Edwardsiella piscicida-like pathogen in cultured grouper

An Edwardsiella sp. was isolated from the kidney of diseased groupers (Epinephelus aeneus and E. marginatus) cultured in Eilat (Israel, Red Sea). Affected fish presented a severe suppurative nephritis with large abscesses occasionally spreading into the surrounding musculature. Biochemical profiles and phenotypic comparisons failed to provide a clear identification to the species level, and genetic analysis of the 16S subunit failed to discriminate between Edwardsiella piscicida, E. tarda and E. ictaluri. Analysis of the gyrB gene, however, placed the grouper isolates into the E. piscicida-like group, a newly recognized taxon which also encompasses the non-motile strains previously classified as atypical E. tarda. Initial genomic analysis revealed the presence of the Edwardsiella type 3 secretion system (T3SS) but also revealed a pathogenicity island encoding a second T3SS with homology to the locus of enterocyte effacement of Escherichia coli. Further analysis revealed 3 different type 6 secretion systems that were also present in all sequenced isolates of Edwardsiella piscicida-like strains. Based on estimated DNA-DNA hybridization values and the average nucleotide index, the grouper strain fits into the E. piscicida-like phylogroup described as E. anguillarum sp. nov. The peculiarities associated with this isolate and the association of other conspecific piscine isolates from multiple marine and brackish water species suggest a link of the entire E. piscicida-like phylogroup to the marine environment.