Vitamin K2 emerges as the key mediator: Cetobacterium somerae ZNN-1 increases muscle protein deposition and improves liver health in Nile tilapia (Oreochromis niloticus)

Abstract Background Cetobacterium somerae ( C. somerae ) is a common indigenous bacterium in the intestine of freshwater fish. Studies have shown that it has the potential to promote protein deposition, but the underlying mechanisms remain unclear. Results Nile tilapia were fed with C. somerae ZNN-1 (10 8 CFU/g feed), which significantly increased the carcass ratio, reduced the hepatosomatic index, and decreased whole-body lipid content. Supplementation of C. somerae ZNN-1 significantly increased the crude protein content in muscle, promoted glucose uptake and utilization in muscle tissue, and activated the phosphorylation of S6K/S6 in muscle tissue. C. somerae ZNN-1 supplementation significantly decreased hepatic total lipid, triglyceride, and free fatty acid contents. Further analysis revealed that C. somerae ZNN-1 supplementation markedly activated the phosphorylation of hepatic AMPK and upregulated the expression of genes involved in hepatic lipolysis and fatty acid β-oxidation. Integrated serum metabolomic, bacterial genomic, and gut metagenomic analyses revealed that C. somerae ZNN-1 synthesized chorismate (CHA), which serves as a precursor for gut microbiota to produce vitamin K 2 (VK2). In vitro experiments demonstrated that VK2 activated the S6K/S6 pathway to promote protein synthesis, while stimulating AMPK phosphorylation and activating lipid catabolism to reduce fat accumulation. Conclusions These findings provide a theoretical basis for the application of C. somerae ZNN-1 in enhancing edible protein content and reducing fat deposition of aquatic animals.