GLYCYRRHIZA URALENSIS ON ANTI-AEROMONAS HYDROPHILA INFECTION OF GIANT SALAMANDER (ANDRIAS DAVIDIANUS)

Chinese giant salamander (Andrias davidianus) is a national second-class key protected species. Wild giant salamanders mostly inhabit in mountain streams. They are widely distributed in Qinling Mountains of Shaanxi Province, China. There are three species of giant salamander namely Chinese giant salamander (A. davidianus), American giant salamander (Cryptobranchus alleganiensis) and Japanese giant salamander (A. japonicus). The septicemia of giant salamander caused by Aeromonas hydrophila is one of the common diseases. A. hydrophila is a common aquatic bacterium, which exists in natural water, but usually does not cause diseases. Only when the immunity of aquatic animals decreases, it is possible to infect the body through the digestive tract or skin wounds. Therefore, if it can enhance the activity of immune cells and immune molecules and improve its autoimmunity, it can effectively block the infection pathway of A. hydrophila and reduce the incidence of sepsis. Most of the used methods of Chinese herbal medicine in aquaculture are direct feeding of crude drugs, and there is no extraction and concentration of immunoactive components. Reports from the poultry industry believe that licorice has immunomodulatory effect on the body, but there is no report on the effect of licorice on the anti-disease ability of amphibian giant salamander. This study carried out this exploration after targeted extraction of the effective components of licorice.The effect of licorice on the anti-A. hydrophila infection of giant salamander was studied by feeding licorice extract to giant salamander. The results showed that from day 28 after continuous feeding, the serum lysozyme activity in the drug group increased firstly and then decreased, showing a parabolic trend. The maximum value was (158.4±34.7) U/mL in the low-dose group on day 42 and (178.3±28.8) U/mL in the high-dose group on day 35, both of which were significantly higher than those in the control group in the same period (P<0.05). The phagocytic activity of renal macrophages in the drug group was significantly higher than that in the control group at the last two sampling periods, i.e. 42d and 56d (P<0.05). The maximum values of low dose group and high dose group appeared at 42 days, which were (59.4±8.5)% and (58.4±5.2)% respectively. Compared with the same period, the leukocyte volume of the drug group was higher than that of the control group. Among them, the leukocyte specific volume of the high-dose group was (5.8±1.7)%, the low-dose group was (5.5±0.8)%, and the high-dose group was (5.9±1.7)%, which were significantly higher than those of the control group at the same time (P<0.05). There was no significant difference in spleen organ coefficient between the drug group and the control group. After the last sampling (56d), the mortality of the control group was 90%, and that of the low-dose group and the high-dose group were 60%, which were lower than that of the control group, while the immune protection rate of the drug group was 33.3%, which was also higher than that of the control group. The results showed that feeding licorice extract could improve the resistance of giant salamander to A. hydrophila to a certain extent.Chinese herbal medicine can indirectly affect the synthesis of protein by affecting the self replication of bacterial DNA and inhibiting its reproduction and growth, and finally inhibit the growth of bacteria. In terms of nonspecific immune defense function of the body, lysozyme, leukocytes and macrophages are extremely important immune molecules and immune cells. They can kill and effectively protect the body by dissolving or swallowing foreign pathogens. In this experiment, licorice improved the survival rate of giant salamander infected by A. hydrophila. At the same time, the activity of serum lysozyme, the number of leukocytes and the phagocytic activity of macrophages increased. Is there a correlation between them? Is the improvement of the survival rate of giant salamander caused by the improvement of these indicators, or is it caused by the fact that Chinese herbal medicine affects the synthesis of bacterial DNA and thus the synthesis of bacterial protein? Further research is needed.