Tetracycline Resistance by Bacteria in Response to Oxytetracycline-Contaminated Catfish Feed

Abstract

During an investigation of the development of tetracycline-resistant bacteria in channel catfish Ictalurus punctatus, a feed contaminated with oxytetracycline was inadvertently used. Tetracycline-resistant gram-negative bacteria were not detected in channel catfish intestinal contents (<1% of isolates) or aquarium water (<3% of isolates) before fish were stocked. However, gram-negative counts of the water increased three orders of magnitude within 1 d after fish were stocked in the aquarium and an additional two orders of magnitude after 10 d of feeding. Tetracycline resistance of the water bacteria ranged from 16 to 40% after an initial increase upon stocking but increased sharply during a 10-d feeding period (>96%). High resistance (>99%) was also observed in fish intestinal bacteria after 7 d of fish feeding. During the remaining 21 d without feeding, bacterial densities and tetracycline resistance remained high in the aquarium water. Although 50 bacterial isolates from feed were tetracycline resistant, differences in species composition from aquarium isolates suggested that the feed was not the source of resistant bacteria in the aquarium. Oxytetracycline was found in two of the three lots of one brad of feed but not in two lots of another brand. Oxytetracycline residues in the feed used in this experiment ranged from 311 to 355 μg/g. The presence of antibiotic contamination in feeds may increase bacterial resistance, thus raising health concerns for humans and fish.