Design, Synthesis, and Antibacterial Activity Evaluation and Mechanism of 1-H-Indole-3-pyrazolamide Derivatives Containing Piperazine

To develop antibacterial agents with novel mechanisms of action, a series of novel 3-pyrazolylindole derivatives incorporating piperazine moieties were designed and synthesized. The in vitro antibacterial activities of these compounds were evaluated against four phytopathogenic bacterial strains: Xanthomonas oryzae pv oryzae (Xoo), Xanthomonas axonopodis pv citri (Xac), Pseudomonas syringae pv actinidiae (Psa), and Ralstonia solanacearum (Rs). All of the synthesized derivatives exhibited moderate antibacterial activity. In particular, compound P16 demonstrated potent inhibitory activity against Xoo and Xac, with EC₅₀ values of 2.54 ± 0.87 and 3.49 ± 0.04 μg/mL, respectively, which were significantly lower than those observed for the positive controls, paclobutrazol and copper thiazole. Compound P16 showed protective and therapeutic efficacies of 86.41% and 94.56%, respectively, against Xoo infection. Furthermore, compound P16 was shown to effectively suppress biofilm formation, bacterial motility, and extracellular polysaccharide production, induce morphological alterations in bacterial cells, and promote reactive oxygen species (ROS) accumulation, ultimately triggering bacterial apoptosis. Mechanistic studies revealed that compound P16 down-regulated the expression of genes associated with the type VI secretion system (T6SS), thereby conferring protection against rice bacterial blight and citrus canker. These results indicate that compound P16 holds significant potential as a lead candidate for further development as an antibacterial agent.