Genetic and molecular analyses of Escherichia coli K1 antigen genes

How bacteria utilize sialic acid during interactions with the host: snip, snatch, dispatch, match and attach

N -glycolylneuraminic acid (Neu5Gc), and its precursor N-acetylneuraminic acid (Neu5Ac), commonly referred to as sialic acids, are two of the most common glycans found in mammals. Humans carry a mutation in the enzyme that converts Neu5Ac into Neu5Gc, and as such, expression of Neu5Ac can be thought of as a 'human specific' trait. Bacteria can utilize sialic acids as a carbon and energy source and have evolved multiple ways to take up sialic acids. In order to generate free sialic acid, many bacteria produce sialidases that cleave sialic acid residues from complex glycan structures. In addition, sialidases allow escape from innate immune mechanisms, and can synergize with other virulence factors such as toxins. Human-adapted pathogens have evolved a preference for Neu5Ac, with many bacterial adhesins, and major classes of toxin, specifically recognizing Neu5Ac containing glycans as receptors. The preference of human-adapted pathogens for Neu5Ac also occurs during biosynthesis of surface structures such as lipo-oligosaccharide (LOS), lipo-polysaccharide (LPS) and polysaccharide capsules, subverting the human host immune system by mimicking the host. This review aims to provide an update on the advances made in understanding the role of sialic acid in bacteria-host interactions made in the last 5-10 years, and put these findings into context by highlighting key historical discoveries. We provide a particular focus on 'molecular mimicry' and incorporation of sialic acid onto the bacterial outer-surface, and the role of sialic acid as a receptor for bacterial adhesins and toxins.

Sequence analysis of nonulosonic acid biosynthetic gene clusters in Vibrionaceae and Moritella viscosa

Nonulosonic acid (NulO) biosynthesis in bacteria is directed by nab gene clusters that can lead to neuraminic, legionaminic or pseudaminic acids. Analysis of the gene content from a set mainly composed of Aliivibrio salmonicida and Moritella viscosa strains reveals the existence of several unique nab clusters, for which the NulO products were predicted. This prediction method can be used to guide tandem mass spectrometry studies in order to verify the products of previously undescribed nab clusters and identify new members of the NulOs family.

Role of capsule and O antigen in the virulence of uropathogenic Escherichia coli

Urinary tract infection (UTI) is one of the most common bacterial infections in humans, with uropathogenic Escherichia coli (UPEC) the leading causative organism. UPEC has a number of virulence factors that enable it to overcome host defenses within the urinary tract and establish infection. The O antigen and the capsular polysaccharide are two such factors that provide a survival advantage to UPEC. Here we describe the application of the rpsL counter selection system to construct capsule (kpsD) and O antigen (waaL) mutants and complemented derivatives of three reference UPEC strains: CFT073 (O6:K2:H1), RS218 (O18:K1:H7) and 1177 (O1:K1:H7). We observed that while the O1, O6 and O18 antigens were required for survival in human serum, the role of the capsule was less clear and linked to O antigen type. In contrast, both the K1 and K2 capsular antigens provided a survival advantage to UPEC in whole blood. In the mouse urinary tract, mutation of the O6 antigen significantly attenuated CFT073 bladder colonization. Overall, this study contrasts the role of capsule and O antigen in three common UPEC serotypes using defined mutant and complemented strains. The combined mutagenesis-complementation strategy can be applied to study other virulence factors with complex functions both in vitro and in vivo.

A new sialidase mechanism: bacteriophage K1F endo-sialidase is an inverting glycosidase

Bacteriophages specific for Escherichia coli K1 express a tailspike protein that degrades the polysialic acid coat of E. coli K1 that is essential for bacteriophage infection. This enzyme is specific for polysialic acid and is a member of a family of endo-sialidases. This family is unusual because all other previously reported sialidases outside of this family are exo- or trans-sialidases. The recently determined structure of an endo-sialidase derived from bacteriophage K1F (endoNF) revealed an active site that lacks a number of the residues that are conserved in other sialidases, implying a new, endo-sialidase-specific catalytic mechanism. Using synthetic trifluoromethylumbelliferyl oligosialoside substrates, kinetic parameters for hydrolysis at a single cleavage site were determined. Measurement of kcat/Km at a series of pH values revealed a dependence on a single protonated group of pKa 5. Mutation of a putative active site acidic residue, E581A, resulted in complete loss of sialidase activity. Direct 1H NMR analysis of the hydrolysis of trifluoromethylumbelliferyl sialotrioside revealed that endoNF is an inverting sialidase. All other wild type sialidases previously reported are retaining glycosidases, implying a new mechanism of sialidase action specific to this family of endo-sialidases.

Cloning, expression, and characterization of sialic acid synthases

The most commonly occurring sialic acid, N-acetylneuraminic acid, is the repeating unit in polysialic acid chain of human neuronal cell adhesion molecule as well as in capsular polysialic acid of neuroinvasive bacteria, Escherichia coli K1 and Neisseria meningitidis. Sialic acid synthesis and polymerization occur in slightly different pathways in animals and bacteria. N-Acetylneuraminic acid (NeuNAc) is synthesized by the condensation of phosphoenolpyruvate and N-acetylmannosamine by NeuNAc synthase in bacteria. The mammalian homologue N-acetylneuraminic acid-9-phosphate (NeuNAc-9-P) synthase uses N-acetylmannosamine-6-phosphate in the condensation reaction to produce NeuNAc-9-P. Both subfamilies of sialic acid synthases possess N-terminal triosephosphate isomerase barrel domain and C-terminal antifreeze protein domain. We report cloning of the genes, expression, purification, and characterization of human NeuNAc-9-P synthase and N. meningitidis NeuNAc synthase. Stability of the purified enzymes and effects of pH and temperature on their activities were evaluated. Enzyme kinetics and preliminary mutagenesis experiments reveal the importance of C-terminal antifreeze protein domain and a conserved cysteine residue for the enzyme activities.

The NeuC protein of Escherichia coli K1 is a UDP N-acetylglucosamine 2-epimerase

The K1 capsule is an essential virulence determinant of Escherichia coli strains that cause meningitis in neonates. Biosynthesis and transport of the capsule, an alpha-2,8-linked polymer of sialic acid, are encoded by the 17-kb kps gene cluster. We deleted neuC, a K1 gene implicated in sialic acid synthesis, from the chromosome of EV36, a K-12-K1 hybrid, by allelic exchange. Exogenously added sialic acid restored capsule expression to the deletion strain (DeltaneuC), confirming that NeuC is necessary for sialic acid synthesis. The deduced amino acid sequence of NeuC showed similarities to those of UDP-N-acetylglucosamine (GlcNAc) 2-epimerases from both prokaryotes and eukaryotes. The NeuC homologue from serotype III Streptococcus agalactiae complements DeltaneuC. We cloned the neuC gene into an intein expression vector to facilitate purification. We demonstrated by paper chromatography that the purified neuC gene product catalyzed the formation of [2-(14)C]acetamidoglucal and [N-(14)C]acetylmannosamine (ManNAc) from UDP-[(14)C]GlcNAc. The formation of reaction intermediate 2-acetamidoglucal with the concomitant release of UDP was confirmed by proton and phosphorus nuclear magnetic resonance spectroscopy. NeuC could not use GlcNAc as a substrate. These data suggest that neuC encodes an epimerase that catalyzes the formation of ManNAc from UDP-GlcNAc via a 2-acetamidoglucal intermediate. The unexpected release of the glucal intermediate and the extremely low rate of ManNAc formation likely were a result of the in vitro assay conditions, in which a key regulatory molecule or protein was absent.

Association of Actinobacillus pleuropneumoniae capsular polysaccharide with virulence in pigs

The capsular polysaccharide (CP) of Actinobacillus pleuropneumoniae is required for virulence of the bacteria in swine. However, a molecular investigation of whether the type or quantity of CP affects A. pleuropneumoniae virulence has not been reported. To initiate this investigation, a DNA region downstream of conserved genes required for CP export in A. pleuropneumoniae serotype 1 was cloned and sequenced. Three open reading frames, designated cps1A, cps1B, and cps1C, were identified that had amino acid homology to bacterial carbohydrate biosynthesis genes. A kanamycin resistance cassette (Kan(r)) was inserted into a 750-bp deletion spanning cps1AB or into a 512-bp deletion in cps1B only, and the constructs were cloned in a suicide vector. The Kan(r) gene was then transferred into the chromosome of strain 4074 by homologous recombination to produce strain 4074Deltacps1N and strain 4074Deltacps1B, respectively. Strain 4074Deltacps1N produced no detectable CP, but strain 4074Deltacps1B made 15% of the serotype 1 CP made by the parent strain, 4074, as determined by enzyme-linked immunosorbent assay and precipitation of free CP. The cps1ABC genes of strain 4074 and the cps5ABC and cps5ABCDE genes of serotype 5a strain J45 were cloned into the shuttle vector pLS88 and electroporated into 4074Deltacps1N to produce 4074Deltacps1N(pABcps101), 4074Deltacps1N(pJMLcps53), and 4074Deltacps1N(pABcps55), respectively. Strain 4074Deltacps1N(pABcps101) produced about 33% of the serotype 1 CP produced by strain 4074. Strains 4074Deltacps1N(pJMLcps53) and 4074Deltacps1N(pABcps55) produced serotype 5a CP in similar quantity or in fourfold excess, respectively, to that produced by strain 4074. With intratracheal challenge in pigs at similar dosages, the order of virulence of strains producing serotype 1 CP (assessed by mortality, lung consolidation, hemorrhage, and fibrinous pleuritis) was the following: strain 4074 > strain 4074Deltacps1N(pABcps101) > or = strain 4074Deltacps1N > strain 4074Deltacps1B. Strain 4074Deltacps1N(pJMLcps53) was less virulent than strain 4074Deltacps1N(pABcps55). However, both strains produced serotype 5a CP in similar or greater quantities than was observed for production of serotype 1 CP by the parent strain, 4074, but were less virulent than the parent strain. Therefore, the amount of serotype 1 or 5a CP produced by isogenic strains of A. pleuropneumoniae correlated with the virulence of the bacteria in pigs. However, virulence was also influenced by the type of CP produced or by its mechanism of expression.

Multiple N-acetyl neuraminic acid synthetase (neuB) genes in Campylobacter jejuni: identification and characterization of the gene involved in sialylation of lipo-oligosaccharide

N-acetyl neuraminic acid (NANA) is a common constituent of Campylobacter jejuni lipo-oligosaccharide (LOS). Such structures often mimic human gangliosides and are thought to be involved in the triggering of Guillain-Barré syndrome (GBS) and Miller-Fisher syndrome (MFS) following C. jejuni infection. Analysis of the C. jejuni NCTC 11168 genome sequence identified three putative NANA synthetase genes termed neuB1, neuB2 and neuB3. The NANA synthetase activity of all three C. jejuni neuB gene products was confirmed by complementation experiments in an Escherichia coli neuB-deficient strain. Isogenic mutants were created in all three neuB genes, and for one such mutant (neuB1) LOS was shown to have increased mobility. C. jejuni NCTC 11168 wild-type LOS bound cholera toxin, indicating the presence of NANA in a LOS structure mimicking the ganglioside GM1. This property was lost in the neuB1 mutant. Gas chromatography-mass spectrometry and fast atom bombardment-mass spectrometry analysis of LOS from wild-type and the neuB1 mutant strain demonstrated the lack of NANA in the latter. Expression of the neuB1 gene in E. coli confirmed that NeuB1 was capable of in vitro NANA biosynthesis through condensation of N-acetyl-D-mannosamine and phosphoenolpyruvate. Southern analysis demonstrated that the neuB1 gene was confined to strains of C. jejuni with LOS containing a single NANA residue. Mutagenesis of neuB2 and neuB3 did not affect LOS, but neuB3 mutants were aflagellate and non-motile. No phenotype was evident for neuB2 mutants in strain NCTC 11168, but for strain G1 the flagellin protein from the neuB2 mutant showed an apparent reduction in molecular size relative to the wild type. Thus, the neuB genes of C. jejuni appear to be involved in the biosynthesis of at least two distinct surface structures: LOS and flagella.

Cloning and mutagenesis of a serotype-specific DNA region involved in encapsulation and virulence of Actinobacillus pleuropneumoniae serotype 5a: concomitant expression of serotype 5a and 1 capsular polysaccharides in recombinant A. pleuropneumoniae serotype 1

A DNA region involved in Actinobacillus pleuropneumoniae serotype 5 capsular polysaccharide (CP) biosynthesis was identified and characterized by using a probe specific for the cpxD gene involved in CP export. The adjacent serotype 5-specific CP biosynthesis region was cloned from a 5.8-kb BamHI fragment and an 8.0-kb EcoRI fragment of strain J45 genomic DNA. DNA sequence analysis demonstrated that this region contained four complete open reading frames, cps5A, cps5B, cps5C, and cps5D. Cps5A, Cps5B, and Cps5C showed low homology with several bacterial glycosyltransferases involved in the biosynthesis of lipopolysaccharide or CP. However, Cps5D had high homology with KdsA proteins (3-deoxy-D-manno-2-octulosonic acid 8-phosphate synthetase) from other gram-negative bacteria. The G+C content of cps5ABC was substantially lower (28%) than that of cps5D and the rest of the A. pleuropneumoniae chromosome (42%). A 2.1-kb deletion spanning the cloned cps5ABC open reading frames was constructed and transferred into the J45 chromosome by homologous recombination with a kanamycin resistance cassette to produce mutant J45-100. Multiplex PCR confirmed the deletion in this region of J45-100 DNA. J45-100 did not produce intracellular or extracellular CP, indicating that cps5A, cps5B, and/or cps5C were involved in CP biosynthesis. However, biosynthesis of the Apx toxins, lipopolysaccharide, and membrane proteins was unaffected by the mutation. Besides lack of CP biosynthesis, and in contrast to J45, J45-100 grew faster, was sensitive to killing in precolostral calf serum, and was avirulent in pigs at an intratracheal challenge dose three times the 50% lethal dose (LD50) of strain J45. At six times the J45 LD50, J45-100 caused mild to moderate lung lesions but not death. Electroporation of cps5ABC into A. pleuropneumoniae serotype 1 strain 4074 generated strain 4074(pJMLCPS5), which expressed both serotype 1 and serotype 5 CP. However, serotype 1 capsule expression was diminished in 4074(pJMLCPS5) in comparison to 4074. The recombinant strain produced significantly less total CP (serotypes 1 and 5 CP combined) in log phase (P = 0.0012) but significantly more total CP in late stationary phase than 4074 (P < 0.0001). In addition, strain 4074(pJMLCPS5) caused less mortality and bacteremia in pigs and mice following respiratory challenge than strain 4074, indicating that virulence was affected by diminished capsule production. These results emphasize the importance of CP in the serum resistance and virulence of A. pleuropneumoniae.

Transcriptional organization and regulation of expression of region 1 of the Escherichia coli K5 capsule gene cluster

The transcriptional organization and regulation of region 1 expression of the Escherichia coli K5 capsule gene cluster were studied. Region 1 was transcribed as an 8.0-kb polycistronic mRNA which was processed to form a separate 1.3-kb transcript encoding the 3'-most gene kpsS. Transcription of region 1 of the E. coli K5 capsule gene cluster was directed from a single promoter 225 bp upstream of a previously unidentified gene, kpsF. The promoter had -35 and -10 consensus sequences typical of an E. coli sigma 70 promoter, with no similarities to binding sites for other sigma factors. Two integration host factor (IHF) binding site consensus sequences were identified 110 bp upstream and 130 bp downstream of the transcription start site. In addition, two AT-rich regions separated by 16 bp identified upstream of the region 1 promoter were conserved upstream of the region 3 promoter. The kpsF gene was 98.8% identical with the kpsF gene identified in the E. coli K1 antigen gene cluster and confirms that the kpsF gene is conserved among group II capsule gene clusters. An intragenic Rho-dependent transcriptional terminator was discovered within the kpsF gene. No essential role for KpsF in the expression of the K5 antigen could be established. The temperature regulation of region 1 expression was at the level of transcription, with no transcription detectable in cells grown at 18 degrees C. Mutations in regulatory genes known to control temperature-dependent expression of a number of virulence genes had no effect on the temperature regulation of region 1 expression. Likewise, RfaH, which is known to regulate expression of E. coli group II capsules had no effect on the expression of region 1. Mutations in the himA and himD genes which encode the subunits of the IHF led to a fivefold reduction in the expression of KpsE at 37 degrees C, confirming a regulatory role for IHF in the expression of region 1 genes.

The biochemistry and genetics of capsular polysaccharide production in bacteria

Bacterial polysaccharides are usually associated with the outer surface of the bacterium. They can form an amorphous layer of extracellular polysaccharide (EPS) surrounding the cell that may be further organized into a distinct structure termed a capsule. Additional polysaccharide molecules such as lipopolysaccharide (LPS) or lipooligosaccharide (LOS) may also decorate the cell surface. Polysaccharide capsules may mediate a number of biological processes, including invasive infections of human beings. Discussed here are the genetics and biochemistry of selected bacterial capsular polysaccharides and the basis of capsule diversity but not the genetics and biochemistry of LPS biosynthesis (for reviews see 100, 140).

Expression and characterization of UDPGlc dehydrogenase (KfiD), which is encoded in the type-specific region 2 of the Escherichia coli K5 capsule genes

Region 2 of the Escherichia coli K5 capsule gene cluster contains four genes (kfiA through -D) which encode proteins involved in the synthesis of the K5 polysaccharide. A DNA fragment containing kfiD was amplified by PCR and cloned into the gene fusion vector pGEX-2T to generate a GST-KfiD fusion protein. The fusion protein was isolated from the cytoplasms of IPTG (isopropyl-beta-D-thiogalactopyranoside)-induced recombinant bacteria by affinity chromatography and cleaved with thrombin. The N-terminal amino acid sequence of the cleavage product KfiD' corresponded to the predicted amino acid sequence of KfiD with an N-terminal glycyl-seryl extension from the cleavage site of the fusion protein. Anti-KfiD antibodies obtained with KfiD' were used to isolate the intact KfiD protein from the cytoplasms of E. coli organisms overexpressing the kfiD gene. The fusion protein, its cleavage product (KfiD'), and overexpressed KfiD converted UDPGlc to UDPGlcA. The KfiD protein could thus be characterized as a UDPglucose dehydrogenase.

Cloning and analysis of gene clusters for production of the Escherichia coli K10 and K54 antigens: identification of a new group of serA-linked capsule gene clusters

The polysaccharide capsules of Escherichia coli have been classified into three groups: I, II, and I/II. The third group, I/II, has been poorly studied and possesses characteristics of both group I and group II capsules. In this report, we describe the cloning of the K10 and K54 capsule gene clusters, two representatives of group I/II capsules. Probes taken from DNA flanking regions 1 and 3 of the group II capsule clusters hybridized to these group I/II clones, confirming that the group I/II capsule genes are flanked by the same DNA and are therefore located in the same serA-linked region of the chromosome as group II capsule gene clusters. Southern blotting showed that homologous sequences were present in both the K10 and K54 capsule gene clusters and in other group I/II strains. No homology was detected between these sequences and the chromosomal DNA of either a group I or a group II strain. Likewise, no homology was detected to the chromosomal DNA of either a K11 or K19 strain, both of which had previously been classified as group I/II strains. In the K10 and K54 capsule gene clusters, these conserved sequences flanked a serotype-specific region in a manner analogous to group II capsule gene organization. Complementation of mutations in the kpsE, kpsD, and kpsC genes in region 1 of the K5 capsule gene cluster by subclones of the K10 and K54 capsule gene clusters indicated that certain stages in the export of group II and I/II capsules may be conserved. In the light of the findings presented here, we suggest that the group I/II capsule gene clusters are sufficiently different from group II capsule gene clusters to justify their renaming as group III.

The revival of interest in mechanisms of bacterial pathogenicity

1. After a long barren period, the study of bacterial pathogenicity is now one of the most popular subjects in microbiology. This is because bacterial diseases remain a major problem in public health despite the advent of antibiotics, and the subject is a fertile field for the application of genetics and molecular biology. 2. Pathogenicity is a multifactorial property. The biological requirements are abilities to: infect mucous surfaces; enter the host through those surfaces; multiply in the environment of the host; interfere with host defences; and damage the host. Each requirement has many facets all of which can be accomplished by a variety of processes. 3. The molecular determinants of the five requirements for pathogenicity can be identified and the relation between their structure and function obtained by a seven step procedure. Genetic manipulation and observations on organisms grown in vivo play major roles in this procedure. Other vital aspects are the availability of good animal models and the design of biological tests for virulence determinants in vitro that are pertinent to the situation in vivo. 4. A survey of the state of studies on bacterial pathogenicity has highlighted some areas of immense erudition and exposed others that need more attention in the future. Research is often at the highest level of molecular biology for: adherence to and entry of epithelial cells; interference with humoral and phagocytic defences; toxins; and direct induction of cytokines and inflammation. The major gaps are: the determinants of competition with commensals on mucous surfaces; spread into deeper tissues; the host supplied nutrients and metabolism underlying growth rate in vivo; the determinants of interference with the immune response in important chronic diseases and carrier states; the determinants of immunopathological reactions that cause damage in chronic disease; and the determinants of change from carrier to invasive state. Areas that are receiving some attention but are worthy of more are: moving through mucus to gain access to mucous surfaces; opportunistic infections; the determinants of mixed infections; and the determinants of host and tissue susceptibility to infection. 5. Current interest in the regulation of production of virulence determinants and the influence on it of environmental factors has raised speculation on the role these factors play in vivo. However, it has not yet provided much information on the host factors specifically involved in particular bacterial infections. The individualistic concept of community, as a relative latecomer to discussions of animal community, is sometimes misconstrued as holding that communities are random assemblages of organisms without biotic interactions among species. Nevertheless, it has increasingly been accepted as supported by studies of diverse taxa and habitats. However, many other ecologists continue to argue for integrated, biotically controlled and evolved communities.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization and localization of the KpsE protein of Escherichia coli K5, which is involved in polysaccharide export

In Escherichia coli with group II capsules, the synthesis and cellular expression of capsular polysaccharide are encoded by the kps gene cluster. This gene cluster is composed of three regions. The central region 2 encodes proteins involved in polysaccharide synthesis, and the flanking regions 1 and 3 direct the translocation of the finished polysaccharide across the cytoplasmic membrane and its surface expression. The kps genes of the K5 polysaccharide, which is a group II capsular polysaccharide, have been cloned and sequenced. Region 1 contains the kpsE, -D, -U, -C, and -S genes. In this communication we describe the KpsE protein, the product of the kpsE gene. A truncated kpsE gene was fused with a truncated beta-galactosidase gene to generate a fusion protein containing the first 375 amino acids of beta-galactosidase and amino acids 67 to 382 of KpsE (KpsE'). This fusion protein was isolated and cleaved with factor Xa, and the purified KpsE' was used to immunize rabbits. Intact KpsE was extracted from the membranes of a KpsE-overexpressing recombinant strain with octyl-beta-glucoside. It was purified by affinity chromatography with immobilized anti-KpsE antibodies. Cytofluorometric analysis using the anti-KpsE antibodies with whole cells and spheroplasts, as well as sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting (immunoblotting) of proteins from spheroplasts and membranes before and after treatment with proteinase K, indicated that the KpsE protein is associated with the cytoplasmic membrane and has an exposed periplasmic domain. By TnphoA mutagenesis and by constructing beta-lactamase fusions to the KpseE protein, it was possible to determine the topology of the KpsE protein within the cytoplasmic membrane.

Nucleotide sequence and genetic analysis of the neuD and neuB genes in region 2 of the polysialic acid gene cluster of Escherichia coli K1

The K1 capsular polysaccharide, a polymer of sialic acid, is an important virulence determinant of extraintestinal pathogenic Escherichia coli. The genes responsible for the synthesis and expression of the polysialic acid capsule of E. coli K1 are located on the 17-kb kps gene cluster, which is functionally divided into three regions. Central region 2 encodes proteins necessary for the synthesis, activation, and polymerization of sialic acid, while flanking regions 1 and 3 are involved in polymer transport to the cell surface. In this study, we identified two genes at the proximal end of region 2, neuD and neuB, which encode proteins with predicted sizes of 22.7 and 38.7 kDa, respectively. Several observations suggest that the neuB gene encodes sialic acid synthase. EV24, a neuB chromosomal mutant that expresses a capsule when provided exogenous sialic acid, could be complemented in trans by the cloned neuB gene. In addition, NeuB has significant sequence similarity to the product of the cpsB gene of Neisseria meningitidis group B, which is postulated to encode sialic acid synthase. We also present data indicating that neuD has an essential role in K1 polymer production. Cells harboring pSR426, which contains all of region 2 but lacks region 1 and 3 genes, produce an intracellular polymer. In contrast, no polymer accumulated in cells carrying a derivative of pSR426 lacking a functional neuD gene. Unlike strains with mutations in neuB, however, neuD mutants are not complemented by exogenous sialic acid, suggesting that NeuD is not involved in sialic acid synthesis. Additionally, cells harboring a mutation in neuD accumulated sialic acid and CMP-sialic acid. We also found no significant differences between the endogenous and exogenous sialyltransferase activities of a neuD mutant and the wild-type organism. NeuD shows significant similarity to a family of bacterial acetyltransferases, leading to the theory that NeuD is an acetyltransferase which may exert its influences through modification of other region 2 proteins.

Topological and mutational analysis of KpsM, the hydrophobic component of the ABC-transporter involved in the export of polysialic acid in Escherichia coli K1

The 17 kb kps gene cluster of Escherichia coli K1, which encodes the information required for synthesis, assembly and translocation of the polysialic acid capsule of E. coli K1, is divided into three functional regions. Region 3 contains two genes, kpsM and kpsT, essential for the transport of capsule polymer across the cytoplasmic membrane. The hydrophobicity profile of KpsM suggests that it is an integral membrane protein while KpsT contains a consensus ATP-binding site. KpsM and KpsT belong to the ATP-binding cassette (ABC) superfamily of membrane transporters. In this study, we investigate the topology of KpsM within the cytoplasmic membrane using beta-lactamase fusions and alkaline phosphatase sandwich fusions. Our analysis provides evidence for a model of KpsM having six membrane-spanning regions, with the N- and C-terminal domains facing the cytoplasm, and a short domain within the third periplasmic loop, which we refer to as the SV-SVI linker localizing in the membrane. Protease digestion studies are consistent with regions of KpsM exposed to the periplasmic space. In vivo cross-linking studies provide support for dimerization of KpsM within the cytoplasmic membrane. Linker-insertion and site-directed mutagenesis define the N-terminus, the first cytoplasmic loop, and the SV-SVI linker as regions that are important for the function of KpsM in K1 polymer transport.

Molecular analysis of the biosynthesis pathway of the alpha-2,8 polysialic acid capsule by Neisseria meningitidis serogroup B

The genes encoding all enzymes necessary for capsular polysaccharide biosynthesis in Neisseria meningitidis B are located on a 5 kb DNA fragment within the chromosomal cps gene cluster. Nucleotide sequence analysis revealed four open reading frames (ORFs), which can encode proteins with molecular masses of 41.4 kDa, 24.9 kDa, 38.3 kDa, and 54.4 kDa, respectively. These ORFs constitute a transcriptional unit as demonstrated by Northern blots. Primer extension analysis revealed that the transcriptional start site is preceded by a nucleotide sequence with homologies to the sigma 70 consensus promoter sequence of Escherichia coli. Functional analysis of the proteins encoded by the ORFs indicated that ORF2 encodes the CMP-NeuNAc synthetase, ORF3 encodes the NeuNAc condensing enzyme, and ORF4 encodes the alpha-2,8 polysialyltransferase. ORF1 encodes an enzyme, which provides a precursor molecule for synthesis of monomeric NeuNAc. In E. coli the subcloned ORFs 2-4 were able to synthesize a high-molecular-weight alpha-2,8 polysialic acid. In contrast, inactivation of ORF1 in the meningococcal genome resulted in a complete loss of capsule production. A regulatory enzyme, the CMP-NeuNAc hydrolase, which cleaves CMP-NeuNAc to CMP and NeuNAc, was not found as a part of the capsular polysaccharide biosynthesis gene operon or within the cps gene cluster.

Molecular cloning and analysis of genes for sialic acid synthesis in Neisseria meningitidis group B and purification of the meningococcal CMP-NeuNAc synthetase enzyme

The gene encoding for the CMP-NeuNAc synthetase enzyme of Neisseria meningitidis group B was cloned by complementation of a mutant of Escherichia coli defective for this enzyme. The gene (neuA) was isolated on a 4.1-kb fragment of meningococcal chromosomal DNA. Determination of the nucleotide sequence of this fragment revealed the presence of three genes, termed neuA, neuB, and neuC, organized in a single operon. The presence of a truncated ctrA gene at one end of the cloned DNA and a truncated gene encoding for the meningococcal sialyltransferase at the other confirmed that the cloned DNA corresponded to region A and part of region C of the meningococcal capsule gene cluster. The predicted amino acid sequence of the meningococcal NeuA protein was 57% homologous to that of NeuA, the CMP-NeuNAc synthetase encoded by E. coli K1. The predicted molecular mass of meningococcal NeuA protein was 24.8 kDa, which was 6 kDa larger than that formerly predicted (U. Edwards and M. Frosch, FEMS Microbiol. Lett. 96:161-166, 1992). Purification of the recombinant meningococcal NeuA protein together with determination of the N-terminal amino acid sequence confirmed that this 24.8-kDa protein was indeed the meningococcal CMP-NeuNAc synthetase. The predicted amino acid sequences of the two other encoded proteins were homologous to those of the NeuC and NeuB proteins of E. coli K1, two proteins involved in the synthesis of NeuNAc. These results indicate that common steps exist in the biosynthesis of NeuNAc in these two microorganisms.

Nucleotide sequence and mutational analysis of the gene encoding KpsD, a periplasmic protein involved in transport of polysialic acid in Escherichia coli K1

The 17-kb kps gene cluster encodes proteins necessary for the synthesis, assembly, and translocation of the polysialic acid capsule of Escherichia coli K1. We previously reported that one of these genes, kpsD, encodes a 60-kDa periplasmic protein that is involved in the translocation of the polymer to the cell surface. The nucleotide sequence of the 2.4-kb BamHI-PstI fragment accommodating the kpsD gene was determined. Sequence analysis showed an open reading frame for a 558-amino-acid protein with a typical N-terminal prokaryotic signal sequence corresponding to the first 20 amino acids. KpsD was overexpressed, partially purified, and used to prepare polyclonal antiserum. A chromosomal insertion mutation was generated in the kpsD gene and results in loss of surface expression of the polysialic acid capsule. Immunodiffusion analysis and electron microscopy indicated that polysaccharide accumulates in the periplasmic space of mutant cells. A wild-type copy of kpsD supplied in trans complemented the chromosomal mutation, restoring extracellular expression of the K1 capsule. However, a kpsD deletion derivative (kpsD delta C11), which results in production of a truncated KpsD protein lacking its 11 C-terminal amino acids, was nonfunctional. Western blot (immunoblot) data from cell fractions expressing KpsD delta C11 suggest that the truncated protein was inefficiently exported into the periplasm and localized primarily to the cytoplasmic membrane.

Cloning, sequencing, expression, and complementation analysis of the Escherichia coli K1 kps region 1 gene, kpsE, and identification of an upstream open reading frame encoding a protein with homology to GutQ

The kps locus for polysialic acid capsule expression in Escherichia coli K1 is composed of a central group of biosynthetic neu genes, designated region 2, flanked on either side by region 1 or region 3 kps genes with poorly defined functions. Chromosomal mutagenesis with MudJ and subsequent complementation analysis, maxicell and in vitro protein expression studies, and nucleotide sequencing identified the region 1 gene, kpsE, which encodes a 39-kDa polypeptide. Polarity of the kpsE::lacZ mutation suggests an operonic structure for region 1. KpsE is homologous to putative polysaccharide-translocation components previously identified in Haemophilus influenzae type b and Neisseria meningitidis group B. An open reading frame upstream of kpsE encodes a 35-kDa polypeptide with homology to GutQ, a putative ATP-binding protein of unknown function encoded by gutQ of the glucitol utilization operon. Whether expression of the gutQ homolog as the potential first gene of region 1 is required for polysialic acid synthesis or localization is presently unknown.

Synthesis of the K5 (group II) capsular polysaccharide in transport-deficient recombinant Escherichia coli

The genes directing the expression of group II capsules in Escherichia coli are organized into three regions. The central region 2 is type specific and thought to determine the synthesis of the respective polysaccharide, whilst the flanking regions 1 and 3 are common to all group II gene clusters and direct the surface expression of the capsular polysaccharide. In this communication we analyze the involvement of region 1 and 3 genes in the synthesis of the capsular KS polysaccharide. Recombinant E. coli strains harboring all KS specific region 2 genes and having various combinations of region 1 and 3 genes were studied using immunoelectron microscopy. Membranes from these bacteria were incubated with UDP[14C]GlcA and UDPGlcNAc in the absence or presence of KS polysaccharide as an exogenous acceptor. It was found that recombinant strains with only gene region 2 did not produce the K5 polysaccharide. Membranes of such strains did not synthesize the polymer and did not elongate K5 polysaccharide added as an exogenous acceptor. An involvement of genes from region 1 (notably kpsC and kpsS) and from region 3 (notably kpsT) in the K5 polysaccharide synthesis was apparent and is discussed.

Genes needed for the modification, polymerization, export, and processing of succinoglycan by Rhizobium meliloti: a model for succinoglycan biosynthesis

The major acidic exopolysaccharide of Rhizobium meliloti, termed succinoglycan, is required for nodule invasion and possibly nodule development. Succinoglycan is a polymer of octasaccharide subunits composed of one galactose residue, seven glucose residues, and acetyl, succinyl, and pyruvyl modifications, which is synthesized on an isoprenoid lipid carrier. A cluster of exo genes in R. meliloti are required for succinoglycan production, and the biosynthetic roles of their gene products have recently been determined (T.L. Reuber and G. C. Walker, Cell 74:269-280, 1993). Our sequencing of 16 kb of this cluster of exo genes and further genetic analysis of this region resulted in the discovery of several new exo genes and has allowed a correlation of the genetic map with the DNA sequence. In this paper we present the sequences of genes that are required for the addition of the succinyl and pyruvyl modifications to the lipid-linked intermediate and genes required for the polymerization of the octasaccharide subunits or the export of succinoglycan. In addition, on the basis of homologies to known proteins, we suggest that ExoN is a uridine diphosphoglucose pyrophosphorylase and that ExoK is a beta(1,3)-beta (1,4)-glucanase. We propose a model for succinoglycan biosynthesis and processing which assigns roles to the products of nineteen exo genes.

Expression of the capsular K5 polysaccharide of Escherichia coli: biochemical and electron microscopic analyses of mutants with defects in region 1 of the K5 gene cluster

The gene cluster of the capsular K5 polysaccharide, a representative of group II capsular antigens of Escherichia coli, has been cloned previously, and three regions responsible for polymerization and surface expression have been defined (I.S. Roberts, R. Mountford, R. Hodge, K. B. Jann, and G. J. Boulnois, J. Bacteriol. 170:1305-1330, 1988). Region 1 has now been sequenced, and five open reading frames (kpsEDUCS) have been defined (C. Pazzani, C. Rosenow, G. J. Boulnois, D. Bronner, K. Jann, and I. S. Roberts, J. Bacteriol. 175:5978-5983, 1993). In this study, we characterized region 1 mutants by immunoelectron microscopy, membrane-associated polymerization activity, cytoplasmic CMP-2-keto-3-deoxyoctonate (KDO) synthetase activity, and chemical analysis of their K5 polysaccharides. Certain mutations within region 1 not only effected polysaccharide transport (lack of region 1 gene products) but also impaired the polymerization capacity of the respective membranes, reflected in reduced amounts of polysaccharide but not in its chain length. KDO and phosphatidic acid (phosphatidyl-KDO) substitution was found with extracellular and periplasmic polysaccharide and not with cytoplasmic polysaccharide. This and the fact that the K5 polysaccharide is formed in a kpsU mutant (defective in capsule-specific K-CMP-KDO synthetase) showed that CMP-KDO is engaged not in initiation of polymerization but in translocation of the polysaccharide.

Molecular analysis of region 1 of the Escherichia coli K5 antigen gene cluster: a region encoding proteins involved in cell surface expression of capsular polysaccharide

The nucleotide sequence of region 1 of the K5 antigen gene cluster of Escherichia coli was determined. This region is postulated to encode functions which, at least in part, participate in translocation of polysaccharide across the periplasmic space and onto the cell surface. Analysis of the nucleotide sequence revealed five genes that encode proteins with predicted molecular masses of 75.7, 60.5, 44, 43, and 27 kDa. The 27-kDa protein was 70.7% homologous to the CMP-2-keto-3-deoxyoctulosonic acid synthetase enzyme encoded by the E. coli kdsB gene, indicating the presence of a structural gene for a similar enzyme within the region 1 operon. The 43-kDa protein was homologous to both the Ctrb and BexC proteins encoded by the Neisseria meningitidis and Haemophilus influenzae capsule gene clusters, respectively, indicating common stages in the expression of capsules in these gram-negative bacteria. However, no homology was detected between the 75.7, 60.5-, and 44-kDa proteins and any of the proteins so far described for the H. influenzae and N. meningitidis capsule gene clusters.

Identification of cpsD, a gene essential for type III capsule expression in group B streptococci

We showed previously that a mutant strain of group B Streptococcus (GBS) defective in capsule production was avirulent. This study describes the derivation of an unencapsulated mutant from a highly encapsulated wild-type strain of type III GBS, COH1, by transposon mutagenesis with Tn916 delta E. The mutant, COH1-13, was sensitive to phagocytic killing by human leukocytes in vitro and was relatively avirulent in a neonatal rat sepsis model compared with the wild-type strain. No capsular polysaccharide was evident in the cytoplasm or on the cell surface of the mutant strain. The Tn916 delta E insertion site in COH1-13 was mapped to the same chromosomal location as the Tn916 insertion site in the unencapsulated type III mutant COH31-15 reported previously. Nucleotide sequencing of DNA flanking the insertion site in COH1-13 revealed an open reading frame, designated cpsD, with significant homology to the rfbP gene of Salmonella typhimurium. RfbP encodes a galactosyl transferase enzyme that catalyses the transfer of galactose to undecaprenol phosphate, the initial step in O-polysaccharide synthesis. A particulate fraction of a lysate of wild-type strain GBS COH1 mediated the transfer of galactose from UDP-galactose to an endogenous acceptor. The galactose-acceptor complex partitioned into organic solvents, suggesting it is lipid in nature or membrane-associated. Galactosyl transferase activity was significantly reduced in the unencapsulated mutant strain COH1-13. These results, together with the similarity in deduced amino acid sequence between cpsD and rfbP suggest that cpsD encodes a galactosyl transferase essential for assembly of the GBS type III capsular polysaccharide.

The NodL and NodJ proteins from Rhizobium and Bradyrhizobium strains are similar to capsular polysaccharide secretion proteins from gram-negative bacteria

The NodL and NodJ nodulation proteins have been described in different Rhizobium and Bradyrhizobium species. The nodLJ genes belong to the nod regulon. Other genes from this regulon are involved in the biosynthesis and modification of lipo-oligosaccharide molecule(s) which are morphogenic signals when acting on legume roots. It has been proposed that the NodL and NodJ proteins belong to a bacterial inner membrane transport system of small molecules. Nucleotide sequencing of Mudll PR13 insertions in the nodulation region of the symbiotic plasmid from a Rhizobium leguminosarum bv. phaseoli strain CE3 has revealed the presence of nodL and nodJ-related sequences downstream of nodC. Computer nucleotide sequence analysis of the entire NodL and NodJ sequences from R. leguminosarum bv. viciae and Bradyrhizobium japonicum strains show that both proteins are similar to the KpsT and KpsM proteins from Escherichia coli K1 and K5 strains, to the BexB and BexA proteins from Haemophilis influenzae and to the CtrD and CtrC proteins from Neisseria meningitidis, respectively. Except for the NodL and NodJ proteins, all of them have been involved in the mechanism of secretion of polysaccharides in each of their harbouring species. On the basis of the similarity found, we propose that the NodL and the NodJ proteins could be involved in the export of a lipo-oligosaccharide.

Genome diversity at the serA-linked capsule locus in Escherichia coli

Individual isolates of Escherichia coli synthesize one of more than 70 chemically distinct polysaccharides which form the capsule. In this article we review the genetics of capsule production in E. coli and highlight what this is beginning to reveal in terms of the genetic basis of the structural diversity of polysaccharides. The serA-linked capsule locus can take three different allelic forms. Two of these are associated with capsule genes and are themselves internally variant, whilst the third form has not so far been implicated in capsule biogenesis. Thus the serA-linked region of the E. coli genome is strikingly polymorphic.

Identification of a genetic locus essential for capsule sialylation in type III group B streptococci

The type III capsular polysaccharide of group B streptococci (GBS) consists of a linear backbone with short side chains ending in residues of N-acetylneuraminic acid, or sialic acid. The presence of sialic acid on the surface of the organism inhibits activation of the alternative pathway of complement and is thought to be an important element in the virulence function of the capsule. We showed previously that a mutant strain of GBS that expressed a sialic acid-deficient, or asialo, form of the type III polysaccharide was avirulent, supporting a virulence function for capsular sialic acid. We now report the derivation of an asialo capsule mutant from a highly encapsulated wild-type strain of type III GBS, strain COH1, by insertional mutagenesis with transposon Tn916 delta E. In contrast to the wild-type strain, the asialo mutant strain COH1-11 was sensitive to phagocytic killing by human leukocytes in vitro and was relatively avirulent in a neonatal rat model of GBS infection. The asialo mutant accumulated free intracellular sialic acid, suggesting a defect subsequent to sialic acid synthesis in the biosynthetic pathway leading to capsule sialylation. The specific biosynthetic defect in mutant strain COH1-11 was found to be in the activation of free sialic acid to CMP-sialic acid: CMP-sialic acid synthetase activity was present in the wild-type strain COH1 but was not detected in the asialo mutant strain COH1-11. One of the two transposon insertions in the asialo mutant COH1-11 mapped to the same chromosomal location as one of the two Tn916 insertions in the previously reported asialo mutant COH31-21, identifying this site as a genetic locus necessary for expression of CMP-sialic acid synthetase activity. These studies demonstrate that the enzymatic synthesis of CMP-sialic acid by GBS is an essential step in sialylation of the type III capsular polysaccharide.

Sequence and expression of the Escherichia coli K1 neuC gene product

The nucleotide sequence of the neuC gene of the Escherichia coli K1 capsule gene cluster encodes a protein with a predicted molecular weight of 44,210 containing 391 amino acids. A chimeric protein with beta-galactosidase fused to the carboxy terminus of the neuC gene product (P7) was constructed and purified. Its amino-terminal sequence confirmed the prediction from the nucleotide sequence that the neuC gene overlaps the distal end of the neuA gene by a single base pair. Both the neuA and neuC genes are coexpressed under the control of a single upstream T7 or tac promoter, suggesting that neuA and neuC are part of an operon.

Identification of two genes, kpsM and kpsT, in region 3 of the polysialic acid gene cluster of Escherichia coli K1

The polysialic acid capsule of Escherichia coli K1, a causative agent of neonatal septicemia and meningitis, is an essential virulence determinant. The 17-kb kps gene cluster, which is divided into three functionally distinct regions, encodes proteins necessary for polymer synthesis and expression at the cell surface. The central region, 2, encodes products required for synthesis, activation, and polymerization of sialic acid, while flanking regions, 1 and 3, are thought to be involved in polymer assembly and transport. In this study, we identified two genes in region 3, kpsM and kpsT, which encode proteins with predicted sizes of 29.6 and 24.9 kDa, respectively. The hydrophobicity profile of KpsM suggests that it is an integral membrane protein, while KpsT contains a consensus ATP-binding domain. KpsM and KpsT belong to a family of prokaryotic and eukaryotic proteins involved with a variety of biological processes, including membrane transport. A previously described kpsT chromosomal mutant that accumulates intracellular polysialic acid was characterized and could be complemented in trans. Results of site-directed mutagenesis of the putative ATP-binding domain of KpsT are consistent with the view that KpsT is a nucleotide-binding protein. KpsM and KpsT have significant similarity to BexB and BexA, two proteins that are essential for polysaccharide capsule expression in Haemophilus influenzae type b. We propose that KpsM and KpsT constitute a system for transport of polysialic acid across the cytoplasmic membrane.

Biosynthesis of the Escherichia coli K5 polysaccharide, a representative of group II capsular polysaccharides: polymerization in vitro and characterization of the product

Biosynthesis of the capsular K5 polysaccharide of Escherichia coli, which has the structure 4)-beta GlcA-1,4-alpha GlcNAc-(1, was studied with membrane preparations from an E. coli K5 wild-type strain and from a recombinant K-12 strain expressing the K5 capsule. Polymerization occurs at the inner face of the cytoplasmic membrane without the participation of lipid-linked oligosaccharides. The serological K5 specificity of the in vitro product was determined with a K5-specific monoclonal antibody in an antigen-binding assay. The K5 polysaccharide, as obtained from the membranes after an in vitro incubation, has 2-keto-3-deoxyoctulosonic acid as the reducing sugar, which indicates that the polysaccharide grows by chain elongation at the nonreducing end.

Biosynthesis of Klebsiella K2 capsular polysaccharide in Escherichia coli HB101 requires the functions of rmpA and the chromosomal cps gene cluster of the virulent strain Klebsiella pneumoniae Chedid (O1:K2)

The genes determining the biosynthesis of type 2 (K2) capsular polysaccharide [3----beta Glc1,4----beta Man(1,3----beta GlcUA) 1,4----alpha Glc1----] of Klebsiella pneumoniae Chedid (O1:K2), which is highly virulent for mice, were cloned and introduced into Escherichia coli HB101 and into four noncapsulated mutants derived from K. pneumoniae reference strains of K1, K7, K9, and K28. The recombinant plasmid pCPS7B06 carried 23 kb of a chromosomal DNA fragment of strain Chedid and encoded a part of the Klebsiella cps gene cluster. However, pCPS7B06 encoded enough genetic information for the production of Klebsiella K2 capsular polysaccharide on the cell surfaces of four noncapsulated mutants of K. pneumoniae. On the other hand, both pCPS7B06 and pROJ3 carrying the rmpA gene locus derived from a resident large plasmid of Chedid were required for the biosynthesis of Klebsiella K2 capsular polysaccharide on the cell surface of E. coli HB101. The insertion inactivation analysis using Tn5 revealed that the cps gene cluster occupied more than 15 kb of the chromosome of Chedid. We conclude that rmpA, which has been known to enhance the biosynthesis of colanic acid in E. coli, is also involved in the biosynthesis of Klebsiella capsular polysaccharide in E. coli HB101.

Map position and genomic organization of the kps cluster for polysialic acid synthesis in Escherichia coli K1

The multigenic kps cluster in Escherichia coli K1 encodes functions for synthesis of a polysialic acid capsule. DNA probes flanking each side of the cluster were hybridized to lambda clones bearing overlapping E. coli W3110 genomic fragments. These fragments covered the region between 60 and 70 map units on the chromosome. The results located kps to an accretion domain near 64 map units and established the orientation of kps cluster genes. Acquisition of kps by the E. coli genome was apparently the result of an ancestral transpositionlike addition event.

Virulence factors in Escherichia coli urinary tract infection

Uropathogenic strains of Escherichia coli are characterized by the expression of distinctive bacterial properties, products, or structures referred to as virulence factors because they help the organism overcome host defenses and colonize or invade the urinary tract. Virulence factors of recognized importance in the pathogenesis of urinary tract infection (UTI) include adhesins (P fimbriae, certain other mannose-resistant adhesins, and type 1 fimbriae), the aerobactin system, hemolysin, K capsule, and resistance to serum killing. This review summarizes the virtual explosion of information regarding the epidemiology, biochemistry, mechanisms of action, and genetic basis of these urovirulence factors that has occurred in the past decade and identifies areas in need of further study. Virulence factor expression is more common among certain genetically related groups of E. coli which constitute virulent clones within the larger E. coli population. In general, the more virulence factors a strain expresses, the more severe an infection it is able to cause. Certain virulence factors specifically favor the development of pyelonephritis, others favor cystitis, and others favor asymptomatic bacteriuria. The currently defined virulence factors clearly contribute to the virulence of wild-type strains but are usually insufficient in themselves to transform an avirulent organism into a pathogen, demonstrating that other as-yet-undefined virulence properties await discovery. Virulence factor testing is a useful epidemiological and research tool but as yet has no defined clinical role. Immunological and biochemical anti-virulence factor interventions are effective in animal models of UTI and hold promise for the prevention of UTI in humans.

The bex locus in encapsulated Haemophilus influenzae: a chromosomal region involved in capsule polysaccharide export

The nucleotide sequence of a 5.1 kb region in the Haemophilus influenzae type b capsulation locus has been determined and found to contain four open reading frames: bexD, bexC, bexB, and bexA. Comparison of the deduced products of bexC, bexB, and bexA to known proteins, and TnphoA mutagenesis, suggests that they form components of an ATP-driven polysaccharide export apparatus. Furthermore, close sequence similarity between BexA and BexB and products of the kpsT and kpsM genes at the Escherichia coli K5 capsulation locus (Smith et al., 1990--accompanying paper) suggests that capsulation genes in these organisms may have a common ancestry.

Further electron microscopic studies on the expression of Escherichia coli group II capsules

The de novo expression of Escherichia coli K1, K5, and K12 capsules was analyzed with immunoelectron microscopy in temperature upshift experiments, with upshift from 18 degrees C (capsule restrictive) to 37 degrees C (capsule permissive). Newly produced capsular polysaccharides appeared at the cell surface atop membrane adhesion sites (Bayer's junctions). After plasmolysis of the bacteria at an early expression stage, the capsular polysaccharides were labeled at discrete sites in the periplasm by the immunogold technique. After temperature upshift in the presence of carbonyl cyanide m-chlorophenylhydrazone (CCCP) or chloramphenicol, the polysaccharides were labeled in the cytoplasm.

Expression of the Escherichia coli K5 capsular antigen: immunoelectron microscopic and biochemical studies with recombinant E. coli

The capsular K5 polysaccharide, a representative of group II capsular antigens of Escherichia coli, has been cloned previously, and three gene regions responsible for polymerization and surface expression have been defined (I. S. Roberts, R. Mountford, R. Hodge, K. B. Jann, and G. J. Boulnois, J. Bacteriol. 170:1305-1310, 1988). In this report, we describe the immunoelectron microscopic analysis of recombinant bacteria expressing the K5 antigen and of mutants defective in either region 1 or region 3 gene functions, as well as the biochemical analysis of the K5 capsular polysaccharide. Whereas the K5 clone expressed the K5 polysaccharide as a well-developed capsule in about 25% of its population, no capsule was observed in whole mount preparations and ultrathin sections of the expression mutants. Immunogold labeling of sections from the region 3 mutant revealed the capsular K5 polysaccharide in the cytoplasm. With the region 1 mutant, the capsular polysaccharide appeared associated with the cell membrane, and, unlike the region 3 mutant polysaccharide, the capsular polysaccharide could be detected in the periplasm after plasmolysis of the bacteria. Polysaccharides were isolated from the homogenized mutants with cetyltrimethylammonium bromide. The polysaccharide from the region 1 mutant had the same size as that isolated from the capsule of the original K5 clone, and both polysaccharides were substituted with phosphatidic acid. The polysaccharide from the region 3 mutant was smaller and was not substituted with phosphatidic acid. These results prompt us to postulate that gene region 3 products are involved in the translocation of the capsular polysaccharide across the cytoplasmic membrane and that region 1 directs the transport of the lipid-substituted capsular polysaccharide through the periplasm and across the outer membrane.

Molecular analysis of a region of the group B streptococcus chromosome involved in type III capsule expression

Type III group B streptococci (GBS) are the most common cause of neonatal sepsis and meningitis in the United States. The important role of the type III polysaccharide capsule and of the terminal sialic acid moiety of the capsule in the virulence of GBS has been demonstrated by using Tn916 mutagenesis. Several of the transposon insertion sites that resulted in defective type III capsule synthesis were located in a 30-kilobase (kb) region of the chromosome. Hybridization analysis of two other type III strains that differed in their relative virulence and of GBS serotypes Ia, Ib, Ic, and II showed that this region of the chromosome was highly conserved. A repetitive 1.4-kb sequence was found only in the 30-kb region of the more virulent type III strain, COH 1. The Escherichia coli maxicell in vivo expression system and an in vitro coupled transcription-translation system successfully identified the proteins expressed from the 30-kb region. Comparison of the proteins expressed from the same DNA fragments in these two assays indicated that some of these proteins may contain leader sequences that would ultimately result in their secretion to the cell surface. Identification and further characterization of the genes and their products will provide the foundation for understanding the genetic and biochemical events in GBS capsular polysaccharide production.