TEM-1 and TEMpUC19β-lactamases can gain activity against ceftazidime and other expanded-spectrum cephalosporins via point mutation. The frequency of emergent resistance to ceftazidime at 4 × MIC was elevated ≥ 250-fold in hyper-mutable, MutS-deficient Escherichia coli harbouring these β-lactamase genes on high- or low-copy plasmids. Moreover, although ceftazidime-resistant mutants, or those with reduced susceptibility, were selected in both the wild-type and mutS hosts, many more mutants in the mutS host showed ceftazidimase-type extended-spectrum β-lactamase (ESBL) activity. This correlated with a G–A point mutation at position 484 in the blaTEM-1 and blaTEM-pUC19 genes, conferring the Arg164His amino-acid substitution found in the TEM-29 ESBL. Non-ESBL mutants lacked changes in blaTEM.