Optimization and characterization of a low-toxicity trans-anethole nanoemulsion for controlling the soil-borne bacterial pathogen Ralstonia solanacearum

Plant-derived trans- anethole (TAN) exhibits favorable antibacterial activity and biocompatibility, but poor solubility, photolysis, and volatility limit its use. Here, we prepared a TAN nanoemulsion (TAN-NE) via low-energy emulsification using non-ionic surfactants (polyethylene glycol tert -octylphenyl ether and polyoxyethylenated castor oil) and a co-surfactant (anhydrous ethanol). TAN-NE had spherical morphology, particle size of 20.68 nm, formulation yield (92.16%), encapsulation efficiency (88.49%), and loading efficiency (3.60%). It showed improved water solubility, photostability, and structural stability under various pH, ionic strength, temperature, and storage conditions. In vitro experiments demonstrated that TAN-NE enhanced the antibacterial efficacy of TAN against Ralstonia solanacearum by inhibiting protein and EPS synthesis, damaging cell integrity, reducing motility and biofilm formation, and the downregulating virulence genes. More importantly, TAN-NE displayed an efficient protective activity of 71.74% against tobacco bacterial wilt on 15-day post inoculation, surpassing those of TAN (52.17%) and thiodiazole copper (42.39%). Additionally, TAN-NE also exhibited good biosafety to earthworms, seeds, tobacco plants and soil environment. Collectively, our study provides a guidance for the nanoemulsion system of insoluble natural product.