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 of 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. 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 MALDI-TOF mass spectrometry for bacterial identification and PCR were employed for the identification of resistance genes. 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). 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.

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.

Background and Aim: Extended-spectrum β-lactamase (ESBL)-producing Escherichia coli represents an increasing One Health threat because of its dissemination among humans, animals, and the environment and its frequent association with multidrug resistance (MDR). Companion animals may serve as reservoirs of clinically important resistant bacteria; however, molecular epidemiological information on ESBL-producing E. coli in domestic cats from Ecuador remains limited. This study aimed to determine the prevalence, antimicrobial susceptibility patterns, β-lactamase gene distribution, and potential risk factors associated with ESBL-producing E. coli carriage in domestic cats from Machala, Ecuador. Materials and Methods: A cross-sectional study was conducted from August 2023 to August 2024 involving 199 domestic cats presented to the Veterinary Clinic of the Universidad Técnica de Machala. Rectal swabs were cultured on CHROMagar™ ESBL, and ESBL production was phenotypically confirmed using the double-disk synergy test according to Clinical and Laboratory Standards Institute criteria. Antimicrobial susceptibility testing against 12 agents was performed using the Kirby–Bauer disk diffusion method. Polymerase chain reaction was used to detect 𝑏𝑙𝑎CTX−M, 𝑏𝑙𝑎SHV, and 𝑏𝑙𝑎TEM. MDR was defined as non-susceptibility to at least one antimicrobial agent in three or more categories. Risk factor associations were evaluated using Pearson’s chi-square or Fisher’s exact test. Results: ESBL-producing E. coli was detected in 83 of 199 cats, corresponding to a prevalence of 41.7% (95% confidence interval: 35.1–48.7). The 𝑏𝑙𝑎CTX−M gene predominated in 60/83 isolates (72.3%), followed by 𝑏𝑙𝑎TEM in 19/83 (22.9%) and 𝑏𝑙𝑎SHV in 1/83 (1.2%). At least one targeted gene was detected in 64/83 isolates (77.1%), whereas 19/83 phenotypically positive isolates lacked all three targeted genes. MDR was observed in 74/83 isolates (89.2%). All isolates remained susceptible to imipenem and meropenem, whereas the highest resistance rates were recorded for trimethoprim–sulfamethoxazole, tetracycline, cefepime, levofloxacin, and doxycycline. None of the evaluated risk factors was significantly associated with ESBL gene positivity. Conclusion: Domestic cats in Machala showed a high prevalence of 𝑏𝑙𝑎CTX−M-dominant, multidrug-resistant ESBL-producing E. coli. These findings indicate widespread community dissemination and support the inclusion of companion animals in Ecuadorian antimicrobial resistance surveillance and stewardship programs under a coordinated One Health framework. Keywords: antimicrobial resistance, companion animals, Ecuador, Escherichia coli, extended-spectrum β-lactamase, multidrug resistance, One Health, blaCTX-M.