JAC Advance Access published online on February 18, 2005
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dki025
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1 Service de Microbiologie, Hôpital Saint Antoine, Assistance Publique-Hôpitaux de Paris, Paris, France; Laboratoire de Bactériologie, EA 2392, UFR Saint Antoine, Université Paris 6, Paris, France
* To whom correspondence should be addressed. Objectives: The aim of this study was to characterize the ampC Methods: S. marcescens SMSA was isolated from an intra-abdominal wound of a patient previously treated with ceftazidime. A susceptible strain, SLS73, was used as a control. Susceptibility testing, PCR, DNA sequencing, molecular cloning, site-directed mutagenesis and determination of kinetic parameters were carried out to investigate the mechanism of resistance to ceftazidime. Results: MICs of ceftazidime were 64 and 0.2 mg/L for SMSA and SLS73, respectively. Sequencing of the ampC gene of SMSA was carried out. When compared with the closest AmpC enzyme, the S. marcescens S3 Conclusions: We present another example of in vivo selection of broad-spectrum resistance by amino acid substitution in the omega loop of chromosomal AmpC
Received October 4, 2004
Revised December 17, 2004
Accepted December 20, 2004
Original article
Resistance to ceftazidime is associated with a S220Y substitution in the omega loop of the AmpC
-lactamase of a Serratia marcescens clinical isolate
2 Service de Microbiologie, Hôpital Louis Mourier, Assistance Publique-Hôpitaux de Paris, Colombes, France
3 CNRS, Unité FRE 2125, Quimper, France
4 Service de Microbiologie, Hôpital Saint Antoine, Assistance Publique-Hôpitaux de Paris, Paris, France
5 Laboratoire de Bactériologie, EA 2392, UFR Saint Antoine, Université Paris 6, Paris, France; Service de Bactériologie, Hôpital Tenon, Assistance Publique-Hôpitaux de Paris, 4 rue de la Chine, 75970 Paris cedex 20, France
Guillaume Arlet, E-mail: guillaume.arlet{at}tnn.ap-hop-paris.fr
Abstract 
-lactamase gene of a clinical isolate of Serratia marcescens resistant to ceftazidime.-lactamase, the novel protein showed E57Q, Q129K and S220Y substitutions. The S220Y substitution is located in the omega loop. Introduced by mutagenesis in the ampC gene of SLS73, this substitution conferred the same level of resistance to ceftazidime. The catalytic efficiency (kcat/Km) of the mutated enzyme toward ceftazidime was increased by about 100-fold.-lactamase in S. marcescens.
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