MICROBIAL RESPONSE TO ANTIBACTERIAL TREATMENT IN MARINE MICROCOSMS

In a previous study of sediments near three salmon open-water net-cage farms in Puget Sound, Washington we determined over a 5-month period the number of cultivable bacteria and the percentage of cultivable bac teria that were resistant to oxytetracycline (OTC), Romet(R) 30 (drug consisting of sulfadimethioxine and ormetoprim), or amoxycillin. These antibacterials are routinely used for the treatment of diseased salmo n. Since we had no control over the administration of medicated feed a t the commercial fish farms, we. supplemented the field studies with m ore controlled laboratory experiments. Twelve seawater-sediment microc osms were established using a seawater flow-through system. The microc osms were constructed using 38-1 aquaria in which sediment from two di fferent sites near a single Puget Sound fish farm were placed in the a quaria and dosed with fish feed containing OTC or Romet(R) 30, The dos e of feed and the treatment schedule of antibacterials was intended to mimic salmon farm conditions. The number of cultivable bacteria, and the percentage of cultivable organisms that were resistant to OTC, Rom et(R) 30, or amoxycillin were determined over a 60-day period. The pop ulation of cultivable organisms in the microcosms that received fish f eed reached levels of 10(7) to 10(8) colony forming units g-l regardle ss of whether the fish feed contained antibacterials. Microcosms that received either Romet(R) 30 or OTC during the treatment period showed increases in both Romet(R) 30- and OTC-resistant bacteria. Amoxycillin -resistance levels also increased in microcosms following treatment wi th OTC. Our results suggest that marine sediments respond with a rapid increase in the number of Romet(R) 30- and OTC-resistant bacteria whe n exposed to either of these antibacterials. Also the addition of fish feed with or without antibacterials to marine sediments caused an inc rease in the concentration of cultivable bacteria. (C) 1997 Elsevier S cience B.V.