Figure 4. Necessary conditions for the long term co-maintenance of two phages (donor and recipient).

For convenience, the growth rate of the donor phage is drawn as a curve with a shallow slope. The important condition for maintenance is that the growth rate of the recipient phage be lower than that of the donor phage when it is at low density, but surpass that of the donor phage when at high density. This property ensures that the recipient phage cannot replace the donor phage but that if the donor phage attains high density, the recipient phage will overtake it. Areas are differentially shaded to distinguish the region in which the donor has the higher growth rate from that in which the recipient has the higher growth rate.