Microbial profiling, antibiotic susceptibility, and residue detection in market-ready tilapia from key aquaculture regions in Zambia

Introduction: Aquaculture contributes substantially to food and nutrition security in Zambia, but bacterial contamination, antimicrobial resistance (AMR), and antimicrobial residues threaten fish safety and production sustainability. This study investigated bacterial profiles, antibiotic susceptibility patterns, and antimicrobial residues in market-ready Oreochromis niloticus (Nile tilapia) from Kafue and Siavonga, two major aquaculture districts in Zambia. Methods: A cross-sectional study was conducted on 118 apparently healthy market-ready Nile tilapia collected from 11 farms representing small-, medium-, and large-scale pond and cage culture systems. Bacterial isolation and presumptive identification were performed using selective culture media, Gram staining, and biochemical tests. Antibiotic susceptibility testing was conducted using the disk diffusion method following Clinical and Laboratory Standards Institute guidelines. Antimicrobial residues in fish muscle were analysed using ultra-high-performance liquid chromatography tandem mass spectrometry. Results: Bacterial occurrence varied by production system and farm scale. Presumptive Vibrio spp. (47.5%), Lactococcus garvieae (37.3%), Salmonella spp. (35.6%), and Escherichia coli (15.3%) were the most frequently detected bacteria. Vibrio spp. was significantly more prevalent in pond culture (p<0.001), while L. garvieae was also significantly associated with production system (p=0.016). E. coli showed 100% resistance to ampicillin, while L. garvieae exhibited complete resistance to tested antibiotics in cage culture. The multiple antibiotic resistance index ranged from 0.04 to 0.48. Only one sample tested positive for penicillin G residue. Conclusion: Market-ready Nile tilapia in Zambia harbour potentially important foodborne and multidrug-resistant bacteria. Strengthened biosecurity, antimicrobial stewardship, and routine AMR and residue surveillance are needed to support safe and sustainable aquaculture production.