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

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 × 107, 3 × 106 and 3 × 105 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.