Microbial resistance to oxytetracycline in sediments from salmon aquaculture sites in the Western Isles Region of the Bay of Fundy

Resistance to the antibiotic oxytetracycline (OTC) was determined for aerobic bacteria cultured from surface sediments collected in August–September 2000 at, near, and distant from salmon aquaculture farm sites in the Western Isles Region of the Bay of Fundy. A simple detection protocol was developed which monitored growth inhibition by following changes in optical density over 72 hours in serially diluted samples placed in microtiter plates. Concentrations of OTC varied from 0 to 160 μg ml−1. Samples (n = 52) collected under and around (within 100 m) salmon farms showed OTC resistance above the threshold for establishing resistance (MIC > 25 μg ml −1). Controls consisting of a type culture (Aeromonas salmonicida), sediment from >100 m away from salmon net pens and from intertidal areas and embayments in New Brunswick and Nova Scotia where salmon aquaculture has not been developed (n = 5) showed no resistance to OTC (MIC <10 μg ml−1). Resistant microbial strains were isolated from farm samples and identified by PCR analysis of the 16S ribosomal gene. The results of this analysis were confirmed by standard serological methods.