Antimicrobial Residues, Antimicrobial Resistant Bacteria and Resistant Genes Isolated in Milk and Beef Marketed in Kabete, Kenya

Abstract

This study investigated the presence of antimicrobial residues, antimicrobial-resistant bacteria, and resistant genes in raw milk and beef sold in Kabete, Kenya. A total of 120 samples (60 milk, 60 beef) were analyzed using high-performance liquid chromatography (HPLC) and PCR-based molecular techniques. Antimicrobial residues were detected in 8.3% of milk and 5% of beef samples, with oxytetracycline and β-lactam antibiotics such as penicillin G and amoxicillin being the most common. Although residue levels were in low concentrations, their presence indicates lapses in observing drug withdrawal periods. Microbiological analysis revealed bacterial contamination in 10% of beef and 18.3% of milk samples. Escherichia coli was the most frequently isolated pathogen in milk, while Staphylococcus aureus and one methicillin-resistant S. aureus (MRSA) strain were detected in beef. Antimicrobial susceptibility testing showed widespread resistance to ampicillin, with some isolates exhibiting multidrug resistant (MDR) against up to five antibiotic classes. Molecular testing identified several resistant genes, including blaZ, blaTEM, blaCTX-M, mecA, tetA, and sul1. Notably, multidrug resistant gene combinations such as blaCTX-M/tetA/sul1 were found in milk isolates, highlighting the potential for transmission of highly resistant bacteria through the food chain. Strong positive correlations were observed between antimicrobial residues, bacterial contamination, and resistant gene presence. The findings underscore significant public health concerns linked to the consumption of contaminated animal products and emphasize the need for routine surveillance, strict enforcement of veterinary drug use regulations, and enhanced farmers’ education on antimicrobial stewardship. This study supports the One Health approach in addressing antimicrobial resistance and calls for integrated measures to ensure food safety and combat the spread of resistance from livestock to humans.