The Two-Component Regulatory System VicRK is Important to Virulence of Streptococcus equi Subspecies equi.

This study aims at evaluating the importance of the two-component regulatory system VicRK to virulence of the horse pathogen Streptococcus equi subspecies equi and the potential of a vicK mutant as a live vaccine candidate using mouse infection models. The vicK gene was deleted by gene replacement. The DeltavicK mutant is attenuated in virulence in both subcutaneous and intranasal infections in mice. DeltavicK grows less slowly than the parent strain but retains the ability of S. equi to resist to phagocytosis by polymorphoneuclear leukocytes, suggesting that the vicK deletion causes growth defect. DeltavicK infection protects mice against reinfection with a wild-type S. equi strain. Intranasal DeltavicK infection induces production of anti-SeM mucosal IgA and systemic IgG. These results indicate that VicRK is important to S. equi growth and virulence and suggest that DeltavicK has the potential to be developed as a live S. equi vaccine.