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 i n Puget Sound, Washington. These compounds include oxytetracycline (OT C), amoxycillin, and Romet(R) 30 (a drug composed of sulfadimethioxine and ormetoprim). In this study we collected surficial sediment sample s at three different commercial salmon net-cage farms during the summe r and early fall of 1992. The three different farms varied in their us e of antibacterials. Before beginning our field investigation we devel oped a modification of Mueller Hinton Agar to enumerate antibacterial- resistant bacteria. A synthetic seawater was added at 70% concentratio n to Mueller Hinton Agar. Because the divalent cations present in seaw ater chelate OTC, the concentration of this antibacterial was increase d 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, amoxyc illin, 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 ne t-cages with densities declining about an order-of-magnitude further a way from the cages. Farm A, which used the greatest amount of antibact erials of the three farms, had the highest percentages of antibacteria l-resistant bacteria in the sediments. At this Farm resistance to Rome t(R) 30 and OTC tended to parallel each other, suggesting either a com mon resistance mechanism or linkage of the genes responsible for the r esistances. Farm C used the least amount of antibacterials and this wa s reflected in that this farm also had the lowest percentage of sedime ntary 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 we re 5% or less for most of the samples. Our results suggest that a back ground of less than 5% of the cultivable bacteria in marine sediments is resistant to OTC, Romet(R) 30, or amoxycillin.