Transferable genes putatively conferring elevated minimum inhibitory concentrations of narasin in Enterococcus faecium from Swedish broilers

• An ABC-type transporter was found in sequenced isolates with increased narasin MIC.
• The transporter was not found in sequenced isolates without increased narasin MIC.
• A similar transporter confers resistance to tetronasin, another polyether ionophore.
• Presence of the ABC-type transporter was correlated to narasin MIC ≥2 mg/L.
• This is the first report of transferrable genes giving increased narasin MIC.

The minimum inhibitory concentration (MIC) of the polyether ionophore antibiotic narasin is elevated in a large proportion of Enterococcus faecium from Swedish broilers. The aim of this study was to identify gene(s) responsible for these elevated MICs.Six plasmids, four conferring vancomycin resistance and elevated MIC of narasin and two only conferring resistance to vancomycin, were sequenced. The genes for a putative mechanism for elevated MIC of narasin was used to design a PCR assay which in turn was used to screen 100 isolates of E. faecium from Swedish broilers.A 5.9 kb area was only found in the plasmids transferring elevated MIC of narasin. This area included two genes coding for an ABC-type transporter; an ‘ABC transporter permease protein’ and an ‘ABC-type multidrug transport system, ATPase component’. These genes are known to confer resistance to the ionophore tetronasin. PCR investigation confirmed a correlation between the presence of the genes and a MIC of narasin ≥2 mg/L.The results of this study indicate that the ABC permease together with the ABC ATPase are responsible for the elevated MIC of narasin present among E. faecium in Swedish broilers. To our knowledge, this is the first report of a putative transferable mechanism for elevated MIC of narasin.