Antibacterial effects of Lippia gracilis Schauer essential oil and its major compounds thymol and carvacrol against Acidovorax citrulli: Impacts on growth, biofilm formation, motility, and membrane permeability

Acidovorax citrulli is a Gram-negative phytopathogenic bacterium that causes bacterial fruit blotch in cucurbit crops, a highly destructive disease for which no commercially available resistant cultivars are available. Essential oils (EOs) are complex mixtures of volatile compounds with diverse biological activities. Lippia gracilis EOs are rich in the monoterpenes thymol and carvacrol, both recognized for their antimicrobial properties. This study evaluated the antibacterial activity of EOs from seven L. gracilis accessions and their major constituents, thymol and carvacrol, against A. citrulli . Minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), bacterial growth, biofilm formation, membrane permeability, and swimming motility were assessed. All EOs inhibited bacterial growth by more than 97.4% at 1,000 μg mL -1 . The lowest MIC values were obtained for LGRA106 EO (500 μg mL -1 ), thymol, and carvacrol (250 μg mL -1 ). LGRA106 EO, thymol, and carvacrol at 1× and 2× MIC significantly reduced bacterial growth within 30 min. Thymol and LGRA106 EO inhibited biofilm formation by more than 95% at their respective MICs and increased bacterial membrane permeability within 5 min of exposure, indicating rapid membrane permeabilization following exposure. In addition, thymol significantly reduced swimming motility at ½×, ¼×, and ⅛× MIC, whereas carvacrol was effective at ½× and ¼× MIC. These findings demonstrate that L. gracilis EOs, particularly LGRA106 EO, together with thymol and carvacrol, exhibit potent antibacterial activity against A. citrulli and represent promising natural alternatives for the sustainable management of bacterial fruit blotch.