quantitative analysis of critical process parameters for glycoprotein synthesis. It was found that sequential induction of oligosaccharyltransferase PglB and carrier protein EPA increased the specific productivity of EPA-2a by a factor of 1.6. Furthermore, by the addition of 10 g/L of the monosaccharide N-acetylglucosamine during induction, glycoconjugate vaccine yield was boosted up to 3.1-fold. The optimum concentration of Mg²⁺ ions for N-glycan transfer was determined to be 10 mM. Finally, optimized parameters were transferred to high cell density cultures with a 46-fold increase of overall yield of glycoconjugate compared to the one in initial shake flask production.

Conclusion

The present study is the first attempt to identify stimulating parameters for improved productivity of S. flexneri 2a bioconjugates. Optimization of glycosylation efficiency will ultimately foster the transfer of lab-scale expression to a cost-effective in vivo production process for a glycoconjugate vaccine against S. flexneri 2a in E. coli. This study is an important step towards this goal and provides a starting point for further optimization studies.

Glyco-engineering O-Antigen-Based Vaccines and Diagnostics in E. coli

The biotechnological relevance of protein glycosylation has exponentially grown in recent years. With the advances in protein glycosylation research, new possibilities for glyco-engineering have arisen, and a wide array of glycans can be designed and potentially transferred to target proteins in the biotechnologically relevant host Escherichia coli. Here we provide insight on how to select the best strains and plasmids. We also describe methods for determination of glycan expression and assembly, protein glycosylation using western blot, and preparation of samples for mass spectrometry.

Current Approaches to Engineering N-Linked Protein Glycosylation in Bacteria

N-Linked protein glycosylation is a common posttranslational protein modification in eukaryotes involved in many biological processes. As glycosylation is also important for the function and the pharmacokinetic properties of many protein therapeutics, there is an increasing interest in expression systems able to produce glycoproteins of well-defined structure. Bacterial expression hosts generally do not glycosylate proteins at all. However, the discovery of bacterial N-glycosylation systems has opened up a new route for the production of therapeutically interesting glycoproteins in glyco-engineered bacteria. This review offers an introduction to the many efforts taken to engineer bacteria in order to produce N-glycoproteins with defined eukaryotic glycan structures, completely novel protein glycoconjugates as well as to establish screening approaches for improvement and adaptation of the glycosylation machinery to specific applications.

Serogroup-specific bacterial engineered glycoproteins as novel antigenic targets for diagnosis of shiga toxin-producing-escherichia coli-associated hemolytic-uremic syndrome

Human infection with Shiga toxin-producing Escherichia coli (STEC) is a major cause of postdiarrheal hemolytic-uremic syndrome (HUS), a life-threatening condition characterized by hemolytic anemia, thrombocytopenia, and acute renal failure. E. coli O157:H7 is the dominant STEC serotype associated with HUS worldwide, although non-O157 STEC serogroups can cause a similar disease. The detection of anti-O157 E. coli lipopolysaccharide (LPS) antibodies in combination with stool culture and detection of free fecal Shiga toxin considerably improves the diagnosis of STEC infections. In the present study, we exploited a bacterial glycoengineering technology to develop recombinant glycoproteins consisting of the O157, O145, or O121 polysaccharide attached to a carrier protein as serogroup-specific antigens for the serological diagnosis of STEC-associated HUS. Our results demonstrate that using these antigens in indirect ELISAs (glyco-iELISAs), it is possible to clearly discriminate between STEC O157-, O145-, and O121-infected patients and healthy children, as well as to confirm the diagnosis in HUS patients for whom the classical diagnostic procedures failed. Interestingly, a specific IgM response was detected in almost all the analyzed samples, indicating that it is possible to detect the infection in the early stages of the disease. Additionally, in all the culture-positive HUS patients, the serotype identified by glyco-iELISAs was in accordance with the serotype of the isolated strain, indicating that these antigens are valuable not only for diagnosing HUS caused by the O157, O145, and O121 serogroups but also for serotyping and guiding the subsequent steps to confirm diagnosis.

Hijacking bacterial glycosylation for the production of glycoconjugates, from vaccines to humanised glycoproteins

OBJECTIVES

Glycosylation or the modification of a cellular component with a carbohydrate moiety has been demonstrated in all three domains of life as a basic post-translational process important in a range of biological processes. This review will focus on the latest studies attempting to exploit bacterial N-linked protein glycosylation for glycobiotechnological applications including glycoconjugate vaccine and humanised glycoprotein production. The challenges that remain for these approaches to reach full biotechnological maturity will be discussed.

KEY FINDINGS

Oligosaccharyltransferase-dependent N-linked glycosylation can be exploited to make glycoconjugate vaccines against bacterial pathogens. Few technical limitations remain, but it is likely that the technologies developed will soon be considered a cost-effective and flexible alternative to current chemical-based methods of vaccine production. Some highlights from current glycoconjugate vaccines developed using this in-vivo production system include a vaccine against Shigella dysenteriae O1 that has passed phase 1 clinical trials, a vaccine against the tier 1 pathogen Francisella tularensis that has shown efficacy in mice and a vaccine against Staphylococcus aureus serotypes 5 and 8. Generation of humanised glycoproteins within bacteria was considered impossible due to the distinct nature of glycan modification in eukaryotes and prokaryotes. We describe the method used to overcome this conundrum to allow engineering of a eukaryotic pentasaccharide core sugar modification within Escherichia coli. This core was assembled by combining the function of the initiating transferase WecA, several Alg genes from Saccharomyces cerevisiae and the oligosaccharyltransferase function of the Campylobacter jejuni PglB. Further exploitation of a cytoplasmic N-linked glycosylation system found in Actinobacillus pleuropneumoniae where the central enzyme is known as N-linking glycosyltransferase has overcome some of the limitations demonstrated by the oligosaccharyltransferase-dependent system.

SUMMARY

Characterisation of the first bacterial N-linked glycosylation system in the human enteropathogen Campylobacter jejuni has led to substantial biotechnological applications. Alternative methods for glycoconjugate vaccine production have been developed using this N-linked system. Vaccines against both Gram-negative and Gram-positive organisms have been developed, and efficacy testing has thus far demonstrated that the vaccines are safe and that robust immune responses are being detected. These are likely to complement and reduce the cost of current technologies thus opening new avenues for glycoconjugate vaccines. These new markets could potentially include glycoconjugate vaccines tailored specifically for animal vaccination, which has until today thought to be non-viable due to the cost of current in-vitro chemical conjugation methods. Utilisation of N-linked glycosylation to generate humanised glycoproteins is also close to becoming reality. This 'bottom up' assembly mechanism removes the heterogeneity seen in current humanised products. The majority of developments reported in this review exploit a single N-linked glycosylation system from Campylobacter jejuni; however, alternative N-linked glycosylation systems have been discovered which should help to overcome current technical limitations and perhaps more systems remain to be discovered. The likelihood is that further glycosylation systems exist and are waiting to be exploited.

Glycoconjugate vaccine containing Escherichia coli O157:H7 O-antigen linked with maltose-binding protein elicits humoral and cellular responses

Glycoconjugate is one of the most efficacious and safest vaccines against bacterial pathogens. Previous studies of glycoconjugates against pathogen E. coli O157:H7 focused more on the humoral responses they elicited. However, little was known about their cellular responses. In this study, we exploited a novel approach based on bacterial protein N-linked glycosylation system to produce glycoconjugate containing Escherichia coli O157:H7 O-antigen linked with maltose-binding protein and examined its humoral and cellular responses in BALB/c mice. The transfer of E. coli O157:H7 O-antigen to MBP was confirmed by western blot and MALDI-TOF MS. Mice injected with glycoconjugate O-Ag-MBP elicited serum bactericidal antibodies including anti-E. coli O157:H7 O-antigen IgG and IgM. Interestingly, O-Ag-MBP also stimulated the secretion of anti-E. coli O157:H7 O-antigen IgA in intestine. In addition, O-Ag-MBP stimulated cellular responses by recruiting Th1-biased CD4+ T cells, CD8+ T cells. Meanwhile, O-Ag-MBP induced the upregulation of Th1-related IFN-γ and downregulation of Th2-related IL-4, and the upregulation of IFN-γ was stimulated by MBP in a dose-dependent manner. MBP showed TLR4 agonist-like properties to activate Th1 cells as carrier protein of O-Ag-MBP. Thus, glycoconjugate vaccine E. coli O157:H7-specific O-Ag-MBP produced by bacterial protein N-linked glycosylation system was able to elicit both humoral and Th1-biased cellular responses.

Production of a recombinant vaccine candidate against Burkholderia pseudomallei exploiting the bacterial N-glycosylation machinery

Vaccines developing immune responses toward surface carbohydrates conjugated to proteins are effective in preventing infection and death by bacterial pathogens. Traditional production of these vaccines utilizes complex synthetic chemistry to acquire and conjugate the glycan to a protein. However, glycoproteins produced by bacterial protein glycosylation systems are significantly easier to produce, and could possible be used as vaccine candidates. In this work, we functionally expressed the Burkholderia pseudomallei O polysaccharide (OPS II), the Campylobacter jejuni oligosaccharyltransferase (OTase), and a suitable glycoprotein (AcrA) in a designer E. coli strain with a higher efficiency for production of glycoconjugates. We were able to produce and purify the OPS II-AcrA glycoconjugate, and MS analysis confirmed correct glycan was produced and attached. We observed the attachment of the O-acetylated deoxyhexose directly to the acceptor protein, which expands the range of substrates utilized by the OTase PglB. Injection of the glycoprotein into mice generated an IgG immune response against B. pseudomallei, and this response was partially protective against an intranasal challenge. Our experiments show that bacterial engineered glycoconjugates can be utilized as vaccine candidates against B. pseudomallei. Additionally, our new E. coli strain SDB1 is more efficient in glycoprotein production, and could have additional applications in the future.

Diversity within the O-linked protein glycosylation systems of acinetobacter species

The opportunistic human pathogen Acinetobacter baumannii is a concern to health care systems worldwide because of its persistence in clinical settings and the growing frequency of multiple drug resistant infections. To combat this threat, it is necessary to understand factors associated with disease and environmental persistence of A. baumannii. Recently, it was shown that a single biosynthetic pathway was responsible for the generation of capsule polysaccharide and O-linked protein glycosylation. Because of the requirement of these carbohydrates for virulence and the non-template driven nature of glycan biogenesis we investigated the composition, diversity, and properties of the Acinetobacter glycoproteome. Utilizing global and targeted mass spectrometry methods, we examined 15 strains and found extensive glycan diversity in the O-linked glycoproteome of Acinetobacter. Comparison of the 26 glycoproteins identified revealed that different A. baumannii strains target similar protein substrates, both in characteristics of the sites of O-glycosylation and protein identity. Surprisingly, glycan micro-heterogeneity was also observed within nearly all isolates examined demonstrating glycan heterogeneity is a widespread phenomena in Acinetobacter O-linked glycosylation. By comparing the 11 main glycoforms and over 20 alternative glycoforms characterized within the 15 strains, trends within the glycan utilized for O-linked glycosylation could be observed. These trends reveal Acinetobacter O-linked glycosylation favors short (three to five residue) glycans with limited branching containing negatively charged sugars such as GlcNAc3NAcA4OAc or legionaminic/pseudaminic acid derivatives. These observations suggest that although highly diverse, the capsule/O-linked glycan biosynthetic pathways generate glycans with similar characteristics across all A. baumannii.

A bacterial engineered glycoprotein as a novel antigen for diagnosis of bovine brucellosis

Brucellosis is a highly contagious zoonosis that affects livestock and human beings. Laboratory diagnosis of bovine brucellosis mainly relies on serological diagnosis using serum and/or milk samples. Although there are several serological tests with different diagnostic performance and capacity to differentiate vaccinated from infected animals, there is still no standardized reference antigen for the disease. Here we validate the first recombinant glycoprotein antigen, an N-formylperosamine O-polysaccharide-protein conjugate (OAg-AcrA), for diagnosis of bovine brucellosis. This antigen can be produced in homogeneous batches without the need of culturing pathogenic brucellae; all characteristics that make it appropriate for standardization. An indirect immunoassay based on the detection of anti O-polysaccharide IgG antibodies in bovine samples was developed coupling OAg-AcrA to magnetic beads or ELISA plates. As a proof of concept and to validate the antigen, we analyzed serum, whole blood and milk samples obtained from non-infected, experimentally infected and vaccinated animals included in a vaccination/infection trial performed in our laboratory as well as more than 1000 serum and milk samples obtained from naturally infected and S19-vaccinated animals from Argentina. Our results demonstrate that OAg-AcrA-based assays are highly accurate for diagnosis of bovine brucellosis, even in vaccinated herds, using different types of samples and in different platforms. We propose this novel recombinant glycoprotein as an antigen suitable for the development of new standard immunological tests for screening and confirmatory diagnosis of bovine brucellosis in regions or countries with brucellosis-control programs.

Activation of duck RIG-I by TRIM25 is independent of anchored ubiquitin

Retinoic acid inducible gene I (RIG-I) is a viral RNA sensor crucial in defense against several viruses including measles, influenza A and hepatitis C. RIG-I activates type-I interferon signalling through the adaptor for mitochondrial antiviral signaling (MAVS). The E3 ubiquitin ligase, tripartite motif containing protein 25 (TRIM25), activates human RIG-I through generation of anchored K63-linked polyubiquitin chains attached to lysine 172, or alternatively, through the generation of unanchored K63-linked polyubiquitin chains that interact non-covalently with RIG-I CARD domains. Previously, we identified RIG-I of ducks, of interest because ducks are the host and natural reservoir of influenza viruses, and showed it initiates innate immune signaling leading to production of interferon-beta (IFN-β). We noted that K172 is not conserved in RIG-I of ducks and other avian species, or mouse. Because K172 is important for both mechanisms of activation of human RIG-I, we investigated whether duck RIG-I was activated by TRIM25, and if other residues were the sites for attachment of ubiquitin. Here we show duck RIG-I CARD domains are ubiquitinated for activation, and ubiquitination depends on interaction with TRIM25, as a splice variant that cannot interact with TRIM25 is not ubiquitinated, and cannot be activated. We expressed GST-fusion proteins of duck CARD domains and characterized TRIM25 modifications of CARD domains by mass spectrometry. We identified two sites that are ubiquitinated in duck CARD domains, K167 and K193, and detected K63 linked polyubiquitin chains. Site directed mutagenesis of each site alone, does not alter the ubiquitination profile of the duck CARD domains. However, mutation of both sites resulted in loss of all attached ubiquitin and polyubiquitin chains. Remarkably, the double mutant duck RIG-I CARD still interacts with TRIM25, and can still be activated. Our results demonstrate that anchored ubiquitin chains are not necessary for TRIM25 activation of duck RIG-I.

Pushing the Bacterial Envelope

Over the past three decades, a powerful array of techniques has been developed for expressing heterologous proteins and saccharides on the surface of bacteria. Surface-engineered bacteria, in turn, have proven useful in a variety of settings, including high-throughput screening, biofuel production, and vaccinology. In this chapter, we provide a comprehensive review of methods for displaying polypeptides and sugars on the bacterial cell surface, and discuss the many innovative applications these methods have found to date. While already an important biotechnological tool, we believe bacterial surface display may be further improved through integration with emerging methodology in other fields, such as protein engineering and synthetic chemistry. Ultimately, we envision bacterial display becoming a multidisciplinary platform with the potential to transform basic and applied research in bacteriology, biotechnology, and biomedicine.

Exploitation of bacterial N-linked glycosylation to develop a novel recombinant glycoconjugate vaccine against Francisella tularensis

Glycoconjugate-based vaccines have proved to be effective at producing long-lasting protection against numerous pathogens. Here, we describe the application of bacterial protein glycan coupling technology (PGCT) to generate a novel recombinant glycoconjugate vaccine. We demonstrate the conjugation of the Francisella tularensis O-antigen to the Pseudomonas aeruginosa carrier protein exotoxin A using the Campylobacter jejuni PglB oligosaccharyltransferase. The resultant recombinant F. tularensis glycoconjugate vaccine is expressed in Escherichia coli where yields of 3 mg l(-1) of culture were routinely produced in a single-step purification process. Vaccination of BALB/c mice with the purified glycoconjugate boosted IgG levels and significantly increased the time to death upon subsequent challenge with F. tularensis subsp. holarctica. PGCT allows different polysaccharide and protein combinations to be produced recombinantly and could be easily applicable for the production of diverse glycoconjugate vaccines.

Expanding the glycoengineering toolbox: the rise of bacterial N-linked protein glycosylation

Glycosylation is the most prevalent post-translational modification found on proteins, occurring in all domains of life. Ever since the discovery of asparagine-linked (N-linked) protein glycosylation pathways in bacteria, major efforts have been made to harness these systems for the creation of novel therapeutics, vaccines, and diagnostics. Recent advances such as the ability to produce designer glycans in bacteria, some containing unnatural sugars, and techniques for evolving glycosylation enzymes have spawned an entirely new discipline known as bacterial glycoengineering. In addition to their biotechnological and therapeutic potential, bacteria equipped with recombinant N-linked glycosylation pathways are improving our understanding of the N-glycosylation process. This review discusses the key role played by microorganisms in glycosciences, particularly in the context of N-linked glycosylation.

In vitro activity of Neisseria meningitidis PglL O-oligosaccharyltransferase with diverse synthetic lipid donors and a UDP-activated sugar

Oligosaccharyltransferases (OTases) are enzymes that catalyze the transfer of an oligosaccharide from a lipid carrier to an acceptor molecule, commonly a protein. OTases are classified as N-OTases and O-OTases, depending on the nature of the glycosylation reaction. The N-OTases catalyze the glycan transfer to amide groups in asparagines in a reaction named N-linked glycosylation. The O-OTases are responsible for protein O-linked glycosylation, which involves the attachment of glycans to hydroxyl groups of serine or threonine residues. These enzymes exhibit a relaxed specificity and are able to transfer a variety of glycan structures to different protein acceptors. This property confers OTases with great biotechnological potential as these enzymes can produce glycoconjugates relevant to the pharmaceutical industry. Furthermore, OTases are thought to be involved in pathogenesis mechanisms. Several aspects of the functionality of OTases are not fully understood. In this work, we developed a novel approach to perform kinetic studies on PglL, the O-OTase from Neisseria meningitidis. We investigated the importance of the acyl moiety of the lipid glycan donor substrate on the functionality of PglL by testing the efficiency of glycosylation reactions using synthetic substrates carrying the same glycan structure but different acyl moieties. We found that PglL can function with many lipids as glycan donors, although the length and the conformation of the lipid moiety significantly influenced the catalytic efficiency. Interestingly, PglL was also able to transfer a monosaccharide employing its nucleotide-activated form, acting as a Leloir glycosyltransferase. These results provide new insights on the function and the evolution of oligosaccharyltransferases.

Development and validation of a novel diagnostic test for human brucellosis using a glyco-engineered antigen coupled to magnetic beads

Brucellosis is a highly contagious zoonosis and still a major human health problem in endemic areas of the world. Although several diagnostic tools are available, most of them are difficult to implement especially in developing countries where complex health facilities are limited. Taking advantage of the identical structure and composition of the Brucella spp. and Yersinia enterocolitica O:9 O-polysaccharide, we explored the application of a recombinant Y. enterocolitica O:9-polysaccharide-protein conjugate (OAg-AcrA) as a novel antigen for diagnosis of human brucellosis. We have developed and validated an indirect immunoassay using OAg-AcrA coupled to magnetic beads. OAg-AcrA was produced and purified with high yields in Y. enterocolitica O:9 cells co-expressing the oligosaccharyltransferase PglB and the protein acceptor AcrA of Campylobacter jejuni without the need for culturing Brucella. Expression of PglB and AcrA in Y. enterocolitica resulted in the transfer of the host O-polysaccharide from its lipid carrier to AcrA. To validate the assay and determine the cutoff values, a receiver-operating characteristic analysis was performed using a panel of characterized serum samples obtained from healthy individuals and patients of different clinical groups. Our results indicate that, using this assay, it is possible to detect infection caused by the three main human brucellosis agents (B. abortus, B. melitensis and B. suis) and select different cutoff points to adjust sensitivity and specificity levels as needed. A cutoff value of 13.20% gave a sensitivity of 100% and a specificity of 98.57%, and a cutoff value of 16.15% resulted in a test sensitivity and specificity of 93.48% and 100%, respectively. The high diagnostic accuracy, low cost, reduced assay time and simplicity of this new glycoconjugate-magnetic beads assay makes it an attractive diagnostic tool for using not only in clinics and brucellosis reference laboratories but also in locations with limited laboratory infrastructure and/or minimally trained community health workers.

Brucellosis is one of the most widespread zoonoses in the world and still represents a mayor animal and human health problem in many endemic areas. Central for early diagnosis and appropriate treatment of the disease in humans is to have an easy to implement, fast and accurate diagnostic test for the disease. Currently, most of the diagnostic tests for human brucellosis are not of simple implementation in areas where laboratory infrastructure or trained personnel are not available. We present here a novel and simple diagnostic test for human brucellosis using a glyco-engineered antigen coupled to magnetic beads. Our results show that, with this new method, it is possible to detect infection caused by the three main human brucellosis agents with high sensitivity and specificity. We additionally demonstrate that this assay allows the selection of different cutoff points so that the desired operating characteristics of the test in terms of diagnostic sensitivity and specificity can be adjusted according to the needs of the operator. We believe that the high accuracy, low cost and simplicity of this new test makes it an attractive diagnostic tool not only for clinics and brucellosis reference laboratories but also for locations with limited laboratory infrastructure and/or minimally trained community health workers.

Genomic and functional analysis of the type VI secretion system in Acinetobacter

The genus Acinetobacter is comprised of a diverse group of species, several of which have raised interest due to potential applications in bioremediation and agricultural purposes. In this work, we show that many species within the genus Acinetobacter possess the genetic requirements to assemble a functional type VI secretion system (T6SS). This secretion system is widespread among Gram negative bacteria, and can be used for toxicity against other bacteria and eukaryotic cells. The most studied species within this genus is A. baumannii, an emerging nosocomial pathogen that has become a significant threat to healthcare systems worldwide. The ability of A. baumannii to develop multidrug resistance has severely reduced treatment options, and strains resistant to most clinically useful antibiotics are frequently being isolated. Despite the widespread dissemination of A. baumannii, little is known about the virulence factors this bacterium utilizes to cause infection. We determined that the T6SS is conserved and syntenic among A. baumannii strains, although expression and secretion of the hallmark protein Hcp varies between strains, and is dependent on TssM, a known structural protein required for T6SS function. Unlike other bacteria, A. baumannii ATCC 17978 does not appear to use its T6SS to kill Escherichia coli or other Acinetobacter species. Deletion of tssM does not affect virulence in several infection models, including mice, and did not alter biofilm formation. These results suggest that the T6SS fulfils an important but as-yet-unidentified role in the various lifestyles of the Acinetobacter spp.