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Objective: In addition to Streptococcus suis serotype 2, Streptococcus suis serotype 9 (SS9) is also a currently prevalent serotype and a zoonotic pathogen. In our previous study, SS9 DNA nuclease (SsnA) was considered as a candidate virulence factor. To clarify the impact of SsnA on SS9 virulence, we constructed ssnA mutant (ΔssnA) and studied its biological functions., Methods: We evaluated the virulence of wild type strain and ΔssnA in a zebrafish infection model and compared the adherence rate to HEp-2 cells, the survival rate in pig blood, and enzymatic activity between wild type stain and ΔssnA., Results: In a zebrafish infection experiment, the 50% lethal dose value of ΔssnA was 11.2-fold higher than that of wild type strain. The adherence rate of ΔssnA to HEp-2 cells was only 60.61% of the wild strain level. The survival rate of ΔssnA in pig blood was declined to 71.88% of wild strain level. The enzymatic activity assay showed that SsnA can degrade both linear and circular DNA., Conclusion: SsnA contributes to SS9 virulence in a zebrafish infection model, the adherence to HEp-2 cells, and the survival in pig blood. SsnA is indeed an essential virulence factor for SS9.

Objective: To prepare the recombinant enolase of Streptococcus suis (SsEno), analyze the effect of SsEno on the antiphagocytosis by antibody-blocking assay in the human blood bactericidal model, and identify the human fibrinogen (hFg)-binding activity of SsEno protein., Methods: SsEno gene was amplified using the primers designed according to 05ZYH33 genome sequences and cloned into the expression vector pET28a to construct recombinant plasmids. The plasmids were transformed into E.coli BL21(DE3) and induced to express by IPTG. The expression level was analyzed by SDS-PAGE. The recombinant proteins were purified by nickel affinity chromatography and identified by Western blotting. High-titer specific antiserum against SsEno was prepared by immunizing rabbits with the purified recombinant proteins. The impact of SsEno on antiphagocytosis was analyzed by antibody-blocking assay in the human blood bactericidal model. In addition, the specific binding activity with hFg was identified by Far-Western blotting and ELISA., Results: The prokaryotic expression vector of hisSsEno (SsEno with His tag) was constructed and high-purity recombinant expressed protein was purified. In specific antibody blocking assay, antiserum against the SsEno significantly decreased the percent of survival bacteria as observed in high virulent strain 05ZYH33. Additionally, hisSsEno was proved to have the specific binding activity with hFg., Conclusion: SsEno was found to be a potential antiphagocytic factor of S. suis with a specific binding to hFg, suggesting that SsEno play an important role in antiphagocytosis of S. suis.

Objective: Streptococcus suis 2 is an emerging zoonotic pathogen responsible for a wide range of life-threatening diseases in pigs and humans. In this study, we investigated the functionality of one of Streptococcus suis 2 sortases, known as the srtBCD., Methods: To obtain the isogenic mutant srtBCD, the competent cells of 05ZYH33 were subjected to electrotrans formation with recombinant plasmid based on the principle of homologous recombination. The resulting mutant strains was further confirmed by a series of PCR and reverse transcription PCR. To better assess the role of srtBCD gene in the virulence of 05ZYH33, cell adherence assays and experimental infection of mice was adopted., Results: A SrtBCD defective mutant of 05ZYH33 was found to be associated with growth curve upon cultivation in standard laboratory used in our in vitro assays. Furthermore, abolishment of the expression of srtBCD result in impaired interactions of S. suis with human laryngeal epithelial cell line. However, there is no differences when infection mice by the WT and mutant strain., Conclusion: These results suggest that srtBCD are critical for the pathogen-host interaction of S. suis 2, but abolishment of srtBCD does not impair the full virulence of 05ZYH33. It is to expect that future study carried out with S. suis 2 to verification the conclusions.

Objective: Glutamyl tRNA Synthetase (Gts) is a protease which catalyzes esterification between tRNA and glutamine. The immunogenicity of Gts was evaluated through immunization and challenge experiment., Methods and Results: We cloned gts from the genome of SC19, and inserted it into prokaryotic expression plasmid pET28a-gts. The recombinant vector was transformed into E. coli BL21. Induced by IPTG, one 58 kD protein, was expressed and purified by using Ni - NTA column (Novagen). The purity of rGtS was 93.3% and the concentration of purified protein was 433 microg/mL. We proved the immunogenicity of recombinant protein rGtS by western blot analysis. We immunized Balb/c mice with rGtS and Freund's adjuvant, and after two boost vaccinations, all mice were challenged with 4 times LD50 amount of SC19 (1.2 x 10(9) CFU). The survive rate of vaccination group is 50% (4/8), significantly higher than blank vector control group (1/8)., Conclusion: These results proved that GtS has certain immunogenicity and can offer partial protection against high dose challenge. Therefore Gts could be a potential candidate of subunit vaccine against Streptococcus suis.

Streptococcus suis serotype 2 (SS2) is an economically important, zoonotic agent causing death and disease in both human and swine. According to published SS2 European strain P1/7 complete genomic sequence, primers for detection and amplification autolysin were designed. Autolysin gene was detected by PCR with genomic DNA of HA9801 (Jiangsu isolate), ZY05719 (Sichuan isolate), ATCC43765 (reference strain), other SS2 isolates from different regions, and strains of other different serotypes (such as serotype 1, 1/2, 7 and 9). PCR results showed that all 27 SS2 virulent isolates harbored gene autolysin, but not in non-virulent SS2 strain. Among other serotypes, only serotype 7 strain had this gene. The complete autolysin genes of HA9801 and ZY05719 were respectively amplified by PCR and sequenced. Their putative protein sequences were analyzed through online software, results showed both had six repeated domain "GBS_Bsp-like" and one domain "N-acetylmuramoyl-L-alanine amidase". In addition, sequence similarity analysis demonstrated that autolysins of the two strains (HA9801 and ZY05719) showed high homologue to that of European strain P1/7 (99.8%), but obviously differed from Canada strain 89/1589, the latter lacked one domain "GBS Bsp-like". Software DNASTAR was used to analyze autolysin protein sequence and predicted its putative antigenicity. Then partial antigentic segment of autolysin was amplified, cloned and inserted into expression vector pET30a(+), and induced by IPTG to express recombinant autolysin. Its reactivity was analyzed by SDS-PAGE and western blot with swine convalescent sera, blot result showed that the recombinant protein had good reactivity and could be considered as a vaccine candidate.

PCR analysis demonstrated the presence of the ad gene in all 29 S. suis strains tested, but none of the seven S. equi subsp. zooepidemicus strains. The fragment of ad gene of virulent isolate SS2-HA9801 was later cloned into pBAD/Myc-HisC vector via restriction endonuclease and then transformed into host strain TOP10. A recombinant protein of 47000Da was highly expressed after induced by L-ararose and purified by Ni-nitrilotriacetic acid affinity chromatography. Western blotting demonstrated that the recombinant protein can reacted to the polyclonal antibody raised against whole-cell protein of SS2-HA9801, which suggested that it possessed some immunogenicity and may be important for further research. Enzymatic assay revealed that the optimum temperature for its activity is 37 degrees C and pH is 6.5. Studies with class-specific inhibitors supported the assignment of a sulfhydryl enzyme with some metallo class characteristics.

Given the lack of effective vaccines to control Streptococcus suis infection and the lack of a rapid and reliable molecular diagnostic assay to detect its infection, S. suis serotype 2 was sequenced partly in an effort to identify important virulence factors. Two new open reading frames were found located between orf2 and mrp. One of new open reading frame (2738 - 3694) that encoded a polypeptide of 319 amino acid residues with a calculated molecular mass of 33.5kDa was identified by Western blot. GenBank database search revealed that the derived amino acid sequence shared low homology with sequences of known function from other genes. Second structure was analyzed by InterPro, PHD, DNAstar software, the deduced protein had functional domains typical of IMP dehydrogenase (IMPDH). The PCR product of the open reading frame was transformed into E. coli BL21 and the fusion protein of 48kDa was expressed. The recombinant protein was reactive with serum from pigs experimentally infected with virulent strains of S. suis type 2, suggesting that the protein is immunogenic. IMPDH activity staining confirmed that the protein has IMPDH function and can catalyze the rate-limiting reaction of GTP biosynthesis, the NAD-dependent reduction of IMP into XMP. Flow cytometry (FCM) revealed that the protein had apparent effect on HEp-2 cell cycle.