Plasmid effects on Escherichia coli metabolism

The idea that plasmids replicate within hosts at the expense of cell metabo lic energy and preformed cellular blocks depicts plasmids as a kind of mole cular parasites that, even when they may eventually provide plasmid-carryin g strains with growth advantages over plasmid-free strains, doom hosts to b ear an unavoidable metabolic burden. Due to the consistency with experiment al data, this idea was rapidly adopted and used as a basis of different hyp otheses to explain plasmid-host interactions. In this article we critically discuss current ideas about plasmid effects on host metabolism, and presen t evidence suggesting that the complex interaction between plasmids and hos ts is related to the alteration of the cellular regulatory status.