Abstract:Flatfish are the major industrial aquaculture marine fish species bred in North China. During the culturing process, members of this species are exposed to infection from a variety of pathogens. Diseases caused by bacterial pathogens are the major cause of harm to cultured flatfish in China. To investigate the bacterial pathogens that affect flatfish, we focused on 124 strains isolated from organs of diseased flatfish in Shandong, Jiangsu, Hebei, Tianjin and other similar places between 1999 and 2012. The 16S rRNA gene of the isolates was sequenced, analyzed using BLAST (GenBank), and subjected to phylogenetic analysis by using Mega 5.05. The results identified 66.90% of isolates as Vibrio (83 strains), 8.90% as Aeromonas (11 strains), 3.20% as Edwardsiella (4 strains), and other 15 genera (26 strains). According to the phylogenetic tree, 66 strains were identified as 16 species; the dominant species were V. alginolyticus, V. harveyi, V. anguillarum, V. damselae, V. furnissii, V. splendidus, A. allosaccharophila, A. salmonicida, and E. tarda. To evaluate the pathogenicity of isolates, the virulence of V. anguillarum and E. tarda strains isolated from diseased fish was further determined by experimental infection based on the 50% lethal dose (LD50) in turbot (Scophthalmus maximus). The results showed that seven V. anguillarum strains and three E. tarda strains were pathogenic to fish. The LD50 values were 105.1 to 106.8 CFU/fish for pathogenic V. anguillarum and 103.4 to 104.1 CFU/fish for pathogenic E. tarda. There were also two strains of V. anguillarum and one strain of E. tarda that showed low virulence to turbot, with an LD50 higher than 107 CFU/fish. The presented results provide significant information to ascertain the bacterial pathogens of flatfish, which is important for establishing strategies for the epidemiology, monitoring, and control of bacterial diseases.
