Group A streptococcal isolate 64/14 expresses surface plasmin-binding structures in addition to Plr

Genetics and Pathogenicity Factors of Group C and G Streptococci

Of the eight phylogenetic groups comprising the genus Streptococcus , Lancefield group C and G streptococci (GCS and GGS, resp.) occupy four of them, including the Pyogenic, Anginosus, and Mitis groups, and one Unnamed group so far. These organisms thrive as opportunistic commensals in both humans and animals but may also be associated with clinically serious infections, often resembling those due to their closest genetic relatives, the group A streptoccci (GAS). Advances in molecular genetics, taxonomic approaches and phylogenomic studies have led to the establishment of at least 12 species, several of which being subdivided into subspecies. This review summarizes these advances, citing 264 early and recent references. It focuses on the molecular structure and genetic regulation of clinically important proteins associated with the cell wall, cytoplasmic membrane and extracellular environment. The article also addresses the question of how, based on the current knowledge, basic research and translational medicine might proceed to further advance our understanding of these multifaceted organisms. Particular emphasis in this respect is placed on streptokinase as the protein determining the host specificity of infection and the Rsh-mediated stringent response with its potential for supporting bacterial survival under nutritional stress conditions.

Glyceradehyde-3-phosphate dehydrogenase as a suitable vaccine candidate for protection against bacterial and parasitic diseases

The enzyme glyceraldehyde-3-P-dehydrogenase (GAPDH) has been identified as having other properties in addition to its key role in glycolysis. The ability of GAPDH to bind to numerous extracellular matrices, modulation of host-immune responses, a role in virulence and surface location has prompted numerous investigators to postulate that GAPDH may be a good vaccine candidate for protection against numerous pathogens. Although immune responses against GAPDH have been described for many microorganisms, vaccines containing GAPDH have been successfully tested in few cases including those against the trematode-Schistosoma mansoni, the helminth-Enchinococcus multilocularis; the nematode filaria- Litomosoides sigmodontis; fish pathogens such as Aeromonas spp., Vibrio spp., Edwarsiella spp., and Streptococcus iniae; and environmental streptococci, namely, Streptococcus uberis and Streptococcus dysgalactiae. Before GAPDH-based vaccines are considered viable options for protection against numerous pathogens, we need to take into account the homology between the host and pathogen GAPDH proteins to prevent potential autoimmune reactions, thus protective GAPDH epitopes unique to the pathogen protein must be identified.

Characterization of streptokinases from group A Streptococci reveals a strong functional relationship that supports the coinheritance of plasminogen-binding M protein and cluster 2b streptokinase

Group Astreptococcus (GAS) strains secrete the protein streptokinase (SK), which functions by activating host human plasminogen (hPg) to plasmin (hPm), thus providing a proteolytic framework for invasive GAS strains. The types of SK secreted by GAS have been grouped into two clusters (SK1 and SK2) and one subcluster (SK2a and SK2b). SKs from cluster 1 (SK1) and cluster 2b (SK2b) display significant evolutionary and functional differences, and attempts to relate these properties to GAS skin or pharynx tropism and invasiveness are of great interest. In this study, using four purified SKs from each cluster, new relationships between plasminogen-binding group A streptococcal M (PAM) protein and SK2b have been revealed. All SK1 proteins efficiently activated hPg, whereas all subclass SK2b proteins only weakly activated hPg in the absence of PAM. Surface plasmon resonance studies revealed that the lower affinity of SK2b to hPg served as the basis for the attenuated activation of hPg by SK2b. Binding of hPg to either human fibrinogen (hFg) or PAM greatly enhanced activation of hPg by SK2b but minimally influenced the already effective activation of hPg by SK1. Activation of hPg in the presence of GAS cells containing PAM demonstrated that PAM is the only factor on the surface of SK2b-expressing cells that enabled the direct activation of hPg by SK2b. As the binding of hPg to PAM is necessary for hPg activation by SK2b, this dependence explains the coinherant relationship between PAM and SK2b and the ability of these particular strains to generate the proteolytic activity that disrupts the innate barriers that limit invasiveness.

Streptococcus gordonii utilizes several distinct gene functions to recruit Porphyromonas gingivalis into a mixed community

Dental plaque biofilm formation proceeds through a developmental pathway initiated by the attachment of pioneer organisms, such as Streptococcus gordonii, to tooth surfaces. Through a variety of synergistic interactions, pioneer organisms facilitate the colonization of later arrivals including Porphyromonas gingivalis, a potential periodontal pathogen. We have investigated genes of S. gordonii required to support a heterotypic biofilm community with P. gingivalis. By screening a plasmid integration library of S. gordonii, genes were identified that are crucial for the accumulation of planktonic P. gingivalis cells into a multispecies biofilm. These genes were further investigated by specific mutation and complementation analyses. The biofilm-associated genes can be grouped into broad categories based on putative function as follows: (i) intercellular or intracellular signalling (cbe and spxB), (ii) cell wall integrity and maintenance of adhesive proteins (murE, msrA and atf), (iii) extracellular capsule biosynthesis (pgsA and atf), and (iv) physiology (gdhA, ccmA and ntpB). In addition, a gene for a hypothetical protein was identified. Biofilm visualization and quantification by confocal microscopy confirmed the role of these genes in the maturation of the multispecies community, including biofilm architectural development. The results suggest that S. gordonii governs the development of heterotypic oral biofilms through multiple genetic pathways.

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.

Characterization of group B streptococcal glyceraldehyde-3-phosphate dehydrogenase: surface localization, enzymatic activity, and protein-protein interactions

During characterization of the surface antigens of serotype III group B streptococci (GBS), a protein with an apparent M(r) of approximately 173,500 migrating on a SDS--polyacrylamide gel was found to have an N-terminal amino acid sequence identical to that of the plasmin receptor (Plr) of group A streptococci, a surface-localized glyceraldehyde-3-phosphate dehydrogenase (GAPDH). This work begins to characterize GBS GAPDH and to assess its functional activity on the cell surface. The 1.0-kb gapC gene of GBS was amplified by PCR. plr and gapC demonstrated 87% homology. An anti-Plr monoclonal antibody reacted with GBS whole cells, suggesting GBS GAPDH is surface localized. Multiple serotypes of GBS demonstrated functional GAPDH on their surfaces. The anti-Plr monoclonal antibody recognized GBS protein bands of approximately 41 and 173.5 kDa, by Western blot. Presumably, these represent monomeric and tetrameric forms of the GAPDH molecule. GBS GAPDH was demonstrated by Western blot analysis to interact with lys- and glu-plasminogens. Fluid-phase GBS GAPDH interacted, by means of ELISA, with immobilized lys-plasminogen, glu-plasminogen, actin, and fibrinogen. Enzymatically active GAPDH, capable of binding cytoskeletal and extracellular matrix proteins, is expressed on the surface of GBS.

Differential recognition of surface proteins in Streptococcus pyogenes by two sortase gene homologs

The interaction of Streptococcus pyogenes (group A streptococcus [GAS]) with its human host requires several surface proteins. In this study, we isolated mutations in a gene required for the surface localization of protein F by transposon mutagenesis of the M6 strain JRS4. This gene (srtA) encodes a protein homologous to Staphylococcus aureus sortase, which covalently links proteins containing an LPXTG motif to the cell wall. The GAS srtA mutant was defective in anchoring the LPXTG-containing proteins M6, protein F, ScpA, and GRAB to the cell surface. This phenotype was complemented when a wild-type srtA gene was provided in trans. The surface localization of T6, however, was unaffected by the srtA mutation. The M1 genome sequence contains a second open reading frame with a motif characteristic of sortase proteins. Inactivation of this gene (designated srtB) in strain JRS4 affected the surface localization of T6 but not M6, protein F, ScpA, or GRAB. This phenotype was complemented by srtB in trans. An srtA probe hybridized with DNA from all GAS strains tested (M types 1, 3, 4, 5, 6, 18, 22, and 50 and nontypeable strain 64/14) and from streptococcal groups C and G, while srtB hybridized with DNA from only a few GAS strains. We conclude that srtA and srtB encode sortase enzymes required for anchoring different subsets of proteins to the cell wall. It seems likely that the multiple sortase homologs in the genomes of other gram-positive bacteria have a similar substrate-specific role.

Analysis of expression of a cytosolic enzyme on the surface of Streptococcus pyogenes

The normally cytosolic glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase, (GAPDH) has been reported to be expressed on the surface of Streptococcus pyogenes, group A, where it can act as a plasmin binding protein (Plr), and potentially a signaling molecule. In studies of wild-type and isogenic mutants, an association between surface expression of antigenic GAPDH/Plr and M and M-related fibrinogen-binding proteins was identified. Inactivation of the mga gene, whose product controls expression of M and M-related proteins also influenced expression of surface GAPDH/Plr. Revertants or pseudorevertants of mga mutants led to concomitant re-expression of surface GAPDH/Plr and M and M-related proteins. Using surface enhanced laser desorption ionization (SELDI) mass spectroscopy, a physical association between GAPDH/Plr and streptococcal fibrinogen-binding proteins was demonstrated. These studies support the hypothesis that surface M and M-related proteins are involved in anchoring GAPDH/Plr on the surface of group A streptococci.

Pathogenesis of group A streptococcal infections

Group A streptococci are model extracellular gram-positive pathogens responsible for pharyngitis, impetigo, rheumatic fever, and acute glomerulonephritis. A resurgence of invasive streptococcal diseases and rheumatic fever has appeared in outbreaks over the past 10 years, with a predominant M1 serotype as well as others identified with the outbreaks. emm (M protein) gene sequencing has changed serotyping, and new virulence genes and new virulence regulatory networks have been defined. The emm gene superfamily has expanded to include antiphagocytic molecules and immunoglobulin-binding proteins with common structural features. At least nine superantigens have been characterized, all of which may contribute to toxic streptococcal syndrome. An emerging theme is the dichotomy between skin and throat strains in their epidemiology and genetic makeup. Eleven adhesins have been reported, and surface plasmin-binding proteins have been defined. The strong resistance of the group A streptococcus to phagocytosis is related to factor H and fibrinogen binding by M protein and to disarming complement component C5a by the C5a peptidase. Molecular mimicry appears to play a role in autoimmune mechanisms involved in rheumatic fever, while nephritis strain-associated proteins may lead to immune-mediated acute glomerulonephritis. Vaccine strategies have focused on recombinant M protein and C5a peptidase vaccines, and mucosal vaccine delivery systems are under investigation.

The potential role for nephritis-associated plasmin receptor in acute poststreptococcal glomerulonephritis

Immunoglobulin G from a patient convalescing from acute poststreptococcal glomerulonephritis (APSGN) bound specific antigenic sites in early APSGN glomeruli. A streptococcal cytoplasmic antigen (preabsorbing antigen, PA-Ag), could selectively preabsorb fluorescein isothiocyanate (FITC)-labeled IgG and prevented glomerular staining. The antigen was purified and identified as an M(r) approximately 43,000 protein with a pI of 4.7 that strongly activated complement C3 (N. Yoshizawa, S. Oshima, I. Sagel, J. Shimizu, and G. Treser, 1992, J. Immunol. 148, 3110-3116). In the present study, a nephritogenic antigen was purified by affinity chromatography using APSGN IgG-immobilized Sepharose followed by chromatography on an anion-exchange resin. Purification was monitored by ELISA and Western blotting using the binding characteristics of the specific antibodies present in APSGN serum. The molecular weight of the purified antigen, named nephritis-associated plasmin receptor (NAPlr), was an M(r) approximately 43,000 protein and the internal amino acid sequence was found to be homologous to those of the plasmin receptor (Plr) of group A streptococci strain 64/14 and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from Bacillus subtilis. The purified NAPlr exhibited GAPDH activity and plasmin(ogen) binding activity. Using FITC-labeled rabbit anti-NAPlr, the antigen was found to be present in the glomeruli of 22 of 22 patients in the early stage of APSGN. Bacterial Plr was also demonstrated in human APSGN glomeruli for the first time using monoclonal antibody to the recombinant Plr protein. Antibody to NAPlr was found in the sera of 46 of 50 (92%) patients within 3 months of onset. These results led us to speculate that NAPlr bound to the glomeruli may contribute to the pathogenesis of APSGN via plasmin and complement activation.

Site-directed mutagenesis of streptococcal plasmin receptor protein (Plr) identifies the C-terminal Lys334 as essential for plasmin binding, but mutation of the plr gene does not reduce plasmin binding to group A streptococci

Plasmin(ogen) binding is a common property of many pathogenic bacteria including group A streptococci. Previous analysis of a putative plasmin receptor protein, Plr, from the group A streptococcal strain 64/14 revealed that it is a glyceraldehyde-3-phosphate dehydrogenase and that the plr gene is present on the chromosome as a single copy. This study continues the functional characterization of Plr as a plasmin receptor. Attempts at insertional inactivation of the plr gene suggested that this single-copy gene may be essential for cell viability. Therefore, an alternative strategy was applied to manipulate this gene in vivo. Site-directed mutagenesis of Plr revealed that a C-terminal lysyl residue is required for wild-type levels of plasmin binding. Mutated Plr proteins expressed in Escherichia coli demonstrated reduced plasmin-binding activity yet retained glyceraldehyde-3-phosphate dehydrogenase activity. A novel integration vector was constructed to precisely replace the wild-type copy of the plr gene with these mutations. Isogenic streptococcal strains expressing altered Plr bound equivalent amounts of plasmin as wild-type streptococci. These data suggest that Plr does not function as a unique plasmin receptor, and underscore the need to identify other plasmin-binding structures on group A streptococci and to assess the importance of the plasminogen system in pathogenesis by inactivation of plasminogen activators and the use of appropriate animal models.