The gene encoding a new mitogenic factor in a Streptococcus pyogenes strain is distributed only in group A streptococci

Nuclease activity: an exploitable biomarker in bacterial infections

INTRODUCTION

In the increasingly challenging field of clinical microbiology, diagnosis is a cornerstone whose accuracy and timing are crucial for the successful management, therapy, and outcome of infectious diseases. Currently employed biomarkers of infectious diseases define the scope and limitations of diagnostic techniques. As such, expanding the biomarker catalog is crucial to address unmet needs and bring about novel diagnostic functionalities and applications.

AREAS COVERED

This review describes the extracellular nucleases of 15 relevant bacterial pathogens and discusses the potential use of nuclease activity as a diagnostic biomarker. Articles were searched for in PubMed using terms: "nuclease", "bacteria", "nuclease activity" or "biomarker". For overview sections, original and review articles between 2000 and 2019 were searched for using terms: "infections", "diagnosis", "bacterial", "burden", "challenges". Informative articles were selected.

EXPERT OPINION

Using the catalytic activity of nucleases offers new possibilities compared to established biomarkers. Nucleic acid activatable reporters in combination with different transduction platforms and delivery methods can be used to detect disease-associated nuclease activity patterns in vitro and in vivo for prognostic and diagnostic applications. Even when these patterns are not obvious or of unknown etiology, screening platforms could be used to identify new disease reporters.

Basic streptococcal superantigens (SPEX/SMEZ or SPEC) are responsible for the mitogenic activity of the so-called mitogenic factor (MF)

The mitogenic factor (MF) of group A streptococci has been reported to be a superantigen stimulating human T cells carrying Vbeta2, 4 and 8 and has been designated streptococcal pyrogenic exotoxin F (SPEF). MF was also shown to possess DNase activity. Here we have purified MF from culture supernatants of different Streptococcus pyogenes strains. Surprisingly, the MF preparations from different strains showed different Vbeta specificities depending on the expression of SPEC or SMEZ3 by the producing strain. Their mitogenic activity decreased upon further purification. In addition, the mitogenic activity could be only neutralized by antibodies against the basic streptococcal superantigens SPEC or SPEX (SMEZ3) but not by antibodies against MF itself although the latter were able to neutralize completely the DNase activity of MF. We found that streptodornase type B (SDB) was expressed in two molecular forms (SDBI and SDBII), differing only by one additional N-terminal arginine at SDBI. MF was found identical to the enzyme SDBII but is devoid of superantigenic properties and should no longer be called a superantigen or a pyrogenic exotoxin.

Analysis of aerobic bacterial strains found in chronic rhinosinusitis using the polymerase chain reaction

INTRODUCTION

Rhinosinusitis is a common disease affecting an estimated 14% of the population. Although there is general agreement in the literature regarding acute rhinosinusitis, chronic rhinosinusitis is not as well studied, and no consensus has been reached regarding the bacterial etiology. The goal of this study was to test chronic rhinosinusitis mucosal specimens using the polymerase chain reaction (PCR) for aerobic bacterial content and to compare the results with standard culture data.

RESULTS

Routine culture samples grew 50% aerobic bacteria, whereas PCR detected 62% aerobic bacteria contamination.

CONCLUSION

PCR detected more bacteria in mucosal samples than in standard culture, but standard culture of this mucosa reflects the general aerobic bacteria found in chronic rhinosinusitis, with no predominant species of aerobic bacteria.

SIGNIFICANCE

The analysis of chronic rhinosinusitis mucosa with the PCR method should give a more accurate picture of the bacteria found in chronic rhinosinusitis so that proper therapy can be instituted.

Purification and biochemical characterization of a basic superantigen (SPEX/SMEZ3) from Streptococcus pyogenes

A potent basic superantigen (designated streptococcal pyrogenic exotoxin X, SPEX/SMEZ3) was purified to homogeneity from culture supernatants of a Streptococcus pyogenes scarlatina strain of type 12 (genotype speA(-), speC(-)) and characterized. Sequence alignments revealed SPEX to be an allele of the streptococcal mitogens type Z (SMEZ). The N-terminal amino acid sequence of SPEX was found with LEVDNNSLLR to be identical to the recently described acidic superantigen SMEZ. Although SPEX/SMEZ genes were present in all of the streptococcal strains tested, a toxin production could only be detected in a small number of strains. The produced toxin concentration in the culture supernatants of positive strains differed between 0 and 20 ng ml(-1). The purified SPEX stimulated human T-lymphocytes with Vbeta8 specificity at extremely low concentrations (lower than 100 pg ml(-1)).

Virulence of Streptococcus canis from canine streptococcal toxic shock syndrome and necrotizing fasciitis

The recent recognition of streptococcal toxic shock syndrome (STSS) and necrotizing fasciitis (NF) in dogs caused by Streptococcus canis highlights our lack of knowledge regarding the mechanisms of virulence of this organism. Fifteen isolates of S. canis from cases of canine STSS and/or NF were examined for the presence of 10 Streptococcus pyogenes-associated virulence genes by Southern hybridizations using gene probes generated by PCR. The isolates lacked DNA with homology to eight of the 10 gene probes (speA, speB, speC, mf, ssa, scp, hasA, ska) under low stringency conditions. Thirteen and 15 of 15 isolates hybridized with streptolysin O and M protein gene probes, respectively. Twelve of 15 S. canis isolates were resistant to phagocytosis in canine blood. Electron microscopy revealed the presence of proteinaceous cell surface fibrillae. These results suggest that S. canis possesses M proteins and encodes streptolysin O, but lacks some of the other recognized virulence genes with significant homology to those in S. pyogenes.

Polymerase chain reaction for Streptococcus pyogenes used to evaluate an optical immunoassay for the detection of group A streptococci in children with pharyngitis

BACKGROUND

In evaluations of sensitive rapid tests for group A streptococci such as the optical immunoassay (OIA), some samples are positive by the antigen test but negative by culture. A method is needed for resolving these discrepant results.

OBJECTIVE

To develop a PCR-based assay to detect group A streptococci and to use it to establish a reference standard for evaluating an OIA for group A streptococcal antigen.

METHODS

A PCR assay that detects a segment of the MF gene of Streptococcus pyogenes was developed for the detection of group A streptococci in throat swabs. Paired swabs were obtained from 200 children with symptomatic pharyngitis and used to perform OIA, agar culture, broth-enhanced culture and PCR. As a reference standard any patient with group A streptococci detected by either culture or PCR was considered to be truly positive.

RESULTS

In comparison to agar and broth-enhanced culture procedures, OIA had sensitivities of 82 and 80% and specificities of 87 and 89%, respectively. Eight (44%) of 18 samples that were positive by OIA but negative by culture were positive for group A streptococci by PCR. Compared with the reference standard, sensitivities were OIA 76%, agar culture 79%, broth-enhanced culture 86% and PCR 96%. The specificity of OIA was 92%.

CONCLUSIONS

PCR can be used to establish a reference standard for evaluating rapid tests for group A streptococci. With this reference standard OIA was nearly as sensitive as but less specific than agar culture for detection of group A streptococci. Maximum detection requires use of both tests.

Mitogenic factor secreted by Streptococcus pyogenes is a heat-stable nuclease requiring His122 for activity

The gene encoding a mitogenic factor, termed MF, was cloned from Streptococcus pyogenes and the recombinant MF was overexpressed in Escherichia coli. Both the natural and recombinant MF had heat-resistant nuclease activity. The nuclease activity of MF was characterized using the recombinant protein. MF showed endonuclease activity, digesting ssDNA, dsDNA and tRNA. The optimal pH for the DNase activity of MF was 9.5. The DNase activity was enhanced approximately tenfold by the simultaneous presence of two divalent cations, Mg2+ and Ca2+, compared to either alone and was inhibited by EDTA or NaCl. The heat stability of MF was biphasic; the DNase activity was heat-stable from 0 to 50 degrees C over 80 degrees C but very unstable at around 60 degrees C. DNA digested by MF possessed 5'-phosphorylated and 3'-hydroxylated termini, identical to those obtained by digestion of DNA by pancreatic deoxyribonuclease I. A mutant clone revealed that His122 was a residue essential to the nuclease activity.

Analysis of human T cell responses to group A streptococci using fractionated Streptococcus pyogenes proteins

Cell extract and spent culture supernatant proteins from Streptococcus pyogenes Manfredo strain (type M5) were each separated to give 22 narrow range molecular weight fractions by blot-elution from SDS-polyacrylamide gels. Eluted samples and unfractionated proteins were screened for T cell stimulatory activity using human peripheral blood mononuclear cells (PBMC) from healthy adults in proliferation assays. Responses were measured in 4- and 7d cultures. Responses to a wide range of cell extract proteins were revealed by fractionation, the degree of response to each fraction varying between donors. Unfractionated culture supernatant proteins elicited proliferative responses by PBMC from all individuals examined. Responses to culture supernatant fractions containing 25-33 kDa proteins could be attributed to known superantigens. Furthermore, samples from culture supernatants containing higher molecular weight fractions (> 45 kDa) elicited responses in 50% of donors in 7d cultures, suggesting that these fractions contained common recall antigens. The efficacy of using electroeluted samples to identify T lymphocyte stimulatory proteins was confirmed by demonstrating that a known superantigen of S. pyogenes Manfredo strain, streptococcal pyrogenic exotoxin C (SPEC), could be fractionated successfully using this method and its activity recovered. Our results show that human T cell responses to group A streptococci involve a remarkably wide range of both cell-associated and released streptococcal proteins.