Effects of oxytetracycline, oxolinic acid, and flumequine on bacteria in an artificial marine fish farm sediment

Oxytetracycline, oxolinic acid, and flumequine are at present the most frequently used antibacterial agents in the treatment of marine farmed fish in Norway. These substances are supplied with the food pellets and a substantial amount ends up in the sediment under the net pens. The effects of these antibacterial agents on the microbial community in a sediment were experimentally examined in tanks containing sediment to which was added either oxytetracycline, oxolinic acid, or flumequine, and supplied with a constant water flow. During the first 20 days of the experiment, approximately one-third of the antibacterial agents disappeared from the sediments. During the next 165 days another 25% vanished. There were elevated numbers of bacteria resistant to oxytetracycline and oxolinic acid a few days after the start of the experiment. The resistance that developed to oxytetracycline was higher and longer lasting than that to oxolinic acid and flumequine. Of the quinolones, flumequine seemed least disposed to induce resistance. The number of bacteria in all treated sediments decreased to 50–67% of the numbers in the control sediment 2 days after medication. Seven days after medication the sulfate reduction rates in the treated sediments were reduced to less than 10% of the rates found in the control. This effect lasted at least 29 days, but after 70 days the inhibition of the sulfate reduction in the treated sediments had disappeared. Key words: antibacterial agents, sediment, bacterial resistance, sulfate reduction.