Abstract:From December 2014 to December 2015, the flora population structural analysis and beneficial bacteria isolation and screening were carried out in large water ponds of a sea cucumber (Apostichopus japonicus) farm. Sixty-six bacterial strains were isolated from the water body and pond sediment. Two bacterial strains Pseudoalteromonas nigrifaciens and Vibrio splendidus associated with the “skin ulcerative syndrome” disease were used as indicators in the antagonistic effect trials. Based on the bacterial extracellular amylase, protease activities and challenge tests, one potential probiotic bacterium YQ-2 was obtained. The results showed that this strain had good ability to inhibit the proliferation of P. nigrifaciens and V. splendidus, evident by the diameter of inhibition zones, 22 mm and 24 mm, respectively; while the diameter of amylase and protease hydrolysis was 22 mm and 36 mm; In addition, the challenge tests revealed that the strain YQ-2 was safe to sea cucumber when it was challenged by bathing the bacterial strain at the concentration of 108 CFU/ml, or fed the food inoculated with the bacterial cells at the concentration of 108 CFU/g once daily for one month. The present study also found that among the sea cucumbers fed the food containing 108 CFU/g bacterial cells, the body weight increased significantly compared with the control group, with the relative growth rate as high as 39.31%. Homogenous analysis using 16S rDNA sequences indicated that the strain YQ-2 had 99% similarity with the Bacillus subtilis strain KLP2015. Thus, the strain YQ-2 was tentatively identified as Bacillus subtilis. The number of B. subtilis in the rearing water was counted in all four seasons and the year-round concentration was about 140–280 CFU/ml, which was higher than other strains. Meanwhile the dominance of YQ-2 was 4.2%, 3.5%, 2.6%, and 4.6% corresponding to Spring, Summer, Autumn and Winter. Furthermore, the dominance of YQ-2 was higher in Spring and Winter compared to Summer and Autumn. Nevertheless, the strain YQ-2 exhibited a strong antibacterial effect on the pathogens P. nigrifaciens and V. splendidus which were the main pathogenic agents causing the “skin ulcer syndrome” disease. Hence, the strain YQ-2 has great application for farming A. japonicus in large water ponds.
