Genetic locus and structural characterization of the biochemical defect in the O-antigenic polysaccharide of the symbiotically deficient Rhizobium etli mutant, CE166. Replacement of N-acetylquinovosamine with its hexosyl-4-ulose precursor

The ROK kinase N-acetylglucosamine kinase uses a sequential random enzyme mechanism with successive conformational changes upon each substrate binding

N-acetyl-d-glucosamine (GlcNAc) is a major component of bacterial cell walls. Many organisms recycle GlcNAc from the cell wall or metabolize environmental GlcNAc. The first step in GlcNAc metabolism is phosphorylation to GlcNAc-6-phosphate. In bacteria, the ROK family kinase N-acetylglucosamine kinase (NagK) performs this activity. Although ROK kinases have been studied extensively, no ternary complex showing the two substrates has yet been observed. Here, we solved the structure of NagK from the human pathogen Plesiomonas shigelloides in complex with GlcNAc and the ATP analog AMP-PNP. Surprisingly, PsNagK showed distinct conformational changes associated with the binding of each substrate. Consistent with this, the enzyme showed a sequential random enzyme mechanism. This indicates that the enzyme acts as a coordinated unit responding to each interaction. Our molecular dynamics modeling of catalytic ion binding confirmed the location of the essential catalytic metal. Additionally, site-directed mutagenesis confirmed the catalytic base and that the metal-coordinating residue is essential. Together, this study provides the most comprehensive insight into the activity of a ROK kinase.

Synthesis of N-acetyl-d-quinovosamine in Rhizobium etli CE3 is completed after its 4-keto-precursor is linked to a carrier lipid

Bacterial O-antigens are synthesized on lipid carriers before being transferred to lipopolysaccharide core structures. Rhizobium etli CE3 lipopolysaccharide is a model for understanding O-antigen biological function. CE3 O-antigen structure and genetics are known. However, proposed enzymology for CE3 O-antigen synthesis has been examined very little in vitro, and even the sugar added to begin the synthesis is uncertain. A model based on mutagenesis studies predicts that 2-acetamido-2,6-dideoxy-d-glucose (QuiNAc) is the first O-antigen sugar and that genes wreV, wreQ and wreU direct QuiNAc synthesis and O-antigen initiation. Previously, synthesis of UDP-QuiNAc was shown to occur in vitro with a WreV orthologue (4,6-hexose dehydratase) and WreQ (4-reductase), but the WreQ catalysis in this conventional deoxyhexose-synthesis pathway was very slow. This seeming deficiency was explained in the present study after WreU transferase activity was examined in vitro. Results fit the prediction that WreU transfers sugar-1-phosphate to bactoprenyl phosphate (BpP) to initiate O-antigen synthesis. Interestingly, WreU demonstrated much higher activity using the product of the WreV catalysis [UDP-4-keto-6-deoxy-GlcNAc (UDP-KdgNAc)] as the sugar-phosphate donor than using UDP-QuiNAc. Furthermore, the WreQ catalysis with WreU-generated BpPP-KdgNAc as the substrate was orders of magnitude faster than with UDP-KdgNAc. The inferred product BpPP-QuiNAc reacted as an acceptor substrate in an in vitro assay for addition of the second O-antigen sugar, mannose. These results imply a novel pathway for 6-deoxyhexose synthesis that may be commonly utilized by bacteria when QuiNAc is the first sugar of a polysaccharide or oligosaccharide repeat unit: UDP-GlcNAc → UDP-KdgNAc → BpPP-KdgNAc → BpPP-QuiNAc.

Structural and immunochemical relatedness suggests a conserved pathogenicity motif for secondary cell wall polysaccharides in Bacillus anthracis and infection-associated Bacillus cereus

Bacillus anthracis (Ba) and human infection-associated Bacillus cereus (Bc) strains Bc G9241 and Bc 03BB87 have secondary cell wall polysaccharides (SCWPs) comprising an aminoglycosyl trisaccharide repeat: →4)-β-d-ManpNAc-(1→4)-β-d-GlcpNAc-(1→6)-α-d-GlcpNAc-(1→, substituted at GlcNAc residues with both α- and β-Galp. In Bc G9241 and Bc 03BB87, an additional α-Galp is attached to O-3 of ManNAc. Using NMR spectroscopy, mass spectrometry and immunochemical methods, we compared these structures to SCWPs from Bc biovar anthracis strains isolated from great apes displaying “anthrax-like” symptoms in Cameroon (Bc CA) and Côte d’Ivoire (Bc CI). The SCWPs of Bc CA/CI contained the identical HexNAc trisaccharide backbone and Gal modifications found in Ba, together with the α-Gal-(1→3) substitution observed previously at ManNAc residues only in Bc G9241/03BB87. Interestingly, the great ape derived strains displayed a unique α-Gal-(1→3)-α-Gal-(1→3) disaccharide substitution at some ManNAc residues, a modification not found in any previously examined Ba or Bc strain. Immuno-analysis with specific polyclonal anti-Ba SCWP antiserum demonstrated a reactivity hierarchy: high reactivity with SCWPs from Ba 7702 and Ba Sterne 34F2, and Bc G9241 and Bc 03BB87; intermediate reactivity with SCWPs from Bc CI/CA; and low reactivity with the SCWPs from structurally distinct Ba CDC684 (a unique strain producing an SCWP lacking all Gal substitutions) and non-infection-associated Bc ATCC10987 and Bc 14579 SCWPs. Ba-specific monoclonal antibody EAII-6G6-2-3 demonstrated a 10–20 fold reduced reactivity to Bc G9241 and Bc 03BB87 SCWPs compared to Ba 7702/34F2, and low/undetectable reactivity to SCWPs from Bc CI, Bc CA, Ba CDC684, and non-infection-associated Bc strains. Our data indicate that the HexNAc motif is conserved among infection-associated Ba and Bc isolates (regardless of human or great ape origin), and that the number, positions and structures of Gal substitutions confer unique antigenic properties. The conservation of this structural motif could open a new diagnostic route in detection of pathogenic Bc strains.

The Biosynthesis of UDP-D-QuiNAc in Bacillus cereus ATCC 14579

N-acetylquinovosamine (2-acetamido-2,6-di-deoxy-d-glucose, QuiNAc) is a relatively rare amino sugar residue found in glycans of few pathogenic gram-negative bacteria where it can play a role in infection. However, little is known about QuiNAc-related polysaccharides in gram-positive bacteria. In a routine screen for bacillus glycan grown at defined medium, it was surprising to identify a QuiNAc residue in polysaccharides isolated from this gram-positive bacterium. To gain insight into the biosynthesis of these glycans, we report the identification of an operon in Bacillus cereus ATCC 14579 that contains two genes encoding activities not previously described in gram-positive bacteria. One gene encodes a UDP-N-acetylglucosamine C4,6-dehydratase, (abbreviated Pdeg) that converts UDP-GlcNAc to UDP-4-keto-4,6-d-deoxy-GlcNAc (UDP-2-acetamido-2,6-dideoxy-α-d-xylo-4-hexulose); and the second encodes a UDP-4-reductase (abbr. Preq) that converts UDP-4-keto-4,6-d-deoxy-GlcNAc to UDP-N-acetyl-quinovosamine in the presence of NADPH. Biochemical studies established that the sequential Pdeg and Preq reaction product is UDP-d-QuiNAc as determined by mass spectrometry and one- and two-dimensional NMR experiments. Also, unambiguous evidence for the conversions of the dehydratase product, UDP-α-d-4-keto-4,6-deoxy-GlcNAc, to UDP-α-d-QuiNAc was obtained using real-time ¹H-NMR spectroscopy and mass spectrometry. The two genes overlap by 4 nucleotides and similar operon organization and identical gene sequences were also identified in a few other Bacillus species suggesting they may have similar roles in the lifecycle of this class of bacteria important to human health. Our results provide new information about the ability of Bacilli to form UDP-QuiNAc and will provide insight to evaluate their role in the biology of Bacillus.

Structural elucidation of the outer core tetrasaccharide isolated from the LPS of Rhizobium leguminosarum bv. trifolii strain 24

The outer core oligosaccharide (OS) was isolated from the lipopolysaccharide (LPS) of Rhizobium leguminosarum bv. trifolii strain 24 after Smith degradation and then studied by sugar and methylation analyses along with NMR and mass spectrometry methods. Negative-ion electrospray (ESI-MS) mass spectrum showed two molecular ions at m/z 686.3 and 728.3, which corresponded to the core OS having the composition Rha2QuiNAcKdh. The mass difference between both ions indicated that the higher molecule mass represented the mono O-acetylated variant of the OS. The sequence of the oligosaccharide was reflected in CID MS/MS spectra. In turn, NMR spectroscopy confirmed the composition and glycosylation pattern of the core OS and provided additional evidence on its structure. 2D NMR experiments revealed that the terminal Rhap is acetylated at position O-2. Moreover, 3-deoxyheptulosonic acid (Kdh), which was detected at the reducing terminus of the OS, was evidently derived from the Kdo as a result of Smith degradation. In addition, the higher intensity of signals for a six-membered pyranose ring of Kdhp over 2,7-anh-Kdhf seemed to indicate prevalence of this form of the sugar in the OS-derived species. Based on the data obtained, the following structure of the outer core tetrasaccharide, which probably links the O-chain polysaccharide to the inner core in the LPS of R. leguminosarum bv. trifolii strain 24, was established: α-L-Rhap-2-OAc*-(1-->3)-α-L-Rhap-(1-->3)-β-D-QuipNAc-(1-->4)-Kdo * ~ 50%. .

Lipopolysaccharide O-chain core region required for cellular cohesion and compaction of in vitro and root biofilms developed by Rhizobium leguminosarum

The formation of biofilms is an important survival strategy allowing rhizobia to live on soil particles and plant roots. Within the microcolonies of the biofilm developed by Rhizobium leguminosarum, rhizobial cells interact tightly through lateral and polar connections, forming organized and compact cell aggregates. These microcolonies are embedded in a biofilm matrix, whose main component is the acidic exopolysaccharide (EPS). Our work shows that the O-chain core region of the R. leguminosarum lipopolysaccharide (LPS) (which stretches out of the cell surface) strongly influences bacterial adhesive properties and cell-cell cohesion. Mutants defective in the O chain or O-chain core moiety developed premature microcolonies in which lateral bacterial contacts were greatly reduced. Furthermore, cell-cell interactions within the microcolonies of the LPS mutants were mediated mostly through their poles, resulting in a biofilm with an altered three-dimensional structure and increased thickness. In addition, on the root epidermis and on root hairs, O-antigen core-defective strains showed altered biofilm patterns with the typical microcolony compaction impaired. Taken together, these results indicate that the surface-exposed moiety of the LPS is crucial for proper cell-to-cell interactions and for the formation of robust biofilms on different surfaces.

In vitro biosynthesis and chemical identification of UDP-N-acetyl-d-quinovosamine (UDP-d-QuiNAc)

N-acetyl-d-quinovosamine (2-acetamido-2,6-dideoxy-d-glucose, QuiNAc) occurs in the polysaccharide structures of many Gram-negative bacteria. In the biosynthesis of QuiNAc-containing polysaccharides, UDP-QuiNAc is the hypothetical donor of the QuiNAc residue. Biosynthesis of UDP-QuiNAc has been proposed to occur by 4,6-dehydration of UDP-N-acetyl-d-glucosamine (UDP-GlcNAc) to UDP-2-acetamido-2,6-dideoxy-d-xylo-4-hexulose followed by reduction of this 4-keto intermediate to UDP-QuiNAc. Several specific dehydratases are known to catalyze the first proposed step. A specific reductase for the last step has not been demonstrated in vitro, but previous mutant analysis suggested that Rhizobium etli gene wreQ might encode this reductase. Therefore, this gene was cloned and expressed in Escherichia coli, and the resulting His6-tagged WreQ protein was purified. It was tested for 4-reductase activity by adding it and NAD(P)H to reaction mixtures in which 4,6-dehydratase WbpM had acted on the precursor substrate UDP-GlcNAc. Thin layer chromatography of the nucleotide sugars in the mixture at various stages of the reaction showed that WbpM converted UDP-GlcNAc completely to what was shown to be its 4-keto-6-deoxy derivative by NMR and that addition of WreQ and NADH led to formation of a third compound. Combined gas chromatography-mass spectrometry analysis of acid hydrolysates of the final reaction mixture showed that a quinovosamine moiety had been synthesized after WreQ addition. The two-step reaction progress also was monitored in real time by NMR. The final UDP-sugar product after WreQ addition was purified and determined to be UDP-d-QuiNAc by one-dimensional and two-dimensional NMR experiments. These results confirmed that WreQ has UDP-2-acetamido-2,6-dideoxy-d-xylo-4-hexulose 4-reductase activity, completing a pathway for UDP-d-QuiNAc synthesis in vitro.

Roles of predicted glycosyltransferases in the biosynthesis of the Rhizobium etli CE3 O antigen

The Rhizobium etli CE3 O antigen is a fixed-length heteropolymer. The genetic regions required for its synthesis have been identified, and the nucleotide sequences are known. The structure of the O antigen has been determined, but the roles of specific genes in synthesizing this structure are relatively unclear. Within the known O-antigen genetic clusters of this strain, nine open reading frames (ORFs) were found to contain a conserved glycosyltransferase domain. Each ORF was mutated, and the resulting mutant lipopolysaccharide (LPS) was analyzed. Tricine SDS-PAGE revealed stepwise truncations of the O antigen that were consistent with differences in mutant LPS sugar compositions and reactivity with O-antigen-specific monoclonal antibodies. Based on these results and current theories of O-antigen synthesis, specific roles were deduced for each of the nine glycosyltransferases, and a model for biosynthesis of the R. etli CE3 O antigen was proposed. In this model, O-antigen biosynthesis is initiated with the addition of N-acetyl-quinovosamine-phosphate (QuiNAc-P) to bactoprenol-phosphate by glycosyltransferase WreU. Glycosyltransferases WreG, WreE, WreS, and WreT would each act once to attach mannose, fucose, a second fucose, and 3-O-methyl-6-deoxytalose (3OMe6dTal), respectively. WreH would then catalyze the addition of methyl glucuronate (MeGlcA) to complete the first instance of the O-antigen repeat unit. Four subsequent repeats of this unit composed of fucose, 3OMe6dTal, and MeGlcA would be assembled by a cycle of reactions catalyzed by two additional glycosyltransferases, WreM and WreL, along with WreH. Finally, the O antigen would be capped by attachment of di- or tri-O-methylated fucose as catalyzed by glycosyltransferase WreB.

Roles for wbtC, wbtI, and kdtA Genes in Lipopolysaccharide Biosynthesis, Protein Glycosylation, Virulence, and Immunogenicity in Francisella tularensis2 Strain SCHU S4

Using a strategy of gene deletion mutagenesis, we have examined the roles of genes putatively involved in lipopolysaccharide biosynthesis in the virulent facultative intracellular bacterial pathogen, Francisella tularensis subspecies tularensis, strain SCHU S4 in LPS biosynthesis, protein glycosylation, virulence and immunogenicity. One mutant, ΔwbtI, did not elaborate a long chain O-polysaccharide (OPS), was completely avirulent for mice, and failed to induce a protective immune response against challenge with wild type bacteria. Another mutant, ΔwbtC, produced a long chain OPS with altered chemical and electrophoretic characteristics. This mutant showed markedly reduced glycosylation of several known glycoproteins. Additionally this mutant was highly attenuated, and elicited a protective immune response against systemic, but not respiratory challenge with wild type SCHU S4. A third mutant, ΔkdtA, produced an unconjugated long chain OPS, lacking a detectable core structure, and which was not obviously expressed at the surface. It was avirulent and elicited partial protection against systemic challenge only.

Localization and structural analysis of a conserved pyruvylated epitope in Bacillus anthracis secondary cell wall polysaccharides and characterization of the galactose-deficient wall polysaccharide from avirulent B. anthracis CDC 684

Bacillus anthracis CDC 684 is a naturally occurring, avirulent variant and close relative of the highly pathogenic B. anthracis Vollum. Bacillus anthracis CDC 684 contains both virulence plasmids, pXO1 and pXO2, yet is non-pathogenic in animal models, prompting closer scrutiny of the molecular basis of attenuation. We structurally characterized the secondary cell wall polysaccharide (SCWP) of B. anthracis CDC 684 (Ba684) using chemical and NMR spectroscopy analysis. The SCWP consists of a HexNAc trisaccharide backbone having identical structure as that of B. anthracis Pasteur, Sterne and Ames, →4)-β-d-ManpNAc-(1 → 4)-β-d-GlcpNAc-(1 → 6)-α-d-GlcpNAc-(1→. Remarkably, although the backbone is fully polymerized, the SCWP is the devoid of all galactosyl side residues, a feature which normally comprises 50% of the glycosyl residues on the highly galactosylated SCWPs from pathogenic strains. This observation highlights the role of defective wall assembly in virulence and indicates that polymerization occurs independently of galactose side residue attachment. Of particular interest, the polymerized Ba684 backbone retains the substoichiometric pyruvate acetal, O-acetate and amino group modifications found on SCWPs from normal B. anthracis strains, and immunofluorescence analysis confirms that SCWP expression coincides with the ability to bind the surface layer homology (SLH) domain containing S-layer protein extractable antigen-1. Pyruvate was previously demonstrated as part of a conserved epitope, mediating SLH-domain protein attachment to the underlying peptidoglycan layer. We find that a single repeating unit, located at the distal (non-reducing) end of the Ba684 SCWP, is structurally modified and that this modification is present in identical manner in the SCWPs of normal B. anthracis strains. These polysaccharides terminate in the sequence: (S)-4,6-O-(1-carboxyethylidene)-β-d-ManpNAc-(1 → 4)-[3-O-acetyl]-β-d-GlcpNAc-(1 → 6)-α-d-GlcpNH(2)-(1→.

Characterization of galacturonosyl transferase genes rgtA, rgtB, rgtC, rgtD, and rgtE responsible for lipopolysaccharide synthesis in nitrogen-fixing endosymbiont Rhizobium leguminosarum: lipopolysaccharide core and lipid galacturonosyl residues confer membrane stability

Rhizobium lipopolysaccharide (LPS) contains four terminally linked galacturonic acid (GalA) residues; one attached to the lipid A and three attached to the core oligosaccharide moiety. Attachment of the GalA residues requires the lipid donor dodecaprenyl-phosphate GalA (Dod-P-GalA), which is synthesized by the GalA transferase RgtE reported here. The galacturonosyl transferases RgtA, -B, and -C utilize Dod-P-GalA to attach GalAs on the LPS core region, and RgtD attaches GalA to the lipid A 4' position. As reported here, the functions of the rgtD and rgtE genes were determined via insertion mutagenesis and structural characterization of the mutant lipid A. The rgtE(-) mutant lacked Dod-P-GalA as determined by mass spectrometry of total lipid extracts and the inability of rgtE(-) mutant membranes to provide the substrate for heterologously expressed RgtA activity. In addition, we created single mutations in each of the rgtA, -B, -C, -D, and -E genes to study the biological function of the GalA residues. The structures of the core oligosaccharide region from each of the rgt mutants were elucidated by glycosyl linkage analysis. Each mutant was assayed for its sensitivity to sodium deoxycholate and to the antimicrobial cationic peptide, polymyxin B (PmxB). The rgt mutants were more sensitive than the parent strain to deoxycholate by varying degrees. However, the rgtA, -B, and -C mutants were more resistant to PmxB, whereas the rgtD and E mutants were less resistant in comparison to the parent strain.

Secondary cell wall polysaccharides from Bacillus cereus strains G9241, 03BB87 and 03BB102 causing fatal pneumonia share similar glycosyl structures with the polysaccharides from Bacillus anthracis

Secondary cell wall polysaccharides (SCWPs) are important structural components of the Bacillus cell wall and contribute to the array of antigens presented by these organisms in both spore and vegetative forms. We previously found that antisera raised to Bacillus anthracis spore preparations cross-reacted with SCWPs isolated from several strains of pathogenic B. cereus, but did not react with other phylogenetically related but nonpathogenic Bacilli, suggesting that the SCWP from B. anthracis and pathogenic B. cereus strains share specific structural features. In this study, SCWPs from three strains of B. cereus causing severe or fatal pneumonia (G9241, 03BB87 and 03BB102) were isolated and subjected to structural analysis and their structures were compared to SCWPs from B. anthracis. Complete structural analysis was performed for the B. cereus G9241 SCWP using NMR spectroscopy, mass spectrometry and derivatization methods. The analyses show that SCWPs from B. cereus G9241 has a glycosyl backbone identical to that of B. anthracis SCWP, consisting of multiple trisaccharide repeats of: →6)-α-d-GlcpNAc-(1 → 4)-β-d-ManpNAc-(1 → 4)-β-d-GlcpNAc-(1→. Both the B. anthracis and pathogenic B. cereus SCWPs are highly substituted at all GlcNAc residues with α- and β-Gal residues, however, only the SCWPs from B. cereus G9241 and 03BB87 carry an additional α-Gal substitution at O-3 of ManNAc residues, a feature lacking in the B. anthracis SCWPs. Both the B. anthracis and B. cereus SCWPs are pyruvylated, with an approximate molecular mass of ≈12,000 Da. The implications of these findings regarding pathogenicity and cell wall structure are discussed.

Structures of the lipopolysaccharides from Rhizobium leguminosarum RBL5523 and its UDP-glucose dehydrogenase mutant (exo5)

Rhizobial lipopolysaccharide (LPS) is required to establish an effective symbiosis with its host plant. An exo5 mutant of Rhizobium leguminosarum RBL5523, strain RBL5808, is defective in UDP-glucose (Glc) dehydrogenase that converts UDP-Glc to UDP-glucuronic acid (GlcA). This mutant is unable to synthesize either UDP-GlcA or UDP-galacturonic acid (GalA) and is unable to synthesize extracellular and capsular polysaccharides, lacks GalA in its LPS and is defective in symbiosis (Laus MC, Logman TJ, van Brussel AAN, Carlson RW, Azadi P, Gao MY, Kijne JW. 2004. Involvement of exo5 in production of surface polysaccharides in Rhizobium leguminosarum and its role in nodulation of Vicia sativa subsp. nigra. J Bacteriol. 186:6617-6625). Here, we determined and compared the structures of the RBL5523 parent and RBL5808 mutant LPSs. The parent LPS core oligosaccharide (OS), as with other R. leguminosarum and Rhizobium etli strains, is a Gal(1)Man(1)GalA(3)Kdo(3) octasaccharide in, which each of the GalA residues is terminally linked. The core OS from the mutant lacks all three GalA residues. Also, the parent lipid A consists of a fatty acylated GlcNGlcNonate or GlcNGlcN disaccharide that has a GalA residue at the 4'-position, typical of other R. leguminosarum and R. etli lipids A. The mutant lipid A lacks the 4'-GalA residue, and the proximal glycosyl residue was only present as GlcNonate. In spite of these alterations to the lipid A and core OSs, the mutant was still able to synthesize an LPS containing a normal O-chain polysaccharide (OPS), but at reduced levels. The structure of the OPS of the mutant LPS was identical to that of the parent and consists of an O-acetylated →4)-α-d-Glcp-(1→3)-α-d-QuipNAc-(1→ repeating unit.

Characterization of the structure and biological functions of a capsular polysaccharide produced by Staphylococcus saprophyticus

Staphylococcus saprophyticus is a common cause of uncomplicated urinary tract infections in women. S. saprophyticus strain ATCC 15305 carries two staphylococcal cassette chromosome genetic elements, SCC(15305RM) and SCC(15305cap). The SCC(15305cap) element carries 13 open reading frames (ORFs) involved in capsular polysaccharide (CP) biosynthesis, and its G+C content (26.7%) is lower than the average G+C content (33.2%) for the whole genome. S. saprophyticus strain ATCC 15305 capD, capL, and capK (capD(Ssp), capL(Ssp), and capK(Ssp)) are homologous to genes encoding UDP-FucNAc biosynthesis, and gtaB and capI(Ssp) show homology to genes involved in UDP-glucuronic acid synthesis. S. saprophyticus ATCC 15305 CP, visualized by immunoelectron microscopy, was extracted and purified using anionic-exchange and size exclusion chromatography. Analysis of the purified CP by (1)H and (13)C nuclear magnetic resonance (NMR) spectroscopy and gas-liquid chromatography revealed two types of branched tetrasaccharide repeating units composed of the following: -4)-beta-Glc-(1-3)-Sug-(1-4)-beta-GlcA-(1- | beta-GlcNAc-(1-2) Sug represents two stereoisomers of 2-acetamido-2,6-dideoxy-hexos-4-ulose residues, one of which has an arabino configuration. The encapsulated ATCC 15305 strain was resistant to complement-mediated opsonophagocytic killing by human neutrophils, whereas the acapsular mutant C1 was susceptible. None of 14 clinical isolates reacted with antibodies to the ATCC 15305 CP. However, 11 of the 14 S. saprophyticus isolates were phenotypically encapsulated based on their resistance to complement-mediated opsonophagocytic killing and their failure to hemagglutinate when cultivated aerobically. Ten of the 14 clinical strains carried homologues of the conserved staphylococcal capD gene or the S. saprophyticus gtaB gene, or both. Our results suggest that some strains of S. saprophyticus are encapsulated and that more than one capsular serotype exists.

Lipopolysaccharides in Rhizobium-legume symbioses

The establishment of nitrogen-fixing symbiosis between a legume plant and its rhizobial symbiont requires that the bacterium adapt to changing conditions that occur with the host plant that both promotes and allows infection of the host root nodule cell, regulates and resists the host defense response, permits the exchange of metabolites, and contributes to the overall health of the host. This adaptive process involves changes to the bacterial cell surface and, therefore, structural modifications to the lipopolysaccharide (LPS). In this chapter, we describe the structures of the LPSs from symbiont members of the Rhizobiales, the genetics and mechanism of their biosynthesis, the modifications that occur during symbiosis, and their possible functions.

Genetic basis for Rhizobium etli CE3 O-antigen O-methylated residues that vary according to growth conditions

The Rhizobium etli CE3 O antigen is a fixed-length heteropolymer with O methylation being the predominant type of sugar modification. There are two O-methylated residues that occur, on average, once per complete O antigen: a multiply O-methylated terminal fucose and 2-O methylation of a fucose residue within a repeating unit. The amount of the methylated terminal fucose decreases and the amount of 2-O-methylfucose increases when bacteria are grown in the presence of the host plant, Phaseolus vulgaris, or its seed exudates. Insertion mutagenesis was used to identify open reading frames required for the presence of these O-methylated residues. The presence of the methylated terminal fucose required genes wreA, wreB, wreC, wreD, and wreF, whereas 2-O methylation of internal fucoses required the methyltransferase domain of bifunctional gene wreM. Mutants lacking only the methylated terminal fucose, lacking only 2-O methylation, or lacking both the methylated terminal fucose and 2-O methylation exhibited no other lipopolysaccharide structural defects. Thus, neither of these decorations is required for normal O-antigen length, transport, or assembly into the final lipopolysaccharide. This is in contrast to certain enteric bacteria in which the absence of a terminal decoration severely affects O-antigen length and transport. R. etli mutants lacking only the methylated terminal fucose were not altered in symbiosis with host Phaseolus vulgaris, whereas mutants lacking only 2-O-methylfucose exhibited a delay in nodule development during symbiosis. These results support previous conclusions that the methylated terminal fucose is dispensable for symbiosis, whereas 2-O methylation of internal fucoses somehow facilitates early events in symbiosis.

Review: Lipopolysaccharide biosynthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa causes serious nosocomial infections, and an important virulence factor produced by this organism is lipopolysaccharide (LPS). This review summarizes knowledge about biosynthesis of all three structural domains of LPS - lipid A, core oligosaccharide, and O polysaccharides. In addition, based on similarities with other bacterial species, this review proposes new hypothetical pathways for unstudied steps in the biosynthesis of P. aeruginosa LPS. Lipid A biosynthesis is discussed in relation to Escherichia coli and Salmonella, and the biosyntheses of core sugar precursors and core oligosaccharide are summarised. Pseudomonas aeruginosa attaches a Common Polysaccharide Antigen and O-Specific Antigen polysaccharides to lipid A-core. Both forms of O polysaccharide are discussed with respect to their independent synthesis mechanisms. Recent advances in understanding O-polysaccharide biosynthesis since the last major review on this subject, published nearly a decade ago, are highlighted. Since P. aeruginosa O polysaccharides contain unusual sugars, sugar-nucleotide biosynthesis pathways are reviewed in detail. Knowledge derived from detailed studies in the O5, O6 and O11 serotypes is applied to predict biosynthesis pathways of sugars in poorly-studied serotypes, especially O1, O4, and O13/O14. Although further work is required, a full understanding of LPS biosynthesis in P. aeruginosa is almost within reach.

Two Kdo-heptose regions identified in Hafnia alvei 32 lipopolysaccharide: the complete core structure and serological screening of different Hafnia O serotypes

Hafnia alvei, a gram-negative bacterium, is an opportunistic pathogen associated with mixed hospital infections, bacteremia, septicemia, and respiratory diseases. Various 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo)-containing fragments different from known structures of core oligosaccharides were previously found among fractions obtained by mild acid hydrolysis of some H. alvei lipopolysaccharides (LPSs). However, the positions of these segments in the LPS structure were not known. Analysis of de-N,O-acylated LPS by nuclear magnetic resonance spectroscopy and mass spectrometry allowed the determination of the location of a Kdo-containing trisaccharide in the structure of H. alvei PCM 32 LPS. It was established that the trisaccharide {L-alpha-D-Hepp-(1-->4)-[alpha-D-Galp6OAc-(1-->7)]-alpha-Kdop-(2-->} is an integral part of the outer-core oligosaccharide of H. alvei 32 LPS. The very labile ketosidic linkage between -->4,7)-alpha-Kdop and -->2)-Glcp in the core oligosaccharide was identified. Screening for this Kdo-containing trisaccharide was performed on the group of 37 O serotypes of H. alvei LPSs using monospecific antibodies recognizing the structure. It was established that this trisaccharide is a characteristic component of the outer-core oligosaccharides of H. alvei 2, 32, 600, 1192, 1206, and 1211 LPSs. The weaker cross-reactions with LPSs of strains 974, 1188, 1198, 1204, and 1214 suggest the presence of similar structures in these LPSs, as well. Thus, we have identified new examples of endotoxins among those elucidated so far. This type of core oligosaccharide deviates from the classical scheme by the presence of the structural Kdo-containing motif in the outer-core region.

Structural characterization of the primary O-antigenic polysaccharide of the Rhizobium leguminosarum 3841 lipopolysaccharide and identification of a new 3-acetimidoylamino-3-deoxyhexuronic acid glycosyl component: a unique O-methylated glycan of uniform size, containing 6-deoxy-3-O-methyl-D-talose, n-acetylquinovosamine, and rhizoaminuronic acid (3-acetimidoylamino-3-deoxy-D-gluco-hexuronic acid)

Rhizobium are Gram-negative bacteria that survive intracellularly, within host membrane-derived plant cell compartments called symbiosomes. Within the symbiosomes the bacteria differentiate to bacteroids, the active form that carries out nitrogen fixation. The progression from free-living bacteria to bacteroid is characterized by physiological and morphological changes at the bacterial surface, a phase shift with an altered array of cell surface glycoconjugates. Lipopolysaccharides undergo structural changes upon differentiation from the free living to the bacteroid (intracellular) form. The array of carbohydrate structures carried on lipopolysaccharides confer resistance to plant defense mechanisms and may serve as signals that trigger the plant to allow the infection to proceed. We have determined the structure of the major O-polysaccharide (OPS) isolated from free living Rhizobium leguminosarum 3841, a symbiont of Pisum sativum, using chemical methods, mass spectrometry, and NMR spectroscopy analysis. The OPS is composed of several unusual glycosyl residues, including 6-deoxy-3-O-methyl-d-talose and 2-acetamido-2deoxy-l-quinovosamine. In addition, a new glycosyl residue, 3-acetimidoylamino-3-deoxy-d-gluco-hexuronic acid was identified and characterized, a novel hexosaminuronic acid that does not have an amino group at the 2-position. The OPS is composed of three to four tetrasaccharide repeating units of -->4)-beta-dGlcp3NAmA-(1-->4)-[2-O-Ac-3-O-Me-alpha-d-6dTalp-(1-->3)]-alpha-l-Fucp-(1-->3)-alpha-l-QuipNAc-(1-->. The unique 3-amino hexuronate residue, rhizoaminuronic acid, is an attractive candidate for selective inhibition of OPS synthesis.

Molecular structure of endotoxins from Gram-negative marine bacteria: an update

Marine bacteria are microrganisms that have adapted, through millions of years, to survival in environments often characterized by one or more extreme physical or chemical parameters, namely pressure, temperature and salinity. The main interest in the research on marine bacteria is due to their ability to produce several biologically active molecules, such as antibiotics, toxins and antitoxins, antitumor and antimicrobial agents. Nonetheless, lipopolysaccharides (LPSs), or their portions, from Gram-negative marine bacteria, have often shown low virulence, and represent potential candidates in the development of drugs to prevent septic shock. Besides, the molecular architecture of such molecules is related to the possibility of thriving in marine habitats, shielding the cell from the disrupting action of natural stress factors. Over the last few years, the depiction of a variety of structures of lipids A, core oligosaccharides and O-specific polysaccharides from LPSs of marine microrganisms has been given. In particular, here we will examine the most recently encountered structures for bacteria belonging to the genera Shewanella, Pseudoalteromonas and Alteromonas, of the gamma-Proteobacteria phylum, and to the genera Flavobacterium, Cellulophaga, Arenibacter and Chryseobacterium, of the Cytophaga-Flavobacterium-Bacteroides phylum. Particular attention will be paid to the chemical features expressed by these structures (characteristic monosaccharides, non-glycidic appendages, phosphate groups), to the typifying traits of LPSs from marine bacteria and to the possible correlation existing between such features and the adaptation, over years, of bacteria to marine environments.

Rhizobium etli CE3 bacteroid lipopolysaccharides are structurally similar but not identical to those produced by cultured CE3 bacteria

Rhizobium etli CE3 bacteroids were isolated from Phaseolus vulgaris root nodules. The lipopolysaccharide (LPS) from the bacteroids was purified and compared with the LPS from laboratory-cultured R. etli CE3 and from cultures grown in the presence of anthocyanin. Comparisons were made of the O-chain polysaccharide, the core oligosaccharide, and the lipid A. Although LPS from CE3 bacteria and bacteroids are structurally similar, it was found that bacteroid LPS had specific modifications to both the O-chain polysaccharide and lipid A portions of their LPS. Cultures grown with anthocyanin contained modifications only to the O-chain polysaccharide. The changes to the O-chain polysaccharide consisted of the addition of a single methyl group to the 2-position of a fucosyl residue in one of the five O-chain trisaccharide repeat units. This same change occurred for bacteria grown in the presence of anthocyanin. This methylation change correlated with the inability of bacteroid LPS and LPS from anthocyanin-containing cultures to bind the monoclonal antibody JIM28. The core oligosaccharide region of bacteroid LPS and from anthocyanin-grown cultures was identical to that of LPS from normal laboratory-cultured CE3. The lipid A from bacteroids consisted exclusively of a tetraacylated species compared with the presence of both tetra- and pentaacylated lipid A from laboratory cultures. Growth in the presence of anthocyanin did not affect the lipid A structure. Purified bacteroids that could resume growth were also found to be more sensitive to the cationic peptides, poly-l-lysine, polymyxin-B, and melittin.

Structural characterization of a K-antigen capsular polysaccharide essential for normal symbiotic infection in Rhizobium sp. NGR234: deletion of the rkpMNO locus prevents synthesis of 5,7-diacetamido-3,5,7,9-tetradeoxy-non-2-ulosonic acid

Many early molecular events in symbiotic infection have been documented, although factors enabling Rhizobium to progress within the plant-derived infection thread and ultimately survive within the intracellular symbiosome compartment as mature nitrogen-fixing bacteroids are poorly understood. Rhizobial surface polysaccharides (SPS), including the capsular polysaccharides (K-antigens), exist in close proximity to plant-derived membranes throughout the infection process. SPSs are essential for bacterial survival, adaptation, and as potential determinants of nodulation and/or host specificity. Relatively few studies have examined the role of K-antigens in these events. However, we constructed a mutant that lacks genes essential for the production of the K-antigen strain-specific sugar precursor, pseudaminic acid, in the broad host range Rhizobium sp. NGR234. The complete structure of the K-antigen of strain NGR234 was established, and it consists of disaccharide repeating units of glucuronic and pseudaminic acid having the structure -->4)-beta-d-glucuronic acid-(1-->4)-beta-5,7-diacetamido-3,5,7,9-tetradeoxy-l-glycero-l-manno-nonulosonic acid-(2-->. Deletion of three genes located in the rkp-3 gene cluster, rkpM, rkpN, and part of rkpO, abolished pseudaminic acid synthesis, yielding a mutant in which the strain-specific K-antigen was totally absent: other surface glycoconjugates, including the lipopolysaccharides, exopolysaccharides, and flagellin glycoprotein appeared unaffected. The NGRDeltarkpMNO mutant was symbiotically defective, showing reduced nodulation efficiency on several legumes. K-antigen production was found to decline after rhizobia were exposed to plant flavonoids, and the decrease coincided with induction of a symbiotically active (bacteroid-specific) rhamnan-LPS, suggesting an exchange of SPS occurs during bacterial differentiation in the developing nodule.

Structural characterization of a flavonoid-inducible Pseudomonas aeruginosa A-band-like O antigen of Rhizobium sp. strain NGR234, required for the formation of nitrogen-fixing nodules

Rhizobium (Sinorhizobium) sp. strain NGR234 contains three replicons, the smallest of which (pNGR234a) carries most symbiotic genes, including those required for nodulation and lipo-chito-oligosaccharide (Nod factor) biosynthesis. Activation of nod gene expression depends on plant-derived flavonoids, NodD transcriptional activators, and nod box promoter elements. Nod boxes NB6 and NB7 delimit six different types of genes, one of which (fixF) is essential for the formation of effective nodules on Vigna unguiculata. In vegetative culture, wild-type NGR234 produces a distinct, flavonoid-inducible lipopolysaccharide (LPS) that is not produced by the mutant (NGRomegafixF); this LPS is also found in nitrogen-fixing bacteroids isolated from V. unguiculata infected with NGR234. Electron microscopy showed that peribacteroid membrane formation is perturbed in nodule cells infected by the fixF mutant. LPSs were purified from free-living NGR234 cultured in the presence of apigenin. Structural analyses showed that the polysaccharide portions of these LPSs are specialized, rhamnose-containing O antigens attached to a modified core-lipid A carrier. The primary sequence of the O antigen is [-3)-alpha-L-Rhap-(1,3)-alpha-L-Rhap-(1,2)-alpha-L-Rhap-(1-]n, and the LPS core region lacks the acidic sugars commonly associated with the antigenic outer core of LPS from noninduced cells. This rhamnan O antigen, which is absent from noninduced cells, has the same primary sequence as the A-band O antigen of Pseudomonas aeruginosa, except that it is composed of L-rhamnose rather than the D-rhamnose characteristic of the latter. It is noteworthy that A-band LPS is selectively maintained on the P. aeruginosa cell surface during chronic cystic fibrosis lung infection, where it is associated with an increased duration of infection.

The structure of the O-polysaccharide of the Pseudoalteromonas rubra ATCC 29570T lipopolysaccharide containing a keto sugar

The structure of the phenol-soluble polysaccharide from Pseudoalteromonas rubra type strain ATCC 29570T has been elucidated using 1H and 13C NMR spectroscopy, including 2D COSY, TOCSY, gNOESY, ROESY, 1H,13C gHMQC and gHMBC experiments. It is concluded that the trisaccharide repeating unit of the polysaccharide has the following structure: [carbohydrate structure: see text] where Sug is 2-acetamido-2,6-dideoxy-D-xylo-hexos-4-ulose, Am is acetimidoyl and Acyl is a malic acid residue, which is O-acetylated in approximately 70% of the units.

2-O-methylation of fucosyl residues of a rhizobial lipopolysaccharide is increased in response to host exudate and is eliminated in a symbiotically defective mutant

When Rhizobium etli CE3 was grown in the presence of Phaseolus vulgaris seed extracts containing anthocyanins, its lipopolysaccharide (LPS) sugar composition was changed in two ways: greatly decreased content of what is normally the terminal residue of the LPS, di-O-methylfucose, and a doubling of the 2-O-methylation of other fucose residues in the LPS O antigen. R. etli strain CE395 was isolated after Tn5 mutagenesis of strain CE3 by screening for mutant colonies that did not change antigenically in the presence of seed extract. The LPS of this strain completely lacked 2-O-methylfucose, regardless of whether anthocyanins were present during growth. The mutant gave only pseudonodules in association with P. vulgaris. Interpretation of this phenotype was complicated by a second LPS defect exhibited by the mutant: its LPS population had only about 50% of the normal amount of O-antigen-containing LPS (LPS I). The latter defect could be suppressed genetically such that the resulting strain (CE395 alpha 395) synthesized the normal amount of an LPS I that still lacked 2-O-methylfucose residues. Strain CE395 alpha 395 did not elicit pseudonodules but resulted in significantly slower nodule development, fewer nodules, and less nitrogenase activity than lps(+) strains. The relative symbiotic deficiency was more severe when seeds were planted and inoculated with bacteria before they germinated. These results support previous conclusions that the relative amount of LPS I on the bacterial surface is crucial in symbiosis, but LPS structural features, such as 2-O-methylation of fucose, also may facilitate symbiotic interactions.