ANTIBACTERIAL AND PREVENTIVE EFFECTS OF TERMINALIA CHEBULA AND METHYL GALLATE AGAINST AEROMONAS HYDROPHILA ISOLATED FROM ALOSA SAPIDISSIMA

To explore effective approaches for preventing and controlling <italic>Aeromonas hydrophila</italic> infection in <italic>Alosa sapidissima</italic>, the pathogenic strain YML1 was first isolated from diseased fish and confirmed as an <italic>A. hydrophila</italic> resistant to β-lactam and tetracycline antibiotics. Antimicrobial susceptibility tests revealed that the water extract of <italic>Terminalia chebula</italic> and methyl gallate produced inhibition zone diameters of (21.00±0.75) and (24.00±1.50) mm, respectively, indicating high sensitivity. The minimum inhibitory concentration (MIC) were 5.00 mg/mL for <italic>T. chebula</italic> and 0.16 mg/mL for methyl gallate, while the minimum bactericidal concentration (MBC) was 40.00 mg/mL and 5.00 mg/mL respectively, demonstrating favorable in vitro antibacterial activity. Further investigation was conducted on the effects of dietary supplementation with <italic>T. chebula</italic> and methyl gallate on immune and antioxidant functions of <italic>A. sapidissima</italic> following challenge with <italic>A. hydrophila</italic>. The results showed that both treatments significantly improved survival rate, with <italic>T. chebula</italic> exhibiting more pronounced effects. Methyl gallate demonstrated potential as an immunostimulant, synergistically upregulating immune-related genes (<italic>lyz</italic>, <italic>tap2a</italic>, <italic>psmb9a</italic>) and antioxidant genes (<italic>gss</italic>, <italic>nrf1</italic>, <italic>cat</italic>), while simultaneously activating the complement system (C3, C4), lysozyme (LZM), and antioxidant enzyme activities (SOD, GSH-Px), thereby establishing a multi-level defense network. <italic>T. chebula</italic> enhanced basal immunity and pathogen recognition capacity of <italic>A. sapidissima</italic> through a multi-pathway, progressive mode by concurrently upregulating antioxidant genes (<italic>gss</italic>, <italic>cat</italic>), innate immune genes (<italic>lyz</italic>, <italic>tlr3</italic>), and antigen processing genes (<italic>tap2a</italic>), and significantly boosting their corresponding enzyme activities (T-SOD, GSH-Px, LZM, C3, C4). This approach effectively established robust defenses while avoiding excessive stress. In conclusion, both <italic>T. chebula</italic> and methyl gallate can effectively alleviate the stress and damage caused by <italic>A. hydrophila</italic> infection by modulating the immune-biochemical indicators and related gene expression in <italic>A. sapidissima</italic>, showing promise as potential feedadditives for bacterial disease prevention in this species.