Plant tissues are natural sources of bioactive molecules to control pathogens that cause infections in plants and humans, as they contain secondary metabolites with antibacterial activities. Especially, terpenoids are responsible for the protection of various plants against herbivores and pathogens. In this study, we studied the antibacterial activity of 12 different terpenoid or related structures in a comparative way and revealed that the phenolic terpenoids are superior to other substituted derivatives against both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria. We also aimed to analyse effect of these compounds on the bacteria cell wall structure and maintenance of homeostasis. We observed an immediate loss of cell membrane integrity and ion leakage upon treatment of both Gram-negative and Gram-positive cells with phenolic terpenoids carvacrol and thymol. After prolonged treatment, leakage of genetic material and increase of membrane permeability for molecules were also observed. Hydroxyl group of the phenol moiety was essential for this effect, since neither the O-methyl derivatives nor the benzylic partners were as effective. We thus propose that perturbation of ion homeostasis upon increase in cell wall permeability is key for the action of terpenoids against bacteria cells.

Antifungal activity and mode of action of nine essential oils (EOs) against S. cerevisiae cells were examined. Antifungal effects of commercial lemon peel, orange peel, tea tree, turpentine, rosemary, peppermint, thyme, oregano and clove oils were determined through Minimum Inhibitory Concentration (MIC), Minimum Fungicidal Concentration (MFC) and inhibition zone measurements. The most effective oil was turpentine oil. Orange peel, thyme and oregano oils were also effective, according to MIC and MFC. Inhibition zone measurements, also revealed oregano, orange peel, thyme, turpentine and clove oils as most efficient ones. Later, membrane damage of yeast cells was studied by measuring the extracellular pH and conductivity in a concentration dependent manner. Orange peel, turpentine, thyme and oregano oils caused a clear increase in extracellular pH of glucose-induced yeast cells and induced an increase in extracellular conductivity. These results point to the deterioration of yeast cell membrane integrity upon exposure to EOs. Since these oils can be used as food preservatives and pharmaceutical agents, it is very important to understand their mode of action and their main target sites in the cell. Thus this research not only opens new perspectives to understand antifungal activity mechanisms of EOs, but also help widen their use.

Plant essential oils are preferred in cosmetics, medicine, food, and beverage industries for various purposes. α-Pinene is found mainly in eucalyptus oils, eugenol is the active ingredient in clove oil, and limonene is the major component in the oil of citrus fruit peels. In this study, we aimed to determine the antifungal activity of α-pinene, eugenol, and limonene against Saccharomyces cerevisiae yeast cells. Besides, we focused on revealing the target side of the compounds on the yeast cells. Firstly, the antifungal activity of compounds was tested via minimum inhibitory concentration (MIC) measurement. After that, we performed a sorbitol effect assay to understand whether it acts on the cell wall or not. With sorbitol, the MIC values were not changed. It means that they are not effective on the yeast cell wall. Then, we measured the extracellular conductivity increase upon treatment with the compounds to understand the effect on the cell membrane. Eugenol and limonene were not changed the extracellular conductivity, and there was no ion leakage from the cell membrane. On the other hand, α-pinene damaged the yeast cell membrane causing a sudden increase in conductivity due to ion leakage. An ergosterol effect assay with α-pinene was performed to detect cell membrane disruption via ergosterol or not. With ergosterol, the MIC value was not changed. α-Pinene must have another target than the ergosterol in the yeast cell membrane. Finally, revealing the mode of action of compounds against yeast cells will provide new insights into their usage in various fields.