Unusual tazobactam-sensitive AmpC beta-lactamase from two Escherichia coli isolates.

Two Escherichia coli isolates were studied. MIC patterns and hydrolysis assays suggested that they hyperproduced AmpC beta-lactamase, but synergy between ceftazidime and tazobactam was greater than between ceftazidime and Ro 48-1256, whereas the converse pattern is typical of AmpC hyperproducers. Studies with purified beta-lactamase from one of the isolates confirmed that tazobactam was a 100-fold stronger inhibitor than for the classical E. coli AmpC enzyme. Moreover, in contrast to typical AmpC types, the new enzyme had greater affinity for cephaloridine than for benzylpenicillin.