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

Authors: ROSE, DIVYA - Mississippi State University; Lafrentz, Benjamin; WOODYARD, ETHAN - Mississippi State University; WALDBIESER, GEOFFREY - US Department Of Agriculture (USDA); WISE, DAVID - Mississippi State University; KHOO, LESTER - Mississippi State University; TEKEDAR, HASAN - Mississippi State University; POUDEL, ASHMITA - Mississippi State University; DHARAN, VANDANA - Mississippi State University; WARE, CYNTHIA - Mississippi State University; LEWIS, MARSHA - Mississippi State University; SOTO, ESTEBAN - University Of California, Davis; HAWKE, JOHN - Louisiana State University; YANONG, ROY - University Of Florida; Aarattuthodi, Suja; GRIFFIN, MATT - Mississippi State University

Submitted to: Diseases of Aquatic Organisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2025
Publication Date: 2/12/2026
Citation: Rose, D., Lafrentz, B.R., Woodyard, E.T., Waldbieser, G.C., Wise, D.J., Khoo, L.H., Tekedar, H., Poudel, A., Dharan, V., Ware, C., Lewis, M., Soto, E., Hawke, J.P., Yanong, R.P., Aarattuthodi, S., Griffin, M.J. 2026. Genotypic, phenotypic, and serologic characterization of Edwardsiella ictaluri isolates from catfish and ornamental fish from the southeastern USA. Diseases of Aquatic Organisms. 165:dao03897. https://doi.org/10.3354/dao03897.
DOI: https://doi.org/10.3354/dao03897

Interpretive Summary: Edwardsiella ictaluri is a gram-negative bacterium that poses a significant threat in channel catfish farming. Although initially thought to only infect channel catfish, it has since been found in other fish species, including non-ictalurid catfish, green knife fish, rainbow trout, and ornamental fish. This study aimed to compare and characterize E. ictaluri isolates from catfish and ornamental fish phenotypically, genotypically and serologically. Phenotypically, bacteria from both catfish and zebrafish were similar in terms of their colony morphology, biochemical profiles, and growth patterns with the main difference being that of zebrafish-derived E. ictaluri isolate auto aggregating at the bottom of broth cultures, while catfish-derived isolate did not. Antibiotic sensitivity tests showed that half of the catfish-derived E. ictaluri isolates were highly sensitive to three antibiotics: Terramycin® (oxytetracycline), Aquaflor® (florfenicol), and Romet® (ormetoprim-sulfadimethoxine), with clear inhibition zones greater than 20 mm. The other half of the catfish isolates showed reduced sensitivity, with inhibition zones less than 10 mm for at least 2 of the 3 tested antibiotics. Ornamental fish bacteria generally responded well to these antibiotics, though some showed intermediate sensitivity to Romet®. Analysis of minimal inhibitory concentrations of multiple antimicrobial drugs revealed that some antibiotics had no effect on bacterial growth even at their highest concentrations and zebrafish derived isolates showed varied responses to a wider range of antibiotics, reflective of the differences in antibiotic use in the two aquaculture industries (food fish and ornamental fish industry). Genetic analyses revealed differences between strains from catfish and ornamental fish. Plasmid profiling revealed that most antibiotic-resistant catfish derived E. ictaluri carried multi drug resistant plasmids. All the catfish derived isolates (both wild type and antibiotic resistant) had the previously described native plasmids of E. ictaluri, with some having additional smaller or larger plasmids. In contrast, ornamental fish isolates had different native plasmids of the length 3.3 kb and 3.9 kb, and no multi-drug resistant (MDR) plasmids were found in these isolates. The presence of genes of Type 3 Secretion System in catfish derived isolates and the absence of the same in ornamental fish derived isolates indicate the difference in virulence mechanism between these two groups. Genomic fingerprinting and multi-locus sequence analysis (MLSA) showed that the bacteria from catfish and ornamental fish formed distinct genetic clusters, highlighting differences in their DNA. Multi Locus Sequence Analysis (MLSA) looks into allelic differences in housekeeping genes of the pathogen and in the current study the bacterial isolates formed different phyletic groups based on host origin. Phylogenetic signal assessment of genes used in previously described MLSA schemes identified 8 informative genes among a total of 20 housekeeping genes that can parse out E. ictaluri isolates derived from different host origins into respective clades. Genome wide comparison between catfish and ornamental fish derived isolates revealed the presence of bacteriophage insertions in ornamental fish derived isolates which are absent in catfish derived E. ictaluri isolates. Comparative genomics also revealed differences in lipopolysaccharide biosynthetic clusters of the pathogen derived from two different host groups warranting the need for serological analysis. Serologically, 5 different protein profiles with slight differences were identified from the whole cell lysate banding patterns. Western blot analysis was used for determining antigen antibody interaction. While the sera obtained from catfish infected with catfish derived E. ictaluri reacted with antigens form catfish der

Technical Abstract: 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.