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Antimicrobial Agents and Chemotherapy, October 2000, p. 2759-2763, Vol. 44, No. 10
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Contribution of Natural Amino Acid Substitutions in SHV Extended-Spectrum beta -Lactamases to Resistance against Various beta -Lactams

Corinne C. Randegger,1 André Keller,1 Michel Irla,1 Akihito Wada,2 and Herbert Hächler1,*

Institute of Medical Microbiology, University of Zürich, CH-8028 Zürich, Switzerland,1 and Department of Bacteriology, National Institute of Infectious Diseases, Shinjuku-ku, Tokyo 162-8640, Japan2

Received 29 November 1999/Returned for modification 28 March 2000/Accepted 16 July 2000

SHV extended-spectrum beta -lactamases (ESBLs) arise through single amino acid substitutions in the parental enzyme, SHV-1. In order to evaluate the effect of genetic dissimilarities around the structural gene on MICs, we had previously devised an isogenic system of strains. Here, we present an extended version of the system that now allows assessment of all major types of SHV beta -lactamases as well as of two types of promoters of various strengths. Moreover, we devised a novel vector, pCCR9, to eliminate interference of the selection marker. A substitution within the signal sequence, I8F found in SHV-7, slightly increased MICs, suggesting more efficient transfer of enzyme precursor into the periplasmic space. We also noted that combination of G238S and E240K yielded higher resistance than G238S alone. However, the influence of the additional E240K change was more pronounced with ceftazidime and aztreonam than with cefotaxime and ceftriaxone. The SHV enzymes characterized by the single change, D179N, such as SHV-8, turned out to be the weakest SHV ESBLs. Only resistance to ceftazidime was moderately increased compared to SHV-1.

* Corresponding author. Mailing address: Institute of Medical Microbiology, University of Zürich, P.O. Box, CH-8028 Zürich, Switzerland. Phone: 41-1-634-26 48. Fax: 41-1-634-4906. E-mail: haechler{at}immv.unizh.ch.

Antimicrobial Agents and Chemotherapy, October 2000, p. 2759-2763, Vol. 44, No. 10
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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