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Antimicrobial Agents and Chemotherapy, September 2006, p. 3003-3010, Vol. 50, No. 9
0066-4804/06/$08.00+0     doi:10.1128/AAC.00137-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Evidence of Antibiotic Resistance Gene Silencing in Escherichia coli

Virve I. Enne,1* Anne A. Delsol,2 John M. Roe,2 and Peter M. Bennett1

Department of Cellular and Molecular Medicine, University of Bristol, Medical Sciences Building, University Walk, Bristol BS8 1TD, United Kingdom,1 Division of Farm Animal Science, Department of Clinical Veterinary Science, University of Bristol, Langford BS40 5DU, United Kingdom2

Received 2 February 2006/ Returned for modification 19 March 2006/ Accepted 11 June 2006

The possibility that unexpressed antibiotic resistance genes are carried by bacterial genomes is seldom investigated. Potential silencing of the resistance genes blaOXA-2, aadA1, sul1, and tetA carried on the plasmid pVE46 in a recent porcine isolate of Escherichia coli was investigated following oral inoculation of the strain into organic piglets. A small proportion of isolates recovered from feces did not express one or more resistance genes, despite retaining the pVE46 plasmid. Different combinations of unexpressed resistance genes were observed, and 12 representative isolates were selected for further study. Surprisingly, in most cases the resistance genes and their promoters, although not expressed, were intact, with fully wild-type sequences. Apart from four isolates exhibiting intermediate-level tetracycline resistance, no mRNA for the unexpressed genes was detected. Silencing of resistance genes was reversible at low frequencies between 10–6 and 10–10. Introduction of the plasmid from silenced isolates to another strain restored expression, indicating that gene silencing was a property of the host chromosome rather than the plasmid itself. When the same recent porcine E. coli strain carrying the unrelated plasmid RP1 was inoculated into piglets, three isolates (of 9,492) that no longer expressed RP1-encoded resistance genes were recovered. As with pVE46, in most cases the coding sequences and promoter regions of these genes were found to be intact, but they were not transcribed. Such gene silencing indicates a previously unrecognized form of transcriptional control that overrides standard expression signals to shut down gene expression. These findings suggest that unexpressed resistance genes may occur in the wild and hence may have clinical implications.

* Corresponding author. Mailing address: Department of Cellular and Molecular Medicine, University of Bristol, Medical Sciences Building, University Walk, Bristol BS8 1TD, United Kingdom. Phone: 44-117-9287522. Fax: 44-117-9287896. E-mail: v.i.enne{at}bristol.ac.uk.

Antimicrobial Agents and Chemotherapy, September 2006, p. 3003-3010, Vol. 50, No. 9
0066-4804/06/$08.00+0     doi:10.1128/AAC.00137-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Liu, J., Keelan, P., Bennett, P. M., Enne, V. I. (2009). Characterization of a novel macrolide efflux gene, mef(B), found linked to sul3 in porcine Escherichia coli. J Antimicrob Chemother 63: 423-426 [Abstract] [Full Text]  
  • Rosengren, L. B., Waldner, C. L., Reid-Smith, R. J. (2009). Associations between Antimicrobial Resistance Phenotypes, Antimicrobial Resistance Genes, and Virulence Genes of Fecal Escherichia coli Isolates from Healthy Grow-Finish Pigs. Appl. Environ. Microbiol. 75: 1373-1380 [Abstract] [Full Text]  


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