Antibacterial Effects of Bulgarian Oregano and Thyme Essential Oils Alone and in Combination with Antibiotics Against Klebsiella pneumoniae and Pseudomonas aeruginosa

Antimicrobial resistance (AMR) poses a global health threat, with multidrug-resistant (MDR) <i>Klebsiella pneumoniae</i> and <i>Pseudomonas aeruginosa</i> contributing to the burden, especially in Bulgaria. This study investigates recent resistance trends of these pathogens in Bulgaria and evaluates the antibacterial effects of commercially available oregano (<i>Origanum vulgare</i>) and thyme (<i>Thymus vulgaris</i>) essential oils (EOs), both alone and in combination with conventional antibiotics. The chemical composition of the EOs was analyzed using gas chromatography-mass spectrometry (GC-MS), revealing carvacrol (84.2%) as the main component in oregano EO and thymol (45.74%) in thyme EO. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values of the EOs were determined using the serial dilution method. Oregano EO exhibited lower MIC and MBC values (0.039-1.250%) compared to thyme EO (0.156-5.000%) against both pathogens. The disk diffusion assay showed that oregano EO exhibits more potent antibacterial effects than thyme EO in <i>Klebsiella pneumoniae</i> and <i>Pseudomonas aeruginosa</i> strains. The findings highlight the potential of EOs as adjunctive therapies to enhance the efficacy of conventional antibiotics against MDR <i>Klebsiella pneumoniae</i> and <i>Pseudomonas aeruginosa</i> strains in Bulgaria.