| Summary: | We previously demonstrated that <i>Lactobacillus casei</i> K17, isolated from Korean kimchi, has high antioxidant levels in vitro and in vivo. However, its effect on <i>Micropterus salmoides</i> is unknown. In this study, we investigated the impact of <i>L. casei</i> K17 supplementation on the lipid metabolism, antioxidant response, liver histology, and fillet quality of <i>M. salmoides</i>. We randomly assigned 450 <i>M. salmoides</i> (33.0 ± 0.5 g) to six diet groups for 69 days. The diets were as follows: 0.85% normal saline; 10% skim milk powder; 1 × 10<sup>8</sup> CFU/g live <i>L. casei</i> K17 (LB); 1 × 10<sup>8</sup> live <i>L. casei</i> K17 protected by skim milk powder (MB); 1 × 10<sup>8</sup> dead <i>L. casei</i> K17 (DB); and <i>L. casei</i> K17 fermentation supernatant. MB significantly improved the crude protein, total collagen, alkaline-insoluble collagen, fiber numbers, hardness, chewiness, and gumminess of <i>M. salmoides</i> fillets (<i>p</i> < 0.05). LB significantly improved crude protein and fiber numbers (<i>p</i> < 0.05). Furthermore, dietary supplementation with LB, MB, and DB maintained normal liver histology, preserved liver function, and increased hepatic and hemal antioxidant status by enhancing antioxidant enzyme activities. Meanwhile, the three diets also promoted lipid metabolism by increasing HDL-C effectiveness and reducing total cholesterol, triglyceride, and low-density lipoprotein cholesterol levels in serum and liver tissues, indicating that dietary supplementation with DB, LB, and MB had hypolipidemic effects on <i>M. salmoides</i>. MB and LB significantly improved fillet quality and LB, MB, and DB improved hemal and hepatic lipid metabolism and antioxidant response and reduced reactive oxygen species production, protecting <i>M. salmoides</i> hepatic cells from injury.
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