Veterinary pharmaceuticals and antibiotic resistance of Escherichia coli isolates in poultry litter from commercial farms and controlled feeding trials

Veterinary pharmaceuticals are commonly used in poultry farming to prevent and treat microbial infections as well as to increase feed efficiency, but their use has created public and environmental health concerns. Poultry litter contains antimicrobial residues and resistant bacteria; when applied as fertilizer, the level and effects of these pharmaceuticals and antimicrobial-resistant bacteria in the environment are of concern. The purpose of this study was to investigate poultry litter for veterinary pharmaceuticals and resistance patterns of Escherichia coli. Litter samples were collected from controlled feeding trials and from commercial farms. Feed additives bacitracin, chlortetracycline, monensin, narasin, nicarbazin, penicillin, salinomycin, and virginiamycin, which were present in the feed on commercial farms and added to the feed in the controlled trials, were extracted in methanol and analyzed by liquid chromatography-mass spectrometry techniques. Sixty-nine E. coli were isolated and identified by API 20E. The susceptibility of the isolates to antibiotics was determined using Avian plates and the Sensititer automated system. This study confirmed the presence of antimicrobial residues in broiler litter from controlled environments as well as commercial farms, ranging from 0.07 to 66 mg/L depending on the compound. Concentrations of individual residues were higher in litter from controlled feeding trials than those from commercial farms. All E. coli isolates from commercial farms were multiresistant to at least 7 antibiotics. Resistance to β-lactam antibiotics (amoxicillin, ceftiofur), tetracyclines, and sulfonamides was the most prevalent. This study concluded that broiler litter is a source of antimicrobial residues and represents a reservoir of multiple antibiotic-resistant E. coli.