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JAC Advance Access originally published online on June 7, 2007
Journal of Antimicrobial Chemotherapy 2007 60(2):269-273; doi:10.1093/jac/dkm191
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© The Author 2007. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Emergence and maintenance of resistance to fluoroquinolones and coumarins in Staphylococcus aureus: predictions from in vitro studies

A. A. Vickers, A. J. O'Neill and I. Chopra*

Antimicrobial Research Centre and Research Institute of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT, UK

Received 19 January 2007; returned 16 April 2007; revised 28 April 2007; accepted 8 May 2007

* Corresponding author. Tel: +44-113-343-5604; Fax: +44-113-343-1407; E-mail: i.chopra{at}leeds.ac.uk

Objectives: Fluoroquinolones and coumarins interfere with the activity of bacterial type II topoisomerase enzymes. We examined the development of resistance to these agents in Staphylococcus aureus and determined the effect of simultaneous topoisomerase IV and DNA gyrase mutations on the biological fitness of the organism. This work aimed to gain insight into how such mutants might arise and survive in the clinical environment.

Methods: Spontaneous mutants resistant to fluoroquinolones and coumarins were selected in S. aureus. Resistance mutations were identified by DNA sequencing of PCR amplicons corresponding to the genes encoding topoisomerase IV and DNA gyrase. In vitro fitness of resistant mutants was compared with the antibiotic-susceptible progenitor strain using pair-wise competition assays.

Results: Mutants simultaneously resistant to both a fluoroquinolone and either of the coumarins, novobiocin or coumermycin A1, could not be recovered following a single-step selection. However, mutants concurrently resistant to both classes of antimicrobial could be generated by step-wise selections. These mutants demonstrated reductions in competitive fitness of up to 36%.

Conclusions: Dual-targeting of topoisomerase IV and DNA gyrase enzymes, for example with the combination of a fluoroquinolone and a coumarin agent, could minimize the emergence of resistance to these drugs in S. aureus. However, resistance-associated fitness costs may not be sufficient to limit the survival of mutants with dual resistance, if they arose in the clinical setting.

Keywords: antibacterial agents , mutation , fitness


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