Antibiotic susceptibility of strains of Aeromonas salmonicida isolated from spanish salmonids

The antibiotic susceptibility of a group of Aeromonas salmonicida strains, isolated from Atlantic salmon (Salmo salar) and Brown trout (Salmo trutta) in some fish farms and rivers of Spain, has been studied. A high percentage of resistant strains was observed, particularly for some antibiotics that often are used in aquaculture, like nalidixic acid and oxytetracycline. Similar results were obtained with chloramphenicol, greatly used some years ago in aquaculture but forbidden in recent years. In contrast, the major part of isolates was highly susceptible to amoxicillin and flumequine or florfenicol, florfenicol being the antibiotic with the highest efficacy. Antibiotic resistance in the isolated Aeromonas salmonicida increases with the age of the animals on which the bacteria has been isolated, while no differences were found in the fish species or origin (fish farm or river).