ANTIBIOTIC-RESISTANCE IN BACTERIA ISOLATED FROM 3 FRESH-WATER FISH FARMS AND AN UNPOLLUTED STREAM IN DENMARK

Resistance profiles against two antibiotics, oxytetracycline and oxoli nic acid, were determined for 296 bacterial strains isolated from thre e fish farms (202 strains) and from an unpolluted stream (94 strains) in Denmark. Resistance against oxytetracycline (25 mug/ml) was seen am ongst 15% (29 strains) from fish farms and for 6% (6 strains) from an unpolluted stream. Oxolinic acid resistance was also more prevalent am ongst fish farm strains where 27% (53 strains) grew in the presence of 12.5 mug/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 th e resistance frequencies found in this study were not statistically di fferent, the use of chemotherapeutics may add to the environmental res istance 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 o f antibiotic. Six percent (13 strains) and 4% (4 strains) from fish fa rms and an unpolluted stream were resistant to both antibiotics. The i solates from the fish farms and the unpolluted stream were identified as Vibrionaceae, Enterobacteriaceae, Alcaligenes spp., Acinetobacter s pp., Pseudomonas spp., Moraxella spp., Moraxella-like and Shewanella p utrefaciens. Fifteen percent were Gram-positive organisms of which one half were resistant to oxolinic acid. Resistance profiles for the Gra m-negative bacteria were, in general, independant of species or groups and no difference was seen between isolates from three different trou t farms exposed to different environmental stress.