In Vitro Synergistic Effects of Selected Essential Oil Components Against Clinical Acinetobacter baumannii Strains

Multidrug-resistant Acinetobacter baumannii isolates pose a threat to healthcare systems worldwide. The antibacterial activity of essential oil components (EOCs)—linalool, geraniol, eugenol, citronellol, 1,8-cineole, carvacrol, and trans-anethole—was evaluated both alone and in combinations against multidrug-resistant A. baumannii clinical strains from wound infections, and against two reference strains. The minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of EOCs were determined using the broth microdilution method. The interactions between EOCs were investigated using the checkerboard microdilution method. Carvacrol and eugenol showed the strongest inhibitory effects against the tested strains. The MICs and MBCs of carvacrol were 1.70–3.39 mg/mL. The MICs of eugenol were 4.10–16.41 mg/mL, while the MBCs were 4.10–32.81 mg/mL. MIC/MBC analysis showed that all tested EOCs had bactericidal activity against A. baumannii in vitro; however, some required higher concentrations. Most EOC mixtures showed synergistic interactions. The combination of eugenol and carvacrol induced the strongest synergistic effect, leading to the greatest reduction in the MICs of both compounds (MIC of carvacrol decreased by 8–30-fold, while the MIC of eugenol decreased by 16–64-fold). Our results indicate that some EOCs, especially when combined, exhibit significant antibacterial activity against A. baumannii.