Virulence and Molecular Typing of Vibrio harveyi Strains Isolated from Cultured Dentex, Gilthead Sea Bream and European Sea Bass

Vibrio harveyi was isolated from internal organs or ulcers of diseased and apparently healthy gilthead sea bream (Sparus aurata) and European sea bass (Dicentrarchus labrax) cultured in several fish farms located on the Spanish Mediterranean coast. The prevalence of the bacterium was significantly higher in European sea bass than in gilthead sea bream, and was closely related to the season in both fish species, occurring almost exclusively on warm months (June to November). After phenotypic characterization, a selection of forty five isolates from gilthead sea bream, sea bass, and several isolates previously obtained from common dentex (Dentex dentex) of the same area, were molecularly typed by automated ribotyping and random amplified polymorphic DNA (RAPD) analysis. Cluster analysis of data established 8 RAPD types and 13 ribotypes among wild isolates, and the combination of both techniques allowed to define fourteen different groups and a clear discrimination of all outbreaks and samplings. Several strains isolated from diseased gilthead sea bream and sea bass and also from asymptomatic sea bream, were tested for virulence in both fish species by intracoelomic injection. All the isolates (11) were pathogenic for sea bass, with nine out of the eleven LD50 values ranging from 1.5 × 105 to 1.6 × 106 cfu/fish. Gilthead sea bream was unaffected by the seven tested strains, even by those more virulent for sea bass, and only one strain caused a 10% mortality at 4.2 × 107 cfu/fish. This is the first report on virulence of V. harveyi for sea bass.
This work was supported by research grants from the Spanish Ministerio de Educación, Cultura y Deporte MAR-98/1000 C02 and FEDER Program 1FD97-979-C02. We are thankful to M.L. Alonso for technical assistance in the samplings.