First Description and Characterisation of <scp> <i>Lactococcus garvieae</i> </scp> Strains Causing Septicaemic Disease in Farmed Sea Bass ( <scp> <i>Dicentrarchus labrax</i> </scp> ; Linnaeus) in Spain

This study presents the first description and characterisation of Lactococcus garvieae isolated from diseased European sea bass (Dicentrarchus labrax) farmed in Spain. During autumn 2023, two fish farms were affected by infectious outbreaks causing moderate cumulative fish mortality (5%-10%). Diseased fish showed clinical signs of haemorrhagic septicaemia, compatible with lactococcosis. Standardised screening tests revealed the presence of Gram-positive cocci in the kidney, spleen and brain of the diseased fish, and negative results for parasites and viruses. Bacterial cultures recovered from the internal organs of all diseased fish were identified as Lactococcus garvieae by phenotypic, genetic, immunological and proteomic analyses. Strains were sensitive to oxytetracycline, florfenicol and erythromycin and resistant to trimethoprim sulfamethoxazole. In addition, representative isolates were virulent to juvenile sea bass and Senegalese sole (Solea senegalensis) after intracoelomic challenge with doses ranging from 10⁶ to 10⁷ CFU/fish. SDS-PAGE and immunoblotting analyses, using rabbit serum anti L. garvieae CECT 5274 and serum from trout immunised with a bivalent oil-based vaccine against L. garvieae-Yersinia ruckeri, showed that the current strains of sea bass shared some antigenic proteins with strains of L. garvieae from other hosts and with the reference strain of L. petauri DSM104842. Our overall results confirm the presence of this emerging pathogen in Spanish marine aquaculture and suggest that commercially available lactococcosis vaccines could confer some protection to sea bass, helping to prevent this new threat.