Variation in the Integration Site for Capsule Gene Cluster in the Genome among Strains of Lactococcus garvieae

Lactococcus garvieae is known as a pathogen of freshwater and marine fish species worldwide. L. garvieae isolates of serotype I have been divided into two serological phenotypes, namely KG- and KG+, which are differentiated by the presence or absence of polysaccharide capsule, and a phenotypic change from KG- to KG+ occurs during repeated subculturing. When we subcultured laboratory collections of L. garvieae KG- strains repeatedly, they were divided into two groups. One group consisted of the strains which changed phenotypically after a relatively small number of times of subculturing, and the other group consisted of those which hardly changed. Genetic analyses revealed that the capsule gene cluster of the strains in the former group was integrated in a plasmid (CPS plasmid) and that in the latter group it was integrated in the chromosome or in both CPS plasmid and chromosome simultaneously. In the present study, we found three novel CPS plasmids. These plasmids were similar in structure to the CPS plasmid pBSLG13015 which was previously found in L. garvieae filefish isolates. In addition, we found a novel integration site of chromosomal capsule gene cluster. The majority of strains isolated before 1991 were those whose capsule gene cluster was integrated in the CPS plasmids.