Unveiling the Past: A Retrospective Detection of Lactococcus petauri in Farmed European Seabass (Dicentrarchus labrax) in the Tyrrhenian Sea

Lactococcus petauri is an emerging bacterial pathogen associated with disease outbreaks in freshwater and marine fish species worldwide. While it has been increasingly reported in countries bordering the Mediterranean Sea, no official records of L. petauri have yet been made in Italy for saltwater species. This study reports, for the first time, the presence of L. petauri in European seabass (Dicentrarchus labrax) farmed in land-based tanks along the south-western coast of Italy, based on a retrospective analysis of previously isolated strains. Although only a single isolate was positive, the finding is of notable significance as it originated from a sample collected in 2012. This temporal aspect highlights the likelihood that L. petauri has been circulating in Italian aquaculture systems for at least a decade. The isolate also exhibited a broad antimicrobial resistance profile, including resistance to amoxicillin, fluoroquinolones and trimethoprim/sulfamethoxazole. The historical misidentification of L. petauri is linked to its clinical similarity with Lactococcus garvieae. This highlights the value of re-analysing archived strains with modern tools and underlines the need for accurate diagnostics in aquaculture. The detection in D. labrax extends the known host range and suggests a longer, undetected presence in the Mediterranean.