Antibiotic residues and antimicrobial resistance patterns of bacteria isolated from beef at Ruti Rufura Slaughterhouse, southwestern Uganda

Abstract

Background: Antimicrobial use in livestock contributes to the emergence of antimicrobial-resistant bacteria and the persistence of antimicrobial residues in foods of animal origin. The coexistence of antimicrobial residues and resistant bacteria in beef poses a major food safety and public health concern. However, data on these hazards in Uganda remain limited. This study assessed antibiotic residues and antimicrobial resistance among bacteria isolated from beef marketed in southwestern Uganda. Methods: A cross-sectional laboratory-based study was conducted from May to August 2025 using beef samples from 92 randomly selected cattle carcasses at Ruti Rufura Slaughterhouse, Mbarara City, Uganda. Antibiotic residues were screened by agar well diffusion and confirmed for penicillin and oxytetracycline using thin-layer chromatography. Bacterial isolates were identified using standard microbiological methods, and antimicrobial susceptibility was determined by the Kirby–Bauer disk diffusion method according to CLSI guidelines. Results: Antibiotic residues were detected in 86/92 (93.5%) samples. Thin-layer chromatography confirmed penicillin residues in 84 (97.7%) and oxytetracycline residues in 71 (82.6%) samples, with 73.1% containing both antibiotics. A total of 125 bacterial isolates representing seven genera were recovered, predominantly Proteus spp. (30.4%), Citrobacter spp. (26.4%), and Escherichia coli (16.8%). Among 75 viable isolates, resistance was highest to ampicillin (97.3%), chloramphenicol (86.7%), tetracycline (82.7%), ceftriaxone (57.3%), and ciprofloxacin (54.7%), while gentamicin showed the highest susceptibility (86.7%). Oxytetracycline residues were significantly associated with tetracycline resistance and penicillin residues with ampicillin resistance (both P < 0.001). Conclusion: Beef marketed in southwestern Uganda frequently contains antimicrobial residues and antimicrobial-resistant bacteria, posing a potential food safety risk. Strengthened antimicrobial stewardship, enforcement of withdrawal periods, and integrated surveillance of antimicrobial residues and resistance in foods of animal origin are needed to support One Health efforts against antimicrobial resistance.