Effects of MS-222 on the physiological and biochemical response, histomorphology, gut microbiota, and flesh quality in largemouth bass (Micropterus salmoides).

This study investigated the effects of MS-222 on biochemical indices, antioxidant status, nonspecific immune responses, histomorphology (gill and liver), gut microbiota, and flesh quality in largemouth bass (Micropterus salmoides). The anesthetic effects of MS-222 at concentrations of 20, 40, 60, 80, and 100 mg/L were first evaluated in fish (~ 468.34 g). Deep sedation was achieved at 40 mg/L, which was identified as suitable for live transportation. Fish were then exposed to 40 mg/L MS-222 for 24 h and transferred to clean water for recovery. Serum, gill, liver, intestine, and dorsal muscle samples were collected at 6, 12, and 24 h during exposure and at the same intervals post-recovery. Serum cortisol, glucose, LDH, and AST levels did not change significantly during sedation or recovery, although BUN levels increased significantly. Antioxidant enzyme activities were not significantly affected, whereas malondialdehyde (MDA) decreased during sedation. MS-222 caused atrophy and hypertrophy of gill filaments, reduced nucleoli, and increased vacuolation in hepatopancreas cells; these changes were not fully reversed after 24 h of recovery. Gut microbial composition was altered, with a reduction in beneficial genera such as Cetobacterium. Flavor-related compounds, including sweet and umami amino acids, betaine, and nucleotides, increased during sedation and recovery. MS-222 also decreased the proportion of monounsaturated fatty acids (MUFAs) and increased polyunsaturated fatty acids (PUFAs). Electronic nose analysis showed distinct changes in muscle flavor during sedation, which diminished during recovery. Overall, MS-222 at 40 mg/L significantly influenced the physiology, biochemistry, and flesh quality of largemouth bass, providing a reference for its application in live transportation.