Application of attenuated Lactococcus garvieae strain lacking a virulence‐associated capsule on its cell surface as a live vaccine in yellowtail Seriola quinqueradiata Temminck and Schlegel

Summary An attenuated Lactococcus garvieae strain lacking a virulence-associated capsule on its cell surface was evaluated for application as a live vaccine. The attenuated strain (MS93003A) was obtained from the parent strain (MS93003V), which produced a well-developed capsule, by culturing on an agar medium supplemented with 2,3,5-triphenyltetrazolium chloride. When live cells of L. garvieae (MS93003A) or formalin-killed cells (MS93003A) were used as an injectable vaccine, protection against virulent L. garvieae (MS93003V) was conferred on Seriola quinqueradiata. The MS93003A cells did not recover their virulence even after in vivo passages in fish. MS93003A live cells also conferred long-lasting protective immunity to S. quinqueradiata against virulent L. garvieae infection.