Influences of immersion bathing in Bacillus velezensis DY-6 on growth performance, non-specific immune enzyme activities and gut microbiota of Apostichopus japonicus

In this study, the influences of immersion bathing in different concentrations of Bacillus velezensis DY-6 on the body weight gain rate and non-specific immune enzyme activities of the coelom fluid of sea cucumber ( Apostichopus japonicus ) were determined in order to obtain the optimum bacterial concentration. The gut microbiota change in A. japonicus was then analyzed through high-throughput sequencing during the immersion bathing in B. velezensis DY-6 at the optimum concentration for 49 d. The results illustrate that the body weight growth rate of all bathing groups was higher than that of the control. The highest growth rate (25.3%) was achieved when the bacterial concentration was 1×10 3 CFU/mL. The activities of non-specific immune enzymes (ACP, AKP, SOD and LZM) of all bathing groups increased, and the activities of the enzymes of groups bathed with the bacterium at 1×10 3 and 1×10 4 CFU/mL reached the highest on day 21 and day 28. Taking the growth rate and economic cost into consideration, the optimum concentration of B. velezensis DY-6 was 1×10 3 CFU/mL. The influences of immersion bathing in B. velezensis DY-6 at 1×10 3 CFU/mL on the gut microbiota of A. japonicus were then evaluated through 16S rDNA sequencing analysis. Results showed that the gut microbiota changed with the addition of B. velezensis DY-6, and the richness and diversity of the gut microbiota peaked twice on day 14 and day 21, respectively. In association with the non-specific immune enzyme activities and if day 28 was selected as the dividing point, the community structure of the gut microbiota could be obviously divided into two types. The correlation analysis revealed that the non-specific immune enzyme activities were correlated significantly to some gut bacteria (in the phyla Firmicutes, Proteobacteria, and Bacteroidetes) after immersion bathing in B. velezensis DY-6. Our findings will provide the theoretical foundation for probiotic application in sea cucumber farming.