Structural elucidation of the O-antigenic polysaccharides from Escherichia coli O21 and the enteroaggregative Escherichia coli strain 105

Glycoengineering directs de novo biomanufacturing of UPEC O21 O-antigen polysaccharide based glycoprotein

Glycoproteins, in which polysaccharides are usually attached to proteins, are an important class of biomolecules that are widely used as therapeutic agents in clinical treatments for decades. Uropathogenic Escherichia coli (UPEC) O21 has been identified as a serogroup that induces urinary tract infections, with a global increasing number among women and young children. Therefore, there is an urgent need to establish protective vaccines against UPEC infection. Herein, we engineered non-pathogenic E. coli MG1655 to achieve robust, cost-effective de novo biosynthesis of O21 O-antigen polysaccharide-based glycoprotein against UPEC O21. Specifically, this glycoengineered E. coli MG1655 was manipulated for high-efficient glucose-glycerol co-utilization and for the gene cluster installation and O-glycosylation machinery assembly. The key pathways of UDP-sugar precursors were also strengthened to enforce more carbon flux towards the glycosyl donors, which enhanced the glycoprotein titer by 5.6-fold. Further optimization of culture conditions yielded glycoproteins of up to 35.34 mg/L. Glycopeptide MS confirmed the preciset biosynthesis of glycoprotein. This glycoprotein elicited antigen-specific IgG immune responses and significantly reduced kidney and bladder colonization. This bacterial cell-based glyco-platform and optimized strategies can provide a guideline for the biosynthesis of other value-added glycoproteins.

Structure and genetics of Escherichia coli O antigens

Escherichia coli includes clonal groups of both commensal and pathogenic strains, with some of the latter causing serious infectious diseases. O antigen variation is current standard in defining strains for taxonomy and epidemiology, providing the basis for many serotyping schemes for Gram-negative bacteria. This review covers the diversity in E. coli O antigen structures and gene clusters, and the genetic basis for the structural diversity. Of the 187 formally defined O antigens, six (O31, O47, O67, O72, O94 and O122) have since been removed and three (O34, O89 and O144) strains do not produce any O antigen. Therefore, structures are presented for 176 of the 181 E. coli O antigens, some of which include subgroups. Most (93%) of these O antigens are synthesized via the Wzx/Wzy pathway, 11 via the ABC transporter pathway, with O20, O57 and O60 still uncharacterized due to failure to find their O antigen gene clusters. Biosynthetic pathways are given for 38 of the 49 sugars found in E. coli O antigens, and several pairs or groups of the E. coli antigens that have related structures show close relationships of the O antigen gene clusters within clades, thereby highlighting the genetic basis of the evolution of diversity.

Structural diversity in Salmonella O antigens and its genetic basis

This review covers the structures and genetics of the 46 O antigens of Salmonella, a major pathogen of humans and domestic animals. The variation in structures underpins the serological specificity of the 46 recognized serogroups. The O antigen is important for the full function and virulence of many bacteria, and the considerable diversity of O antigens can confer selective advantage. Salmonella O antigens can be divided into two major groups: those which have N-acetylglucosamine (GlcNAc) or N-acetylgalactosamine (GalNAc) and those which have galactose (Gal) as the first sugar in the O unit. In recent years, we have determined 21 chemical structures and sequenced 28 gene clusters for GlcNAc-/GalNAc-initiated O antigens, thus completing the structure and DNA sequence data for the 46 Salmonella O antigens. The structures and gene clusters of the GlcNAc-/GalNAc-initiated O antigens were found to be highly diverse, and 24 of them were found to be identical or closely related to Escherichia coli O antigens. Sequence comparisons indicate that all or most of the shared gene clusters were probably present in the common ancestor, although alternative explanations are also possible. In contrast, the better-known eight Gal-initiated O antigens are closely related both in structures and gene cluster sequences.

Structural studies and biosynthetic aspects of the O-antigen polysaccharide from Escherichia coli O174

The structure of the repeating unit of the O-antigenic polysaccharide (PS) from Escherichia coli O174 has been determined. Component analysis together with (1)H and (13)C NMR spectroscopy experiments were employed to elucidate the structure. Inter-residue correlations were determined by (1)H,(13)C-heteronuclear multiple-bond correlation and (1)H,(1)H-NOESY experiments. The PS is composed of tetrasaccharide repeating units with the following structure: [formula see text] Cross-peaks of low intensity were present in the NMR spectra consistent with a β-D-GlcpNAc-(1→2)-β-D-GlcpA(1→ structural element at the terminal part of the polysaccharide, which on average is composed of ∼15 repeating units. Consequently the biological repeating unit has a 3-substituted N-acetyl-D-galactosamine residue at its reducing end.

Structural studies of the O-antigenic polysaccharides from the enteroaggregative Escherichia coli strain 94/D4 and the international type strain Escherichia coli O82

The structure of the O-antigen polysaccharides (PS) from the enteroaggregative Escherichia coli strain 94/D4 and the international type strain E. coli O82 have been determined. Component analysis and (1)H, (13)C, and (31)P NMR spectroscopy experiments were employed to elucidate the structure. Inter-residue correlations were determined by (1)H, (13)C-heteronuclear multiple-bond correlation, and (1)H, (1)H-NOESY experiments. D-GroA as a substituent is linked via its O-2 in a phosphodiester-linkage to O-6 of the alpha-D-Glcp residue. The PS is composed of tetrasaccharide repeating units with the following structure: -->4)-alpha-D-Glcp6-(P-2-D-GroA)-(1-->4)-beta-D-Galp-(1-->4)-beta-D-Glcp-(1-->3)-beta-D-GlcpNAc-(1--> Cross-peaks of low intensity from an alpha-D-Glcp residue were present in the NMR spectra and spectral analysis indicates that they originate from the terminal residue of the polysaccharide. Consequently, the biological repeating unit has a 3-substituted N-acetyl-D-glucosamine residue at its reducing end. Enzyme immunoassay using specific anti-E. coli O82 rabbit sera showed identical reactivity to the LPS of the two strains, in agreement with the structural analysis of their O-antigen polysaccharides.

Characterization of Escherichia coli O3 and O21 O antigen gene clusters and development of serogroup-specific PCR assays

Escherichia coli O3 and O21 are associated with enteroaggregative E. coli (EAEC). EAEC strains are often non-typable using the routine agglutination method due to their aggregative phenotype. Typing of E. coli O3 and O21 may also be impeded by cross-reactions with O152 or O83. In this study, the O antigen gene clusters of E. coli O3 and O21 were characterized, and PCR assays based on O antigen specific genes wzx (encoding O unit flippase) and wzy (encoding O unit polymerase) from each strain were developed. By screening against all 186 known E. coli O serotypes, the PCR assays were shown to be highly specific to O3 and O21 respectively. The sensitivity of the assays was determined to be 1 pg per microl of chromosomal DNA and 2 CFU per 10 g of water samples. The PCR assays were also applied to 658 clinical E. coli isolates, and 100% of detection accuracy was obtained. The PCR assays developed here are suitable for the detection and identification of E. coli O3 and O21 strains in environmental and clinical samples.

Structural studies of the O-antigenic polysaccharides from the enteroaggregative Escherichia coli strain 87/D2 and international type strains from E. coli O128

The O-antigen of the lipopolysaccharide (LPS) from the enteroaggregative Escherichia coli strain 87/D2 has been determined by component analysis together with NMR spectroscopy. The polysaccharide has pentasaccharide repeating units in which all the residues have the galacto-configuration. The repeating unit of the O-antigen, elucidated using the O-deacylated LPS, is branched with the following structure: Analysis of the 1H NMR spectrum of the LPS revealed O-acetyl groups (approximately 0.7 per repeating unit) distributed over two positions. Subsequent analysis showed that the galactose residue carries acetyl groups at either O-3 or O-4 in a ratio of approximately 2:1. The international reference strain from E. coli O128ab was investigated and the repeating unit of the O-antigens has the following structure: Analysis of the 1H NMR spectrum of the LPS revealed O-acetyl groups (approximately one per repeating unit) distributed over two positions. The integrals of the resonances for the O-acetyl groups indicated similarities between the O-antigen from E. coli O128ab and that of E. coli strain 87/D2, whereas the O-acetyl substitution pattern in the E. coli O128ac O-antigen differed slightly. Enzyme immunoassay using specific anti-E. coli O128ab and anti-E. coli O128ac rabbit sera confirmed the results.

The structures ofEscherichia coliO-polysaccharide antigens

Escherichia coli is usually a non-pathogenic member of the human colonic flora. However, certain strains have acquired virulence factors and may cause a variety of infections in humans and in animals. There are three clinical syndromes caused by E. coli: (i) sepsis/meningitis; (ii) urinary tract infection and (iii) diarrhoea. Furthermore the E. coli causing diarrhoea is divided into different 'pathotypes' depending on the type of disease, i.e. (i) enterotoxigenic; (ii) enteropathogenic; (iii) enteroinvasive; (iv) enterohaemorrhagic; (v) enteroaggregative and (vi) diffusely adherent. The serotyping of E. coli based on the somatic (O), flagellar (H) and capsular polysaccharide antigens (K) is used in epidemiology. The different antigens may be unique for a particular serogroup or antigenic determinants may be shared, resulting in cross-reactions with other serogroups of E. coli or even with other members of the family Enterobacteriacea. To establish the uniqueness of a particular serogroup or to identify the presence of common epitopes, a database of the structures of O-antigenic polysaccharides has been created. The E. coli database (ECODAB) contains structures, nuclear magnetic resonance chemical shifts and to some extent cross-reactivity relationships. All fields are searchable. A ranking is produced based on similarity, which facilitates rapid identification of strains that are difficult to serotype (if known) based on classical agglutinating methods. In addition, results pertinent to the biosynthesis of the repeating units of O-antigens are discussed. The ECODAB is accessible to the scientific community at http://www.casper.organ.su.se/ECODAB/.