Probiotic Potential of Indigenous Cetobacterium somerae R9 in Mud Crab (Scylla paramamosain)

Probiotics provide an efficient and relatively safe method for preventing disease and increasing production in aquaculture. During the screening of beneficial bacteria in the economically important mud crab (Scylla paramamosain), an indigenous gut strain, Cetobacterium somerae R9, was isolated for the first time as a butyrate producer. This study aimed to evaluate the probiotic potential of C. somerae R9 both in vitro and in vivo. In vitro assays revealed that C. somerae R9 exhibited grow at pH 7-9, NaCl concentrations of 0.5-2.5%, and bile salt concentrations of 0.4-1.0%, and displayed susceptibility to most of the antibiotics tested. In the in vivo study, dietary supplementation with C. somerae R9, either alone or in combination with prebiotics (resistant starch and galactooligosaccharides), enhanced growth, improved antioxidative status (evidenced by elevated SOD and CAT activity and reduced MDA content), reduced hepatopancreatic damage (reduced AST activity), and maintained intestinal integrity. Supplementation also selectively enriched beneficial gut microbiota (e.g., members of Fusobacteriota). Transcriptome analysis showed that C. somerae R9 appeared to activate the PI3K-Akt signaling pathway, focal adhesion, and ECM-receptor interaction, while the combination of C. somerae R9 and prebiotics activates complement and coagulation cascades, amino sugar and nucleotide sugar metabolism, and protein digestion and absorption. Furthermore, mud crabs fed diets supplemented with C. somerae R9 exhibited significantly higher survival after challenge with Vibrio parahaemolyticus, with the synbiotic providing greater benefits than the probiotic alone. These findings collectively suggest that C. somerae R9 is a promising probiotic candidate (used either alone or in combination with prebiotics) for mud crab aquaculture.