Molecular basis of group A streptococcal virulence

The diagnosis of necrotizing fasciitis

PURPOSE OF REVIEW

A delay in the diagnosis and appropriate treatment of necrotizing fasciitis has clearly been demonstrated to increase mortality. However, paucity of specific cutaneous signs makes early recognition extremely difficult. This review highlights recent developments in the approaches to the diagnosis of necrotizing fasciitis.

RECENT FINDINGS

A clinical staging of necrotizing fasciitis is proposed to better define the progression of the disease. Several clinical subtypes of necrotizing fasciitis have been described recently with hyperacute and sub-acute variants. Imaging techniques, such as magnetic resonance imaging and frozen section biopsies, have been reported to be of value in the early recognition of necrotizing fasciitis. However availability and cost limit the routine use of these tests. Several diagnostic adjuncts that have been developed recently to help in early recognition will be discussed. These include the fasciitis LRINEC (laboratory risk indicator for necrotizing fasciitis) score and transcutaneous tissue oxygen saturation monitoring. Some may have the potential for widespread application in the assessment of severe soft tissue infections.

SUMMARY

Delayed recognition, with consequent massive soft tissue loss and sepsis, remains a deadly pitfall in the management of necrotizing fasciitis. With a better understanding of the clinical manifestations and the potential use and limitations of various diagnostic adjuncts available for the assessment of equivocal cases of soft tissue infections, it is hoped that a clear and logical approach to the diagnosis of necrotizing fasciitis may be developed.

Group A Streptococcus transcriptome dynamics during growth in human blood reveals bacterial adaptive and survival strategies

The molecular basis for bacterial responses to host signals during natural infections is poorly understood. The gram-positive bacterial pathogen group A Streptococcus (GAS) causes human mucosal, skin, and life-threatening systemic infections. During the transition from a throat or skin infection to an invasive infection, GAS must adapt to changing environments and host factors. To better understand how GAS adapts, we used transcript profiling and functional analysis to investigate the transcriptome of a wild-type serotype M1 GAS strain in human blood. Global changes in GAS gene expression occur rapidly in response to human blood exposure. Increased transcription was observed for many genes that likely enhance bacterial survival, including those encoding superantigens and host-evasion proteins regulated by a multiple gene activator called Mga. GAS also coordinately expressed genes involved in proteolysis, transport, and catabolism of oligopeptides to obtain amino acids in this protein-rich host environment. Comparison of the transcriptome of the wild-type strain to that of an isogenic deletion mutant (DeltacovR) mutated in the two-component regulatory system designated CovR-CovS reinforced the hypothesis that CovR-CovS has an important role linking key biosynthetic, catabolic, and virulence functions during transcriptome restructuring. Taken together, the data provide crucial insights into strategies used by pathogenic bacteria for thwarting host defenses and surviving in human blood.

Large-scale screen highlights the importance of capsule for virulence in the zoonotic pathogen Streptococcus iniae

Zoonotic pathogens have the unique ability to cross the species barrier, causing disease in both humans and specific animal hosts. Streptococcus iniae is a zoonotic pathogen of both fish and humans, and the clinical presentations of S. iniae infections in fish and humans are very similar to those caused by various human-specific streptococcal pathogens. Virulence mechanisms required for infection by this pathogen of either host have yet to be determined. Using the previously reported zebrafish infectious disease model, we performed a large-scale screening to determine genes required for systemic infection. Screening 1,128 signature-tagged transposon mutants through the zebrafish model allowed identification of 41 potential mutants that were unable to survive within the host environment. Greater than 50% of the mutants that could be identified through homology searches were highly homologous to genes found in other human-specific streptococcal pathogens, while 32% were found to have no homology to any sequences found in the databases, suggesting as yet unknown gram-positive bacterial virulence factors. A large percentage of the insertions were found to be located in several putative capsule synthesis genes, an important virulence component for other systemic pathogens. Density gradient assays demonstrated that several of these putative capsule mutants have dissimilar buoyant densities, suggesting different levels of capsule synthesis. Putative capsule mutants were also less resistant to phagocytosis in whole-blood assays than wild-type S. iniae. Our initial large-scale characterization of S. iniae virulence highlights the importance of the capsule for successful infection.

Mutational analysis of the group A streptococcal operon encoding streptolysin S and its virulence role in invasive infection

The pathogen group A Streptococcus (GAS) produces a wide spectrum of infections including necrotizing fasciitis (NF). Streptolysin S (SLS) produces the hallmark beta-haemolytic phenotype produced by GAS. The nine-gene GAS locus (sagA-sagI) resembling a bacteriocin biosynthetic operon is necessary and sufficient for SLS production. Using precise, in-frame allelic exchange mutagenesis and single-gene complementation, we show sagA, sagB, sagC, sagD, sagE, sagF and sagG are each individually required for SLS production, and that sagE may further serve an immunity function. Limited site-directed mutagenesis of specific amino acids in the SagA prepropeptide supports the designation of SLS as a bacteriocin-like toxin. No significant pleotrophic effects of sagA deletion were observed on M protein, capsule or cysteine protease production. In a murine model of NF, the SLS-negative M1T1 GAS mutant was markedly diminished in its ability to produce necrotic skin ulcers and spread to the systemic circulation. The SLS toxin impaired phagocytic clearance and promoted epithelial cell cytotoxicity, the latter phenotype being enhanced by the effects of M protein and streptolysin O. We conclude that all genetic components of the sag operon are required for expression of functional SLS, an important virulence factor in the pathogenesis of invasive M1T1 GAS infection.

Involvement of immunoglobulin FcgammaRIIA and FcgammaRIIIB gene polymorphisms in susceptibility to rheumatic fever

OBJECTIVES

To assess the impact of the human FcgammaRIIA and FcgammaRIIIB gene polymorphisms on the risk of rheumatic fever (RF).

DESIGNS AND METHODS

FcgammaRIIA-R/H-131 and FcgammaRIIIB-NA1/NA2 genotypes were determined using polymerase chain reaction in 66 RF cases and 117 healthy controls in this case control study.

RESULTS

Compared with healthy controls, the RR genotype was enriched in the entire group of RF cases (odds ratio [OR] 4.98, 95% confidence interval [95% CI] 1.81-13.70). RF patients were more frequently HR heterozygotes rather than HH homozygotes (OR 3.09 vs. 0.11). The results of this study show that patients who have RF are more likely to have the RR and HR genotypes than control children. These probabilities show that RR is associated with the greatest risk for rheumatic fever and HR is associated with an intermediate risk. For the distribution of FcgammaRIIIB NA2 genotypes, a nonsignificant increase was found in RF patients (39.31% vs. 51.51%; OR 1.64, P = 0.1226).

CONCLUSION

The FcgammaRIIA-R/H-131 polymorphism may be an important marker in determining predisposition to RF.

Eating Oneself and Uninvited Guests

The eukaryotic cell uses an evolutionarily conserved lysosomal pathway of self-digestion (autophagy) for survival when extracellular nutrients are limited. In this issue of Cell, new evidence indicates that autophagy is used to for survival when intracellular nutrients are limited by growth factor deprivation (Lum et al., 2005). Other recent studies indicate that the autophagy machinery is also used to degrade foreign microbial invaders (xenophagy).

Epidemiology and molecular characterization of Streptococcus pyogenes recovered from scarlet fever patients in central Taiwan from 1996 to 1999

One hundred seventy-nine Streptococcus pyogenes isolates recovered from scarlet fever patients from 1996 to 1999 in central Taiwan were characterized by emm, Vir, and pulsed-field gel electrophoresis (PFGE) typing methods. The protocols for Vir and PFGE typing were standardized. A database of the DNA fingerprints for the isolates was established. Nine emm or emm-like genes, 19 Vir patterns, and 26 SmaI PFGE patterns were detected among the isolates. Among the three typing methods, PFGE was the most discriminatory. However, it could not completely replace Vir typing because some isolates with identical PFGE patterns could be further differentiated into several Vir patterns. The prevalent emm types were emm4 (n = 81 isolates [45%]), emm12 (n = 64 [36%]), emm1 (n = 14 [8%]), and emm22 (n = 13 [7%]). Some emm type isolates could be further differentiated into several emm-Vir-PFGE genotypes; however, only one genotype in each emm group was usually predominant. DNA from nine isolates was resistant to SmaI digestion. Further PFGE analysis with SgrAI showed that the SmaI digestion-resistant strains could be derived from indigenous strains by horizontal transfer of exogenous genetic material. The emergence of the new strains could have resulted in an increase in scarlet fever cases in central Taiwan since 2000. The emm sequences, Vir, and PFGE pattern database will serve as a basis for information for the long-term evolutionary study of local S. pyogenes strains.

Autophagy defends cells against invading group A Streptococcus

We found that the autophagic machinery could effectively eliminate pathogenic group A Streptococcus (GAS) within nonphagocytic cells. After escaping from endosomes into the cytoplasm, GAS became enveloped by autophagosome-like compartments and were killed upon fusion of these compartments with lysosomes. In autophagy-deficient Atg5-/- cells, GAS survived, multiplied, and were released from the cells. Thus, the autophagic machinery can act as an innate defense system against invading pathogens.

Peripheral blood T cell responses to keratin peptides that share sequences with streptococcal M proteins are largely restricted to skin-homing CD8(+) T cells

The association of psoriasis with Streptococcus pyogenes throat infections suggests a potential antigenic target for the T cells that are known to infiltrate psoriatic skin. Streptococcal M protein share an extensive sequence homology with the human epidermal keratins. Keratin 17 (K17), while being mostly absent from uninvolved skin, is up-regulated in psoriatic lesions. Consequentially, M-protein-primed T cells may recognize up-regulated keratin epitopes via molecular mimicry. Using in vitro lymphocyte culture and cytokine flow cytometry we demonstrate that HLA-Cw*0602(+) psoriasis patients had significant CD8(+) T cell interferon (IFN)-gamma responses to peptides from the K17 and M6 protein selected on the basis of sequence homology and predicted HLA-Cw*0602 binding. These responses were about 10 times more frequent in the skin-homing cutaneous lymphocyte-associated antigen-expressing (CLA(+)) subset of CD8(+) T cells. CD4(+) T cells showed only borderline responses. CLA(+) CD8(+) T cells from Cw6(+) non-psoriatic individuals responded to some M6 peptides but rarely to K17 peptides. Cw6(-) psoriasis patients showed a response that was intermediate between Cw6(+) patients and controls. These findings indicate that psoriatic individuals have CD8(+) T cells that recognize keratin self-antigens and that epitopes shared by streptococcal M proteins and human keratins may be targets for the CD8(+) T cells that infiltrate psoriatic skin lesions.

Streptococcus pyogenes and human neutrophils: a paradigm for evasion of innate host defense by bacterial pathogens

Human polymorphonuclear leukocytes (PMNs) are the first line of defense against invading microorganisms. Although most invading bacteria are eliminated by PMNs, some have evolved complex strategies to prevent normal PMN function. This review focuses on the interaction of human PMNs with Streptococcus pyogenes as a paradigm for successful pathogen evasion mechanisms.

Histopathologic changes in kidney and liver correlate with streptococcal pyrogenic exotoxin B production in the mouse model of group A streptococcal infection

Previous studies show that isogenic mutants deficient in streptococcal pyrogenic exotoxin B (SPE B) cause less mortality and skin tissue damage than wild-type strains of Streptococcus pyogenes when inoculated into mice via an air pouch. In this study, the growth and dissemination of bacteria, pathologic changes in various organs, and their correlation with SPE B production were examined. Bacterial numbers in the air pouch from wild-type strain NZ131-infected mice increased at 48 h, while those from speB mutant SW510-infected mice continuously reduced. Mice infected with NZ131 developed bacteremia and greater dissemination in the kidney, liver, and spleen; those infected with SW510 showed either no or slight bacteremia and dissemination. Co-inoculation of SW510 with recombinant SPE B showed a higher bacterial count in the air pouch, bacteremia, and organ dissemination compared to co-inoculation with a C192S mutant lacking protease activity. The histopathologic changes examined showed lesions in kidney and liver in the NZ131-infected but not in SW510-infected mice. The elevation in sera of BUN, AST, and ALT correlated positively with renal and liver impairment. Taken together, SPE B produced during S. pyogenes infection plays a pathogenic role. A direct effect of SPE B on vessel permeability change was also demonstrated.

Identification of srv, a PrfA-like regulator of group A streptococcus that influences virulence

We have identified a Crp/Fnr-like transcriptional regulator of Streptococcus pyogenes that when inactivated attenuates virulence. The gene, named srv for streptococcal regulator of virulence, encodes a 240-amino-acid protein with 53% amino acid similarity to PrfA, a transcriptional activator of virulence in Listeria monocytogenes.

The pathogenesis of streptococcal infections: from tooth decay to meningitis

The development of bacterial disease has been likened to a 'molecular arms race', in which the host tries to eliminate the bacteria, while the bacteria try to survive in the host. Although most bacteria do not cause disease, some cause serious human infection in a large proportion of encounters. Between these two extremes are bacteria that can coexist with humans in a carriage state but, under appropriate circumstances, cause disease. The streptococci exemplify this group of organisms, and by studying them we can begin to address why bacteria cause such a wide spectrum of disease.

Acute rheumatic fever: a chink in the chain that links the heart to the throat?

Acute rheumatic fever (ARF) remains a major problem in tropical regions, resource-poor countries, and minority indigenous communities. It has long been thought that group A streptococcal (GAS) pharyngitis alone was responsible for acute rheumatic fever; this belief has been supported by laboratory and epidemiological evidence gathered over more than 60 years, mainly in temperate climates where GAS skin infection is uncommon. GAS strains have been characterised as either rheumatogenic or nephritogenic based on phenotypic and genotypic properties. Primary prevention strategies and vaccine development have long been based on these concepts. The epidemiology of ARF in Aboriginal communities of central and northern Australia challenges this view with reported rates of ARF and rheumatic heart disease (RHD) that are among the highest in the world. GAS throat colonisation is uncommon, however, and symptomatic GAS pharyngitis is rare; pyoderma is the major manifestation of GAS infection. Typical rheumatogenic strains do not occur. Moreover, group C and G streptococci have been shown to exchange key virulence determinants with GAS and are more commonly isolated from the throats of Aboriginal children. We suggest that GAS pyoderma and/or non-GAS infections are driving forces behind ARF in these communities and other high-incidence settings. The question needs to be resolved as a matter of urgency because current approaches to controlling ARF/RHD in Aboriginal communities have clearly been ineffective. New understanding of the pathogenesis of ARF would have an immediate effect on primary prevention strategies and vaccine development.

Effect of a bacterial pheromone peptide on host chemokine degradation in group A streptococcal necrotising soft-tissue infections

BACKGROUND

Necrotising soft-tissue infections due to group A streptococcus (GAS) are rare (about 0.2 cases per 100000 people). The disease progresses rapidly, causing severe necrosis and hydrolysis of soft tissues. Histopathological analysis of necrotic tissue debrided from two patients (one with necrotising fasciitis and one with myonecrosis) showed large quantities of bacteria but no infiltrating neutrophils. We aimed to investigate whether the poor neutrophil chemotaxis was linked with the ability of group A streptococcus (GAS) to degrade host chemokines.

METHODS

We did RT-PCR, ELISA, and dot-blot assays to establish whether GAS induces synthesis of interleukin 8 mRNA, but subsequently degrades the released chemokine protein. Class-specific protease inhibitors were used to characterise the protease that degraded the chemokine. We used a mouse model of human soft-tissue infections to investigate the pathogenic relevance of GAS chemokine degradation, and to test the therapeutic effect of a GAS pheromone peptide (SilCR) that downregulates activity of chemokine protease.

FINDINGS

The only isolates from the necrotic tissue were two beta-haemolytic GAS strains of an M14 serotype. A trypsin-like protease released by these strains degraded human interleukin 8 and its mouse homologue MIP2. When innoculated subcutaneously in mice, these strains produced a fatal necrotic soft-tissue infection that had reduced neutrophil recruitment to the site of injection. The M14 GAS strains have a missense mutation in the start codon of silCR, which encodes a predicted 17 aminoacid pheromone peptide, SilCR. Growth of the M14 strain in the presence of SilCR abrogated chemokine proteolysis. When SilCR was injected together with the bacteria, abundant neutrophils were recruited to the site of infection, bacteria were cleared without systemic spread, and the mice survived. The therapeutic effect of SilCR was also obtained in mice challenged with M1 and M3 GAS strains, a leading cause of invasive infections.

INTERPRETATION

The unusual reduction in neutrophils in necrotic tissue of people with GAS soft-tissue infections is partly caused by a GAS protease that degrades interleukin 8. In mice, degradation can be controlled by administration of SilCR, which downregulates GAS chemokine protease activity. This downregulation increases neutrophil migration to the site of infection, preventing bacterial spread and development of a fulminant lethal systemic infection.