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

The Double Life of Group B Streptococcus: Asymptomatic Colonizer and Potent Pathogen

Group B streptococcus (GBS) is a β-hemolytic gram-positive bacterium that colonizes the lower genital tract of approximately 18% of women globally as an asymptomatic member of the gastrointestinal and/or vaginal flora. If established in other host niches, however, GBS is highly pathogenic. During pregnancy, ascending GBS infection from the vagina to the intrauterine space is associated with preterm birth, stillbirth, and fetal injury. In addition, vertical transmission of GBS during or after birth results in life-threatening neonatal infections, including pneumonia, sepsis, and meningitis. Although the mechanisms by which GBS traffics from the lower genital tract to vulnerable host niches are not well understood, recent advances have revealed that many of the same bacterial factors that promote asymptomatic vaginal carriage also facilitate dissemination and virulence. Furthermore, highly pathogenic GBS strains have acquired unique factors that enhance survival in invasive niches. Several host factors also exist that either subdue GBS upon vaginal colonization or alternatively permit invasive infection. This review summarizes the GBS and host factors involved in GBS's state as both an asymptomatic colonizer and an invasive pathogen. Gaining a better understanding of these mechanisms is key to overcoming the challenges associated with vaccine development and identification of novel strategies to mitigate GBS virulence.

Phenotypic and molecular analysis of nontypeable Group B streptococci: identification of cps2a and hybrid cps2a/cps5 Group B streptococcal capsule gene clusters

The Group B streptococcus (GBS) can express a capsular polysaccharide (CPS). There are ten recognized CPSs (Ia, Ib, and II-IX). A GBS isolate is considered nontypeable (NT) when CPS cannot be identified as one of ten types. Two groups of GBS NT isolates were studied, isolates without surface sialic acid (sia(-)) and isolates with surface sialic acid (sia(+)). The first objective was to characterize NT sia(-) isolates that failed CPS identification by an immunodiffusion antisera typing assay and a RT-PCR capsule typing assay. NT sia(-) isolates were characterized by assaying phenotypic changes and identifying covR/S mutations that may potentially have a role in the altered phenotypes. The second objective was to characterize NT sia(+) isolates that failed to identify as one of the ten CPS types by an immundiffusion antisera-based typing assay and a RT-PCR capsule typing assay yet expressed capsule. Fifteen NT sia(-) isolates displayed increased β hemolysis/orange pigmentation, decreased CAMP activity, inability to form biofilm, and susceptibility to phagocytosis by human blood. DNA sequence analysis of the covR/S genes in the sia(-) isolates found mutations in 14 of 15 isolates assayed. These mutations in the covR/S genes may potentially contribute to lack of expression of phenotypic traits assayed in vitro. For the three NT sia(+) isolates, whole-genome sequence analyses identified two isolates with cps gene clusters identical to the recently described and uncommon CPSIIa type. The third isolate possessed a hybrid cluster containing cps genes for both CPSIIa and CPSV suggesting recombination between these two gene clusters.

Streptococcus gallolyticus subsp. gallolyticus endocarditis isolate interferes with coagulation and activates the contact system

Streptococcus gallolyticus subsp. gallolyticus, formerly classified as S. bovis biotype I, is an increasing cause of bacteremia and infective endocarditis in the elderly. The physiopathology of infective endocarditis is poorly understood and involves immune and coagulation systems. In this study, we found that S. gallolyticus subsp. gallolyticus activates the human contact system, which in turn has two consequences: cleavage of high-molecular-weight kininogen (HK) resulting in release of the potent pro-inflammatory peptide bradykinin, and initiation of the intrinsic pathway of coagulation. S. gallolyticus subsp. gallolyticus was found to bind and activate factors of the human contact system at its surface, leading to a significant prolongation of the intrinsic coagulation time and to the release of bradykinin. High-affinity binding of factor XII to the bacterial Pil1 collagen binding protein was demonstrated with a K_D of 13 nM. Of note, Pil1 expression was exclusively found in S. gallolyticus subsp. gallolyticus, further supporting an essential contribution of this pilus in virulence.

Identification of Group B Streptococcus Capsule Type by Use of a Dual Phenotypic/Genotypic Assay

The group B streptococcus (GBS) capsular polysaccharide (CPS) is an important virulence factor which is also used for GBS typing. There are 10 CPS types (Ia, Ib, and II to IX). GBS that do not phenotypically type are considered nontypeable. All genes required for CPS synthesis are found on the GBS cps operon, which contains a highly variable CPS-determining region (cpsG-cpsK). The objective of this study was development of an assay to detect sialic acid on the GBS cell surface, followed by a genotypic PCR CPS typing assay. Sialic acid is located at the terminal end of the side chain of all known GBS CPS types. Sialic acid can be bound to commercially available lectins such as slug Limax flavus lectin. Biotinylated L. flavus-streptavidin-peroxidase complex was used in an enzyme immunoassay and dot blot assay to detect sialic acid. This was followed by a PCR typing scheme that was developed to target the serotype-determining region of the cps locus for Ia, Ib, and II to IX. Sialic acid from the CPS types Ia, Ib, and II to IX was detectable on the GBS cell surfaces of all previously identified CPS-typed GBS strains assayed. This was followed by the real-time PCR typing assay which successfully identified CPS Ia, Ib, and II to IX types. The combination of phenotypic and genotypic assays provides an accurate tool for detection of CPS expression and assignment of CPS typing. These assays have the potential to be used for CPS typing in large-scale epidemiological studies.

Streptococcus agalactiae capsule polymer length and attachment is determined by the proteins CpsABCD

The production of capsular polysaccharides (CPS) or secreted exopolysaccharides is ubiquitous in bacteria, and the Wzy pathway constitutes a prototypical mechanism to produce these structures. Despite the differences in polysaccharide composition among species, a group of proteins involved in this pathway is well conserved. Streptococcus agalactiae (group B Streptococcus; GBS) produces a CPS that represents the main virulence factor of the bacterium and is a prime target in current vaccine development. We used this human pathogen to investigate the roles and potential interdependencies of the conserved proteins CpsABCD encoded in the cps operon, by developing knock-out and functional mutant strains. The mutant strains were examined for CPS quantity, size, and attachment to the cell surface as well as CpsD phosphorylation. We observed that CpsB, -C, and -D compose a phosphoregulatory system where the CpsD autokinase phosphorylates its C-terminal tyrosines in a CpsC-dependent manner. These Tyr residues are also the target of the cognate CpsB phosphatase. An interaction between CpsD and CpsC was observed, and the phosphorylation state of CpsD influenced the subsequent action of CpsC. The CpsC extracellular domain appeared necessary for the production of high molecular weight polysaccharides by influencing CpsA-mediated attachment of the CPS to the bacterial cell surface. In conclusion, although having no impact on cps transcription or the synthesis of the basal repeating unit, we suggest that these proteins are fine-tuning the last steps of CPS biosynthesis (i.e. the balance between polymerization and attachment to the cell wall).

Molecular mapping of the cell wall polysaccharides of the human pathogen Streptococcus agalactiae

The surface of many bacterial pathogens is covered with polysaccharides that play important roles in mediating pathogen-host interactions. In Streptococcus agalactiae, the capsular polysaccharide (CPS) is recognized as a major virulence factor while the group B carbohydrate (GBC) is crucial for peptidoglycan biosynthesis and cell division. Despite the important roles of CPS and GBC, there is little information available on the molecular organization of these glycopolymers on the cell surface. Here, we use atomic force microscopy (AFM) and transmission electron microscopy (TEM) to analyze the nanoscale distribution of CPS and GBC in wild-type (WT) and mutant strains of S. agalactiae. TEM analyses reveal that in WT bacteria, peptidoglycan is covered with a very thin (few nm) layer of GBC (the "pellicle") overlaid by a 15-45 nm thick layer of CPS (the "capsule"). AFM-based single-molecule mapping with specific antibody probes shows that CPS is exposed on WT cells, while it is hardly detected on mutant cells impaired in CPS production (ΔcpsE mutant). By contrast, both TEM and AFM show that CPS is over-expressed in mutant cells altered in GBC expression (ΔgbcO mutant), indicating that the production of the two surface glycopolymers is coordinated in WT cells. In addition, AFM topographic imaging and molecular mapping with specific lectin probes demonstrate that removal of CPS (ΔcpsE), but not of GBC (ΔgbcO), leads to the exposure of peptidoglycan, organized into 25 nm wide bands running parallel to the septum. These results indicate that CPS forms a homogeneous barrier protecting the underlying peptidoglycan from environmental exposure, while the presence of GBC does not prevent peptidoglycan detection. This work shows that single-molecule AFM, combined with high-resolution TEM, represents a powerful platform for analysing the molecular arrangement of the cell wall polymers of bacterial pathogens.

Microbial protein-tyrosine kinases

Microbial ester kinases identified in the past 3 decades came as a surprise, as protein phosphorylation on Ser, Thr, and Tyr amino acids was thought to be unique to eukaryotes. Current analysis of available microbial genomes reveals that "eukaryote-like" protein kinases are prevalent in prokaryotes and can converge in the same signaling pathway with the classical microbial "two-component" systems. Most microbial tyrosine kinases lack the "eukaryotic" Hanks domain signature and are designated tyrosine kinases based upon their biochemical activity. These include the tyrosine kinases termed bacterial tyrosine kinases (BY-kinases), which are responsible for the majority of known bacterial tyrosine phosphorylation events. Although termed generally as bacterial tyrosine kinases, BY-kinases can be considered as one family belonging to the superfamily of prokaryotic protein-tyrosine kinases in bacteria. Other members of this superfamily include atypical "odd" tyrosine kinases with diverse mechanisms of protein phosphorylation and the "eukaryote-like" Hanks-type tyrosine kinases. Here, we discuss the distribution, phylogeny, and function of the various prokaryotic protein-tyrosine kinases, focusing on the recently discovered Mycobacterium tuberculosis PtkA and its relationship with other members of this diverse family of proteins.

SecA localization and SecA-dependent secretion occurs at new division septa in group B Streptococcus

Exported proteins of Streptococcus agalactiae (GBS), which include proteins localized to the bacterial surface or secreted into the extracellular environment, are key players for commensal and pathogenic interactions in the mammalian host. These proteins are transported across the cytoplasmic membrane via the general SecA secretory pathway and those containing the so-called LPXTG sorting motif are covalently attached to the peptidoglycan by sortase A. How SecA, sortase A, and LPXTG proteins are spatially distributed in GBS is not known. In the close relative Streptococcus pyogenes, it was shown that presence of the YSIRKG/S motif (literally YSIRKX3Gx2S) in the signal peptide (SP) constitutes the targeting information for secretion at the septum. Here, using conventional and deconvolution immunofluorescence analyses, we have studied in GBS strain NEM316 the localization of SecA, SrtA, and the secreted protein Bsp whose signal peptide contains a canonical YSIRKG/S motif (YSLRKykfGlaS). Replacing the SP of Bsp with four other SPs containing or not the YSIRKG/S motif did not alter the localized secretion of Bsp at the equatorial ring. Our results indicate that secretion and cell wall-anchoring machineries are localized at the division septum. Cell wall- anchored proteins displayed polar (PilB, Gbs0791), punctuate (CspA) or uniform distribution (Alp2) on the bacterial surface. De novo secretion of Gbs0791 following trypsin treatment indicates that it is secreted at the septum, then redistributed along the lateral sides, and finally accumulated to the poles. We conclude that the ±YSIRK SP rule driving compartimentalized secretion is not true in S. agalactiae.

Group B Streptococcus induces a caspase-dependent apoptosis in fetal rat lung interstitium

Group B Streptococcus (GBS) is an important pathogen and is associated with sepsis and meningitis in neonates and infants. An ex vivo model that facilitates observations of GBS interactions with multiple host cell types over time was used to study its pathogenicity. GBS infections were associated with profound reductions in fetal lung; explant size, and airway branching. Elevated levels of apoptosis subsequent to GBS infections were observed by whole-mount confocal immunofluorescence using activated-caspase-3-antibodies and terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) assays. The caspase inhibitor Z-VAD-FMK abolished the increase in TUNEL-positive cells associated with GBS infections, indicating that the GBS-induced apoptosis was caspase-dependent. Digital image analyses revealed that both GBS and the active form of caspase-3 were distributed primarily within the lung interstitium, suggesting that these tissues are important targets for GBS. Antibodies to the active form of caspase-3 colocalized with both macrophage- and erythroblast-markers, suggesting that these hematopoietic cells are vulnerable to GBS-mediated pathogenesis. These studies suggest that GBS infections profoundly alter lung morphology and caspase-dependent hematopoietic cell apoptosis within the lung interstitium play roles in GBS pathophysiology in this model.

Chemical inhibition of bacterial protein tyrosine phosphatase suppresses capsule production

Capsule polysaccharide is a major virulence factor for a wide range of bacterial pathogens, including Streptococcus pneumoniae. The biosynthesis of Wzy-dependent capsules in both gram-negative and -positive bacteria is regulated by a system involving a protein tyrosine phosphatase (PTP) and a protein tyrosine kinase. However, how the system functions is still controversial. In Streptococcus pneumoniae, a major human pathogen, the system is present in all but 2 of the 93 serotypes found to date. In order to study this regulation further, we performed a screen to find inhibitors of the phosphatase, CpsB. This led to the observation that a recently discovered marine sponge metabolite, fascioquinol E, inhibited CpsB phosphatase activity both in vitro and in vivo at concentrations that did not affect the growth of the bacteria. This inhibition resulted in decreased capsule synthesis in D39 and Type 1 S. pneumoniae. Furthermore, concentrations of Fascioquinol E that inhibited capsule also lead to increased attachment of pneumococci to a macrophage cell line, suggesting that this compound would inhibit the virulence of the pathogen. Interestingly, this compound also inhibited the phosphatase activity of the structurally unrelated gram-negative PTP, Wzb, which belongs to separate family of protein tyrosine phosphatases. Furthermore, incubation with Klebsiella pneumoniae, which contains a homologous phosphatase, resulted in decreased capsule synthesis. Taken together, these data provide evidence that PTPs are critical for Wzy-dependent capsule production across a spectrum of bacteria, and as such represents a valuable new molecular target for the development of anti-virulence antibacterials.

Tyrosine phosphorylation and bacterial virulence

Protein phosphorylation on tyrosine has emerged as a key device in the control of numerous cellular functions in bacteria. In this article, we review the structure and function of bacterial tyrosine kinases and phosphatases. Phosphorylation is catalyzed by autophosphorylating adenosine triphosphate-dependent enzymes (bacterial tyrosine (BY) kinases) that are characterized by the presence of Walker motifs. The reverse reaction is catalyzed by three classes of enzymes: the eukaryotic-like phosphatases (PTPs) and dual-specific phosphatases; the low molecular weight protein-tyrosine phosphatases (LMW-PTPs); and the polymerase–histidinol phosphatases (PHP). Many BY kinases and tyrosine phosphatases can utilize host cell proteins as substrates, thereby contributing to bacterial pathogenicity. Bacterial tyrosine phosphorylation/dephosphorylation is also involved in biofilm formation and community development. The Porphyromonas gingivalis tyrosine phosphatase Ltp1 is involved in a restraint pathway that regulates heterotypic community development with Streptococcus gordonii. Ltp1 is upregulated by contact with S. gordonii and Ltp1 activity controls adhesin expression and levels of the interspecies signal AI-2.

The CovS/CovR acid response regulator is required for intracellular survival of group B Streptococcus in macrophages

Group B Streptococcus (GBS) is a leading cause of neonatal meningitis and septicemia. The ability of this organism to survive inside phagocytic cells is poorly understood but thought to be an important step for the establishment of disease in the host. Here, we demonstrate that GBS shows prolonged survival within J774 macrophages and that the capacity to survive is not significantly changed across a diverse range of strains representing different serotypes, multilocus sequence types (MLST), and sites of clinical isolation. Using staining for the lysosome-associated membrane protein (LAMP) and by pharmacological inhibition of phagosome acidification, we demonstrate that streptococci reside in a phagosome and that acidification of the phagosome is required for GBS to survive intracellularly. Moreover, we show that the GBS two-component system CovS/CovR, which is the major acid response regulator in this organism, is required for survival inside the phagosome.

Analogies and homologies in lipopolysaccharide and glycoprotein biosynthesis in bacteria

Bacteria generate and attach countless glycan structures to diverse macromolecules. Despite this diversity, the mechanisms of glycoconjugate biosynthesis are often surprisingly similar. The focus of this review is on the commonalities between lipopolysaccharide (LPS) and glycoprotein assembly pathways and their evolutionary relationship. Three steps that are essential for both pathways are completed by membrane proteins. These include the initiation of glycan assembly through the attachment of a first sugar residue onto the lipid carrier undecaprenyl pyrophosphate, the translocation across the plasma membrane and the final transfer onto proteins or lipid A-core. Two families of initiating enzymes have been described: the polyprenyl-P N-acetylhexosamine-1-P transferases and the polyprenyl-P hexosamine-1-P transferases, represented by Escherichia coli WecA and Salmonella enterica WbaP, respectively. Translocases are either Wzx-like flippases or adenosine triphosphate (ATP)-binding cassette transporters (ABC transporters). The latter can consist either of two polypeptides, Wzt and Wzm, or of a single polypeptide homolog to the Campylobacter jejuni PglK. Finally, there are two families of conjugating enzymes, the N-oligosaccharyltransferases (N-OTase), best represented by C. jejuni PglB, and the O-OTases, including Neisseria meningitidis PglL and the O antigen ligases involved in LPS biosynthesis. With the exception of the N-OTases, probably restricted to glycoprotein synthesis, members of all these transmembrane protein families can be involved in the synthesis of both glycoproteins and LPS. Because many translocation and conjugation enzymes display relaxed substrate specificity, these bacterial enzymes could be exploited in engineered living bacteria for customized glycoconjugate production, generating potential vaccines and therapeutics.

The Cell Wall Teichuronic Acid Synthetase (TUAS) Is an Enzyme Complex Located in the Cytoplasmic Membrane of Micrococcus luteus

The cell wall teichuronic acid (TUA) of Micrococcus luteus is a long-chain polysaccharide composed of disaccharide repeating units [-4-β-D-ManNAcAp-(1→6)α-D-Glcp−1-]_n, which is covalently anchored to the peptidoglycan on the inner cell wall and extended to the outer surface of the cell envelope. An enzyme complex responsible for the TUA chain biosynthesis was purified and characterized. The 440 kDa enzyme complex, named teichuronic acid synthetase (TUAS), is an octomer composed of two kinds of glycosyltransferases, Glucosyltransferase, and ManNAcA-transferase, which is capable of catalyzing the transfer of disaccharide glycosyl residues containing both glucose and the N-acetylmannosaminuronic acid residues. TUAS displays hydrophobic properties and is found primarily associated with the cytoplasmic membrane. The purified TUAS contains carotinoids and lipids. TUAS activity is diminished by phospholipase digestion. We propose that TUAS serves as a multitasking polysaccharide assembling station on the bacterial membrane.

Enterococcus faecalis capsular polysaccharide serotypes C and D and their contributions to host innate immune evasion

It has become increasingly difficult to treat infections caused by Enterococcus faecalis due to its high levels of intrinsic and acquired antibiotic resistance. However, few studies have explored the mechanisms that E. faecalis employs to circumvent the host innate immune response and establish infection. Capsular polysaccharides are important virulence factors that are associated with innate immune evasion. We demonstrate, using cultured macrophages (RAW 264.7), that capsule-producing E. faecalis strains of either serotype C or D are more resistant to complement-mediated opsonophagocytosis than unencapsulated strains. We show that differences in opsonophagocytosis are not due to variations in C3 deposition but are due to the ability of capsule to mask bound C3 from detection on the surface of E. faecalis. Similarly, E. faecalis capsule masks lipoteichoic acid from detection, which correlates with decreased tumor necrosis factor alpha production by cultured macrophages in the presence of encapsulated strains compared to that in the presence of unencapsulated strains. Our studies confirm the important role of the capsule as a virulence factor of E. faecalis and provide several mechanisms by which the presence of the capsule influences evasion of the innate immune response and suggest that the capsule could be a potential target for developing alternative therapies to treat E. faecalis infections.

Protein kinase and phosphatase signaling in Mycobacterium tuberculosis physiology and pathogenesis

Mycobacterium tuberculosis (Mtb), the etiological agent of tuberculosis (TB), evades the antimicrobial defenses of the host and survives within the infected individual through a complex set of strategies. These include active prevention of host cellular killing processes as well as overwhelming adaptive gene expression. In the past decade, we have gained an increased understanding of how mycobacteria not only have the ability to adapt to a changing host environment but also actively interfere with the signaling machinery within the host cell to counteract or inhibit parts of the killing apparatus employed by the macrophage. Mtb is able to sense its environment via a set of phospho-signaling proteins which mediate its response and interaction with the host in a coordinated manner. In this review, we summarize the current knowledge about selected Mtb serine, threonine, and tyrosine kinase and phosphatase signaling proteins, focusing on the protein kinases, PknG and PtkA, and the protein phosphatase, PtpA.

Capsular polysaccharide production in Enterococcus faecalis and contribution of CpsF to capsule serospecificity

Many bacterial species produce capsular polysaccharides that contribute to pathogenesis through evasion of the host innate immune system. The gram-positive pathogen Enterococcus faecalis was previously reported to produce one of four capsule serotypes (A, B, C, or D). Previous studies describing the four capsule serotypes of E. faecalis were based on immunodetection methods; however, the underlying genetics of capsule production did not fully support these findings. Previously, it was shown that capsule production for serotype C (Maekawa type 2) was dependent on the presence of nine open reading frames (cpsC to cpsK). Using a novel genetic system, we demonstrated that seven of the nine genes in the cps operon are essential for capsule production, indicating that serotypes A and B do not make a capsular polysaccharide. In support of this observation, we showed that serotype C and D capsule polysaccharides mask lipoteichoic acid from detection by agglutinating antibodies. Furthermore, we determined that the genetic basis for the difference in antigenicity between serotypes C and D is the presence of cpsF in serotype C strains. High-pH anion-exchange chromatography with pulsed amperometric detection analysis of serotype C and D capsules indicated that cpsF is responsible for glucosylation of serotype C capsular polysaccharide in E. faecalis.

Acquisition of factor H by a novel surface protein on group B Streptococcus promotes complement degradation

Binding of the host complement regulator, factor H (FH), by some pathogenic microbes constitutes an important virulence mechanism, whereby complement is broken down to help microbes survive in the host. Although it has been hypothesized for the past two decades that GBS type III binds FH via sialic acid present on its capsule, neither the binding of FH to GBS has been demonstrated nor the mechanism of interaction identified. We observed that FH bound to both wild-type and capsule or sialic acid-deficient GBS that were used as negative controls. Wild-type and acapsular GBS were incubated with serum or pure FH degraded almost 90% of C3b, suggesting that the GBS-bound FH maintained cofactor activity. In addition, dot-blot analysis showed approximately 5-10% of C5 and C9 formation, as compared to an Escherichia coli control, suggesting breakdown at the C3b level. Protease treatment of the bacteria completely abolished binding of FH. Using overlay assays and mass spectroscopic analysis, we identified the FH receptor as the streptococcal histidine triad (SHT) surface protein. The ability of binding FH to SHT was further confirmed by using recombinant SHT. This report describes the identification of the SHT as an FH-binding protein on the surface of GBS type III, revealing a novel mechanism by which the bacterium acquires FH to evade complement opsonization.

Evasion of macrophage scavenger receptor A-mediated recognition by pathogenic streptococci

PRR recognize conserved structures on pathogenic microbes and are important for the defense against invading microorganisms. However, accumulating evidence indicates that many pathogens have evolved mechanisms to avoid recognition by PRR. One type of PRR is the macrophage scavenger receptor A (SR-A), which has been shown to play an important role in recognition and non-opsonic phagocytosis of pathogenic bacteria. The bacterial ligands for SR-A have been suggested to be LPS or lipoteichoic acid. Here, we use murine bone marrow-derived macrophages to analyze the role of SR-A in non-opsonic phagocytosis of two major Gram-positive pathogens, Streptococcus agalactiae (group B streptococcus; GBS) and Streptococcus pyogenes. We show that the polysaccharide capsule of GBS and the surface M protein of S. pyogenes, two important virulence factors, prevent SR-A-mediated non-opsonic phagocytosis of streptococci. The sialic acid moiety of the GBS capsule was crucial for its ability to prevent recognition by SR-A. Moreover, we show that a ligand on GBS recognized by SR-A in the absence of capsule is the surface lipoprotein Blr. These findings represent the first example of a microbial strategy to prevent recognition by SR-A and suggest that bacterial surface proteins may be of importance as ligands for SR-A.

L-Ficolin/mannose-binding lectin-associated serine protease complexes bind to group B streptococci primarily through N-acetylneuraminic acid of capsular polysaccharide and activate the complement pathway

Group B streptococci (GBS) are the most common cause of neonatal sepsis and meningitis. Most infants who are colonized with GBS at birth do not develop invasive disease, although many of these uninfected infants lack protective levels of capsular polysaccharide (CPS)-specific antibody. The lectin pathway of complement is a potential mechanism for initiating opsonization of GBS with CPS-specific antibody-deficient serum. In this study, we determined whether mannose-binding lectin (MBL)/MBL-associated serine protease (MASP) complexes and L-ficolin/MASP complexes bind to different strains of GBS to activate the lectin pathway, and we identified the molecules recognized by lectins on the GBS surface. We found that MBL did not bind to any GBS examined, whereas L-ficolin bound to GBS cells of many serotypes. L-ficolin binding to GBS cells correlated with the CPS content in serotypes Ib, III (restriction digestion pattern types III-2 and III-3), and V but not with the group B-specific polysaccharide (GBPS) content or with the lipoteichoic acid (LTA) content. L-ficolin bound to purified CPS and GBPS in a concentration-dependent manner but not to purified LTA. All strains to which L-ficolin/MASP complexes bound consumed C4. When N-acetylneuraminic acid (NeuNAc) was selectively removed from GBS cells by treatment with neuraminidase, the reduction in L-ficolin binding was correlated with the amount of NeuNAc removed. Additionally, L-ficolin was able to bind to wild-type strains but was able to bind only weakly to unencapsulated mutants and a mutant strain in which the CPS lacks NeuNAc. We concluded that L-ficolin/MASP complexes bind to GBS primarily through an interaction with NeuNAc of CPS.

Effects of complement regulators bound to Escherichia coli K1 and Group B Streptococcus on the interaction with host cells

Escherichia coli K1 and Group B Streptococcus (GBS) are the most common bacteria that cause meningitis during the neonatal period. Complement, the first line of defence in the host, acts on these bacteria to opsonize with various components of complement for subsequent presentation to phagocytes. To counteract these opsonization effects, E. coli and GBS bind to the complement regulators C4 binding protein and Factor H, respectively. Nonetheless, the deposition of complement components on these two bacteria from neonatal serum and their effect on the host cell interaction is unclear. Here we demonstrated that the deposition of complement proteins from adult serum prevented the invasion of E. coli into human brain microvascular endothelial cells, whereas the invasion of GBS was enhanced. In contrast, treatment with cord serum had no effect on the invasion of both these bacteria. We also examined the effect of the deposited complement proteins on phagocytosis using THP-1 cells and THP-1 cells differentiated into macrophages. Escherichia coli treated with adult serum neither attached nor entered these cells, whereas GBS was phagocytosed and survived efficiently. We further demonstrate that the inhibitory effect of complement proteins is the result of the bound complement inhibitors C4b-binding protein, in the case of E. coli, and Factor H, in the case of GBS. Taken together, these results suggest that E. coli and GBS utilize contrasting mechanisms of complement-mediated interactions with their target cells for successful establishment of disease.

UDP-Acetyl-Mannosamine Dehydrogenase Is an Endogenous Protein Substrate of Staphylococcus aureus Protein-Tyrosine Kinase Activity

The in silico analysis of the amino acid sequences deduced from the complete genome sequence of Staphylococcus aureus suggested the presence of two protein tyrosine kinase activities, each split into two distinct polypeptides, respectively Cap5A1/Cap5B1 and Cap5A2/Cap5B2, like in some other Gram-positive bacteria. To check this prediction, the corresponding genes were cloned and overexpressed, and the four corresponding proteins were purified by affinity chromatography and assayed for phosphorylating activity in vitro. Individually, none of them was found to autophosphorylate. However, when Cap5B2 was incubated in the presence of Cap5A2 or, with a larger efficiency, in the presence of Cap5A1, this protein exhibited intensive autokinase activity, occurring selectively at tyrosine residues. On the other hand, whatever the protein combination assayed, Cap5B1 did not present any phosphorylating activity. In search of a possible role for the phosphorylation reaction mediated by Cap5B2, an endogenous substrate of this kinase was characterized. This substrate, termed Cap5O, is the enzyme UDP-acetyl-mannosamine dehydrogenase involved in the cascade of reactions leading to the synthesis of the bacterial capsule. It represents the first endogenous substrate for a tyrosine kinase activity so far identified in S. aureus. The analysis of its dehydrogenase activity showed that it was positively controlled by its phosphorylation at tyrosine.

Chloramphenicol is a substrate for a novel nitroreductase pathway in Haemophilus influenzae

The p-nitroaromatic antibiotic chloramphenicol has been used extensively to treat life-threatening infections due to Haemophilus influenzae and Neisseria meningitidis; its mechanism of action is the inhibition of protein synthesis. We found that during incubation with H. influenzae cells and lysates, chloramphenicol is converted to a 4-aminophenyl allylic alcohol that lacks antibacterial activity. The allylic alcohol moiety undergoes facile re-addition of water to restore the 1,3-diol, as well as further dehydration driven by the aromatic amine to form the iminoquinone. Several Neisseria species and most chloramphenicol-susceptible Haemophilus species, but not Escherichia coli or other gram-negative or gram-positive bacteria we examined, were also found to metabolize chloramphenicol. The products of chloramphenicol metabolism by species other than H. influenzae have not yet been characterized. The strains reducing the antibiotic were chloramphenicol susceptible, indicating that the pathway does not appear to mediate chloramphenicol resistance. The role of this novel nitroreductase pathway in the physiology of H. influenzae and Neisseria species is unknown. Further understanding of the H. influenzae chloramphenicol reduction pathway will contribute to our knowledge of the diversity of prokaryotic nitroreductase mechanisms.

Streptococcus iniae capsule impairs phagocytic clearance and contributes to virulence in fish

Surface capsular polysaccharides play a critical role in protecting several pathogenic microbes against innate host defenses during infection. Little is known about virulence mechanisms of the fish pathogen Streptococcus iniae, though indirect evidence suggests that capsule could represent an important factor. The putative S. iniae capsule operon contains a homologue of the cpsD gene, which is required for capsule polymerization and export in group B Streptococcus and Streptococcus pneumoniae. To elucidate the role of capsule in the S. iniae infectious process, we deleted cpsD from the genomes of two virulent S. iniae strains by allelic exchange mutagenesis to generate the isogenic capsule-deficient DeltacpsD strains. Compared to wild-type S. iniae, the DeltacpsD mutants had a predicted reduction in buoyancy and cell surface negative charge. Transmission electron microscopy confirmed a decrease in the abundance of extracellular capsular polysaccharide. Gas-liquid chromatography-mass spectrometry analysis of the S. iniae extracellular polysaccharides showed the presence of l-fucose, d-mannose, d-galactose, d-glucose, d-glucuronic acid, N-acetyl-d-galactosamine, and N-acetyl-d-glucosamine, and all except mannose were reduced in concentration in the isogenic mutant. The DeltacpsD mutants were highly attenuated in vivo in a hybrid striped bass infection challenge despite being more adherent and invasive to fish epithelial cells and more resistant to cationic antimicrobial peptides than wild-type S. iniae. Increased susceptibility of the S. iniae DeltacpsD mutants to phagocytic killing in whole fish blood and by a fish macrophage cell line confirmed the role of capsule in virulence and highlighted its antiphagocytic function. In summary, we report a genetically defined study on the role of capsule in S. iniae virulence and provide preliminary analysis of S. iniae capsular polysaccharide sugar components.

The streptococcal Blr and Slr proteins define a family of surface proteins with leucine-rich repeats: camouflaging by other surface structures

Regions with tandemly arranged leucine-rich repeats (LRRs) have been found in many prokaryotic and eukaryotic proteins, in which they provide a remarkably versatile framework for the formation of ligand-binding sites. Bacterial LRR proteins include the recently described Slr protein of Streptococcus pyogenes, which is related to internalin A of Listeria monocytogenes. Here, we show that strains of the human pathogen Streptococcus agalactiae express a protein, designated Blr, which together with Slr defines a family of internalin A-related streptococcal LRR proteins. Analysis with specific antibodies demonstrated that Blr is largely inaccessible on S. agalactiae grown in vitro, but surface exposure was increased approximately 100-fold on mutants lacking polysaccharide capsule. In S. pyogenes, surface exposure of Slr was not affected in a mutant lacking hyaluronic acid capsule but was increased >20-fold in mutants lacking M protein or protein F. Thus, both Blr and Slr are efficiently camouflaged by other surface structures on bacteria grown in vitro. When Blr and Slr exposed on the bacterial surface were compared, they exhibited only little immunological cross-reactivity, in spite of extensive residue identity, suggesting that their surface-exposed parts have been under evolutionary pressure to diverge functionally and/or antigenically. These data identify a family of immunologically diverse streptococcal LRR proteins that show unexpected complexity in their interactions with other bacterial surface components.

DNA polymorphism and molecular subtyping of the capsular gene cluster of group B streptococcus

Serotyping and other phenotypic methods are often used to characterize the capsular polysaccharide of group B streptococci (GBS). We describe a capsular genotyping method that utilizes PCR of capsular polysaccharide synthesis genes (cps) and restriction enzyme digestion. This method facilitates the detection of DNA polymorphism in cps genes and correlates well with serotyping.

Role of Protein Phosphorylation on Serine/Threonine and Tyrosine in the Virulence of Bacterial Pathogens

Bacterial pathogens have developed a diversity of strategies to interact with host cells, manipulate their behaviors, and thus to survive and propagate. During the process of pathogenesis, phosphorylation of proteins on hydroxyl amino acids (serine, threonine, tyrosine) occurs at different stages, including cell-cell interaction and adherence, translocation of bacterial effectors into host cells, and changes in host cellular structure and function induced by infection. The phosphorylation reactions are catalyzed in a reversible fashion by specific protein kinases and phosphatases that belong to either the invading bacterial cells or the infected eukaryotic host cells. Among the various virulence factors involved in bacterial pathogenesis, special attention has been paid recently to the cell wall components, exopolysaccharides. A major breakthrough has been made by showing the existence of a biological link between the activity of certain protein-tyrosine kinases/phosphatases and the production and/or transport of surface polysaccharides. In addition, genetic studies have revealed a key role played by some serine/threonine kinases in pathogenesis. Considering the structural organization and membrane topology of these different kinases, it can be envisaged that they operate as one-component systems in signal transduction pathways, in the form of single proteins containing input and output domains on the same polypeptide chain. From a general standpoint, the demonstration of a direct relationship between protein phosphorylation on serine/threonine/tyrosine and bacterial virulence represents a novel concept of great importance in deciphering the molecular and cellular mechanisms that underlie pathogenesis.

Protein-Tyrosine Phosphorylation in Bacillus subtilis

In recent years bacterial protein-tyrosine kinases have been found to phosphorylate a growing number of protein substrates, including RNA polymerase sigma factors, UDP-glucose dehydrogenases and single-stranded DNA-binding proteins. The activity of these protein substrates was affected by tyrosine phosphorylation, indicating that this post-translational modification could regulate physiological processes ranging from stress response and exopolysaccharide synthesis to DNA metabolism. Some interesting work in this field was done in Bacillus subtilis, and we here present the current state of knowledge on protein-tyrosine phosphorylation in this gram-positive model organism. With its two kinases, two kinase modulators, three phosphatases and at least four different tyrosine-phosphorylated substrates, B. subtilis is the bacterium with the highest number of presently known participants in the global network of protein-tyrosine phosphorylation. We discuss the approaches currently used to chart this network: ranging from studies of substrate specificity and the physiological role of tyrosine phosphorylation of individual enzymes to the global approaches at the level of systems biology.

Blood-brain barrier invasion by group B Streptococcus depends upon proper cell-surface anchoring of lipoteichoic acid

Group B streptococci (GBSs) are the leading cause of neonatal meningitis. GBSs enter the CNS by penetrating the blood-brain barrier (BBB), which consists of specialized human brain microvascular endothelial cells (hBMECs). To identify GBS factors required for BBB penetration, we generated random mutant libraries of a virulent strain and screened for loss of hBMEC invasion in vitro. Two independent hypo-invasive mutants possessed disruptions in the same gene, invasion associated gene (iagA), which encodes a glycosyltransferase homolog. Allelic replacement of iagA in the GBS chromosome produced a 4-fold decrease in hBMEC invasiveness. Mice challenged with the GBS DeltaiagA mutant developed bacteremia comparably to WT mice, yet mortality was significantly lower (20% vs. 90%), as was the incidence of meningitis. The glycolipid diglucosyldiacylglycerol, a cell membrane anchor for lipoteichoic acid (LTA) and predicted product of the IagA glycosyltransferase, was absent in the DeltaiagA mutant, which consequently shed LTA into the media. Attenuation of virulence of the DeltaiagA mutant was found to be independent of TLR2-mediated signaling, but bacterial supernatants from the DeltaiagA mutant containing released LTA inhibited hBMEC invasion by WT GBS. Our data suggest that LTA expression on the GBS surface plays a role in bacterial interaction with BBB endothelium and the pathogenesis of neonatal meningitis.

CpsE from type 2 Streptococcus pneumoniae catalyzes the reversible addition of glucose-1-phosphate to a polyprenyl phosphate acceptor, initiating type 2 capsule repeat unit formation

The majority of the 90 capsule types made by the gram-positive pathogen Streptococcus pneumoniae are assembled by a block-type mechanism similar to that utilized by the Wzy-dependent O antigens and capsules of gram-negative bacteria. In this mechanism, initiation of repeat unit formation occurs by the transfer of a sugar to a lipid acceptor. In S. pneumoniae, this step is catalyzed by CpsE, a protein conserved among the majority of capsule types. Membranes from S. pneumoniae type 2 strain D39 and Escherichia coli containing recombinant Cps2E catalyzed incorporation of [14C]Glc from UDP-[14C]Glc into a lipid fraction in a Cps2E-dependent manner. The Cps2E-dependent glycolipid product from both membranes was sensitive to mild acid hydrolysis, suggesting that Cps2E was catalyzing the formation of a polyprenyl pyrophosphate Glc. Addition of exogenous polyprenyl phosphates ranging in size from 35 to 105 carbons to D39 and E. coli membranes stimulated Cps2E activity. The stimulation was due, in part, to utilization of the exogenous polyprenyl phosphates as an acceptor. The glycolipid product synthesized in the absence of exogenous polyprenyl phosphates comigrated with a 60-carbon polyprenyl pyrophosphate Glc. When 10 or 100 microM UMP was added to reaction mixtures containing D39 membranes, Cps2E activity was inhibited 40% and 80%, respectively. UMP, which acted as a competitive inhibitor of UDP-Glc, also stimulated Cps2E to catalyze the reverse reaction, with synthesis of UDP-Glc from the polyprenyl pyrophosphate Glc. These data indicated that Cps2E was catalyzing the addition of Glc-1-P to a polyprenyl phosphate acceptor, likely undecaprenyl phosphate.

Sialylation of group B streptococcal capsular polysaccharide is mediated by cpsK and is required for optimal capsule polymerization and expression

Several bacterial pathogens have evolved the means to escape immune detection by mimicking host cell surface carbohydrates that are crucial for self/non-self recognition. Sialic acid, a terminal residue on these carbohydrates, inhibits activation of the alternate pathway of complement by recruiting the immune modulating molecule factors H, I, and iC3b. Sialylation of capsular polysaccharide (CPS) is important for virulence of group B streptococci (GBS), a significant human pathogen. We previously reported that cpsK, a gene within the cps locus of type III GBS, could complement a sialyltransferase deficient lst mutant of Haemophilus ducreyi, implicating its role in sialylation of the GBS capsule. To explore the function of cpsK in GBS capsule production, we created a mutant in cpsK. Immunoblot analysis and enzyme-linked immunosorbent assay using anti-type III CPS antisera demonstrated that the mutant CPS did not contain sialic acid. This was confirmed by high-performance liquid chromatography after mild acid hydrolysis of the CPS. Although increased CPS chain length was seen for this strain, CPS production was <20% of the parental isolate. An episomal cpsK copy restored synthesis of sialo-CPS to wild-type levels. These data support our hypothesis that cpsK encodes the GBS CPS sialyltransferase and provide further evidence that lack of CPS oligosaccharide sialylation reduces the amount of CPS expressed on the cell surface. These observations also imply that one or more of the components involved in synthesis or transport of oligosaccharide repeating units requires a sialo-oligosaccharide for complete activity.

CovS/CovR of group B streptococcus: a two-component global regulatory system involved in virulence

In this study, we carried out a detailed structural and functional analysis of a Streptococcus agalactiae (GBS) two-component system which is orthologous to the CovS/CovR (CsrS/CsrR) regulatory system of Streptococcus pyogenes. In GBS, covR and covS are part of a seven gene operon transcribed from two promoters that are not regulated by CovR. A DeltacovSR mutant was found to display dramatic phenotypic changes such as increased haemolytic activity and reduced CAMP activity on blood agar. Adherence of the DeltacovSR mutant to epithelial cells was greatly increased and analysis by transmission electron microscopy revealed the presence at its surface of a fibrous extracellular matrix that might be involved in these intercellular interactions. However, the DeltacovSR mutant was unable to initiate growth in RPMI and its viability in human normal serum was greatly impaired. A major finding of this phenotypic analysis was that the CovS/CovR system is important for GBS virulence, as a 3 log increase of the LD(50) of the mutant strain was observed in the neonate rat sepsis model. The pleiotropic phenotype of the DeltacovSR mutant is in full agreement with the large number of genes controlled by CovS/CovR as seen by expression profiling analysis, many of which encode potentially secreted or cell surface-associated proteins: 76 genes are repressed whereas 63 were positively regulated. CovR was shown to bind directly to the regulatory regions of several of these genes and a consensus CovR recognition sequence was proposed using both DNase I footprinting and computational analyses.

Encapsulated Streptococcus suis inhibits activation of signaling pathways involved in phagocytosis

Streptococcus suis capsular type 2 is an important zoonotic agent of meningitis. Previous studies reported that, in contrast to nonencapsulated mutants, encapsulated S. suis is able to resist phagocytosis. However, the mechanisms by which S. suis avoids phagocytosis are unknown. To elucidate the signaling pathway(s) involved in S. suis antiphagocytosis, we compared the ability of an encapsulated strain and its nonencapsulated mutant to induce the activation of Akt and protein kinase C (PKC), which are downstream kinases of the phosphatidylinositol 3-kinase (PI-3K) pathway, known to be involved in the phagocytosis processes. The results demonstrated high levels of Akt and PKCalpha phosphorylation after infection of J774 macrophages with the nonencapsulated mutant, whereas the encapsulated strain showed reduced activation of PI-3K/Akt/PKCalpha signaling pathway, as well as several protein tyrosine events. These results correlated with the number of intracellular bacteria. Macrophages pretreated with specific PI-3K or PKC inhibitors showed reduced levels of Akt and PKCalpha phosphorylation, resulting in 50% reduction of phagocytosis. The role of phosphatases in the antiphagocytic mechanisms was evaluated by using phosphatase inhibitors, as well as SHP-1-deficient macrophages. Only in the absence of SHP-1 did the phagocytosis of encapsulated S. suis significantly increase, leading to Akt phosphorylation levels similar to those observed with the nonencapsulated strain, indicating activation of this important SH2 domain-containing tyrosine phosphatase by encapsulated S. suis. Finally, when purified S. suis capsular polysaccharide (CPS) was added to macrophages, no phosphorylation events were observed. In addition, CPS and encapsulated S. suis were able to inhibit the uptake of the nonencapsulated mutant. These results suggest the importance of CPS in the mechanisms, whereby S. suis downmodulates phagocytosis.

Biosynthesis and assembly of Group 1 capsular polysaccharides in Escherichia coli and related extracellular polysaccharides in other bacteria

Extracellular and capsular polysaccharides (EPSs and CPSs) are produced by a wide range of bacteria, including important pathogens of humans, livestock, and plants. These polymers are major surface antigens and serve a variety of roles in virulence, depending on the biology of the producing organism. In addition to their importance in disease, some EPSs also have industrial applications as gelling and emulsifying agents. An understanding of the processes involved in the synthesis and regulation of CPSs and EPSs therefore potentially contributes to an understanding of the disease state, surface expression of protective antigens, and modulation of polymer structure to give defined physical properties. Escherichia coli has provided important model systems for EPS and CPS biosynthesis. Here we describe current knowledge concerning assembly of the Group 1 CPSs of E. coli and the conservation of similar mechanisms in other bacteria.

A novel streptococcal surface protease promotes virulence, resistance to opsonophagocytosis, and cleavage of human fibrinogen

Group B streptococcus (GBS) is an important human pathogen. In this study, we sought to identify mechanisms that may protect GBS from host defenses in addition to its capsular polysaccharide. A gene encoding a cell-surface-associated protein (cspA) was characterized from a highly virulent type III GBS isolate, COH1. Its sequence indicated that it is a subtilisin-like extracellular serine protease homologous to streptococcal C5a peptidases and caseinases of lactic acid bacteria. The wild-type strain cleaved the alpha chain of human fibrinogen, whereas a cspA mutant, TOH121, was unable to cleave fibrinogen. We observed aggregated material when COH1 was incubated with fibrinogen but not when the mutant strain was treated similarly. This suggested that the product(s) of fibrinogen cleavage have strong adhesive properties and may be similar to fibrin. The cspA gene was present among representative clinical isolates from all nine capsular serotypes, as revealed by Southern blotting. A cspA(-) mutant was ten times less virulent in a neonatal rat sepsis model of GBS infections, as measured by LD(50) analysis. In addition, the cspA(-) mutant was significantly more sensitive than the wild-type strain to opsonophagocytic killing by human neutrophils in vitro. Taken together, the results suggest that cleavage of fibrinogen by CspA may increase the lethality of GBS infection, potentially by protecting the bacterium from opsonophagocytic killing.

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.

Mechanisms, organisms and markers of infection in pregnancy

Premature delivery is still a significant problem in Obstetrics. It has multiple causes, with around 50% thought due to infection. Of note infection as a pathogenesis is more likely in those pre-term births occurring <30 weeks gestation and is largely sub-clinical. Potential pathogens largely arise from the ascending route and from the endogenous vaginal flora, causing chorioamnionitis. Resultant morbidity from the release of endo+/exotoxins from such pathogens, the stimulation and production of inflammatory cytokine pathways, prostaglandins, metalloproteinases includes maternal sepsis (chorioamnionitis, septicaemia, post-partum endometritis), pre-term delivery (infant pre-maturity and its consequences, increased susceptibility to cerebral palsy and neonatal sepsis). As well, infection increases mortality due to fetal loss (extreme pre-maturity) as well as severe neonatal sepsis.

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.

Tyrosine phosphorylation of protein kinase Wzc from Escherichia coli K12 occurs through a two-step process

In bacteria, several proteins have been shown to autophosphorylate on tyrosine residues, but little is known on the molecular mechanism of this modification. To get more information on this matter, we have analyzed in detail the phosphorylation of a particular autokinase, protein Wzc, from Escherichia coli K12. The analysis of the hydropathic profile of this protein indicates that it is composed of two main domains: an N-terminal domain, including two transmembrane alpha-helices, and a C-terminal cytoplasmic domain. The C-terminal domain alone can undergo autophosphorylation and thus appears to harbor the protein-tyrosine kinase activity. By contrast, the N-terminal domain is not phosphorylated when incubated either alone or in the presence of the C-domain, and does not influence the extent of phosphorylation of the C-domain. The C-domain contains six different sites of phosphorylation. Among these, five are located at the C-terminal end of the molecule in the form of a tyrosine cluster (Tyr(708), Tyr(710), Tyr(711), Tyr(713), and Tyr(715)), and one site is located upstream, at Tyr(569). The Tyr(569) residue can autophosphorylate through an intramolecular process, whereas the tyrosine cluster cannot. The phosphorylation of Tyr(569) results in an increased protein kinase activity of Wzc, which can, in turn, phosphorylate the five terminal tyrosines through an intermolecular process. It is concluded that protein Wzc autophosphorylates by using a cooperative two-step mechanism that involves both intra- and interphosphorylation. This mechanism may be of biological significance in the signal transduction mediated by Wzc.

Contribution of Mn-cofactored superoxide dismutase (SodA) to the virulence of Streptococcus agalactiae

Superoxide dismutases convert superoxide anions to molecular oxygen and hydrogen peroxide, which, in turn, is metabolized by catalases and/or peroxidases. These enzymes constitute one of the major defense mechanisms of cells against oxidative stress and hence play a role in the pathogenesis of certain bacteria. We previously demonstrated that group B streptococci (GBS) possess a single Mn-cofactored superoxide dismutase (SodA). To analyze the role of this enzyme in the pathogenicity of GBS, we constructed a sodA-disrupted mutant of Streptococcus agalactiae NEM316 by allelic exchange. This mutant was subsequently cis complemented by integration into the chromosome of pAT113/Sp harboring the wild-type sodA gene. The SOD specific activity detected by gel analysis in cell extracts confirmed that active SODs were present in the parental and complemented strains but absent in the sodA mutant. The growth rates of these strains in standing cultures were comparable, but the sodA mutant was extremely susceptible to the oxidative stress generated by addition of paraquat or hydrogen peroxide to the culture medium and exhibited a higher mutation frequency in the presence of rifampin. In mouse bone marrow-derived macrophages, the sodA mutant showed an increased susceptibility to bacterial killing by macrophages. In a mouse infection model, after intravenous injection the survival of the sodA mutant in the blood and the brain was markedly reduced in comparison to that of the parental and complemented strains whereas only minor effects on survival in the liver and the spleen were observed. These results suggest that SodA plays a role in GBS pathogenesis.

Phosphorylation of Wzc, a tyrosine autokinase, is essential for assembly of group 1 capsular polysaccharides in Escherichia coli

Wzc proteins are tyrosine autokinases. They are found in some important bacterial pathogens of humans and livestock as well as plant-associated bacteria, and are often encoded within gene clusters determining synthesis and assembly of capsular and extracellular polysaccharides. Autophosphorylation of Wzc(cps) is essential for assembly of the serotype K30 group 1 capsule in Escherichia coli O9a:K30, although a genetically unlinked Wzc(cps)-homologue (Etk) can also participate with low efficiency. While autophosphorylation of Wzc(cps) is required for assembly of high molecular weight K30 capsular polysaccharide, it is not essential for either the synthesis of the K30 repeat units or for activity of the K30 polymerase enzyme. Paradoxically, the cognate phosphotyrosine protein phosphatase for Wzc(cps), Wzb(cps), is also required for capsule expression. The tyrosine-rich domain at the C terminus of Wzc(cps) was identified as the site of phosphorylation and autophosphorylation of Wzc requires a functional Walker A motif. Intermolecular transphosphorylation of Wzc(cps) was detected in strains expressing a combination of mutant Wzc(cps) derivatives. The N- and C-terminal domains of Wzc(cps) were expressed independently to mimic the situation found naturally in Gram-positive bacteria. In this format, both domains were required for phosphorylation of the Wzc(cps) C terminus, and for capsule assembly. Regulation by a post-translational phosphorylation event represents a new dimension in the assembly of bacterial cell-surface polysaccharides.

Functional analysis in type Ia group B Streptococcus of a cluster of genes involved in extracellular polysaccharide production by diverse species of streptococci

Several species of streptococci produce extracellular polysaccharides in the form of secreted exopolysaccharides or cell-associated capsules. Although the biological properties and repeating unit structures of these polysaccharides are diverse, sequence analysis of the genes required for their production has revealed a surprising degree of conservation among five genes found in the capsule gene cluster of each of several polysaccharide-producing streptococci. To determine the function of these conserved genes, we characterized a series of isogenic mutants derived from a wild-type strain of type Ia group B Streptococcus by selectively inactivating each gene. Inactivation of cpsIaE resulted in an acapsular phenotype, consistent with previous work that identified the cpsIaE product as the glycosyltransferase that initiates synthesis of the polysaccharide repeating unit. Mutants in cpsIaA, cpsIaB, cpsIaC, or cpsIaD produced type Ia capsular polysaccharide, but in reduced amounts compared with the wild type. Analysis of the mutant polysaccharides and of capsule gene transcription in the mutant strains provided evidence that cpsIaA encodes a transcriptional activator that regulates expression of the capsule gene operon. Mutants in cpsIaC or cpsIaD produced polysaccharide of reduced molecular size but with an identical repeating unit structure as the wild-type strain. We conclude that CpsA to -D are not required for polysaccharide repeating unit biosynthesis but rather that they direct the coordinated polymerization and export of high molecular weight polysaccharide.

Pathogenesis of neonatal Streptococcus agalactiae infections

Streptococcus agalactiae is an important human pathogen causing severe neonatal infections. During the course of infection, S. agalactiae colonizes and invades a number of different host compartments. Bacterial molecules including the polysaccharide capsule, the hemolysin, the C5a peptidase, the C-proteins, the hyaluronate lyase and a number of unknown bacterial components determine the interaction with host tissues. This review summarizes our current knowledge about these interactions.

Genotyping of the capsule gene cluster (cps) in nontypeable group B streptococci reveals two major cps allelic variants of serotypes III and VII

Forty group B Streptococcus (GBS) isolates obtained from Europe and the United States previously reported to be nontypeable (NT) by capsule serotype determination were subjected to buoyant density gradient centrifugation. From nearly half of the isolates capsule-expressing variants could be selected. For characterization of the remaining NT-GBS isolates, the capsule operon (cps) was amplified by the long-fragment PCR technique and compared by restriction fragment length polymorphism (RFLP) analysis. The patterns from serotype reference isolates (n = 32) were first determined and used as a comparison matrix for the NT-GBS isolates. Using two restriction enzymes, SduI and AvaII, cluster analysis revealed a high degree of similarity within serotypes but less than 88% similarity between serotypes. However, serotypes III and VII were each split in two distant RFLP clusters, which were designated III(1) and III(2) and VII(1) and VII(2), respectively. Among the isolates that remained NT after repeated Percoll gradient selections, two insertional mutants were revealed. Both were found in blood isolates and harbored insertion sequence (IS) elements within cpsD: one harbored IS1548, and the other harbored IS861. All other NT-GBS isolates could, by cluster analysis, be referred to different serotypes by comparison to the RFLP reference matrix. In pulsed-field gel electrophoresis of SmaI-restricted chromosomal DNA, patterns from allelic type 1 and 2 isolates were essentially distributed in separate clusters in serotypes III and VII. A covariation with insertion sequence IS1548 in the hylB gene was suggested for serotype III, since allelic type III(1) harboring IS1548 in hylB, clustered separately. The variation in serotype VII was not dependent on the presence of IS1548, which was not detected at any position in the type VII chromosome.

The serotype of type Ia and III group B streptococci is determined by the polymerase gene within the polycistronic capsule operon

Streptococcus agalactiae is a primary cause of neonatal morbidity and mortality. Essential to the virulence of this pathogen is the production of a type-specific capsular polysaccharide (CPS) that enables the bacteria to evade host immune defenses. The identification, cloning, sequencing, and functional characterization of seven genes involved in type III capsule production have been previously reported. Here, we describe the cloning and sequencing of nine additional adjacent genes, cps(III)FGHIJKL, neu(III)B, and neu(III)C. Sequence comparisons suggested that these genes are involved in sialic acid synthesis, pentasaccharide repeating unit formation, and oligosaccharide transport and polymerization. The type III CPS (cpsIII) locus was comprised of 16 genes within 15.5 kb of contiguous chromosomal DNA. Primer extension analysis and investigation of mRNA from mutants with polar insertions in their cpsIII loci supported the hypothesis that the operon is transcribed as a single polycistronic message. The translated cpsIII sequences were compared to those of the S. agalactiae cpsIa locus, and the primary difference between the operons was found to reside in cps(III)H, the putative CPS polymerase gene. Expression of cps(III)H in a type Ia strain resulted in suppression of CPS Ia synthesis and in production of a CPS which reacted with type III-specific polyclonal antibody. Likewise, expression of the putative type Ia polymerase gene in a type III strain reduced synthesis of type III CPS with production of a type Ia immunoreactive capsule. Based on the similar structures of the oligosaccharide repeating units of the type Ia and III capsules, our observations demonstrated that cps(Ia)H and cps(III)H encoded the type Ia and III CPS polymerases, respectively. Additionally, these findings suggested that a single gene can confer serotype specificity in organisms that produce complex polysaccharides.

Group B streptococci and other gram-positive cocci bind to cytokeratin 8

Group B streptococci (GBS) adhere to surface receptors present on epithelial cells; these receptors include fibronectin and laminin. To identify other possible receptors, plasma membranes from A549 cells, a respiratory tract epithelial cell line, were prepared. These plasma membranes were tested in a protein blot analysis using radiolabeled GBS as a probe. GBS adhered to two species, with molecular masses of 50 kDa (p50) and 57 kDa (p57). We concluded that p50 and p57 correspond to two forms of cytokeratin 8 (CK8) on the basis of the following results: (i) protein blot results demonstrated that p50 and p57 exactly comigrated with two forms of CK8 after separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (PAGE); (ii) p50 and p57 exactly comigrated with CK8 after separation by two-dimensional PAGE; (iii) CK8 in solution bound to GBS, as demonstrated by immunoblot analysis of proteins from A549 lysates that bound to GBS in a liquid-phase assay; and (iv) radiolabeled GBS bound to A549 lysate-derived CK8 that had been captured in anti-CK8-coated microtiter wells. CK8 bound to COH1-13, an acapsular mutant of COH1, demonstrating that adherence is not mediated by capsular polysaccharide. Trypsin-treated GBS did not bind to CK8, indicating that adherence is mediated via a protein on the surface of GBS. Soluble CK8 bound to six of six GBS strains tested. Soluble CK8 also bound to Staphylococcus aureus, Lactococcus lactis, Enterococcus faecalis, and Streptococcus pyogenes. We hypothesize that adherence of GBS to cytokeratin may be important for maintenance of colonization at sites of keratinized epithelium, such as the vagina, or for adherence of these bacteria to damaged epithelial cells at other sites.

Characterization of the capsular polysaccharide biosynthesis locus of Streptococcus pneumoniae type 19F

We have used a combination of plasmid insertion/rescue and inverse Polymerase Chain Reaction (PCR) to clone the region of the Streptococcus pneumoniae type 19F chromosome encoding biosynthesis of type 19F capsular polysaccharide (cps19f), which was then subjected to sequence analysis. The cps19f locus is located in the S. pneumoniae chromosome between dexB and aliA, and consists of 15 open reading frames (ORFs), designated cps19fA to cps19fO, that appear to be arranged as a single transcriptional unit. Insertion-duplication mutants in 13 of the 15 ORFs have been constructed in a smooth type 19F strain, all of which resulted in a rough (unencapsulated) phenotype, confirming that the operon is essential for capsule production. Comparison with sequence databases has allowed us to propose functions for 12 of the cps19f gene products, and a biosynthetic pathway for type 19F capsular polysaccharide. Southern hybridization analysis indicated that cps19fA and cps19fB were the only cps genes found in all 16 S. pneumoniae serotypes/groups tested. The region from cps19fG to cps19fK was found only in members of serogroup 19, and within this cps19fI was unique to type 19F.