Structure of the has operon promoter and regulation of hyaluronic acid capsule expression in group A Streptococcus

Development of Streptococcus equisimilis Group G Mutant Strains with Ability to Produce Low Polydisperse and Low-Molecular-Weight Hyaluronic Acid

Background

Background: Hyaluronic acid (HA), a natural polymer with wide applications in biomedicine and cosmetics, is mainly produced by Streptococcal fermentation at industrial scale. In the present study, chemical random mutagenesis was used for development of Streptococcus equisimilis group G mutant strains with high HA productivity.

Methods

Methods: The optimum of the pH of culture condition and cultivation time for HA production by wild strain group G were assessed. At first, two rounds of mutation at different concentrations of NTG was used for mutagenesis. Then, the nonhemolytic and hyaluronidase-negative mutants were screened on the blood and HA agar. HA productivity and molecular weight were determined by carbazole assay, agarose gel electrophoresis and specific staining. Moreover, stability of the high producer mutants was evaluated within 10 generations.

Results

Results: The results showed that the wild-type strain produced 1241 ± 2.1 µg/ml of HA at pH 5.5 and 4 hours of cultivation, while the screened mutants showed a 16.1-45.5% increase in HA production. Two mutant strains, named Gm2-120-21-3 (2470 ± 8.1 µg/ml) and Gm2-120-21-4 (2856 ± 4.2 µg/ml), indicated the highest titer and a consistent production. The molecular weight (Mw) of HA for the mutants was less than 160 kDa, considering as a low Mw HA.

Conclusion

Conclusion: The mutant strains producing a low polydisperse, as well as low Mw of HA with high titer might be regarded as potential industrial strains for HA production after further safety investigations.

Survival Strategies of Streptococcus pyogenes in Response to Phage Infection

Bacteriophages exert strong evolutionary pressure on their microbial hosts. In their lytic lifecycle, complete bacterial subpopulations are utilized as hosts for bacteriophage replication. However, during their lysogenic lifecycle, bacteriophages can integrate into the host chromosome and alter the host’s genomic make-up, possibly resulting in evolutionary important adjustments. Not surprisingly, bacteria have evolved sophisticated immune systems to protect against phage infection. Streptococcus pyogenes isolates are frequently lysogenic and their prophages have been shown to be major contributors to the virulence of this pathogen. Most S. pyogenes phage research has focused on genomic prophages in relation to virulence, but little is known about the defensive arsenal of S. pyogenes against lytic phage infection. Here, we characterized Phage A1, an S. pyogenes bacteriophage, and investigated several mechanisms that S. pyogenes utilizes to protect itself against phage predation. We show that Phage A1 belongs to the Siphoviridae family and contains a circular double-stranded DNA genome that follows a modular organization described for other streptococcal phages. After infection, the Phage A1 genome can be detected in isolated S. pyogenes survivor strains, which enables the survival of the bacterial host and Phage A1 resistance. Furthermore, we demonstrate that the type II-A CRISPR-Cas system of S. pyogenes acquires new spacers upon phage infection, which are increasingly detectable in the absence of a capsule. Lastly, we show that S. pyogenes produces membrane vesicles that bind to phages, thereby limiting the pool of phages available for infection. Altogether, this work provides novel insight into survival strategies employed by S. pyogenes to combat phage predation.

Streptococci and the complement system: interplay during infection, inflammation and autoimmunity

Streptococci are a broad group of Gram-positive bacteria. This genus includes various human pathogens causing significant morbidity and mortality. Two of the most important human pathogens are Streptococcus pneumoniae (pneumococcus) and Streptococcus pyogenes (group A streptococcus or GAS). Streptococcal pathogens have evolved to express virulence factors that enable them to evade complement-mediated attack. These include factor H-binding M (S. pyogenes) and pneumococcal surface protein C (PspC) (S. pneumoniae) proteins. In addition, S. pyogenes and S. pneumoniae express cytolysins (streptolysin and pneumolysin), which are able to destroy host cells. Sometimes, the interplay between streptococci, the complement, and antistreptococcal immunity may lead to an excessive inflammatory response or autoimmune disease. Understanding the fundamental role of the complement system in microbial clearance and the bacterial escape mechanisms is of paramount importance for understanding microbial virulence, in general, and, the conversion of commensals to pathogens, more specifically. Such insights may help to identify novel antibiotic and vaccine targets in bacterial pathogens to counter their growing resistance to commonly used antibiotics.

Increase in Invasive Group A Streptococcal Disease and Emergence of Mucoid Strains in a Pediatric Population: February-June 2017

Background

Infection with group A Streptococcus (GAS) can cause severe systemic and locally invasive disease. Invasive group A streptococcal (iGAS) disease incidence varies both seasonally and year-to-year, and it may exhibit clustered outbreaks. We observed an upswing in iGAS cases at a tertiary care Children’s Hospital, prompting further characterization of local iGAS disease.

Methods

Cases of iGAS disease were abstracted from the medical record by manual chart review of all positive screening tests and cultures for GAS over a 4-year span. Incidence rates per 1000 hospital admissions and per 100 positive GAS tests were calculated and compared. Selected isolates were further characterized by whole-genome sequencing.

Results

Significant year-to-year differences in per-admission iGAS incidence rate were observed in February and June, although per-positive test incidence rates were not significantly different. Whole-genome sequencing revealed 2 dominant serotypes—emm3 and emm6—with high rates of mucoid phenotype and systemic bacteremia.

Conclusions

We document a significant but transient increase in iGAS disease incidence in 2 months of 2017. Genome sequencing revealed 2 dominant serotypes associated with mucoid phenotypes and severe disease, highlighting the dynamic nature of iGAS disease pattern.

Capsular Polysaccharide of Group A Streptococcus

Most clinical isolates of Streptococcus pyogenes elaborate a capsular polysaccharide, which is composed of hyaluronic acid, a high-molecular-mass polymer of alternating residues of N-acetyl glucosamine and glucuronic acid. Certain strains, particularly those of the M18 serotype, produce abundant amounts of capsule, resulting in formation of large, wet-appearing, translucent or "mucoid" colonies on solid media, whereas strains of M-types 4 and 22 produce none. Studies of acapsular mutant strains have provided evidence that the capsule enhances virulence in animal models of infection, an effect attributable, at least in part, to resistance to complement-mediated opsonophagocytic killing by leukocytes. The presence of the hyaluronic acid capsule may mask adhesins on the bacterial cell wall. However, the capsule itself can mediate bacterial attachment to host cells by binding to the hyaluronic-acid binding protein, CD44. Furthermore, binding of the S. pyogenes capsule to CD44 on host epithelial cells can trigger signaling events that disrupt cell-cell junctions and facilitate bacterial invasion into deep tissues. This article summarizes the biochemistry, genetics, regulation, and role in pathogenesis of this important virulence determinant.

Streptococcal Lancefield polysaccharides are critical cell wall determinants for human Group IIA secreted phospholipase A2 to exert its bactericidal effects

Human Group IIA secreted phospholipase A2 (hGIIA) is an acute phase protein with bactericidal activity against Gram-positive bacteria. Infection models in hGIIA transgenic mice have suggested the importance of hGIIA as an innate defense mechanism against the human pathogens Group A Streptococcus (GAS) and Group B Streptococcus (GBS). Compared to other Gram-positive bacteria, GAS is remarkably resistant to hGIIA activity. To identify GAS resistance mechanisms, we exposed a highly saturated GAS M1 transposon library to recombinant hGIIA and compared relative mutant abundance with library input through transposon-sequencing (Tn-seq). Based on transposon prevalence in the output library, we identified nine genes, including dltA and lytR, conferring increased hGIIA susceptibility. In addition, seven genes conferred increased hGIIA resistance, which included two genes, gacH and gacI that are located within the Group A Carbohydrate (GAC) gene cluster. Using GAS 5448 wild-type and the isogenic gacI mutant and gacI-complemented strains, we demonstrate that loss of the GAC N-acetylglucosamine (GlcNAc) side chain in the ΔgacI mutant increases hGIIA resistance approximately 10-fold, a phenotype that is conserved across different GAS serotypes. Increased resistance is associated with delayed penetration of hGIIA through the cell wall. Correspondingly, loss of the Lancefield Group B Carbohydrate (GBC) rendered GBS significantly more resistant to hGIIA-mediated killing. This suggests that the streptococcal Lancefield antigens, which are critical determinants for streptococcal physiology and virulence, are required for the bactericidal enzyme hGIIA to exert its bactericidal function.

RocA truncation underpins hyper-encapsulation, carriage longevity and transmissibility of serotype M18 group A streptococci

Group A streptococcal isolates of serotype M18 are historically associated with epidemic waves of pharyngitis and the non-suppurative immune sequela rheumatic fever. The serotype is defined by a unique, highly encapsulated phenotype, yet the molecular basis for this unusual colony morphology is unknown. Here we identify a truncation in the regulatory protein RocA, unique to and conserved within our serotype M18 GAS collection, and demonstrate that it underlies the characteristic M18 capsule phenotype. Reciprocal allelic exchange mutagenesis of rocA between M18 GAS and M89 GAS demonstrated that truncation of RocA was both necessary and sufficient for hyper-encapsulation via up-regulation of both precursors required for hyaluronic acid synthesis. Although RocA was shown to positively enhance covR transcription, quantitative proteomics revealed RocA to be a metabolic regulator with activity beyond the CovR/S regulon. M18 GAS demonstrated a uniquely protuberant chain formation following culture on agar that was dependent on excess capsule and the RocA mutation. Correction of the M18 rocA mutation reduced GAS survival in human blood, and in vivo naso-pharyngeal carriage longevity in a murine model, with an associated drop in bacterial airborne transmission during infection. In summary, a naturally occurring truncation in a regulator explains the encapsulation phenotype, carriage longevity and transmissibility of M18 GAS, highlighting the close interrelation of metabolism, capsule and virulence.

Group A streptococcus is an important human pathogen which produces a polysaccharide capsule that confers resistance to killing by white blood cells and allows bacterial adherence to host epithelial surfaces. Serotype M18 isolates over-produce capsule, creating a unique and characteristic appearance when grown on blood agar. This feature may underlie the waves of infectious pharyngitis and subsequent onset of rheumatic fever associated with this serotype. The reason for hyper-encapsulation of M18 GAS is unknown. Here we show that a naturally-occurring truncation in an important regulatory protein, RocA, underlies serotype M18 hyper-encapsulation. By correcting the truncation we were able to reverse hyper-encapsulation, modify the 3-D structural morphology of bacteria within colonies and alter the overall protein expression pattern of the bacterium. We were able to reproduce characteristics of M18 streptococci in a different serotype strain by introducing the same truncation mutation. It was also possible to show that the truncation in RocA led to prolonged nasopharyngeal carriage of GAS in mice and also promoted bacterial airborne transmission. Thus, the propensity for M18 isolates to be associated with outbreaks of pharyngitis and rheumatic fever may be accounted for by the level of encapsulation induced by truncation of the regulatory protein RocA.

Production of hyaluronic acid by mutant strains of group C Streptococcus

This study addresses the influence of upstream region sequence on the strength of has operon promoter in highly encapsulated S. equi subsp. zooepidemicus (SEZ). For this purpose, seven different strains were constructed. Each strain carries a point mutation in one of the following positions upstream of the has promoter: -43, -44, -49, and -50 bp. To facilitate measuring of the recombinant promoter relative strength, ß-glucuronidase gene was used as a reporter gene. Three mutations located in positions -49 and -50: AT, GT, and AG, positively impacted has promoter strength when compared to the wild type sequence GG. Conversely, two other mutations: TG and TT, exhibited a slight inhibitory effect. Further, three different strains carrying chromosomal mutations in the has promoter region were constructed. In two cases, the has operon is under the control of a stronger promoter and in the third strain the has operon is controlled by a weaker promoter. The laboratory fermenter scale cultivations confirmed the increase of hyaluronan yields for SEZPhasAG and SEZPhas2G, resulting 116 and 105 %, respectively. As expected, the yield of the hyaluronic acid of SEZPhas2B strain fell to 41 %.

A conserved UDP-glucose dehydrogenase encoded outside the hasABC operon contributes to capsule biogenesis in group A Streptococcus

Group A Streptococcus (GAS) is a human-specific bacterial pathogen responsible for serious morbidity and mortality worldwide. The hyaluronic acid (HA) capsule of GAS is a major virulence factor, contributing to bloodstream survival through resistance to neutrophil and antimicrobial peptide killing and to in vivo pathogenicity. Capsule biosynthesis has been exclusively attributed to the ubiquitous hasABC hyaluronan synthase operon, which is highly conserved across GAS serotypes. Previous reports indicate that hasA, encoding hyaluronan synthase, and hasB, encoding UDP-glucose 6-dehydrogenase, are essential for capsule production in GAS. Here, we report that precise allelic exchange mutagenesis of hasB in GAS strain 5448, a representative of the globally disseminated M1T1 serotype, did not abolish HA capsule synthesis. In silico whole-genome screening identified a putative HasB paralog, designated HasB2, with 45% amino acid identity to HasB at a distant location in the GAS chromosome. In vitro enzymatic assays demonstrated that recombinant HasB2 is a functional UDP-glucose 6-dehydrogenase enzyme. Mutagenesis of hasB2 alone slightly decreased capsule abundance; however, a ΔhasB ΔhasB2 double mutant became completely acapsular. We conclude that HasB is not essential for M1T1 GAS capsule biogenesis due to the presence of a newly identified HasB paralog, HasB2, which most likely resulted from gene duplication. The identification of redundant UDP-glucose 6-dehydrogenases underscores the importance of HA capsule expression for M1T1 GAS pathogenicity and survival in the human host.

Structure of the has operon promoter and the effect of mutations on the has promoter strength in Streptococcus equi subsp. zooepidemicus

The purpose of this study is to determine the effect of the corresponding nucleotides from Streptococcus pyogenes on the has promoter strength in highly encapsulated strain S. equi subsp. zooepidemicus (SEZ) and detect an empowering mutations in SEZ. Eight different strains of SEZ carrying nucleotide mutations in the -73 to -38 region upstream of the has promoter were constructed. The significant activity decrease to 36-1% was observed after the introduction of mutations in the promoter region from -44 to -38 site. The exception was observed in mutation in -49 site when no significant decrease was observed. When nucleotides TTT were used in positions -73 the promoter became weaker, whereas no significant effect was observed after using nucleotides CCC (96%). Unfortunately, introduction of these mutations into chromosome SEZ has no empowering effect. Six strains, which carried nucleotide sequences of different lengths upstream from the transcription start of hasA promoter, were constructed to determine the minimum upstream region required for the maximum transcription efficiency of the has operon. No change of the activity of the has promoter constructs containing as few as 101 nucleotides upstream from the transcription start point was observed.

Distinct signatures of diversifying selection revealed by genome analysis of respiratory tract and invasive bacterial populations

Many pathogens colonize different anatomical sites, but the selective pressures contributing to survival in the diverse niches are poorly understood. Group A Streptococcus (GAS) is a human-adapted bacterium that causes a range of infections. Much effort has been expended to dissect the molecular basis of invasive (sterile-site) infections, but little is known about the genomes of strains causing pharyngitis (streptococcal "sore throat"). Additionally, there is essentially nothing known about the genetic relationships between populations of invasive and pharyngitis strains. In particular, it is unclear if invasive strains represent a distinct genetic subpopulation of strains that cause pharyngitis. We compared the genomes of 86 serotype M3 GAS pharyngitis strains with those of 215 invasive M3 strains from the same geographical location. The pharyngitis and invasive groups were highly related to each other and had virtually identical phylogenetic structures, indicating they belong to the same genetic pool. Despite the overall high degree of genetic similarity, we discovered that strains from different host environments (i.e., throat, normally sterile sites) have distinct patterns of diversifying selection at the nucleotide level. In particular, the pattern of polymorphisms in the hyaluronic acid capsule synthesis operon was especially different between the two strain populations. This finding was mirrored by data obtained from full-genome analysis of strains sequentially cultured from nonhuman primates. Our results answer the long-standing question of the genetic relationship between GAS pharyngitis and invasive strains. The data provide previously undescribed information about the evolutionary history of pathogenic microbes that cause disease in different anatomical sites.

Induction of group A Streptococcus virulence by a human antimicrobial peptide

Group A streptococci (Streptococcus pyogenes or GAS) freshly isolated from individuals with streptococcal sore throat or invasive ("flesh-eating") infection often grow as mucoid colonies on primary culture but lose this colony appearance after laboratory passage. The mucoid phenotype is due to abundant production of the hyaluronic acid capsular polysaccharide, a key virulence determinant associated with severe GAS infections. These observations suggest that signal(s) from the human host trigger increased production of capsule and perhaps other virulence factors during infection. Here we show that subinhibitory concentrations of the human antimicrobial cathelicidin peptide LL-37 stimulate expression of the GAS capsule synthesis operon (hasABC). Up-regulation is mediated by the CsrRS 2-component regulatory system: it requires a functional CsrS sensor protein and can be antagonized by increased extracellular Mg(2+), the other identified environmental signal for CsrS. Up-regulation was also evident for other CsrRS-regulated virulence genes, including the IL-8 protease PrtS/ScpC and the integrin-like/IgG protease Mac/IdeS, findings that suggest a coordinated GAS virulence response elicited by this antimicrobial immune effector peptide. LL-37 signaling through CsrRS led to a marked increase in GAS resistance to opsonophagocytic killing by human leukocytes, an in vitro measure of enhanced GAS virulence, consistent with increased expression of the antiphagocytic capsular polysaccharide and Mac/IdeS. We propose that the human cathelicidin LL-37 has the paradoxical effect of stimulating CsrRS-regulated virulence gene expression, thereby enhancing GAS pathogenicity during infection. The ability of GAS to sense and respond to LL-37 may explain, at least in part, the unique susceptibility of the human species to streptococcal infection.

PerR confers phagocytic killing resistance and allows pharyngeal colonization by group A Streptococcus

The peroxide response transcriptional regulator, PerR, is thought to contribute to virulence of group A Streptococcus (GAS); however, the specific mechanism through which it enhances adaptation for survival in the human host remains unknown. Here, we identify a critical role of PerR-regulated gene expression in GAS phagocytosis resistance and in virulence during pharyngeal infection. Deletion of perR in M-type 3 strain 003Sm was associated with reduced resistance to phagocytic killing in human blood and by murine macrophages in vitro. The increased phagocytic killing of the perR mutant was abrogated in the presence of the general oxidative burst inhibitor diphenyleneiodonium chloride (DPI), a result that suggests PerR-dependent gene expression counteracts the phagocyte oxidative burst. Moreover, an isogenic perR mutant was severely attenuated in a baboon model of GAS pharyngitis. In competitive infection experiments, the perR mutant was cleared from two animals at 24 h and from four of five animals by day 14, in sharp contrast to wild-type bacteria that persisted in the same five animals for 28 to 42 d. GAS genomic microarrays were used to compare wild-type and perR mutant transcriptomes in order to characterize the PerR regulon of GAS. These studies identified 42 PerR-dependent loci, the majority of which had not been previously recognized. Surprisingly, a large proportion of these loci are involved in sugar utilization and transport, in addition to oxidative stress adaptive responses and virulence. This finding suggests a novel role for PerR in mediating sugar uptake and utilization that, together with phagocytic killing resistance, may contribute to GAS fitness in the infected host. We conclude that PerR controls expression of a diverse regulon that enhances GAS resistance to phagocytic killing and allows adaptation for survival in the pharynx.

Mg(2+) signalling defines the group A streptococcal CsrRS (CovRS) regulon

CsrRS (or CovRS) is a two-component system implicated in the control of multiple virulence determinants in the important human pathogen, group A Streptococcus (GAS). Earlier studies suggested that extracellular Mg(2+) signalled through the presumed sensor histidine kinase, CsrS. We now confirm those findings, as complementation of a csrS mutant restored Mg(2+)-dependent gene regulation. Moreover, we present strong evidence that Mg(2+) signals through CsrS to regulate an extensive and previously undefined repertoire of GAS genes. The effect of Mg(2+) on regulation of global gene expression was evaluated using genomic microarrays in an M-type 3 strain of GAS and in an isogenic csrS mutant. Unexpectedly, of the 72 genes identified in the Mg(2+)-stimulated CsrRS regulon, 42 were absent from the CsrR regulon (the latter being defined by comparison of wild-type and CsrR mutant transcriptomes at low Mg(2+)). We observed CsrS-dependent regulation of 72 of the 73 genes whose expression changed in response to elevated extracellular Mg(2+) in wild-type bacteria, a result that identifies CsrS as the principal, if not exclusive, sensor for extracellular Mg(2+) in GAS. To our knowledge, this study is the first to characterize global gene regulation by a GAS two-component system in response to a specific environmental stimulus.

Identification of a glycosaminoglycan binding region of the alpha C protein that mediates entry of group B Streptococci into host cells

Group B Streptococcus (GBS) frequently colonizes the human gastrointestinal and gynecological tracts and less frequently causes deep tissue infections. The transition between colonization and infection depends upon the ability of the organism to cross epithelial barriers. The alpha C protein (ACP) on the surface of GBS contributes to this process. A virulence factor in mouse models of infection, and prototype for a family of Gram-positive bacterial surface proteins, ACP facilitates GBS entry into human cervical epithelial cells and movement across cell layers. ACP binds to host cell surface glycosaminoglycan (GAG). From crystallography, we have identified a cluster of basic residues (BR2) that is a putative GAG binding area in Domain 2, near the junction of the N-terminal domain of ACP and the first of a series of tandem amino acid repeats. D2-R, a protein construct including this region, binds to cells similarly to full-length ACP. We now demonstrate that the predicted charged BR2 residues confer GAG binding; site-directed mutagenesis of these residues (Arg(172), Arg(185), or Lys(196)) eliminates cell-binding activity of construct D2-R. In addition, we have constructed a GBS strain expressing a variant ACP with a charge-neutralizing substitution at residue 185. This strain enters host cells less effectively than does the wild-type strain and similarly to an ACP null mutant strain. The point mutant strain transcytoses similarly to the wild-type strain. These data indicate that GAG-binding activity underlies ACP-mediated cellular entry of GBS. GBS entry into host cells and transcytosis of host cells may occur by distinct mechanisms.

Evolution of transcription regulatory genes is linked to niche specialization in the bacterial pathogen Streptococcus pyogenes

Streptococcus pyogenes is a highly prevalent bacterial pathogen, most often giving rise to superficial infections at the throat or skin of its human host. Three genotype-defined subpopulations of strains exhibiting strong tropisms for either the throat or skin (specialists) or having no obvious tissue site preference (generalists) are recognized. Since the microenvironments at the throat and skin are distinct, the signal transduction pathways leading to the control of gene expression may also differ for throat versus skin strains of S. pyogenes. Two loci (mga and rofA/nra) encoding global regulators of virulence gene expression are positioned 300 kb apart on the genome; each contains alleles forming two major sequence clusters of approximately 25 to 30% divergence that are under balancing selection. Strong linkage disequilibrium is observed between sequence clusters of the transcription regulatory loci and the subpopulations of throat and skin specialists, against a background of high recombination rates among housekeeping genes. A taxonomically distinct commensal species (Streptococcus dysgalactiae subspecies equisimilus) shares highly homologous rof alleles. The findings provide strong support for a mechanism underlying niche specialization that involves orthologous replacement of regulatory genes following interspecies horizontal transfer, although the directionality of gene exchange remains unknown.

Survey of emm-like gene sequences from pharyngeal isolates of group C and group G streptococci collected in Spain

We conducted a nationwide surveillance of the variable 5' emm-like (M-like protein gene) sequences from 214 pharyngeal group C and group G streptococci. Almost 75% of the isolates exhibited emm or emm-like sequences previously described. We identified six new 5' emm-like regions, and almost 23% of the isolates were nontypeable. Five emm-like sequences accounted for more than 50% of the isolates in group C and group G, suggesting horizontal gene transfer between strains of different species.

Regulation of virulence by a two-component system in group B streptococcus

Group B Streptococcus (GBS) is frequently carried in the gastrointestinal or genitourinary tract as a commensal organism, yet it has the potential to cause life-threatening infection in newborn infants, pregnant women, and individuals with chronic illness. Regulation of virulence factor expression may affect whether GBS behaves as an asymptomatic colonizer or an invasive pathogen, but little is known about how such factors are controlled in GBS. We now report the characterization of a GBS locus that encodes a two-component regulatory system similar to CsrRS (or CovRS) in Streptococcus pyogenes. Inactivation of csrR, encoding the putative response regulator, in two unrelated wild-type strains of GBS resulted in a marked increase in production of beta-hemolysin/cytolysin and a striking decrease in production of CAMP factor, an unrelated cytolytic toxin. Quantitative RNA hybridization experiments revealed that these two phenotypes were associated with a marked increase and decrease in expression of the corresponding genes, cylE and cfb, respectively. The CsrR mutant strains also displayed increased expression of scpB encoding C5a peptidase. Similar, but less marked, changes in gene expression were observed in CsrS (putative sensor component) mutants, evidence that CsrR and CsrS constitute a functional two-component system. Experimental infection studies in mice demonstrated reduced virulence of both CsrR and CsrS mutant strains relative to the wild type. Together, these results indicate that CsrRS regulates expression of multiple GBS virulence determinants and is likely to play an important role in GBS pathogenesis.

Streptococcus pyogenes pharyngeal isolates with reduced susceptibility to ciprofloxacin in Spain: mechanisms of resistance and clonal diversity

A survey of emm gene sequences and an analysis of the pulsed-field electrophoretic profiles of 30 Streptococcus pyogenes isolates with reduced susceptibilities to ciprofloxacin detected the prevalence of isolates with emm type 6 and considerable genetic diversity among isolates. The mechanism of ciprofloxacin resistance in these isolates was based on point mutations in topoisomerase IV subunit C encoded by parC, mainly replacement of serine-79 by alanine.

Interaction between Complement Regulators andStreptococcus pyogenes: Binding of C4b-Binding Protein and Factor H/Factor H-Like Protein 1 to M18 Strains Involves Two Different Cell Surface Molecules

Streptococcus pyogenes, or group A Streptococcus, is one of the most frequent causes of pharyngitis and skin infections in humans. Many virulence mechanisms have been suggested to be involved in the infectious process. Among them is the binding to the bacterial cell surface of the complement regulatory proteins factor H, factor H-like protein 1 (FHL-1), and C4b-binding protein. Previous studies indicate that binding of these three regulators to the streptococcal cell involves the M protein encoded by the emm gene. M-type 18 strains are prevalent among clinical isolates and have been shown to interact with all three complement regulators simultaneously. Using isogenic strains lacking expression of the Emm18 or the Enn18 proteins, we demonstrate in this study that, in contradistinction to previously described S. pyogenes strains, M18 strains bind the complement regulators factor H, FHL-1, and C4b-binding protein through two distinct cell surface proteins. Factor H and FHL-1 bind to the Emm18 protein, while C4BP binds to the Enn18 protein. We propose that expression of two distinct surface structures that bind complement regulatory proteins represents a unique adaptation of M18 strains that enhances their resistance to opsonization by human plasma and increases survival of this particular S. pyogenes strain in the human host. These new findings illustrate that S. pyogenes has evolved diverse mechanisms for recruitment of complement regulatory proteins to the bacterial surface to evade immune clearance in the human host.

Rheumatic fever-associated Streptococcus pyogenes isolates aggregate collagen

Acute rheumatic fever is a serious autoimmune sequel of Streptococcus pyogenes infection. This study shows that serotype M3 and M18 S. pyogenes isolated during outbreaks of rheumatic fever have the unique capability to bind and aggregate human basement membrane collagen type IV. M3 protein is identified as collagen-binding factor of M3 streptococci, whereas M18 isolates bind collagen through a hyaluronic acid capsule, revealing a novel function for M3 protein and capsule. Following in vivo mouse passage, conversion of a nonencapsulated and collagen-binding negative M1 S. pyogenes into an encapsulated, collagen-binding strain further supports the crucial role of capsule in mediating collagen binding. Collagen binding represents a novel colonization mechanism, as it is demonstrated that S. pyogenes bind to collagen matrix in vitro and in vivo. Moreover, immunization of mice with purified recombinant M3 protein led to the generation of anti-collagen type IV antibodies. Finally, sera from acute rheumatic fever patients had significantly increased titers of anti-collagen type IV antibodies as compared with healthy controls. These findings may suggest a link between the potential of rheumatogenic S. pyogenes isolates to bind collagen, and the presence of collagen-reactive autoantibodies in the serum of rheumatic fever patients, which may form a basis for post-streptococcal rheumatic disease. These anti-collagen antibodies may form a basis for poststreptococcal rheumatic disease.

Survey of emm gene sequences from pharyngeal Streptococcus pyogenes isolates collected in Spain and their relationship with erythromycin susceptibility

We conducted a nationwide survey of the variable 5' emm (M protein gene) sequences from 614 pharyngeal Streptococcus pyogenes isolates susceptible (299 isolates) and resistant (315 isolates) to erythromycin that were isolated in Spain from 1996 to 1999. Almost 98% of these isolates had emm sequences in agreement with previously recorded M antigen association. We only identified a new 5' emm sequence in 17 isolates. Nine different emm types accounted for 85% of the S. pyogenes isolates susceptible to erythromycin. By contrast, only 3 emm types accounted for 70% of the erythromycin-resistant isolates. Further characterization of these isolates by ribotyping and pulsed-field gel electrophoresis indicated that high frequency of erythromycin resistance in Spain is due to few clones.

Molecular basis of group A streptococcal virulence

The group A streptococcus (GAS) (Streptococcus pyogenes) is among the most common and versatile of human pathogens. It is responsible for a wide spectrum of human diseases, ranging from trivial to lethal. The advent of modern techniques of molecular biology has taught much about the organism's virulence, and the genomes of several GAS types have now been deciphered. Surface structures of GAS including a family of M proteins, the hyaluronic acid capsule, and fibronectin-binding proteins, allow the organism to adhere to, colonise, and invade human skin and mucus membranes under varying environmental conditions. M protein binds to complement control factors and other host proteins to prevent activation of the alternate complement pathway and thus evade phagocytosis and killing by polymorphonuclear leucocytes. Extracellular toxins, including superantigenic streptococcal pyrogenic exotoxins, contribute to tissue invasion and initiate the cytokine storm felt responsible for illnesses such as necrotising fasciitis and the highly lethal streptococcal toxic shock syndrome. Progress has been made in understanding the molecular epidemiology of acute rheumatic fever but less is understood about its basic pathogenesis. The improved understanding of GAS genetic regulation, structure, and function has opened exciting possibilities for developing safe and effective GAS vaccines. Studies directed towards achieving this long-sought goal are being aggressively pursued.

The CsrR/CsrS two-component system of group A Streptococcus responds to environmental Mg2+

Group A streptococci control expression of key virulence determinants via the two-component sensorregulator system CsrRCsrS. The membrane-bound sensor CsrS is thought to respond to previously unknown environmental signal(s) by controlling phosphorylation of its cognate regulator component CsrR. Phosphorylation of CsrR increases its affinity for binding to the promoter regions of Csr-regulated genes to repress transcription. Here we show that environmental Mg(2+) concentration is a potent and specific stimulus for CsrRCsrS-mediated regulation. We studied the effect of divalent cations on expression of the Csr-regulated hyaluronic acid capsule genes (hasABC) by measuring chloramphenicol acetyltransferase (CAT) activity in a reporter strain of group A Streptococcus carrying a has operon promoter-cat fusion. Addition of Mg(2+), but not of Ca(2+), Mn(2+), or Zn(2+), repressed capsule gene expression by up to 80% in a dose-dependent fashion. The decrease in capsule gene transcription was associated with a marked reduction in cell-associated capsular polysaccharide. RNA hybridization analysis demonstrated reduced expression of the Csr-regulated hasABC operon, streptokinase (ska), and streptolysin S (sagA) during growth in the presence of 15 mM Mg(2+) for the wild-type strain 003CAT but not for an isogenic csrS mutant. We propose that Mg(2+) binds to CsrS to induce phosphorylation of CsrR and subsequent repression of virulence gene expression. The low concentration of Mg(2+) in extracellular body fluids predicts that the CsrRCsrS system is maintained in the inactive state during infection, thereby allowing maximal expression of critical virulence determinants in the human host.

Regulation of capsule gene expression by group A Streptococcus during pharyngeal colonization and invasive infection

Capsular polysaccharide production by group A Streptococcus (GAS) is controlled by transcription of the has operon that encodes the enzymes uniquely required for synthesis of the hyaluronic acid polysaccharide. To investigate the regulation of capsule gene expression during infection, we developed a reporter strain of GAS in which the has operon promoter directed transcription of green fluorescent protein (GFP). Gfp expression was triggered within minutes after introduction of the reporter strain into the peritoneal cavity of mice, as evidenced by the recovery of highly fluorescent GAS from the peritoneum 1 h after challenge. Capsule gene expression was also stimulated in the bloodstream of infected mice, as intensely fluorescent bacteria were observed in blood samples collected after either intraperitoneal or intravenous challenge. Using a similar approach, we also observed rapid induction of capsule gene expression in bacteria inoculated into the pharynx of baboons. Compared to the inoculum, increased green fluorescence was recorded in bacteria recovered from throat swabs collected 1 h after inoculation in all five animals studied. We conclude that introduction of GAS into the pharynx or into deep tissues results in rapid induction of has operon expression, a critical adaptive response that enhances GAS survival in the infected host.

A mutation in the 5' untranslated region increases stability of norA mRNA, encoding a multidrug resistance transporter of Staphylococcus aureus

NorA, a multidrug efflux pump in Staphylococcus aureus, protects the cell from multiple drugs, including quinolones. The flqB mutation (T-->G) in the 5' untranslated region upstream of norA causes norA overexpression of 4.9-fold in cis, as measured in norA::blaZ fusions. The transcriptional initiation site of norA was unchanged in mutant and wild-type strains, but the half-life of norA mRNA was increased 4.8-fold in the flqB mutant compared to the wild-type strain. Computer-generated folding of the first 68 nucleotides of the norA transcript predicts an additional stem-loop and changes in a putative RNase III cleavage site in the flqB mutant.

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

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

Pathogenesis of 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.

Inactivation of the cysteine protease SpeB affects hyaluronic acid capsule expression in group A streptococci

The human pathogen Streptococcus pyogenes expresses several virulence factors that are required for the pathogens survival within the host and the concomitant development of disease. To examine the influence of one virulence factor, the extracellular cysteine protease SpeB, on the expression of other virulence factors, the speB structural gene of a serotype M3 and M49 strain was inactivated. Morphologic examination, quantification of extracellular hyaluronic acid capsule, and Northern blot analysis of the isogenic speB -mutants revealed a strain-dependent decrease of hyaluronic acid capsule production and an increase in superoxide dismutase transcription. The transcription of streptolysin O (slo), di- and oligo-peptide permease (dpp, opp), hyaluronidase (hyl), streptokinase (ska) and streptococcal pyrogenic exotoxin A (speA) was unaffected.

Role of group A streptococcal virulence factors in adherence to keratinocytes

To evaluate the role of putative group A streptococcal virulence factors in the initiation of skin infections, we compared the adherence of a wild-type M49-protein skin-associated strain to that of a series of 16 isogenic mutants created by insertional inactivation of virulence genes. None of the mutants, including the M-protein-deficient (emm mutant) strain, displayed reduced adherence to early-passage cultured human keratinocytes, but adherence of the mutant lacking hyaluronic acid capsule expression (has mutant) was increased 13-fold. In contrast, elimination of capsule expression in M2-, M3-, and M18-protein has mutants increased adherence only slightly (1.3- to 2.3-fold) compared to their respective wild-type strains. A mutant with inactivation of both emm and has displayed high-level adherence (34.9 +/- 4.1%) equal to that of the has mutant strain (40.7 + 8.0%), confirming the lack of involvement of M49 protein in attachment. Moreover, adherence of the M49-protein-deficient (emm mutant) and wild-type strains was increased to the same level (57 and 55%, respectively) following enzymatic digestion of their hyaluronic acid capsule. Adherence of mutants lacking oligopeptide permease (Opp) expression was increased 3.8- to 5.5-fold, in association with decreased cell-associated hyaluronic acid capsule. Finally, soluble CD46 failed to inhibit adherence of M49- and M52-serotype skin strains. We conclude that (i) bacterial M protein and keratinocyte CD46 do not mediate adherence of M49 skin-associated Streptococcus pyogenes to epidermal keratinocytes, (ii) hyaluronic acid capsule impedes the interaction of bacterial adhesins with keratinocyte receptors, (iii) modulation of capsule expression may be important in the pathogenesis of skin infections, and (iv) the molecular interactions in attachment of skin strains of S. pyogenes to keratinocytes are unique and remain unidentified.

Absence of SpeB production in virulent large capsular forms of group A streptococcal strain 64

Passage in human blood of group A streptococcal isolate 64p was previously shown to result in the enhanced expression of M and M-related proteins. Similarly, when this isolate was injected into mice via an air sac model for skin infection, organisms recovered from the spleens showed both increased expression of M and M-related proteins and increased skin-invasive potential. We show that these phenotypic changes were not solely the result of increased transcription of the mRNAs encoding the M and M-related gene products. Rather, the altered expression was associated with posttranslational modifications of the M and M-related proteins that occur in this strain, based on the presence or absence of another virulence protein, the streptococcal cysteine protease SpeB. The phenotypic variability also correlates with colony size variation. Large colonies selected by both regimens expressed more hyaluronic acid, which may explain differences in colony morphology. All large-colony variants were SpeB negative and expressed three distinct immunoglobulin G (IgG)-binding proteins in the M and M-related protein family. Small-colony variants were SpeB positive and bound little IgG through their M and M-related proteins because these proteins, although made, were degraded or altered in profile by the SpeB protease. We conclude that passage in either human blood or a mouse selects for a stable, phase-varied strain of group A streptococci which is altered in many virulence properties.

A two-component regulatory system, CsrR-CsrS, represses expression of three Streptococcus pyogenes virulence factors, hyaluronic acid capsule, streptolysin S, and pyrogenic exotoxin B

Certain Tn916 insertions in the chromosome of an M1-type, nonmucoid Streptococcus pyogenes isolate (MGAS166) were previously shown to result in stable mucoidy with increased expression of the capsular synthetic genes. The transposon insertions in these strains are directly upstream of an apparent operon encoding a two-component regulatory system, designated csrR-csrS. Compared with MGAS166, these mucoid mutants are more hemolytic and cause significantly more tissue damage in a murine model of skin infection. To extend these observations, we constructed an in-frame deletion in the gene encoding the response regulator, csrR, and we evaluated the expression of other known S. pyogenes virulence factors. We discovered that csrR mutants have enhanced transcription of sagA, a gene associated with streptolysin S (SLS) and speB, the gene encoding pyrogenic exotoxin B (SpeB). The mutants also express substantially higher SLS activity and SpeB antigen in late-exponential-phase cultures. There is no change in expression of emm, scpA, sic, or cpa (genes encoding other S. pyogenes virulence factors). CsrR- strains but not the wild-type parental strain produce necrotizing lesions in a mouse model of subcutaneous infection. A double mutant with deletions in both csrR and the capsular synthesis genes caused fewer and smaller necrotic skin lesions than the csrR mutants. However, this nonmucoid csrR strain was more likely than the wild type to yield necrotic lesions, suggesting that mucoidy contributes to virulence in this model of infection but that there are other csrR-regulated factors involved in the production of necrotic lesions.

Characterization of a two-component system in Streptococcus pyogenes which is involved in regulation of hyaluronic acid production

Hyaluronic acid production by group A streptococci is regulated by transcriptional control. In this study, transposon mutagenesis of an unencapsulated strain yielded an encapsulated mutant. Two genes homologous to sensors and response regulators of bacterial two-component systems were identified downstream of the transposon insertion. Inactivation of the putative sensor gene, csrS, in three different unencapsulated strains yielded encapsulated mutant strains. Electrophoretic mobility shift assays determined factor(s) in a cytoplasmic extract of an unencapsulated group A streptococcal strain was binding to a double-stranded DNA fragment derived from the has operon promoter. In contrast, similarly prepared cytoplasmic extracts from a csrS deletion mutant did not shift the fragment. The putative response regulator, CsrR, was partially purified and was shown to bind the has operon promoter fragment. The affinity and specificity of CsrR for the fragment were increased significantly after incubation with acetyl phosphate. DNase I footprinting determined that the acetyl phosphate-treated CsrR was binding to key sequences in the promoter and the coding region of hasA. Therefore, a two-component system is repressing the production of hyaluronic acid in group A streptococci using a phosphorylation-dependent binding interaction between the response regulator CsrR and the promoter region of the has operon.

Identification of csrR/csrS, a genetic locus that regulates hyaluronic acid capsule synthesis in group A Streptococcus

The hyaluronic acid capsule of group A Streptococcus (GAS) is an important virulence factor, but little is known about mechanisms that regulate capsule expression. Transposon Tn916 mutagenesis of the poorly encapsulated M-type 3 GAS strain DLS003 produced a transconjugant that exhibited a mucoid colony morphology, reflecting increased hyaluronic acid capsule production. Analysis of chromosomal DNA sequence immediately downstream of the transposon insertion identified two open reading frames, designated csrR and csrS, which exhibited sequence similarity to bacterial two-component regulatory systems. We constructed an in-frame deletion mutation within csrR, which encodes the putative response component. Replacement of the native csrR gene in the DLS003 chromosome with the mutant allele resulted in a sixfold increase in capsule production and a corresponding increase in transcription of the has operon, which contains the essential genes for hyaluronic acid synthesis. Increased capsule production by the csrR mutant strain was associated with enhanced resistance to complement-mediated opsonophagocytic killing in vitro and with a 500-fold increase in virulence in mice. These results establish CsrR as a negative regulator of hyaluronic acid capsule synthesis and suggest that it is part of a two-component regulatory system that influences capsule expression and virulence.

Molecular analysis of the capsule gene region of group A Streptococcus: the hasAB genes are sufficient for capsule expression

Enzymes directing the biosynthesis of the group A streptococcal hyaluronic acid capsule are encoded in the hasABC gene cluster. Inactivation of hasC, encoding UDP-glucose pyrophosphorylase in the heavily encapsulated group A streptococcal strain 87-282, had no effect on capsule production, indicating that hasC is not required for hyaluronic acid synthesis and that an alternative source of UDP-glucose is available for capsule production. Nucleotide sequence and deletion mutation analysis of the 5.5 kb of DNA upstream of hasA revealed that this region is not required for capsule expression. Many (10 of 23) group A streptococcal strains were found to contain insertion element IS1239' approximately 50 nucleotides upstream of the -35 site of the hasA promoter. The presence of IS1239' upstream of hasA did not prevent capsule expression. These results elucidate the molecular architecture of the group A streptococcal chromosomal region upstream of the has operon, indicate that hasABC are the sole components of the capsule gene cluster, and demonstrate that hasAB are sufficient to direct capsule synthesis in group A streptococci.