Fish feed as a source of oxytetracycline-resistant bacteria in the sediments under fish farms

Concentrations of oxytetracycline and the frequency of oxytetracycline resistance in the environmental microflora were monitored following the therapeutic use of this agent at a marine fish farm. 529 kg of oxytetracycline were administered over a 24 day period at an average of 1.4 kg per cage per day. Three days after the end of the therapy 4.6 ± 3.7 μg/g oxytetracycline were detected in the sediments and the frequency of resistance in the sediment microflora was 9.0 ± 5.3%. A rise in the frequency of resistance in this flora to 26 ± 8.7% occurred 24 days after the therapy. This rise was not associated with any increase in the concentrations of oxytetracycline in the sediment. At this time the frequency of resistance in the flora isolated from mussels suspended above the sediments (36 ± 8.5%) was significantly (P = 0.005) higher than that present in the sediment flora. The feed used on the farm 24 days after the end of therapy was shown to contain 4.6 × 10⁴ oxytetracycline-resistant cfu/g. The distribution of phenotypic groups in the oxytetracycline-resistant flora in this feed and in the sediments during the peak in resistance were compared with those from other marine environments. These data demonstrated that resistant flora in feed can, under certain circumstances, significantly contribute to the resistant flora detected in sediments under fish cages.