This study aimed to evaluate the effects of Mentha piperita methanolic extract (MPE) on Cyprinus carpio intestinal microbiota, including total microorganisms gram-negative bacteria, lactic acid bacteria, and fungi count. Liver and intestinal histopathology, and the activity of liver enzymes, were also used to evaluate the possible side effects of MPE. A total of 96 healthy C. carpio fries (76.76±20.26 g) were allocated to four treatment groups with three replications in a completely randomized design. The fries were fed with diets containing 0, 0.5, 1, and 2% extract for 56 days at the rate of 2% of body weight during the experiment. Results showed a significant decrease in total microorganisms, enteric gram-negative bacteria, and total fungi counts (P < 0.05). The total lactic acid bacteria count in 0.5% treatment was significantly lower than in control and 2% treated fish (P <0.05). MPE did not affect AST, leading to a significant increase in ALT levels. Simultaneously, ALP represented significantly higher activity in the control group (P > 0.05). Microscopic findings revealed marked lesions, including congestion and cell degeneration in the livers of the three groups of fish fed with the extract. The intestinal folds were shortened and blunted in the treatment groups. Furthermore, the intestinal mucosa was necrotic, and the lamina propria was significantly thickened with mononuclear inflammatory cells (P <0.05). Although MPE significantly affects intestinal microbiota, its consumption at 2% is not recommended for C. carpio due to the lesions made in the liver and intestine.

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