ECA-immunogenicity of Proteus mirabilis strains

A New Look at the Enterobacterial Common Antigen Forms Obtained during Rough Lipopolysaccharides Purification

Enterobacterial common antigen (ECA) is a conserved antigen expressed by enterobacteria. It is built by trisaccharide repeating units: →3)-α-D-Fucp4NAc-(1→4)-β-D-ManpNAcA-(1→4)-α-D-GlcpNAc-(1→ and occurs in three forms: as surface-bound linear polysaccharides linked to a phosphoglyceride (ECA_PG) or lipopolysaccharide - endotoxin (ECA_LPS), and cyclic form (ECA_CYC). ECA maintains, outer membrane integrity, immunogenicity, and viability of enterobacteria. A supernatant obtained after LPS ultracentrifugation was reported as a source for ECA isolation, but it has never been assessed for detailed composition besides ECA_CYC. We used mild acid hydrolysis and gel filtration, or zwitterionic-hydrophilic interaction liquid (ZIC^®HILIC) chromatography combined with mass spectrometry for purification, fractionation, and structural analysis of rough Shigella sonnei and Escherichia coli R1 and K12 crude LPS preparations. Presented work is the first report concerning complex characteristic of all ECA forms present in LPS-derived supernatants. We demonstrated high heterogeneity of the supernatant-derived ECA that contaminate LPS purified by ultracentrifugation. Not only previously reported O-acetylated tetrameric, pentameric, and hexameric ECA_CYC have been identified, but also devoid of lipid moiety linear ECA built from 7 to 11 repeating units. Described results were common for all selected strains. The origin of linear ECA is discussed against the current knowledge about ECA_PG and ECA_LPS.

Lipopolysaccharide-Linked Enterobacterial Common Antigen (ECALPS) Occurs in Rough Strains of Escherichia coli R1, R2, and R4

Enterobacterial common antigen (ECA) is a conserved surface antigen characteristic for Enterobacteriaceae. It is consisting of trisaccharide repeating unit, →3)-α-d-Fucp4NAc-(1→4)-β-d-ManpNAcA-(1→4)-α-d-GlcpNAc-(1→, where prevailing forms include ECA linked to phosphatidylglycerol (ECA_PG) and cyclic ECA (ECA_CYC). Lipopolysaccharide (LPS)-associated form (ECA_LPS) has been proved to date only for rough Shigella sonnei phase II. Depending on the structure organization, ECA constitutes surface antigen (ECA_PG and ECA_LPS) or maintains the outer membrane permeability barrier (ECA_CYC). The existence of LPS was hypothesized in the 1960–80s on the basis of serological observations. Only a few Escherichia coli strains (i.e., R1, R2, R3, R4, and K-12) have led to the generation of anti-ECA antibodies upon immunization, excluding ECA_PG as an immunogen and conjecturing ECA_LPS as the only immunogenic form. Here, we presented a structural survey of ECA_LPS in E. coli R1, R2, R3, and R4 to correlate previous serological observations with the presence of ECA_LPS. The low yields of ECA_LPS were identified in the R1, R2, and R4 strains, where ECA occupied outer core residues of LPS that used to be substituted by O-specific polysaccharide in the case of smooth LPS. Previously published observations and hypotheses regarding the immunogenicity and biosynthesis of ECA_LPS were discussed and correlated with presented herein structural data.

Enterobacterial Common Antigen: Synthesis and Function of an Enigmatic Molecule

The outer membrane (OM) of Gram-negative bacteria poses a barrier to antibiotic entry due to its high impermeability. Thus, there is an urgent need to study the function and biogenesis of the OM. In Enterobacterales, an order of bacteria with many pathogenic members, one of the components of the OM is enterobacterial common antigen (ECA). We have known of the presence of ECA on the cell surface of Enterobacterales for many years, but its properties have only more recently begun to be unraveled. ECA is a carbohydrate antigen built of repeating units of three amino sugars, the structure of which is conserved throughout Enterobacterales. There are three forms of ECA, two of which (ECAPG and ECALPS) are located on the cell surface, while one (ECACYC) is located in the periplasm. Awareness of the importance of ECA has increased due to studies of its function that show it plays a vital role in bacterial physiology and interaction with the environment. Here, we review the discovery of ECA, the pathways for the biosynthesis of ECA, and the interactions of its various forms. In addition, we consider the role of ECA in the host immune response, as well as its potential roles in host-pathogen interaction. Furthermore, we explore recent work that offers insights into the cellular function of ECA. This review provides a glimpse of the biological significance of this enigmatic molecule.

Swarmer Cell Development of the Bacterium Proteus mirabilis Requires the Conserved Enterobacterial Common Antigen Biosynthesis Gene rffG

Individual cells of the bacterium Proteus mirabilis can elongate up to 40-fold on surfaces before engaging in a cooperative surface-based motility termed swarming. How cells regulate this dramatic morphological remodeling remains an open question. In this paper, we move forward the understanding of this regulation by demonstrating that P. mirabilis requires the gene rffG for swarmer cell elongation and subsequent swarm motility. The rffG gene encodes a protein homologous to the dTDP-glucose 4,6-dehydratase protein of Escherichia coli, which contributes to enterobacterial common antigen biosynthesis. Here, we characterize the rffG gene in P. mirabilis, demonstrating that it is required for the production of large lipopolysaccharide-linked moieties necessary for wild-type cell envelope integrity. We show that the absence of the rffG gene induces several stress response pathways, including those controlled by the transcriptional regulators RpoS, CaiF, and RcsB. We further show that in rffG-deficient cells, the suppression of the Rcs phosphorelay, via loss of RcsB, is sufficient to induce cell elongation and swarm motility. However, the loss of RcsB does not rescue cell envelope integrity defects and instead results in abnormally shaped cells, including cells producing more than two poles. We conclude that an RcsB-mediated response acts to suppress the emergence of shape defects in cell envelope-compromised cells, suggesting an additional role for RcsB in maintaining cell morphology under stress conditions. We further propose that the composition of the cell envelope acts as a checkpoint before cells initiate swarmer cell elongation and motility.IMPORTANCEProteus mirabilis swarm motility has been implicated in pathogenesis. We have found that cells deploy multiple uncharacterized strategies to handle cell envelope stress beyond the Rcs phosphorelay when attempting to engage in swarm motility. While RcsB is known to directly inhibit the master transcriptional regulator for swarming, we have shown an additional role for RcsB in protecting cell morphology. These data support a growing appreciation that the Rcs phosphorelay is a multifunctional regulator of cell morphology in addition to its role in microbial stress responses. These data also strengthen the paradigm that outer membrane composition is a crucial checkpoint for modulating entry into swarm motility. Furthermore, the rffG-dependent moieties provide a novel attractive target for potential antimicrobials.

First evidence for a covalent linkage between enterobacterial common antigen and lipopolysaccharide in Shigella sonnei phase II ECALPS

Enterobacterial common antigen (ECA) is expressed by Gram-negative bacteria belonging to Enterobacteriaceae, including emerging drug-resistant pathogens such as Escherichia coli, Klebsiella pneumoniae, and Proteus spp. Recent studies have indicated the importance of ECA for cell envelope integrity, flagellum expression, and resistance of enteric bacteria to acetic acid and bile salts. ECA, a heteropolysaccharide built from the trisaccharide repeating unit, →3)-α-D-Fucp4NAc-(1→4)-β-D-ManpNAcA-(1→4)-α-D-GlcpNAc-(1→, occurs as a cyclic form (ECA(CYC)), a phosphatidylglycerol (PG)-linked form (ECA(PG)), and an endotoxin/lipopolysaccharide (LPS)-associated form (ECA(LPS)). Since the discovery of ECA in 1962, the structures of ECA(PG) and ECA(CYC) have been completely elucidated. However, no direct evidence has been presented to support a covalent linkage between ECA and LPS; only serological indications of co-association have been reported. This is paradoxical, given that ECA was first identified based on the capacity of immunogenic ECA(LPS) to elicit antibodies cross-reactive with enterobacteria. Using a simple isolation protocol supported by serological tracking of ECA epitopes and NMR spectroscopy and mass spectrometry, we have succeeded in the first detection, isolation, and complete structural analysis of poly- and oligosaccharides of Shigella sonnei phase II ECA(LPS). ECA(LPS) consists of the core oligosaccharide substituted with one to four repeating units of ECA at the position occupied by the O-antigen in the case of smooth S. sonnei phase I. These data represent the first structural evidence for the existence of ECA(LPS) in the half-century since it was first discovered and provide insights that could prove helpful in further structural analyses and screening of ECA(LPS) among Enterobacteriaceae species.

Bacterial cell surface structures in Yersinia enterocolitica

Yersinia enterocolitica is a widespread member of the family of Enterobacteriaceae that contains both non-virulent and virulent isolates. Pathogenic Y. enterocolitica strains, especially belonging to serotypes O:3, O:5,27, O:8 and O:9 are etiologic agents of yersiniosis in animals and humans. Y. enterocolitica cell surface structures that play a significant role in virulence have been subject to many investigations. These include outer membrane (OM) glycolipids such as lipopolysaccharide (LPS) and enterobacterial common antigen (ECA) and several cell surface adhesion proteins present only in virulent Y. enterocolitica, i.e., Inv, YadA and Ail. While the yadA gene is located on the Yersinia virulence plasmid the Ail, Inv, LPS and ECA are chromosomally encoded. These structures ensure the correct architecture of the OM, provide adhesive properties as well as resistance to antimicrobial peptides and to host innate immune response mechanisms.

Immunization with an anti-idiotypic antibody against the broadly lipopolysaccharide-reactive antibody WN1 222-5 induces Escherichia coli R3-core-type specific antibodies in rabbits

The mouse monoclonal antibody (mAb) WN1 222-5 recognizes a carbohydrate epitope in the inner core region of LPS that is shared by all strains of Escherichia coli and Salmonella enterica and is able to neutralize their endotoxic activity in vitro and in vivo. Immunization of mice with mAb WN1 222-5 yielded several anti-idiotypic mAbs one of which (mAb S81-19) competitively inhibited binding of mAb WN1 222-5 to E. coli and Salmonella LPS. After immunization of rabbits with mAb S81-19, the serological responses towards LPS were characterized at intervals over two years. Whereas the serological response against the anti-idiotype developed as expected, the anti-anti-idiotypic response against LPS developed slowly and antibodies appeared after 200 d that bound to E. coli LPS of the R3 core-type and neutralized its TNF-α inducing capacity for human peripheral mononuclear cells. We describe the generation of a novel anti-idiotypic antibody that can induce LPS core-reactive antibodies upon immunization in rabbits and show that it is possible, in principle, to obtain LPS neutralizing antibodies by anti-idiotypic immunization against the mAb WN1 222-5. The mimicked epitope likely shares common determinants with the WN1 222-5 epitope, yet differences with respect to either affinity or specificity do exist, as binding to smaller oligosaccharides of the inner core was not observed.