Structure, assembly and regulation of expression of capsules in Escherichia coli

Molecular basis of bacterial outer membrane permeability revisited

Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Positive correlation between tyrosine phosphorylation of CpsD and capsular polysaccharide production in Streptococcus pneumoniae

CpsA, CpsB, CpsC, and CpsD are part of a tyrosine phosphorylation regulatory system involved in modulation of capsule synthesis in Streptococcus pneumoniae and many other gram-positive and gram-negative bacteria. Using an immunoblotting technique, we observed distinct laddering patterns of S. pneumoniae capsular polysaccharides of various serotypes and found that transfer of the polymer from the membrane to the cell wall was independent of size. Deletion of cps2A, cps2B, cps2C, or cps2D in the serotype 2 strain D39 did not affect the ability to transfer capsule to the cell wall. Deletion of cps2C or cps2D, which encode two domains of an autophosphorylating tyrosine kinase, resulted in the production of only short-chain polymers. The function of Cps2A is unknown, and the polymer laddering pattern of the cps2A deletion mutants appeared similar to that of the parent, although the total amount of capsule was decreased. Loss of Cps2B, a tyrosine phosphatase and a kinase inhibitor, resulted in an increase in capsule amount and a normal ladder pattern. However, Cps2B mutants exhibited reduced virulence following intravenous inoculation of mice and were unable to colonize the nasopharynx, suggesting a diminished capacity to sense or respond to these environments. In D39 and its isogenic mutants, the amounts of capsule and tyrosine-phosphorylated Cps2D (Cps2D approximately P) correlated directly. In contrast, restoration of type 2 capsule production followed by deletion of cps2B in Rx1, a laboratory passaged D39 derivative containing multiple uncharacterized mutations, resulted in decreased capsule amounts but no alteration in Cps2D approximately P levels. Thus, a factor outside the capsule locus, which is either missing or defective in the Rx1 background, is important in the control of capsule synthesis.

The RcsC sensor kinase is required for normal biofilm formation in Escherichia coli K-12 and controls the expression of a regulon in response to growth on a solid surface

Bacteria are often found associated with surfaces as sessile bacterial communities called biofilms, and the formation of a biofilm can be split up into different stages each requiring the expression of specific genes. The production of extracellular polysaccharides (EPS) is important for the maturation of biofilms and is controlled by the Rcs two-component pathway in Escherichia coli (and other Gram-negative bacteria). In this study, we show, for the first time, that the RcsC sensor kinase is required for normal biofilm development in E. coli. Moreover, using a combination of DNA macroarray technology and transcriptional fusion analysis, we show that the expression of > 150 genes is controlled by RcsC in E. coli. In silico analyses of the RcsC regulon predicts that 50% of the genes encode proteins that are either localized to the envelope of E. coli or have activities that affect the structure/properties of the bacterial surface, e.g. the production of colanic acid. Moreover, we also show that RcsC is activated during growth on a solid surface. Therefore, we suggest that the RcsC sensor kinase may play an important role in the remodelling of the bacterial surface during growth on a solid surface and biofilm formation.

A novel outer membrane protein, Wzi, is involved in surface assembly of the Escherichia coli K30 group 1 capsule

Escherichia coli group 1 K antigens form a tightly associated capsule structure on the cell surface. Although the general features of the early steps in capsular polysaccharide biosynthesis have been described, little is known about the later stages that culminate in assembly of a capsular structure on the cell surface. Group 1 capsule biosynthesis gene clusters (cps) in E. coli and Klebsiella pneumoniae include a conserved open reading frame, wzi. The wzi gene is the first of a block of four conserved genes (wzi-wza-wzb-wzc) found in all group 1 K-antigen serotypes. Unlike wza, wzb, and wzc homologs that are found in gene clusters responsible for production of exopolysaccharides (i.e., predominantly cell-free polymer) in a range of bacteria, wzi is found only in systems that assemble capsular polysaccharides. The predicted Wzi protein shows no similarity to any other known proteins in the databases, but computer analysis of Wzi predicted a cleavable signal sequence. Wzi was expressed with a C-terminal hexahistidine tag, purified, and used for the production of specific antibodies that facilitated localization of Wzi to the outer membrane. Circular dichroism spectroscopy indicates that Wzi consists primarily of a beta-barrel structure, and dynamic light scattering studies established that the protein behaves as a monomer in solution. A nonpolar wzi chromosomal mutant retained a mucoid phenotype and remained sensitive to lysis by a K30-specific bacteriophage. However, the mutant showed a significant reduction in cell-bound polymer, with a corresponding increase in cell-free material. Furthermore, examination of the mutant by electron microscopy showed that it lacked a coherent capsule structure. It is proposed that the Wzi protein plays a late role in capsule assembly, perhaps in the process that links high-molecular-weight capsule to the cell surface.

Translocation of group 1 capsular polysaccharide in Escherichia coli serotype K30. Structural and functional analysis of the outer membrane lipoprotein Wza

The late steps in assembly of capsular polysaccharides (CPS) and their translocation to the bacterial cell surface are not well understood. The Wza protein was shown previously to be required for the formation of the prototype group 1 capsule structure on the surface of Escherichia coli serotype K30 (Drummelsmith, J., and Whitfield, C. (2000) EMBO J. 19, 57-66). Wza is a conserved outer membrane lipoprotein that forms multimers adopting a ringlike structure, and collective evidence suggests a role for these structures in the export of capsular polymer across the outer membrane. Wza was purified in the native form and with a C-terminal hexahistidine tag. WzaHis6 was acylated and functional in capsule assembly, although its efficiency was slightly reduced in comparison to the native Wza protein. Ordered two-dimensional crystals of WzaHis6 were obtained after reconstitution of purified multimers into lipids. Electron microscopy of negatively stained crystals and Fourier filtering revealed ringlike multimers with an average outer diameter of 8.84 nm and an average central cavity diameter of 2.28 nm. Single particle analysis yielded projection structures at an estimated resolution of 3 nm, favoring a structure for the WzaHis6 containing eight identical subunits. A derivative of Wza (Wza*) in which the original signal sequence was replaced with that from OmpF showed that the native acylated N terminus of Wza is critical for formation of normal multimeric structures and for their competence for CPS assembly, but not for targeting Wza to the outer membrane. In the presence of Wza*, CPS accumulated in the periplasm but was not detected on the cell surface. Chemical cross-linking of intact cells suggested formation of a transmembrane complex minimally containing Wza and the inner membrane tyrosine autokinase Wzc.

β-Lactam induction of colanic acid gene expression inEscherichia coli

An unexpected observation led us to examine the relationship between beta-lactam exposure and synthesis of colonic acid capsular polysaccharide in Escherichia coli. Strains containing a cps-lacZ transcriptional fusion were challenged with antibiotics having various modes of action, and gene expression was detected by a disk-diffusion assay and in broth cultures. The cps genes were induced by a subset of beta-lactams but not by agents inhibiting protein synthesis or DNA replication, indicating that cps expression was specific and not due to stresses accompanying cell death or by a general inhibition of peptidoglycan synthesis. A narrow concentration just below the MIC triggered cps expression in liquid culture, suggesting the response may be triggered by near-lethal levels of antibiotic. Because colanic acid is important for maturation of biofilm architecture, antibiotics that increase its synthesis might exacerbate the formation or persistence of biofilms.

Escherichia coli YrbH is a D-arabinose 5-phosphate isomerase

A gene encoding for arabinose 5-phosphate isomerase (API), which catalyzes the interconversion of d-ribulose 5-phosphate (Ru5P) and d-arabinose 5-phosphate (A5P), has been identified from the genome of Escherichia coli K-12. API is the first enzyme in the biosynthesis of 3-deoxy-d-manno-octulosonate (KDO), a sugar moiety located in the lipopolysaccharide layer of most Gram-negative bacteria. The API gene yrbH is located next to the recently identified specific KDO 8-P phosphatase gene, yrbI. The 328-amino acid open reading frame yrbH was cloned, overexpressed, and characterized. The purified recombinant enzyme is a tetramer and is sensitive to inhibition by zinc cations. API has optimal activity at pH 8.4 and catalytic residues with estimated pKa values of 6.55 +/- 0.04 and 10.34 +/- 0.07. The enzyme is specific for A5P and Ru5P, with apparent Km values of 0.61 +/- 0.06 mm for A5P and 0.35 +/- 0.08 mm for Ru5P. The apparent kcat in the A5P to Ru5P direction is 157 +/- 4 s-1, and in the Ru5P to A5P direction it is 255 +/- 16 s-1. The value of Keq (Ru5P/A5P) is 0.50 +/- 0.06. Homology searches of the E. coli genome suggest yrbH may be one of multiple genes that encode proteins with API activity.

Role of capsular colanic acid in adhesion of uropathogenic Escherichia coli

Urinary tract infections are the most common urologic disease in the United States and one of the most common bacterial infections of any organ system. Biofilms persist in the urinary tract and on catheter surfaces because biofilm microorganisms are resistant to host defense mechanisms and antibiotic therapy. The first step in the establishment of biofilm infections is bacterial adhesion; preventing bacterial adhesion represents a promising method of controlling biofilms. Evidence suggests that capsular polysaccharides play a role in adhesion and pathogenicity. This study focuses on the role of physiochemical and specific binding interactions during adhesion of colanic acid exopolysaccharide mutant strains. Bacterial adhesion was evaluated for isogenic uropathogenic Escherichia coli strains that differed in colanic acid expression. The atomic force microscope (AFM) was used to directly measure the reversible physiochemical and specific binding interactions between bacterial strains and various substrates as bacteria initially approach the interface. AFM results indicate that electrostatic interactions were not solely responsible for the repulsive forces between the colanic acid mutant strains and hydrophilic substrates. Moreover, hydrophobic interactions were not found to play a significant role in adhesion of the colanic acid mutant strains. Adhesion was also evaluated by parallel-plate flow cell studies in comparison to AFM force measurements to demonstrate that prolonged incubation times alter bacterial adhesion. Results from this study demonstrate that the capsular polysaccharide colanic acid does not enhance bacterial adhesion but rather blocks the establishment of specific binding as well as time-dependent interactions between uropathogenic E. coli and inert substrates.

Purification and characterization of KpsT, the ATP-binding component of the ABC-capsule exporter of Escherichia coli K1

The K1 capsule, an alpha(2,8)-linked polymer of sialic acid, is an important virulence determinant of invasive Escherichia coli. The 17-kb kps gene cluster of E. coli K1 encodes the information necessary for capsule expression at the cell surface. Two proteins, KpsM and KpsT, play a role in the transport of capsular polysaccharide across the cytoplasmic membrane, utilizing the energy from ATP hydrolysis. They belong to the ATP-binding cassette superfamily of transport proteins. In this study, we purified KpsT in its native form and show that the purified protein is able to bind ATP, undergo an ATP-dependent conformational change and hydrolyze ATP. Protease accessibility studies demonstrate the in vivo interaction between KpsM and KpsT.

Detection of conserved N-linked glycans and phase-variable lipooligosaccharides and capsules from campylobacter cells by mass spectrometry and high resolution magic angle spinning NMR spectroscopy

Glycomics, the study of microbial polysaccharides and genes responsible for their formation, requires the continuous development of rapid and sensitive methods for the identification of glycan structures. In this study, methods for the direct analysis of sugars from 108 to 1010 cells are outlined using the human gastrointestinal pathogen, Campylobacter jejuni. Using capillary-electrophoresis coupled with sensitive electrospray mass spectrometry, we demonstrate variability in the lipid A component of C. jejuni lipooligosaccharides (LOSs). In addition, these sensitive methods have permitted the detection of phase-variable LOS core structures that were not observed previously. High resolution magic angle spinning (HR-MAS) NMR was used to examine capsular polysaccharides directly from campylobacter cells and showed profiles similar to those observed for purified polysaccharides analyzed by solution NMR. This method also exhibited the feasibility of campylobacter serotyping, mutant verification, and preliminary sugar analysis. HR-MAS NMR examination of growth from individual colonies of C. jejuni NCTC11168 indicated that the capsular glycan modifications are also phase-variable. These variants show different staining patterns on deoxycholate-PAGE and reactivity with immune sera. One of the identified modifications was a novel -OP=O(NH2)OMe phosphoramide, not observed previously in nature. In addition, HR-MAS NMR detected the N-linked glycan, GalNAc-alpha1,4-GalNAc-alpha1,4-[Glc-beta1,3-]GalNAc-alpha1,4-GalNAc-alpha1,4-GalNAc-alpha1,3-Bac, where Bac is 2,4-diacetamido-2,4,6-trideoxy-d-glucopyranose, in C. jejuni and Campylobacter coli. The presence of this common heptasaccharide in multiple campylobacter isolates demonstrates the conservation of the N-linked protein glycosylation pathway in this organism and describes the first report of HR-MAS NMR detection of N-linked glycans on glycoproteins from intact bacterial cells.

Striking complexity of lipopolysaccharide defects in a collection of Sinorhizobium meliloti mutants

Although the role that lipopolysaccharide (LPS) plays in the symbiosis between Sinorhizobium meliloti and alfalfa has been studied for over a decade, its function in this process remains controversial and poorly understood. This is largely due to a lack of mutants affected by its synthesis. In one of the definitive studies concerning this issue, Clover et al. (R. H. Clover, J. Kieber, and E. R. Signer, J. Bacteriol. 171:3961-3967, 1989) identified a series of mutants with putative LPS defects, judged them to be symbiotically proficient on Medicago sativa, and concluded that LPS might not have a symbiotic function in S. meliloti. The mutations in these strains were never characterized at the molecular level nor was the LPS from most of them analyzed. We have transduced these mutations from the Rm2011 background from which they were originally isolated into the sequenced strain Rm1021 and have characterized the resulting strains in greater detail. We found the LPS from these mutants to display a striking complexity of phenotypes on polyacrylamide electrophoresis gels, including additional rough LPS bands and alterations in the molecular weight distribution of the smooth LPS. We found that some of the mutants contain insertions in genes that are predicted to be involved in the synthesis of carbohydrate components of LPS, including ddhB, lpsB, lpsC, and lpsE. The majority, however, code for proteins predicted to be involved in a wide variety of functions not previously recognized to play a role in LPS synthesis, including a possible transcription elongation factor (GreA), a possible queuine synthesis protein, and a possible chemotaxis protein. Furthermore, using more extensive assays, we have found that most of these strains have symbiotic deficiencies. These results support more recent findings that alterations in LPS structure can affect the ability of S. meliloti to form an effective symbiosis.

Consensus-degenerate hybrid oligonucleotide primers for amplification of priming glycosyltransferase genes of the exopolysaccharide locus in strains of the Lactobacillus casei group

A primer design strategy named CODEHOP (consensus-degenerate hybrid oligonucleotide primer) for amplification of distantly related sequences was used to detect the priming glycosyltransferase (GT) gene in strains of the Lactobacillus casei group. Each hybrid primer consisted of a short 3' degenerate core based on four highly conserved amino acids and a longer 5' consensus clamp region based on six sequences of the priming GT gene products from exopolysaccharide (EPS)-producing bacteria. The hybrid primers were used to detect the priming GT gene of 44 commercial isolates and reference strains of Lactobacillus rhamnosus, L. casei, Lactobacillus zeae, and Streptococcus thermophilus. The priming GT gene was detected in the genome of both non-EPS-producing (EPS(-)) and EPS-producing (EPS(+)) strains of L. rhamnosus. The sequences of the cloned PCR products were similar to those of the priming GT gene of various gram-negative and gram-positive EPS(+) bacteria. Specific primers designed from the L. rhamnosus RW-9595M GT gene were used to sequence the end of the priming GT gene in selected EPS(+) strains of L. rhamnosus. Phylogenetic analysis revealed that Lactobacillus spp. form a distinctive group apart from other lactic acid bacteria for which GT genes have been characterized to date. Moreover, the sequences show a divergence existing among strains of L. rhamnosus with respect to the terminal region of the priming GT gene. Thus, the PCR approach with consensus-degenerate hybrid primers designed with CODEHOP is a practical approach for the detection of similar genes containing conserved motifs in different bacterial genomes.

Detection and genetic analysis of group II capsules in Aeromonas hydrophila

The genetic organization and sequences of the group II capsule gene cluster of Aeromonas hydrophila PPD134/91 have been determined previously. The purified capsular polysaccharides can increase the ability of avirulent strain PPD35/85 to survive in naive tilapia serum but have no inhibitory effect on the adhesion of PPD134/91 to carp epithelial cells. In this study, the presence of group II capsules among 33 randomly chosen A. hydrophila strains was examined by electron microscopy and genetic analysis. Ten strains were found to produce group II capsules. A PCR detection system was developed to identify two types of group II capsules (IIA and IIB) based on their genetic organization in the region II gene clusters. Group IIA capsules in the authors' collection of A. hydrophila strains are mainly found in the O : 18 and O : 34 serogroups, while group IIB capsules are found in the O : 21 and O : 27 serogroups. The presence of group II capsules in A. hydrophila strongly correlates with the serum and phagocyte survival abilities (seven out of ten strains). The results indicate that the authors' PCR detection system can constitute a reliable assay for the classification of group II capsules in A. hydrophila.

Molecular organization of exopolysaccharide (EPS) encoding genes on the lactococcal bacteriophage adsorption blocking plasmid, pCI658

The lactococcal plasmid pCI658 (58 kb) isolated from Lactococcus lactis ssp. cremoris HO2 encodes the production of a hydrophilic exopolysaccharide (EPS) which consists primarily of galactose and glucuronic acid and which interferes with adsorption of phages ø712 and øc2 to cell surface receptors. Examination of the nucleotide sequence of a 21.8-kb region of the plasmid revealed a large genetic cluster consisting of at least 23 putative EPS biosynthetic determinants in addition to the presence of insertion sequences at the 5(') and 3(') ends. According to homology searches, the genes were organized in specific regions involved in regulation, synthesis and export of the EPS. The predicted products of individual genes exhibited significant homology to exopolysaccharide, capsular polysaccharide (CPS), and lipopolysaccharide (LPS) gene products from a variety of Gram positive and Gram negative bacteria. Evidence of a gene encoding UDP-glucose dehydrogenase is also presented and this is the first description of such a gene in Lactococcus.

The multidrug/oligosaccharidyl-lipid/polysaccharide (MOP) exporter superfamily

The multidrug/oligosaccharidyl-lipid/polysaccharide (MOP) exporter superfamily (TC #2.A.66) consists of four previously recognized families: (a) the ubiquitous multi-drug and toxin extrusion (MATE) family; (b) the prokaryotic polysaccharide transporter (PST) family; (c) the eukaryotic oligosaccharidyl-lipid flippase (OLF) family and (d) the bacterial mouse virulence factor family (MVF). Of these four families, only members of the MATE family have been shown to function mechanistically as secondary carriers, and no member of the MVF family has been shown to function as a transporter. Establishment of a common origin for the MATE, PST, OLF and MVF families suggests a common mechanism of action as secondary carriers catalyzing substrate/cation antiport. Most protein members of these four families exhibit 12 putative transmembrane alpha-helical segments (TMSs), and several have been shown to have arisen by an internal gene duplication event; topological variation is observed for some members of the superfamily. The PST family is more closely related to the MATE, OLF and MVF families than any of these latter three families are related to each other. This fact leads to the suggestion that primordial proteins most closely related to the PST family were the evolutionary precursors of all members of the MOP superfamily. Here, phylogenetic trees and average hydropathy, similarity and amphipathicity plots for members of the four families are derived and provide detailed evolutionary and structural information about these proteins. We show that each family exhibits unique characteristics. For example, the MATE and PST families are characterized by numerous paralogues within a single organism (58 paralogues of the MATE family are present in Arabidopsis thaliana), while the OLF family consists exclusively of orthologues, and the MVF family consists primarily of orthologues. Only in the PST family has extensive lateral transfer of the encoding genes occurred, and in this family as well as the MVF family, topological variation is a characteristic feature. The results serve to define a large superfamily of transporters that we predict function to export substrates using a monovalent cation antiport mechanism.

Identification of genetic loci required for capsular expression in Vibrio vulnificus

Transposon mutagenesis of an encapsulated, virulent strain of Vibrio vulnificus 1003(O) led to the identification of four genetic regions that are essential to capsular polysaccharide (CPS) expression and virulence. Of the four regions, three are believed to be part of a capsule gene locus comprised of biosynthesis, polymerization, and transport genes clustered on a single chromosomal fragment. Genes indicating a Wzy-dependent system of polymerization and transmembrane export are present, suggesting that the CPS of V. vulnificus is lipid linked. The fourth region, while it contains a gene essential for CPS expression, is characteristic of an integron-gene cassette region, similar to the super integron of V. cholerae. It is not believed to be part of a CPS gene locus and is located in a region of the chromosome separate from the putative CPS loci. It is comprised of open reading frames (ORFs) carrying genes of unknown function surrounded by direct repeats. This region also contains IS492, an insertion sequence located numerous times throughout a region of the genome, demonstrating a restriction fragment length polymorphism among an encapsulated and nonencapsulated morphotype of V. vulnificus. Collectively, 22 ORFs were recognized: 13 capsule synthesis genes, 4 insertion sequences, 1 truncated biosynthesis gene, and 4 genes of unknown function. This study has led to the identification of previously unrecognized genetic loci that may help to increase the understanding of capsular genetics and antigenic diversity among V. vulnificus strains.

Transcriptional organization and regulation of the Escherichia coli K30 group 1 capsule biosynthesis (cps) gene cluster

Escherichia coli group 1 capsules are important virulence determinants, yet little is known about the transcriptional organization or regulation of their biosynthetic (cps) operons. Transcription of the prototype serotype K30 cluster is modulated by the JUMPStart-RfaH antitermination mechanism, with the cps promoter being localized to a region immediately upstream of the JUMPStart sequence. A putative stem-loop structure located within the K30 cps cluster separates conserved genes with products that are required for surface expression of capsule from serotype-specific genes encoding enzymes for polymer repeat-unit synthesis and polymerization. This putative stem-loop structure significantly reduces transcription in a termination-probe vector and may contribute to differential expression of the cps genes. Previous work indicated that increased amounts of group 1 capsular polysaccharide synthesis resulted from the overexpression of the Rcs (regulator of capsule synthesis) proteins. However, neither overexpression of the transcriptional activator RcsB nor an rcsB::aadA chromosomal insertion altered the level of transcription measured by cps::lacZ fusions. In the group 1 strains examined, an RcsAB box was found immediately upstream of galF, a gene involved in the production of sugar nucleotide precursors. Overexpression of RcsB was found to result in a threefold increase in transcription of a galF::lacZ chromosomal fusion. Moreover, overexpression of GalF gave rise to a two- to threefold increase in cell-free as well as cell-associated capsule, without affecting cps::lacZ activity. These results indicate that transcription of the E. coli group 1 capsule cluster itself is not regulated by the Rcs system and may, in fact, be constitutive. However, the Rcs system can potentially influence levels of capsular polysaccharide production by increasing galF transcription and influencing the available pool of biosynthetic precursors.

Involvement of a protein tyrosine kinase in production of the polymeric bioemulsifier emulsan from the oil-degrading strain Acinetobacter lwoffii RAG-1

The genes associated with the biosynthesis of the polymeric bioemulsifier emulsan, produced by the oil-degrading Acinetobacter lwoffii RAG-1 are clustered within a 27-kbp region termed the wee cluster. This report demonstrates the involvement of two genes of the wee cluster of RAG-1, wzb and wzc, in emulsan biosynthesis. The two gene products, Wzc and Wzb were overexpressed and purified. Wzc exhibited ATP-dependent autophosphorylating protein tyrosine kinase activity. Wzb was found to be a protein tyrosine phosphatase capable of dephosphorylating the phosphorylated Wzc. Using the synthetic substrate p-nitrophenyl phosphate (PNPP) Wzb exhibited a V(max) of 12 micromol of PNPP min(-1) mg(-1) and a K(m) of 8 mM PNPP at 30 degrees C. The emulsifying activity of mutants lacking either wzb or wzc was 16 and 15% of RAG-1 activity, respectively, suggesting a role for the two enzymes in emulsan production. Phosphorylation of Wzc was found to occur within a cluster of five tyrosine residues at the C terminus. Colonies from a mutant in which these five tyrosine residues were replaced by five phenylalanine residues along with those of a second mutant, which also lacked Wzb, exhibited a highly viscous colony consistency. Emulsan activity of these mutants was 25 and 24% of that of RAG-1, respectively. Neither of these mutants contained cell-associated emulsan. However, they did produce an extracellular high-molecular-mass galactosamine-containing polysaccharide. A model is proposed in which subunit polymerization, translocation and release of emulsan are all associated and coregulated by tyrosine phosphorylation.

RmpA2, an activator of capsule biosynthesis in Klebsiella pneumoniae CG43, regulates K2 cps gene expression at the transcriptional level

The rmpA2 gene, which encodes an activator for capsular polysaccharide (CPS) synthesis, was isolated from a 200-kb virulence plasmid of Klebsiella pneumoniae CG43. Based on the sequence homology with LuxR at the carboxyl-terminal DNA-binding motif, we hypothesized that RmpA2 exerts its effect by activating the expression of cps genes that are responsible for CPS biosynthesis. Two luxAB transcriptional fusions, each containing a putative promoter region of the K. pneumoniae K2 cps genes, were constructed and were found to be activated in the presence of multicopy rmpA2. The activation is likely due to direct binding of RmpA2 to the cps gene promoter through its C-terminal DNA binding motif. Moreover, the loss of colony mucoidy in a K. pneumoniae strain deficient in RcsB, a regulator for cps gene expression, could be recovered by complementing the strain with a multicopy plasmid carrying rmpA2. The CPS production in Lon protease-deficient K. pneumoniae significantly increased, and the effect was accompanied by an increase of RmpA2 stability. The expression of the rmpA2 gene was negatively autoregulated and could be activated when the organism was grown in M9 minimal medium. An IS3 element located upstream of the rmpA2 was required for the full activation of the rmpA2 promoter. In summary, our results suggest that the enhancement of K2 CPS synthesis in K. pneumoniae CG43 by RmpA2 can be attributed to its transcriptional activation of K2 cps genes, and the expression level of rmpA2 is autoregulated and under the control of Lon protease.

Impact of phosphorylation of specific residues in the tyrosine autokinase, Wzc, on its activity in assembly of group 1 capsules in Escherichia coli

Wzc(CPS) is a tyrosine autokinase essential for the assembly of a high-molecular-weight (HMW) group 1 capsular polysaccharide (CPS) in Escherichia coli. Homologues of Wzc participate in the formation of CPS and exopolysaccharides in a variety of gram-positive and gram-negative bacteria. Phosphorylation of tyrosine residues in the Wzc(CPS) C terminus is essential for HMW CPS assembly. Overexpression of Wzb(CPS) (phosphatase) in a wild-type background caused a 3.7-fold decrease in the amount of cell-associated K30 CPS produced, confirming the importance of Wzc(CPS) phosphorylation for capsule assembly. In this study, the tyrosine-rich region was dissected in an attempt to identify residues critical for Wzc(CPS) phosphorylation and/or capsule expression. Site-directed mutagenesis demonstrated that no single tyrosine residue in this region is sufficient for detectable phosphorylation of Wzc(CPS) in vivo or for HMW CPS expression. Furthermore, no single tyrosine residue is essential for phosphorylation or capsule assembly, since removal of any one tyrosine residue has no detectable effect. Altering combinations of tyrosine residues (from two to five) led to Wzc(CPS) derivatives that were still competent for phosphorylation but that could not support assembly of HMW CPS, showing that phosphorylation of Wzc per se is not an accurate measure of its ability to function in capsule assembly. One interpretation of these data is that the overall level of phosphorylation in this region, rather than the precise combination of residues accessible to phosphorylation, is important for the activity of Wzc(CPS). Tyrosine 569, a residue shown to modulate the in vitro phosphorylation of Wzc(CA) from E. coli K-12, was also mutated. The derivative with this mutation still functioned in capsule assembly. Quantitation of K30(CPS) from this mutant revealed no difference in the amount of polymer produced. Finally, dithiobis(succinimidylpropionate) cross-linking was used to confirm that Wzc(CPS) forms complexes in vivo, independent of the phosphorylation state of the protein.

O-antigen expression in Salmonella enterica serovar Typhi is regulated by nitrogen availability through RpoN-mediated transcriptional control of the rfaH gene

The authors previously reported increased expression of the Salmonella enterica serovar Typhi (S. typhi) rfaH gene when the bacterial cells reach stationary phase. In this study, using a lacZ fusion to the rfaH promoter region, they demonstrate that growth-dependent regulation of rfaH expression occurs at the level of transcription initiation. It was also observed that production of the lipopolysaccharide (LPS) O-antigen by S. typhi Ty2 correlated with the differential expression of rfaH during bacterial growth. This was probably due to the increased cellular levels of RfaH, since expression of the distal gene in the O-antigen gene cluster of S. typhi Ty2, wbaP, was also increased during stationary growth, as demonstrated by RT-PCR analysis. Examination of the sequences upstream of the rfaH coding region revealed homologies to potential binding sites for the RcsB/RcsA dimer of the RcsC/YopJ/RcsB phosphorelay regulatory system and for the RpoN alternative sigma factor. The expression of the rfaH gene in rpoN and rcsB mutants of S. typhi Ty2 was measured. The results indicate that inactivation of rpoN, but not of rcsB, suppresses the growth-phase-dependent induction of rfaH expression. Furthermore, production of beta-galactosidase mediated by the rfaH-lacZ fusion increased approximately fourfold when bacteria were grown in a nitrogen-limited medium. Nitrogen limitation was also shown to increase the expression of the O-antigen by the wild-type S. typhi Ty2, as demonstrated by a similar electrophoretic profile to that observed during the stationary phase of growth in rich media. It is therefore concluded that the relationship between LPS production and nitrogen limitation parallels the pattern of rfaH regulation under the control of RpoN and is consistent with the idea that RpoN modulates LPS formation via its effect on rfaH gene expression during bacterial growth.

Role of Vibrio cholerae O139 surface polysaccharides in intestinal colonization

Since the first occurrence of O139 Vibrio cholerae as a cause of cholera epidemics, this serogroup has been investigated intensively, and it has been found that its pathogenicity is comparable to that of O1 El Tor strains. O139 isolates express a thin capsule, composed of a polymer of repeating units structurally identical to the lipopolysaccharide (LPS) O side chain. In this study, we investigated the role of LPS O side chain and capsular polysaccharide (CPS) in intestinal colonization by with genetically engineered mutants. We constructed CPS-negative, CPS/LPS O side chain-negative, and CPS-positive/LPS O side chain-negative mutants. Furthermore, we constructed two mutants with defects in LPS core oligosaccharide (OS) assembly. Loss of LPS O side chain or CPS resulted in a approximately 30-fold reduction in colonization of the infant mouse small intestine, indicating that the presence of both LPS O side chain and CPS is important during the colonization process. The strain lacking both CPS and LPS O side chain and a CPS-positive, LPS O side chain-negative core OS mutant were both essentially unable to colonize. To characterize the role of surface polysaccharides in survival in the host intestine, resistance to several antimicrobial substances was investigated in vitro. These investigations revealed that the presence of CPS protects the cell against attack of the complement system and that an intact core OS is necessary for survival in the presence of bile.

The structures of the lipooligosaccharide and capsule polysaccharide of Campylobacter jejuni genome sequenced strain NCTC 11168

Campylobacter jejuni infections are one of the leading causes of human gastroenteritis and are suspected of being a precursor to Guillain-Barré and Miller-Fisher syndromes. Recently, the complete genome sequence of C. jejuni NCTC 11168 was described. In this study, the molecular structure of the lipooligosaccharide and capsular polysaccharide of C. jejuni NCTC 11168 was investigated. The lipooligosaccharide was shown to exhibit carbohydrate structures analogous to the GM1a and GM2 carbohydrate epitopes of human gangliosides (shown below): The high Mr capsule polysaccharide was composed of beta-d-Ribp, beta-d-GalfNAc, alpha-d-GlcpA6(NGro), a uronic acid amidated with 2-amino-2-deoxyglycerol at C-6, and 6-O-methyl-d-glycero-alpha-l-gluco-heptopyranose as a side-branch (shown below): The structural information presented here will aid in the identification and characterization of specific enzymes that are involved in the biosynthesis of these structures and may lead to the discovery of potential therapeutic targets. In addition, the correlation of carbohydrate structure with gene complement will aid in the elucidation of the role of these surface carbohydrates in C. jejuni pathogenesis.

Structural organization of the protein-tyrosine autokinase Wzc within Escherichia coli cells

Protein Wzc from Escherichia coli is a member of a newly defined family of protein-tyrosine autokinases that are essential for surface polysaccharide production in both Gram-negative and Gram-positive bacteria. Although the catalytic mechanism of the autophosphorylation of Wzc was recently described, the in vivo structural organization of this protein remained unclear. Here, we have determined the membrane topology of Wzc by performing translational fusions of lacZ and phoA reporter genes to the wzc gene. It has been shown that Wzc consists of two main structural domains: an N-terminal domain, bordered by two transmembrane helices, which is located in the periplasm of cells, and a C-terminal domain, harboring all phosphorylation sites of the protein, which is located in the cytoplasm. In addition, it has been demonstrated for the first time that Wzc can oligomerize in vivo to form essentially trimers and hexamers. Cross-linking experiments performed on strains expressing various domains of Wzc have shown that the cytoplasmic C-terminal domain is sufficient to generate oligomerization of Wzc. Mutant proteins, modified in either the ATP-binding site or the different phosphorylation sites, i.e. rendered unable to undergo autophosphorylation, have appeared to oligomerize into high molecular mass species identical to those formed by the wild-type protein. It was concluded that phosphorylation of Wzc is not essential to its oligomerization. These data, connected with the phosphorylation mechanism of Wzc, may be of biological significance in the regulatory role played by this kinase in polysaccharide synthesis.

Staphylococcus aureus contains two low-molecular-mass phosphotyrosine protein phosphatases

The analysis of the different amino acid sequences deduced from the complete genome sequence of the gram-positive bacterium Staphylococcus aureus suggested the presence of two eukaryotic-protein-like low-molecular-mass phosphotyrosine protein phosphatases, which are usually found in gram-negative bacteria. To check this prediction, the corresponding genes were cloned and overexpressed in an Escherichia coli system. Two distinct proteins with an apparent molecular mass of 23 kDa each, PtpA and PtpB, were produced and then purified by affinity chromatography and assayed for enzymatic properties. As expected, they both exhibited phosphatase activity in vitro, with a maximum value at a pH of around 6.2 and at a temperature of 40 degrees C. In addition, their kinetic constants, their specificity for phosphotyrosine residues, and their sensitivity to two phosphatase inhibitors, N-ethylmaleimide and orthovanadate, matched those of acid low-molecular-mass phosphotyrosine protein phosphatases.

Type 1 fimbriae and extracellular polysaccharides are preeminent uropathogenic Escherichia coli virulence determinants in the murine urinary tract

Escherichia coli is the leading cause of urinary tract infections (UTIs). Despite the association of numerous bacterial factors with uropathogenic E. coli (UPEC), few such factors have been proved to be required for UTI in animal models. Previous investigations of urovirulence factors have relied on prior identification of phenotypic characteristics. We used signature-tagged mutagenesis (STM) in an unbiased effort to identify genes that are essential for UPEC survival within the murine urinary tract. A library of 2049 transposon mutants of the prototypic UPEC strain CFT073 was constructed using mini-Tn5km2 carrying 92 unique tags and screened in a murine model of ascending UTI. After initial screening followed by confirmation in co-infection experiments, 19 survival-defective mutants were identified. These mutants were recovered in numbers 101- to 106-fold less than the wild type in the bladder, kidneys or urine or at more than one site. The transposon junctions from each attenuated mutant were sequenced and analysed. Mutations were found in: (i) the type 1 fimbrial operon; (ii) genes involved in the biosyn-thesis of extracellular polysaccharides including group I capsule, group II capsule and enterobacterial common antigen; (iii) genes involved in metabolic pathways; and (iv) genes with unknown function. Five of the genes identified are absent from the genome of the E. coli K-12 strain. Mutations in type 1 fimbrial genes resulted in severely attenuated colonization, even in the case of a mutant with an insertion upstream of the fim operon that affected the rate of fimbrial switching from the 'off' to the 'on' phase. Three mutants had insertions in a new type II capsule biosynthesis locus on a pathogenicity island and were impaired in the production of capsule in vivo. An additional mutant with an insertion in wecE was unable to synthesize enterobacterial common antigen. These results confirm the pre-eminence of type 1 fimbriae, establish the importance of extracellular polysaccharides in the pathogenesis of UTI and identify new urovirulence determinants.

Sodium dodecyl sulfate hypersensitivity of clpP and clpB mutants of Escherichia coli

We studied the hypersensitivity of clpP and clpB mutants of Escherichia coli to sodium dodecyl sulfate (SDS). Both wild-type E. coli MC4100 and lon mutants grew in the presence of 10% SDS, whereas isogenic clpP and clpB single mutants could not grow above 0.5% SDS and clpA and clpX single mutants could not grow above 5.0% SDS. For wild-type E. coli, cellular ClpP levels as determined by Western immunoblot analysis increased ca. sixfold as the levels of added SDS increased from 0 to 2%. Capsular colanic acid, measured as uronic acid, increased ca. sixfold as the levels of added SDS increased from 2 to 10%. Based on these findings, 3 of the 19 previously identified SDS shock proteins (M. Adamowicz, P. M. Kelley, and K. W. Nickerson, J. Bacteriol. 173:229-233, 1991) are tentatively identified as ClpP, ClpX, and ClpB.

Molecular cloning and characterization of genes for Shigella sonnei form I O polysaccharide: proposed biosynthetic pathway and stable expression in a live salmonella vaccine vector

The gene region for biosynthesis of Shigella sonnei form I O polysaccharide (O-Ps) and flanking sequences, totaling >18 kb, was characterized by deletion analysis to define a minimal construct for development of Salmonella-based live vaccine vector strains. Lipopolysaccharide (LPS) expression and DNA sequence studies of plasmid deletion derivatives indicated form I O-Ps expression from a 12.3-kb region containing a putative promoter and 10 contiguous open reading frames (ORFs), one of which is the transposase of IS630. A detailed biosynthetic pathway, consistent with the predicted functions of eight of the nine essential ORFs and the form I O-Ps structure, is proposed. Further sequencing identified partial IS elements (i.e., IS91 and IS630) and wzz upstream of the form I coding region and a fragment of aqpZ and additional full or partial IS elements (i.e., IS629, IS91, and IS911) downstream of this region. The stability of plasmid-based form I O-Ps expression was greater from low-copy vectors than from high-copy vectors and was enhanced by deletion of the downstream IS91 from plasmid inserts. Both core-linked (i.e., LPS) and non-core-linked (i.e., capsule-like) surface expression of form I O-Ps were detected by Western blotting and silver staining of polyacrylamide gel electrophoresis-separated Shigella and Escherichia coli extracts. However, salmonellae, which have a core that is chemically dissimilar to that of shigellae, expressed only non-core-linked surface-associated form I O-Ps. Finally, attenuated Salmonella enterica serovar Typhi live vaccine vector candidates, containing minimal-sized form I operon constructs, elicited immune protection in mice against virulent S. sonnei challenge, thereby supporting the promise of live, oral vaccines for the prevention of shigellosis.

Structural characterization of Escherichia coli sialic acid synthase

Sialic acid synthase encoded by the neuB gene of Escherichia coli catalyzes the condensation of N-acetylmannosamine and phosphoenolpyruvate to form N-acetylneuraminic acid. This report demonstrates the first structural information on sialic acid synthase by CD, MALDI-TOF, and chemical cross-linking studies. Also, a specific cleavage by endogenous protease(s) has been identified at Lys(280) of the enzyme (40 kDa) by LC-MS and N-terminal sequencing analyses. The cleavage results in the formation of two inactive fragments of 33 and 7 kDa. The structural analysis indicates that the fragmentation is associated with a significant change of the enzyme from a tetrameric to trimeric form, and alterations in both secondary and native quaternary structures.

Never say never again: protein glycosylation in pathogenic bacteria

In recent years, accumulating evidence for glycosylated bacterial proteins has overthrown an almost dogmatic belief that prokaryotes are not able to synthesize glycoproteins. Now it is widely accepted that eubacteria express glycoproteins. Although, at present, detailed information about glycosylation and structure-function relationships is available for only few eubacterial proteins, the variety of different components and structures observed already indicates that the variations in bacterial glycoproteins seem to exceed the rather limited display found in eukaryotes. Numerous virulence factors of bacterial pathogens have been found to be covalently modified with carbohydrate residues, thereby identifying these factors as true glycoproteins. In several bacterial species, gene clusters suggested to represent a general protein glycosylation system have been identified. In other cases, genes encoding highly specific glycosyltransferases have been found to be directly linked with virulence genes. These findings raise interesting questions concerning a potential role of glycosylation in pathogenesis. In this review, we will therefore focus on protein glycosylation in Gram-negative bacterial pathogens.

Molecular cloning and characterization of chondroitin polymerase from Escherichia coli strain K4

Escherichia coli strain K4 produces the K4 antigen, a capsule polysaccharide consisting of a chondroitin backbone (GlcUA beta(1-3)-GalNAc beta(1-4))(n) to which beta-fructose is linked at position C-3 of the GlcUA residue. We molecularly cloned region 2 of the K4 capsular gene cluster essential for biosynthesis of the polysaccharide, and we further identified a gene encoding a bifunctional glycosyltransferase that polymerizes the chondroitin backbone. The enzyme, containing two conserved glycosyltransferase sites, showed 59 and 61% identity at the amino acid level to class 2 hyaluronan synthase and chondroitin synthase from Pasteurella multocida, respectively. The soluble enzyme expressed in a bacterial expression system transferred GalNAc and GlcUA residues alternately, and polymerized the chondroitin chain up to a molecular mass of 20 kDa when chondroitin sulfate hexasaccharide was used as an acceptor. The enzyme exhibited apparent K(m) values for UDP-GlcUA and UDP-GalNAc of 3.44 and 31.6 microm, respectively, and absolutely required acceptors of chondroitin sulfate polymers and oligosaccharides at least longer than a tetrasaccharide. In addition, chondroitin polymers and oligosaccharides and hyaluronan polymers and oligosaccharides served as acceptors for chondroitin polymerization, but dermatan sulfate and heparin did not. These results may lead to elucidation of the mechanism for chondroitin chain synthesis in both microorganisms and mammals.

Vibrio cholerae and cholera: out of the water and into the host

The facultative human pathogen Vibrio cholerae can be isolated from estuarine and aquatic environments. V. cholerae is well recognized and extensively studied as the causative agent of the human intestinal disease cholera. In former centuries cholera was a permanent threat even to the highly developed populations of Europe, North America, and the northern part of Asia. Today, cholera still remains a burden mainly for underdeveloped countries, which cannot afford to establish or to maintain necessary hygienic and medical facilities. Especially in these environments, cholera is responsible for significant mortality and economic damage. During the last three decades, intensive research has been undertaken to unravel the virulence properties and to study the epidemiology of this significant human pathogen. More recently, researchers have been elucidating the environmental lifestyle of V. cholerae. This review provides an overview of the current knowledge of both the host- and environment-specific physiological attributes of V. cholerae.

Organization and characterization of the capsule biosynthesis locus of Streptococcus pneumoniae serotype 9V

The capsular polysaccharide (CPS) synthesis locus of Streptococcus pneumoniae serotype 9V was amplified by long-range PCR and sequenced. The locus was 17368 bp in size and contained 15 ORFs. The genetic organization of the cluster shared many features with other S. pneumoniae capsule loci, including the presence of four putative regulatory genes at the 5' end. Comparative sequence analyses allowed putative functions to be assigned to each of the gene products. The ORFs appeared to encode, besides the four regulatory genes, five glycosyltransferases, two O-acetyltransferases, an N-acetylglucosamine 2-epimerase, a glucose 6-dehydrogenase, an oligosaccharide transporter protein and a polysaccharide repeating unit polymerase. These functions covered the steps proposed in the CPS biosynthesis of serotype 9V. TLC of carbohydrate intermediates formed after incubation of bacterial membrane preparations with 14C-labelled precursors demonstrated that the fifth ORF (cps9vE) encoded a UDP-glucosyl-1-phosphate transferase. This function was confirmed with the help of a cps9vE mutant that carried a deletion of a guanine residue located adjacent to a stretch of adenines. The identification and characterization of the serotype 9V locus is a major step in unravelling the 9V capsule biosynthesis pathway and broadens the insight into the genetic diversity of the S. pneumoniae capsule loci.

Construction and characterization of an acapsular mutant of Mannheimia haemolytica A1

The nmaA and nmaB genes, which code for UDP-GlcNAc-2-epimerase and UDP-ManNAc-dehydrogenase, respectively, are involved in capsular polysaccharide biosynthesis in Mannheimia haemolytica A1. A chloramphenicol resistance (Cm(r)) cassette cloned behind an M. haemolytica A1 promoter, plpcat, was created and used to interrupt nmaA and nmaB. A 1.3-kbp DNA fragment that encompasses part of nmaA and nmaB was replaced by the 1.0-kbp plpcat, resulting in a knockout mutant which is Cm(r) and unable to synthesize N-acetylmannosamine (ManNAc) and N-acetylmannosaminuronic acid (ManNAcA). The DNA replacement was confirmed by Southern hybridization and PCR analyses of the nmaA and nmaB loci. Electron microscopy examination of the mutant showed the absence of capsular materials compared to the parent strain. The loss of NmaA and NmaB activity was confirmed by analysis of carbohydrate moieties using capillary electrophoresis. Serum sensitivity assays indicated that the acapsular mutant is as resistant as the encapsulated parent to complement-mediated killing by colostrum-deprived calf serum but is more sensitive to killing by immune bovine serum. Analysis of lipopolysaccharide prepared from the acapsular mutant and encapsulated parent confirmed that these strains have long O-polysaccharide chains, possibly conferring resistance to serum-mediated killing.

Novel Aeromonas hydrophila PPD134/91 genes involved in O-antigen and capsule biosynthesis

The sequences of the O-antigen and capsule gene clusters of the virulent Aeromonas hydrophila strain PPD134/91 were determined. The O-antigen gene cluster is 17,296 bp long and comprises 17 genes. Seven pathway genes for the synthesis of rhamnose and mannose, six transferase genes, one O unit flippase gene, and one O-antigen chain length determinant gene were identified by amino acid sequence similarity. PCR and Southern blot analysis were performed to survey the distribution of these 17 genes among 11 A. hydrophila strains of different serotypes. A. hydrophila PPD134/91 might belong to serotype O:18, as represented by JCM3980; it contained all the same O-antigen genes as JCM3980 (97 to 100% similarity at the DNA and amino acid levels). The capsule gene cluster of A. hydrophila PPD134/91 is 17,562 bp long and includes 13 genes, which were assembled into three distinct regions similar to those of the group II capsule gene cluster of Escherichia coli and other bacteria. Regions I and III contained four and two capsule transport genes, respectively. Region II had five genes which were highly similar to capsule synthesis pathway genes found in other bacteria. Both the purified O-antigen and capsular polysaccharides increased the ability of the avirulent A. hydrophila strain PPD35/85 to survive in naïve tilapia serum. However, the purified surface polysaccharides had no inhibitory effect on the adhesion of A. hydrophila PPD134/91 to carp epithelial cells.

Expression of the Streptococcus pneumoniae type 3 synthase in Escherichia coli. Assembly of type 3 polysaccharide on a lipid primer

Synthesis of the type 3 capsular polysaccharide of Streptococcus pneumoniae is catalyzed by the membrane-localized type 3 synthase, which utilizes UDP-Glc and UDP-GlcUA to form high molecular mass [3-beta-d-GlcUA-(1-->4)-beta-d-Glc-(1-->](n). Expression of the synthase in Escherichia coli resulted in synthesis of a 40-kDa protein that was reactive with antibody directed against the C terminus of the synthase and was the same size as the native enzyme. Membranes isolated from E. coli contained active synthase, as demonstrated by the ability to incorporate Glc and GlcUA into a high molecular mass polymer that could be degraded by type 3 polysaccharide-specific depolymerase. As in S. pneumoniae, the membrane-bound synthase from E. coli catalyzed a rapid release of enzyme-bound polysaccharide when incubated with either UDP-Glc or UDP-GlcUA alone. The recombinant enzyme expressed in E. coli was capable of releasing all of the polysaccharide from the enzyme, although the chains remained associated with the membrane. The recombinant enzyme was also able to reinitiate polysaccharide synthesis following polymer release by utilizing a lipid primer present in the membranes. At low concentrations of UDP-Glc and UDP-GlcUA (1 microm in the presence of Mg(2+) and 0.2 microm in Mn(2+)), novel glycolipids composed of repeating disaccharides with linkages consistent with type 3 polysaccharide were synthesized. As the concentration of the UDP-sugars was increased, there was a marked transition from glycolipid to polymer formation. At UDP-sugar concentrations of either 5 microm (with Mg(2+)) or 1.5 microm (with Mn(2+)), 80% of the incorporated sugar was in polymer form, and the size of the polymer increased dramatically as the concentration of UDP-sugars was increased. These results suggest a cooperative interaction between the UDP-precursor-binding site(s) and the nascent polysaccharide-binding site, resulting in a non-processive addition of sugars at the lower UDP-sugar concentrations and a processive reaction as the substrate concentrations increase.

CpsB is a modulator of capsule-associated tyrosine kinase activity in Streptococcus pneumoniae

Tyrosine phosphorylation is associated with polysaccharide synthesis in a number of Gram-positive and Gram-negative bacteria. In Streptococcus pneumoniae, CpsB, CpsC, and CpsD affect tyrosine phosphorylation and are critical for the production of a mature capsule in vitro. To characterize the interactions between these proteins and the phosphorylation event they modulate, cps2B, cps2C, and cps2D from the capsule type 2 S. pneumoniae D39 were cloned and expressed both individually and in combination in Escherichia coli. Cps2D purified from E. coli was not phosphorylated unless it was co-expressed with its cognate transmembrane domain, Cps2C. Purified phosphorylated Cps2D had tyrosine kinase activity and could phosphorylate both dephosphorylated Cps2D and an exogenous substrate (poly-Glu-Tyr) in the absence of ATP. Cps2B exhibited phosphatase activity against both purified phosphorylated Cps2D and p-nitrophenyl phosphate. An additional role for Cps2B as an inhibitor of Cps2D phosphorylation was demonstrated in both co-expression experiments in E. coli and in vitro experiments where it blocked the transphosphorylation of Cps2D even in the presence of the phosphatase inhibitor sodium orthovanadate. cps2C and cps2D deletion mutants in S. pneumoniae produced no detectable mature capsule during laboratory culture. Both were avirulent in systemic mouse infections and were unable to colonize the nasopharynx, suggesting that the failure to produce capsule was not dependent on the environment. Based on these results, we propose a model for capsule regulation where CpsB, CpsC, CpsD, and ATP form a stable complex that enhances capsule synthesis.

The rkp-3 gene region of Sinorhizobium meliloti Rm41 contains strain-specific genes that determine K antigen structure

The rkp-3 region is indispensable for capsular polysaccharide (K antigen) synthesis in Sinorhizobium meliloti Rm41. Strain Rm41 produces a K antigen of strain-specific structure, designated as the KR5 antigen. The data in this report show that the rkp-3 gene region comprises 10 open reading frames involved in bacterial polysaccharide synthesis and export. The predicted amino acid sequences for the rkpL-Q gene products are homologous to enzymes involved in the production of specific sugar moieties, while the putative products of the rkpRST genes show a high degree of similarity to proteins required for transporting polysaccharides to the cell surface. Southern analysis experiments using gene-specific probes suggest that genes involved in the synthesis of the precursor sugars are unique in strain Rm41, whereas sequences coding for export proteins are widely distributed among Sinorhizobium species. Mutations in the rkpL-Q genes result in a modified K antigen pattern and impaired symbiotic capabilities. On this basis, we suggest that these genes are required for the production of the KR5 antigen that is necessary for S. meliloti Rm41 exoB (AK631)-alfalfa (Medicago sativa) symbiosis.

Lipopolysaccharide endotoxins

Bacterial lipopolysaccharides (LPS) typically consist of a hydrophobic domain known as lipid A (or endotoxin), a nonrepeating "core" oligosaccharide, and a distal polysaccharide (or O-antigen). Recent genomic data have facilitated study of LPS assembly in diverse Gram-negative bacteria, many of which are human or plant pathogens, and have established the importance of lateral gene transfer in generating structural diversity of O-antigens. Many enzymes of lipid A biosynthesis like LpxC have been validated as targets for development of new antibiotics. Key genes for lipid A biosynthesis have unexpectedly also been found in higher plants, indicating that eukaryotic lipid A-like molecules may exist. Most significant has been the identification of the plasma membrane protein TLR4 as the lipid A signaling receptor of animal cells. TLR4 belongs to a family of innate immunity receptors that possess a large extracellular domain of leucine-rich repeats, a single trans-membrane segment, and a smaller cytoplasmic signaling region that engages the adaptor protein MyD88. The expanding knowledge of TLR4 specificity and its downstream signaling pathways should provide new opportunities for blocking inflammation associated with infection.

Bacterial translocation in neonatal rats: the relation between intestinal flora, translocated bacteria, and influence of milk

BACKGROUND

A high incidence of bacterial translocation in neonates results not only from immaturity of host-defense functions, but also from the dominant colonization of aerobic bacteria in the intestine. Bacterial colonization develops differently among breast-fed, formula-fed, premature, and full-term infants. The purpose of this study was to examine the incidence of bacterial translocation and to identify the translocated bacterial species, relating these findings to the intestinal microflora and to the type of feeding in neonatal rats.

METHODS

Animals were divided into three groups: breast-fed normal pups (MR group), formula-fed pups fed via an intragastric cannula implanted esophageally (AR group), and breast-fed pups after the removal of the cannula (Sham group). Artificial rearing was achieved using a machine feeding system. Culture and identification of the bacteria in the intestine, mesenteric lymph nodes, liver, portal blood, and lungs were made using a simplified version of Mitsuoka's method.

RESULTS

At 14 days of age, the dominant bacteria in the feces of the MR and Sham Groups were Enterobacteriaceae, Lactobacillus, and Enterococcus, but Enterobacteriaceae and Clostridium were significantly more common in the AR group than in the MR group. The dominant bacteria in the mesenteric lymph nodes were Enterobacteriaceae, Lactobacillus, and Staphylococcus. The extent of systemic bacterial translocation decreased earlier in the Sham group than in the AR group.

CONCLUSIONS

The frequency with which species of bacteria were cultured from mesenteric lymph nodes and other peripheral sites did not mirror the composition of the intestinal flora. Among the translocated bacteria, Staphylococcus may be especially hard to recognize and difficult for the host-defense systems to destroy. Breast-feeding inhibited systemic bacterial translocation in the suckling period of the rat.

Identification of a group 1-like capsular polysaccharide operon for Vibrio vulnificus

Virulence of Vibrio vulnificus correlates with changes in colony morphology that are indicative of a reversible phase variation for expression of capsular polysaccharide (CPS). Encapsulated variants are virulent with opaque colonies, whereas phase variants with reduced CPS expression are attenuated and are translucent. Using TnphoA mutagenesis, we identified a V. vulnificus CPS locus, which included an upstream ops element, a wza gene (wza(Vv)), and several open reading frames with homology to CPS biosynthetic genes. This genetic organization is characteristic of group 1 CPS operons. The wza gene product is required for transport of CPS to the cell surface in Escherichia coli. Polar transposon mutations in wza(Vv) eliminated expression of downstream biosynthetic genes, confirming operon structure. On the other hand, nonpolar inactivation of wza(Vv) was specific for CPS transport, did not alter CPS biosynthesis, and could be complemented in trans. Southern analysis of CPS phase variants revealed deletions or rearrangements at this locus. A survey of environmental isolates indicated a correlation between deletions in wza(Vv) and loss of virulent phenotype, suggesting a genetic mechanism for CPS phase variation. Full virulence in mice required surface expression of CPS and supported the essential role of capsule in the pathogenesis of V. vulnificus.

A non-Golgi alpha 1,2-fucosyltransferase that modifies Skp1 in the cytoplasm of Dictyostelium

Skp1 is a subunit of the SCF-E3 ubiquitin ligase that targets cell cycle and other regulatory factors for degradation. In Dictyostelium, Skp1 is modified by a pentasaccharide containing the type 1 blood group H trisaccharide at its core. To address how the third sugar, fucose alpha1,2-linked to galactose, is attached, a proteomics strategy was applied to determine the primary structure of FT85, previously shown to copurify with the GDP-Fuc:Skp1 alpha 1,2-fucosyltransferase. Tryptic-generated peptides of FT85 were sequenced de novo using Q-TOF tandem mass spectrometry. Degenerate primers were used to amplify FT85 genomic DNA, which was further extended by a novel linker polymerase chain reaction method to yield an intronless open reading frame of 768 amino acids. Disruption of the FT85 gene by homologous recombination resulted in viable cells, which had altered light scattering properties as revealed by flow cytometry. FT85 was necessary and sufficient for Skp1 fucosylation, based on biochemical analysis of FT85 mutant cells and Escherichia coli that express FT85 recombinantly. FT85 lacks sequence motifs that characterize all other known alpha 1,2-fucosyltransferases and lacks the signal-anchor sequence that targets them to the secretory pathway. The C-terminal region of FT85 harbors motifs found in inverting Family 2 glycosyltransferase domains, and its expression in FT85 mutant cells restores fucosyltransferase activity toward a simple disaccharide substrate. Whereas most prokaryote and eukaryote Family 2 glycosyltransferases are membrane-bound and oriented toward the cytoplasm where they glycosylate lipid-linked or polysaccharide precursors prior to membrane translocation, the soluble, eukaryotic Skp1-fucosyltransferase modifies a protein that resides in the cytoplasm and nucleus.

The complete sequence of the 1,683-kb pSymB megaplasmid from the N2-fixing endosymbiont Sinorhizobium meliloti

Analysis of the 1,683,333-nt sequence of the pSymB megaplasmid from the symbiotic N(2)-fixing bacterium Sinorhizobium meliloti revealed that the replicon has a high gene density with a total of 1,570 protein-coding regions, with few insertion elements and regions duplicated elsewhere in the genome. The only copies of an essential arg-tRNA gene and the minCDE genes are located on pSymB. Almost 20% of the pSymB sequence carries genes encoding solute uptake systems, most of which were of the ATP-binding cassette family. Many previously unsuspected genes involved in polysaccharide biosynthesis were identified and these, together with the two known distinct exopolysaccharide synthesis gene clusters, show that 14% of the pSymB sequence is dedicated to polysaccharide synthesis. Other recognizable gene clusters include many involved in catabolic activities such as protocatechuate utilization and phosphonate degradation. The functions of these genes are consistent with the notion that pSymB plays a major role in the saprophytic competence of the bacteria in the soil environment.

Elongation of alternating alpha 2,8/2,9 polysialic acid by the Escherichia coli K92 polysialyltransferase

We have chosen E. coli K92, which produces the alternating structure alpha(2-8)neuNAc alpha(2-9)neuNAc as a model system for studying bacterial polysaccharide biosynthesis. We have shown that the polysialyltransferase encoded by the K92 neuS gene can synthesize both alpha(2-8) and alpha(2-9) neuNAc linkages in vivo by 13C-nuclear magnetic resonance analysis of polysaccharide isolated from a heterologous strain containing the K92 neuS gene. The K92 polysialyltransferase is associated with the membrane in lysates of cells harboring the neuS gene in expression vectors. Although the enzyme can transfer sialic acid to the nonreducing end of oligosaccharides with either linkage, it is unable to initiate chain synthesis without exogenously added polysialic acid. Thus, the polysialyltransferase encoded by neuS is not sufficient for de novo synthesis of polysaccharide but requires another membrane component for initiation. The acceptor specificity of this polysialyltransferase was studied using sialic acid oligosaccharides of various structures as exogenous acceptors. The enzyme can transfer to the nonreducing end of all bacteria polysialic acids, but has a definite preference for alpha(2-8) acceptors. Gangliosides containing neuNAc alpha(2-8)neuNAc are elongated, whereas monsialylated gangliosides are not. Disialylgangliosides are better acceptors than short oligosaccharides, suggesting a lipid-linked oligosaccharide may be preferred in the elongation reaction. These studies show that the K92 polysialyltransferase catalyzes an elongation reaction that involves transfer of sialic acid from CMP-sialic acid to the nonreducing end of two different acceptor substrates.