Genotypic, phenotypic, and serologic characterization of Edwardsiella ictaluri isolates from catfish and ornamental fish from the southeastern USA

This study explores the phenotypic, genotypic, and serological differences among Edwardsiella ictaluri isolated from farm-raised catfish Ictalurus spp. and ornamental fish (Danio spp. and Puntius tetrazona) in the USA. Plasmid analyses revealed diverse plasmid profiles, with varied antimicrobial resistance genes indicating diverse resistance and virulence mechanisms. Analysis of complete genome sequences revealed multiple virulence factors, including the Type III and Type VI secretion systems, with varied presence between the catfish and ornamental derived strains. Molecular analyses, including multi locus sequence analysis (MLSA) and repetitive extragenic palindromic-PCR, identified discrete genotypic differences, identifying an optimally informative MLSA scheme for demonstrating intraspecific relationships among E. ictaluri conspecifics. Serological assessments revealed marked differences between catfish and ornamental isolates, indicating distinct antigenic profiles and suggesting that E. ictaluri vaccines derived from catfish isolates may have limited efficacy against ornamental isolates, supporting anecdotal reports from the industry. This research advances our understanding of this important bacterial fish pathogen, laying a foundation to help guide approaches in vaccine design and disease mitigation in catfish and ornamental fish moving forward.