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Open Access Research article

Fitness of Escherichia coli strains carrying expressed and partially silent IncN and IncP1 plasmids

Bruce Humphrey1, Nicholas R Thomson2, Christopher M Thomas3, Karen Brooks2, Mandy Sanders2, Anne A Delsol4, John M Roe4, Peter M Bennett1 and Virve I Enne15*

Author Affiliations

1 Bristol Centre for Antimicrobial Research, Department of Cellular and Molecular Medicine, University of Bristol, Medical Sciences Building, University Walk, Bristol, BS8 1TD, UK

2 Pathogen Genomics, Wellcome Trust Sanger Institute, Genome Campus, Hinxton, Cambridge, CB10 1SA, UK

3 School of Biosciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK

4 Division of Animal Health and Husbandry, Department of Clinical Veterinary Science, University of Bristol, Langford, BS40 5DU, UK

5 Centre for Immunology and Infectious Disease, Blizard Institute, Barts and The London School of Medicine and Dentistry, 4 Newark Street, London, E1 2AT, UK

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BMC Microbiology 2012, 12:53  doi:10.1186/1471-2180-12-53

Published: 4 April 2012

Abstract

Background

Understanding the survival of resistance plasmids in the absence of selective pressure for the antibiotic resistance genes they carry is important for assessing the value of interventions to combat resistant bacteria. Here, several poorly explored questions regarding the fitness impact of IncP1 and IncN broad host range plasmids on their bacterial hosts are examined; namely, whether related plasmids have similar fitness impacts, whether this varies according to host genetic background, and what effect antimicrobial resistance gene silencing has on fitness.

Results

For the IncP1 group pairwise in vitro growth competition demonstrated that the fitness cost of plasmid RP1 depends on the host strain. For the IncN group, plasmids R46 and N3 whose sequence is presented for the first time conferred remarkably different fitness costs despite sharing closely related backbone structures, implicating the accessory genes in fitness. Silencing of antimicrobial resistance genes was found to be beneficial for host fitness with RP1 but not for IncN plasmid pVE46.

Conclusions

These findings suggest that the fitness impact of a given plasmid on its host cannot be inferred from results obtained with other host-plasmid combinations, even if these are closely related.