Isolation, Identification, and Biological Characteristics of a Pathogenic Bacterial Strain from Cage-Cultured Black Rockfish (Sebastes schlegelii)

Abstract:

In July 2016, a severe disease outbreak occurred in offshore-cage cultured black rockfish (Sebastes schlegelii) in Changdao County, Shangdong Province, which resulted in significant mortality. The main symptom of diseased rockfish was skin ulcers on the body. In order to determine the cause of the disease, pathogenic bacteria were isolated from diseased fish and identified by various methods including morphological observation, traditional physiological and biochemical tests, and gyrB gene sequence alignment. A dominant bacterial strain was isolated from ulcerative skin and was marked as BZ01. The strain formed a subtransparent colony on TSB solid medium plate and a green colony on TCBS selective medium plate. Its shape was short and rod-like with a single polar flagellum when observed by transmission electron microscopy (TEM). An artificial infection experiment was carried out to verify its pathogenicity to black rockfish. The results demonstrated that this strain of BZ01 had strong pathogenicity to black rockfish and could cause death. The artificial infected black rockfishes showed the same symptoms as those of natural infection. The value of LD50 was calculated as 2.07106 CFU/ml. Analysis of the gyrB gene sequence suggested that the strain of BZ01 was most closely related to Vibrio. In gyrB gene sequencing, it clustered into Vibrio rotiferianus in the phylogenetic tree with 100% confidence. Based on these results, this strain was identified as V. rotiferianus. At the same time, the drug sensitivity test showed that this strain of BZ01 was highly sensitive to tetracyclines (tetracycline, doxycycline), quinolones (norfloxacin, ofloxacin, fleroxacin, nalidixic acid), coumarin (streptonivicin), and peptidyl transferase (florfenicol), while medium sensitive or resistant to macrolides, peptides, sulfonamides, β-lactams, and aminoglycosides. This is the first report that V. rotiferianus could infect offshore-cage cultured rockfish in China, which would provide an effective reference for disease control in cultured S. schlegelii.