Characteristic patterns of susceptibility to β-lactam antibiotics are associated with different biovars of . To elucidate the basis for these differences, the β-lactamases of strains of biovars 4 (n = 63), 2 (n = 12) and 5 (n = 10) were characterised. PCR fragments were generated from the β-lactamase A () and B () genes; in addition, β-lactamase induction tests were performed with imipenem as the inducer and β-lactamase inhibition assays were undertaken with aztreonam and clavulanic acid. All the strains yielded PCR amplification fragments with primers to and . Biovar 4 strains had uniform patterns of β-lactamase induction and inhibition: uninduced biovar 4 strains predominantly expressed BlaA, but low-level expression of BlaB was also detected; after induction, biovar 4 strains predominantly produced BlaB. β-Lactamase expression varied between and within biovars 2 and 5: uninduced strains predominantly expressed either BlaA or BlaB, or exclusively BlaB; after induction BlaB was predominantly or exclusively expressed. Both the basal and induced levels of β-lactamase varied within biovars 2 and 5. Some biovar 5 strains were not inducible; these predominantly produced BlaA. The results of this study show that biovar 2, 4 and 5 strains contain both and , but that the expression of the enzymes is regulated differently between the biovars, and varies within biovars 2 and 5. There was some correlation between antibiogram and the clusters defined from the β-lactamase induction and inhibition tests, but it was not possible to predict β-lactamase expression profiles from MIC data.


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