alpha-Enolase of Streptococcus pneumoniae is a plasmin(ogen)-binding protein displayed on the bacterial cell surface

Serum antibodies to putative proteinase maturation protein A in children with acute otitis media

The pneumococcal genes encoding for the surface associated proteins have been proposed to be important for pneumococcal protein vaccine development. We cloned the full-length putative proteinase maturation protein A gene SP098l/ppmA (as published by Tettelin et al. in 2001) and produced the encoded protein in high levels in E. coli. The purified recombinant PpmA was used as an antigen in Western blotting to study systemic antibody responses to PpmA in animals and in children with acute otitis media (AOM). In children, the geometric mean titers of serum IgG antibodies against PpmA increased with age and differed significantly in relation to pneumococcal findings in middle ear fluid and/or nasopharyngeal aspirate. The serum IgG antibody titers against PpmA were low in children with Streptococcus pneumoniae cultured in the middle ear, and the highest in children with pneumococci in the nasopharynx, without them being found in the middle ear fluid. We conclude that PpmA is immunogenic in humans, and therefore an interesting antigen to study further in developing pneumococcal multicomponent protein vaccines.

Catabolite control protein A (CcpA) contributes to virulence and regulation of sugar metabolism in Streptococcus pneumoniae

We characterized the role of catabolite control protein A (ccpA) in the physiology and virulence of Streptococcus pneumoniae. S. pneumoniae has a large percentage of its genome devoted to sugar uptake and metabolism, and therefore, regulation of these processes is likely to be crucial for fitness in the nasopharynx and may play a role during invasive disease. In many bacteria, carbon catabolite repression (CCR) is central to such regulation, influencing hierarchical sugar utilization and growth rates. CcpA is the major transcriptional regulator in CCR in several gram-positive bacteria. We show that CcpA functions in CCR of lactose-inducible beta-galactosidase activity in S. pneumoniae. CCR of maltose-inducible alpha-glucosidase, raffinose-inducible alpha-galactosidase, and cellobiose-inducible beta-glucosidase is unaffected in the ccpA strain, suggesting that other regulators, possibly redundant with CcpA, control these systems. The ccpA strain is severely attenuated for nasopharyngeal colonization and lung infection in the mouse, establishing its role in fitness on these mucosal surfaces. Comparison of the cell wall fraction of the ccpA and wild-type strains shows that CcpA regulates many proteins in this compartment that are involved in central and intermediary metabolism, a subset of which are required for survival and multiplication in vivo. Both in vitro and in vivo defects were complemented by providing ccpA in trans. Our results demonstrate that CcpA, though not a global regulator of CCR in S. pneumoniae, is required for colonization of the nasopharynx and survival and multiplication in the lung.

Adherence molecules of pathogenic pneumococci

Adherence molecules are key players in pathogen-host interactions. These are usually surface-exposed structures that facilitate adherence to host cells, or target host serum proteins of the extracellular matrix. Our knowledge of the function of pneumococcal cell-surface structures, and the basic mechanisms underlying their interaction with host receptor molecules has dramatically increased, through molecular and structural analysis of adherence molecules. In particular, choline-binding proteins have received considerable attention because of their versatility, and their sophisticated role in the interaction with host proteins. Interestingly, subversion of host-protein functions to facilitate host invasion and immune evasion has also been attributed to intracellular or surface-exposed proteins of the pathogen. Many of these molecules do not possess the classic features of bacterial surface proteins.

Inhibition of cell surface export of group A streptococcal anchorless surface dehydrogenase affects bacterial adherence and antiphagocytic properties

Surface dehydrogenase (SDH) is an anchorless, multifunctional protein displayed on the surfaces of group A Streptococcus (GAS) organisms. SDH is encoded by a single gene, sdh (gap or plr) that is essential for bacterial survival. Hence, the resulting nonfeasibility of creating a knockout mutant is a major limiting factor in studying its role in GAS pathogenesis. An insertion mutagenesis strategy was devised in which a nucleotide sequence encoding a hydrophobic tail of 12 amino acids ((337)IVLVGLVMLLLS(348)) was added at the 3' end of the sdh gene, successfully creating a viable mutant strain (M1-SDH(HBtail)). In this mutant strain, the SDH(HBtail) protein was not secreted in the medium but was retained in the cytoplasm and to some extent trapped within the cell wall. Hence, SDH(HBtail) was not displayed on the GAS surface. The mutant strain, M1-SDH(HBtail), grew at the same rate as the wild-type strain. The SDH(HBtail) protein displayed the same GAPDH activity as the wild-type SDH protein. Although the whole-cell extracts of the wild-type and mutant strains showed similar GAPDH activities, cell wall extracts of the mutant strain showed 5.5-fold less GAPDH activity than the wild-type strain. The mutant strain, M1-SDH(HBtail), bound significantly less human plasminogen, adhered poorly to human pharyngeal cells, and lost its innate antiphagocytic activity. These results indicate that the prevention of the cell surface export of SDH affects the virulence properties of GAS. The anchorless SDH protein, thus, is an important virulence factor.

Adherence to human vaginal epithelial cells signals for increased expression of Trichomonas vaginalis genes

Host parasitism by Trichomonas vaginalis is complex, and the adhesion to vaginal epithelial cells (VECs) by trichomonads is preparatory to colonization of the vagina. Since we showed increased synthesis of adhesins after contact with VECs (A. F. Garcia, et al., Mol. Microbiol. 47:1207-1224, 2003) and more recently demonstrated up-regulated gene expression in VECs after parasite attachment (A. S. Kucknoor, et al., Cell. Microbiol. 7:887-897, 2005), we hypothesized that enhanced expression of adhesin and other genes would result from signaling of trichomonads following adherence. In order to identify the genes that are up-regulated, we constructed a subtraction cDNA library enriched for differentially expressed genes from the parasites that were in contact with the host cells. Thirty randomly selected cDNA clones representing the differentially regulated genes upon initial contact of parasites with host cells were sequenced. Several genes encoded functional proteins with specific functions known to be associated with colonization, such as adherence, change in morphology, and gene transcription and translation. Interestingly, genes unique to trichomonads with unknown functions were also up-regulated. Semiquantitative reverse transcription-PCR (RT-PCR) confirmed expression of select genes. An increased amount of protein was demonstrated by immunoblotting with monoclonal antibody. Finally, we showed the transcriptional regulation of some genes by iron by using RT-PCR. To our knowledge, this is the first report addressing the differential regulation of T. vaginalis genes immediately upon contact with VECs.

Correlation of probioticLactobacillus salivariusgrowth phase with its cell wall-associated proteome

Lactobacillus salivarius subsp. salivarius UCC118 is a probiotic bacterium that was originally isolated from human intestinal tissues and was subsequently shown in a pilot study to alleviate symptoms associated with mild-moderate Crohn's disease. Strain UCC118 can adhere to animal and human intestinal tissue, and to both healthy and inflamed ulcerative colitis mucosa, irrespective of location in the gut. In this study, an enzymatic technique has been combined with proteomic analysis to correlate bacterial growth phase with the presence of factors present in the cell wall of the bacterium. Using PAGE electrophoresis, it was determined that progression from lag to log to stationary growth phases in vitro correlated with increasing prominence of an 84kD protein associated with in vitro adherence ability. Isolated proteins from the 84kD band region were further separated by two-dimensional electrophoresis, resolving this band into 20 individual protein spots at differing isoelectric points. The protein moieties were excised, trypsin digested and subjected to tandem mass spectrometry. The observed proteins are analogous to those reported to be associated with the Listeria monocytogenes cell-wall proteome, and include DnaK, Ef-Ts and pyruvate kinase. These data suggest that at least some of the beneficial attributes of probiotic lactobacilli, and in particular this strain, may be due to nonpathogenic mimicry of pathogens and potentially be mediated through a form of attenuated virulence.

The streptococcal lipoprotein rotamase A (SlrA) is a functional peptidyl-prolyl isomerase involved in pneumococcal colonization

Streptococcus pneumoniae expresses two surface-exposed lipoproteins, PpmA and SlrA, which share homology with distinct families of peptidyl-prolyl isomerases (PPIases). In this study, we demonstrated for the first time that the lipoprotein cyclophilin, SlrA, can catalyze the cis-trans isomerization of proline containing tetrapeptides and that SlrA contributes to pneumococcal colonization. The substrate specificity of SlrA is typical for prokaryotic and eukaryotic cyclophilins, with Suc-Ala-Ala-Pro-Phe-p-nitroanilide (pNA) being the most rapidly catalyzed substrate. In a mouse pneumonia model the slrA knock-out D39DeltaslrA did not cause significant differences in the survival times of mice compared with the isogenic wild-type strain. In contrast, a detailed analysis of bacterial outgrowth over time in the nasopharynx, airways, lungs, blood, and spleen showed a rapid elimination of slrA mutants from the upper airways but did not reveal significant differences in the lungs, blood, and spleen. These results suggested that SlrA is involved in colonization but does not contribute significantly to invasive pneumococcal disease. In cell culture infection experiments, the absence of SlrA impaired adherence to pneumococcal disease-specific epithelial and endothelial non-professional cell lines. Adherence of the slrA mutant could not be restored by exogenously added SlrA. Strikingly, deficiency in SlrA did not reduce binding activity to host target proteins, but resulted in enhanced uptake by professional phagocytes. In conclusion, SlrA is a functional, cyclophilin-type PPIase and contributes to pneumococcal virulence in the first stage of infection, namely, colonization of the upper airways, most likely by modulating the biological function of important virulence proteins.

Role of HtrA in surface protein expression and biofilm formation by Streptococcus mutans

The HtrA surface protease in gram-positive bacteria is involved in the processing and maturation of extracellular proteins and degradation of abnormal or misfolded proteins. Inactivation of htrA has been shown to affect the tolerance to thermal and environmental stress and to reduce virulence. We found that inactivation of Streptococcus mutans htrA by gene-replacement also resulted in a reduced ability to withstand exposure to low and high temperatures, low pH, and oxidative and DNA damaging agents. The htrA mutation affected surface expression of several extracellular proteins including glucan-binding protein B (GbpB), glucosyltransferases, and fructosyltransferase. In addition, htrA mutation also altered the surface expression of enolase and glyceraldehyde-3-phosphate dehydrogenease, two glycolytic enzymes that are known to be present on the streptococcal cell surface. As expected, microscopic analysis of in vitro grown biofilm structure revealed that the htrA deficient biofilms adopted a much more granular patchy appearance, rather than the relatively smooth confluent layer normally seen in the wild type. These results suggest that HtrA plays an important role in the biogenesis of extracellular proteins including surface associated glycolytic enzymes and in biofilm formation of S. mutans.

A recombinant enolase from Anisakis simplex is differentially recognized in natural human and mouse experimental infections

A 1,963-bp cDNA was isolated from an Anisakis simplex cDNA library by immunoscreening with a hyperimmune rabbit serum raised against a crude extract of A. simplex L3 larvae. The open reading frame encodes a putative protein of 436 amino acid residues, which exhibits high similarity (70-80%) to enolase molecules from various other organisms, including helminth parasites. After subcloning and expression of the A. simplex cDNA in PGEX-4T-3, the resulting glutathione S-transferase fusion protein, purified by glutathione-Sepharose-4B chromatography, showed functional enolase activity. The immunogenicity of the recombinant A. simplex enolase was analyzed by immunoblotting using sera obtained from (a) mice immunized with crude extracts (CE) of A. simplex, or other nematode species, (b) mice immunized with excretory-secretory (ES) antigens from A. simplex, or (c) mice infected with L3 larvae by the intraperitoneal route. In addition, we used ELISA, to investigate the presence of IgG1 and IgE antibodies against this molecule in sera from patients infected with A. simplex. Mouse sera obtained after infection with L3 or raised against CE antigens, but not sera raised against ES antigens, showed strong reactivity with the recombinant A. simplex enolase. We also obtained good reactivity in Western blotting with sera from mice immunized with CE antigens from Ascaris suum and Toxocara canis, but not with sera from mice immunized with CE antigens from Trichuris muris, Trichinella spiralis or Hysterothylacium aduncum. In contrast to the experimental infections/immunizations in mice, we were unable to detect anti-enolase IgE antibodies in sera from human patients infected with A.simplex (15 sera), and the levels of anti-enolase IgG1 antibodies in these sera were low and apparently nonspecific. These results seem to indicate that, during natural infection in humans, A. simplex larvae do not offer sufficient antigenic stimulus to induce anti-enolase antibodies.

Antibodies against human muscle enolase recognize a 45-kDa bacterial cell wall outer membrane enolase-like protein

Enolase, is a glycolytic enzyme ubiquitous in higher organisms, where it forms tissue specific dimers of isoforms, also found in the cytoplasm of fermentative bacteria. The aim of this work was to identify enolase-like proteins in the cell wall of some Gram-negative bacteria using antibodies against human beta-enolase, an isoenzyme specific to skeletal and heart muscles. Cell wall outer membrane protein (OMP) preparations were obtained from 9 strains of Enterobacteriaceae and one of Pseudomonas aeruginosa. Specific enzymatic enolase activity was detected in the supernatant fractions of cytosolic and inner membrane material, but not in purified OMP preparations. Rabbit polyclonal antibodies specific against human beta-enolase were prepared and purified using immobilized human beta-enolase in affinity chromatography. In SDS-polyacrylamide gel electrophoresis and immunoblotting assay of purified OMP preparations, rabbit anti-enolase antibody interacted specifically with a few OMPs, of which a 45-kDa band also interacted with human sera of patients presenting Buerger disease and atherosclerosis. The most distinct interaction of human sera was observed with a 45-kDa OMP of Klebsiella pneumoniae. This protein was further isolated from K. pneumoniae cell mass in two ways, namely preparative SDS-polyacrylamide gel electrophoresis and specific affinity chromatography using immobilized affinity-purified rabbit antibody raised against human beta-enolase. The data obtained from tandem mass spectrometry tryptic peptide analysis and sequence comparison of human and bacterial enolases using protein databases, could reveal the similarity in the epitopes between membrane enolase-like protein from Klebsiella and human beta-enolase. The results show that the protein present in all studied strains has a common epitope on human beta-enolase. These data raise the question whether such a bacterial protein might be a marker for detecting and monitoring damage to skeletal and heart muscles.

Comparative secretome analyses of three Bacillus anthracis strains with variant plasmid contents

Bacillus anthracis, the causative agent of anthrax, secretes numerous proteins into the extracellular environment during infection. A comparative proteomic approach was employed to elucidate the differences among the extracellular proteomes (secretomes) of three isogenic strains of B. anthracis that differed solely in their plasmid contents. The strains utilized were the wild-type virulent B. anthracis RA3 (pXO1(+) pXO2(+)) and its two nonpathogenic derivative strains: the toxigenic, nonencapsulated RA3R (pXO1(+) pXO2(-)) and the totally cured, nontoxigenic, nonencapsulated RA3:00 (pXO1(-) pXO2(-)). Comparative proteomics using two-dimensional gel electrophoresis followed by computer-assisted gel image analysis was performed to reveal unique, up-regulated, or down-regulated secretome proteins among the strains. In total, 57 protein spots, representing 26 different proteins encoded on the chromosome or pXO1, were identified by peptide mass fingerprinting. S-layer-derived proteins, such as Sap and EA1, were most frequently observed. Many sporulation-associated enzymes were found to be overexpressed in strains containing pXO1(+). This study also provides evidence that pXO2 is necessary for the maximal expression of the pXO1-encoded toxins lethal factor (LF), edema factor (EF), and protective antigen (PA). Several newly identified putative virulence factors were observed; these include enolase, a high-affinity zinc uptake transporter, the peroxide stress-related alkyl hydroperoxide reductase, isocitrate lyase, and the cell surface protein A.

PavA of Streptococcus pneumoniae modulates adherence, invasion, and meningeal inflammation

Pneumococcal adherence and virulence factor A (PavA) is displayed to the cell outer surface of Streptococcus pneumoniae and mediates pneumococcal binding to immobilized fibronectin. PavA, which lacks a typical gram-positive signal sequence and cell surface anchorage motif, is essential for pneumococcal virulence in a mouse infection model of septicemia. In this report the impact of PavA on pneumococcal adhesion to and invasion of eukaryotic cells and on experimental pneumococcal meningitis was investigated. In the experimental mouse meningitis model, the virulence of the pavA knockout mutant of S. pneumoniae D39, which did not show alterations of subcellular structures as indicated by electron microscopic studies, was strongly decreased. Pneumococcal strains deficient in PavA showed substantially reduced adherence to and internalization of epithelial cell lines A549 and HEp-2. Similar results were obtained with human brain-derived microvascular endothelial cells and human umbilical vein-derived endothelial cells. Attachment and internalization of pneumococci were not significantly affected by preincubation or cocultivations of pneumococci with anti-PavA antisera. Pneumococcal adherence was also not significantly affected by the addition of PavA protein. Complementation of the pavA knockout strain with exogenously added PavA polypeptide did not restore adherence of the mutant. These data suggest that PavA affects pneumococcal colonization by modulating expression or function of important virulence determinants of S. pneumoniae.

Competence-programmed predation of noncompetent cells in the human pathogen Streptococcus pneumoniae: genetic requirements

Natural competence for genetic transformation is the best-characterized feature of the major human pathogen Streptococcus pneumoniae. Recent studies have shown the virulence of competence-deficient mutants to be attenuated, but the nature of the connection between competence and virulence remained unknown. Here we document the release, triggered by competent cells, of virulence factors (e.g., the cytolytic toxin pneumolysin) from noncompetent cells. This phenomenon, which we name allolysis, involves a previously undescribed bacteriocin system consisting of a two-peptide bacteriocin, CibAB, and its immunity factor, CibC; the major autolysin, LytA, and lysozyme, LytC; and a proposed new amidase, CbpD. We show that CibAB are absolutely required for allolysis, whereas LytA and LytC can be supplied either by the competent cells or by the targeted cells. We propose that allolysis constitutes a competence-programmed mechanism of predation of noncompetent cells, which benefits to the competent cells and contributes to virulence by coordinating the release of virulence factors.

Comparative proteome analysis of secretory proteins from pathogenic and nonpathogenicListeriaspecies

Extracellular proteins of bacterial pathogens play a crucial role in the infection of the host. Here we present the first comprehensive validation of the secretory subproteome of the Gram positive pathogen Listeria monocytogenes using predictive bioinformatic and experimental proteomic approaches. The previous original signal peptide (SP) prediction (Glaser et al., Science 2001, 294, 849-852) has been greatly improved by an in-depth analysis using seven different bioinformatic tools. Subsequent careful classification of the resulting data gives a probability dependent annotation of 121 putatively secreted proteins of which 45 are novel. Complementary proteomic analysis using both two-dimensional gel electrophoresis/matrix assisted laser desorption/ionization mass spectrometry and high performance liquid chromatography/electrospray ionization-mass spectrometry has identified 105 proteins in the culture supernatant of L. monocytogenes. Among these, we were able to detect all the currently known virulence factors with an SP showing the importance of this subproteome and demonstrating the reliability of the techniques used. The comparison between the L. monocytogenes wildtype and the nonpathogenic species Listeria innocua was performed to reveal proteins probably involved in pathogenicity and/or the adaptation to their respective lifestyles. In addition to the eight known virulence factors, all of which have no orthologous genes in L. innocua, eight additional proteins have been identified that exhibit the typical key feature defining the known listerial virulence factors. Further significant differences between the two species are evident in the group of cell wall and secretory proteins that warrant further study. Our investigation clearly demonstrates that the major difference between the pathogenic and nonpathogenic species, noted in the comparative genome analysis, manifests itself strongest in the secretome.

Bacterial metastasis: the host plasminogen system in bacterial invasion

Several pathogenic bacterial species intervene with the mammalian proteolytic plasminogen-plasmin system. Recent developments have been made in understanding the structure and the virulence-associated functions of bacterial plasminogen receptors and activators, in particular by using plasminogen-deficient or transgenic gain-of-function mice. Bacteria can affect the regulation of the plasminogen system by degrading circulating plasmin inhibitors and by influencing the expression levels of mammalian plasminogen activators and activation inhibitors. Interaction with the plasminogen system promotes damage of extracellular matrices as well as bacterial spread and organ invasion during infection, suggesting common mechanisms in migration of eukaryotic and prokaryotic cells.

Identification of new potential vaccine candidates against Chlamydia pneumoniae by multiple screenings

Chlamydia are intracellular bacteria associated to serious human disease. A vaccine has proved difficult to obtain so far, and current opinions agree that multi-antigen combinations may be required to induce optimal protective responses. In order to identify new potential vaccine candidates, we recently screened the Chlamydia pneumoniae (Cpn) genome and described 53 recombinant proteins which elicited antibodies binding to purified Cpn cells. We now report that six proteins in this group can also induce in vitro neutralizing antibodies. Antibody specificity for the corresponding antigens was assessed by immunoblot analysis of 2DE Cpn protein maps. Furthermore, four of the six in vitro neutralizing antigens (Pmp2, Pmp10, OmpH-like and enolase) could inhibit Cpn dissemination in a hamster model. The results show that these Cpn proteins are immunoaccessible in infectious EBs, and recommend further investigation on their value as vaccine components.

Identification of enolase as a laminin-binding protein on the surface of Staphylococcus aureus

We have previously demonstrated that Staphylococcus aureus, a highly invasive bacteria, presents a 52-kDa surface protein that mediates its binding to laminin. In order to better characterize this receptor, we excised this putative laminin receptor from two-dimensional (2-D) PAGE and used it as antigen for raising a mouse hyperimmune serum which was for screening an S. aureus expression library. A single clone of 0.3 kb was obtained, and its sequence revealed 100% homology with S. aureus alpha-enolase. Moreover, amino acid sequencing of the 52-kDa protein eluted from the 2-D gel indicated its molecular homology with alpha-enolase, an enzyme that presents a high evolutionary conservation among species. In parallel, monoclonal antibodies raised against the S. aureus 52-kDa band also recognized yeast alpha-enolase in western blot analysis. These monoclonal antibodies were also able to promote capture of iodine-labeled bacteria when adsorbed to a solid phase, and this capture was inhibited by the addition of excess rabbit muscle alpha-enolase. Finally, the cell surface localization of S. aureus alpha-enolase was further confirmed by flow cytometry. Hence, alpha-enolase might play a critical role in the pathogenesis of S. aureus by allowing its adherence to laminin-containing extracellular matrix.

Characterization of an iron-regulated alpha-enolase of Bacteroides fragilis

This study describes the identification, cloning and molecular characterization of the alpha-enolase P46 of Bacteroides fragilis. The gram-negative anaerobic bacterium B. fragilis is a member of the commensal flora of the human intestine but is also frequently found in severe intra-abdominal infections. Several virulence factors have been described that may be involved in the development of these infections. Many of these virulence factors are upregulated under conditions of iron- or heme-starvation. We found a major protein of 46 kDa (P46) that is upregulated under iron-depleted conditions. This protein was identified as an alpha-enolase. Alpha-enolases in several gram-positive bacteria and eukaryotic cells are located at the cell surface and function as plasminogen-binding proteins. Localization studies demonstrated that P46 is mainly located in the cytoplasm and partly associated with the inner membrane (IM). Under iron-restricted conditions, however, P46 is localized primarily in the IM fraction. Plasminogen-binding to B. fragilis cells did occur but was not P46 dependent. A 60-kDa protein was identified as a putative plasminogen-binding protein in B. fragilis.

Streptococcus mutans surface alpha-enolase binds salivary mucin MG2 and human plasminogen

Matrix-assisted laser desorption ionization-time of flight mass spectrometry analysis identified enolase as a cell surface component of Streptococcus mutans, which was confirmed by enzyme-linked immunosorbent assay, Western blotting, and transmission electron microscopy. Surface enolase was demonstrated to bind to human plasminogen and salivary mucin MG2. The results suggested a role for enolase in S. mutans attachment, clearance, or breach of the bloodstream barrier.

Glycolytic enzymes associated with the cell surface of Streptococcus pneumoniae are antigenic in humans and elicit protective immune responses in the mouse

Streptococcus pneumoniae is a leading cause of otitis media, sinusitis, pneumonia, bacteraemia and meningitis worldwide. The drawbacks associated with the limited number of various capsular polysaccharides that can be included in the polysaccharide-based vaccines focuses much attention on pneumococcal proteins as vaccine candidates. We extracted an enriched cell wall fraction from S. pneumoniae WU2. Approximately 150 soluble proteins could be identified by 2D gel electrophoresis. The proteins were screened by 2D-Western blotting using sera that were obtained longitudinally from children attending day-care centres at 18, 30 and 42 months of age and sera from healthy adult volunteers. The proteins were further identified using matrix-assisted laser desorption ionization-time of flight mass spectrometry. Seventeen proteins were antigenic in children and adults, of which 13 showed an increasing antibody response with age in all eight children analysed. Two immunogenic proteins, fructose-bisphosphate aldolase (FBA) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and a control protein with known low immunogenicity, heat shock protein 70 (DnaK), were expressed in Escherichia coli, purified and used to immunize mice. Mouse antibodies elicited to the recombinant (r) FBA and rGAPDH were cross-reactive with several genetically unrelated strains of different serotypes and conferred protection to respiratory challenge with virulent pneumococci. In addition, the FBA used in this study (NP_345117) does not have a human ortholog and warrants further investigation as a candidate for a pneumococcal vaccine. In conclusion, the immunoproteomics based approach utilized in the present study appears to be a suitable tool for identification of novel S. pneumoniae vaccine candidates.

Plasmin(ogen)-binding α-Enolase from Streptococcus pneumoniae: Crystal Structure and Evaluation of Plasmin(ogen)-binding Sites

Alpha-enolases are ubiquitous cytoplasmic, glycolytic enzymes. In pathogenic bacteria, alpha-enolase doubles as a surface-displayed plasmin(ogen)-binder supporting virulence. The plasmin(ogen)-binding site was initially traced to the two C-terminal lysine residues. More recently, an internal nine-amino acid motif comprising residues 248 to 256 was identified with this function. We report the crystal structure of alpha-enolase from Streptococcus pneumoniae at 2.0A resolution, the first structure both of a plasminogen-binding and of an octameric alpha-enolase. While the dimer is structurally similar to other alpha-enolases, the octamer places the C-terminal lysine residues in an inaccessible, inter-dimer groove restricting the C-terminal lysine residues to a role in folding and oligomerization. The nine residue plasminogen-binding motif, by contrast, is exposed on the octamer surface revealing this as the primary site of interaction between alpha-enolase and plasminogen.

Activation of plasminogen by Streptococcus mutans

Streptococcus mutans, a member of the viridans streptococci, is the etiologic agent of dental caries and is also a causative agent of subacute infective endocarditis. The generation of proteolytic molecules, such as plasmin, may be important in the pathogenesis of endocarditis caused by S. mutans. In this study, we demonstrate that S. mutans cells have the ability to bind and activate plasminogen to plasmin. Incubation of S. mutans cells with plasminogen was found to be sufficient for the activation of plasminogen, which suggests the presence of an endogenously produced plasminogen activator. The plasmin activity generated by S. mutans cells was shown to be inhibited by epsilon-aminocaproic acid, lysine, aprotinin, and alpha(2)-macroglobulin. We also show that S. mutans cells have the ability to bind and activate plasminogen from human plasma as well as human serum. The plasmin activity generated on the surface of S. mutans cells could degrade the extracellular matrix molecule, fibronectin.

Biofilm-specific surface properties and protein expression in oral Streptococcus sanguis

OBJECTIVE

Oral streptococci are primary colonisers of the tooth surface and are abundant in dental plaque biofilms. Bacteria growing in these relatively dense, surface-associated communities are phenotypically quite distinct from their planktonic counterparts. The purpose of the present study was to develop a method to investigate biofilm-specific surface protein expression by Streptococcus sanguis to help provide a better understanding of the critical events in plaque development.

DESIGN

Biofilm cells were grown on the surface of glass beads in a biofilm device fed with mucin-containing artificial saliva. Planktonic cells were grown in continuous culture at approximately the same growth rate. Surface hydrophobicity of biofilm and planktonic cells was determined by hexadecane partitioning, and expression of streptococcal fibronectin adhesin CshA was determined in ELISA using specific antiserum. Antisera raised to glutaraldehyde-fixed whole biofilm or planktonic grown cells were used to screen an expression library of S. sanguis genomic DNA, and isolated clones were sequenced.

RESULTS

Phenotypic analysis of biofilm and planktonic cells confirmed that mode of growth affected surface properties of S. sanguis. Thus, hydrophobicity and CshA expression was significantly elevated in biofilm cells. Library screening with biofilm antiserum yielded 32 recombinant clones representing 21 different S. sanguis proteins involved in adhesion and colonisation, carbohydrate utilisation or bacterial metabolism. In differential analysis of four selected Escherichia coli clones, biofilm antiserum reacted five times stronger than planktonic antiserum with cell-free extracts of clones encoding homologues of CshA and Cna collagen adhesin of Staphylococcus aureus, suggesting that these surface proteins are up-regulated in biofilm cells. In contrast, both antisera reacted equally strongly with cell-free extracts of the remaining two clones (encoding dihydrofolate synthase and an unknown protein).

CONCLUSIONS

The method described represents a useful means for determining bacterial protein expression in biofilms based on a combination of molecular and immunological techniques. Surface expression of putative fibronectin and collagen adhesins was up-regulated in biofilm cells.

Development of antibodies against pneumococcal proteins α-enolase, immunoglobulin A1 protease, streptococcal lipoprotein rotamase A, and putative proteinase maturation protein A in relation to pneumococcal carriage and Otitis Media

Surface associated pneumococcal proteins alpha-enolase (Eno), immunoglobulin A1 protease (Iga), streptococcal lipoprotein rotamase A (SlrA), and putative proteinase maturation protein A (PpmA) have potential as candidates for future protein-based anti-pneumococcal vaccines. The immunogenicity of these proteins were studied in a cohort of 329 children during their first two years of life. During the first recorded episode of otitis media, acute and convalescent phase sera were available from 151 children. Concentrations of antibodies against Eno, Iga, SlrA and PpmA were measured by EIA and detected in 99% (300/302), 95% (288/302), 95% (288/302), and 83% (251/302) of the sera, respectively. There were no statistically significant differences between the groups of children with and without a history of pneumococcal contact or with respect to the type of pneumococcal contact. Despite a mean overall decrease in the antibody titers in the convalescent sera following AOM, several children were able to respond with a more than twofold increase in antibody titer in response to AOM. The majority of the children with increased antibody concentrations appeared in the groups, which were colonized with pneumococci at the time of serum collection, but were recorded as having no prior contact with pneumococci. In conclusion, SlrA, PpmA, Eno and Iga are immunogenic proteins that elicit antibody responses early in life. No significant correlation between antibody titers to these proteins and pneumococcal carriage or infection was found. Presumably, this results from the presence of cross-reactive epitopes on commensal bacteria.

Use of the surface proteins GapC and Mig ofStreptococcus dysgalactiaeas potential protective antigens against bovine mastitis

Streptococcus dysgalactiae is a significant pathogen associated with bovine mastitis in lactating and nonlactating dairy cows, causing a severe inflammatory response of the mammary gland, which results in major economic losses to the dairy industry. Two proteins from S. dysgalactiae strain SDG8 were tested for their protective capacity against a homologous bacterial challenge in a dry cow model. The first was a bovine plasmin receptor protein (GapC), which shares 99.4% sequence identity to the plasmin-binding Plr protein of group A streptococci. The second protein product was Mig, a α2-M-, IgG-, and IgA-binding protein present on the cell surface of SDG8. We investigated the efficacy of immunization with purified recombinant forms of GapC and Mig by measuring the number of somatic cells and assessing the presence of the challenge strain in mammary secretions following challenge. In this model, we found that, although the number of quarters containing SDG8 was significantly reduced in the GapC- but not in the Mig-immunized animals, the somatic cell counts from teat secretions were significantly decreased in both the GapC and Mig vaccinates.Key words: Streptococcus dysgalactiae, bovine mastitis, Mig, GapC, GAPDH.

Glyceraldehyde-3-phosphate dehydrogenase of Streptococcus pneumoniae is a surface-displayed plasminogen-binding protein

The recruitment of plasminogen endows the bacterial cell surface of Streptococcus pneumoniae with proteolytic activity. In this study we demonstrate specific plasmin- and plasminogen-binding activity for the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which is located in the cytoplasm as well as on the surface of pneumococci. GAPDH exhibits a high affinity for plasmin and a significantly lower affinity for plasminogen.

Identification of enolase as a plasminogen-binding protein in excretory-secretory products ofFasciola hepatica

We have followed a combined proteomic approach to identify proteins of Fasciola hepatica that could be involved in host-parasite interactions. Using two-dimensional gel electrophoresis, far Western immunoblot and mass spectrometry analyses, we have identified the enolase enzyme, present in the excretory/secretory materials of F. hepatica, as a human plasminogen-binding protein. This enzyme has an apparent molecular weight of 47 kDa with pI ranging from 6.2 to 7.2. These results suggest that enolase could act as a plasminogen receptor.

Enolase from Streptococcus sobrinus is an immunosuppressive protein

A strategy of Streptococcus sobrinus, a major agent of dental caries, to survive and colonize the host consists of the production of a protein that suppresses the specific antibody responses. We have cloned the gene coding for a protein with immunosuppressive activity. It contains an open reading frame of 1302 base pairs encoding a polypeptide with 434 amino acid residues and a molecular mass of 46910 Da. The gene product is homologous to enolases from several organisms. The polypeptide was expressed in Escherichia coli as a hexahistidine-tagged protein and purified in a fluoride-sensitive enzymatically active form. Pretreatment of mice with the S. sobrinus recombinant enolase suppresses a primary immune response against T-cell dependent antigens. This immunosuppressive effect is specific to the antigen used in the immunization, as it is not observed when the immune response against other antigens is analysed. Furthermore, the S. sobrinus recombinant enolase stimulates an early production of interleukin-10, an anti-inflammatory cytokine, and not the pro-inflammatory cytokine IFN-gamma. These observations indicate that enolase acts in the suppression of the specific host immune response against S. sobrinus infection.

Plasminogen-mediated group A streptococcal adherence to and pericellular invasion of human pharyngeal cells

Alpha-enolase (SEN) is a strong plasminogen-binding protein on the surface of group A streptococci (GAS). By flow cytometry and immunofluorescence analyses and using human enolase-specific antibody, human pharyngeal cells (Detroit 562) also were found to express enolase on their surface. Detroit 562 cells preferentially bound to Lys-plasminogen and this binding was inhibited in the presence of a lysine analog, epsilon-aminocaproic acid and by carboxypeptidase-B treatment suggesting that the C-terminal lysine residue of the putative pharyngeal cell receptor(s) may play an important role in plasminogen-binding. The increased plasminogen-binding in the presence of free enolase indicated the presence of an enolase/SEN-specific receptor on the pharyngeal cell surface. GAS, when precoated with Lys-plasminogen, adhered to pharyngeal cells significantly more in numbers than when precoated with fibronectin or laminin. Similarly, GAS adhered also significantly more in numbers to pharyngeal cells which were precoated with Lys-plasminogen. GAS adhered similarly in high numbers when incubated with pharyngeal cells in the presence of soluble plasminogen. The de novo pharyngeal cell-bound protease activity, created as a result of activation of bound plasminogen by t-PA, indicated its potential role in pericellular fibrinolytic activity. Further GAS with tPA-activated plasminogen bound on their surface penetrated through Transwell-grown pharyngeal cells in significantly higher numbers. Together, the results presented in this study highlight a novel function of plasminogen in streptococcal adherence to pharyngeal cells and a newly discovered streptococcal ability to pericellularly invade pharyngeal cells as a result of tPA/endogenous plasminogen activator-mediated proteolytic activity.

Role of the C-terminal lysine residues of streptococcal surface enolase in Glu- and Lys-plasminogen-binding activities of group A streptococci

Streptococcal surface enolase (SEN) is a major plasminogen-binding protein of group A streptococci. Our earlier biochemical studies have suggested that the region responsible for this property is likely located at the C-terminal end of the SEN molecule. In the present study, the gene encoding SEN was cloned from group A streptococci M6 isolate D471. A series of mutations in the sen gene corresponding to the C-terminal region (428KSFYNLKK435) of the SEN molecule were created by either deleting one or more terminal lysine residues or replacing them with leucine. All purified recombinant SEN proteins with altered C-terminal ends were found to be enzymatically active and were analyzed for their Glu- and Lys-plasminogen-binding activities. Wild-type SEN bound to Lys-plasminogen with almost three times more affinity than to Glu-plasminogen. However, the recombinant mutant SEN proteins with a deletion of Lys434-435 or with K435L and K434-435L replacements showed a significant decrease in Glu- and Lys-plasminogen-binding activities. Accordingly, a streptococcal mutant expressing SEN-K434-435L showed a significant decrease in Glu- and Lys-plasminogen-binding activities. Biochemical and functional analyses of the isogenic mutant strain revealed a significant decrease in its abilities to cleave a chromogenic tripeptide substrate, acquire plasminogen from human plasma, and penetrate the extracellular matrix. Together, these data indicate that the last two C-terminal lysine residues of surface-exposed SEN contribute significantly to the plasminogen-binding activity of intact group A streptococci and hence to their ability to exploit host properties to their own advantage in tissue invasion.

Ectodomains 3 and 4 of human polymeric Immunoglobulin receptor (hpIgR) mediate invasion of Streptococcus pneumoniae into the epithelium

Streptococcus pneumoniae binds to the ectodomain of the human polymeric Ig receptor (pIgR), also known as secretory component (SC), via a hexapeptide motif in the choline-binding protein SpsA. The SpsA-pIgR interaction mediates adherence and internalization of the human pathogen into epithelial cells. In this study the results of SpsA binding to human, mouse, and chimeric SC strongly supported the human specificity of this unique interaction and suggested that binding sites in the third and fourth Ig-like domain of human SC (D3 and D4, respectively) are involved in SpsA-pIgR complex formation. Binding of SpsA to SC-derived synthetic peptides indicated surface-located potential binding motifs in D3 and D4. Adherence and uptake of pneumococci or SpsA-coated latex beads depended on the SpsA hexapeptide motif as well as SpsA-binding sites in D3 and D4 of human pIgR. The involvement of D3 and D4 in adherence and invasion was demonstrated by the lack of binding of SpsA-coated latex beads to transfected epithelial cells expressing mutated pIgR. Finally, blocking experiments with chimeric human-mouse SC as well as synthetic peptides indicated the participation of D3 and a key role of D4 in pneumococcal invasion.

Effect of acidic pH on expression of surface-associated proteins of Streptococcus oralis

Streptococcus oralis, a member of the mitis group of oral streptococci, is implicated in the pathogenesis of infective endocarditis and is the predominant aciduric non-mutans-group streptococcus in dental plaque. We undertook to identify the most abundant surface-associated proteins of S. oralis and to investigate changes in protein expression when the organism was grown under acidic culture conditions. Surface-associated proteins were extracted from cells grown in batch culture, separated by two-dimensional gel electrophoresis, excised, digested with trypsin, and analyzed by matrix-assisted laser desorption ionization-time of flight mass spectrometry and liquid chromatography-tandem mass spectrometry. Putative functions were assigned by homology to a translated genomic database of Streptococcus pneumoniae. A total of 27 proteins were identified; these included a lipoprotein, a ribosome recycling factor, and the glycolytic enzymes phosphoglycerate kinase, fructose bisphosphate aldolase, glyceraldehyde-3-phosphate dehydrogenase, and enolase. The most abundant protein, phosphocarrier protein HPr, was present as three isoforms. Neither lactate dehydrogenase nor pyruvate oxidase, dominant intracellular proteins, were present among the proteins on the gels, demonstrating that proteins in the surface-associated pool did not arise as a result of cell lysis. Eleven of the proteins identified were differentially expressed when cells were grown at pH 5.2 versus pH 7.0, and these included superoxide dismutase, a homologue of dipeptidase V from Lactococcus lactis, and the protein translation elongation factors G, Tu, and Ts. This study has extended the range of streptococcal proteins known to be expressed at the cell surface. Further investigations are required to ascertain their functions at this extracellular location and determine how their expression is influenced by other environmental conditions.

Identification of a novel plasmin(ogen)-binding motif in surface displayed alpha-enolase of Streptococcus pneumoniae

The interaction of Streptococcus pneumoniae with human plasmin(ogen) represents a mechanism to enhance bacterial virulence by capturing surface-associated proteolytic activity in the infected host. Plasminogen binds to surface displayed pneumococcal alpha-enolase (Eno) and is subsequently activated to the serine protease plasmin by host-derived tissue plasminogen activator (tPA) or urokinase (uPA). The C-terminal lysyl residues of Eno at position 433 and 434 were identified as a binding site for the kringle motifs of plasmin(ogen) which contain lysine binding sites. In this report we have identified a novel internal plamin(ogen)-binding site of Eno by investigating the protein-protein interaction. Plasmin(ogen)-binding activity of C-terminal mutated Eno proteins used in binding assays as well as surface plasmon resonance studies suggested that an additional binding motif of Eno is involved in the Eno-plasmin(ogen) complex formation. The analysis of spot synthesized synthetic peptides representing Eno sequences identified a peptide of nine amino acids located between amino acids 248-256 as the minimal second binding epitope mediating binding of plasminogen to Eno. Binding of radiolabelled plasminogen to viable pneumococci was competitively inhibited by a synthetic peptide FYDKERKVYD representing the novel internal plasmin(ogen)-binding motif of Eno. In contrast, a synthetic peptide with amino acid substitutions at critical positions in the internal binding motif identified by systematic mutational analysis did not inhibit binding of plasminogen to pneumococci. Pneumococcal mutants expressing alpha-enolase with amino acid substitutions in the internal binding motif showed a substantially reduced plasminogen-binding activity. The virulence of these mutants was also attenuated in a mouse model of intranasal infection indicating the significance of the novel plasminogen-binding motif in the pathogenesis of pneumococcal diseases.

Molecular cloning of an alpha-enolase from the human filarial parasite Onchocerca volvulus that binds human plasminogen

Enolase represents a multifunctional protein involved in basic energy metabolism and plasminogen binding and activation at the surface of prokaryotic pathogens. A complete cDNA of 1615 bp of an alpha-enolase from Onchocerca volvulus (Ov-ENO) was isolated using a PCR-based approach. The open reading frame encoded for 435 amino acids and the high degree of conservation included the crucial amino acid residues that participate in the formation of the catalytic site, Mg(2+) binding site, and a hydrophobic motif reported to relate to surface expression. A 1089-bp fragment was expressed in a N-terminal 6 x His-tag expression vector in Escherichia coli. By immunohistological analysis using anti-Ov-ENO rabbit antibodies, native enolase could be detected in most tissues of adult O. volvulus, microfilariae, and infective larvae. Intense staining was observed in the muscles, where the energy consumption is high. The purified recombinant protein fragment revealed plasminogen binding activity in a blot-overlay assay employing anti-plasminogen antibodies. In sera from individuals infected with O. volvulus, IgG antibodies reactive with recombinant Ov-ENO were demonstrated by immunoblot and enzyme-linked immunosorbent analyses. The plasminogen-binding property of O. volvulus alpha-enolase may support plasmin-mediated proteolysis including degradation of host's extracellular matrix thereby promoting the migration of larval stages through tissues. The recognition by antibodies in sera of O. volvulus-infected persons indicate an involvement of the protein in the interaction between the parasite and the human host.

Functional analysis of a PcsB-deficient mutant of group B streptococcus

Group B streptococcus (GBS) is the major cause of bacterial sepsis and meningitis in neonates and poses a significant threat to parturient women. Recently, we identified in GBS the polypeptide PcsB, which is a protein required for cell separation of GBS, and which is also involved in the antibiotic sensitivity of these bacteria. In the present study, the introduction of the pcsB-carrying plasmid pATpcsB into the PcsB-deficient GBS mutant Sep1 restored the phenotype and the antibiotic susceptibility of this strain to that of the GBS wild-type. Although Northern blots revealed a four- to five-fold increased transcription of pcsB in pATpcsB-carrying GBS strains, overexpression of pcsB did not result in higher amounts of PcsB in the cell wall and in the culture supernatant of GBS, indicating regulatory mechanisms that control the translation or secretion of PcsB in these bacteria. In the culture supernatant of mutant Sep1 significant amounts of enolase were identified. As this protein was also present in extracts of cell wall-bound proteins from the GBS wild-type, it can be speculated that GBS can translocate enolase across the cytoplasmic membrane. Northern blot analysis exhibited similar expression of the enolase gene in the GBS strains 6313 and Sep1, indicating that mutant Sep1 is impaired in the anchoring of this protein to its cell wall.

Differential expression of the enolase gene under in vivo versus in vitro growth conditions of Aeromonas hydrophila

Aeromonas hydrophila is an emerging human pathogen that leads to gastroenteritis and other invasive diseases. By using a murine peritoneal culture (MPC) model, we identified via restriction fragment differential display PCR (RFDDPCR) five genes of A. hydrophila that were differentially expressed under in vivo versus in vitro growth conditions. The gene encoding enolase was among those five genes that were differentially up regulated. Enolase is a glycolytic enzyme and its surface expression was recently shown to be important in the pathogenesis of a gram-positive bacterium Streptococcus pyogenes. By Western blot analysis and Immunogold staining, we demonstrated secretion and surface expression of enolase in A. hydrophila. We also showed that the whole cells of A. hydrophila had strong enolase activity. Using an enzyme-linked immunosorbant assay and sandwich Western blot analysis, we demonstrated binding of enolase to human plasminogen, which is involved in the fibrinolytic system of the host. We cloned the A. hydrophila enolase gene, which exhibited 62% homology at the DNA level and 57% homology at the amino acid level when compared to S. pyogenes enolase. This is a first report describing the increased expression of enolase gene in vivo that could potentially contribute to the pathogenesis of A. hydrophila infections.

Candida albicans binds human plasminogen: identification of eight plasminogen-binding proteins

Several microbial pathogens augment their invasive potential by binding and activating human plasminogen to generate the proteolytic enzyme plasmin. Yeast cells and cell wall proteins (CWP) of the human pathogenic fungus Candida albicans bound plasminogen with a K(d) of 70 +/- 11 nM and 112 +/- 20 nM respectively. Bound plasminogen could be activated to plasmin by mammalian plasminogen activators; no C. albicans plasminogen activator was detected. Binding of plasminogen to CWP and whole cells was inhibited by epsilon ACA, indicating that binding was predominantly to lysine residues. Candida albicans mutant strains defective in protein glycosylation did not show altered plasminogen binding, suggesting that binding was not mediated via a surface lectin. Binding was sensitive to digestion by basic carboxypeptidase, implicating C-terminal lysine residues in binding. Proteomic analysis identified eight major plasminogen-binding proteins in isolated CWP. Five of these (phosphoglycerate mutase, alcohol dehydrogenase, thioredoxin peroxidase, catalase, transcription elongation factor) had C-terminal lysine residues and three (glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase and fructose bisphosphate aldolase) did not. Activation of plasminogen could potentially increase the capacity of this pathogenic fungus for tissue invasion and necrosis. Although surface-bound plasmin(ogen) degraded fibrin, no direct evidence for a role in invasion of endothelial matrix or in penetration and damage of endothelial cells was found.

Housekeeping enzymes as virulence factors for pathogens

Housekeeping enzymes are ubiquitously present in almost all living beings to perform essential metabolic functions for the purpose of survival. These enzymes have been characterized in detail for many years. In recent years, there has been a number of reports indicating that some of these enzymes perform a variety of other functions. In case of many pathogens, certain enzymes play a role to enhance virulence. To perform such a function, enzymes must be located on the surface of pathogens. Although they do not have the typical signal sequence or membrane anchoring mechanisms, they do get secreted and are displayed on the surface, probably by their reassociation. Once on the surface, these enzymes interact with host components, such as fibronectin and plasminogen, or interact directly with the host cells, to trigger signal transduction and thereby enable the pathogens to colonize, persist and invade the host tissue. Therefore, certain housekeeping enzymes may act as putative virulence factors and targets for the development of new strategies to control the infection by using agents that can block their secretion and/or reassociation.

The viable but nonculturable state and starvation are different stress responses of Enterococcus faecalis, as determined by proteome analysis

The protein expression patterns of exponentially growing, starved, and viable but nonculturable (VBNC) Enterococcus faecalis cells were analyzed to establish whether differences exist between the VBNC state and other stress responses. The results indicate that the protein profile of VBNC cells differs from that of either starved or exponentially growing bacteria. This demonstrates that the VBNC state is a distinct physiological phase within the life cycle of E. faecalis, which is activated in response to multiple environmental stresses.

Identification and characterization of two temperature-induced surface-associated proteins of Streptococcus suis with high homologies to members of the Arginine Deiminase system of Streptococcus pyogenes

The present study was performed to identify stress-induced putative virulence proteins of Streptococcus suis. For this, protein expression patterns of streptococci grown at 32, 37, and 42 degrees C were compared by one- and two-dimensional gel electrophoresis. Temperature shifts from 32 and 37 to 42 degrees C induced expression of two cell wall-associated proteins with apparent molecular masses of approximately 47 and 53 kDa. Amino-terminal sequence analysis of the two proteins indicated homologies of the 47-kDa protein with an ornithine carbamoyltransferase (OCT) from Streptococcus pyogenes and of the 53-kDa protein with the streptococcal acid glycoprotein (SAGP) from S. pyogenes, an arginine deiminase (AD) recently proposed as a putative virulence factor. Cloning and sequencing the genes encoding the putative OCT and AD of S. suis, octS and adiS, respectively, revealed that they had 81.2 (octS) and 80.2% (adiS) identity with the respective genes of S. pyogenes. Both genes belong to the AD system, also found in other bacteria. Southern hybridization analysis demonstrated the presence of the adiS gene in all 42 serotype 2 and 9 S. suis strains tested. In 9 of these 42 strains, selected randomly, we confirmed expression of the AdiS protein, homologous to SAGP, by immunoblot analysis using a specific antiserum against the SAGP of S. pyogenes. In all strains AD activity was detected. Furthermore, by immunoelectron microscopy using the anti-S. pyogenes SAGP antiserum we were able to demonstrate that the AdiS protein is expressed on the streptococcal surface in association with the capsular polysaccharides but is not coexpressed with them.

The viable but nonculturable state and starvation are different stress responses of Enterococcus faecalis, as determined by proteome analysis

The protein expression patterns of exponentially growing, starved, and viable but nonculturable (VBNC) Enterococcus faecalis cells were analyzed to establish whether differences exist between the VBNC state and other stress responses. The results indicate that the protein profile of VBNC cells differs from that of either starved or exponentially growing bacteria. This demonstrates that the VBNC state is a distinct physiological phase within the life cycle of E. faecalis, which is activated in response to multiple environmental stresses.

Is the GehD lipase from Staphylococcus epidermidis a collagen binding adhesin?

The opportunistic human pathogen Staphylococcus epidermidis is the major cause of nosocomial biomaterial infections. S. epidermidis has the ability to attach to indwelling materials coated with extracellular matrix proteins such as fibrinogen, fibronectin, vitronectin, and collagen. To identify the proteins necessary for S. epidermidis attachment to collagen, we screened an expression library using digoxigenin-labeled collagen as well as two monoclonal antibodies generated against the Staphylococcus aureus collagen-adhesin, Cna, as probes. These monoclonal antibodies recognize collagen binding epitopes on the surface of S. aureus and S. epidermidis cells. Using this approach, we identified GehD, the extracellular lipase originally found in S. epidermidis 9, as a collagen-binding protein. Despite the monoclonal antibody cross-reactivity, the GehD amino acid sequence and predicted structure are radically different from those of Cna. The mature GehD circular dichroism spectra differs from that of Cna but strongly resembles that of a mammalian cell-surface collagen binding receptor, known as the alpha(1) integrin I domain, suggesting that they have similar secondary structures. The GehD protein is translated as a preproenzyme, secreted, and post-translationally processed into mature lipase. GehD does not have the conserved LPXTG C-terminal motif present in cell wall-anchored proteins, but it can be detected in lysostaphin cell wall extracts. A recombinant version of mature GehD binds to collagens type I, II, and IV adsorbed onto microtiter plates in a dose-dependent saturable manner. Recombinant, mature GehD protein and anti-GehD antibodies can inhibit the attachment of S. epidermidis to immobilized collagen. These results provide evidence that GehD may be a bi-functional molecule, acting not only as a lipase but also as a cell surface-associated collagen adhesin.

Evidence for nuclear localisation of two stage-specific isoenzymes of enolase in Toxoplasma gondii correlates with active parasite replication

The protozoan parasite Toxoplasma gondii has a complex life cycle involving the developmental transition between the asexual exo-enteric stages (tachyzoites and bradyzoites) and the coccidian (sexual and asexual) forms (schizonts, macrogametes and microgametes). Previous work has established the stage-specific expression of certain proteins including two glycolytic isoenzymes of enolase and lactate dehydrogenase in T. gondii. Here we describe the expression and subcellular localisation of the two isoforms of enolase (ENO1 and ENO2) and lactate dehydrogenase (LDH1 and LDH2) in vivo using immunocytochemistry. In mice, proliferating parasites in the lung expressed ENO2 and LDH1 and were characterised as tachyzoites by the presence of a tachyzoite specific surface antigen (SAG1). In contrast, ENO1 and LDH2 were expressed by bradyzoites present in tissue cysts in the brain characterised by the presence of the bradyzoite specific antigen (BAG1). During stage conversion (tachyzoite/bradyzoite), the isoenzyme changes occur at an early stage when the bradyzoites are actively proliferating and thus may not simply be reflecting reduced metabolic needs. When the coccidian stages were examined in the cat intestine, they were negative for SAG1, BAG1, LDH2 and ENO1 but were similar to the tachyzoite in strongly expressing LDH1 and ENO2. The isoenzymes LDH1 and LDH2 were exclusively expressed in the cytoplasm. In contrast, it was observed that the strongest labelling for both ENO1 and ENO2 was observed in the nucleus with less intense but specific cytoplasmic staining. Immunoelectron microscopy confirmed the cytoplasmic location of LDH and the predominantly nuclear location of enolase. During early intracellular proliferation and development, all stages of the life cycle (tachyzoite, bradyzoite and coccidian stages) exhibited very strong nuclear labelling for enolase but this was markedly reduced in mature parasites to levels below that seen in the cytoplasm. The specific nuclear localisation of enolases appears to be associated with nuclear activity (transcription and/or division) and may play some part in the control of gene regulation during parasite proliferation and differentiation in addition to its role in glycolysis.

Purification of native alpha-enolase from Streptococcus pneumoniae that binds plasminogen and is immunogenic

Many pathogenic bacteria express plasminogen receptors on their surface, which may play a role in the dissemination of organisms by binding plasminogen that, when converted to plasmin, can digest extracellular matrix proteins. A 45-kDa protein was purified from Streptococcus pneumoniae and confirmed as an alpha-enolase by its ability to catalyse the dehydration of 2-phospho-D-glycerate to phosphoenolpyruvate and by N-terminal sequencing. The activity of alpha-enolase was found in the cytoplasm and in whole cells. Activity was also demonstrated in cell wall fractions, which confirmed that alpha-enolase is a cytoplasmic antigen also expressed on the surface of S. pneumoniae. The plasminogen-binding activity of alpha-enolase was examined by Western blot, which showed that purified alpha-enolase was able to bind human plasminogen. Immunoblots of the purified 45-kDa alpha-enolase with 22 sera from patients with pneumococcal disease showed binding in 15 cases, indicating that pneumococcal enolase is immunogenic.

Anchorless adhesins and invasins of Gram-positive bacteria: a new class of virulence factors

Bacterial adherence to and invasion of eukaryotic cells are important mechanisms of pathogenicity. Most Gram-positive bacteria interact with the components of the host extracellular matrix (ECM) to adhere to, colonize and invade cells and tissues. The bacterial proteins that bind to components of the ECM harbour signal sequences for their secretion and mechanisms of anchoring to the host cell surface. However, in recent years, some cell-surface adhesins and invasins of Gram-positive bacteria have been described that do not possess a signal sequence or a membrane anchor. These proteins are secreted by an as-yet-unknown mechanism and are probably localized on the bacterial surface by reassociation. These anchorless but surface-located adhesins and invasins represent a new class of virulence factors.

The pavA gene of Streptococcus pneumoniae encodes a fibronectin-binding protein that is essential for virulence

Streptococcus pneumoniae colonizes the nasopharynx in up to 40% of healthy subjects, and is a leading cause of middle ear infections (otitis media), meningitis and pneumonia. Pneumococci adhere to glycosidic receptors on epithelial cells and to immobilized fibronectin, but the bacterial adhesins mediating these reactions are largely uncharacterized. In this report we describe a novel pneumococcal protein PavA, which binds fibronectin and is associated with pneumococcal adhesion and virulence. The pavA gene, present in 64 independent isolates of S. pneumoniae tested, encodes a 551 amino acid residue polypeptide with 67% identical amino acid sequence to Fbp54 protein in Streptococcus pyogenes. PavA localized to the pneumococcal cell outer surface, as demonstrated by immunoelectron microscopy, despite lack of conventional secretory or cell-surface anchorage signals within the primary sequence. Full-length recombinant PavA polypeptide bound to immobilized human fibronectin in preference to fluid-phase fibronectin, in a heparin-sensitive interaction, and blocked binding of wild-type pneumococcal cells to fibronectin. However, a C-terminally truncated PavA' polypeptide (362 aa residues) failed to bind fibronectin or block pneumococcal cell adhesion. Expression of pavA in Enterococcus faecalis JH2-2 conferred > sixfold increased cell adhesion levels to fibronectin over control JH2-2 cells. Isogenic mutants of S. pneumoniae, either abrogated in PavA expression or producing a 42 kDa C-terminally truncated protein, showed up to 50% reduced binding to immobilized fibronectin. Inactivation of pavA had no effects on growth rate, cell morphology, cell-surface physico-chemical properties, production of pneumolysin, autolysin, or surface proteins PspA and PsaA. Isogenic pavA mutants of encapsulated S. pneumoniae D39 were approximately 104-fold attenuated in virulence in the mouse sepsis model. These results provide evidence that PavA fibronectin-binding protein plays a direct role in the pathogenesis of pneumococcal infections.