Smyth, C.; Leigh, R. J.; Do, T. T.; Walsh, F.

Plasmids play a crucial role in the spread of antimicrobial resistance genes (ARGs) across One Health due to their ability to transfer a wide range of ARGs within and across bacterial species and biomes. We sequenced 173 circularised plasmids transferred from wastewater treatment plant (WWTP) effluent into Escherichia coli and subsequently characterised their genetic content. Multiple multidrug resistant plasmids were identified with a significant number of mega plasmids (>100Kb). Plasmids existing in isolation were rare and almost all existed with other plasmids. Our results suggest that positive epistasis promotes plasmid persistence in WWTP populations in a similar manner to that identified in vitro via infectious transmission, varying properties against plasmid community backgrounds, interactions with a range of other plasmids, source-sink spill-over transmission within the plasmid community rather than the host bacteria and compensatory mutations. We have demonstrated that the plasmid paradox solutions apply to plasmid communities in addition to plasmid host interactions. Our study identified that rather than existing as lone entities plasmids co-exist in small packs, the protection is afforded to the pack not by all members but by one or two and many plasmids coast within this pack as they contain no obvious advantage to the host. Our findings show that we need to enter a new paradigm and study plasmids in packs rather than as single entities in order to understand their transmission across One Health.