Fig 1. Addictive antibiotic resistance plasmids.

The replicon (rep, solid circle), antitoxin (AT, arrowhead) and toxin (T, arrow) genes of a PSK/addiction system, an antibiotic resistance gene (Ab^R) and corresponding antibiotic (Ab, solid blocks) are shown. (A) An addictive plasmid is stable in the absence of antibiotic selection. (B) An addictive plasmid can be displaced by an incompatible plasmid. (C) A compatible plasmid providing specific antitoxin (non-addictive compatible) leads to loss of addictive resistance plasmids from some cells. (D) An incompatible non-addictive interference plasmid providing specific antitoxin (non-addictive incompatible) ensures that all bacterial cells are ultimately free of both plasmid types.