Retrospective Identification of Lactococcus petauri Among Archived Isolates From Farmed Fish in Japan: Pathogenicity...

Lactococcosis, caused by Lactococcus garvieae, L. formosensis, and L. petauri, is an important bacterial disease of farmed fish worldwide. In Japan, L. petauri has not previously been reported as a cause of lactococcosis, although lactococcosis cases not attributable to either L. garvieae or L. formosensis have been recorded. The identity, pathogenicity, and epidemiological status of these bacteria remain unclear. We retrospectively examined 16 unidentified Lactococcus isolates from diseased fish farmed in Japan between 1978 and 2021. All isolates clustered within the L. petauri clade in phylogenetic analysis based on concatenated sequences of seven housekeeping genes. Multilocus sequence analysis assigned these isolates to seven sequence types, including five novel sequence types. Two previously reported multiplex PCR assays developed to differentiate Lactococcus species did not accurately identify these L. petauri isolates. Intraperitoneal injection of representative isolate L20-09 induced dose-dependent mortality in yellowtail Seriola quinqueradiata and striped jack Pseudocaranx dentex. The median lethal dose (LD₅₀) values were 5.20 × 10⁶ CFU/fish in yellowtail and 1.16 × 10³ CFU/fish in striped jack. These findings indicate that pathogenic L. petauri has been present in farmed fish in Japan and may have been overlooked during routine diagnosis.