Sub-inhibitory polyether ionophores enhance resistance plasmid transfer and transiently perturb the broiler gut...

Chronic sub-inhibitory antimicrobial exposures may shape antibiotic resistance (AMR) dissemination at the animal, food and environment interface. Polyether ionophore coccidiostats remain widely used in poultry production, yet their influence on AMR dissemination at sub-inhibitory exposure is unclear.

To determine whether sub-minimum inhibitory concentration (MIC) polyether ionophores enhance resistance plasmid transfer in vitro and to characterize their effects on gut microbiota and resistome dynamics in vivo during and after administration.

We investigated the effects of representative polyether ionophores at sub-MICs on resistance spreading phenotypes in vitro and gut resistome dynamics in VREfm-challenged broilers. In vitro plasmid conjugation and related phenotypes were quantified, and in vivo caecal microbiota and resistome were profiled by 16S rRNA gene sequencing and shotgun metagenomics.

Sub-MIC polyether ionophores increased plasmid conjugation, copy number and biofilm formation in Enterococcus spp., whereas no comparable effects were observed in Escherichia coli. In vivo, salinomycin temporarily disrupted caecal microbiota development and, at Day 20, suppression of indigenous taxa (e.g. Faecalibacterium) was accompanied by a transient surge in VREfm colonization and vanA abundance; resistome expansion was non-persistent. After salinomycin cessation, recovery of beneficial genera like Akkermansia was associated with reduction of the total resistance gene burden towards pre-treatment baseline by Day 42.

Polyether ionophores can promote resistance dissemination phenotypes in vitro, but gut ecological resilience may limit long-term impacts after cessation of exposure under recommended dosing conditions. The transient resistome surge during the treatment suggests increased shedding and potential environmental dissemination via manure, warranting surveillance and risk assessment.