Abstract
Since 2012, Lactococcus garvieae strains which do not agglutinate with anti-KG– phenotype rabbit serum have been isolated from cultured yellowtail Seriola quinqueradiata and greater amberjack S. dumerili in Japan. In this study, pathogenicity of non-agglutinating L. garvieae was confirmed by intraperitoneal injection into these fish species. Cross-protective responses of yellowtail to the KG– phenotype and non-agglutinating strains were also examined. Yellowtail were immunized with formalin-killed cells (FKC) prepared from both types. Three weeks after immunization, the fish were challenged with the KG– phenotype and non-agglutinating strains. Each type of FKC provided effective protection against infection with the homologous strain. The protection level of the KG– phenotype FKC was relatively lower against the non-agglutinating strain than against the homologous KG– phenotype and the non-agglutinating type FKC was ineffective against the KG– phenotype. In conclusion, we propose two serotypes (I and II) of L. garvieae isolated from marine fish species in Japan. Serotype I which agglutinates with anti-KG– phenotype serum is typical L. garvieae and divided into Ia and Ib, which are KG– (capsulated) and KG+ (non-capsulated) phenotypes, respectively. Serotype II which does not agglutinate with the anti-KG– serum is a new type in fish pathogenic L. garvieae.
