Phenotypic and Genotypic Characterization and Comparison of Edwardsiella ictaluri Isolates Derived from Catfish and Ornamental Fish Species

1,4Divya Johnson, 2,4Matt J. Griffin, 2,4Ethan T. Woodyard, 2,4Lester H. Khoo, 3,4Geoffrey C. Waldbieser, 5Roy P. E. Yanong, 6John P. Hawke and 1,4Suja Aarattuthodi

 1 Department of Wildlife, Fisheries, and Aquaculture, Mississippi State University, MS, 38776, 2 College of Veterinary Medicine, Mississippi State University, Stoneville, MS 38776, 3 U.S. Department of Agriculture, Agriculture Research Service, Stoneville, MS 38776, 4Thad Cochran National Warmwater Aquaculture Center, Delta Research and Extension Center, Mississippi State University, Stoneville, MS 38776, 5Tropical Aquaculture Laboratory, University of Florida, Ruskin, FL, 33570, 6School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA  70803

ABSTRACT

The gram negative enteric pathogen, Edwardsiella ictaluri, causes significant economic losses to the U. S. catfish industry. Historically regarded host-specific to catfish, E. ictaluri outbreaks have been reported in other aquacultured species as well, including ornamental fish raised in the southeastern U. S. This expanding host range necessitated a comprehensive phenotypic and genotypic characterization of E. ictaluri isolates from U.S. catfish (n=50) and ornamental (n=42) aquaculture. Morphological, biochemical, and protein profiles among isolates were largely similar. While the ornamental E. ictaluri isolates autoaggregated in broth, the catfish derived isolates displayed turbid growth, with some of the isolates showing a slightly increased motility in semisolid media. Ornamental fish-derived isolates demonstrated robust zones of inhibition (>20 mm) for Aquaflor® [florfenicol], Terramycin® [oxytetracycline], and Romet® [5:1 sulfadimethoxine/ormetoprim], suggesting a lack of acquired resistance. Similarly, most catfish isolates demonstrated marked susceptibility to these antibiotics and other tested antimicrobials. A selected group of catfish-derived isolates displayed reduced susceptibility to Aquaflor®, Terramycin®, and Romet® (as reported by the Aquatic Research and Diagnostic Laboratory of the Thad Cochran National Warmwater Aquaculture Center at Stoneville, MS). Between host groups, there were stark differences in plasmid profiles of wild-type isolates. However, within groups, native plasmid profiles were largely homogeneous and consistent with previous published reports. Two distinct multi-drug resistant (MDR) plasmids were identified from the selected catfish-derived isolates, one (135,268 bp) which was previously reported from E. ictaluri and another (117,449 bp) reported from the closely related E. piscicida. Ornamental isolates were devoid of  MDR plasmids.  These distinct, yet stable plasmid arsenals among the two groups suggest limited Edwardsiella plasmid trafficking between the catfish and ornamental fish industries.  Whole genome analysis revealed discrete genomic differences of E. ictaluri among host groups, including the presence of a unique Type 4 secretory pathway and putative phage elements in ornamental fish derived isolates. Phylogenetic signal assessment was performed on three previously established MLST schemes initially developed to establish interspecfic relationships among Edwardsiella spp. Signal assement of individual gene components from each MLST approach identified optimal MLST gene targets for delineating intraspecific variability among E. ictaluri strains.  These data evinced catfish and ornamental fish-derived isolates represent two discrete phyletic lineages. Clonality of these groups indicates a high degree of genetic stability among E. ictaluri isolates within these two respective industries. Study results provide important baseline data that can be used to enhance management and preventative strategies for this enigmatic pathogen affecting two diverse U.S. aquaculture industries.