Antibacterial mechanisms of magnolol against Streptococcus agalactiae and immunomodulatory effects in Schizothorax prenanti.

Streptococcus agalactiae is an important zoonotic pathogen with an expanding host range and increasing antibiotic resistance, presenting a significant risk to the aquaculture in China. Therefore, developing effective treatment approaches and reducing the risk of antibiotics is crucial. Chinese medicine compounds have shown great potential in the prevention and control of aquatic animal diseases. In this study, we investigated the antibacterial activity and mechanism of magnolol against S. agalactiae (SICAU SA001) derived from Schizothorax prenanti. The in vitro experiments showed that the minimum inhibitory concentration (MIC) of magnolol against S. agalactiae was 16 μg/mL, and the minimum bactericidal concentration (MBC) was 32 μg/mL. Under the treatment with magnolol at 1× MIC, the biofilm formation was reduced by 61.98%, the conductivity increased by 30.59%, and the extracellular DNA (eDNA) release increased by 2.86-fold. The integrated omics analysis and molecular docking found that magnolol can interfere with the expression of ribosomal proteins in S. agalactiae, and inhibit the gene and protein expression of the phosphotransferase system (PTS). The in vivo experiments showed that magnolol conferred a relative protection rate of 54.55% against S. agalactiae infection in S. prenanti, significantly reducing bacterial loads in the liver, spleen, and kidney while alleviating S. agalactiae-induced histopathological damage. Furthermore, magnolol can significantly increase the activities of non-specific immune enzymes (acid phosphatase, catalase, and superoxide dismutase), and suppress the overexpression of pro-inflammatory cytokines (IL-6, IL-1β, and IL-8). In summary, magnolol exhibits favorable inhibitory effect on S. agalactiae, mainly by disrupting the cell membrane function, interfering with the ribosome and the PTS, and enhancing the immunity of the host to exert its antibacterial effect. Our study provides a promising strategy for the antibiotic-free control of S. agalactiae and supports the sustainable development of Schizothorax aquaculture.