Figure 3.

Modulation of growth rate can create an implicit feedback loop with two resulting subpopulations of microbes. A. A toxin expressed from a plasmid is unequally partitioned into two daughter cells. With higher toxin, there is slower growth, allowing, in turn, more toxin buildup. The result of the feedback loop is a non-linear relationship between toxin promoter strength and growth rate. A mathematical model predicts two resulting subpopulations of cells growing at different rates for some conditions (red curve; dashed portion represents unstable intermediate steady state) and unimodal populations with nonlinear toxin response in other conditions (black curve) ³⁵. B. A synthetic system in E. coli with autoregulating T7 RNA polymerase that also upregulates a fluorescent protein (CFP) as a readout. A second, implicit feedback loop arises from the metabolic burden of gene expression. Microcolonies of the synthetic strain exhibit bimodal fluorescence (visible as both dark and green cells) as a result of bistability³⁶. [Part B copied directly from Tan et al.]