Antibacterial and antioxidant activity of clove oil against Streptococcus iniae infection in Nile tilapia (Oreochromis niloticus) and its effect on hepatic hepcidin expression
This study was designed to evaluate the modulating effect dietary clove essential oil (CL) has on the antioxidant and immunological status of Nile tilapia following Streptococcus iniae (Si) infection. Fish were placed on either control or (1.5 and 3%) CL-supplemented diets for 4 weeks. After sampling, the remaining fish in the control group were divided into 2 groups: an unchallenged (negative control) and an Si-challenged positive control. On the other hand, the remaining fish in CL-supplemented groups were challenged with Si, and mortality was checked for two weeks before the final sampling. Serum immunological parameters, tissue antioxidants, and oxidative stress markers were determined. Moreover, hepatic hepcidin expression was also measured in different groups. The obtained results showed improvements in blood phagocytic, bactericidal, lysozyme, and respiratory burst activities in CL-supplemented fish before and after the Si challenge. Si-challenge caused a remarkable increase in tissue malondialdehyde (MDA) levels that was inhibited by CL supplementation. The activities of glutathione peroxidase (GPx) and superoxide dismutase (SOD) in tissues were significantly elevated in a dose-dependent manner in CL-supplemented groups in both pre- and post-challenge experiments; renal SOD did not show any differences. Hepatic nitric oxide (NO) level was significantly decreased in CL-supplemented fish in a dose-dependent manner. In the post-challenge experiment, nitrosative stress was apparent in the liver and kidney; however, CL supplementation was sufficient to reverse it. Interestingly, a remarkable induction of the hepatic hepcidin expression was observed in all CL-supplemented groups in the pre-challenge experiment and Si-challenged fish, underscoring the role of CL as an antibacterial through inducing hepatic hepcidin expression to combat S. iniae infection. CL-supplementation was associated with lower mortality rates after Si-challenge, which was more pronounced in CL-3% supplemented fish. In conclusion, our results demonstrate that CL has a potent antioxidant role via increasing antioxidant enzymes' activities and antagonizing lipid peroxidation. Moreover, CL has an immune-stimulant effect by inducing the hepatic hepcidin expression and immunological markers in response to S. iniae infection.