الفهرس 
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Abstract
This study was carried out to investigate the effect of two medicinal plants (garlic and thyme) on the immune response of Nile tilapia (O. niloticus) against bacterial infection. First, ethanolic extracts of garlic (Allium sativum) and thyme (Thymus vulgaris) were prepared. Then the in-vitro antimicrobial activity of four different concentrations (2, 5, 10 and 20 mg) of both extracts was tested against some bacterial fish pathogens (Aeromonas hydrophila, Pseudomonas aeruginosa and Pseudomonas putida). Then the effect dietary administration of garlic and/or thyme extracts, at three different doses (1, 2.5 and 5 gm/kg diet), for 30 days, on some immune parameters and antioxidant activities of Nile tilapia (Oreochromis niloticus) was assessed in comparison to oxytetracycline (OTC) (0.1 gm/kg diet). Lastly, a challenge test was done by I/P injection of fish by A. hydrophila to determine the relative level of protection of garlic and/or thyme extracts and OTC against bacterial infection through recording the clinical signs, P/M lesions, mortality rate and the histopathological examination.
The obtained results can be summarized as following:
1- In-vitro antimicrobial test revealed that, garlic and thyme ethanolic extracts at the four different concentrations (2, 5, 10 and 20 mg) showed inhibitory activity against Aeromonas hydrophila, Pseudomonas aeruginosa and Pseudomonas putida with appearance of inhibition zones.
2- Feeding of Nile tilapia (Oreochromis niloticus) on garlic and/or thyme ethanolic extracts (1, 2.5 and 5 gm/kg diet) for 30 days resulted in increases in the serum Lysozyme activity, IgM level, SOD activity and Total antioxidant capacity when compared to that of the control group which did not receive garlic or thyme extract. However, Nitric oxide level was increased only in group (T2) that received garlic extract at (2.5 gm/kg diet) and also in groups (T5 and T6) that received thyme extracts at (2.5 and 5 gm/kg diet), respectively. Anti-protease activity showed increases with garlic extract (5 gm/kg diet), thyme extract (2.5 and 5 gm/kg diet) and garlic and thyme mixture (1.25+1.25 g/kg diet) and (2.5+2.5 g/kg diet).