Microbiological characteristics of Vibrio scophthalmi isolates from diseased olive flounder Paralichthys olivaceus

In 2005, massive mortality occurred in olive flounder Paralichthys olivaceus farms in Korea, and five isolates were collected from diseased fish. In this study, microbiological and pathogenic characteristics of these isolates were studied. The isolates gave negative results in lysine and ornithine decarboxylase, ortho-nitrophenyl-??-galactoside, and citrate tests, and positive results in urease, esculinase, and nitrate reduction tests. The isolates produced acid from adipate, fructose, d-glucose, and maltose, and gave positive results in alkaline phosphatase, esterase lipase, leucine arylamidase, and naphthol-AS-BI-phosphohydrolase. According to genetic analysis, 16S rRNA gene sequences showed 98?C100?% identity with both Vibrio scophthalmi and V. ichthyoenteri. The dnaJ gene sequences presented a higher identity with V. scophthalmi than with V. ichthyoenteri. Thus, the isolates were identified as V. scophthalmi. Pathogenicity of the five isolates in olive flounder was different and LD₅₀ values were from 10⁶ to 10⁸?CFU/g fish. Symptoms included darkening of skin, hemorrhage of liver and intestine, ascites, and distended abdomen. Histopathological changes included hemopoiesis dilatation and epithelial hyaline droplets in kidney, macrophage infiltration and ellipsoid dilatation in spleen, vascular dilatation, submucosal edema, and serosa inflammation of intestine. Cumulative mortality was 25?% for fish singly infected by isolate A19008 or Streptococcus parauberis, and increased to 87.5?% in super-infection group with these two pathogens.