Objectives: 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.