Amoxicillin Resistance in Scottish Isolates of Aeromonas-Salmonicida

Eighty isolates of Aeromonas salmonicida, recovered from separate outbreaks of furunculosis in famed and wild salmon in Scotland during 1988 and 1989, were examined for susceptibility to the beta-lactam antibiotic amoxycillin. Susceptibility was determined in terms of minimum inhibitory concentration (MIC). All of the A. salmonicida subsp. salmonicida isolates investigated were susceptible to amoxycillin, with MICs of 0.30-1.50 mg l-1. All of the A. salmonicida subsp. achromogenes isolates tested were resistant to amoxycillin, with MICs in excess of 500 mg l-1. The A. salmonicida subsp. achromogenes produced a beta-lactamase enzyme with a pl of approximately 8.0. The enzyme was inducible and its production was unaffected by plasmid curing with ethidium bromide, suggesting that resistance was chromosomal rather than plasmid mediated