Live Streptococcus suis type 5 strain XS045 provides cross-protection against infection by strains of types 2 and 9.

Streptococcus suis is one of the common pathogens causing diseases in pigs and covers 35 serotypes with the type 2 strains being more pathogenic and zoonotic. Existing inactivated or subunit vaccines, in clinical use or under trial, could not provide cross protection against other serotypes. We identified a natural low-virulence S. suis type 5 strain XS045 as a live vaccine candidate because it is highly adhesive to the cultured HEp-2 cells, but with no apparent pathogenicity in mice and piglets. We further demonstrate that subcutaneous administration of the live XS045 strain to mice induced high antibody responses and was able to provide cross protection against challenges by a type 2 strain HA9801 (100% protection) and a type 9 strain JX13 (85% protection). Induction of high-titer antibodies with opsonizing activity as well as their cross-reactivity to surface proteins of the types 2 and 9 strains and anti-adhesion effect could be the mechanisms of cross protection. This is the first report that a live vaccine candidate S. suis type 5 strain could induce cross-protection against strains of types 2 and 9. This candidate strain is to be further examined for safety in pigs of different ages and breeds as well as for its protection against other serotypes or other strains of the type 2, a serotype of particular importance from public health concern.