Comparative genomics reveals that a fish pathogenic bacterium Edwardsiella tarda has acquired the locus of enterocyte effacement LEE through horizontal gene transferReport as inadecuate

Comparative genomics reveals that a fish pathogenic bacterium Edwardsiella tarda has acquired the locus of enterocyte effacement LEE through horizontal gene transfer - Download this document for free, or read online. Document in PDF available to download.

BMC Genomics

, 14:642

Comparative and evolutionary genomics


BackgroundEdwardsiella tarda is an enterobacterium which causes edwardsiellosis, a fatal disease of cultured fishes such as red sea bream, eel, and flounder. Preventing the occurrence of E. tarda infection has thus been an important issue in aquaculture. E. tarda has been isolated from other animals and from many environments; however, the relationship between the genotype and evolutionary process of this pathogen is not fully understood. To clarify this relationship, we sequenced and compared the genomes of pathogenic and non-pathogenic E. tarda strains isolated from fish, human, and eel pond using next-generation sequencing technology.

ResultsEight strains of E. tarda were sequenced with high accuracy >99.9% with coverages from 50- to 400-fold. The obtained reads were mapped to a public reference genome. By comparing single nucleotide and insertion-deletion polymorphisms, we found that an attenuated strain of E. tarda had a loss-of-function mutation in a gene related to the type III secretion system T3SS, suggesting that this gene is involved in the virulence of E. tarda. A comprehensive gene comparison indicated that fish pathogenic strains possessed a type VI secretion system T6SS and pilus assembly genes in addition to the T3SS. Moreover, we found that an E. tarda strain isolated from red sea bream harbored two pathogenicity islands of T3SS and T6SS, which were absent in other strains. In particular, this T3SS was homologous to the locus of enterocyte effacement LEE in enteropathogenic and enterohemorrhagic Escherichia coli. Evolutionary analysis suggested that this locus, here named Et-LEE E. tarda LEE, was introgressed into the E. tarda genome through horizontal transfer.

ConclusionsWe found significant differences in the presence-absence of virulence-related genes among E. tarda strains, reflecting their evolutionary relationship. In particular, a single genotype previously proposed for fish-pathogenic strains may be further divided into two subgroups. Furthermore, the current study demonstrated, for the first time, that a fish pathogenic bacterium carried a LEE-like pathogenicity island which was previously reported only in zoonotic pathogenic enterobacteria. These findings will contribute to the exploration of strain-specific drug targets against E. tarda in aquafarms, while also shedding light on the evolution of pathogenesis in enterobacteria.

Electronic supplementary materialThe online version of this article doi:10.1186-1471-2164-14-642 contains supplementary material, which is available to authorized users.

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Author: Yoji Nakamura - Tomokazu Takano - Motoshige Yasuike - Takamitsu Sakai - Tomomasa Matsuyama - Motohiko Sano


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