<em>In Vitro</em> Evaluation of the Antimicrobial Activity of <em>Origanum vulgare</em> Essential Oil Against Esbl-Producing Strains of <em>Escherichia coli</em> and <em>Klebsiella pneumoniae</em>

Background/Objectives: One of the greatest threats to global public health is antimicrobial resistance (AMR), due to the increasing number of infections caused by extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae. Escherichia coli and Klebsiella pneumoniae ESBL-producing bacteria possess resistance mechanisms that inactivate β-lactam antibiotics by hydrolyzing their β-lactam ring, thereby limiting conventional therapeutic options. In response to this problem, the objective in this exploratory in vitro study was to evaluate the antimicrobial activity of Origanum vulgare (oregano) essential oil and its interaction with the antibiotic cefepime using in vitro methods. Methods: Antimicrobial susceptibility tests were performed, including determination of the minimum inhibitory concentration by the microdilution method with statistical analysis, and evaluation of the fractional inhibitory concentration index using the checkerboard method. In addition, advanced methods such as bacterial identification by MALDI-TOF mass spectrometry and PCR were employed for the identification of resistance genes. Results: The studied strains exhibited both phenotypic and genotypic resistance. The MIC of the essential oil was 1024 µg/mL for ESBL-producing E. coli and 2048 µg/mL for ESBL-producing K. pneumoniae, whereas the ATCC strains showed higher susceptibility. The FICI values indicated synergism in E. coli (FICI = 0.188) and an additive effect in K. pneumoniae (FICI = 0.563). Conclusions: Oregano essential oil exhibits antimicrobial activity and the ability to potentiate the effect of cefepime, suggesting its potential as a therapeutic adjuvant. Additional studies are required, including a larger number of strains, cytotoxicity analyses, and clinical validation.