In vitro antibacterial activity of essential oils from Carum copticum and Ziziphora clinopodioides plants against the phytopathogen Pseudomonas syringae pv. syringae

Abstract The present study aimed to determine the in vitro antibacterial activity and the chemical composition of essential oils (EOs) from two medicinal plants (seeds of Carum copticum L. and leaves of Ziziphora clinopodioides Lam) against the bacterial plant pathogen Pseudomonas syringae pv. syringae. The sensitivity of P. syringae to the two types of EOs was investigated through three methods including disc diffusion, well diffusion, and vapor phase tests. Gas chromatography (GC) and gas chromatography attached to a mass spectrometer (GC-MS) were used to identify and quantify the essential oil compounds. The greatest growth inhibition zones for disc diffusion, well diffusion, and vapor tests were observed for the highest essential oil volume (50 µL) of Z. clinopodioides as 0.925 cm, 1.75 cm, and 1.45 cm, respectively. Concerning the EOs from C. copticum seeds, the diameters of growth inhibition zones were 0.775 cm, 1 cm, and 0 cm, respectively. Based on the obtained results, the highest antibacterial activity, with a growth inhibition zone of 1.75 cm, belonged to 50 µL EOs of Z. clinopodioides on Luria-Bertani (LB) agar medium. The minimum bactericidal concentration (MBC) and minimum inhibitory concentration (MIC) for the EOs of Z. clinopodioides were 8 µL mL−1 and 1 µL mL−1 in LB agar medium. The MBC and MIC values for EOs of C. copticum were 16 µL mL−1 and 1 µL mL−1, in LB agar medium, respectively. According to the chromatography results, 16 different compounds were identified in the EOs of C. copticum, of which thymol (54.6%) was the major constituent. Moreover, 27 different compounds were identified in the Z. clinopodioides EOs, of which pulegone (23%) and 1,8 cineole (20.3%) were the main essential oil constituents. The results demonstrated that the EOs of Z. clinopodioides plants had higher chemical diversity and displayed stronger antibacterial activities against P. syringe pv. syringae, compared to those of C. copticum.