Antibacterial and preventive effects of Terminalia chebula Retz. aqueous extract and methyl gallate in American shad Alosa sapidissima (Wilson, 1981) against Aeromonas hydrophila YML1.
American shad, Alosa sapidissima (Wilson, 1981), is an economically important emerging species in recirculating aquaculture systems and photovoltaic aquaculture. However, Aeromonas hydrophila poses a threat to the healthy development of its aquaculture industry. This study investigated the effects of 95.5 mg·L-1 aqueous extract from Terminalia chebula Retz. and 318.3 mg·L-1 methyl gallate (Experiment I) and methyl gallate at 31.8-127.3 mg·L-1 (Experiment II) together with A. hydrophila YML1 challage groups (1.0 × 107 and 1.58 × 105 CFU·mL-1 in Experiments I and II) on liver morphology, hepatic enzyme profiles, and transcriptional responses in American shad. Results for Experiment I showed after high-dose pathogen infection, vacuolar degeneration and inflammatory cell infiltration were observed. In addition, caspase-3, tumor necrosis factor α (TNF-α), and malondialdehyde (MDA) contents increased significantly, and the steroid biosynthesis pathway was significantly enriched via the transcriptional analysis. However, inflammation and apoptosis remained evident after high-dose A. hydrophila YML1 infection. After treatment with 95.5 mg·L-1 T. chebula Retz. extract, triphosphopyridine nucleotide (NADPH), Fructose-1,6-bisphosphatase (FBP), and phospho fructo kinase 1 (PFK) levels decreased significantly. After treatment with 318.3 mg·L-1 methyl gallate, nicotinamide adenine dinucleotide (NADH), NADPH, PFK, caspase-3, TNF-α, and MDA contents decreased significantly. For Experiment II, caspase-3 and TNF-α also decreased in the 31.8 mg·L-1 group at 24-96 h and in the 127.3 mg·L-1 group at 96 h. The peroxisome proliferator-activated receptor (PPAR) signaling pathway and fatty acid metabolism were also significantly enriched after methyl gallate treatment.Different expression genes (DEGs) in the cytokine-cytokine receptor interaction pathway showed a time- and dose-dependent pattern. At 48 h, the MAPK signaling pathway and apoptosis were enriched in the 95.5 mg·L-1 group, whereas at 96 h the PPAR-MAPK (mitogen-activated protein kinase) signaling pathway was enriched, with downregulation of fabp7b, soat2, cpt1ab2, and pparg. These changes were associated with inflammatory cell infiltration and increased MDA contents. Enhanced fatty acid degradation, possibly via cpt2, together with reduced fatty acid transporter transcription, may have alleviated liver injury in shad.