(A) Vertical transmission. TA systems increase plasmid prevalence in growing bacterial populations by post-segregational killing (PSK). PSK⁺ plasmid is shown in purple, left panel. Daughter bacteria that inherit a plasmid copy at cell division grow normally. If daughter bacteria do not inherit a plasmid copy, degradation of the labile antitoxin proteins by the host ATP-dependent proteases will liberate the stable toxin. This will lead to the selective killing of the plasmid-free bacteria (in gray). When considering only vertical transmission, TA systems increase the prevalence of the plasmid in the population as compared with plasmids devoid of TA systems (PSK⁻ plasmid in black, right panel). (B) Horizontal transmission. Plasmid–plasmid competition. The PSK⁺ plasmid (in purple) and the PSK⁻ plasmid (in black) belong to the same incompatibility group and are conjugative. Under conditions in which conjugation occurs, conjugants containing both plasmids are generated. Because the two plasmids are incompatible, they can not be maintained in the same bacteria. The “loss” of the PSK⁺ plasmid will lead to the killing of bacteria containing the PSK⁻ plasmid through the PSK mechanism (in gray), thereby outcompeting the PSK⁻ plasmid. On the contrary, the loss of the PSK⁻ plasmid will be without any deleterious effect on the PSK⁺ plasmid. Through multiple events of conjugation, the fitness of the PSK⁺ plasmid will be increased (arrow).

The chromosomally encoded anti-addiction system is represented in black; the PSK⁺ plasmid in purple. In this model, the antitoxin of the chromosomally encoded TA system is able to counteract the toxin of the plasmid-encoded system. Therefore, daughter bacteria that do not inherit a plasmid copy at cell division will survive post-segregational killing.