Immune and antioxidative effects of dietary licorice (Glycyrrhiza glabra L.) on performance of Nile tilapia, Oreochromis niloticus (L.) and its susceptibility to Aeromonas hydrophila infection

Licorice (Glycyrrhiza glabra L.) is considered as one of the most extensively used medicinal plants because of its innumerable pharmacological functions. In the present study, licorice root powder (LRP) was incorporated in practical diets for Nile tilapia, Oreochromis niloticus (L.), at levels of 0 (control), 5, 10, and 20 g/kg diet and its effects on growth, biochemical, antioxidants, and immune responses were investigated. For this, fish (11.5 +/- 0.8 g) were fed on experimental diets up to apparent satiation thrice a day for 60 days. After that, fish were intrapreatinally injected with pathogenic bacteria Aeromonas hydrophila and fish mortality was observed for 10 days. The dietary LRP significantly improved growth parameters (weight gain, weight gain %, specific growth rate, and feed intake but did not affect the feed conversion ratio) as compared with the control group. Additionally, serum biochemical parameters (glucose, total protein, albumin, and globulin), antioxidants (catalase, superoxide dismutase, and glutathione peroxidase), and immunological responses (lysozyme activity, respiratory burst, and total immunoglobulin) were significantly improved with respect to the control group. Conversely, the markers of the hepato-renal functions, such as activities of serum alanine and aspartate aminotransferase and values of creatinine and uric acid significantly decreased in LRP-fed fish in relation to the control group. Meanwhile, fish survivability after bacterial challenge significantly increased as inclusion levels of dietary LRP increased with no mortality recorded in fish group fed 10-20 g/kg diet. The control fish group was more susceptible to bacterial infection showing the lowest survival rate (15%). Conclusively, the current study found that the LRP inclusion in a diet at a level of 10 g/kg diet was the optimum for Nile tilapia where this level reduced the diet's cost by 8.5% of the control diet.