Genomic characterization, stress tolerance and cardiac transcriptomic responses of a virulent Edwardsiella piscicida strain from farmed largemouth bass

Abstract Edwardsiella piscicida is an important intracellular pathogen in finfish aquaculture. During a mortality episode in farmed largemouth bass ( Micropterus salmoides ) in Foshan, South China, we isolated a virulent E. piscicida strain (B202203044) from diseased liver and characterized its pathogenicity, stress tolerance and genomic features. The strain was identified as E. piscicida based on biochemical profiling, species‐specific PCR and 16S rRNA sequencing, and caused typical Edwardsiellosis signs in experimentally infected bass. Intraperitoneal challenge caused dose‐dependent mortality, reaching 100%, 50% and 20% at 3 × 10 7 , 3 × 10 6 and 3 × 10 5 CFU/fish, respectively, and produced severe lesions in the liver, spleen and intestine. B202203044 displayed resistance to several commonly used antibiotics but remained susceptible to trimethoprim and cotrimoxazole. A genome assembly of strain B202203044 (3.85 Mb; 3395 predicted coding sequences) encoded 21 predicted two‐component‐system‐associated modules, complete type III and type VI secretion‐system gene clusters and 143 putative virulence‐associated genes. CAZy annotation identified 209 predicted glycosyltransferase‐associated genes, suggesting a substantial repertoire of functions related to glycoconjugate synthesis. Comparative genomics with four published E. piscicida genomes revealed high overall synteny but distinct sets of strain‐specific gene families. To explore host responses, we infected a largemouth bass cardiac cell line and performed RNA‐seq. Infection induced 4276 differentially expressed genes (2258 upregulated and 2018 downregulated; false discovery rate (FDR) <0.05), with enrichment of innate immune (mitogen‐activated protein kinase (MAPK), retinoic acid‐inducible gene I (RIG‐I)‐like, cytosolic DNA‐sensing, apoptosis/necroptosis, phagosome) and adhesion/cytoskeleton‐related pathways (extracellular matrix (ECM)–receptor interaction, focal adhesion). qPCR of eight representative genes confirmed RNA‐seq trends. Together, these data provide an integrated genomic and host transcriptomic characterization of a virulent E. piscicida strain from farmed largemouth bass.