[Effect of the 10 kb sequence of piscine Streptococcus agalactiae on bacterial virulence]

From the previous comparative genomic analysis, we found a specific unknown 10 kb sequence (including 11 Open reading Frames) in Chinese piscine strain GD201008-001 genome. To study the role of 10 kb in the pathogenicity of piscine S. agalactiae, the 10 kb sequence was deleted from the GD201008-001 genome.

The isogenic mutant Δ10 kb was constructed by using the temperature-sensitive Streptococcus-E. coli shuttle vector pSET4s. We compared the growth characteristics, adherence to HEp-2 cell and bacterial virulence in a zebrafish infection model between wild strain and mutant. Meanwhile the expressions of the known virulence genes from GD201008-001 and Δ10 kb were also quantified by real-time PCR.

The Δ10 kb showed no significant differences in bacterial morphology and adherence to HEp-2 cells compared with the wild-type strain, but the speed of growth was slightly slower than the wild strain. Furthermore the 50% lethal dose of Δ10 kb was decreased up to 10-fold (P < 0.001) of the parental strain in a zebrafish infection model, and the expressions of the virulence genes, PI-2b and neul, were significantly increased in the mutant.

These findings demonstrated that the 10 kb sequence of piscine Streptococcus agalactiae exerts a significant effect on bacterial virulence and probably regulates the virulence genes expression of GD20 1008-001.