Deletion of pcnB affects antibiotic susceptibility in resistant Escherichia coli by reducing copy number of ColE1-family plasmids.

Plasmids play a major role in the spread of antibiotic resistance genes in bacteria. Plasmid copy number (PCN) is often tightly regulated. In plasmids of the ColE1-type, this regulation happens by a negative feedback mechanism using an antisense RNA. Here, we employed a sequencing-based method for determining PCN to demonstrate that copy number of different ColE1-family plasmids harboring antibiotic resistance genes increases during antibiotic treatment. Further, we show that deletion of the gene pcnB reduces the copy number of ColE1-family plasmids in E. coli MG1655, which in turn results in a reduced resistance to antimicrobials of the classes aminoglycosides, β-lactams and tetracyclines. In the absence of antibiotic selection, the deletion of pcnB also decreased the number of ColE1-type plasmids in a bacterial population. Hence, PcnB, which polyadenylates RNA, marking it for decay, represents a potential drug and helper-drug target that could be used to reduce PCN to re-sensitize bacteria with multi-copy-number resistance-plasmids to treatment with different antimicrobials.