A bacterial plasmid, ColE1 carries a gene that encodes for colicin. This is not generally expressed, but if the bacteria undergoes an SOS response (a pretty good sign that the ship is sinking as far as this particular organism is concerned) the plasmid expresses huge amounts of colicin leading to the death of the host and colicin being flung everywhere. Normally this would kill any bacteria nearby, but if they're carrying ColE1 as well they're safe - it also encodes a colicin resistance gene. Any bacteria that aren't will get wiped out, freeing up space for bacteria carrying the plasmid to live in and thereby increasing the number of plasmids in the population.

Plasmids are sneaky buggers.