title: Retrospective evidence for a biological cost of vancomycin resistance determinants in the absence of glycopeptide selective pressures.
abstract: OBJECTIVE
  To estimate the relative fitness differences between glycopeptide-resistant Enterococcus faecium (GREF) and glycopeptide-susceptible E. faecium (GSEF) from yearly surveillance data on the occurrence of GREF in Danish poultry farm environments.
  
  
  METHODS
  A population genetic model was adapted to retrospectively estimate the biological fitness cost of acquired resistance. Maximization of a likelihood function was used to predict the longitudinal persistence of acquired resistance.
  
  
  RESULTS
  Our analysis suggests strong selection against GREF following the 1995 ban on the glycopeptide growth promoter avoparcin. However, parameterizing the model with two selection coefficients suggesting a reduced negative effect of the acquired resistance on bacterial fitness over time significantly improved the fit of the model. Our analyses suggest that the acquired glycopeptide resistance will persist for >25 years.
  
  
  CONCLUSIONS
  Acquired resistance determinants in commensal E. faecium populations in Danish farm environments are likely to persist for decades, even in the absence of glycopeptide use.
authors.affiliation: Department of Pharmacy, Faculty of Health Sciences, University of Tromsø, 9037 Tromsø, Norway. paal.johnsen@uit.no
authors.forename: P J
authors.initials: PJ
authors.lastname: Johnsen
authors.forename: J P
authors.initials: JP
authors.lastname: Townsend
authors.forename: T
authors.initials: T
authors.lastname: Bøhn
authors.forename: G S
authors.initials: GS
authors.lastname: Simonsen
authors.forename: A
authors.initials: A
authors.lastname: Sundsfjord
authors.forename: K M
authors.initials: KM
authors.lastname: Nielsen
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D002353:Carrier State / Q000382:microbiology / Q000662:veterinary*; D003718:Denmark; D016984:Enterococcus faecium / Q000254:growth & development*; D006020:Glycopeptides / Q000494:pharmacology; D016908:Gram-Positive Bacterial Infections / Q000382:microbiology / Q000662:veterinary*; D011200:Poultry / Q000382:microbiology; D020713:Vancomycin Resistance
chemical_list: D000900:Anti-Bacterial Agents; D006020:Glycopeptides