First identification, molecular characterization, and pathogenicity assessment of Lactococcus garvieae isolated from cultured pompano in Taiwan.

Lactococcosis, caused by Lactococcus garvieae, is an acute hemorrhagic septicemia in fish recorded in marine and freshwater aquaculture during the summer months. In 2020-2021, several sea cage Pompano farms recorded sudden fish mortality events. Based on the results of phenotypic and biochemical tests, L. garvieae was predicted to be the cause. PCR with L. garvieae specific primers (pLG1 and pLG2) targeting the 16S rRNA region further confirmed the etiological agent as L. garvieae after amplifying an 1100 base pairs (bp) product. Furthermore, the 16S rRNA sequences of the two representative strains (AOD109-196-2B and AOD110-215-2B) shared 99.81% identity with L. garvieae (GenBank accession number: MT597707.1). The genetic profiles of the strains were classified using pulsed-field gel electrophoresis after digestion with SmaI and ApaI, which clustered our strains under the same pulsotype. Multiplex PCR targeting the capsule gene cluster and serotype-specific PCR collectively showed that the strains were non-capsulated; thus, they belonged to serotype I. An experimental infection was designed to fulfil Koch's postulates by infecting healthy Pompano with case-driven L. garvieae strains (AOD109-196-2B and AOD110-215-2B) with a cumulative mortality of 70%. Overall, L. garvieae infection in Pompano emphasizes the need for better monitoring and control procedures in aquaculture settings.