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Antimicrobial Agents and Chemotherapy, July 2004, p. 2652-2658, Vol. 48, No. 7
0066-4804/04/$08.00+0     DOI: 10.1128/AAC.48.7.2652-2658.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Inhibitor-Sensitive AmpC ß-Lactamase Variant Produced by an Escherichia coli Clinical Isolate Resistant to Oxyiminocephalosporins and Cephamycins

Department of Bacterial Pathogenesis and Infection Control, National Institute of Infectious Diseases, Tokyo,¹ Suntory Institute for Bioorganic Research, Osaka, Japan²

Received 28 June 2003/ Returned for modification 8 November 2003/ Accepted 3 March 2004

Escherichia coli HKY28, a ceftazidime-resistant strain isolated from a urine specimen in Japan, produced an inhibitor-sensitive AmpC ß-lactamase variant. The deduced amino acid sequence of the enzyme contained a number of substitutions and a tripeptide deletion (Gly286-Ser287-Asp288) compared with the sequence of native AmpC of E. coli. When the deletion was reverted by a 9-base insertion at the relevant site of ampC in the clone, the typical inhibitor-resistant phenotype of AmpC was restored, while at the same time the levels of resistance to ceftazidime, cefpirome, and cefepime were reduced eightfold or more. Molecular modeling studies indicated that a structural change took place in the H-10 helix as a result of the deletion, and this change caused an alteration of the substrate binding site, leading to a unique phenotype analogous to that of inhibitor-sensitive class A extended-spectrum ß-lactamases. The degree of inhibition was greater with sulbactam and tazobactam than with clavulanic acid. To our knowledge, this is the first report to have characterized an E. coli ampC that encodes chromosomal AmpC ß-lactamase sensitive to the available ß-lactamase inhibitors.

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