Successful Spread of Mcr-1-bearing IncX4 Plasmids is Associated with Variant in Replication Protein of IncX4 Plasmids

IncX4 plasmids are one of the most epidemiologically successful vehicles for mcr-1 spread. Here we found that the IncX4 plasmids carried two different replication proteins encoded by genes pir-1 and pir-2, respectively, but mcr-1 was only carried by IncX4 plasmid encoding pir-1. The copy number of pir-2 encoding plasmids (3.15 +/- 0.9 copies) are higher than that of pir-1 encoding plasmids (0.85 +/- 0.5 copies). When mcr-1 was cloned into IncX4 plasmid encoding pir-2, the higher copy number of these plasmids resulted in increased expression of mcr-1 and a greater fitness burden on their host cells. However, these plasmids exhibited a lower rate of invasion into the bacterial population compared with mcr-1-positive plasmids encoding the pir-1 gene. These findings collectively explain the absence of mcr-1 in all IncX4 plasmids encoding pir-2. Our results further confirmed that low -copy numbers are important for the spread of mcr-1 plasmid from the perspective of natural evolution. (c) 2024 The Author(s). Published by Elsevier Ltd on behalf of International Society for Antimicrobial Chemotherapy. This is an open access article under the CC BY -NC -ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )