Frequency and distribution of resistance to oxytetracycline in microorganisms isolated from marine fish farm sediments following therapeutic use of oxytetracycline.

The background level of resistance to oxytetracycline in sediments free of anthropogenic influences was determined on 2216 V agar with 25 mug.g-1 oxytetracycline. The mean frequency of resistance in 153 samples taken in Galway Bay was 1.2 + - 1.8%. The impact of oxytetracycline therapy on the frequency of resistance in the sediments under a marine fish farm was investigated on two occasions. In the first investigation, oxytetracycline was detected at a concentration of 9.9 + - 2.9-mug.g-1 in the sediments under a cage that received 865 g oxytetracycline per day for 10 days, but no significant rise in resistance frequency was detected. In the second investigation, oxytetracycline was detected at a concentration of 10.9 + - 6.5 mug.g-1 in the sediments under a cage block that received 175 kg oxytetracycline over 12 days. The frequency of resistance reached 16.0 + - 8.9% after the treatment. The frequency declined at an exponential rate (r2=0.89) with a half-life of 26 days. At 73 days after the end of therapy the frequency, in under-cage samples, was not significantly higher than the background level. At the end of the therapy elevated frequencies of resistance were detected up to 75 m from the edge of the cage block and in samples where the levels of oxytetracycline were below the limit of detection (1.2 mug.g-1). Thirty-three days after the end of the therapy the frequency of resistance in all samples not directly under the cages was not significantly higher than in samples taken from sediments free of anthropogenic influence.