Phylogenetic investigation of Edwardsiella tarda with multilocus sequence typing (MLST) and pulsed field gel electrophoresis (PFGE) typing methods

• PFGE and MLST methods were established to reveal the relationships of E. tarda.
• E. tarda isolated from humans and fish could be clustered into two clades.
• E tarda turbot/flounder isolates were distinct from eel isolates in China.

Edwardsiella tarda is the etiologic agent of Edwardsiellosis, a devastating fish disease prevailing in worldwide aquaculture industries. Here we developed the SpeI-based pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) assays to investigate the genetic diversity of Edwardsiella isolates obtained from aquaculture farms and to get insights into the epidemiological relationships of E. tarda isolates in China. The results showed a strong congruence of clustering profiles obtained by PFGE and MLST methods. This investigation also indicated distinct phylogenetic profiles of E. tarda isolates from different hosts and different geographical regions in China with the following conclusions: i) E. tarda isolates collected from flatfish (turbot or flounder) in Northern China were phylogenetically distinct from the eel isolates from Southern China, and ii) E. tarda isolates from humans and fish (except strain DT from Oscar fish) were clustered into two distinct clades. dnaK-based phylogenetic analysis of the 67 Edwardsiella strains from all over the world further confirmed the above inference as well as the previous findings that there is misclassification in Edwardsiella species and Edwardsiella ictaluri is close to fish isolates of E. tarda while showing distant phylogenetic relationship to human isolates of E. tarda. Our data highlighted the intraspecies relationships of E. tarda and indicated potential application of PFGE and MLST methods in tracing source of infection and enhancing understanding of epidemiological relationships among isolates of various origins.