Lactococcus garvieae strains from yellowtail Seriola quinqueradiata carry different lengths offimbriae on their cell surface

摘要

Lactococcus garvieae is a serious bacterial fish pathogen of yellowtail Seriolaquinqueradiata. Lactococcus garvieae strains isolated from S. quinqueradiata have been divided into non-agglutinating (capsulated strains; KG- phenotype) and agglutinating (non-capsulated or microcapsu-lated strains; KG+ phenotype) phenotypes using rabbit antiserum raised against non-capsulated phenotype cells. The capsulated strain is more virulent to yellowtail than the non-capsulated phenotype strain with resistance to the opsonophago-cytosis of the yellowtail phagocytic cells and serum killing. Therefore, a cell capsule is thought to be one of the virulence factors in L. garvieae cells from yellowtail. Recently, oral or injectable vaccines have been developed for preventing L. garvieae infection in yellowtail. Although strong immunity is induced infish by immunization of vaccines with formalin-killed L. garvieae cells, protective mechanisms in fish by the vaccine are still under investigation. In a previous investigation, the cell-surface properties of strains of L. garvieae cells were examined.Two capsular types were found: one with a highly developed capsule and one with a microcapsule carrying fimbriae-like structures projecting from the cell surface. Although both capsular-type cells induced a strongimmunity in fish, another strain had neither cell capsular nor fimbriae-like structures on its cell surface, thus providing incomplete immunity for the fish. However, the reason why antigens elicit strong immunity for fish is still unknown, and it is possible that cell surface components are strongly related to inducing protection in fish. The aim of the present investigation was to re-examine cell-surface properties of various L. garvieae strains under different culture conditions.