Cloning and expression of a surface immunogenic protein in S treptococcus dysgalactiae isolated from fish and its application in enzyme‐linked immunosorbent assays to diagnose S . dysgalactiae infections in fish

Lancefield group C S treptococcus dysgalactiae ( GCSD ) causes severe necrotic lesions in the caudal peduncle in the genus S eriola farmed in J apan. To develop a sero‐diagnostic method for GCSD infection in farmed fish, we attempted to identify a surface immunogenic protein that induces an antibody after infection with GCSD by immunoblot analysis using sera collected from infected fish. A protein obtained from sodium dodecyl sulfate ( SDS ) extracts of GCSD was identified as S . dysgalactiae surface immunogenic protein ( S d‐ S ip). S d‐ S ip exhibited more than 94% homology with a surface antigen or a hypothetical protein from S . dysgalactiae mammalian isolates at the nucleotide sequence level. Expression of the recombinant S d‐ S ip (r S d‐ S ip) was confirmed by immunoblot analysis, that is, its reactivity to GCSD ‐infected sera. Antibody detection ELISA using r S d‐ S ip and their usefulness for diagnosis of GCSD infection were examined. GCSD ‐infected sera collected from farmed amberjack, S eriola dumerili ( R isso), showed strong reaction with immobilized r S d‐ S ip. Meanwhile, sera immunized by other pathogenic bacteria of fish were showed ELISA values similar to those of non‐infected sera. These results of this study suggest that the antibody detection ELISA using r S d‐ S ip is an effective diagnostic method for GCSD infection in fish.