Bacillus baekryungensis MS1 regulates the growth, non-specific immune parameters and gut microbiota of the sea cucumber Apostichopus japonicus

Winter is a high incidence period of skin ulceration syndrome in the sea cucumber Apostichopus japonicus. Disease control during the overwintering of sea cucumber can help increase yield and reduce losses. The purpose of this study was to study the effect of the low temperature-resistant probiotic Bacillus baekryungensis MS1 on the growth and immune parameters of sea cucumbers and preliminarily investigate the molecular mechanism of the effects. A low temperature-resistant bacterium, B. baekryungensis MS1, was isolated from a sea cucumber pond in winter and used for culture experiments. After 10 days of prefeeding, the experiment was divided into the control group (fed with commercial diet) and the MS1 group (fed with diet containing B. baekryungensis MS1 at 10(7) cfu g(-1)) for a total of 60 days. The specific growth rate was measured at the end of the culture period to evaluate the growth performance of the sea cucumber. Samples were taken on days 30 and 60 to determine the immune parameters (including superoxide dismutase activity, catalase activity, alkaline phosphatase activity, acid phosphatase activity, nitric oxide synthetase activity, phagocytosis and respiratory burst), aquaculture water microbiota and gut microbiota of the sea cucumber. Finally, transcriptome sequencing and qRT-PCR verification of the two groups of sea cucumbers were performed to study the mechanism of B. baekryungensis MS1 to improve the immunity of the sea cucumber. The results showed that after 60 days of feeding, B. baekryungensis MS1 significantly improved the growth performance and immune enzyme activity and formed a healthier structure of the gut microbiota in the sea cucumber. The challenge test showed that B. baekryungensis MS1 significantly reduced the mortality of sea cucumbers infected with Vthrio splendidus. Transcriptome and gene expression analysis indicated that B. baekryungensis MS1 activated the ubiquitin-mediated proteolysis pathway and inhibited the mTOR signaling pathway to regulate the immunity of the sea cucumber. In summary, the low temperature-resistant bacterium B. baekryungensis MS1 could be applied for the aquiculture of sea cucumber in winter to improve health status and resist pathogenic bacteria such as V. splendidus.