Isolation and identification of the pathogentic bacterium causing red body from cultured Turbot Scophthalmus maximus

A dominant strain of the bacteria causing red body of turbot was isolated from the diseased turbot and designated as H1.In an artificial infection test ,all fish of the experimental group died in 15 days after intramuscularly being injected with a high bacterial suspention(1.41×109CFU/mL), and the low bacterial suspention(1.41×CFU103/mL)did not cause any death, while the control group showed no signs all through the experimentation. The LD50 was calculated as 2.82×105CFU/mL.The moribund experimental fish had gross signs similar to the natural infected fish. The bacterium re- isolated from the challenged fish also had the same characteristics as H1, which proved that the isolated H1 was the pathogenic bacterium that triggered this red body disease. Different methods were used to identify the pathogenic bacterium. The bacterium is negative, rod in shape, peritrichous flagella under electronic microscope. Traditional biochemical identification revealed that H1 exhibited relatedness to Edwardsiella tarda, while the identification result by API 32E system indicated that H1 was E. tarda with 99.9% reliability . Its 16S ribosomal RNA gene sequences were analyzed , and it is shown from the anlysis results that this strain of bacterium has a close phylogenetic relation (99%)to E.tarda. In summary, the pathogenic bacterium was identified as E. tarda.