title: Fitness of Escherichia coli strains carrying expressed and partially silent IncN and IncP1 plasmids.
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.
authors.affiliation: Bristol Centre for Antimicrobial Research, Department of Cellular and Molecular Medicine, University of Bristol, Medical Sciences Building, University Walk, Bristol, BS8 1TD, UK.
authors.forename: Bruce
authors.initials: B
authors.lastname: Humphrey
authors.forename: Nicholas R
authors.initials: NR
authors.lastname: Thomson
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Thomas
authors.forename: Karen
authors.initials: K
authors.lastname: Brooks
authors.forename: Mandy
authors.initials: M
authors.lastname: Sanders
authors.forename: Anne A
authors.initials: AA
authors.lastname: Delsol
authors.forename: John M
authors.initials: JM
authors.lastname: Roe
authors.forename: Peter M
authors.initials: PM
authors.lastname: Bennett
authors.forename: Virve I
authors.initials: VI
authors.lastname: Enne
mesh_terms: D000818:Animals; D004269:DNA, Bacterial / Q000235:genetics; D024881:Drug Resistance, Bacterial / Q000235:genetics*; D004926:Escherichia coli / Q000235:genetics* / Q000472:pathogenicity; D020868:Gene Silencing; D056084:Genetic Fitness; D008969:Molecular Sequence Data; D010957:Plasmids / Q000235:genetics*; D013552:Swine
chemical_list: D004269:DNA, Bacterial