Serological Characterization of Streptococcus iniae Strains Isolated from Cultured Fish in Japan

Streptococcus iniae is known as an important marine and freshwater fish pathogen worldwide. In this paper, we characterized two serological phenotypes in Japanese S. iniae isolates, mainly from Japanese flounder Paralichthys olivaceus. The two phenotypes, designated K+ and K-, were distinguished by the presence or absence of capsule. K- cells agglutinated both with anti-K- and K+ type sera, whereas K+ cells agglutinated only with anti-K+ type serum. These two types were indistinguishable by any ordinary biological or biochemical characterization tests. Immunodiffusion test demonstrated the common antigens and a K+ type-specific antigen in the autoclave-extracts of cells. Capsule was observed on the K+ cells, from which acidic polysaccharides were detected. Strains of K+ type were virulent for Japanese flounder, whereas those of K- type were avirulent. There is a direct relationship between the serological phenotypes and the existence of capsule in S. iniae with link to its virulence.