Dietary oregano essential oil improved the immune response, activity of digestive enzymes, and intestinal microbiota of the koi carp, Cyprinus carpio

The objective of this research was to evaluate the effects of dietary oregano essential oil (OEO) on the nonspecific immunity and intestinal bacterial community of Cyprinus carpio (C. carpio). We randomly assigned 300 fish to one of the four treatments: basal diet (0 mg/kg OEO, control), OEO-L (basal diet plus 500 mg/kg OEO), OEO-M (basal diet plus 1500 mg/kg OEO), and OEO-H (basal diet plus 4500 mg/kg OEO). Blood and intestinal samples were collected at the end of the experiment to investigate the immune response, digestive enzyme activity levels, and intestinal microbiota. Dietary supplementation with OEO significantly increased the levels of lysozyme (P < .0001), complement C3 (P < .0001), complement C4 (P < .0001), superoxide dismutase activity (SOD; P < .0001), and glutathione peroxidase (GPx; P < .0001), but significantly decreased malonaldehyde (MDA) levels (P < .0001). OEO supplementation also significantly increased the activity levels of protease (P < .0001), lipase (P < .0001), and amylase (P = .0071), but significantly downregulated the relative expression of tumor necrosis factor alpha (TNF-alpha) (P = .0002) and transforming growth factor beta (TGF-beta) (P = .0064). After Aeromonas hydrophila (A. hydrophila) injection, the 7-d cumulative survival rate of C. carpio was significantly increased by oral OEO administration (P = .005). Bacterial community composition in the OEO-H group was distinct from that in the control (ANOSIM, R = 0.3165, P = .011). We identified 22 taxa, which were differentially abundant between the OEO-H group and the control, as potential biomarkers. The genera Propionibacterium, Brevinema, and Cotynebacterium_1 were enriched in the OEO-H group, whereas Vibrio was enriched in the control. Thus, in C. carpio, dietary OEO increased digestive enzyme activity and antioxidant capability, stimulated immunomodulatory effects, and enhanced disease resistance. These beneficial effects were probably due to OEO-mediated alternations in the structure of the C. carpio gut microbiota.