Colicin diversity: a result of eco-evolutionary dynamics

Colicins are plasmids that are carried in Escherichia coli. They code for a toxic protein and for proteins that confer on the host immunity against th is toxin. When bacteria carry plasmids their growth rate is reduced. At the same time, the production of toxins makes it possible for colicinogenic ba cteria to invade bacterium strains that are not immune. In natural bacteriu m populations there is a high diversity of colicin types. The reason for th e maintenance of this diversity has been the subject of much recent debate. We have studied a simple eco-evolutionary model of the interaction of bacte ria with colicins and show that high diversity of colicins is to be expecte d. We find two different dynamical modes each with a high diversity: a hype rimmunity merle and a multitoxicity, mode. Bacteria are immune to most toxi ns in the first mode but in fact produce very few toxins. In the second mod e bacteria are immune only to those toxins that they actually produce. In t he second mode toxin levels per bacterium are much higher, whereas immunity levels per bacterium are lower. (C) 1999 Academic Press.