Isolation and Identification of the Pathogenic Bacteria of Skin Ulcerative Syndrome (SUS) from Apostichopus japonicus Juveniles in Summer and Study on Its Pathogenic Threshold

Abstract:

In the summer of 2013, there was an outbreak of skin ulcerative syndrome (SUS) in Apostichopus japonicus juveniles at a hatchery in Shandong Province. The symptoms included weak adhesion, rotten spines, viscera eversion, skin ulceration, and body autolysis. From the skin lesions of diseased juveniles we isolated a dominant strain HP130917A-1 with a median lethal concentration of 1.2×106 CFU/ml. This strain exhibited high pathogenicity and caused the same symptoms as SUS. The identification of this pathogen was verified using morphological observation, physiological and biochemical test, and 16S rDNA gene sequence analysis. The results demonstrated that HP130917A-1 was Vibiro alginolyticus and the causative pathogen associated with SUS. The microbial community structure was also analyzed using 16S rDNA gene sequencing, from which six major strains were found in the rearing system including V. alginolyticus, V. natriegens, V. parahaemolyticus, Bacillus subtilis, Bacillus thuringiensis and Staphylococcus equorum. V. alginolyticus was predominant among six major strains during the 50 days. On the 50th day of the experiment, the amount of V. alginolyticus reached the highest value of 1.31×105 CFU/ml in the substrates, which was considered as the pathogenic threshold of causing sea cucumber SUS. These results provided insights into the mechanisms of SUS and the health management of sea cucumber A. japonicus. Based on our findings we recommend healthy management strategies such as regular monitor of the amount of Vibrios, suitable frequency changing of the substrates, and the application of probiotics or herbal medicines.