In Vitro Antimicrobial Activity of Selected Essential Oils Against Clinically Relevant MDR Bacterial Pathogens, Including ESKAPE Species

Background/Objectives: Essential oils (EOs) are volatile, biologically active compounds naturally extracted from aromatic plants. Known for their broad-spectrum antimicrobial properties, EOs are gaining increasing attention as potential alternative or complementary agents in clinical practice, especially in the face of rising antibiotic resistance. EOs act through multiple mechanisms, including disruption of microbial membranes, interference with enzyme activity, and inhibition of quorum sensing pathways, making them promising antimicrobial candidates against multidrug-resistant organisms (MDROs). Methods: In this study, the in vitro antimicrobial activity of seven essential oil batches, including Eucalyptus globulus, Lavandula hybrida, Melaleuca alternifolia, Pelargonium graveolens, Coriandrum sativum, Cymbopogon winterianus, and Cinnamomum zeylanicum, was evaluated against five reference strains and ten clinical strains mostly belonging to ESKAPE species, plus Stenotrophomonas maltophilia, Achromobacter xylosoxidans, and Burkholderia cepacia complex. Results: The tested EO batches displayed heterogeneous antimicrobial activity: Cinnamomum zeylanicum exhibited the strongest and broadest antibacterial effect, showing consistently low minimum inhibitory concentration (MIC) values across all tested strains, including carbapenemase-producing and non-fermenting Gram-negative bacteria. The remaining oils showed moderate-to-limited activity, particularly against Gram-negative strains. The two Klebsiella pneumoniae strains producing either KPC or NDM carbapenemases, and the Pseudomonas aeruginosa PAO-1 strain, showed the highest MIC values for all EOs, except for C. zeylanicum, for which MICs were 0.125 and 0.25% (v/v), respectively. Conclusions: The consistently low MIC values of the C. zeylanicum batch against the panel of tested strains suggested a potential for this essential oil and for its bioactive constituents as promising candidates against several MDROs.