Multilocus Sequence Typing-Based Molecular Epidemiology of Lactococcus garvieae and Lactococcus formosensis in Japanese Amberjack, Greater Amberjack and Striped Jack Aquaculture.

Lactococcosis is a major bacterial disease that affects Japanese Seriola aquaculture; however, its molecular epidemiology remains poorly understood. This study characterized 62 Lactococcus isolates recovered from Japanese amberjack, greater amberjack and striped jack between 2016 and 2025 using multiplex polymerase chain reaction, multilocus sequence typing (MLST) and erythromycin susceptibility testing. An epidemiological shift was observed from the dominant ST17-serotype I lineage (Lactococcus garvieae) to ST56- and ST115-serotype II lineages (Lactococcus formosensis), with the emergence of ST95-serotype III (L. garvieae). MLST demonstrated an association between sequence type, serotype and erythromycin susceptibility. ST56 remained susceptible to erythromycin, whereas ST115 showed erythromycin resistance, harboured the erm(B) gene and represented a previously unreported lineage. Furthermore, goeBURST and phylogenetic analyses revealed that ST56 and ST115 belonged to the same clonal complex and differed at a single locus, indicating recent diversification within the endemic serotype II lineage. Conversely, ST95 formed a singleton lineage, consistent with a distinct evolutionary origin. ST115 detection at multiple aquaculture sites suggests dissemination through fish movement, including imported and translocated juveniles. These findings indicate changes in Lactococcus population structure in Japanese aquaculture and highlight the need for molecular surveillance, improved vaccines and biosecurity to limit the spread of emerging antimicrobial-resistant lineages.