Inappropriate and uncontrolled use of antibiotics in humans and animals leads to the emergence of multi-resistant bacteria. Before the discovery of antibiotics, plant extracts and essential oils were used for therapeutic purposes. Today, due to increasing antibiotic resistance, many studies are frequently carried out on the antimicrobial activities of natural active substances that can be a source for new drug candidates. The aim of this study was to investigate the antibacterial activity of components such as alpha-pinene (alpha-PN), p-cymene (p-CYM), carvacrol (CAR), thymol (TY) and eugenol (EG) found in the essential oils of many plants and their synergistic interaction with antibiotics. In this study, the antibacterial activity of these essential oil components and antibiotics in clinical use such as gentamicin (GEN), tetracycline (TET), tigecycline (TGC) and linezolid (LZD), against Staphylococcus aureus [methicillin resistant S.aureus (MRSA), and methicillin sensitive S.aureus (MSSA)], Escherichia coli and Pseudomonas aeruginosa strains were determined by disc diffusion and microdilution method. In addition, the interaction between the essential oil components and antibiotics was also determined by the checkerboard method. While CAR, TY and EG components showed strong antibacterial activity, the antibacterial activity of alpha PN and p-CYM was found to be weak. Combinations of alpha-pinene, carvacrol, thymol and eugenol with gentamicin and tetracycline mostly showed synergistic interactions against all bacteria. In alpha PN, CAR, TY and EG with GEN and TET, synergistic/partial synergistic interaction was observed against S.aureus strains, while indifferent interaction was detected in E.coli and P.aeruginosa strains. The combination of alpha PN and p-CYM with TGC showed synergistic interaction against E.coli and P.aeruginosa strains, and additive and indifferent interaction against S.aureus strains. On the other hand, synergistic interaction was observed against all bacterial strains in combinations of TGC and CAR, TY and EG components. Antagonistic interaction was not detected in any of the tested component-antibiotic combinations against the bacteria used in our study. A synergistic interaction between natural bioactive components and commonly used antibiotics may contribute to the effectiveness of antibiotics and components at lower doses, minimizing their potential toxic side effects and reducing treatment costs. However, more research is needed in terms of their pharmacokinetic and toxic properties to evaluate the therapeutic application potential of phytochemicals.
