In vitro efficacy of six antibiotics and plasma concentrations of two quinolones in Atlantic salmon (Salmo salar)

The minimum inhibitory concentrations (MICs) of six antibiotics were determined for Aeromonas salmonicida, Vibrio anguillarum and Vibrio ordalii isolated from disease outbreaks in fish on the Atlantic and Pacific coasts. MICs were performed at three incubation temperatures (10, 20, 30$\sp\circ$C). Incubation temperature influenced the MIC for some antibiotics. The most pronounced case was a two-fold or greater increase in the MIC of oxytetracycline against Vibrio spp. with increasing temperature. Changes in resistance patterns were examined in the isolates collected from 1985 to 1990. The in vitro efficacy of the antibiotics examined ranged from the potent quinolones (oxolinic acid and sarafloxacin) to ormetoprim-sulfadimethoxine and oxytetracycline, to the relatively ineffective drugs erythromycin and streptomycin.

The plasmid profiles of oxytetracycline and streptomycin resistant and sensitive bacterial isolates were examined by agarose gel electrophoresis. Resistant isolates were examined when grown in the presence and in the absence of antibiotic. Resistance to these antibiotics did not appear to be plasmid-mediated. (Abstract shortened by UMI.).