The extract of Cirsium setosum exerts antimicrobial activities on largemouth bass infected by Aeromonas veronii via regulating inflammatory and antioxidant responses.

Aeromonas veronii (A. veronii), a type of Gram-negative, rod-shaped, facultative anaerobic bacterium, is commonly found in aquatic environments. It is a species with strong pathogenicity within the Aeromonas genus, which belongs to the family Vibrionaceae, the order Vibrionales, and the genus Aeromonas. Cirsium setosum is a Chinese traditional medicine with various biological activities, including anti-inflammatory, antihemorrhagic, antimicrobial, sedative functions, enhancement of strong heart function etc. effects. Therefore, this study evaluated the antimicrobial activity and mechanism of Cirsium setosum extract (CSE) on A. veronii using methods such as minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) assays, growth inhibition assay, measurement of the inhibition zone diameter, bacterial protein synthesis assay, Real time PCR, Cell membrane leakage assay for nucleic acids. The results showed that CSE exhibited significant inhibitory effects on A. veronii in vitro: The MIC and MBC of CSE on A. veronii is 62.5 mg/mL; And CSE achieves its antibacterial effect by inhibiting growth, influencing the permeability of the cell membrane, affecting the expression of important virulence factors, and regulating the synthesis of certain proteins. Additionally, this study evaluated the antimicrobial activity of CSE in largemouth bass via oral administration. After oral administration, the mortality rate of largemouth bass (Micropterus salmoides, MS) infected A. veronii was significantly reduced. Pathological examination revealed alleviated tissue damage in the liver, spleen, intestine, and head kidney of the treated fish. RT-qPCR analysis demonstrated that dietary supplementation with CSE enhanced the immune response in largemouth bass, as evidenced by significant downregulation of inflammatory cytokine expression and upregulation of antioxidant factor expression in the liver, spleen, intestine, and head kidney. These findings indicate that CSE exerts anti-inflammatory and antioxidant effects, thereby improving the disease resistance of largemouth bass.