Antibiotic Residues and Antibiotic-Resistant Bacteria in Pig Slurry used to Fertilize Agricultural Fields

Pig manure may contain antibiotic residues, antibiotic-resistant bacteria or pathogens, which may reach the environment upon fertilization. During this study, 69 antibiotic residues belonging to 12 classes were quantified in 89 pig slurry samples. These samples were also studied for the presence of Salmonella and for E. coli resistant to meropenem, colistin, ciprofloxacin, or cefotaxim. The obtained isolates were further tested for antibacterial susceptibility. No antibiotic residues were detected in four samples, whereas in the other samples, up to 12 antibiotics were found. The most frequently detected antibiotic residues were doxycycline, sulfadiazine, and lincomycin. Doxycycline was found in the highest concentration with a mean of 1476 μg/kg manure (range: 18–13632 μg/kg). Tylosin and oxytetracycline were found with mean concentrations of 784 μg/kg (range: 17–5599 μg/kg) and 482 μg/kg (range: 11–3865 μg/kg), respectively. Lincomycin, had a mean concentration of 177 μg/kg manure (range: 9–3154 μg/kg). All other 18 antibiotic residues were found with mean concentrations of less than 100 μg/kg manure. Fifty-one slurry samples harbored Salmonella; 35% of the Salmonella isolates were sensitive to a panel of 14 antibiotics, whereas the other 65% were resistant up to five antibiotics. For E. coli, 52 manure samples contained E. coli isolates which were resistant to ciprofloxacin and 22 resistant to cefotaxime. All ciprofloxacin and cefotaxime-resistant isolates were multi-resistant, with resistance up to nine and eight antibiotics, respectively. This research indicates that pig slurry used for fertilization often contains antibiotic residues and antibiotic-resistant bacteria, including pathogens.