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 fre e of anthropogenic influences was determined on 2216 V agar with 25 mu g.g-1 oxytetracycline. The mean frequency of resistance in 153 samples taken in Galway Bay was 1.2 +/- 1.8%. The impact of oxytetracycline t herapy on the frequency of resistance in the sediments under a marine fish farm was investigated on two occasions. In the first investigatio n, 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 w as detected. In the second investigation, oxytetracycline was detected at a concentration of 10.9 +/- 6.5 mug.g-1 in the sediments under a c age block that received 175 kg oxytetracycline over 12 days. The frequ ency of resistance reached 16.0 +/- 8.9% after the treatment. The freq uency 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-cag e samples, was not significantly higher than the background level. At the end of the therapy elevated frequencies of resistance were detecte d 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 r esistance in all samples not directly under the cages was not signific antly higher than in samples taken from sediments free of anthropogeni c influence.