From In Vitro Screening to In Vivo Evaluation of Essential-Oil-Based Formulation Against Avian Pathogenic Escherichia coli

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Antimicrobial resistance in avian pathogenic Escherichia coli (APEC) poses a significant challenge to poultry production, highlighting the need for effective alternatives to conventional antimicrobials. This study aimed to evaluate the antibacterial activity of selected essential oils (EOs) against multidrug-resistant APEC isolates and assess the in vivo effects of an oregano–thyme EO-based formulation in experimentally infected broilers. Six EOs were screened in vitro using the broth microdilution method, and oregano and thyme EOs, showing the strongest activity (MIC 0.003% and 0.003–0.007%, respectively), were further evaluated in two combinations. The more active combination was formulated for administration via drinking water and tested in 420 Ross 308 broilers experimentally challenged with APEC. Birds were allocated to untreated control, infected untreated, EO-treated, enrofloxacin-treated, and enrofloxacin + reduced-dose EO formulation groups. Mortality did not differ significantly among groups. A binomial GLMM identified significant differences in faecal consistency among experimental groups (likelihood-ratio χ2 = 32.90, df = 4, p < 0.001), while no significant differences were observed in body weight, feed intake, or overall FCR. Significant differences were detected in AST, glucose, total bilirubin, and calcium concentrations. Overall, the EO-based formulation demonstrated antibacterial activity in vitro and was associated with changes in intestinal status following APEC challenge, while no consistent improvement in productive performance was observed. These findings support further investigation of EO-based formulations as potential non-antibiotic approaches for managing APEC-associated intestinal disturbances in broiler chickens.