Structural studies of the capsular polysaccharide and lipopolysaccharide O-antigen of Aeromonas salmonicida strain 80204-1 produced under in vitro and in vivo growth conditions

Surface Glucan Structures in Aeromonas spp

Aeromonas spp. are generally found in aquatic environments, although they have also been isolated from both fresh and processed food. These Gram-negative, rod-shaped bacteria are mostly infective to poikilothermic animals, although they are also considered opportunistic pathogens of both aquatic and terrestrial homeotherms, and some species have been associated with gastrointestinal and extraintestinal septicemic infections in humans. Among the different pathogenic factors associated with virulence, several cell-surface glucans have been shown to contribute to colonization and survival of Aeromonas pathogenic strains, in different hosts. Lipopolysaccharide (LPS), capsule and α-glucan structures, for instance, have been shown to play important roles in bacterial–host interactions related to pathogenesis, such as adherence, biofilm formation, or immune evasion. In addition, glycosylation of both polar and lateral flagella has been shown to be mandatory for flagella production and motility in different Aeromonas strains, and has also been associated with increased bacterial adhesion, biofilm formation, and induction of the host proinflammatory response. The main aspects of these structures are covered in this review.

Structure of the capsule and lipopolysaccharide O-antigen from the channel catfish pathogen, Aeromonas hydrophila

A hypervirulent A. hydrophila (vAh) pathotype has been identified as the etiologic agent responsible for disease outbreaks in farmed carp species and channel catfish (Ictalurus punctatus) in China and the Southeastern United States, respectively. The possible route of infection has previously been unknown; however, virulence is believed to be multifactorial, involving the production/secretion of several virulence factors, including a high molecular weight group 4 capsular polysaccharide. Here we present chemical structural evidence of a novel capsule- and LPS-associated O-antigen found present in vAh isolated during these disease outbreaks. In this study, the chemical structure of the vAh O-antigen was determined by chemical analysis, Smith degradation, mass spectrometry, and 2D proton and carbon nuclear magnetic resonance (NMR) spectroscopy and found to be unique among described bacterial O-antigens. The O-antigen consists of hexasaccharide repeating units featuring a 4)-α-l-Fucp-(1-3)-β-d-GlcpNAc-(1-4)-α-l-Fucp-(1-4)-β-d-Glcp-(1- backbone, substituted with single residue side chains of α-d-Glcp and α-d-Quip3NAc linked to O-3 of the two fucose residues. The polysaccharide is partially O-acetylated on O-6 of the 4-substituted β-Glcp residue.

A cationic cysteine-hydrazide as an enrichment tool for the mass spectrometric characterization of bacterial free oligosaccharides

In Campylobacterales and related ε-proteobacteria with N-linked glycosylation (NLG) pathways, free oligosaccharides (fOS) are released into the periplasmic space from lipid-linked precursors by the bacterial oligosaccharyltransferase (PglB). This hydrolysis results in the same molecular structure as the oligosaccharide that is transferred to a protein to be glycosylated. This allowed for the general elucidation of the fOS-branched structures and monosaccharides from a number of species using standard enrichment and mass spectrometry methods. To aid characterization of fOS, hydrazide chemistry has often been used for chemical modification of the reducing part of oligosaccharides resulting in better selectivity and sensitivity in mass spectrometry; however, the removal of the unreacted reagents used for the modification often causes the loss of the sample. Here, we develop a more robust method for fOS purification and characterize glycostructures using complementary tandem mass spectrometry (MS/MS) analysis. A cationic cysteine hydrazide derivative was synthesized to selectively isolate fOS from periplasmic fractions of bacteria. The cysteine hydrazide nicotinamide (Cyhn) probe possesses both thiol and cationic moieties. The former enables reversible conjugation to a thiol-activated solid support, while the latter improves the ionization signal during MS analysis. This enrichment was validated on the well-studied Campylobacter jejuni by identifying fOS from the periplasmic extracts. Using complementary MS/MS analysis, we approximated data of a known structure of the fOS from Campylobacter concisus. This versatile enrichment technique allows for the exploration of a diversity of protein glycosylation pathways.

The main Aeromonas pathogenic factors

The members of the Aeromonas genus are ubiquitous, water-borne bacteria. They have been isolated from marine waters, rivers, lakes, swamps, sediments, chlorine water, water distribution systems, drinking water and residual waters; different types of food, such as meat, fish, seafood, vegetables, and processed foods. Aeromonas strains are predominantly pathogenic to poikilothermic animals, and the mesophilic strains are emerging as important pathogens in humans, causing a variety of extraintestinal and systemic infections as well as gastrointestinal infections. The most commonly described disease caused by Aeromonas is the gastroenteritis; however, no adequate animal model is available to reproduce this illness caused by Aeromonas. The main pathogenic factors associated with Aeromonas are: surface polysaccharides (capsule, lipopolysaccharide, and glucan), S-layers, iron-binding systems, exotoxins and extracellular enzymes, secretion systems, fimbriae and other nonfilamentous adhesins, motility and flagella.

Diversity in the protein N-glycosylation pathways within the Campylobacter genus

The foodborne bacterial pathogen, Campylobacter jejuni, possesses an N-linked protein glycosylation (pgl) pathway involved in adding conserved heptasaccharides to asparagine-containing motifs of >60 proteins, and releasing the same glycan into its periplasm as free oligosaccharides. In this study, comparative genomics of all 30 fully sequenced Campylobacter taxa revealed conserved pgl gene clusters in all but one species. Structural, phylogenetic and immunological studies showed that the N-glycosylation systems can be divided into two major groups. Group I includes all thermotolerant taxa, capable of growth at the higher body temperatures of birds, and produce the C. jejuni-like glycans. Within group I, the niche-adapted C. lari subgroup contain the smallest genomes among the epsilonproteobacteria, and are unable to glucosylate their pgl pathway glycans potentially reminiscent of the glucosyltransferase regression observed in the O-glycosylation system of Neisseria species. The nonthermotolerant Campylobacters, which inhabit a variety of hosts and niches, comprise group II and produce an unexpected diversity of N-glycan structures varying in length and composition. This includes the human gut commensal, C. hominis, which produces at least four different N-glycan structures, akin to the surface carbohydrate diversity observed in the well-studied commensal, Bacteroides. Both group I and II glycans are immunogenic and cell surface exposed, making these structures attractive targets for vaccine design and diagnostics.

The structural diversity of carbohydrate antigens of selected gram-negative marine bacteria

Marine microorganisms have evolved for millions of years to survive in the environments characterized by one or more extreme physical or chemical parameters, e.g., high pressure, low temperature or high salinity. Marine bacteria have the ability to produce a range of biologically active molecules, such as antibiotics, toxins and antitoxins, antitumor and antimicrobial agents, and as a result, they have been a topic of research interest for many years. Among these biologically active molecules, the carbohydrate antigens, lipopolysaccharides (LPSs, O-antigens) found in cell walls of gram-negative marine bacteria, show great potential as candidates in the development of drugs to prevent septic shock due to their low virulence. The structural diversity of LPSs is thought to be a reflection of the ability for these bacteria to adapt to an array of habitats, protecting the cell from being compromised by exposure to harsh environmental stress factors. Over the last few years, the variety of structures of core oligosaccharides and O-specific polysaccharides from LPSs of marine microrganisms has been discovered. In this review, we discuss the most recently encountered structures that have been identified from bacteria belonging to the genera Aeromonas, Alteromonas, Idiomarina, Microbulbifer, Pseudoalteromonas, Plesiomonas and Shewanella of the Gammaproteobacteria phylum; Sulfitobacter and Loktanella of the Alphaproteobactera phylum and to the genera Arenibacter, Cellulophaga, Chryseobacterium, Flavobacterium, Flexibacter of the Cytophaga-Flavobacterium-Bacteroides phylum. Particular attention is paid to the particular chemical features of the LPSs, such as the monosaccharide type, non-sugar substituents and phosphate groups, together with some of the typifying traits of LPSs obtained from marine bacteria. A possible correlation is then made between such features and the environmental adaptations undertaken by marine bacteria.

Structural characterization of the lipopolysaccharide O-antigen from atypical isolate of Vibrio anguillarum strain 1282

Vibrio anguillarum is a Gram-negative bacterium associated with vibriosis in Atlantic cod (Gadus morhua L.). Although farmed cod in Norway is routinely vaccinated against the infection, outbreaks of V. anguillarum-associated vibriosis still occur. Here, we describe the structural characterization of the LPS O-chain polysaccharide (O-PS) from atypical isolates of V. anguillarum strain 1282 and show that it is distinct from that previously established for V. anguillarum serotype O2. The structure of the purified O-PS was shown by 1D/2D NMR ((1)H, (13)C) spectroscopy and CE-MS studies to be a high-molecular mass linear polymer of tetrasaccharide repeating units, composed of 2-acetamido-3-(N-formyl-L-alanyl)amido-2,3-dideoxy-D-glucuronamide [GlcNAc3N(Fo-L-Ala)AN], 2-acetamido-3-acetamidino-2,3-dideoxy-D-mannuronic acid (ManNAc3NAmA), 3-acetamido-3-dideoxy-D-quinovose (Qui3NAc), and 2,4-diacetamido-2,4-dideoxy-D-fucose (FucNAc4NAc). [carbohydrate structure: see text]. NMR analysis of the partial hydrolysis-derived oligosaccharides confirmed the presence of an O-acetyl group at position O-4 of GlcNAc3N(Fo-L-Ala)AN and established that the above-mentioned structure represents the biological repeating unit of the O-PS. In addition, it was demonstrated that some of 2,3-diamino-2,3-dideoxy-glucuronamide in the O-PS was present in the form of 2,3-diamino-2,3-dideoxy-glucose.

Application of capillary electrophoresis mass spectrometry to the characterization of bacterial lipopolysaccharides

Capillary electrophoresis (CE) is a high-resolution technique for the separation of complex biological mixtures and has been widely applied to biological analyses. The coupling of capillary electrophoresis with mass spectrometry (MS) provides a powerful approach for rapid identification of target analytes present at trace levels in biological matrices, and for structural characterization of complex biomolecules. Here we review the analytical potential of combined capillary electrophoresis electrospray mass spectrometry (CE-MS) for the analysis of bacterial lipopolysaccharides (LPS). This hyphened methodology facilitates the determination of closely related LPS glycoform and isoform families by exploiting differences in their unique molecular conformations and ionic charge distributions by electrophoretic separation. On-line CE-MS also provides an additional avenue to improve detection limits, which has been successfully applied to directly probe oligosaccharide LPS glycoform populations of bacteria isolated from infected animal models without the need for further passage.

Structural characterization of the lipid A region of Aeromonas salmonicida subsp. salmonicida lipopolysaccharide

The lipid A components of Aeromonas salmonicida subsp. salmonicida from strains A449, 80204-1 and an in vivo rough isolate were isolated by mild acid hydrolysis of the lipopolysaccharide. Structural studies carried out by a combination of fatty acid, electrospray ionization-mass spectrometry and nuclear magnetic resonance analyses confirmed that the structure of lipid A was conserved among different isolates of A. salmonicida subsp. salmonicida. All analyzed strains contained three major lipid A molecules differing in acylation patterns corresponding to tetra-, penta- and hexaacylated lipid A species and comprising 4'-monophosphorylated beta-2-amino-2-deoxy-d-glucopyranose-(1-->6)-2-amino-2-deoxy-d-glucopyranose disaccharide, where the reducing end 2-amino-2-deoxy-d-glucose was present primarily in the alpha-pyranose form. Electrospray ionization-tandem mass spectrometry fragment pattern analysis, including investigation of the inner-ring fragmentation, allowed the localization of fatty acyl residues on the disaccharide backbone of lipid A. The tetraacylated lipid A structure containing 3-(dodecanoyloxy)tetradecanoic acid at N-2',3-hydroxytetradecanoic acid at N-2 and 3-hydroxytetradecanoic acid at O-3, respectively, was found. The pentaacyl lipid A molecule had a similar fatty acid distribution pattern and, additionally, carried 3-hydroxytetradecanoic acid at O-3'. In the hexaacylated lipid A structure, 3-hydroxytetradecanoic acid at O-3' was esterified with a secondary 9-hexadecenoic acid. Interestingly, lipid A of the in vivo rough isolate contained predominantly tetra- and pentaacylated lipid A species suggesting that the presence of the hexaacyl lipid A was associated with the smooth-form lipopolysaccharide.

Structure of a capsular polysaccharide isolated from Salmonella enteritidis

Salmonella enteritidis is a food-borne enteric human pathogen that can form a complex protective extracellular matrix. We describe here a component of this matrix which is distinct from other known salmonella extracellular polysaccharides such as cellulose and colanic acid. We have used glycosyl composition and linkage analysis, as well as 1D and 2D NMR spectroscopy to determine the structure of this polysaccharide. We propose that the primary saccharide in the S. enteritidis capsule has a branched tetrasaccharide repeating unit having the following structure: -->3)-alpha-D-Galp-(1-->2)-[alpha-Tyvp-(1-->3)]-alpha-D-Manp-(1-->4)-alpha-L-Rhap-(1-->. This structure is partially substituted on both tyvelose and galactose with a glucose-containing side chain. It further bears considerable similarity to the O antigen from this organism, a feature found in a number of other capsules from Gram-negative bacteria. In addition, we have detected fatty acids at levels that indicate the presence of a lipid anchor.

Structural and serological characterization of the O-chain polysaccharide of Aeromonas salmonicida strains A449, 80204 and 80204-1

The O-chain polysaccharide (O-PS) of Aeromonas salmonicida was studied by a combination of compositional, methylation, CE-ESMS and one- and two-dimensional NMR analyses. It was found to be a branched polymer of trisaccharide-repeating units composed of L-rhamnose (Rha), D-glucose (Glc), 2-acetamido-2-deoxy-D-mannose (ManNAc) and O-acetyl group (OAc) and having the following structure: CE-ESMS analysis of A. salmonicida cells from strains A449, 80204 and 80204-1 grown under different conditions confirmed that the O-PS structure was conserved. ELISA-based serological study with native LPS-specific antisera performed on the native O-PS and its O-deacetylated and periodate-oxidized derivatives confirmed the importance of the O-PS backbone structure as an immunodominant determinant.

In-source fragmentation and analysis of polysaccharides by capillary electrophoresis/mass spectrometry

In order to develop a robust and easy-to-use technique for characterization of bacterial polysaccharides, a pseudo-hydrolysis strategy was investigated. Based on in-source collision-induced dissociation, polysaccharide molecular ions were fragmented within the orifice-skimmer region of an electrospray ionization (ESI) mass spectrometer. The fragment ions thus generated were then analyzed similarly to the conventional ESI mass spectrometry approach. MS/MS scanning was applied to obtain product-ion spectra of the primary fragments for sequencing. To further improve the sensitivity and separation of polysaccharides from other components in the samples, a pressure-assisted capillary electrophoresis/mass spectrometry (CE/MS) system was employed. Using bacterial polysaccharides as model compounds, the mass spectra obtained for polysaccharide repeating units generated through chemical hydrolysis and in-source fragmentation were directly compared, both in positive and negative ion modes. With the additional separation of impurities provided by CE, the success of this technique has been demonstrated for structural analysis of O-chain polysaccharides (O-PS) and capsular polysaccharides (CPS). In-source fragmentation was applied to promote the formation of structurally relevant repeating units of heterogeneous CPS that would remain undetected using conventional ESI conditions. This approach was proven to be particularly useful for probing the subtle structural differences in monosaccharide composition and functionalities arising across bacterial serotypes.