Antibacterial properties of lemon essential oil and five types of mint on pathogenic bacteria Xanthomonas citri subsp. citri, X. gardneri and X. perforans

Citrus canker disease caused by Xanthomonas citri subsp. citri and bacterial leaf spots caused by X. gardneri and X. perforans are critical bacterial diseases. The control of these pathogens is a serious challenge due to the problems caused by the restrictions on the use of poisons and antibiotics, and the issue of toxic resistance. Therefore, plant essential oils can be a suitable alternative to control plant pathogens. In the present study, the effect of Lippia citriodora, Mentha piperita, M. aquatica, M. saulavens, M. spicata and M. pulegium essential oils against X. citri subsp. citri, X. gardneri and X. perforans have been evaluated.

After plant essential oils extraction by clevenger, their constituent compounds were identified by GC-MS. Then, the antibacterial effect of essential oils was surveyed using the disk diffusion method. Also, the Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC), the combined effect of plant essential oils, and the effect of M. pulegium essential oil against X. gardneri were investigated using Transmission electron microscopy.

The results of GC-MS essential oils analysis showed that a total of 28, 28, 30, 36, 39, and 42 compounds were identified in M. saulavens, M. piperita, M. pulegium, M. aquatica, M. spicata, and L. citriodora essential oil compounds, respectively. The highest and lowest amount of inhibitory zone was related to oregano species on the bacteria X. citri subsp. citri with an amount of 24 ± 2.08 mm and apple mint essential oil on X. gardneri with an amount of 5.66 ± 0.66 mm, respectively. MIC and MBC for the examined essential oils against the pathogenic bacteria were between 1-5 and 2-6 μg/ml, respectively. Also, the synergistic effect was observed between different essential oils such as M. piperita- L. citriodora, M. pulegium- M. spicata, M. pulegium- M. aquatic, M. aquatic- M. piperita, M. aquatic- M. pulegium, M. pulegium- M. saulavens on pathogenic bacteria. Transmission electron microscope photos related to the effect of M. pulegium essential oil against X. gardneri bacteria showed complete cell destruction, cell wall damage, swollen nuclear, area and changes in the cytoplasm density of bacterial cells.

Essential oils are regarded safe substances when considering the control limitations of bacterial infections, bacterial resistance to toxins, and environmental issues associated with the use of toxins. Also, due to the different compositions of essential oils and the observation of synergistic properties between them in this study, the risk of developing resistance to them decreases, and their antibacterial effect increases.