Oleanolic acid combats Streptococcus iniae via cell membrane damage and oxidative stress, transcriptomic and...

Streptococcus iniae is a significant pathogen that affects aquatic animals, resulting in substantial economic losses in aquaculture. Oleanolic acid, a pentacyclic triterpenoid derived from Swertia mileensis, shows strong antibacterial properties. This study conducted a series of phenotypic experiments and transcriptome analyses to investigate the antibacterial effect and mechanism of oleanolic acid on S. iniae DY2307. The results show that the MIC and MBC of oleanolic acid against S. iniae DY2307 are 16 μg/mL and 32 μg/mL, respectively. Phenotypic analysis demonstrated that oleanolic acid disrupted the cell membranes and cell walls, decreased ATP production (p < 0.001), increased ROS levels (p < 0.001), disrupted biofilm (p < 0.01), and hindered protein synthesis. Transcriptomic profiling enriched 906 DEGs, including 452 upregulated and 454 downregulated genes. KEGG enrichment analysis revealed that the phosphate transferase system (PTS), glycolysis/glycogenesis, tricarboxylic acid (TCA) cycle, pentose phosphate pathway (PPP), and the genes for other critical metabolic and transport pathways of S. iniae DY2307 were all downregulated. Simultaneously, intricate changes took place in pathways like the two-component system, ribosomes, quorum sensing, and biofilms. Our study demonstrates that oleanolic acid combats S. iniae primarily through inducing cell membrane damage and oxidative stress, laying the foundation for its use as an antibiotic alternative in preventing and controlling S. iniae.