Mutants of the TEM-1 β-lactamase conferring resistance to ceftazidime

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Abstract
Spontaneous ceftazidime resistant mutants were obtained from an Escherichia coli K12 J62-2 expressing the TEM-1 β-lactamase (mutation frequency = 10−9). These mutants produced β-lactamases with similar molecular weights, kinetic parameters and isoelectric points (pI) to the β-lactamases produced by ceftazidime resistant clinical isolates which have recently been identified in this laboratory. Mutant enzyme A focused as a doublet band at pI 5·3 with an additional weak pI 5·4 band. The doublet co-focused with the TEM-E2 β-lactamase, produced by a ceftazidime resistant Klebsiella oxytoca isolate, which was originally obtained in a Liverpool hospital. Mutant enzyme B had a pI identical to the TEM-E1 β-lactamase produced by a ceftazidime resistant clinical isolate of E. coli found in Belgium. These results suggest that the two β-lactamases in the clinical strains may have come from simple mutations of the TEM-1 β-lactamase gene.