Parasitic plasmid-host dynamics and host competition in flowing habitats

•Competition and coexistence were examined for two bacterial species, each potentially carrying a fitness-reducing, parasitic plasmid that was vertically transmitted with possible loss through segregation.•It was numerically shown that parasitic plasmids can mediate coexistence of competing host species, in conditions where plasmid-free hosts could not coexist.•Mathematically, we can study the possibility of coexistence of plasmid-free and plasmid-bearing subpopulations of one species, along with plasmid-bearing and plasmid-free populations of the other species.•Numerical construction of a coexistence example suggests that it arises only for a narrow parameter range. Competition and coexistence were examined for two bacterial species, each potentially carrying a fitness-reducing, parasitic plasmid that was vertically transmitted with possible loss through segregation. Here, the fitness reduction of hosts was due to a toxin produced by plasmid-bearing cells and inhibiting plasmid-free cells. These populations were placed in a flow reactor habitat representing an idealized mammal gut. It was numerically shown that parasitic plasmids can mediate coexistence of competing host species, in conditions where plasmid-free hosts could not coexist. Numerical construction of a coexistence example suggests that it arises only for a narrow parameter range. In particular, both rates of segregation and the growth costs of plasmid carriage must be relatively low.