Phenotypic diversity in the alpha C protein of group B streptococci

The long and the short of Periscope Proteins

Bacteria sense, interact with, and modify their environmental niche by deploying a molecular ensemble at the cell surface. The changeability of this exposed interface, combined with extreme changes in the functional repertoire associated with lifestyle switches from planktonic to adherent and biofilm states necessitate dynamic variability. Dynamic surface changes include chemical modifications to the cell wall; export of diverse extracellular biofilm components; and modulation of expression of cell surface proteins for adhesion, co-aggregation and virulence. Local enrichment for highly repetitive proteins with high tandem repeat identity has been an enigmatic phenomenon observed in diverse bacterial species. Preliminary observations over decades of research suggested these repeat regions were hypervariable, as highly related strains appeared to express homologues with diverse molecular mass. Long-read sequencing data have been interrogated to reveal variation in repeat number; in combination with structural, biophysical and molecular dynamics approaches, the Periscope Protein class has been defined for cell surface attached proteins that dynamically expand and contract tandem repeat tracts at the population level. Here, I review the diverse high-stability protein folds and coherent interdomain linkages culminating in the formation of highly anisotropic linear repeat arrays, so-called rod-like protein 'stalks', supporting roles in bacterial adhesion, biofilm formation, cell surface spatial competition, and immune system modulation. An understanding of the functional impacts of dynamic changes in repeat arrays and broader characterisation of the unusual protein folds underpinning this variability will help with the design of immunisation strategies, and contribute to synthetic biology approaches including protein engineering and microbial consortia construction.

Group B streptococcus — a pathogen not restricted to neonates

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Invasive group B streptococcal (GBS) infections can occur in any age group, not only neonates.

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Most of the patients are elderly or have severe concomitant diseases.

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Screening-based intrapartum antibiotic prophylaxis (based on maternal risk factors) significantly decreases very early-onset infections.

Objectives

This was a retropective study of invasive group B streptococcal (GBS) infections isolated from blood, cerebrospinal fluid (CSF), synovial fluid, peritoneal fluid, pleural fluid, pericardial fluid and corpus vitreum in a defined region in southwest Sweden over a 14-year period.

Design

Information on all invasive GBS infections was obtained from all four bacteriological laboratories in the region, with data obtained from individual patient records.

Results

GBS was isolated from normally sterile body fluids in 1244 samples (579 from females and 665 from males) from 1101 patients. Of these patients, 196 were neonates. The incidence in neonates (0–27 days) was 7.3 per 100 000 live births per year, but there was a significant decrease from 2012 when risk-factor-based intrapartum antibibiotic prophylaxis was implemented. The great majority of neonatal infections were very early-onset infections. The incidence rates in children (28 days to 17 years), adults (18–64 years) and elderly patients (≥ 65 years) were 1.3, 3.6, and 12.9 per 100 000 per year, respectively. The majority of children and adults had severe underlying diseases, but severe infections were also seen in individuals with no risk factors.

Conclusions

GBS is an important pathogen in all age groups. Intrapartum antibiotic prophylaxis significantly decreases very early-onset infections.

Surface Structures of Group B Streptococcus Important in Human Immunity

The surface of the Gram-positive opportunistic pathogen Streptococcus agalactiae, or group B Streptococcus (GBS), harbors several carbohydrate and protein antigens with the potential to be effective vaccines. Capsular polysaccharides of all clinically-relevant GBS serotypes coupled to immunogenic proteins of both GBS and non-GBS origin have undergone extensive testing in animals that led to advanced clinical trials in healthy adult women. In addition, GBS proteins either alone or in combination have been tested in animals; a fusion protein construct has recently advanced to human clinical studies. Given our current understanding of the antigenicity and immunogenicity of the wide array of GBS surface antigens, formulations now exist for the generation of viable vaccines against diseases caused by GBS.

The MiiA motif is a common marker present in polytopic surface proteins of oral and urinary tract invasive bacteria

Many surface virulence factors of bacterial pathogens show mosaicism and confounding phylogenetic origin. The Streptococcus gordonii platelet-binding GspB protein, the Streptococcus sanguinis SrpA adhesin and the Streptococcus pneumoniae DiiA protein, share an imperfect 27-residue motif. Given the disparate domain architectures of these proteins and its association to invasive disease, this motif was named MiiA from Multiarchitecture invasion-involved motif A. MiiA is predicted to adopt a beta-sheet folding, probably related to the Ig-like fold, with a symmetrical positioning of two conserved aspartic residues. A specific hidden Markov model profiling MiiA was built, which specifically detected the motif in proteins from 58 species, mainly in cell-wall proteins from Gram-positive bacteria. These proteins contained one to ten MiiA motifs, which were embedded within larger repeat units of 70-82 residues. MiiA motifs combined to other domains and elements such as coiled-coils and low-complexity regions. The species carrying MiiA-proteins included commensals from the urogenital tract and the oral cavity, which can cause opportunistic endocarditis and sepsis. Intra-protein MiiA repeats showed a complex mixture of orthologal, paralogal and inter-species relationships, suggestive of a multistep origin. Presence of these repeats in proteins involved in oligosaccharide recognition and lifestyle of species suggest a putative function for MiiA repeats in sugars binding, probably those present in receptors of epithelial and blood cells. MiiA modules appear to have been transferred horizontally between species co-habiting in the same niche to create their own MiiA-containing determinants. The present work provides a global study and a catalog of potential MiiA virulence factors that should be analyzed experimentally.

Interaction of Streptococcus agalactiae and Cellular Innate Immunity in Colonization and Disease

Streptococcus agalactiae (Group B streptococcus, GBS) is highly adapted to humans, where it is a normal constituent of the intestinal and vaginal flora. Yet, GBS has highly invasive potential and causes excessive inflammation, sepsis, and death at the beginning of life, in the elderly and in diabetic patients. Thus, GBS is a model pathobiont that thrives in the healthy host, but has not lost its potential virulence during coevolution with mankind. It remains incompletely understood how the innate immune system contains GBS in the natural niches, the intestinal and genital tracts, and which molecular events underlie breakdown of mucocutaneous resistance. Newborn infants between days 7 and 90 of life are at risk of a particularly striking sepsis manifestation (late-onset disease), where the transition from colonization to invasion and dissemination, and thus from health to severe sepsis is typically fulminant and not predictable. The great majority of late-onset sepsis cases are caused by one clone, GBS ST17, which expresses HvgA as a signature virulence factor and adhesin. In mice, HvgA promotes the crossing of both the mucosal and the blood–brain barrier. Expression levels of HvgA and other GBS virulence factors, such as pili and toxins, are regulated by the upstream two-component control system CovR/S. This in turn is modulated by acidic epithelial pH, high glucose levels, and during the passage through the mouse intestine. After invasion, GBS has the ability to subvert innate immunity by mechanisms like glycerinaldehyde-3-phosphate-dehydrogenase-dependent induction of IL-10 and β-protein binding to the inhibitory phagocyte receptors sialic acid binding immunoglobulin-like lectin 5 and 14. On the host side, sensing of GBS nucleic acids and lipopeptides by both Toll-like receptors and the inflammasome appears to be critical for host resistance against GBS. Yet, comprehensive models on the interplay between GBS and human immune cells at the colonizing site are just emerging.

Expression, purification and structural analysis of a fibrinogen receptor FbsA from Streptococcus agalactiae

Streptococcus agalactiae is a leading cause of bacterial sepsis and meningitis in neonates. FbsA, a fibrinogen receptor of S. agalactiae is highly repetitive protein with each repeat containing 16 amino acids. The protein sequence of FbsA shows no homology to any known fibrinogen binding protein from other bacterial species, making it a unique fibrinogen receptor. FbsA is cloned, expressed in E. coli and purified. The recombinant protein shows a laddering pattern in SDS-PAGE gel because of its poor stability in solution. The instability of the protein is probably because of the presence Gln-Gly dipeptide in each repeat. The circular dichroism study of FbsA has shown that the protein is composed of alpha helices predominantly and random coils to a lesser extent, which agrees with the predicted secondary structure. Ab initio modeling of a single repeat shows that FbsA is made up of mainly alpha helix and the structural model of multiple repeats (3 or 4) suggests that the protein might adopt some form of a repeating helical structure and the overall conformation of the molecule might change depending on the number of repeats.

Regulation of CovR expression in Group B Streptococcus impacts blood-brain barrier penetration

Group B Streptococcus (GBS) is an important cause of invasive infections in humans. The pathogen encodes a number of virulence factors including the pluripotent beta-haemolysin/cytolysin (beta-H/C). As GBS has the disposition of both a commensal organism and an invasive pathogen, it is important for the organism to appropriately regulate beta-H/C and other virulence factors in response to the environment. GBS can repress transcription of beta-H/C using the two-component system, CovR/CovS. Recently, we described that the serine/threonine kinase Stk1 can phosphorylate CovR at threonine 65 to relieve repression of beta-H/C. In this study, we show that infection with CovR-deficient GBS strains resulted in increased sepsis. Although CovR-deficient GBS showed decreased ability to invade the brain endothelium in vitro, they were more proficient in induction of permeability and pro-inflammatory signalling pathways in brain endothelium and penetration of the blood-brain barrier (BBB) in vivo. Microarray analysis revealed that CovR positively regulates its own expression and regulates the expression of 153 genes. Collectively, our results suggest that the positive feedback loop which regulates CovR transcription modulates host cell interaction and immune defence and may facilitate the transition of GBS from a commensal organism to a virulent meningeal pathogen.

Putative novel surface-exposed Streptococcus agalactiae protein frequently expressed by the group B streptococcus from Zimbabwe

Group B streptococci (GBS) express a variety of surface-exposed and strain-variable proteins which function as phenotypic markers and as antigens which are able to induce protective immunity in experimental settings. Among these proteins, the chimeric and immunologically cross-reacting alpha-like proteins are particularly important. Another protein, R3, which has been less well studied, occurred at a frequency of 21.5% in GBS from Zimbabwe and, notably, occurred in serotype V strains at a frequency of 75.9%. Working with rabbit antiserum raised against the R3 reference strain ATCC 49447 (strain 10/84; serotype V/R3) to detect the expression of the R3 protein, we recorded findings which suggested that strain 10/84 expressed a strain-variable protein antigen, in addition to R3. The antigen was detected by various enzyme-linked immunosorbent assay-based tests by using acid extract antigens or GBS whole-cell coats and by whole-cell-based Western blotting. We named the putative novel antigen the Z antigen. The Z antigen was a high-molecular-mass antigen that was susceptible to degradation by pepsin and trypsin but that was resistant to m-periodate oxidation and failed to show immunological cross-reactivity with any of a variety of other GBS protein antigens. The Z antigen was expressed by 33/121 (27.2%) of strains of a Zimbabwean GBS strain collection and by 64.2% and 72.4% of the type Ib and type V strains, respectively, and was occasionally expressed by GBS of other capsular serotypes. Thus, the putative novel GBS protein named Z showed distinct capsular antigen associations and presented as an important phenotypic marker in GBS from Zimbabwe. It may be an important antigen in GBS from larger areas of southern Africa. Its prevalence in GBS from Western countries is not known.

Identification of serine/threonine kinase substrates in the human pathogen group B streptococcus

All living organisms respond to changes in their internal and external environment for their survival and existence. Signaling is primarily achieved through reversible phosphorylation of proteins in both prokaryotes and eukaryotes. A change in the phosphorylation state of a protein alters its function to enable the control of cellular responses. A number of serine/threonine kinases regulate the cellular responses of eukaryotes. Although common in eukaryotes, serine/threonine kinases have only recently been identified in prokaryotes. We have described that the human pathogen Group B Streptococcus (GBS, Streptococcus agalactiae) encodes a single membrane-associated, serine/threonine kinase (Stk1) that is important for virulence of this bacterium. In this study, we used a combination of phosphopeptide enrichment and mass spectrometry to enrich and identify serine (S) and threonine (T) phosphopeptides of GBS. A comparison of S/T phosphopeptides identified from the Stk1 expressing strains to the isogenic stk1 mutant indicates that 10 proteins are potential substrates of the GBS Stk1 enzyme. Some of these proteins are phosphorylated by Stk1 in vitro and a site-directed substitution of the phosphorylated threonine to an alanine abolished phosphorylation of an Stk1 substrate. Collectively, these studies provide a novel approach to identify serine/threonine kinase substrates for insight into their signaling in human pathogens like GBS.

The group B streptococcal alpha C protein binds alpha1beta1-integrin through a novel KTD motif that promotes internalization of GBS within human epithelial cells

Group B Streptococcus (GBS) is the leading cause of bacterial pneumonia, sepsis and meningitis among neonates and a cause of morbidity among pregnant women and immunocompromised adults. GBS epithelial cell invasion is associated with expression of alpha C protein (ACP). Loss of ACP expression results in a decrease in GBS internalization and translocation across human cervical epithelial cells (ME180). Soluble ACP and its 170 amino acid N-terminal region (NtACP), but not the repeat protein RR', bind to ME180 cells and reduce internalization of wild-type GBS to levels obtained with an ACP-deficient isogenic mutant. In the current study, ACP colocalized with alpha(1)beta(1)-integrin, resulting in integrin clustering as determined by laser scanning confocal microscopy. NtACP contains two structural domains, D1 and D2. D1 is structurally similar to fibronectin's integrin-binding region (FnIII10). D1's (KT)D146 motif is structurally similar to the FnIII10 (RG)D1495 integrin-binding motif, suggesting that ACP binds alpha(1)beta(1)-integrin via the D1 domain. The (KT)D146A mutation within soluble NtACP reduced its ability to bind alpha(1)beta(1)-integrin and inhibit GBS internalization within ME180 cells. Thus ACP binding to human epithelial cell integrins appears to contribute to GBS internalization within epithelial cells.

Alpha C protein-specific immunity in humans with group B streptococcal colonization and invasive disease

Alpha C protein, found in 76% of non-type III strains of group B Streptococcus (GBS), elicits antibodies protective against alpha C-expressing strains in experimental animals, making it an appealing carrier for a GBS conjugate vaccine. We determined whether natural exposure to alpha C elicits antibodies in women. Geometric mean concentrations of alpha C-specific IgM and IgG were similar by ELISA in sera from 58 alpha C GBS strain colonized and 174 age-matched non-colonized women (IgG 245 and 313 ng/ml; IgM 257 and 229 ng/ml, respectively), but acute sera from 13 women with invasive alpha C-expressing GBS infection had significantly higher concentrations (IgM 383 and IgG 476 ng/ml [p=0.036 and 0.038, respectively]). Convalescent sera from 5 of these women 16-49 days later had high alpha C-specific IgM and IgG concentrations (1355 and 4173 ng/ml, respectively). In vitro killing of alpha C-expressing GBS correlated with total alpha C-specific antibody concentration. Invasive disease but not colonization elicits alpha C-specific IgM and IgG in adults.

Distribution of novel and previously investigated virulence genes in colonizing and invasive isolates of Streptococcus agalactiae

Although Streptococcus agalactiae has emerged as an important cause of invasive disease, relatively little is known regarding the genetic basis of virulence of this organism. Three novel genes with characteristics suggesting a role in virulence were identified via comparison of sequenced genomes of S. agalactiae. The presence of these genes and of the previously identified genes bac, bca, rib, and spb1 was determined, and isolates were assigned a binary genetic signature. It was found that isolates containing spb1, previously suggested to be limited to serotype III-3, were represented by 18 different genetic signatures and several serotypes, and that the presence of both sbp1 and rib was more predictive of invasive disease than spb1 alone. Additionally, bac-positive isolates, reported to be genetically homogeneous, were represented by 14 different genetic signatures. Finally, the majority of serotype V isolates examined contained zero or only one of the genes tested, suggesting that much remains undiscovered regarding important virulence factors in isolates of this serotype.

The frequency of genes encoding three putative group B streptococcal virulence factors among invasive and colonizing isolates

BACKGROUND

Group B Streptococcus (GBS) causes severe infections in very young infants and invasive disease in pregnant women and adults with underlying medical conditions. GBS pathogenicity varies between and within serotypes, with considerable variation in genetic content between strains. Three proteins, Rib encoded by rib, and alpha and beta C proteins encoded by bca and bac, respectively, have been suggested as potential vaccine candidates for GBS. It is not known, however, whether these genes occur more frequently in invasive versus colonizing GBS strains.

METHODS

We screened 162 invasive and 338 colonizing GBS strains from different collections using dot blot hybridization to assess the frequency of bca, bac and rib. All strains were defined by serotyping for capsular type, and frequency differences were tested using the Chi square test.

RESULTS

Genes encoding the beta C protein (bac) and Rib (rib) occurred at similar frequencies among invasive and colonizing isolates, bac (20% vs. 23%), and rib (28% vs. 20%), while the alpha (bca) C protein was more frequently found in colonizing strains (46%) vs, invasive (29%). Invasive strains were associated with specific serotype/gene combinations.

CONCLUSION

Novel virulence factors must be identified to better understand GBS disease.

Abundance of the delta subunit of RNA polymerase is linked to the virulence of Streptococcus agalactiae

Group B streptococcus (GBS) remains a major cause of morbidity and mortality among newborn children. The bacterium is a commensal organism colonizing the rectum and the gastrointestinal and urogenital tracts of adults, but it can be transmitted to neonates by an ascending infection of the maternal genital tract or during parturition. We previously reported that a transposon insertion disrupting rpoE resulted in the decreased survival of the mutant in the neonatal rat sepsis model of GBS infection. rpoE encodes the delta protein, a subunit of RNA polymerase (RNAP) that has been characterized in Bacillus species. In this study, we confirm the association of the delta protein with purified GBS RNAP and show that it is expressed in strains representing all nine serotypes. Flow cytometric analysis of a reporter strain containing a transcriptional fusion of the rpoE promoter to gfp revealed that, in vitro, this gene is continuously expressed. Analysis of delta expression in the transposon mutant by quantitative Western blotting revealed a 10-fold reduction in relative abundance (which was linked to the attenuation in virulence that was observed for this mutant) compared to that for the wild-type strain. These data suggest that a minimum intracellular concentration of delta is necessary for this organism to cause disease.

Characterization of a novel leucine-rich repeat protein antigen from group B streptococci that elicits protective immunity

Group B streptococci (GBS) usually behave as commensal organisms that asymptomatically colonize the gastrointestinal and urogenital tracts of adults. However, GBS are also pathogens and the leading bacterial cause of life-threatening invasive disease in neonates. While the events leading to transmission and disease in neonates remain unclear, GBS carriage and level of colonization in the mother have been shown to be significant risk factors associated with invasive infection. Surface antigens represent ideal vaccine targets for eliciting antibodies that can act as opsonins and/or inhibit colonization and invasion. Using a genetic screen for exported proteins in GBS, we identified a gene, designated lrrG, that encodes a novel LPXTG anchored surface antigen containing leucine-rich repeat (LRR) motifs found in bacterial invasins and other members of the LRR protein family. Southern blotting showed that lrrG was present in all GBS strains tested, representing the nine serotypes, and revealed the presence of an lrrG homologue in Streptococcus pyogenes. Recombinant LrrG protein was shown in vitro to adhere to epithelial cells in a dose-dependent manner, suggesting that it may function as an adhesion factor in GBS. More importantly, immunization with recombinant LrrG elicited a strong immunoglobulin G response in CBA/ca mice and protected against lethal challenge with virulent GBS. The data presented in this report suggest that this conserved protein is a highly promising candidate antigen for use in a GBS vaccine.

Surface proteins of Streptococcus agalactiae and related proteins in other bacterial pathogens

Streptococcus agalactiae (group B Streptococcus) is the major cause of invasive bacterial disease, including meningitis, in the neonatal period. Although prophylactic measures have contributed to a substantial reduction in the number of infections, development of a vaccine remains an important goal. While much work in this field has focused on the S. agalactiae polysaccharide capsule, which is an important virulence factor that elicits protective immunity, surface proteins have received increasing attention as potential virulence factors and vaccine components. Here, we summarize current knowledge about S. agalactiae surface proteins, with emphasis on proteins that have been characterized immunochemically and/or elicit protective immunity in animal models. These surface proteins have been implicated in interactions with human epithelial cells, binding to extracellular matrix components, and/or evasion of host immunity. Of note, several S. agalactiae surface proteins are related to surface proteins identified in other bacterial pathogens, emphasizing the general interest of the S. agalactiae proteins. Because some S. agalactiae surface proteins elicit protective immunity, they hold promise as components in a vaccine based only on proteins or as carriers in polysaccharide conjugate vaccines.

Molecular pathogenesis of neonatal group B streptococcal infection: no longer in its infancy

The process of human infection by group B Streptococcus (GBS) is complex and multifactorial. While this bacterium has adapted well to asymptomatic colonization of adult humans, it remains a potentially devastating pathogen to susceptible infants. Advances in molecular techniques and refinement of in vitro and in vivo model systems have elucidated key elements of the pathogenic process, from initial attachment to the maternal vaginal epithelium to penetration of the newborn blood-brain barrier. Sequencing of two complete GBS genomes has provided additional context for interpretation of experimental data and comparison to other well-studied pathogens. Here we review recent discoveries regarding GBS virulence mechanisms, many of which are revealed or magnified by the unique circumstances of the birthing process and the deficiencies of neonatal immune defence. Appreciation of the formidable array of GBS virulence factors underscores why this bacterium remains at the forefront of neonatal pathogens.

Alpha C protein of group B Streptococcus binds host cell surface glycosaminoglycan and enters cells by an actin-dependent mechanism

Group B Streptococcus (GBS) colonizes mucosal surfaces of the human gastrointestinal and gynecological tracts and causes disease in a wide range of patients. Invasive illness occurs after organisms traverse an epithelial boundary and enter deeper tissues. Previously we have reported that the alpha C protein (ACP) on the surface of GBS mediates GBS entry into ME180 cervical epithelial cells and GBS translocation across layers of these cells. We now demonstrate that ACP interacts with host cell glycosaminoglycan (GAG); the interaction of ACP with ME180 cells is inhibited if cells are pretreated with sodium chlorate, an inhibitor of sulfate incorporation, or with heparitinases. The interaction is also inhibited in the presence of soluble heparin or heparan sulfate or host cell-derived GAG. In addition, ACP binds soluble heparin specifically in inhibition and dot blot assays. After interaction with host GAG, soluble ACP enters ME180 cells and fractionates to the eukaryotic cell cytosol. These events are inhibited in cells pretreated with cytochalasin D or with Clostridium difficile toxin B. These data indicate that full-length ACP interacts with ME180 cell GAG and enters the eukaryotic cell cytosol by a mechanism that involves Rho GTPase-dependent actin rearrangements. We suggest that these molecular interactions drive ACP-mediated translocation of GBS across epithelial barriers, thereby facilitating invasive GBS infection.

Molecular analysis of group B protective surface protein, a new cell surface protective antigen of group B streptococci

Group B streptococci (GBS) express various surface antigens designated c, R, and X antigens. A new R-like surface protein from Streptococcus agalactiae strain Compton R has been identified by using a polyclonal antiserum raised against the R protein fraction of this strain to screen a lambda Zap library. DNA sequence analysis of positive clones allowed the prediction of the primary structure of a 105-kDa protein designated BPS protein (group B protective surface protein) that exhibited typical features of streptococcal surface proteins such as a signal sequence and a membrane anchor region but did not show significant similarity with other known sequences. Immunogold electron microscopy using a BPS-specific antiserum confirmed the surface location of BPS protein on S. agalactiae strain Compton R. Anti-BPS antibodies did not cross-react with R1 and R4 proteins expressed by two variant type III GBS strains but reacted with the parental streptococcal strain in Western blot and immunoprecipitation analyses. Separate R3 and BPS immunoprecipitation bands were observed when a cell extract of strain Compton R was tested with an antiserum against Compton R previously cross-absorbed to remove R4 antibodies. Immunization of mice with recombinant BPS protein by the subcutaneous route produced an efficient antigen-specific response, and immunized animals survived challenge with a lethal dose of a virulent strain. Therefore, BPS protein represents a new R-like protective antigen of GBS.

Polymorphisms in the cell wall-spanning domain of the C protein beta-antigen in clinical Streptococcus agalactiae isolates are caused by genetic instability of repeating DNA sequences

The C protein alpha- and beta-antigens are immunodominant components of the surface of Streptococcus agalactiae, the most frequent cause of neonatal sepsis. Both proteins are thought to contribute significantly to virulence of S. agalactiae. They are mainly expressed by serotypes Ia, Ib, and II. The C protein beta-antigen (Cbeta-protein) binds to the Fc portion of human IgA and seems to be of importance in bacterial resistance to mucosal immune defense mechanisms. In this study, PCR analysis of S. agalactiae isolates obtained from 189 neonates and 112 pregnant women revealed the presence of the Cbeta-protein gene in 19% and 22% of the isolates, respectively. Size polymorphisms of the PCR products within the gene region encoding the cell wall-spanning domain indicated a high degree of genetic variability. Thirteen different variants of the amplified region were differentiated among the 60 Cbeta-protein-positive isolates by sequence analysis. In all variants, the polymorphisms were caused by insertions and deletions of repetitive DNA elements that did not alter the open reading frame. Comparison of the Cbeta-protein gene polymorphisms showed a significantly higher rate of isolates carrying deletions >50 bp in serotype Ib than in serotype II isolates (p = 0.001); this was also true for neonatal isolates analyzed separately (p = 0.01). Neonatal isolates carried a higher rate of large deletions when compared with maternal isolates; this difference, however, did not reach statistical significance (p = 0.08). We hypothesize that polymorphisms in the cell wall-spanning domain of the Cbeta-protein are of functional relevance with regard to maternofetal transmission of the pathogen.

Tandem repeat deletion in the alpha C protein of group B streptococcus is recA independent

Group B streptococci (GBS) contain a family of protective surface proteins characterized by variable numbers of repeating units within the proteins. The prototype alpha C protein of GBS from the type Ia/C strain A909 contains a series of nine identical 246-bp tandem repeat units. We have previously shown that deletions in the tandem repeat region of the alpha C protein affect both the immunogenicity and protective efficacy of the protein in animal models, and these deletions may serve as a virulence mechanism in GBS. The molecular mechanism of tandem repeat deletion is unknown. To determine whether RecA-mediated homologous recombination is involved in this process, we identified, cloned, and sequenced the recA gene homologue from GBS. A strain of GBS with recA deleted, A909DeltarecA, was constructed by insertional inactivation in the recA locus. A909DeltarecA demonstrated significant sensitivity to UV light, and the 50% lethal dose of the mutant strain in a mouse intraperitoneal model of sepsis was 20-fold higher than that of the parent strain. The spontaneous rate of tandem repeat deletion in the alpha C protein in vitro, as well as in our mouse model of immune infection, was studied using A909DeltarecA. We report that tandem repeat deletion in the alpha C protein does occur in the absence of a functional recA gene both in vitro and in vivo, indicating that tandem repeat deletion in GBS occurs by a recA-independent recombinatorial pathway.

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.

Identification of group B streptococcal Sip protein, which elicits cross-protective immunity

A protein of group B streptococci (GBS), named Sip for surface immunogenic protein, which is distinct from previously described surface proteins, was identified after immunological screening of a genomic library. Immunoblots using a Sip-specific monoclonal antibody indicated that a protein band with an approximate molecular mass of 53 kDa which did not vary in size was present in every GBS strain tested. Representatives of all nine GBS serotypes were included in the panel of strains. Cloning and sequencing of the sip gene revealed an open reading frame of 1,305 nucleotides coding for a polypeptide of 434 amino acid residues, with a calculated pI of 6. 84 and molecular mass of 45.5 kDa. Comparison of the nucleotide sequences from six different strains confirmed with 98% identity that the sip gene is highly conserved among GBS isolates. N-terminal amino acid sequencing also indicated the presence of a 25-amino-acid signal peptide which is cleaved in the mature protein. More importantly, immunization with the recombinant Sip protein efficiently protected CD-1 mice against deadly challenges with six GBS strains of serotypes Ia/c, Ib, II/R, III, V, and VI. The data presented in this study suggest that this highly conserved protein induces cross-protective immunity against GBS infections and emphasize its potential as a universal vaccine candidate.

Mosaicism in the alpha-like protein genes of group B streptococci

Members of a family of repeat-containing surface proteins of group B streptococci (GBS) defined by the alpha C and Rib proteins exhibit size variability and cross-reactivity and have been studied as potential vaccine components. We report evidence of horizontal DNA transfer with subsequent recombination as a mechanism generating diversity within this antigen family. Alp2 and Alp3 are additional members of the alpha C protein family identified in strains of the emerging GBS serotypes V and VIII. Each contains an overall genetic organization highly similar to that of the alpha C and Rib proteins, including a tandem repeat region and conserved N- and C-terminal regions. Among different strains, protein size varies according to the number of tandem repeats within the corresponding gene. Unlike the alpha C and Rib proteins, however, the newly described alpha-like proteins contain other regions, including one similar to the IgA-binding region of the GBS beta C protein, a nontandem repeat region, and an isolated repeat highly homologous to the alpha C repeat. Sequence analysis of the regions flanking the alpha C protein gene on a 13.7-kb insert reveals several ORFs that are likely to be involved in basic metabolic pathways. Analysis of corresponding flanking regions in other GBS strains, including the parent strains of the newly described alpha-like proteins, shows striking conservation among all strains studied. These findings indicate that the alpha-like proteins are encoded by mosaic variants at a single genomic locus and suggest that recombination after horizontal DNA transfer is a means of generating diversity within this protein family.

Group B streptococcal surface proteins as targets for protective antibodies: identification of two novel proteins in strains of serotype V

Strains of group B streptococcus (GBS) express surface proteins that confer protective immunity. In particular, most strains of the four classical capsular serotypes (Ia, Ib, II, and III) express either of the Rib and alpha proteins, two members of the same protein family. Here, we report a study of surface proteins expressed by strains of serotype V, which has recently emerged as an important serotype among GBS strains causing serious disease. Two novel GBS proteins were identified, purified, and characterized. One of these proteins, designated Fbs, was immunologically unrelated to other GBS surface proteins. This approximately 110-kDa protein was found in 15 of 49 (31%) type V isolates but in few strains of other serotypes. The Fbs proteins expressed by different strains showed limited variation in size. The most common surface protein among type V strains, found in 29 of 49 (59%) isolates, was designated Rib-like, since it cross-reacted with Rib but was not immunologically identical to Rib. Characterization of this Rib-like protein showed that the N-terminal sequence (12 residues) was identical to that of alpha, although these two proteins lacked cross-reactivity. The biochemical and immunological properties of the Rib-like GBS protein indicate that it is closely related to the R28 protein of Streptococcus pyogenes. Importantly, passive and active immunization experiments with mice showed that the Fbs and Rib-like proteins are targets for protective antibodies. These two proteins are therefore of interest for analysis of pathogenic mechanisms and for vaccine development.

Properties and distribution of the putative R3 protein of Streptococcus agalactiae

Strain-variable Streptococcus agalactiae (group B streptococci; GBS) proteins exposed at the bacterial cell surface are important markers in GBS serotyping. These proteins include the c proteins c(alpha) and c(beta) and the R proteins R1 through R4, of which R1 and R4 have been studied most extensively. This study presents the characteristics of a protein which was expressed by a capsular antigen type V GBS strain shown by means of polyclonal and monoclonal antibody testing. Examination of a number of reference and prototype strains by fluorescent antibody testing and Western blotting provided evidence that the serotype V-derived protein was the R3 protein of GBS, previously defined on the basis of immunoprecipitation assays. The putative R3 protein formed ladder-like banding patterns on Western blotting with polypeptides in the 30 kDa to > or = 140 kDa range, was destroyed by pepsin digestion, and partially degraded by trypsin digestion. The protein was expressed by 10 (6.5%) of 153 clinical GBS strains tested, the expression being restricted to isolates of the capsular antigen types II, III, and V. Some isolates expressed both the c(beta) and the R3 protein. Expression in combination with c(alpha) or R4 protein synthesis was not detected. Inclusion of the anti-R3 monoclonal antibody among antibody reagents for GBS serotyping will enhance the discriminatory power of this typing method.

The R28 protein of Streptococcus pyogenes is related to several group B streptococcal surface proteins, confers protective immunity and promotes binding to human epithelial cells

The R28 protein is a surface molecule expressed by some strains of Streptococcus pyogenes (group A streptococcus). Here, we present evidence that R28 may play an important role in virulence. Sequence analysis demonstrated that R28 has an extremely repetitive sequence and can be viewed as a chimera derived from the three surface proteins Rib, alpha and beta of the group B streptococcus (GBS). Thus, the gene encoding R28 may have originated in GBS. The R28 protein promotes adhesion to human epithelial cells, as shown by experiments with an R28-negative mutant and by the demonstration that antibodies to highly purified R28 inhibited adhesion. In a mouse model of lethal intraperitoneal S. pyogenes infection, antibodies to R28 conferred protective immunity. However, the virulence of an R28-negative mutant was similar to that of the parental strain in the intraperitoneal infection model. Together, these data indicate that R28 represents a novel type of adhesin expressed by S. pyogenes and that R28 may also act as a target for protective antibodies at later stages of an infection. We consider the hypothesis that R28 played a pathogenetic role in the well-known epidemics of childbed fever (puerperal fever), which were caused by S. pyogenes. A role for R28 in these epidemics is suggested by epidemiological data.

Alpha C protein as a carrier for type III capsular polysaccharide and as a protective protein in group B streptococcal vaccines

The alpha C protein, a protective surface protein of group B streptococci (GBS), is present in most non-type III GBS strains. Conjugate vaccines composed of the alpha C protein and type III capsular polysaccharide (CPS) might be protective against most GBS infections. In this study, the type III CPS was covalently coupled to full-length, nine-repeat alpha C protein (resulting in III-alpha9r conjugate vaccine) or to two-repeat alpha C protein (resulting in III-alpha2r conjugate vaccine) by reductive amination. Initial experiments with the III-alpha9r vaccine showed that it was poorly immunogenic in mice with respect to both vaccine antigens and was suboptimally efficacious in providing protection in mice against challenge with GBS. Therefore, modified vaccination protocols were used with the III-alpha2r vaccine. Female mice were immunized three times with 0.5, 5, or 20 microgram of the III-alpha2r vaccine with an aluminum hydroxide adjuvant and bred. Ninety-five percent of neonatal mice born to dams immunized with the III-alpha2r vaccine survived challenge with GBS expressing type III CPS, and 60% survived challenge with GBS expressing wild-type (nine-repeat) alpha C protein; 18 and 17%, respectively, of mice in the negative control groups survived (P, <0.0001). These protection levels did not differ significantly from those obtained with the type III CPS-tetanus toxoid conjugate vaccine and the unconjugated two-repeat alpha C protein, which protected 98 and 58% of neonates from infection with GBS expressing type III CPS or the alpha C protein, respectively. Thus, the two-repeat alpha C protein in the vaccine was immunogenic and simultaneously enhanced the immunogenicity of type III CPS. III-alpha vaccines may be alternatives to GBS polysaccharide-tetanus toxoid vaccines, eliciting additional antibodies protective against GBS infection.

Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

Comparison of three different methods in monoclonal antibody-based detection of Streptococcus agalactiae protein serotype markers

Surface-exposed proteins are important serotype markers in Streptococcus agalactiae (group B streptococci; GBS). The proteins include the c proteins c(alpha) and c(beta), the R4 protein and a protein provisionally called P. For all of these markers, protein-specific monoclonal antibodies (MAbs) have been generated. We have compared whole-cell-based fluorescent antibody testing (FAT), ELISA, and dot blotting for MAb-based detection of these proteins by testing a panel of 52 GBS isolates of different capsular antigen types. Of a total of 208 observations with each of the tests, positive signalling in the dot assay was observed in 32.2%, with ELISA in 27.8%, and with FAT in 26.4% of the recordings. Discordant results were noted most frequently with the c(beta) and c(alpha) MAbs. In the case of c(alpha) the reason for the discordant test results was further examined and it appeared that this could be attributed to low level expression of the c(alpha) protein, although structural variations of c(alpha) proteins cannot be excluded. Our findings favour dot blotting as the method of choice although we consider all three methods acceptable for serotyping of GBS.

Infection-derived Enterococcus faecalis strains are enriched in esp, a gene encoding a novel surface protein

We report the identification of a new cell wall-associated protein of Enterococcus faecalis. Studies on the distribution of the gene encoding this novel surface protein, Esp, reveal a significant (P < 0.001) enrichment in infection-derived E. faecalis isolates. Interestingly, the esp gene was not identified in any of 34 clinical E. faecium isolates or in 4 other less pathogenic enterococcal species tested. Analysis of the structural gene among various E. faecalis isolates reveals the existence of alternate forms of expression of the Esp protein. The deduced primary structure of the Esp protein from strain MMH594, inferred to be 1,873 amino acids (aa) with a predicted mass of approximately 202 kDa, reveals a core region consisting of repeat units that make up 50% of the protein. Esp bears global organizational similarity to the Rib and C alpha proteins of group B streptococci. Identity among Esp, Rib, and C alpha proteins is strikingly localized to a stretch of 13 aa within repeats of similar length. The high degree of conservation of this 13-residue sequence suggests that it plays an important role in the natural selection for this trait among infection-derived E. faecalis and group B streptococcal isolates.

Deletion of repeats in the alpha C protein enhances the pathogenicity of group B streptococci in immune mice

The alpha C protein is a protective surface-associated antigen of group B streptococci (GBS). The prototype alpha C protein of GBS (strain A909) contains nine identical tandem repeats, each comprising 82 amino acids, flanked by N- and C-terminal domains. Clinical isolates of GBS show variable numbers of repeats with a normal distribution and a median of 9 to 10 repeats. Here, we show that escape mutants of GBS expressing one-repeat alpha C protein were 100-fold more pathogenic than GBS expressing wild-type nine-repeat alpha C protein in neonatal mice whose dams were immunized with antiserum elicited to nine-repeat alpha C protein (50% lethal doses of 1.6 x 10(3) and 1.8 x 10(5), respectively; P = 0.0073). There was no difference in pathogenicity in nonimmune mice. Enzyme-linked immunosorbent assay inhibition showed that nine-repeat but not one-repeat alpha C protein is readily available for antibody binding on the surface of intact GBS. Immune electron microscopy studies with antibodies to the capsular polysaccharide (CPS) and to the alpha C protein demonstrated localization of the nine-repeat alpha C protein and the CPS at similar distances from the cell wall. The one-repeat alpha C protein was visualized poorly and only in close proximity to the cell wall, thus suggesting that antibody binding to the protein was hindered by CPS or other cell surface components. We concluded that deletion in the repeat region of the alpha C protein enhanced the pathogenicity of GBS in immune mice by (i) loss of a protective (conformational) epitope(s) and (ii) loss of antibody binding to the alpha C protein due to a decrease in antigen size relative to cell wall components and/or CPS.

Immunogenicity and protective efficacy of the alpha C protein of group B streptococci are inversely related to the number of repeats

Infection by group B streptococci (GBS) is an important cause of bacterial disease in neonates. Alpha C protein is a protective cell surface-associated protein of GBS. This protein contains a repeat region flanked by N and C termini. Variable expression of tandem repeating units of alpha C proteins had been found among clinical isolates of GBS. We examined the effect of the number of repeats on the immunogenicity of the alpha C protein and its ability to elicit protection from GBS infection in a neonatal mouse model. Mice were immunized with purified alpha C proteins of constructs containing various numbers of repeats (n = 1, 2, 9, and 16) and the N- and C-terminal regions. Both the N-terminal and the repeat regions contain protective and opsonic epitopes. Antibody responses to the alpha C protein constructs with various numbers of repeats were tested with enzyme-linked immunosorbent assay plates coated with either native, nine-repeat alpha C protein or "repeatless" N-terminal antigen. An inverse relationship was found between the number of repeats and the immunogenicity of the alpha C protein; this effect was most pronounced on titers of antibody to the N-terminal region. An inverse relationship was also observed between the number of repeats and protective efficacy, i.e., mouse dams immunized with 5 microg of one- or nine-repeat alpha C protein transferred protective immunity to 65 or 11% of their pups, respectively (P < 0.0001). Thus, the presence of multiple repeats appears to lessen the antibody response to the complete alpha C protein, and especially the antibody response to its N-terminal region, and suggests a mechanism whereby repeat elements contribute to the evasion of host immunity.

Inactivation of the alpha C protein antigen gene, bca, by a novel shuttle/suicide vector results in attenuation of virulence and immunity in group B Streptococcus

The alpha C protein of group B Streptococcus (GBS) is a major surface-associated antigen. Although its role in the biology and virulence of GBS has not been defined, it is opsonic and capable of eliciting protective immunity. The alpha C protein is widely distributed among clinical isolates and is a potential protein carrier and antigen in conjugate vaccines to prevent GBS infections. The structural gene for the alpha C protein, bca, has been cloned and sequenced. The protein encoded by bca is related to a class of surface-associated proteins of gram-positive cocci involved in virulence and immunity. To investigate the potential roles of the alpha C protein, bca null mutants were generated in which the bca gene was replaced with a kanamycin resistance cassette via homologous recombination using a novel shuttle/suicide vector. Studies of lethality in neonatal mice showed that the virulence of the bca null mutants was attenuated 5- to 7-fold when compared with the isogenic wild-type strain A909. Significant differences in mortality occurred in the first 24 h, suggesting that the role of the alpha antigen is important in the initial stages of the infection. In contrast to A909, bca mutants were no longer killed by polymorphonuclear leukocytes in the presence of alpha-specific antibodies in an in vitro opsonophagocytic assay. In contrast to previous studies, alpha antigen expression does not appear to play a role in resistance to opsonophagocytosis in the absence of alpha-specific antibodies. In addition, antibodies to the alpha C protein did not passively protect neonatal mice from lethal challenge with bca mutants, suggesting that these epitopes are uniquely present within the alpha antigen as expressed from the bca gene. Therefore, the alpha C protein is important in the pathogenesis of GBS infection and is a target for protective immunity in the development of GBS vaccines.

Characterization of two distinct opsonic and protective epitopes within the alpha C protein of the group B Streptococcus

Group B Streptococcus (GBS) is a major cause of neonatal sepsis, meningitis in early infancy, postpartum endometritis, and serious invasive infections in adults in the United States. We previously cloned, sequenced, and characterized the alpha antigen gene, bca, and showed that the alpha C protein of GBS is a trypsin-resistant, surface-associated polypeptide that contains a signal sequence, a unique N terminus, nine identical tandem repeats, and a C-terminal membrane anchor structure. Polyclonal antiserum raised to the recombinant alpha C protein and an opsonic monoclonal antibody, 4G8, raised to the native protein from GBS have been shown to be protective in a mouse model. The binding site of 4G8 has now been localized to the tandem repeat region of the alpha C protein. To determine whether the N terminus of the alpha C protein contains additional opsonic and/or protective epitopes, the sequence corresponding to the alpha C protein N terminus was subcloned into a pET vector, the expressed peptide from Escherichia coli was purified by Ni2+ affinity chromatography, and rabbit polyclonal antibodies were raised to the purified recombinant peptide. Antibodies to the alpha C protein N terminus were shown to be opsonic by an in vitro opsonophagocytosis assay. In addition, 69% of newborn mouse pups from mothers passively immunized with the antiserum to the recombinant N-terminal polypeptide of the alpha C protein were protected against lethal challenge with GBS A909. These data indicate that at least two distinct regions of the alpha C protein, the N terminus and the tandem repeat region, contain opsonic and protective epitopes.

A protective surface protein from type V group B streptococci shares N-terminal sequence homology with the alpha C protein

Infection by group B streptococci (GBS) is an important cause of bacterial disease in neonates, pregnant women, and nonpregnant adults. Historically, serotypes Ia, Ib, II, and III have been most prevalent among disease cases; recently, type V strains have emerged as important strains in the United States and elsewhere. In addition to type-specific capsular polysaccharides, many GBS strains possess surface proteins which demonstrate a laddering pattern on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and resistance to trypsin digestion. These include the alpha C protein, the R proteins, and protein Rib. Some of these proteins elicit protective antibodies in animals. We demonstrate a trypsin-resistant laddering protein purified from a type V GBS strain by mutanolysin extraction and column chromatography. This protein contains a major 90-kDa band and a series of smaller bands spaced approximately 10 kDa apart on SDS-PAGE. Cross-reactivity of the type V protein with the alpha C protein and with R1 was demonstrated on Western blot (immunoblot). N-terminal sequence analysis of the protein revealed residue identity with 17 of 18 residues at corresponding positions on the alpha protein. Western blot of SDS extracts of 41 clinical type V isolates with rabbit antiserum to the protein demonstrated a homologous protein in 25 isolates (61%); two additional strains exhibited a heterologous pattern which was also demonstrated with 4G8, a monoclonal antibody directed to the alpha C protein repeat region. Rabbit antiserum raised to the type V protein conferred protection in neonatal mice against a type V strain bearing a homologous protein. These data support the hypothesis that there exists a family of trypsin-resistant, laddering GBS surface proteins which may play a role in immunity to GBS infection.

Experimental vaccination against group B streptococcus, an encapsulated bacterium, with highly purified preparations of cell surface proteins Rib and alpha

Encapsulated bacteria cause some of the most common diseases in humans. Although the polysaccharide capsules of these pathogens have attracted the most attention with regard to vaccine development, recent evidence suggests that bacterial surface proteins may also be used to confer protective immunity. We have analyzed this possibility in group B streptococcus (GBS), an encapsulated bacterium that is the major cause of invasive bacterial disease in the neonatal period. Previous work has shown that the majority of GBS strains causing invasive infections express the Rib protein, and that most strains lacking Rib express a protein designated alpha. Here we report that active immunization with highly purified preparations of Rib or alpha protected mice against lethal infection with strains expressing the corresponding protein. Vaccination with the Rib protein protected against two strains of capsular type III and two strains of type II, and vaccination with the alpha protein protected against one strain of type II and one strain of type Ib. The mice vaccinated with Rib or alpha showed a good immunoglobulin G response to the immunogen. These data suggest that a vaccine against GBS disease may be based on cell surface proteins and support the notion that proteins may be used for immunization against encapsulated bacteria.

Variation in repeat number within the alpha C protein of group B streptococci alters antigenicity and protective epitopes

Variable expression of repeating units of the protective alpha C proteins among clinical isolates of group B streptococci (GBS) may have implications for vaccine development. In this study, alpha C protein genes containing various numbers of repeats (1,2,9, and 16) were cloned in a T7 overexpression vector in Escherichia coli. Expression was induced by isopropyl-beta-D-thiogalactopyranoside, and proteins were purified by anion-exchange, gel filtration, or affinity chromatography or by isoelectric focusing. Rabbits were immunized with purified 1-,2-,9-, or 16-repeat proteins. All proteins appeared to be highly immunogenic. Enzyme-linked immunosorbent assay inhibition with 9-repeat protein as the coating antigen and 9-repeat-antigen-elicited antiserum showed that a 200-fold-higher concentration of 1-repeat antigen than of 9- or 16-repeat antigen was required for 50% inhibition of antibody-antigen binding. The concentration of 2-repeat antigen required for 50% inhibition was intermediate relative to the concentrations of 1- and 9-repeat antigens. These results suggested that antibodies to 9-repeat antigen recognized predominantly a conformational epitope(s) contained in proteins with higher numbers of repeats (9 or 16) but lost considerable binding affinities for an epitope(s) contained in alpha C proteins with fewer repeats (1 or 2). Similar results were obtained with antiserum to 16-repeat antigen. However, antibodies to 1- and 2-repeat antigens recognized 1-,2-,9-,and 16-repeat antigens with equal binding affinities. This finding suggested that 1- and 2-repeat-elicited antibodies recognized an epitope(s) on individual repeats. Loss of repeating units from the alpha C proteins may result in decreased protection because the loss of epitopes (including conformational epitopes) gives the microorganisms the opportunity to escape host antibodies. If 1- and 2-repeat-elicited antibodies bind all alpha C proteins with equal affinity, regardless of their repeat number, they may prevent GBS strains with fewer repeats from escaping host immunity. Protection data obtained with antisera to the proteins with different repeat numbers support this hypothesis: mouse pups challenged with GBS strain A909 were better protected when immunized with 1- or 2-repeat-elicited antiserum (76 and 75%, respectively) than when immunized with 9- or 16-repeat-elicited antiserum (41 and 48%, respectively).

Molecular diversity among the trypsin resistant surface proteins of group B streptococci

The alpha (alpha) component of the c protein and R proteins are trypsin resistant, but antigenically distinct, proteins on the cell surface of some but not all strains of group B streptococci (GBS). These two classes of proteins, along with the group and type polysaccharide, can be used to characterize strains of GBS. Four species of R protein (R1 through R4) have been described. We studied trypsin extracts from numerous strains of GBS by immunodiffusion in agarose and polyacrylamide gel electrophoresis/Western blot. Sera monospecific for alpha, R1 and R4 were used to immunoprecipitate/blot the proteins. The molecular weight of the blotted proteins was determined. Although by immunodiffusion the proteins within a class were identical to each other, great heterogeneity in size and blotting pattern was found within each class. Variation was independent of the polysaccharide serotype. Multiple molecular weight species were seen for alpha, R1 and R4 proteins. For a given strain, the various forms of alpha or R1 appeared to form a multiple size ladder; those of R4 were fewer and closer in size. The highest form of alpha ranged from 85 to 170 kDa, with 45 kDa being the highest form for some rare GBS strains. For R4 the predominant and highest form varied from 84 to 197 kDa, whereas some strains with R1 had the highest form over 200 kDa. Our results indicated that despite similarities, there is great diversity among the alpha, R1 and R4 trypsin resistant proteins of GBS.

Identification of a family of streptococcal surface proteins with extremely repetitive structure

The group B Streptococcus (GBS) causes the majority of life-threatening bacterial infections in newborn children. Most GBS strains isolated from such infections express a surface protein, designated Rib, that confers protective immunity and therefore is of interest for analysis of pathogenetic mechanisms. Sequence analysis demonstrated that Rib has an exceptionally long signal peptide (55 amino acid residues) and 12 repeats (79 amino acid residues each) that account for >80% of the sequence of the mature protein. The repeats are identical even at the DNA level, indicating that an efficient mechanism operates to maintain a highly repetitive structure in Rib. The structure of Rib is similar to that of alpha, a previously characterized surface protein that is common among GBS strains lacking Rib. However, highly purified preparations of Rib and alpha did not cross-react immunologically, although the two proteins show extensive amino acid residue identity (47% in the repeat region). When analyzed in Western blots, Rib and alpha give rise to a regularly spaced ladder pattern, apparently due to hydrolysis of acid-labile Asp-Pro bonds in the repeats. We conclude that Rib and alpha are members of a novel family of streptococcal surface proteins with unusual repetitive structure.

Group B streptococci escape host immunity by deletion of tandem repeat elements of the alpha C protein

Group B streptococci (GBS) are the most common cause of neonatal sepsis, pneumonia, and meningitis. The alpha C protein is a surface-associated antigen; the gene (bca) for this protein contains a series of tandem repeats (each encoding 82 aa) that are identical at the nucleotide level and express a protective epitope. We previously reported that GBS isolates from two of 14 human maternal and neonatal pairs differed in the number of repeats contained in their alpha C protein; in both pairs, the alpha C protein of the neonatal isolate was smaller in molecular size. We now demonstrate by PCR that the neonatal isolates contain fewer tandem repeats. Maternal isolates were susceptible to opsonophagocytic killing in the presence of alpha C protein-specific antiserum, whereas the discrepant neonatal isolates proliferated. An animal model was developed to further study this phenomenon. Adult mice passively immunized with antiserum to the alpha C protein were challenged with an alpha C protein-expressing strain of GBS. Splenic isolates of GBS from these mice showed a high frequency of mutation in bca--most commonly a decrease in repeat number. Isolates from non-immune mice were not altered. Spontaneous deletions in the repeat region were observed at a much lower frequency (6 x 10(-4)); thus, deletions in that region are selected for under specific antibody pressure and appear to lower the organism's susceptibility to killing by antibody specific to the alpha C protein. This mechanism of antigenic variation may provide a means whereby GBS evade host immunity.

Group B streptococci adhere to a variant of fibronectin attached to a solid phase

Group B streptococci (GBS) are the leading cause of neonatal pneumonia and meningitis. Adherence of GBS to host tissues may play an important role in the pathogenesis of infection. The host molecules which mediate GBS adherence to host tissues are unknown. Many bacterial pathogens adhere to fibronectin, an important component of the extracellular matrix (ECM). Some pathogens adhere to both immobilized and soluble fibronectin, while others adhere to immobilized fibronectin, but not to soluble fibronectin. Previous data indicated that GBS do not adhere to soluble fibronectin. We studied the ability of GBS to adhere to immobilized fibronectin. Forty-five per cent of the input inoculum of COH1, a virulent GBS isolate, adhered to fibronectin immobilized on polystyrene. COH1 did not adhere to the other ECM proteins tested (laminin, type I collagen, vitronectin, and tenascin). Nine out of nine GBS strains from human sources tested adhered specifically to fibronectin at levels varying from 4-60%. We considered the possibility that GBS were adherent to a contaminant in the fibronectin preparation. Properties of fibronectin, including the presence of an immunologic epitope of fibronectin and binding to collagen, were verified to be properties of the molecule to which GBS adhere. COH1 adhered to fibronectin captured by a monoclonal antibody to fibronectin (FN-15), confirming that the molecule to which GBS adhere bears immunologic determinants of fibronectin. Adherence of COH1 to fibronectin was inhibited by collagen, confirming that the molecule to which GBS adhere binds to collagen. These data strongly suggest that GBS adhere to fibronectin, and not to a contaminant.(ABSTRACT TRUNCATED AT 250 WORDS)

Maternal immunization of mice with group B streptococcal type III polysaccharide-beta C protein conjugate elicits protective antibody to multiple serotypes

Group B streptococcal infection is a major cause of neonatal mortality. Antibody to the capsular polysaccharide protects against invasive neonatal disease, but immunization with capsular polysaccharides fails to elicit protective antibody in many recipients. Conjugation of the polysaccharide to tetanus toxoid has been shown to increase immune response to the polysaccharide. In animal models, C proteins of group B streptococci are also protective determinants. We examined the ability of the beta C protein to serve in the dual role of carrier for the polysaccharide and protective immunogen. Type III polysaccharide was covalently coupled to beta C protein by reductive amination. Immunization of rabbits with the polysaccharide-protein conjugate elicited high titers of antibody to both components, and the serum induced opsonophagocytic killing of type III, Ia/C, and Ib/C strains of group B streptococci. Female mice were immunized with the conjugate vaccine and then bred; 93% of neonatal pups born to these dams vaccinated with conjugate survived type III group B streptococcal challenge and 76% survived type Ia/C challenge, compared with 3% and 8% survival, respectively, in controls (P < 0.001). The beta C protein acted as an effective carrier for the type III polysaccharide while simultaneously induced protective immunity against beta C protein--containing strains of group B streptococci.

Protein rib: a novel group B streptococcal cell surface protein that confers protective immunity and is expressed by most strains causing invasive infections

The group B Streptococcus, an important cause of invasive infections in the neonate, is classified into four major serotypes (Ia, Ib, II, and III) based on the structure of the polysaccharide capsule. Since the capsule is a known virulence factor, it has been extensively studied, in particular in type III strains, which cause the majority of invasive infections. Two cell surface proteins, alpha and beta, have also been studied in detail since they confer protective immunity, but these proteins are usually not expressed by type III strains. We describe here a cell surface protein, designated protein Rib (resistance to proteases, immunity, group B), that confers protective immunity and is expressed by most strains of type III. Protein Rib was first identified as a distinct 95-kD protein in extracts of a type III strain, and was purified to homogeneity from that strain. Rabbit antiserum to protein Rib was used to demonstrate that it is expressed on the cell surface of 31 out of 33 type III strains, but only on 1 out of 25 strains representing the other three serotypes. Mouse protection tests showed that antiserum to protein Rib protects against lethal infection with three different strains expressing this antigen, including a strain representing a recently identified high virulence type III clone. Protein Rib is immunologically unrelated to the alpha and beta proteins, but shares several features with the alpha protein. Most importantly, the NH2-terminal amino acid sequences of the Rib and alpha proteins are identical at 6 out of 12 positions. In addition, both protein Rib and the alpha protein are relatively resistant to trypsin (and Rib is also resistant to pepsin) and both proteins vary greatly in size between different clinical isolates. Finally, both protein Rib and the alpha protein exhibit a regular ladderlike pattern in immunoblotting experiments, which may reflect a repetitive structure. Taken together, these data suggest that the Rib and alpha proteins are members of a family of proteins with related structure and function. Since protein Rib confers protective immunity, it may be valuable for the development of a protein vaccine against the group B Streptococcus, an encapsulated bacterium.

Properties and type antigen patterns of group B streptococcal isolates from pigs and nutrias

All 59 group B streptococcal cultures isolated from pigs and nutrias reacted with group B-specific antiserum and gave a positive CAMP reaction in the zone of staphylococcal beta-lysin. Most of the cultures were pigmented; all cultures hydrolyzed Na hippurate and utilized salicin, maltose, and saccharose but not esculin, mannitol, or inulin. Fifty-three percent of the group B streptococci from pigs and none of those from nutrias were lactose positive. Serotyping revealed that most of the group B streptococci from pigs were of serotype III and most of those from nutrias were of type Ia/c. Protein c was present as c beta antigen. All group B streptococci were susceptible to penicillin and bacitracin (10 U), and most of the porcine cultures were resistant to tetracycline. According to these results, group B streptococci from pigs and nutrias differ from bovine and human group B streptococci and seem to play no role in cross-infections between animals or between animals and humans.

Protection of neonatal mice from group B streptococcal infection by maternal immunization with beta C protein

Group B streptococci (GBS) cause the majority of cases of neonatal sepsis and meningitis in the United States. Immunization of women of childbearing age is one strategy under consideration for the prevention of neonatal disease. The beta C protein, a 130-kDa antigen present in many clinical isolates of GBS, was purified from GBS by extraction into sodium dodecyl sulfate (SDS)-containing buffer, preparative SDS-polyacrylamide gel electrophoresis, and electroelution. Purified beta C protein antigen (25 micrograms) with Freund's adjuvant was used to immunize rabbits. Rabbits developed enzyme-linked immunosorbent assay titers of > 1:1.6 x 10(6), and sera from immunized rabbits were administered to pregnant mice. Their neonatal pups were then challenged with a strain of GBS expressing beta C protein; 68% of these pups were protected by immune antiserum, whereas no controls were protected (P < 0.001). The immune serum (diluted 1:100) facilitated opsonophagocytic killing of GBS strains expressing the beta C protein but not those that do not express the antigen (mean log kill +/- standard deviation = 0.71 +/- 0.8 log10 CFU for beta+ strains and 0.09 +/- 0.2 for beta- strains; P = 0.02). In subsequent experiments, adult female mice were actively immunized with two doses of 2, 5, or 10 micrograms of beta C protein 2 months prior to mating. One- to two-day-old offspring of these dams were challenged with GBS and were protected in a dose-dependent manner, with 96% survival in the high-dose (10-micrograms) group and 20% survival in a sham-immunized control group (P < 0.001). Thus, active immunization of mice with the GBS beta C protein confers protection against lethal infection with beta+ GBS to their offspring.

Large, identical, tandem repeating units in the C protein alpha antigen gene, bca, of group B streptococci

Group B Streptococcus (GBS) is the leading cause of neonatal sepsis and meningitis in the United States. The surface-associated C protein alpha antigen of GBS is thought to have a role in both virulence and immunity. We previously cloned the C protein alpha antigen structural gene (named bca for group B, C protein, alpha) into Escherichia coli. Western blots of both the native alpha antigen and the cloned gene product demonstrate a regularly laddered pattern of heterogeneous polypeptides. The nucleotide sequence of the bca locus reveals an open reading frame of 3060 nucleotides encoding a precursor protein of 108,705 Da. Cleavage of a putative signal sequence of 41 amino acids yields a mature protein of 104,106 Da. The 20,417-Da N-terminal region of the alpha antigen shows no homology to previously described protein sequences and is followed by a series of nine tandem repeating units that make up 74% of the mature protein. Each repeating unit is identical and consists of 82 amino acids with a molecular mass of 8665 Da, which is encoded by 246 nucleotides. The size of the repeating units corresponds to the observed size differences in the heterogeneous ladder of alpha C proteins expressed by GBS. The C-terminal region of the alpha antigen contains a membrane anchor domain motif that is shared by a number of Gram-positive surface proteins. The large region of identical repeating units in bca defines protective epitopes and may play a role in generating phenotypic and genotypic diversity of the alpha antigen.