A new Edwardsiella taxon was recently described from fishes of Europe and Asia. Phenotypically similar to E. tarda, extensive genetic and phenotypic characterization determined this new strain does not belong to any established Edwardsiella taxa, leading to the adoption of a new taxon, E. piscicida. Concurrent research in the USA also identified 2 genetically distinct taxa within the group of organisms traditionally classified as E. tarda. Comparisons of gyrB sequences between US isolates and E. piscicida from Europe and Asia identified several US isolates with > 99.6% similarity to the gyrB sequence of the E. piscicida type strain (ET883) but < 87% similarity to the E. tarda type strain (ATCC #15947). A discriminatory PCR was developed for the identification of E. tarda and 2 genetic variants of E. piscicida (E. piscicida and E. piscicida-like species). Using these PCR assays, a survey was conducted of 44 archived bacterial specimens from disease case submissions to the Aquatic Research and Diagnostic Laboratory (Stoneville, MS, USA) between 2007 and 2012. All 44 isolates, originally identified phenotypically and biochemically as E. tarda, were identified as E. piscicida by PCR. Repetitive sequence-mediated PCR (rep-PCR) analysis of these archived specimens suggests they are largely homogenous, similar to what has been observed for E. ictaluri. The gyrB sequence data, coupled with the E. piscicida specific-PCR and rep-PCR data, confirms that E. piscicida has been isolated from fish disease cases in the southeastern USA. Moreover, our survey data suggests E. piscicida may be more prevalent in catfish aquaculture than E. tarda.

Edwardsiella tarda, a Gram-negative member of the family Enterobacteriaceae, has been implicated in significant losses in aquaculture facilities worldwide. Here, we assessed the intra-specific variability of E. tarda isolates from 4 different fish species in the eastern United States. 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 more similarity to E. ictaluri than DNA group I, which contained the reference E. tarda strain (ATCC #15947). Conventional PCR analysis using published E. tarda-specific primer sets yielded variable results, with several primer sets producing no observable amplification of target DNA from some isolates. Fluorometric determination of G+C content demonstrated 56.4% G+C content for DNA group I, 60.2% for DNA group II, and 58.4% for E. ictaluri. Surprisingly, these isolates were indistinguishable using conventional biochemical techniques, with all isolates demonstrating phenotypic characteristics consistent with E. tarda. Analysis using two commercial test kits identified multiple phenotypes, although no single metabolic characteristic could reliably discriminate between genetic groups. Additionally, anti-microbial susceptibility and fatty acid profiles did not demonstrate remarkable differences between groups. The significant genetic variation (<90% similarity at gyrA, gyrB, pho, phi and pgm; <40% similarity by rep-PCR) between these groups suggests organisms from DNA group II may represent an unrecognized, genetically distinct taxa of Edwardsiella that is phenotypically indistinguishable from E. tarda.

The δ-chain of catfish IgD was initially characterized as a unique chimeric molecule containing a rearranged VDJ spliced to Cμ1, seven C domain-encoding exons (δ1–δ7), and a transmembrane tail. The presence of cDNA forms showing splicing of δ7 to an exon encoding a secretory tail was interpreted to indicate that membrane (δm) and secreted (δs) forms were likely expressed from a single gene by alternative RNA processing. Subsequent cloning and sequence analyses have unexpectedly revealed the presence of three δ C region genes, each linked to a μ gene or pseudogene. The first (IGHD1) is located 1.6 kb 3′ of the functional Cμ (IGHM1). The second (IGHD3) is positioned immediately downstream of a pseudo Cμ (IGHM3P), ∼725 kb 5′ of IGHM1. These two δ genes are highly similar in sequence and each contains a tandem duplication of δ2-δ3-δ4. However, IGHD1 has a terminal exon encoding the transmembrane region, whereas IGHD3 has a single terminal exon encoding a secreted tail. The occurrence of IGHD3 immediately downstream of a μ pseudogene indicates that the putative δs product may not be expressed as a chimeric μδ molecule. Western blots and protein sequencing data indicate that an IGHD3-encoded protein is expressed in catfish serum. Thus, catfish δm transcripts appear to originate from IGHD1, whereas δs transcripts originate from IGHD3 rather than, as previously inferred, from a single expressed δ gene. The third δ (IGHD2) is associated with a pseudo Cμ (IGHM2P); its presence is inferred by Southern blot analyses.

Twelve cDNA libraries from two species of catfish have been sequenced, resulting in the generation of nearly 500,000 ESTs.

No abstract