The protective immune response of yellowtail Seriola quinqueradiata to the bacterial fish pathogen Lactococcus garvieae.

Yellowtail Seriola quinqueradiata were immunized with 2 different Lactococcus garvieae bacterin, formalin-killed KG- phenotype cells (capsulated phenotype) and formalin-killed KG+ phenotype cells (unencapsulated phenotype). These 2 injected vaccines conferred long-term protection to yellowtail against an artificial infection of an encapsulated Lactococcus garvieae strain with long-lasting agglutinating titres against KG+ phenotype cells. However, no agglutinating titres or low agglutinating titres against KG- phenotype cells were detected in fish given each of these bacterin. These results suggested that a capsule in KG- phenotype cells apparently affects their immunogenicity, but the antigens which conferred protection to fish against lactococcal infection may be located on the surface of KG+ phenotype cells, and are not cell capsules in KG- phenotype cells. The protection offered by a formalin-killed KG+ phenotype cell vaccine would not appear to be strain specific. Encapsulated L. garvieae cells were well phagocytosed, and fimbrie-like appendages were seen in KG- phenotype cells after treatment with yellowtail immune serum.