Abstract
Polyether ionophores are antimicrobial compounds used as feed additives in poultry feed to control diseases caused by coccidia. In addition to the anticoccidial activity of these compounds, polyether ionophores also contain antibacterial properties. Resistance to the polyether ionophore narasin was recently shown to exist on mobile plasmids in Enterococcus faecium and the resistance mechanism was suggested to be associated with a two-gene operon encoding an ABC-type transporter. In this study we demonstrate that the genes encoding the putative narasin resistance mechanism confers reduced susceptibility to the polyether ionophores narasin, salinomycin and maduramicin, but not to monensin and suggest that this resistance mechanism should be referred to as NarAB. Importantly, NarAB does not affect the susceptibility of E. faecium to any of the tested antimicrobial compounds that are used in clinical medicine. However, we show that conjugation in the presence of certain polyether ionophores increases the number of vancomycin resistant E. faecium suggesting that narasin and certain other polyether ionophores can contribute to the persistence of VRE in poultry populations.
Highlights
Polyether ionophores are used worldwide as feed additives in conventional rearing of poultry to control coccidiosis caused by the genus Eimeria (Kadykalo et al, 2018)
Considering the close association between narasin resistance and vancomycin resistance (Norm/Norm-Vet, 2001–2014; SVARM, 2011, 2013) it is possible that the use of narasin in animal husbandry has contributed to maintaining the VRE reservoir in animals in Europe
The bacteria were dislodged from the filter membranes and suspended in Brain heart infusion (BHI) broth, 10-fold serially diluted and plated on BHI agar supplemented with relevant antimicrobials
Summary
Polyether ionophores are used worldwide as feed additives in conventional rearing of poultry to control coccidiosis caused by the genus Eimeria (Kadykalo et al, 2018). In the early 1990s, a community reservoir of VRE was discovered in Europe, which raised concern for the public health (van Belkun et al., 1996; Endtz et al, 1997) It is believed it emerged as a result of widespread use of the glycopeptide avoparcin in animal husbandry (Aarestrup, 1995; Klare et al, 1995a,b; Aarestrup et al., 2000). Considering the close association between narasin resistance and vancomycin resistance (Norm/Norm-Vet, 2001–2014; SVARM, 2011, 2013) it is possible that the use of narasin (or other polyether ionophores) in animal husbandry has contributed to maintaining the VRE reservoir in animals in Europe. Due to a decision by the Norwegian broiler industry, broilers have been reared without in-feed anticoccidials since 2016 This may have contributed to a dramatic reduction in narasin resistant isolates in broilers in 2018 (Norm/Norm-Vet, 2019). Turkeys on the other hand, still receive in-feed monensin and the proportion of narasin-resistant E. faecium isolates from turkeys remained on a high level in 2018 (Norm/Norm-Vet, 2019)
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