Increased copy number couples the evolution of plasmid horizontal transmission and plasmid-encoded antibiotic resistance

Significance Antimicrobial resistance (AMR) is typically caused by genes present on plasmids, genetic parasites that can rapidly spread between bacterial cells. We demonstrate that plasmids can evolve increased transmission when hosts are abundant by increasing the number of plasmid molecules present within cells. A side effect of having more plasmids in a cell is a greater degree of cellular AMR. Selection for higher cellular levels of AMR in turn resulted in increased transmission via the same mechanism. Opportunities for plasmid transmission thus promote the evolution of plasmids that are highly infectious and confer high levels of AMR on their hosts.