Fig. 1: Different modeled mechanisms of plasmid transfer lead to distinct ecological phase diagrams, but all such mechanisms leave individual populations susceptible to runaway plasmid invasion.

A At each division, plasmids are randomly segregated between daughter cells. Original plasmid copy number is regenerated if at least one plasmid remains in a daughter cell. B Schematic of plasmid transfer mechanisms. Left: spread of plasmids by plasmid-containing cells conjugating with plasmid-free cells. Right: spread of plasmids by extracellular plasmids infecting plasmid-free cells via transformation. C Phase diagram for conjugative plasmids as a function of plasmid cost, \({\Delta}\), and \(\gamma _{\mathrm{c}}\); \(\delta \,=\, 0.1\), \(S \,=\, 1\), \(p_\ell \,=\, 0\), and \(\alpha \,=\, 1\) (see Eq. 4). D Phase diagram for transformative plasmids as a function of \({\Delta}\) and \(\gamma _{\mathrm{t}}\). Parameters as in C with \(\delta _{\mathrm{p}} \,=\, 0.3\) and \(n_{{\mathrm{eff}}} \,=\, 0.6\) (see Eq. 9). See “Methods” for details. E In model multiplasmid cells, plasmid types segregate independently. If at least one plasmid of a given type remains in a daughter cell, the full copy number of that plasmid type is regenerated. F Fitness cost as a function of number of unique plasmid types in a cell for multiplicative case \({\Delta}_{{\mathrm{tot}}} \,=\, 1 \,-\, (1 \,-\, {\Delta})^m\) with \({\Delta} \,=\, 0.05\). G Steady-state distribution of number of plasmid types per cell at different conjugation rates, measured relative to \(\gamma _{\mathrm{c}}^ \ast\) (the critical conjugation rate necessary for invasion of a single plasmid into a plasmid-free population, see Eq. 4). Results for eight unique plasmid types with \(\delta \,=\, 1\), \({\Delta} \,=\, 0.1\), \(\alpha \,=\, 1\), \(S \,=\, 1\), and \(p_\ell \,=\, 0.05\).