Antibacterial activity of some plant-derived essential oils against plant pathogenic bacteria

Essential oils (EOs) are natural products being considered as an alternative to chemicals to control plant pathogenic bacteria (PPB). In the present study, the EOs from Thymus kotschyanus Boiss. & Hohen., Thymus daenensis Celak, Ferulago angulata (Schlecht.) Boiss., Ziziphora clinopodioides Lam., Eucalyptus camaldulensis Dehnh., Echinophora cinerea (Boiss.) Hedge et Lamond., and Trachyspermum ammi L. were obtained and their major constituents were identified by gas chromatography/mass spectrometry (GC-MS). The effect of the EOs against nine species of PPB was evaluated using a disk diffusion assay. T. ammi EO showed the highest antibacterial activity with an inhibition zone of 52.25 and 46.25 mm against Xanthomonas citri pv. citri and Pectobacterium carotovorum subsp. carotovorum, respectively. All EOs exhibited an anti-biofilm formation effect on Erwinia amylovora. Minimum inhibitory concentration and minimum bactericidal concentration of the EOs were determined using the macro-dilution method. Synergistic effects of the EOs were determined using the serial dilution checkerboard method. Transmission electron microscopy revealed that exposure of the PPB to the EOs caused unified cell structure including bacterial aggregation due to the lysis of the cell wall, shrinkage, and deformation. In conclusion, the present results suggest that the EOs might be a promising source of antibacterial activity against PPB.