3 BETA-LACTAMASES ISOLATED FROM AEROMONAS-SALMONICIDA, INCLUDING A CARBAPENEMASE NOT DETECTABLE BY CONVENTIONAL METHODS

The beta-lactamases of seven strains of Aeromonas salmonicida subsp. a chromogenes resistant to amoxicillin (MIC > 1024 mg/l) and responsible for furunculosis in farmed Atlantic salmon in Scotland were examined to establish the mechanisms of beta-lactam resistance. Separation of a cell-free extract on an isoelectric focusing gel stained with the chr omogenic cephalosporin nitrocefin showed the presence of two beta lact amases, one with a pI of 7.9 and the other with a pI of 6.0. Hydrolysi s assays of cell-free extracts of these strains demonstrated carbapene mase, penicillinase and cephalosporinase activity. However, when the b eta-lactamases were separated by anion exchange chromategraphy, the ca rbapenemase activity could not be retrieved in either of the peak frac tions containing the separated enzymes that had been visualised by nit rocefin. Consequently, a novel carbapenemase was discovered which cann ot be defected with nitrocefin.