Microbial response to antibacterial treatment in marine microcosms

Abstract

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 bacteria that were resistant to oxytetracycline (OTC), Romet ® 30 (drug consisting of sulfadimethioxine and ormetoprim), or amoxycillin. These antibacterials are routinely used for the treatment of diseased salmon. Since we had no control over the administration of medicated feed at the commercial fish farms, we supplemented the field studies with more controlled laboratory experiments. Twelve seawater-sediment microcosms were established using a seawater flow-through system. The microcosms were constructed using 38-1 aquaria in which sediment from two different sites near a single Puget Sound fish farm were placed in the aquaria and dosed with fish feed containing OTC or Romet ® 30. The dose 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, Romet ® 30, or amoxycillin were determined over a 60-day period. The population of cultivable organisms in the microcosms that received fish feed reached levels of 10 7 to 10 8 colony forming units g −1 regardless of whether the fish feed contained antibacterials. Microcosms that received either Romet ® 30 or OTC during the treatment period showed increases in both Romet ® 30- and OTC-resistant bacteria. Amoxycillin-resistance levels also increased in microcosms following treatment with OTC. Our results suggest that marine sediments respond with a rapid increase in the number of Romet ® 30- and OTC-resistant bacteria when exposed to either of these antibacterials. Also the addition of fish feed with or without antibacterials to marine sediments caused an increase in the concentration of cultivable bacteria.