Plasmid encoded β-lactamases resistant to inhibition by clavulanic acid produced by calf faecal coliforms

Abstract Two new plasmid encoded β-lactamase enzymes produced by a strain of Escherichia coli and a strain of Citrobacter freundii isolated from calf faeces have been characterised. Both enzymes were similar to TEM-1 in terms of substrate and inhibition profiles and physical properties but differed from TEM-1 in being far less susceptible to the β-lactamase inhibitors clavulanic acid or tazobactam. In each case transfer of the the plasmid E coli K12 rendered it clinically resistant to the combination of amoxycillin and clavulanic acid. The β-lactamase from the E coli had an iso-electric point (pI) of 5·4 and was encoded on a plasmid of 95 Kbp which also mediated resistance to tetracycline, sulphonamides, apramycin, streptomycin and gentamicin. The β-lactamase from the C freundii had a pI of 5·2 and was encoded on a 75 Kbp plasmid which also mediated resistance to trimethoprim, chloramphenicol, apramycin, gentamicin and tobramycin.