Virulence attenuation of capsular polysaccharide-deleted mutants of Streptococcus iniae in Japanese flounder Paralichthys olivaceus

Abstract

Streptococcus iniae is known as an important bacterial pathogen causing serious damage in fish culture industry worldwide. Although the capsule of this pathogen is suggested to play a role in the disease process, it has not been verified yet. In this study we created isogenic uncapsulated mutants of S. iniae NUF631 by Tn916 transposon mutagenesis and investigated the role of capsule in the virulence and resistance to phagocytic activities of peritoneal macrophages of Japanese flounder Paralichthys olivaceus. Mutants transformed from K + phenotype to K- phenotype were all uncapsulated. They exhibited neither K+ type-specific precipitation line in an immunodiffusion test nor ladder-like banding pattern characteristic for bacterial acidic polysaccharides in polyacrylamide gel electrophoresis. Their virulence to Japanese flounder was highly attenuated. In contrast to the mutant strains, the parent capsulated strain resisted phagocytic and bactericidal activities of the macrophages and prevented chemiluminescence reaction of the macrophages upon phagocytosis. From these results, it is verified that the S. iniae capsule is an important factor for intramacrophage survival of the pathogen. © 2007 The Japanese Society of Fish Pathology.