Antibiotic resistance in bacteria isolated from three freshwater fish farms and an unpolluted stream in Denmark

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Resistance profiles against two antibiotics, oxytetracycline and oxolinic acid, were determined for 296 bacterial strains isolated from three fish farms (202 strains) and from an unpolluted stream (94 strains) in Denmark. Resistance against oxytetracycline (25 μg/ml) was seen amongst 15% (29 strains) from fish farms and for 6% (6 strains) from an unpolluted stream. Oxolinic acid resistance was also more prevalent amongst fish farm strains where 27% (53 strains) grew in the presence of 12.5 μg/ml oxolinic acid. Only 16% (15 strains) of isolates from the unpolluted stream were resistant against oxolinic acid. Bacteria from the fish farms may spread with the water circulation and, although the resistance frequencies found in this study were not statistically different, the use of chemotherapeutics may add to the environmental resistance pool. Two types of inhibition curves were observed. One showed instant inhibition at a given concentration of antibiotic. The other curve showed a slow decrease in growth with increasing concentration of antibiotic. Six percent (13 strains) and 4% (4 strains) from fish farms and an unpolluted stream were resistant to both antibiotics. The isolates from the fish farms and the unpolluted stream were identified as Vibrionaceae, Enter-obacteriaceae, Alcaligenes spp., Acinetobacter spp., Pseudomonas spp., Moraxella spp., Moraxella-like and Shewanella putrefaciens. Fifteen percent were Gram-positive organisms of which one half were resistant to oxolinic acid. Resistance profiles for the Gram-negative bacteria were, in general, independant of species or groups and no difference was seen between isolates from three different trout farms exposed to different environmental stress.