Elucidation on potential utilization of Forsythia suspensa fruits in freshwater bivalve (Hyriopsis cumingii) culture against Aeromonas veronii infection

Plant extracts have been implicated in biological studies for their antibacterial, antioxidant, and anti-inflammatory properties. The curative efficacy of fruit extracts (Forsythia suspensa and Prunus mume) and three common antibiotics against Aeromonas veronii (alone or in extract-antibiotic combination) were clarified in vitro by microplate assays and checkerboard methods. Then the extracts with high antibacterial activity (F. suspensa) in vitro were exposed to Hyriopsis cumingii with A. veronii infection for in vivo assessments of their protective efficacy. Subsequently, oxidative stress markers, immune enzymes, and mRNA expression of immune genes in H. cumingii were investigated. Finally, principal component analysis of the physiological biomarkers and genetic markers that respond to A. veronii infection and extract exposures was conducted. The FICI values in vitro implied the following: P. mume combined with cephalexin was antagonistic, as opposed to other combinations whose effects were commutative or indifferent. In vivo results demonstrated that A. veronii caused lipid oxidative damage led to significant decreases in antioxidant defenses and induced immune responses in H. cumingii. F. suspensa significantly enhanced alanine transaminase activities and reduced malondialdehyde levels caused by A. veronii infection in gills and hepatopancreas. It also promoted antioxidant defenses, including superoxide dismutase activities and peroxidase contents, inhibiting increased alkaline phosphatase and acid phosphatase activities. Notably, 0.6% of F. suspensa extract (dry fruits: wet H. cumingii, w/w) reversed the upregulation of glutathione S-transferase and small heat shock protein mRNA levels in hepatopancreas and remarkably improved the survival rate of H. cumingii after the infection. Building on the research, F. suspensa may represent a promising substitute for offering effective protection to H. cumingii against bacterial diseases and contribute to the transformation and upgrading of the freshwater pearl culture industry.