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Antimicrobial Agents and Chemotherapy, February 2008, p. 441-445, Vol. 52, No. 2
0066-4804/08/$08.00+0     doi:10.1128/AAC.00359-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Selection of SHV Extended-Spectrum-β-Lactamase-Dependent Cefotaxime and Ceftazidime Resistance in Klebsiella pneumoniae Requires a Plasmid-Borne bla_SHV Gene

David S. Hammond,^1, Tegan Harris,^1, Jan Bell,² John Turnidge,² and Philip M. Giffard^1*

Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Queensland,¹ Women's and Children's Hospital, Adelaide, South Australia, Australia²

Received 16 March 2007/ Returned for modification 16 July 2007/ Accepted 31 October 2007

In Klebsiella pneumoniae, it is common for plasmid-located and chromosome-located bla_SHV copies to coexist within single cells. The plasmid-borne genes are mainly derived from two separate IS26-mediated mobilizations of bla_SHV. The objective of this study was to test the hypothesis that the presence of a non-extended-spectrum β-lactamase (non-ESBL) encoding plasmid-borne form of bla_SHV facilitates the cefotaxime (CTX)-mediated selection of ESBL-expressing mutants, even when there is a chromosomal copy of the same gene. Twenty-one diverse ESBL-negative, bla_TEM-negative K. pneumoniae clinical isolates were tested for the IS26 insertions characteristic of the two mobilization events. The isolates were then tested for their ability to be selected for ESBL-mediated CTX resistance by serial subculturing with a doubling of the CTX concentration at every subculture. Fourteen isolates possessed neither of the IS26 insertions. None of these became ESBL positive, and all died during the course of the experiment, despite possessing chromosomal bla_SHV copies. The other isolates all became ESBL positive and grew abundantly up to a CTX concentration of 128 µg/ml. Similar results were obtained with ceftazidime. ESBL expression was associated with the appearance of the expected GA mutation at position 1 of codon 238 and also with bla_SHV copy number amplification. It was concluded that plasmid-borne bla_SHV greatly facilitates the selection of ESBL expression, even when the same gene is on the chromosome, and that gene dosage effects are likely to contribute to this phenomenon.

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* Corresponding author. Mailing address: Institute of Health and Biomedical Innovation, Queensland University of Technology, Cnr Blamey St. and Musk Ave., Kelvin Grove, Queensland 4059, Australia. Phone: 61 7 31386194. Fax: 61 7 38641534. E-mail: p.giffard{at}qut.edu.au

Published ahead of print on 12 November 2007.

D.S.H. and T.H. contributed equally to this study.

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Antimicrobial Agents and Chemotherapy, February 2008, p. 441-445, Vol. 52, No. 2
0066-4804/08/$08.00+0     doi:10.1128/AAC.00359-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

This article has been cited by other articles:

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