Oxytetracycline (OTC) is administered in the poultry industry for the treatment of digestive and respiratory diseases. The use of OTC may contribute to the selection of resistant bacteria in the gastrointestinal tract of birds or in the environment. To determine the effect of OTC on the selection of resistant Escherichia coli strains post-treatment, bacteria were isolated from droppings and litter sampled from untreated and treated birds. Bacterial susceptibility to tetracyclines was determined by the Kirby–Bauer test. A total of 187 resistant isolates were analyzed for the presence of tet(A), (B), (C), (D), (E), and (M) genes by PCR. Fifty-four strains were analyzed by PFGE for subtyping. The proportion of tetracycline-resistant E. coli strains isolated was 42.88%. The susceptibility of the strains was treatment-dependent. A high clonal diversity was observed, with the tet(A) gene being the most prevalent, followed by tet(C). Even at therapeutic doses, there is selection pressure on resistant E. coli strains. The most prevalent resistance genes were tet(A) and tet(C), which could suggest that one of the main mechanisms of resistance of E. coli to tetracyclines is through active efflux pumps.

Objective: We set out to evaluate the persistence of sulfachloropyridazine, oxytetracycline, and enrofloxacin in broiler chicken litter following therapeutic oral treatment and its association with the isolation of Escherichia coli resistant to these antimicrobials. Methods: Forty broiler chickens were raised under controlled conditions and divided into three experimental groups, each treated with a different antimicrobial, in addition to an untreated/control group. Litter samples were collected post treatment, analyzed by UPLC-MS/MS, and processed for the isolation of E. coli. The antimicrobial susceptibility of E. coli was assessed using the Kirby–Bauer disk diffusion method. Results: Chemical analysis detected concentrations of antimicrobials throughout post treatment, reaching maxima of 42,910.14 μg kg⁻¹, 92,712 μg kg⁻¹, and 9567 μg kg⁻¹ for sulfachloropyridazine, oxytetracycline plus 4-epi-oxytetracycline, and enrofloxacin plus ciprofloxacin, respectively. It was estimated that the concentrations of sulfachloropyridazine, oxytetracycline, and enrofloxacin would persist in broiler litter for 61, 244, and 514 days, respectively. A very strong association was observed between the presence of antimicrobial residues and the antimicrobial resistance of E. coli (p-value < 0.0001, and Cramer’s coefficient of 0.47), and an independence between the level of residue concentration and susceptibility (p-value 0.5262). Conclusions: The persistence of antimicrobial residues contributes to the selection of resistant bacteria, regardless of persistent antimicrobial concentrations. These findings highlight the need for stricter regulations on poultry litter management, including residue thresholds and resistance monitoring, to minimize environmental and public health risks. Proper treatment of poultry litter is essential to ensure its sustainable and safe re-use in agricultural systems.

Introduction: Tylosin, a veterinary antimicrobial belonging to the macrolide family, is commonly used in the poultry industry. Residues generated from its use can be present in the litter and droppings of treated birds. Due to the diverse uses of poultry byproducts, such as fertilizing agricultural soils or incorporation into the diets of other animal species, there is a risk to public health, as the presence of antimicrobial residues favors the development of antimicrobial resistance, which is a global problem. Objective: This study aimed to evaluate the dissemination of tylosin residues from the litter and droppings of treated birds and untreated birds in a controlled broiler environment. Methods: Bird droppings and litter samples were collected and analyzed using HPLC-MS/MS to detect and quantify tylosin residues. Results: The residue concentrations detected in the dropping matrix only exceeded the Limits of Quantification (LOQ = 4 µg kg⁻¹) in the treated group. The litter matrix had statistically significant differences between the study groups. The persistence of tylosin residues in the litter of birds at day 42 was 290.16 µg kg⁻¹ in the treated group (A) and 9.35 µg kg⁻¹ in the adjacent untreated group (B.1). Conclusions: The results indicate that exposure distance influences tylosin residue dissemination.

Ampicillin residues in animal-derived foods may cause allergic reactions and promote antimicrobial resistance in consumers; however, data on residue behavior in poultry remain limited. This study aimed to evaluate the depletion of ampicillin in muscle and skin plus fat of broiler chickens. Thirty birds were treated with ampicillin intramuscularly (20 mg kg⁻¹ every 24 h for three days) and sacrificed at 0.5, 1, 2, 5, and 9 days post-administration. Samples were analyzed by liquid chromatography coupled with tandem mass spectrometry, a method successfully validated according to Commission Implementing Regulation (EU) 2021/808, VICH GL49 and GL2. Quantification was performed by linear regression from matrix-matched calibration curves. Residue depletion was evaluated following the European Medicines Agency guidelines. Ampicillin residues in muscle were detected only during the first 24 h post-administration (6.50–8.48 µg kg⁻¹). Residues in skin plus fat remained detectable until day 5 post-administration (6.87–59.88 µg kg⁻¹). Based on this, the withdrawal period calculated for skin plus fat was 9 days considering EU maximum residue limit (MRL) and 19 days considering method limit of quantification, with 95% confidence. These results provide critical data on ampicillin residue kinetics under controlled experimental conditions, supporting risk assessments and the establishment of MRLs in broiler chickens by the Codex Alimentarius.