Large plasmids of Escherichia coli and Salmonella encode highly diverse arrays of accessory genes on common replicon families

► Large plasmids are abundant in the ECOR and SAR reference collections. ► Two plasmid replicon families (IncF and IncI1) are predominant in these strains. ► Most of the plasmids have unique RFLP patterns, suggesting highly variable genomes. ► The virulence plasmid pSLT is the only plasmid type found in at least four strains.