Serological differentiation of Streptococcus parauberis strains isolated from cultured Japanese flounder in Japan

Streptococcus parauberis, a Gram-positive bacterium, is known as the causative agent of mastitis in cows as well as streptococcicosis in turbot Scophthalmus maximus and Japanese flounder Paralichthys olivaceus. It has recently become one of the most important pathogens of cultured Japanese flounder in western Japan. In this study, we investigated the serological nature of S. parauberis strains isolated from Japanese flounder in Japan. As a result, the strains were distinctly divided into two groups, serotype I represented by strain KRS-02083 and serotype II represented by strain KRS-02109, by an agglutination test with rabbit antisera. Autoclave-extracts of bacterial cells derived from each serotype formed a type-specific precipitation line with each corresponding antiserum in an agar double diffusion test. Acidic polysaccharides were detected in the autoclave-extracts of the bacterial cells. Electron microscopy revealed capsular structure on the bacterial cells pretreated with each corresponding antiserum. This finding suggests that capsular polysaccharide antigens serve to differentiate two serotypes.