Evolution of bacterial surface exclusion against incompatible plasmids.

Many conjugative transferable plasmids exhibit surface exclusion against plasmids of the same incompatibility group. A mathematical model is developed to calculate under which conditions surface exclusion against incompatible plasmids can evolve. It appears that plasmids inducing surface exclusion can evolve and even replace non-excluding plasmids if the copy number is low and the transfer rate high provided that the cost of surface exclusion is small. They can more easily expel the non-excluding plasmids if the possession of a plasmid is not very harmful for a bacterium and if the rate at which plasmids are lost is small.