Streptococcus suis Infections in Humans: What is the prognosis for Western countries? (Part I)

Capsular serotypes, virulence-associated genes and antimicrobial susceptibility of Streptococcus suis isolates from pigs in Korea

Streptococcus suis is an important pig pathogen with potential for human transmission. The serotype distributions and phenotypic characteristics vary over time and among regions; however, little is known about the characteristics of S. suis isolates in Korea. In this study, 240 S. suis isolates collected from pigs in Korea in 2009–2010 were serotyped by coagglutination tests, subsequently screened for three virulence-associated genes (mrp, epf and sly) and tested for antimicrobial susceptibility. As for 80 isolates, the serotypes of which were relevant to human infections, clonal complexes (CCs) were further identified by PCR. Serotype 3 was the most prevalent (15.8%), followed by serotype 2 (15.0%), with geographical variation for each serotype. Overall, 55.4% of the isolates carried mrp, whereas only 3.8% carried epf. CC25 was the most prevalent (41.3%) and was related to serotypes 2 and 9. The isolates showed higher susceptibility to ampicillin (93.4%) and ceftiofur (90.8%) than to the other antimicrobial agents tested. The highest resistance rate was observed to tetracycline (98.0%), followed by erythromycin (88.8%). In addition, the resistance to certain antimicrobials was significantly associated, in part, with virulence-associated genes or serotypes. Therefore, continuous characterization of S. suis is essential for the benefit of veterinary and human medicine.

Targeting TREM-1 Signaling in the Presence of Antibiotics is Effective Against Streptococcal Toxic-Shock-Like Syndrome (STSLS) Caused by Streptococcus suis

Streptococcus suis (S.suis), a major swine pathogen, is also a severe threat to human health. Infection with highly virulent strains of S. suis can cause human Streptococcal toxic-shock-like syndrome (STSLS), which is associated with high serum pro-inflammatory cytokine levels and a high mortality rate. Our previous study indicated that highly virulent S. suis infection could activate the TREM-1 signaling pathway, which promotes host clearance of S. suis during early infection. However, it remained to be elicited whether TREM-1 signaling could be a target against STSLS in the presence of antibiotic. In the present study, mice were infected with a highly virulent S. suis strain and then treated with rTREM-1 (the recombinant extracellular domain of TREM-1) to block TREM-1 signaling, antibiotics, both rTREM-1 and antibiotics, or PBS. The survival rates, clinical signs, serum IL-1β and TNF-α levels, and serum bacterial loads were evaluated. Treatment with rTREM-1 could aggravate the outcome of infection as described previously. Although the conventional treatment with antibiotics contributed to effective S. suis clearance, it did not improve survival significantly. In comparison, due to the reduction of the exaggerated pro-inflammatory response, treatment combined with rTREM-1 and antibiotics not only led to efficient bacterial clearance but also alleviated inflammation. In conclusion, TREM-1 signaling contributed to severe inflammatory response and benefited S. suis clearance. Therefore, blocking TREM-1 signaling could still be a target for the treatment of STSLS in the presence of antibiotics.

Reappraisal of the taxonomy of Streptococcus suis serotypes 20, 22 and 26: Streptococcus parasuis sp. nov

In order to clarify the taxonomic position of serotypes 20, 22 and 26 of Streptococcus suis , biochemical and molecular genetic studies were performed on isolates (SUT-7, SUT-286T, SUT-319, SUT-328 and SUT-380) reacted with specific antisera of serotypes 20, 22 or 26 from the saliva of healthy pigs as well as reference strains of serotypes 20, 22 and 26. Comparative recN gene sequencing showed high genetic relatedness among our isolates, but marked differences from the type strain S. suis NCTC 10234T, i.e. 74.8–75.7 % sequence similarity. The genomic relatedness between the isolates and other strains of species of the genus Streptococcus , including S. suis, was calculated using the average nucleotide identity values of whole genome sequences, which indicated that serotypes 20, 22 and 26 should be removed taxonomically from S. suis and treated as a novel genomic species. Comparative sequence analysis revealed 99.0–100 % sequence similarities for the 16S rRNA genes between the reference strains of serotypes 20, 22 and 26, and our isolates. Isolate STU-286T had relatively high 16S rRNA gene sequence similarity with S. suis NCTC 10234T (98.8 %). SUT-286T could be distinguished from S. suis and other closely related species of the genus Streptococcus using biochemical tests. Due to its phylogenetic and phenotypic similarities to S. suis we propose naming the novel species Streptococcus parasuis sp. nov., with SUT-286T ( = JCM 30273T = DSM 29126T) as the type strain.

Identification and characterization of a new cell surface protein possessing factor H-binding activity in the swine pathogen and zoonotic agent Streptococcus suis

Streptococcus suis is a major swine pathogen and an emerging zoonotic agent. The ability of pathogenic bacteria to bind the complement regulator factor H on their cell surface may allow them to avoid complement attack and phagocytosis. The aim of this study was to characterize a new cell surface protein possessing factor H-binding activity in S. suis serotype 2. The capacity of S. suis to bind the complement regulator factor H on its surface was demonstrated by ELISA. Using a factor I–cofactor assay, it was found that the functional activity of factor H bound to S. suis was kept. Since the product of gene SSU0186 in S. suis P1/7 shared similarity with a Streptococcus pneumoniae protein (named PspC) possessing factor H-binding activity, it was proposed as a putative factor H receptor in S. suis. SSU0186 has a 1686 bp open reading frame encoding a 561 amino acid protein containing the Gram-positive cell wall anchoring motif (LPXTG) at the carboxy-terminal, an amino-terminal signal sequence, an α-helix domain, a proline-rich region and a G5 domain. The SSU0186 gene was cloned in Escherichia coli and the purified recombinant factor H-binding protein showed a molecular mass of 95 kDa, as determined by SDS-PAGE. The protein possessed the functional property of binding factor H. Sera from S. suis-infected pigs reacted with the recombinant factor H receptor, suggesting that it is produced during the course of infections. In conclusion, we identified a novel S. suis cell surface protein that binds the complement factor H. This cell surface protein may help S. suis to resist complement attack and phagocytosis and contribute to pathogenesis.

Genome sequence of the swine pathogen Streptococcus suis serotype 2 strain S735

Streptococcus suis is a major swine pathogen responsible for significant, worldwide economic losses in the swine industry, in addition to being an emerging zoonotic agent. Strains of serotype 2 are the most commonly associated with infections causing meningitis, endocarditis, and septicemia. Here we present the genome sequence of S. suis serotype 2 strain S735.

Reappraisal of the taxonomy of Streptococcus suis serotypes 20, 22, 26, and 33 based on DNA-DNA homology and sodA and recN phylogenies

To date, Streptococcus suis was divided into thirty-three serotypes based on its polysaccharide capsular antigens. Although 16S rRNA sequence similarities of serotypes 20, 22, 26, and 33 reference strains to the type strain NCTC 10234(T) were below the threshold value of 98.5% to assign them to S. suis species, no strong evidence support to reclassification. Here, their taxonomic identities were determined by DNA-DNA hybridization assays and by partial sequencing of the sodA and recN genes. Our results confirmed that the serotype 20, 22, 26, and 33 reference strains were distantly related to the type strain NCTC 10234(T) and the whole sequence strain P1/7 of S. suis. Moreover, the reference strains of serotypes 20, 22, and 26 were closely related to each other but distinct from the serotype 33 reference strain. Sequencing analyses of sodA and recN of a total 33 serotype reference strains showed that the serotype 20, 22, and 26 reference strains and the serotype 33 reference strain did not fall with not only other serotypes of S. suis, but also other streptococcal species (63 strains of 56 species for sodA and 87 strains of 55 species for recN). The evidence further substantiates the view that the reference strains of serotypes 20, 22, 26 and 33 should be taxonomically removed from S. suis, although their taxonomic designations and determinative phenotypic characteristics are yet to be addressed.

Characterization of porcine dendritic cell response to Streptococcus suis

Streptococcus suis is a major swine pathogen and important zoonotic agent causing mainly septicemia and meningitis. However, the mechanisms involved in host innate and adaptive immune responses toward S. suis as well as the mechanisms used by S. suis to subvert these responses are unknown. Here, and for the first time, the ability of S. suis to interact with bone marrow-derived swine dendritic cells (DCs) was evaluated. In addition, the role of S. suis capsular polysaccharide in modulation of DC functions was also assessed. Well encapsulated S. suis was relatively resistant to phagocytosis, but it increased the relative expression of Toll-like receptors 2 and 6 and triggered the release of several cytokines by DCs, including IL-1β, IL-6, IL-8, IL-12p40 and TNF-α. The capsular polysaccharide was shown to interfere with DC phagocytosis; however, once internalized, S. suis was readily destroyed by DCs independently of the presence of the capsular polysaccharide. Cell wall components were mainly responsible for DC activation, since the capsular polysaccharide-negative mutant induced higher cytokine levels than the wild-type strain. The capsular polysaccharide also interfered with the expression of the co-stimulatory molecules CD80/86 and MHC-II on DCs. To conclude, our results show for the first time that S. suis interacts with swine origin DCs and suggest that these cells might play a role in the development of host innate and adaptive immunity during an infection with S. suis serotype 2.

The SspA subtilisin-like protease of Streptococcus suis triggers a pro-inflammatory response in macrophages through a non-proteolytic mechanism

Background

Streptococcus suis is a major swine pathogen worldwide that causes meningitis, septicemia, arthritis, and endocarditis. Using animal models, a surface-associated subtilisin-like protease (SspA) has recently been shown to be an important virulence factor for S. suis. In this study, we hypothesized that the S. suis SspA subtilisin-like protease may modulate cytokine secretion by macrophages thus contributing to the pathogenic process of meningitis.

Results

Phorbol 12-myristate 13-acetate-differentiated U937 macrophages were stimulated with recombinant SspA prior to monitor cytokine secretion by ELISA. Our results indicated that the recombinant SspA was able to dose-dependently induce IL-1β, IL-6, TNF-α, CXCL8 and CCL5 secretion in macrophages. The heat-inactivated protease was still able to induce cytokine secretion suggesting a non-proteolytic mechanism of macrophage activation. Using specific kinase inhibitors, evidence were bought that cytokine secretion by macrophages stimulated with the recombinant SspA involves the mitogen-activated protein kinase signal transduction pathway. While stimulation of macrophages with low concentrations of recombinant SspA was associated to secretion of high amounts of CCL5, the use of recombinant SspA at a high concentration resulted in low amounts of CCL5 detected in the conditioned medium. This was found to be associated with a proteolytic degradation of CCL5 by SspA. The ability of SspA to induce cytokine secretion in macrophages was confirmed using a mutant of S. suis deficient in SspA expression.

Conclusion

In conclusion, this study identified a new mechanism by which the S. suis SspA may promote central nervous system inflammation associated with meningitis.