Isolation and identification of fish pathogen Edwardsiella tarda from mariculture in China

The causative agent was isolated from diseased turbots (Scophthalmus maximus) stricken by a high‐mortality outbreak of bacterial septicaemia occurring in a mariculture farm in Yantai, a northern coastal city of China. Seven pure isolates, namely EH‐15, EH‐103, EH‐107, EH‐202, EH‐203, EH‐305 and EH‐306, belonged to Edwardsiella tarda. The phenotypic features of the cultures were analysed extensively. Three of the isolates showed high 16S rDNA sequence similarities with E. tarda sequence (GenBank accession no. EF467289). However, unlike the E. tarda ATCC 15947, all the isolates, except EH‐15, contained a novel large plasmid sized about 23.7 kb. Furthermore, pathogenicity of the isolates was addressed by experimental challenges with fish models. The isolates exhibited strong virulence to swordtail fish with LD₅₀ ranging between 3.8 × 10³ and 3.8 × 10⁵ CFU g^?1, and EH‐202 displaying the lowest LD₅₀ value among them. Antibiotic susceptibilities of E. tarda isolates were assayed. Compared with E. tarda ATCC 15947, the isolates displayed strong resistance to chloramphenicol, and the probable dominant chloramphenicol resistance determinant was cat III. Depicting the main biological properties of turbot‐borne E. tarda strains in China, the study provided useful information for further unveiling their pathogenic mechanisms.