Identification of the gene encoding a 38-kilodalton immunogenic and protective antigen of Streptococcus suis

In our continued effort to search for a Streptococcus suis protein(s) that can serve as a vaccine candidate or a diagnostic reagent, we constructed and screened a gene library with a polyclonal antibody raised against the whole-cell protein of S. suis type 2. A clone that reacted with the antibody was identified and characterized. Analysis revealed that the gene encoding the protein is localized within a 2.0-kbp EcoRI DNA fragment. The nucleotide sequence contained an open reading frame that encoded a polypeptide of 445 amino acid residues with a calculated molecular mass of 46.4 kDa. By in vitro protein synthesis and Western blot experiments, the protein exhibited an electrophoretic mobility of approximately 38 kDa. At the amino acid level the deduced primary sequence shared homology with sequences of unknown function from Streptococcus pneumoniae (89%), Streptococcus mutans (86%), Lactococcus lactis (80%), Listeria monocytogenes (74%), and Clostridium perfringens (64%). Except for strains of serotypes 20, 26, 32, and 33, Southern hybridization analysis revealed the presence of the gene in strains of other S. suis serotypes and demonstrated restriction fragment length differences caused by a point mutation in the EcoRI recognition sequence. We confirmed expression of the 38-kDa protein in the hybridization-positive isolates using specific antiserum against the purified protein. 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 and may serve as an antigen of diagnostic importance for the detection of most S. suis infections. Pigs immunized with the recombinant 38-kDa protein mounted antibody responses to the protein and were completely protected against challenge with a strain of a homologous serotype, the wild-type virulent strain of S. suis type 2, suggesting that it may be a good candidate for the development of a vaccine that can be used as protection against S. suis infection. Analysis of the cellular fractions of the bacterium by Western blotting revealed that the protein was present in the surface and cell wall extracts. The functional role of the protein with respect to pathogenesis and whether antibodies against the antigen confer protective immunity against diseases caused by strains of other pathogenic S. suis capsular types remains to be determined.