Antibacterial residues, bacterial contaminants and their antibacterial susceptibility in raw milk vended in Kampala, Uganda

Abstract

The widespread use of antibacterials in animal production, combined with poor regulation in low-income countries, has led to the presence of antibacterial residues in food products like raw milk. In Uganda, the failure to observe withdrawal periods and the bacterial contamination of raw milk pose significant public health risks. Although bacterial contamination has been partially studied, data on antibacterial residues in Ugandan milk remain limited, raising ongoing concerns about milk safety. This study aims to determine the prevalence of antibacterial residues and assess bacterial contamination in raw milk sold across selected urban selling points in Kampala, Uganda. A cross-sectional study was conducted with 92 raw milk samples from urban selling points in Kampala. These samples were collected and analysed for antibacterial residues using LC-MS/MS at the Directorate of Government Analytical Laboratories, Bacterial contamination and antibiotic susceptibility patterns were analysed at the Veterinary Microbiology Research Laboratory located at the College of Veterinary Medicine, Animal Resources and biosecurity (COVAB). Data were recorded in Excel, cleaned, coded, and exported to RStudio (Version 2024.12.0+467) for analysis. Descriptive statistics and were used. The study revealed that 23.9% of the samples contained antibacterial residues. Oxytetracycline was the most common contaminant, followed by sulfamethoxazole, pyrazinamide, tylosin, amoxicillin, tetracycline, and penicillin, with concentrations of 8.7%, 7.6%, 6.5%, 3.2%, and 1.1%, respectively. Additionally, 14.1% of the samples had antibacterial residue concentrations greater than the MRL set. Bacterial contamination analysis revealed that 94.6% of the samples were contaminated with E. coli, 94.6%, S. aureus, 89.1%, and no sample with Salmonella. Dual-contamination was observed in 65.9% of the raw milk samples. Of the contaminated samples, 64.1% and 25% exceeded the satisfactory levels of 105 E. coli and 104 S. aureus, respectively. E. coli showed resistance to Ampicillin, 43.3%, and no resistance was observed with Azithromycin, while S. aureus was 11.7% and 48.3% resistant to SXT and tetracycline, respectively. The levels of antibacterial residues in the milk samples were within acceptable limits; however, bacterial contamination levels were considerably elevated.