Structure of the lipopolysaccharide core of Vibrio vulnificus type strain 27562

Discovery of a Monoclonal Antibody That Targets Cell-Surface Pseudaminic Acid of Acinetobacter baumannii with Direct Bactericidal Effect

The therapeutic effects of antibodies include neutralization of pathogens, activation of the host complement system, and facilitation of phagocytosis of pathogens. However, antibody alone has never been shown to exhibit bactericidal activity. In this study, we developed a monoclonal antibody that targets the bacterial cell surface component Pseudaminic acid (Pse). This monoclonal antibody, Pse-MAB1, exhibited direct bactericidal activity on Acinetobacter baumannii strains, even in the absence of the host complements or other immune factors, and was able to confer a protective effect against A. baumannii infections in mice. This study provides new insight into the potential of developing monoclonal antibody-based antimicrobial therapy of multidrug resistant bacterial infections, especially those which occurred among immunocompromised patients.

Synthetic Pseudaminic-Acid-Based Antibacterial Vaccine Confers Effective Protection against Acinetobacter baumannii Infection

Acinetobacter baumannii exhibits resistance to most first-line antibiotics; thus, development of new antibacterial agents is urgently required. Pseudaminic acid exists as the surface glycan of A. baumannii. In this study, we chemically synthesized pseudaminic acid, conjugated it to carrier protein CRM197 using the OPA (ortho-phthalaldehyde) chemistry, and obtained three Pse-CRM197 conjugates with different Pse loadings. These Pse-CRM197 conjugates were found to stimulate high immune responses in mice, which protected the vaccinated mice from infections caused by Pse-producing A. baumannii. Our data indicate that chemically synthesized Pse-CRM197 conjugates can be developed into vaccines against Pse-bearing pathogens, thus offering a feasible alternative for the control of clinical infections caused by multidrug-resistant (MDR) A. baumannii, for which current treatment options are extremely limited.

Structural and Biosynthetic Diversity of Nonulosonic Acids (NulOs) That Decorate Surface Structures in Bacteria

Nonulosonic acids (NulOs) are a diverse family of 9-carbon α-keto acid sugars that are involved in a wide range of functions across all branches of life. The family of NulOs includes the sialic acids as well as the prokaryote-specific NulOs. Select bacteria biosynthesize the sialic acid N-acetylneuraminic acid (Neu5Ac), and the ability to produce this sugar and its subsequent incorporation into cell-surface structures is implicated in a variety of bacteria-host interactions. Furthermore, scavenging of sialic acid from the environment for energy has been characterized across a diverse group of bacteria, mainly human commensals and pathogens. In addition to sialic acid, bacteria have the ability to biosynthesize prokaryote-specific NulOs, of which there are several known isomers characterized. These prokaryotic NulOs are similar in structure to Neu5Ac but little is known regarding their role in bacterial physiology. Here, we discuss the diversity in structure, the biosynthesis pathways, and the functions of bacteria-specific NulOs. These carbohydrates are phylogenetically widespread among bacteria, with numerous structurally unique modifications recognized. Despite the diversity in structure, the NulOs are involved in similar functions such as motility, biofilm formation, host colonization, and immune evasion.

Structural and functional characterization of a modified legionaminic acid involved in glycosylation of a bacterial lipopolysaccharide

Nonulosonic acids (NulOs) are a diverse family of α-keto acid carbohydrates present across all branches of life. Bacteria biosynthesize NulOs among which are several related prokaryotic-specific isomers and one of which, N-acetylneuraminic acid (sialic acid), is common among all vertebrates. Bacteria display various NulO carbohydrates on lipopolysaccharide (LPS), and the identities of these molecules tune host-pathogen recognition mechanisms. The opportunistic bacterial pathogen Vibrio vulnificus possesses the genes for NulO biosynthesis; however, the structures and functions of the V. vulnificus NulO glycan are unknown. Using genetic and chemical approaches, we show here that the major NulO produced by a clinical V. vulnificus strain CMCP6 is 5-N-acetyl-7-N-acetyl-d-alanyl-legionaminic acid (Leg5Ac7AcAla). The CMCP6 strain could catabolize modified legionaminic acid, whereas V. vulnificus strain YJ016 produced but did not catabolize a NulO without the N-acetyl-d-alanyl modification. In silico analysis suggested that Leg5Ac7AcAla biosynthesis follows a noncanonical pathway but appears to be present in several bacterial species. Leg5Ac7AcAla contributed to bacterial outer-membrane integrity, as mutant strains unable to produce or incorporate Leg5Ac7AcAla into the LPS have increased membrane permeability, sensitivity to bile salts and antimicrobial peptides, and defects in biofilm formation. Using the crustacean model, Artemia franciscana, we demonstrate that Leg5Ac7AcAla-deficient bacteria have decreased virulence potential compared with WT. Our data indicate that different V. vulnificus strains produce multiple NulOs and that the modified legionaminic acid Leg5Ac7AcAla plays a critical role in the physiology, survivability, and pathogenicity of V. vulnificus CMCP6.

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Metabolic Labeling of Pseudaminic Acid-Containing Glycans on Bacterial Surfaces

The rise in antibiotic-resistant bacteria is causing worldwide concerns. The urgent need for new antibacterial drugs calls for new thinking and strategies to explore novel, narrow-spectrum, and pathogen-specific antibacterial targets. Legionaminic acid (Leg) and pseudaminic acid (Pse) are nonulosonic acid carbohydrates with structural similarity to eukaryotic sialic acid, and are distributed in numerous pathogenic Gram-negative bacteria as components of cell surface-associated glycans. They are involved in the host interaction, pathogenicity, antiphage defense mechanism, and immune escape mechanism. To further explore their biological significance, we developed a synthesis of 2-acetamido-4-azidoacetamido-2,4,6-trideoxy-l-altrose (Alt-4NAz) and 2-azidoacetamido-4-acetamido-2,4,6-trideoxy-l-altrose (Alt-2NAz), among which Alt-4NAz served as an effective chemical reporter to realize bacterial Pse metabolic labeling. The effectiveness of this chemical reporter has been demonstrated in Pseudomonas aeruginosa, Vibrio vulnificus, and Acinetobacter baumannii strains. Expectedly, this strategy can provide a useful assay to detect phenotypic presence of Pse biosynthesis and screen for agents targeting this pathway.

Tannerella forsythia strains display different cell-surface nonulosonic acids: biosynthetic pathway characterization and first insight into biological implications

Tannerella forsythia is an anaerobic, Gram-negative periodontal pathogen. A unique O-linked oligosaccharide decorates the bacterium's cell surface proteins and was shown to modulate the host immune response. In our study, we investigated the biosynthesis of the nonulosonic acid (NulO) present at the terminal position of this glycan. A bioinformatic analysis of T. forsythia genomes revealed a gene locus for the synthesis of pseudaminic acid (Pse) in the type strain ATCC 43037 while strains FDC 92A2 and UB4 possess a locus for the synthesis of legionaminic acid (Leg) instead. In contrast to the NulO in ATCC 43037, which has been previously identified as a Pse derivative (5-N-acetimidoyl-7-N-glyceroyl-3,5,7,9-tetradeoxy-l-glycero-l-manno-NulO), glycan analysis of strain UB4 performed in this study indicated a 350-Da, possibly N-glycolyl Leg (3,5,7,9-tetradeoxy-d-glycero-d-galacto-NulO) derivative with unknown C5,7 N-acyl moieties. We have expressed, purified and characterized enzymes of both NulO pathways to confirm these genes’ functions. Using capillary electrophoresis (CE), CE–mass spectrometry and NMR spectroscopy, our studies revealed that Pse biosynthesis in ATCC 43037 essentially follows the UDP-sugar route described in Helicobacter pylori, while the pathway in strain FDC 92A2 corresponds to Leg biosynthesis in Campylobacter jejuni involving GDP-sugar intermediates. To demonstrate that the NulO biosynthesis enzymes are functional in vivo, we created knockout mutants resulting in glycans lacking the respective NulO. Compared to the wild-type strains, the mutants exhibited significantly reduced biofilm formation on mucin-coated surfaces, suggestive of their involvement in host-pathogen interactions or host survival. This study contributes to understanding possible biological roles of bacterial NulOs.

Novel coordination of lipopolysaccharide modifications in Vibrio cholerae promotes CAMP resistance

In the environment and during infection, the human intestinal pathogen Vibrio cholerae must overcome noxious compounds that damage the bacterial outer membrane. The El Tor and classical biotypes of O1 V. cholerae show striking differences in their resistance to membrane disrupting cationic antimicrobial peptides (CAMPs), such as polymyxins. The classical biotype is susceptible to CAMPs, but current pandemic El Tor biotype isolates gain CAMP resistance by altering the net charge of their cell surface through glycine modification of lipid A. Here we report a second lipid A modification mechanism that only functions in the V. cholerae El Tor biotype. We identify a functional EptA ortholog responsible for the transfer of the amino-residue phosphoethanolamine (pEtN) to the lipid A of V. cholerae El Tor that is not functional in the classical biotype. We previously reported that mildly acidic growth conditions (pH 5.8) downregulate expression of genes encoding the glycine modification machinery. In this report, growth at pH 5.8 increases expression of eptA with concomitant pEtN modification suggesting coordinated regulation of these LPS modification systems. Similarly, efficient pEtN lipid A substitution is seen in the absence of lipid A glycinylation. We further demonstrate EptA orthologs from non-cholerae Vibrio species are functional.

Host-like carbohydrates promote bloodstream survival of Vibrio vulnificus in vivo

Sialic acids are found on all vertebrate cell surfaces and are part of a larger class of molecules known as nonulosonic acids. Many bacterial pathogens synthesize related nine-carbon backbone sugars; however, the role(s) of these non-sialic acid molecules in host-pathogen interactions is poorly understood. Vibrio vulnificus is the leading cause of seafood-related death in the United States due to its ability to quickly access the host bloodstream, which it can accomplish through gastrointestinal or wound infection. However, little is known about how this organism persists systemically. Here we demonstrate that sialic acid-like molecules are present on the lipopolysaccharide of V. vulnificus, are required for full motility and biofilm formation, and also contribute to the organism's natural resistance to polymyxin B. Further experiments in a murine model of intravenous V. vulnificus infection demonstrated that expression of nonulosonic acids had a striking benefit for bacterial survival during bloodstream infection and dissemination to other tissues in vivo. In fact, levels of bacterial persistence in the blood corresponded to the overall levels of these molecules expressed by V. vulnificus isolates. Taken together, these results suggest that molecules similar to sialic acids evolved to facilitate the aquatic lifestyle of V. vulnificus but that their emergence also resulted in a gain of function with life-threatening potential in the human host.

Vibrio vulnificus MO6-24/O lipopolysaccharide stimulates superoxide anion, thromboxane B₂, matrix metalloproteinase-9, cytokine and chemokine release by rat brain microglia in vitro

Although human exposure to Gram-negative Vibrio vulnificus (V. vulnificus) lipopolysaccharide (LPS) has been reported to result in septic shock, its impact on the central nervous system's innate immunity remains undetermined. The purpose of this study was to determine whether V. vulnificus MO6-24/O LPS might activate rat microglia in vitro and stimulate the release of superoxide anion (O₂⁻), a reactive oxygen species known to cause oxidative stress and neuronal injury in vivo. Brain microglia were isolated from neonatal rats, and then treated with either V. vulnificus MO6-24/O LPS or Escherichia coli O26:B6 LPS for 17 hours in vitro. O₂⁻ was determined by cytochrome C reduction, and matrix metalloproteinase-2 (MMP-2) and MMP-9 by gelatinase zymography. Generation of cytokines tumor necrosis factor alpha (TNF-α), interleukin-1 alpha (IL-1α), IL-6, and transforming growth factor-beta 1 (TGF-β1), chemokines macrophage inflammatory protein (MIP-1α)/chemokine (C-C motif) ligand 3 (CCL3), MIP-2/chemokine (C-X-C motif) ligand 2 (CXCL2), monocyte chemotactic protein-1 (MCP-1)/CCL2, and cytokine-induced neutrophil chemoattractant-2alpha/beta (CINC-2α/β)/CXCL3, and brain-derived neurotrophic factor (BDNF), were determined by specific immunoassays. Priming of rat microglia by V. vulnificus MO6-24/O LPS in vitro yielded a bell-shaped dose-response curve for PMA (phorbol 12-myristate 13-acetate)-stimulated O₂⁻ generation: (1) 0.1-1 ng/mL V. vulnificus LPS enhanced O₂⁻ generation significantly but with limited inflammatory mediator generation; (2) 10-100 ng/mL V. vulnificus LPS maximized O₂⁻ generation with concomitant release of thromboxane B2 (TXB2), matrix metalloproteinase-9 (MMP-9), and several cytokines and chemokines; (3) 1000-100,000 ng/mL V. vulnificus LPS, with the exception of TXB2, yielded both attenuated O₂⁻ production, and a progressive decrease in MMP-9, cytokines and chemokines investigated. Thus concentration-dependent treatment of neonatal brain microglia with V. vulnificus MO6-24/O LPS resulted in a significant rise in O₂⁻ production, followed by a progressive decrease in O₂⁻ release, with concomitant release of lactic dehydrogenase (LDH), and generation of TXB2, MMP-9, cytokines and chemokines. We hypothesize that the inflammatory mediators investigated may be cytotoxic to microglia in vitro, by an as yet undetermined autocrine mechanism. Although V. vulnificus LPS was less potent than E. coli LPS in vitro, inflammatory mediator release by the former was clearly more efficacious. Finally, we hypothesize that should V. vulnificus LPS gain entry into the CNS, it would be possible that microglia might become activated, resulting in high levels of O₂⁻ as well as neuroinflammatory TXB2, MMP-9, cytokines and chemokines.

The sugar 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) as a characteristic component of bacterial endotoxin — a review of its biosynthesis, function, and placement in the lipopolysaccharide core

The sugar 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) is a characteristic component of bacterial lipopolysaccharide (LPS, endotoxin). It connects the carbohydrate part of LPS with C6 of glucosamine or 2,3-diaminoglucose of lipid A by acid-labile α-ketosidic linkage. The number of Kdo units present in LPS, the way they are connected, and the occurrence of other substituents (P, PEtn, PPEtn, Gal, or β-l-Ara4N) account for structural diversity of the inner core region of endotoxin. In a majority of cases, Kdo is crucial to the viability and growth of bacterial cells. In this paper, the biosynthesis of Kdo and the mechanism of its incorporation into the LPS structure, as well as the location of this unique component in the endotoxin core structures, have been described.

O-antigen and core carbohydrate of Vibrio fischeri lipopolysaccharide: composition and analysis of their role in Euprymna scolopes light organ colonization

Vibrio fischeri exists in a symbiotic relationship with the Hawaiian bobtail squid, Euprymna scolopes, where the squid provides a home for the bacteria, and the bacteria in turn provide camouflage that helps protect the squid from night-time predators. Like other gram-negative organisms, V. fischeri expresses lipopolysaccharide (LPS) on its cell surface. The structure of the O-antigen and the core components of the LPS and their possible role in colonization of the squid have not previously been determined. In these studies, an O-antigen ligase mutant, waaL, was utilized to determine the structures of these LPS components and their roles in colonization of the squid. WaaL ligates the O-antigen to the core of the LPS; thus, LPS from waaL mutants lacks O-antigen. Our results show that the V. fischeri waaL mutant has a motility defect, is significantly delayed in colonization, and is unable to compete with the wild-type strain in co-colonization assays. Comparative analyses of the LPS from the wild-type and waaL strains showed that the V. fischeri LPS has a single O-antigen repeat composed of yersiniose, 8-epi-legionaminic acid, and N-acetylfucosamine. In addition, the LPS from the waaL strain showed that the core structure consists of L-glycero-D-manno-heptose, D-glycero-D-manno-heptose, glucose, 3-deoxy-D-manno-octulosonic acid, N-acetylgalactosamine, 8-epi-legionaminic acid, phosphate, and phosphoethanolamine. These studies indicate that the unusual V. fischeri O-antigen sugars play a role in the early phases of bacterial colonization of the squid.

Genomic and metabolic profiling of nonulosonic acids in Vibrionaceae reveal biochemical phenotypes of allelic divergence in Vibrio vulnificus

Nonulosonic acids (NulOs) encompass a large group of structurally diverse nine-carbon backbone α-keto sugars widely distributed among the three domains of life. Mammals express a specialized version of NulOs called sialic acids, which are displayed in prominent terminal positions of cell surface and secreted glycoconjugates. Within bacteria, the ability to synthesize NulOs has been demonstrated in a number of human pathogens and is phylogenetically widespread. Here we examine the distribution, diversity, evolution, and function of NulO biosynthesis pathways in members of the family Vibrionaceae. Among 27 species of Vibrionaceae examined at the genomic level, 12 species contained nab gene clusters. We document examples of duplication, divergence, horizontal transfer, and recombination of nab gene clusters in different Vibrionaceae lineages. Biochemical analyses, including mass spectrometry, confirmed that many species do, in fact, produce di-N-acetylated NulOs. A library of clinical and environmental isolates of Vibrio vulnificus served as a model for further investigation of nab allele genotypes and levels of NulO expression. The data show that lineage I isolates produce about 20-fold higher levels of NulOs than lineage II isolates. Moreover, nab gene alleles found in a subset of V. vulnificus clinical isolates express 40-fold higher levels of NulOs than nab alleles associated with environmental isolates. Taken together, the data implicate the family Vibrionaceae as a "hot spot" of NulO evolution and suggest that these molecules may have diverse roles in environmental persistence and/or animal virulence.

The lipid A from Vibrio fischeri lipopolysaccharide: a unique structure bearing a phosphoglycerol moiety

Vibrio fischeri, a bioluminescent marine bacterium, exists in an exclusive symbiotic relationship with the Hawaiian bobtail squid, Euprymna scolopes, whose light organ it colonizes. Previously, it has been shown that the lipopolysaccharide (LPS) or free lipid A of V. fischeri can trigger morphological changes in the juvenile squid's light organ that occur upon colonization. To investigate the structural features that might be responsible for this phenomenon, the lipid A from V. fischeri ES114 LPS was isolated and characterized by multistage mass spectrometry (MS(n)). A microheterogeneous mixture of mono- and diphosphorylated diglucosamine disaccharides was observed with variable states of acylation ranging from tetra- to octaacylated forms. All lipid A species, however, contained a set of conserved primary acyl chains consisting of an N-linked C14:0(3-OH) at the 2-position, an unusual N-linked C14:1(3-OH) at the 2'-position, and two O-linked C12:0(3-OH) fatty acids at the 3- and 3'-positions. The fatty acids found in secondary acylation were considerably more variable, with either a C12:0 or C16:1 at the 2-position, C14:0 or C14:0(3-OH) at the 2'-position, and C12:0 or no substituent at the 3'-position. Most surprising was the presence of an unusual set of modifications at the secondary acylation site of the 3-position consisting of phosphoglycerol (GroP), lysophosphatidic acid (GroP bearing C12:0, C16:0, or C16:1), or phosphatidic acid (GroP bearing either C16:0 + C12:0 or C16:0 + C16:1). Given their unusual nature, it is possible that these features of the V. fischeri lipid A may underlie the ability of E. scolopes to recognize its symbiotic partner.

Bacterial catabolism of nonulosonic (sialic) acid and fitness in the gut

The term nonulosonic acid or sialic acid encompasses a varied group of nine-carbon amino sugars widely distributed among mammals and higher metazoans. Among bacteria, the ability to synthesize sialic acid was first examined in a small number of human pathogenic species that deposit sialic acid on their outer surface. New phylogenomic data suggest that the ability to synthesize sialic acid and sialic acid-like compounds is not a novel bacterial innovation but a much more widespread ancient trait. In contrast, the genes required for the catabolism of sialic acid are found only among pathogenic and commensal bacterial species. This ability to utilize sialic acid as a carbon source is correlated with bacterial virulence, especially, in the sialic acid rich environment of the gut. In this article, we present the most recent findings in sialobiology with a focus on sialic acid catabolism.

The diversity of the core oligosaccharide in lipopolysaccharides

Bacterial lipopolysaccharides (LPSs) are the major component of the outer membrane of Gram-negative bacteria. They have a structural role since they contribute to the cellular rigidity by increasing the strength of cell wall and mediating contacts with the external environment that can induce structural changes to allow life in different conditions. Furthermore, the low permeability of the outer membrane acts as a barrier to protect bacteria from host-derived antimicrobial compounds. Lipopolysaccharides are amphiphilic macromolecules generally comprising three defined regions distinguished by their genetics, structures and function: the lipid A, the core oligosaccharide and a polysaccharide portion, the O-chain. In some Gram-negative bacteria LPS can terminate with the core portion to form rough type LPS (R-LPS, LOS). The core oligosaccharide is an often branched and phosphorylated heterooligosaccharide with less than fifteen sugars, more conserved in the inner region, proximal to the lipid A, and often carrying non-stoichiometric substitutions leading to variation and micro-heterogeneity. The core oligosaccharide contributes to the bacterial viability and stability of the outer membrane, can assure the serological specificity and possesses antigenic properties.

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.