| Streptococcus suis is an invasive porcine pathogen associated with septicemia, meningitis, pneumonia, endocarditis, arthritis and other diseases. The pathogen constitutes a major health problem and great loss in the swine industry worldwide. Thirty-three serotypes (types 1-31,33 and 1/2) have been described based on capsular polysaccharides, and S. suis serotype 2 (SS2) is considered to be the most pathogenic as well as the most prevalent capsular type in diseased pigs. SS2 might also cause diseases even death in humans. Recently, two large-scale outbreaks of human SS2 in China (25 cases with 14 deaths in Jiangsu in 1998, and 204 cases with 38 deaths in Sichuan in 2005) showed that Streptococcus suis is actually a great threat to human health. Although a set of virulence traits have been identified, molecular mechanisms of invasion and damage of host tissues by the bacteria are still far from clear. Studies on virulence-associated factor contribute to explain the pathogenicity of S. suis.It has been shown that the polysaccharide capsule is a great recognition virulence factor of S.suis. However, some avirulent strains seem to be fully encapsulated. This suggests that some other virulence factors may play important role on virulence of S.suis. Some Secreted proteins may have important functions in pathogenesis of S.suis. These secreted proteins can damage host cells depending on diffusion in tissue in order to reach critical concentration but do not require bacteria-host cell contact. SLY has been believed to be one of the most important secreted proteins. In addition to its cytotoxic activity, SLY plays critical role on the process of S.suis infecting host cell by blocking the complement-mediated phagocytosis and killing role. SLY contribute to Streptococcus suis type 2 (SS2) invasion and cell lysis and lead to meningitis. Furthermore, s/y-positive strains are usually more prone to lead to systemic infection. Due to these functions, some scholars believe that the SLY may be the most important virulence associated factors of Europe and Asia strains.It has been pointed out that an ORF in many virulent serotype 2 and 9 strains encodes a putative hemolysin type III. However, at present there is no experimental evidence for expression of any other active hemolysin except suilysin. Therefore this expriment was conducted to study the function of hemolysin type III gene(slyrp) of Streptococcus suis type 2 to investigate the role of slyrp in cleavage of red blood cells and pathogenicity of SS2. The SS205ZY isogenic suilysin mutant Aslyrp was constructed through homologous recombination, as control, Aslyrp and the two genes knock out mutant Asly/slyrp were construct. The hemolysis capability and virulence to mice between wild type strain and the gene deleted mutants were compared. The results indicated that the isogenic sly mutant showed significant decreased hemolysis capability; the isogenic slyrp mutant showed slight decreased hemolysis capability and the the two genes knock out mutant Asly/slyrp showed no hemolysis capability. The sly gene knock out mutant showed relative lower virulent to mice; slyrp gene knock out mutant showed the same virulent to mice as wild type strain; the two genes knock out mutant Asly/slyrp showed considerable decreased virulent to mice.The results indicated that:sly plays an important role in invading and lysis of host cells by SS2, and slyrp does synergistic effect to sly. Slyrp plays relative role but not decisive role in SS2 infection. Sly is the main virulence factor, but apparently is not the only virulence factor that leads to the high pathogenicity of SS2. |
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