Streptococcal M protein: molecular design and biological behavior

Streptococcal inhibitor of complement (SIC) modulates fibrinolysis and enhances bacterial survival within fibrin clots

Some strains of the bacterial pathogen Streptococcus pyogenes secrete protein SIC (streptococcal inhibitor of complement), including strains of the clinically relevant M1 serotype. SIC neutralizes the effect of a number of antimicrobial proteins/peptides and interferes with the function of the host complement system. Previous studies have shown that some S. pyogenes proteins bind and modulate coagulation and fibrinolysis factors, raising the possibility that SIC also may interfere with the activity of these factors. Here we show that SIC interacts with both human thrombin and plasminogen, key components of coagulation and fibrinolysis. We found that during clot formation, SIC binds fibrin through its central region and that SIC inhibits fibrinolysis by interacting with plasminogen. Flow cytometry results indicated that SIC and plasminogen bind simultaneously to S. pyogenes bacteria, and fluorescence microscopy revealed co-localization of the two proteins at the bacterial surface. As a consequence, SIC-expressing bacteria entrapped in clots inhibit fibrinolysis, leading to delayed bacterial escape from the clots as compared with mutant bacteria lacking SIC. Moreover, within the clots SIC-expressing bacteria were protected against killing. In an animal model of subcutaneous infection, SIC-expressing bacteria exhibited a delayed systemic spread. These results demonstrate that the bacterial protein SIC interferes with coagulation and fibrinolysis and thereby enhances bacterial survival, a finding that has significant implications for S. pyogenes virulence.

Evaluation of safety and immunogenicity of a group A streptococcus vaccine candidate (MJ8VAX) in a randomized clinical trial

BACKGROUND

Group A streptococcus (GAS) is a serious human pathogen that affects people of different ages and socio-economic levels. Although vaccination is potentially one of the most effective methods to control GAS infection and its sequelae, few prototype vaccines have been investigated in humans. In this study, we report the safety and immunogenicity of a novel acetylated peptide-protein conjugate vaccine candidate MJ8VAX (J8-DT), when delivered intramuscularly to healthy adults.

METHODS

A randomized, double-blinded, controlled Phase I clinical trial was conducted in 10 healthy adult participants. Participants were randomized 4:1 to receive the vaccine candidate (N = 8) or placebo (N = 2). A single dose of the vaccine candidate (MJ8VAX), contained 50 μg of peptide conjugate (J8-DT) adsorbed onto aluminium hydroxide and re-suspended in PBS in a total volume of 0.5 mL. Safety of the vaccine candidate was assessed by monitoring local and systemic adverse reactions following intramuscular administration. The immunogenicity of the vaccine was assessed by measuring the levels of peptide (anti-J8) and toxoid carrier (anti-DT)-specific antibodies in serum samples.

RESULTS

No serious adverse events were reported over 12 months of study. A total of 13 adverse events (AEs) were recorded, two of which were assessed to be associated with the vaccine. Both were mild in severity. No local reactogenicity was recorded in any of the participants. MJ8VAX was shown to be immunogenic, with increase in vaccine-specific antibodies in the participants who received the vaccine. The maximum level of vaccine-specific antibodies was detected at 28 days post immunization. The level of these antibodies decreased with time during follow-up. Participants who received the vaccine also had a corresponding increase in anti-DT serum antibodies.

CONCLUSIONS

Intramuscular administration of MJ8VAX was demonstrated to be safe and immunogenic. The presence of DT in the vaccine formulation resulted in a boost in the level of anti-DT antibodies.

TRIAL REGISTRATION

ACTRN12613000030774.

Contribution of cryptic epitopes in designing a group A streptococcal vaccine

A successful vaccine needs to target multiple strains of an organism. Streptococcus pyogenes is an organism that utilizes antigenic strain variation as a successful defence mechanism to circumvent the host immune response. Despite numerous efforts, there is currently no vaccine available for this organism. Here we review and discuss the significant obstacles to vaccine development, with a focus on how cryptic epitopes may provide a strategy to circumvent the obstacles of antigenic variation.

Linear and branched polyacrylates as a delivery platform for peptide-based vaccines

AIM

Peptide-based vaccines are designed to carry the minimum required antigen to trigger the desired immune responses; however, they are usually poorly immunogenic and require appropriate delivery system.

RESULTS

Peptides, B-cell epitope (J14) derived from group A streptococcus M-protein and universal T-helper (PADRE) epitope, were conjugated to a variety of linear and branched polyacrylates. All produced conjugates formed submicron-sized particles and induced a high level of IgG titres in mice after subcutaneous immunization. These polymer-peptide conjugates demonstrated high opsonization capacity against group A streptococcus clinical isolates.

CONCLUSION

We have successfully demonstrated that submicron-sized polymer-peptide conjugates were capable of inducing strong humoral immune responses after single immunization.

Streptococcal pharyngitis and rheumatic heart disease: the superantigen hypothesis revisited

Streptococcus pyogenes is a human-specific and globally prominent bacterial pathogen that despite causing numerous human infections, this bacterium is normally found in an asymptomatic carrier state. This review provides an overview of both bacterial and human factors that likely play an important role in nasopharyngeal colonization and pharyngitis, as well as the development of acute rheumatic fever and rheumatic heart disease. Here we highlight a recently described role for bacterial superantigens in promoting acute nasopharyngeal infection, and discuss how these immune system activating toxins could be crucial to initiate the autoimmune process in rheumatic heart disease.

Streptococcus pyogenes Infection and the Human Proteome with a Special Focus on the Immunoglobulin G-cleaving Enzyme IdeS

Infectious diseases are characterized by a complex interplay between host and pathogen, but how these interactions impact the host proteome is unclear. Here we applied a combined mass spectrometry-based proteomics strategy to investigate how the human proteome is transiently modified by the pathogen Streptococcus pyogenes, with a particular focus on bacterial cleavage of IgG in vivo. In invasive diseases, S. pyogenes evokes a massive host response in blood, whereas superficial diseases are characterized by a local leakage of several blood plasma proteins at the site of infection including IgG. S. pyogenes produces IdeS, a protease cleaving IgG in the lower hinge region and we find highly effective IdeS-cleavage of IgG in samples from local IgG poor microenvironments. The results show that IdeS contributes to the adaptation of S. pyogenes to its normal ecological niches. Additionally, the work identifies novel clinical opportunities for in vivo pathogen detection.

Bivalent mucosal peptide vaccines administered using the LCP carrier system stimulate protective immune responses against Streptococcus pyogenes infection

Despite the broad knowledge about the pathogenicity of Streptococcus pyogenes there is still a controversy about the correlate of protection in GAS infections. We aimed in further improving the immune responses stimulated against GAS comparing different vaccine formulations including bis-(3',5')-cyclic dimeric adenosine monophosphate (c-di-AMP) and BPPCysMPEG, a derivative of the macrophage-activating lipopeptide (MALP-2), as adjuvants, respectively, to be administered with and without the universal T helper cell epitope P25 along with the optimized B cell epitope J14 of the M protein and B and T cell epitopes of SfbI. Lipopeptide based nano carrier systems (LCP) were used for efficient antigen delivery across the mucosal barrier. The stimulated immune responses were efficient in protecting mice against a respiratory challenge with a lethal dose of a heterologous S. pyogenes strain. Moreover, combination of the LCP based peptide vaccine with c-di-AMP allowed reduction of antigen dose at the same time maintaining vaccine efficacy.

Towards Identifying Protective B-Cell Epitopes: The PspA Story

Pneumococcal surface protein A (PspA) is one of the most abundant cell surface protein of Streptococcus pneumoniae (S. pneumoniae). PspA variants are structurally and serologically diverse and help evade complement-mediated phagocytosis of S. pneumoniae, which is essential for its survival in the host. PspA is currently been screened for employment in the generation of more effective (serotype independent) vaccine to overcome the limitations of polysaccharide based vaccines, providing serotype specific immune responses. The cross-protection eliciting regions of PspA localize to the α-helical and proline rich regions. Recent data indicate significant variation in the ability of antibodies induced against the recombinant PspA variants to recognize distinct S. pneumoniae strains. Hence, screening for the identification of the topographical repertoire of B-cell epitopes that elicit cross-protective immune response seems essential in the engineering of a superior PspA-based vaccine. Herein, we revisit epitope identification in PspA and the utility of hybridoma technology in directing the identification of protective epitope regions of PspA that can be used in vaccine research.

Allergenicity of vertebrate tropomyosins: Challenging an immunological dogma

With the exception of tilapia tropomyosin, other anecdotic reports of tropomyosin recognition of vertebrate origin are generally not accompanied by clinical significance and a dogmatic idea is generally accepted about the inexistence of allergenicity of vertebrate tropomyosins, based mainly on sequence similarity evaluations with human tropomyosins. Recently, a specific work-up of a tropomyosin sensitised patient with seafood allergy, demonstrated that the IgE-recognition of tropomyosin from different fish species can be clinically relevant. We hypothesise that some vertebrate tropomyosins could be relevant allergens. The hypothesis is based on the molecular evolution of the proteins and it was tested by in silico methods. Fish, which are primitive vertebrates, could have tropomyosins similar to those of invertebrates. If the hypothesis is confirmed, tropomyosin should be included in different allergy diagnosis tools to improve the medical protocols and management of patients with digestive or cutaneous symptoms after fish intake.

Targeted Proteomics and Absolute Protein Quantification for the Construction of a Stoichiometric Host-Pathogen Surface Density Model

Sepsis is a systemic immune response responsible for considerable morbidity and mortality. Molecular modeling of host-pathogen interactions in the disease state represents a promising strategy to define molecular events of importance for the transition from superficial to invasive infectious diseases. Here we used the Gram-positive bacterium Streptococcus pyogenes as a model system to establish a mass spectrometry based workflow for the construction of a stoichiometric surface density model between the S. pyogenes surface, the surface virulence factor M-protein, and adhered human blood plasma proteins. The workflow relies on stable isotope labeled reference peptides and selected reaction monitoring mass spectrometry analysis of a wild-type strain and an M-protein deficient mutant strain, to generate absolutely quantified protein stoichiometry ratios between S. pyogenes and interacting plasma proteins. The stoichiometry ratios in combination with a novel targeted mass spectrometry method to measure cell numbers enabled the construction of a stoichiometric surface density model using protein structures available from the protein data bank. The model outlines the topology and density of the host-pathogen protein interaction network on the S. pyogenes bacterial surface, revealing a dense and highly organized protein interaction network. Removal of the M-protein from S. pyogenes introduces a drastic change in the network topology, validated by electron microscopy. We propose that the stoichiometric surface density model of S. pyogenes in human blood plasma represents a scalable framework that can continuously be refined with the emergence of new results. Future integration of new results will improve the understanding of protein-protein interactions and their importance for bacterial virulence. Furthermore, we anticipate that the general properties of the developed workflow will facilitate the production of stoichiometric surface density models for other types of host-pathogen interactions.

Variable region in streptococcal M-proteins provides stable binding with host fibrinogen for plasminogen-mediated bacterial invasion

Dimeric M-proteins (M-Prt) in group A Streptococcus pyogenes (GAS) are surface-expressed virulence factors implicated in processes that contribute to the pathogenicity of infection. Sequence analyses of various GAS M-Prts have shown that they contain a highly conserved sortase A-dependent cell wall-anchored C terminus, whereas the surface-exposed N terminus is highly variable, a feature used for identification and serotyping of various GAS strains. This variability also allows for strain-specific responses that suppress host defenses. Previous studies have indeed identified the N-terminal M-Prt B-domain as the site interacting with antiphagocytotic human-host fibrinogen (hFg). Herein, we show that hFg strongly interacts with M-Prts containing highly variable B-domains. We further demonstrate that specific GAS clinical isolates display high affinity for the D-domain of hFg, and this interaction allowed for subsequent surface binding of human-host plasminogen (hPg) to the E-domain of hFg. This GAS surface-bound hPg is then activated by GAS-secreted streptokinase, leading to the generation of an invasive proteolytic bacterial surface. Our results underscore the importance of the human fibrinolytic system in host-pathogen interactions in invasive GAS infections.

Group A Streptococcal M1 Protein Provides Resistance against the Antimicrobial Activity of Histones

Histones are essential elements of chromatin structure and gene regulation in eukaryotes. An unexpected attribute of these nuclear proteins is their antimicrobial activity. A framework for histone release and function in host defense in vivo was revealed with the discovery of neutrophil extracellular traps, a specialized cell death process in which DNA-based structures containing histones are extruded to ensnare and kill bacteria. Investigating the susceptibility of various Gram-positive pathogens to histones, we found high-level resistance by one leading human pathogen, group A Streptococcus (GAS). A screen of isogenic mutants revealed that the highly surface-expressed M1 protein, a classical GAS virulence factor, was required for high-level histone resistance. Biochemical and microscopic analyses revealed that the N-terminal domain of M1 protein binds and inactivates histones before they reach their cell wall target of action. This finding illustrates a new pathogenic function for this classic GAS virulence factor, and highlights a potential innate immune evasion strategy that may be employed by other bacterial pathogens.

The Streptococcus gordonii Adhesin CshA Protein Binds Host Fibronectin via a Catch-Clamp Mechanism

Adherence of bacteria to biotic or abiotic surfaces is a prerequisite for host colonization and represents an important step in microbial pathogenicity. This attachment is facilitated by bacterial adhesins at the cell surface. Because of their size and often elaborate multidomain architectures, these polypeptides represent challenging targets for detailed structural and functional characterization. The multifunctional fibrillar adhesin CshA, which mediates binding to both host molecules and other microorganisms, is an important determinant of colonization by Streptococcus gordonii, an oral commensal and opportunistic pathogen of animals and humans. CshA binds the high-molecular-weight glycoprotein fibronectin (Fn) via an N-terminal non-repetitive region, and this protein-protein interaction has been proposed to promote S. gordonii colonization at multiple sites within the host. However, the molecular details of how these two proteins interact have yet to be established. Here we present a structural description of the Fn binding N-terminal region of CshA, derived from a combination of X-ray crystallography, small angle X-ray scattering, and complementary biophysical methods. In vitro binding studies support a previously unreported two-state "catch-clamp" mechanism of Fn binding by CshA, in which the disordered N-terminal domain of CshA acts to "catch" Fn, via formation of a rapidly assembled but also readily dissociable pre-complex, enabling its neighboring ligand binding domain to tightly clamp the two polypeptides together. This study presents a new paradigm for target binding by a bacterial adhesin, the identification of which will inform future efforts toward the development of anti-adhesive agents that target S. gordonii and related streptococci.

Null Mutations of Group A Streptococcus Orphan Kinase RocA: Selection in Mouse Infection and Comparison with CovS Mutations in Alteration of In Vitro and In Vivo Protease SpeB Expression and Virulence

Group A Streptococcus (GAS) acquires mutations of the virulence regulator CovRS in human and mouse infections, and these mutations result in the upregulation of virulence genes and the downregulation of the protease SpeB. To identify in vivo mutants with novel phenotypes, GAS isolates from infected mice were screened by enzymatic assays for SpeB and the platelet-activating factor acetylhydrolase Sse, and a new type of variant that had enhanced Sse expression and normal levels of SpeB production was identified (the variants had a phenotype referred to as enhanced Sse activity [Sse^A+] and normal SpeB activity [SpeB^A+]). Sse^A+ SpeB^A+ variants had transcript levels of CovRS-controlled virulence genes comparable to those of a covS mutant but had no covRS mutations. Genome resequencing of an Sse^A+ SpeB^A+ isolate identified a C605A nonsense mutation in orphan kinase gene rocA, and 6 other Sse^A+ SpeB^A+ isolates also had nonsense mutations or small indels in rocA RocA and CovS mutants had similar levels of enhancement of the expression of CovRS-controlled virulence genes at the exponential growth phase; however, mutations of RocA but not mutations of CovS did not result in the downregulation of speB transcription at stationary growth phase or in subcutaneous infection of mice. GAS with RocA and CovS mutations caused greater enhancement of the expression of hasA than spyCEP in mouse skin infection than wild-type GAS did. RocA mutants ranked between wild-type GAS and CovS mutants in skin invasion, inhibition of neutrophil recruitment, and virulence in subcutaneous infection of mice. Thus, GAS RocA mutants can be selected in subcutaneous infections in mice and exhibit gene expression patterns and virulences distinct from those of CovS mutants. The findings provide novel information for understanding GAS fitness mutations in vivo, virulence gene regulation, in vivo gene expression, and virulence.

Genome Sequence of the Uncommon Streptococcus pyogenes M/emm66 Strain STAB13021, Isolated from Clonal Clustered Cases in French Brittany

Here, we announce the complete annotated genome sequence of the invasive Streptococcus pyogenes strain M/emm66, isolated in 2013 from a subcutaneous abscess in new clustered cases in French Brittany.

Mechanisms of Host-Pathogen Protein Complex Formation and Bacterial Immune Evasion of Streptococcus suis Protein Fhb

Streptococcus suis serotype 2 (S. suis 2)-induced sepsis and meningitis are often accompanied by bacteremia. The evasion of polymorphonuclear leukocyte-mediated phagocytic clearance is central to the establishment of bacteremia caused by S. suis 2 and is facilitated by the ability of factor H (FH)-binding protein (Fhb) to bind FH on the bacterial surface, thereby impeding alternative pathway complement activation and phagocytic clearance. Here, C3b/C3d was found to bind to Fhb, along with FH, forming a large immune complex. The formation of this immune complex was mediated by domain II of Fhb via electrostatic and hydrophobic interactions, which, to our knowledge, is a new type of interaction. Interestingly, Fhb was found to be associated with the cell envelope and also present in the culture supernatant, where secreted Fhb inhibited complement activation via interactions with domain II, thereby enhancing antiphagocytic clearance by polymorphonuclear leukocytes. Thus, Fhb is a multifunctional bacterial protein, which binds host complement component C3 as well as FH and interferes with innate immune recognition in a secret protein manner. S. suis 2 therefore appears to have developed a new strategy to combat host innate immunity and enhance survival in host blood.

Predicted Coverage and Immuno-Safety of a Recombinant C-Repeat Region Based Streptococcus pyogenes Vaccine Candidate

The C-terminal region of the M-protein of Streptococcus pyogenes is a major target for vaccine development. The major feature is the C-repeat region, consisting of 35–42 amino acid repeat units that display high but not perfect identity. SV1 is a S. pyogenes vaccine candidate that incorporates five 14mer amino acid sequences (called J14_i variants) from differing C-repeat units in a single recombinant construct. Here we show that the J14_i variants chosen for inclusion in SV1 are the most common variants in a dataset of 176 unique M-proteins. Murine antibodies raised against SV1 were shown to bind to each of the J14_i variants present in SV1, as well as variants not present in the vaccine. Antibodies raised to the individual J14_i variants were also shown to bind to multiple but different combinations of J14_i variants, supporting the underlying rationale for the design of SV1. A Lewis Rat Model of valvulitis was then used to assess the capacity of SV1 to induce deleterious immune response associated with rheumatic heart disease. In this model, both SV1 and the M5 positive control protein were immunogenic. Neither of these antibodies were cross-reactive with cardiac myosin or collagen. Splenic T cells from SV1/CFA and SV1/alum immunized rats did not proliferate in response to cardiac myosin or collagen. Subsequent histological examination of heart tissue showed that 4 of 5 mice from the M5/CFA group had valvulitis and inflammatory cell infiltration into valvular tissue, whereas mice immunised with SV1/CFA, SV1/alum showed no sign of valvulitis. These results suggest that SV1 is a safe vaccine candidate that will elicit antibodies that recognise the vast majority of circulating GAS M-types.

Tissue tropisms in group A Streptococcus: what virulence factors distinguish pharyngitis from impetigo strains?

PURPOSE OF REVIEW

Group A streptococci (GAS) are a common cause of pharyngitis and impetigo, and distinct throat strains and skin strains have been long recognized. This review aims to describe recent advances in molecular differences between throat and skin strains, and the pathogenic mechanisms used by virulence factors that may distinguish between these two groups.

RECENT FINDINGS

Recent findings include a new typing scheme for GAS strains based on sequence clusters of genes encoding the entire surface-exposed portion of M protein; correlations between emm-based typing schemes, clinical disease and surface adhesins; covalent bond formation mediated by GAS pili and other adhesins in binding to host ligands; a key role for superantigens in oropharyngeal infection via binding major histocompatibility complex class II antigen; and migration of GAS-specific Th17 cells from the upper respiratory tract to the brain, which may be relevant to autoimmune sequelae.

SUMMARY

The gap between molecular markers of disease (correlation) and virulence mechanisms (causation) in the establishment of tissue tropisms for GAS infection currently remains wide, but the gap also continues to narrow. Whole genome sequencing combined with mutant construction and improvements in animal models for oropharyngeal infection by GAS may help pave the way for new discoveries.

Collagen-binding proteins of Streptococcus mutans and related streptococci

The ability of Streptococcus mutans to interact with collagen through the expression of collagen-binding proteins (CBPs) bestows this oral pathogen with an alternative to the sucrose-dependent mechanism of colonization classically attributed to caries development. Based on the abundance and distribution of collagen throughout the human body, stringent adherence to this molecule grants S. mutans with the opportunity to establish infection at different host sites. Surface proteins, such as SpaP, WapA, Cnm and Cbm, have been shown to bind collagen in vitro, and it has been suggested that these molecules play a role in colonization of oral and extra-oral tissues. However, robust collagen binding is not achieved by all strains of S. mutans, particularly those that lack Cnm or Cbm. These observations merit careful dissection of the contribution from these different CBPs towards tissue colonization and virulence. In this review, we will discuss the current understanding of mechanisms used by S. mutans and related streptococci to colonize collagenous tissues, and the possible contribution of CBPs to infections in different sites of the host.

Molecular Mechanisms of Inhibition of Streptococcus Species by Phytochemicals

This review paper summarizes the antibacterial effects of phytochemicals of various medicinal plants against pathogenic and cariogenic streptococcal species. The information suggests that these phytochemicals have potential as alternatives to the classical antibiotics currently used for the treatment of streptococcal infections. The phytochemicals demonstrate direct bactericidal or bacteriostatic effects, such as: (i) prevention of bacterial adherence to mucosal surfaces of the pharynx, skin, and teeth surface; (ii) inhibition of glycolytic enzymes and pH drop; (iii) reduction of biofilm and plaque formation; and (iv) cell surface hydrophobicity. Collectively, findings from numerous studies suggest that phytochemicals could be used as drugs for elimination of infections with minimal side effects.

Types of Recombinant Vaccines

The original scientific strategy behind vaccinology has historically been to “isolate, inactivate, and inject,” first invoked by Louis Pasteur.

The N-terminal domain of the thermo-regulated surface protein PrpA of Enterococcus faecium binds to fibrinogen, fibronectin and platelets

Enterococcus faecium is a commensal of the mammalian gastrointestinal tract, but is also found in non-enteric environments where it can grow between 10 °C and 45 °C. E. faecium has recently emerged as a multi-drug resistant nosocomial pathogen. We hypothesized that genes involved in the colonization and infection of mammals exhibit temperature-regulated expression control and we therefore performed a transcriptome analysis of the clinical isolate E. faecium E1162, during mid-exponential growth at 25 °C and 37 °C. One of the genes that exhibited differential expression between 25 °C and 37 °C, was predicted to encode a peptidoglycan-anchored surface protein. The N-terminal domain of this protein is unique to E. faecium and closely related enterococci, while the C-terminal domain is homologous to the Streptococcus agalactiae surface protein BibA. This region of the protein contains proline-rich repeats, leading us to name the protein PrpA for proline-rich protein A. We found that PrpA is a surface-exposed protein which is most abundant during exponential growth at 37 °C in E. faecium E1162. The heterologously expressed and purified N-terminal domain of PrpA was able to bind to the extracellular matrix proteins fibrinogen and fibronectin. In addition, the N-terminal domain of PrpA interacted with both non-activated and activated platelets.

Development of Phage-Based Antibody Fragment Reagents for Affinity Enrichment of Bacterial Immunoglobulin G Binding Proteins

Disease and death caused by bacterial infections are global health problems. Effective bacterial strategies are required to promote survival and proliferation within a human host, and it is important to explore how this adaption occurs. However, the detection and quantification of bacterial virulence factors in complex biological samples are technically demanding challenges. These can be addressed by combining targeted affinity enrichment of antibodies with the sensitivity of liquid chromatography-selected reaction monitoring mass spectrometry (LC-SRM MS). However, many virulence factors have evolved properties that make specific detection by conventional antibodies difficult. We here present an antibody format that is particularly well suited for detection and analysis of immunoglobulin G (IgG)-binding virulence factors. As proof of concept, we have generated single chain fragment variable (scFv) antibodies that specifically target the IgG-binding surface proteins M1 and H of Streptococcus pyogenes. The binding ability of the developed scFv is demonstrated against both recombinant soluble protein M1 and H as well as the intact surface proteins on a wild-type S. pyogenes strain. Additionally, the capacity of the developed scFv antibodies to enrich their target proteins from both simple and complex backgrounds, thereby allowing for detection and quantification with LC-SRM MS, was demonstrated. We have established a workflow that allows for affinity enrichment of bacterial virulence factors.

Prevalence of emm types and antimicrobial susceptibility of Streptococcus dysgalactiae subsp. equisimilis in Austria

OBJECTIVES

An increase of severe infections caused by Streptococcus dysgalactiae subsp. equisimilis (SDSE) similar to infections caused by Streptococcus pyogenes has been reported over the last years. Little is known about infections with SDSE in Austria. Therefore, we investigated a collection of 113 SDSE invasive and non-invasive isolates from different infection sites and type of infections as well as patients' characteristics.

METHODS

The isolates were phenotypically identified and emm typed using the enlarged emm database from the Centers for Disease Control and Prevention. Additionally, 13 antimicrobial agents were tested using EUCAST guidelines and virulence genes were investigated.

RESULTS

Severe SDSE infections were most common in elderly men with underlying diseases especially diabetes mellitus. With VitekMS identification of SDSE isolates was successful to the species level only. Emm typing revealed 24 different emm types, one new type and one new subtype. StG485, stG6, stC74a, stG643, and stG480 were the predominant types in this study, stC74a and stG652 in invasive infections and stG643, stC74a and stG485 in non-invasive infections. Resistance was observed to tetracycline (62%), macrolides (13%) with one M phenotype, and clindamycin (12%) presenting 6 constitutive MLS(B) phenotypes and 8 inducible MLS(B) phenotypes. Levofloxacin resistance was detected only in one isolate. All isolates tested for virulence genes were positive for scpA, ska, saga and slo. Superantigenic genes were negative except speG(dys) (positive 17/34; 50%).

CONCLUSION

This paper presents the first report of SDSE infections in Austria. Severe SDSE infections were found mainly in elderly men with underlying diseases. SDSE isolates demonstrated substantial emm type diversity without association with infections site or invasiveness. Analysis of virulence genes showed no significant difference between invasive and non-invasive infections.

Streptococcus pyogenes Sortase Mutants Are Highly Susceptible to Killing by Host Factors Due to Aberrant Envelope Physiology

Cell wall anchored virulence factors are critical for infection and colonization of the host by Gram-positive bacteria. Such proteins have an N-terminal leader sequence and a C-terminal sorting signal, composed of an LPXTG motif, a hydrophobic stretch, and a few positively charged amino acids. The sorting signal halts translocation across the membrane, allowing sortase to cleave the LPXTG motif, leading to surface anchoring. Deletion of sortase prevents the anchoring of virulence factors to the wall; the effects on bacterial physiology however, have not been thoroughly characterized. Here we show that deletion of Streptococcus pyogenes sortase A leads to accumulation of sorting intermediates, particularly at the septum, altering cellular morphology and physiology, and compromising membrane integrity. Such cells are highly sensitive to cathelicidin, and are rapidly killed in blood and plasma. These phenomena are not a loss-of-function effect caused by the absence of anchored surface proteins, but specifically result from the accumulation of sorting intermediates. Reduction in the level of sorting intermediates leads to a return of the sortase mutant to normal morphology, while expression of M protein with an altered LPXTG motif in wild type cells leads to toxicity in the host environment, similar to that observed in the sortase mutant. These unanticipated effects suggest that inhibition of sortase by small-molecule inhibitors could similarly lead to the rapid elimination of pathogens from an infected host, making such inhibitors much better anti-bacterial agents than previously believed.

Application of an enzyme-labeled antigen method for visualizing plasma cells producing antibodies against Strep A, a carbohydrate antigen of Streptococcus pyogenes, in recurrent tonsillitis

Streptococcus pyogenes is the main causative pathogen of recurrent tonsillitis. Histologically, lesions of recurrent tonsillitis contain numerous plasma cells. Strep A is an antigenic carbohydrate molecule on the cell wall of S. pyogenes. As expected, plasma cells in subjects with recurrent tonsillitis secrete antibodies against Strep A. The enzyme‐labeled antigen method is a novel histochemical technique that visualizes specific antibody‐producing cells in tissue sections by employing a biotin‐labeled antigen as a probe. The purpose of the present study was to visualize plasma cells producing antibodies reactive with Strep A in recurrent tonsillitis. Firstly, the lymph nodes of rats immunized with boiled S. pyogenes were paraformaldehyde‐fixed and specific plasma cells localized in frozen sections with biotinylated Strep A. Secondly, an enzyme‐labeled antigen method was used on human tonsil surgically removed from 12 patients with recurrent tonsillitis. S. pyogenes genomes were PCR‐detected in all 12 specimens. The emm genotypes belonged to emm12 in nine specimens and emm1 in three. Plasma cells producing anti‐Strep A antibodies were demonstrated in prefixed frozen sections of rat lymph nodes, 8/12 human specimens from patients with recurrent tonsillitis but not in two control tonsils. In human tonsils, Strep A‐reactive plasma cells were observed within the reticular squamous mucosa and just below the mucosa, and the specific antibodies belonged to either IgA or IgG classes. Our technique is effective in visualizing immunocytes producing specific antibodies against the bacterial carbohydrate antigen, and is thus a novel histochemical tool for analyzing immune reactions in infectious disorders.

Increased β-haemolytic group A streptococcal M6 serotype and streptodornase B-specific cellular immune responses in Swedish narcolepsy cases

BACKGROUND

Type 1 narcolepsy is a neurological disorder characterized by excessive daytime sleepiness and cataplexy associated with the HLA allele DQB1*06:02. Genetic predisposition along with external triggering factors may drive autoimmune responses, ultimately leading to the selective loss of hypocretin-positive neurons.

OBJECTIVE

The aim of this study was to investigate potential aetiological factors in Swedish cases of postvaccination (Pandemrix) narcolepsy defined by interferon-gamma (IFNγ) production from immune cells in response to molecularly defined targets.

METHODS

Cellular reactivity defined by IFNγ production was examined in blood from 38 (HLA-DQB1*06:02(+) ) Pandemrix-vaccinated narcolepsy cases and 76 (23 HLA-DQB1*06:02(+) and 53 HLA-DQB1*06:02(-) ) control subjects, matched for age, sex and exposure, using a variety of different antigens: β-haemolytic group A streptococcal (GAS) antigens (M5, M6 and streptodornase B), influenza (the pandemic A/H1N1/California/7/09 NYMC X-179A and A/H1N1/California/7/09 NYMC X-181 vaccine antigens, previous Flu-A and -B vaccine targets, A/H1N1/Brisbane/59/2007, A/H1N1/Solomon Islands/3/2006, A/H3N2/Uruguay/716/2007, A/H3N2/Wisconsin/67/2005, A/H5N1/Vietnam/1203/2004 and B/Malaysia/2506/2004), noninfluenza viral targets (CMVpp65, EBNA-1 and EBNA-3) and auto-antigens (hypocretin peptide, Tribbles homolog 2 peptide cocktail and extract from rat hypothalamus tissue).

RESULTS

IFN-γ production was significantly increased in whole blood from narcolepsy cases in response to streptococcus serotype M6 (P = 0.0065) and streptodornase B protein (P = 0.0050). T-cell recognition of M6 and streptodornase B was confirmed at the single-cell level by intracellular cytokine (IL-2, IFNγ, tumour necrosis factor-alpha and IL-17) production after stimulation with synthetic M6 or streptodornase B peptides. Significantly, higher (P = 0.02) titres of serum antistreptolysin O were observed in narcolepsy cases, compared to vaccinated controls.

CONCLUSION

β-haemolytic GAS may be involved in triggering autoimmune responses in patients who developed narcolepsy symptoms after vaccination with Pandemrix in Sweden, characterized by a Streptococcus pyogenes M-type-specific IFN-γ cellular immune response.

Sequence variability is correlated with weak immunogenicity in Streptococcus pyogenes M protein

The M protein of Streptococcus pyogenes, a major bacterial virulence factor, has an amino-terminal hypervariable region (HVR) that is a target for type-specific protective antibodies. Intriguingly, the HVR elicits a weak antibody response, indicating that it escapes host immunity by two mechanisms, sequence variability and weak immunogenicity. However, the properties influencing the immunogenicity of regions in an M protein remain poorly understood. Here, we studied the antibody response to different regions of the classical M1 and M5 proteins, in which not only the HVR but also the adjacent fibrinogen-binding B repeat region exhibits extensive sequence divergence. Analysis of antisera from S. pyogenes-infected patients, infected mice, and immunized mice showed that both the HVR and the B repeat region elicited weak antibody responses, while the conserved carboxy-terminal part was immunodominant. Thus, we identified a correlation between sequence variability and weak immunogenicity for M protein regions. A potential explanation for the weak immunogenicity was provided by the demonstration that protease digestion selectively eliminated the HVR-B part from whole M protein-expressing bacteria. These data support a coherent model, in which the entire variable HVR-B part evades antibody attack, not only by sequence variability but also by weak immunogenicity resulting from protease attack.

Direct Host Plasminogen Binding to Bacterial Surface M-protein in Pattern D Strains of Streptococcus pyogenes Is Required for Activation by Its Natural Coinherited SK2b Protein

Streptokinase (SK), secreted by Group A Streptococcus (GAS), is a single-chain ∼47-kDa protein containing three consecutive primary sequence regions that comprise its α, β, and γ modules. Phylogenetic analyses of the variable β-domain sequences from different GAS strains suggest that SKs can be arranged into two clusters, SK1 and SK2, with a subdivision of SK2 into SK2a and SK2b. SK2b is secreted by skin-tropic Pattern D M-protein strains that also express plasminogen (human Pg (hPg)) binding Group A streptococcal M-protein (PAM) as its major cell surface M-protein. SK2a-expressing strains are associated with nasopharynx tropicity, and many of these strains express human fibrinogen (hFg) binding Pattern A-C M-proteins, e.g. M1. PAM interacts with hPg directly, whereas M1 binds to hPg indirectly via M1-bound hFg. Subsequently, SK is secreted by GAS and activates hPg to plasmin (hPm), thus generating a proteolytic surface on GAS that enhances its dissemination. Due to these different modes of hPg/hPm recognition by GAS, full characterizations of the mechanisms of activation of hPg by SK2a and SK2b and their roles in GAS virulence are important topics. To more fully examine these subjects, isogenic chimeric SK- and M-protein-containing GAS strains were generated, and the virulence of these chimeric strains were analyzed in mice. We show that SK and M-protein alterations influenced the virulence of GAS and were associated with the different natures of hPg activation and hPm binding. These studies demonstrate that GAS virulence can be explained by disparate hPg activation by SK2a and SK2b coupled with the coinherited M-proteins of these strains.

Ante-partum necrotising myometritis due to Streptococcal toxic shock

Group A streptococcus (GAS) causes severe infections in obstetric patients. A rare complication is rapidly progressive necrotising myometritis. Postpartum necrotising myometritis has been previously described; however, antenatal development of such a condition is extremely rare. We present a patient who developed antenatal necrotising myometritis and toxic shock syndrome (TSS) due to GAS during the first trimester of pregnancy, eventually requiring hysterectomy and bilateral oophorectomy. We discuss the rare complication of ante-partum necrotising myometritis, as well as the antibiotic therapy, and treatment of TSS associated with severe Group A Streptococcal infections.

Molecular typing of Chinese Streptococcus pyogenes isolates

Streptococcus pyogenes causes human infections ranging from mild pharyngitis and impetigo to serious diseases including necrotizing fasciitis and streptococcal toxic shock syndrome. The objective of this study was to compare molecular emm typing and pulsed field gel electrophoresis (PFGE) with multiple-locus variable-number tandem-repeat analysis (MLVA) for genotyping of Chinese S. pyogenes isolates. Molecular emm typing and PFGE were performed using standard protocols. Seven variable number tandem repeat (VNTR) loci reported in a previous study were used to genotype 169 S. pyogenes geographically-diverse isolates from China isolated from a variety of disease syndromes. Multiple-locus variable-number tandem-repeat analysis provided greater discrimination between isolates when compared to emm typing and PFGE. Removal of a single VNTR locus (Spy2) reduced the sensitivity by only 0.7%, which suggests that Spy2 was not informative for the isolates screened. The results presented support the use of MLVA as a powerful epidemiological tool for genotyping S. pyogenes clinical isolates.

Complete Genome Sequence of Streptococcus pyogenes M/emm44 Strain STAB901, Isolated in a Clonal Outbreak in French Brittany

We report the complete genome sequence of an invasive isolate of Streptococcus pyogenes M/emm44, belonging to a clonal outbreak that occurred in French Brittany. The genome is composed of 1,795,608 bp, with a GC content of 38.5%, has 1,358 identified coding sequences (CDSs), and harbors a novel Tn916-like transposon (Tn6253).

A systematic and functional classification of Streptococcus pyogenes that serves as a new tool for molecular typing and vaccine development

Streptococcus pyogenes ranks among the main causes of mortality from bacterial infections worldwide. Currently there is no vaccine to prevent diseases such as rheumatic heart disease and invasive streptococcal infection. The streptococcal M protein that is used as the substrate for epidemiological typing is both a virulence factor and a vaccine antigen. Over 220 variants of this protein have been described, making comparisons between proteins difficult, and hindering M protein-based vaccine development. A functional classification based on 48 emm-clusters containing closely related M proteins that share binding and structural properties is proposed. The need for a paradigm shift from type-specific immunity against S. pyogenes to emm-cluster based immunity for this bacterium should be further investigated. Implementation of this emm-cluster-based system as a standard typing scheme for S. pyogenes will facilitate the design of future studies of M protein function, streptococcal virulence, epidemiological surveillance, and vaccine development.

Management of invasive group A streptococcal infections

Invasive group A streptococcal (GAS) disease in children includes deep soft tissue infection, bacteraemia, bacteraemic pneumonia, meningitis and osteomyelitis. The expression of toxins and super antigens by GAS can complicate infection by triggering an overwhelming systemic inflammatory response, referred to as streptococcal toxic shock syndrome (STSS). The onset and progression of GAS disease can be rapid, and the associated mortality high. Prompt antibiotics therapy and early surgical debridement of infected tissue are essential. Adjunctive therapy with intravenous immunoglobulin and hyperbaric therapy may improve outcomes in severe disease. Nosocomial outbreaks and secondary cases in close personal contacts are not uncommon; infection control measures and consideration of prophylactic antibiotics to those at high risk are important aspects of disease control. To reduce a substantial part of the global burden of GAS disease, an affordable GAS vaccine with efficacy against a broad number of strains is needed.

The role of coagulation/fibrinolysis during Streptococcus pyogenes infection

The hemostatic system comprises platelet aggregation, coagulation and fibrinolysis and is a host defense mechanism that protects the integrity of the vascular system after tissue injury. During bacterial infections, the coagulation system cooperates with the inflammatory system to eliminate the invading pathogens. However, pathogenic bacteria have frequently evolved mechanisms to exploit the hemostatic system components for their own benefit. Streptococcus pyogenes, also known as Group A Streptococcus, provides a remarkable example of the extraordinary capacity of pathogens to exploit the host hemostatic system to support microbial survival and dissemination. The coagulation cascade comprises the contact system (also known as the intrinsic pathway) and the tissue factor pathway (also known as the extrinsic pathway), both leading to fibrin formation. During the early phase of S. pyogenes infection, the activation of the contact system eventually leads to bacterial entrapment within a fibrin clot, where S. pyogenes is immobilized and killed. However, entrapped S. pyogenes can circumvent the antimicrobial effect of the clot by sequestering host plasminogen on the bacterial cell surface that, after conversion into its active proteolytic form, plasmin, degrades the fibrin network and facilitates the liberation of S. pyogenes from the clot. Furthermore, the surface-localized fibrinolytic activity also cleaves a variety of extracellular matrix proteins, thereby enabling S. pyogenes to migrate across barriers and disseminate within the host. This review summarizes the knowledge gained during the last two decades on the role of coagulation/fibrinolysis in host defense against S. pyogenes as well as the strategies developed by this pathogen to evade and exploit these host mechanisms for its own benefit.

Survey of the bp/tee genes from clinical group A streptococcus isolates in New Zealand - implications for vaccine development

Group A streptococcus (GAS) is responsible for a wide range of diseases ranging from superficial infections, such as pharyngitis and impetigo, to life-threatening diseases, such as toxic shock syndrome and acute rheumatic fever (ARF). GAS pili are hair-like extensions protruding from the cell surface and consist of highly immunogenic structural proteins: the backbone pilin (BP) and one or two accessory pilins (AP1 and AP2). The protease-resistant BP builds the pilus shaft and has been recognized as the T-antigen, which forms the basis of a major serological typing scheme that is often used as a supplement to M typing. A previous sequence analysis of the bp gene (tee gene) in 39 GAS isolates revealed 15 different bp/tee types. In this study, we sequenced the bp/tee gene from 100 GAS isolates obtained from patients with pharyngitis, ARF or invasive disease in New Zealand. We found 20 new bp/tee alleles and four new bp/tee types/subtypes. No association between bp/tee type and clinical outcome was observed. We confirmed earlier reports that the emm type and tee type are associated strongly, but we also found exceptions, where multiple tee types could be found in certain M/emm type strains, such as M/emm89. We also reported, for the first time, the existence of a chimeric bp/tee allele, which was assigned into a new subclade (bp/tee3.1). A strong sequence conservation of the bp/tee gene was observed within the individual bp/tee types/subtypes (>97 % sequence identity), as well as between historical and contemporary New Zealand and international GAS strains. This temporal and geographical sequence stability provided further evidence for the potential use of the BP/T-antigen as a vaccine target.

Zebrafish as a model for zoonotic aquatic pathogens

Aquatic habitats harbor a multitude of bacterial species. Many of these bacteria can act as pathogens to aquatic species and/or non-aquatic organisms, including humans, that come into contact with contaminated water sources or colonized aquatic organisms. In many instances, the bacteria are not pathogenic to the aquatic species they colonize and are only considered pathogens when they come into contact with humans. There is a general lack of knowledge about how the environmental lifestyle of these pathogens allows them to persist, replicate and produce the necessary pathogenic mechanisms to successfully transmit to the human host and cause disease. Recently, the zebrafish infectious disease model has emerged as an ideal system for examining aquatic pathogens, both in the aquatic environment and during infection of the human host. This review will focus on how the zebrafish has been used successfully to analyze the pathogenesis of aquatic bacterial pathogens.

Closed Genome Sequence of Noninvasive Streptococcus pyogenes M/emm3 Strain STAB902

We report a closed genome sequence of group A Streptococcus genotype emm3 (GAS M/emm3) strain STAB902, isolated from a superficial pyodermatitis. The genome is composed of 1,892,124 bp, 6 integrated prophages, and has 1,858 identified coding sequences (CDSs). It has been fitted with the two available invasive GAS M/emm3 strains.

Sec-secretion and sortase-mediated anchoring of proteins in Gram-positive bacteria

Signal peptide-driven secretion of precursor proteins directs polypeptides across the plasma membrane of bacteria. Two pathways, Sec- and SRP-dependent, converge at the SecYEG translocon to thread unfolded precursor proteins across the membrane, whereas folded preproteins are routed via the Tat secretion pathway. Gram-positive bacteria lack an outer membrane and are surrounded by a rigid layer of peptidoglycan. Interactions with their environment are mediated by proteins that are retained in the cell wall, often through covalent attachment to the peptidoglycan. In this review, we describe the mechanisms for both Sec-dependent secretion and sortase-dependent assembly of proteins in the envelope of Gram-positive bacteria. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Dimerization is not a determining factor for functional high affinity human plasminogen binding by the group A streptococcal virulence factor PAM and is mediated by specific residues within the PAM a1a2 domain

A emm53 subclass of Group A Streptococcus pyogenes (GAS) interacts tightly with human plasma plasminogen (hPg) and plasmin (hPm) via the kringle 2 (K2hPg) domain of hPg/hPm and the N-terminal a1a2 regions of a GAS coiled-coil M-like protein (PAM). Previous studies have shown that a monomeric PAM fragment, VEK30 (residues 97-125 + Tyr), interacted specifically with isolated K2hPg. However, the binding strength of VEK30 (KD = 56 nm) was ∼60-fold weaker than that of full-length dimeric PAM (KD = 1 nm). To assess whether this attenuated binding was due to the inability of VEK30 to dimerize, we defined the minimal length of PAM required to dimerize using a series of peptides with additional PAM residues placed at the NH2 and COOH termini of VEK30. VEK64 (PAM residues 83-145 + Tyr) was found to be the smallest peptide that adopted an α-helical dimer, and was bound to K2hPg with nearly the same affinity as PAM (KD = 1-2 nm). However, addition of two PAM residues (Arg(126)-His(127)) to the COOH terminus of VEK30 (VEK32) maintained a monomeric peptidic structure, but exhibited similar K2hPg binding affinity as full-length dimeric PAM. We identified five residues in a1a2 (Arg(113), His(114), Glu(116), Arg(126), His(127)), mutation of which reduced PAM binding affinity for K2hPg by ∼ 1000-fold. Replacement of these critical residues by Ala in the GAS genome resulted in reduced virulence, similar to the effects of inactivating the PAM gene entirely. We conclude that rather than dimerization of PAM, the five key residues in the binding domain of PAM are essential to mediate the high affinity interaction with hPg, leading to increased GAS virulence.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes mild human infections such as pharyngitis and impetigo and serious infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, repeated GAS infections may trigger autoimmune diseases, including acute poststreptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease. Combined, these diseases account for over half a million deaths per year globally. Genomic and molecular analyses have now characterized a large number of GAS virulence determinants, many of which exhibit overlap and redundancy in the processes of adhesion and colonization, innate immune resistance, and the capacity to facilitate tissue barrier degradation and spread within the human host. This improved understanding of the contribution of individual virulence determinants to the disease process has led to the formulation of models of GAS disease progression, which may lead to better treatment and intervention strategies. While GAS remains sensitive to all penicillins and cephalosporins, rising resistance to other antibiotics used in disease treatment is an increasing worldwide concern. Several GAS vaccine formulations that elicit protective immunity in animal models have shown promise in nonhuman primate and early-stage human trials. The development of a safe and efficacious commercial human vaccine for the prophylaxis of GAS disease remains a high priority.

Beta-hemolytic streptococcal infections in trauma patients

PURPOSE

β-hemolytic streptococci (βHS) causes a diverse array of human infections. The molecular epidemiology of β-hemolytic streptococcal infections in trauma patients has not been studied. This study reports the molecular and clinical epidemiology of β-hemolytic streptococcal infections at a level 1 trauma centre of India.

METHODS

A total of 117 isolates of βHS were recovered from clinical samples of trauma patients. The isolates were identified to species level and subjected to antimicrobial susceptibility testing. Polymerase chain reaction (PCR) assay was done to detect exotoxin virulence genes. The M protein gene (emm gene) types of GAS strains were ascertained by sequencing.

RESULTS

Group A Streptococcus (GAS) was the most common isolate (64 %), followed by group G Streptococcus (23 %). A large proportion of GAS produced speB (99 %), smeZ (91 %), speF (95 %) and speG (87 %). smeZ was produced by 22 % of GGS. A total of 25 different emm types/subtypes were seen in GAS, with emm 11 being the most common. Resistance to tetracycline (69 %) and erythromycin (33 %) was commonly seen in GAS.

CONCLUSIONS

β-hemolytic streptococcal infections in Indian trauma patients are caused by GAS and non-GAS strains alike. A high diversity of emm types was seen in GAS isolates, with high macrolide and tetracycline resistance. SpeA was less commonly seen in Indian GAS isolates. There was no association between disease severity and exotoxin gene production.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes mild human infections such as pharyngitis and impetigo and serious infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, repeated GAS infections may trigger autoimmune diseases, including acute poststreptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease. Combined, these diseases account for over half a million deaths per year globally. Genomic and molecular analyses have now characterized a large number of GAS virulence determinants, many of which exhibit overlap and redundancy in the processes of adhesion and colonization, innate immune resistance, and the capacity to facilitate tissue barrier degradation and spread within the human host. This improved understanding of the contribution of individual virulence determinants to the disease process has led to the formulation of models of GAS disease progression, which may lead to better treatment and intervention strategies. While GAS remains sensitive to all penicillins and cephalosporins, rising resistance to other antibiotics used in disease treatment is an increasing worldwide concern. Several GAS vaccine formulations that elicit protective immunity in animal models have shown promise in nonhuman primate and early-stage human trials. The development of a safe and efficacious commercial human vaccine for the prophylaxis of GAS disease remains a high priority.

Streptococcus dysgalactiae subsp. equisimilis bacteremia: an emerging infection

The importance of group C and G Streptococcus dysgalactiae subspecies equisimilis (S. dysgalactiae subsp. equisimilis) as a significant pathogen has recently been better recognized. S. dysgalactiae subsp. equisimilis disease can range in severity from milder skin and soft-tissue conditions such as wound infection, erysipelas, and cellulitis, to life-threatening necrotizing fasciitis and streptococcal toxic shock syndrome, thus sharing the clinical picture with S. pyogenes. The most common clinical manifestation of bacteremia is cellulitis. An increase in the incidence of S. dysgalactiae subsp. equisimilis bacteremia has been recognized. Invasive forms of this infection are most commonly found in elderly patients with underlying comorbidities and skin breakdown. The case fatality in bacteremia has been reported to be 15-18%. In this review, the epidemiology, clinical characteristics, and emm types of S. dysgalactiae subsp. equisimilis bacteremia are summarized.

Molecular basis of host specificity in human pathogenic bacteria

Pathogenic bacteria display various levels of host specificity or tropism. While many bacteria can infect a wide range of hosts, certain bacteria have strict host selectivity for humans as obligate human pathogens. Understanding the genetic and molecular basis of host specificity in pathogenic bacteria is important for understanding pathogenic mechanisms, developing better animal models and designing new strategies and therapeutics for the control of microbial diseases. The molecular mechanisms of bacterial host specificity are much less understood than those of viral pathogens, in part due to the complexity of the molecular composition and cellular structure of bacterial cells. However, important progress has been made in identifying and characterizing molecular determinants of bacterial host specificity in the last two decades. It is now clear that the host specificity of bacterial pathogens is determined by multiple molecular interactions between the pathogens and their hosts. Furthermore, certain basic principles regarding the host specificity of bacterial pathogens have emerged from the existing literature. This review focuses on selected human pathogenic bacteria and our current understanding of their host specificity.