Antibacterial resistant bacteria in surficial sediments near salmon net-cage farms in Puget Sound, Washington

Antibacterials are used in medicated fish feed at fish farms located in Puget Sound, Washington. These compounds include oxytetracycline (OTC), amoxycillin, and Romet(R) 30 (a drug composed of sulfadimethioxine and ormetoprim). In this study we collected surficial sediment samples at three different commercial salmon net-cage farms during the summer and early fall of 1992. The three different farms varied in their use of antibacterials. Before beginning our field investigation we developed a modification of Mueller Hinton Agar to enumerate antibacterial-resistant bacteria. A synthetic seawater was added at 70% concentration to Mueller Hinton Agar. Because the divalent cations present in seawater chelate OTC, the concentration of this antibacterial was increased in the seawater medium relative to concentrations typically used in clinical microbiology laboratories. We enumerated the total number of cultivable bacteria, and bacteria resistant to oxytetracycline, amoxycillin, or Romet(R) 30 by plating aliquots of sediment samples onto the marine Mueller Hinton Agar. The highest numbers of bacteria (10(6) to 10(8) colony forming units g(-1)) were generally found nearest the net-cages with densities declining about an order-of-magnitude further away from the cages. Farm A, which used the greatest amount of antibacterials of the three farms, had the highest percentages of antibacterial-resistant bacteria in the sediments. At this Farm resistance to Romet(R) 30 and OTC tended to parallel each other, suggesting either a common resistance mechanism or linkage of the genes responsible for the resistances. Farm C used the least amount of antibacterials and this was reflected in that this farm also had the lowest percentage of sedimentary bacteria that were resistant to the antibacterials. Although the total number of bacteria at Farm C was in a similar range as found at Farms A and B, the percentages of antibacterial resistant bacteria were 5% or less for most of the samples. Our results suggest that a background of less than 5% of the cultivable bacteria in marine sediments is resistant to OTC, Romet(R) 30, or amoxycillin.