Location: Warmwater Aquaculture Research UnitTitle: Comparative analysis of Edwardsiella tarda isolates from fish in the eastern United States suggests the existence of two genetically distinct species, Edwardsiella tarda and Edwardsiella pseudotarda sp. nov) Author
Submitted to: Veterinary Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/28/2013
Publication Date: 8/1/2013
Citation: Griffin, M.J., Quiniou, S., Cody, T., Tabuchi, M., Ware, C., Cipriano, R.C. 2013. Comparative analysis of Edwardsiella tarda isolates from fish in the eastern United States suggests the existence of two genetically distinct species, Edwardsiella tarda and Edwardsiella pseudotarda sp. nov. Veterinary Microbiology. 165(3-4):358-372. Interpretive Summary: Edwardsiella tarda is a bacterial pathogen attributed to disease related losses in farm-raised catfish and cultured fishes worldwide. A survey was performed on multiples isolates of Edwardsiella tarda from 4 different fish species in the eastern United States, including channel catfish. This research revealed that bacteria previously described as Edwardsiella tarda based on biochemical testing were assembling into two very different genetic groups, representing two genetically distinct species of bacteria that are indistinguishable by conventional diagnostic methods. We proposed to split those two groups and denominate the second group as Edwardsiella pseudotarda. Although the biological consequences of these genetic differences is not currently known, this work will provide the framework for future research investigating the distinct characteristics of both groups and determine which factors are important in causing disease.
Technical Abstract: Edwardsiella tarda, a Gram-negative member of the family Enterobacteriaceae, is often implicated in significant losses in aquaculture facilities worldwide. Here, we assessed the intra-specific variability of a collection of E. tarda isolates from 4 different fish species in the eastern United States. All isolates used in this analysis were identified as E. tarda by traditional biochemical screening and two commercial biochemical identification kits. Repetitive sequence mediated PCR (rep-PCR) using 4 different primer sets (ERIC I & II, ERIC II, BOX, and GTG5) and multi-locus sequence analysis of 16S SSU rDNA, groEl , gyrA, gyrB, pho, pgi, pgm, and rpoA gene fragments identified two distinct genotypes of E. tarda (DNA group I; DNA group II). Isolates that fell into DNA group II demonstrated a higher degree of similarity to E. ictaluri than the other DNA group (I), which contained the reference E. tarda strain (ATCC #15947). Conventional PCR analysis using several published E. tarda-specific primer sets yielded variable results between the two DNA groups, with no amplification from some isolates for several primer sets. Surprisingly, these isolates were indistinguishable using conventional biochemical techniques, with all isolates demonstrating phenotypic characteristics consistent with E. tarda. Analysis using two commercial biochemical test kits identified multiple phenotypes, although no single characteristic could reliably discriminate between genetic groups. Additionally, anti-microbial susceptibility and fatty acid profiles did not demonstrate any remarkable differences. The significant genetic variation (<90% similarity at gyrA, gyrB, pho, phi and pgm; <40% similarity by rep-PCR) between these isolates suggests organisms traditionally identified as E. tarda represent two genetically distinct yet phenotypically indistinguishable species of Edwardsiella. Based on these genetic differences we propose that isolates falling into DNA group II be characterized as Edwardsiella pseudotarda sp. Nov