Amoxycillin resistance in Scottish isolates of Aeromonas salmonicida

Eighty isolates of Aeromonas salmonicida, recovered from separate outbreaks of furunculosis in farmed and wild salmon in Scotland during 1988 and 1989, were examined for susceptibility to the β‐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 0.30–1.50mg1‐1. All of the A. salmonicida subsp. achromogenes isolates tested were resistant to amoxycillin, with MICs in excess of 500mgl‐1. The A. salmonicida subsp. achromogenes produced a β‐lactamase enzyme with a pI 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.