Comparative Genomics of Edwardsiella piscicida in the Japanese Flounder (Paralichthys olivaceus): Discovery and Implications of a Novel Genomic Island.

Edwardsiella piscicida is a significant pathogen that poses a particular threat to Japanese flounder (Paralichthys olivaceus) aquaculture in Japan and other countries. The damage is caused by the pathogen's ability to evade host immune defences and establish intracellular infections, intensified by its genomic plasticity and capacity for horizontal gene transfer. To investigate evolutionary adaptations between one older (2019) and four recent (2023) E. piscicida strains from the same geographical locations, we performed comparative genomic analysis of five isolates using high-quality hybrid genome assemblies and compared them with 27 Edwardsiella reference genomes. Pangenome analysis identified distinct novel genomic islands (GIs) specific to the 2023 strains. These GIs (~100 kb in size) shared 85 gene clusters encoding multiple antibiotic resistance genes, phage defence systems, mobilisation genes, and mercury resistance. In addition, they encoded integrases, transposases, and conjugative transfer genes, suggesting they function as integrative and conjugative elements (ICEs), a type of mobile genetic element. Phenotypic characterisation showed the 2023 strains carrying novel GI increased antibiotic resistance, but no significant difference in virulence in Japanese flounder infection trials. These findings highlight the recent genomic diversification of E. piscicida in aquaculture and the importance of monitoring emerging GIs driving antibiotic resistance and environmental persistence.