Bacterial Septicaemia And Examination Of Pathogen From Flounder, Paralichthys Olivaceus L.

Flounder, Paralichthys olivaceus,is the main farmed species in the aquaculture industry of northern China. Recently, more and more bacterial diseases of flounder occurred, and the spread of some pathogenic bacteria is wide because of rearing large scale, stocking high density, lacking formal vaccination and introduction, lacking effective control measures to infectious diseases, and easy transmission of pathogenic bacteria in special water environment of fishes. Infection by pathogenic bacteria often exhibited widespread areas, high morbidity and mortality, and caused large economic damages to aquaculture production.Recently, we conducted examination to seven cases of flounder. The results showed that all fishes expressed bacterial septicaemia. The phenotypic information of 130 pure cultures, including morphological characteristics, physiological and biochemical characteristics, the G+C ratio of the DNA for representative strains, were studied extensively. In addition, molecular identification analysis of the nucleotide sequence of the 16S rRNA gene was applied, and the phylogenetic tree representing genetic relatedness among the isolates and publicly available 16S rRNA gene sequence from GenBank database was constructed.130 strains Edwardsiella tarda (20 strains were negative, 110 strains were positive) were examined in this study, the representative strains (HC010907-1 and HC010830-1) were selected for the molecular identification by sequencing partial 16S rRNA gene. The results indicated that the sequenced 16S rRNA gene of strain HC010907-1 (GenBank accession no. AY775313) is 1414 bp in length, the sequenced 16S rRNA gene of strain HC010830-1 (GenBank accession no. AY775314) is 1419 bp in length. 12 strains were identified as Vibrio ichthyoenteri, the sequenced 16S rRNA gene of the representative strain (HQ010223-1) (GenBank accession no. DQ003270) is 1429 bp in length. 10 strains were identified as Lactococcus garvieae, the sequenced 16S rRNA gene of the representative strain (T030817-1) (GenBank accession no. DQ010113) is 1427 bp in length.The nine cases of flounder (seven cases were infected by E. tarda; one was infected by V. ichthyoenteri; one was infected by L. garvieae) were examined systematically, including clinical signs, pathological changes and biological characterization of pathogenic bacteria. In addition, we isolated the serum homology, clarified the serotype of isolates and conducted the serological methods for identification. The rabbit antiserum was raised against the selected representative strains. Examinations of immune serology, including the slide agglutination tests and indirect fluorescent antibody technique, were conducted. The result showed that this method was feasibility. Artificial infection experiments of representative strains from different farms were conducted, the results showed the corresponding primitive causal agent of the diseases. The detective technology was constructed in characteristics of diseases, isolation and identification of pathogen, and examination of immunology. Sensitive antimicrobial agents for control these bacteria were selected by susceptibility tests. Data and method were obtained for examination and control of bacterial septicaemia, which caused by Edwardsiella tarda, Vibrio ichthyoenteri and Lactococcus garvieae.