Bacterial resistance to oxytetracycline in Chilean salmon farming

The use of oxytetracycline for preventing and controlling bacterial pathogens in salmon farming is frequent in Chile, yet no studies have been performed to evaluate the ecological impact of its intensive and prolonged use. In this work, the frequency of oxytetracycline-resistant bacteria from water, pelletized feed and fingerlings from four Chilean freshwater Atlantic salmon farms, as well as the level of resistance of selected strains was investigated. Viable bacterial counts were performed by a spread plate method and antibiotic-resistant bacteria were counted in culture media supplemented with the selected antibiotic. Resistance levels of selected strains isolated in media containing antibiotic were determined using an agar plate dilution method. High proportions of low- and high-level oxytetracycline-resistant bacteria (selected in agar plates containing 30 and 100 mug/ml, respectively), mainly from pelletized feed and effluent samples of the fish farms were found. The highest proportions of resistant bacteria were found in the effluent samples, and were significantly higher ( P < 0.05, Tukey's test) than those from the other samples studied. On the contrary, influent samples exhibited the lowest proportions of resistant bacteria. One hundred and three resistant Gram-negative isolates, which represented the oxytetracycline-resistant bacterial population, were randomly selected on TSA containing 30 or 100 mug/ml of oxytetracycline, from salmon farms and pellet samples, and streaked for purification on TSA plates without oxytetracycline. A large number of non-fermenting bacteria (77.7%) were identified. Among these, Pseudomonas fluorescens (28.2%), Stenotrophomonas maltophilia (5.8%), Sphingomonas paucimobilis (5.8%), Acinetobacter lwoffii (4.8%), and Pseudomonas putida (4.8%) were the most frequent. Also, an important number of strains of Aeromonas hydrophila (9.7%), Burkholderia cepacia (3.9%), Brevundimonas vesicularis (3.9%), Acinetobacter johnsonii (2.9%), Pantoea sp. (2.9%) and Moraxella sp. (2.9%) were found. P fluorescens and A. hydrophila predominated in salmon fingerlings, whereas A. lwoffii and S. maltophilia were predominant in pellet samples. Selected strains exhibited high levels of oxytetracycline resistance, with minimum inhibitory concentrations (MICs) ranging from 64 to 2048 mug/ml, whereas MIC90 of oxytetracycline varied between 1024 and 2048 mug/ml. This study shows the presence of an important population of oxytetracycline-resistant bacteria in the microflora of Chilean salmon farms. Therefore, die environment of these farms might play important roles as reservoirs of bacteria carrying genetic determinants for hi.-h-level tetracycline resistance, prompting an important risk to public health for workers involved in fish culturing and processing. (C) 2002 Elsevier Science B.V. All rights reserved.