Edwardsiella piscicida sp. nov., a novel species pathogenic to fish

This study describes a novel species within the genus Edwardsiella based on phenotypic and genetic characterization of fish pathogenic Edwardsiella isolates previously identified as E. tarda.

Phenotypic characterization, DNA-DNA hybridization and phylogenetic analysis of representative Edwardsiella isolates from fish previously identified as E. tarda were conducted and compared with E. tarda type strain (ATCC 15947(T)). Phenotypically, strains from fish grow with pin-point colonies producing slight β-haemolysis under the colony. In contrast to the E. tarda type strain, fish strains did not [corrected] degrade β-methyl-D-glucoside [corrected] (with the exception of NCIMB 2034), citric acid and L-proline. [corrected]. With the exception of strain ETK01, all fish strains were highly pathogenic to zebra fish, while ATCC 15947(T) and NCIMB 2034 were nonpathogenic. DNA-DNA hybridization (DDH) levels between representative fish isolates and the E. tarda type strain ranged from 15 to 43·6%, while NCIMB 2034 hybridised with the type strain at the level of 63·2%. DDH values between the various fish isolates ranged from 68·2 to 93·9% defining a new and separate DNA hybridization group differing from the E. tarda type strain consistent with the findings of phylogenetic analysis, in which the fish isolates comprised a separate clade.

Phenotypical and genetic characterizations demonstrated that Edwardsiella isolates from fish described in this study do not belong to the species E. tarda or any of the previously established taxa within the genus Edwardsiella. The fish related strains studied here (excluding NCIMB 2034) represent, therefore, a novel species within the genus Edwardsiella for which we propose the name Edwardsiella piscicida sp. nov, with strain ET883(T) (NCIMB 14824(T) = CCUG 62929) as the type strain.

The current finding will improve the diagnosis, understanding of the epidemiology and in establishment of effective control measures against this serious fish pathogen.