Abstract
Lactococcus garvieae is recognized as a crucial bacterial pathogen of freshwater and marine fish species. It has been divided into two serological phenotypes, namely KG− and KG+. Difference of the two phenotypes is owing to the presence or absence of polysaccharide capsule, and a phenotypic change from KG− to KG+ occurs during stocking of isolates for a long period or by repeated subculturing. We found that the phenotypic change occurred more readily in L. garvieae isolates from cultured filefish, thread-sail filefish Stephanolepis cirrhifer and black scraper Thamnaconus modestus, than those from other fish species. Thus we studied the gene cluster for capsular polysaccharide biosynthesis (capsule gene cluster) of a filefish isolate, strain BSLG13015, and revealed that the strain possessed the same capsule gene cluster as those from other fish species, but that it was integrated in a newly identified plasmid. The plasmid, a size of 31,654 bp and circular, was named pBSLG13015. It was detected in all of KG− filefish isolates but not in KG+ filefish isolates or L. garvieae from other fish species. It is highly probable that the easier change from KG− to KG+ in L. garvieae filefish isolates is attributed to the loss of the plasmid.
