Bacterial quorum sensing allows graded and bimodal cellular responses to variations in population density

Quorum sensing (QS) is a mechanism of cell to cell communication that connects gene expression to environmental conditions (e.g. density) in many bacterial species, mediated by diffusible signal molecules. Current functional studies focus on a dichotomy of QS on/off (or, quorate / sub-quorate) states, overlooking the potential for intermediate, graded responses to shifts in the environment. Here, we track QS regulated protease (lasB) expression and show that Pseudomonas aeruginosa can deliver a graded behavioral response to fine-scale variation in population density, on both the population and single-cell scales. On the population scale, we see a graded response to variation in environmental population density. On the single-cell scale, we see significant bimodality at higher densities, with separate OFF and ON sub-populations that respond differentially to changes in density; static OFF cells and increasing intensity of expression among ON cells. While the QS-controlled behavioral output is graded, the underlying multi-signal dynamics display a threshold shift in signal concentration with increasing density, reflecting the onset of positive signal auto-regulation at intermediate densities. Together these results indicate that QS can tune gene expression to graded environmental change, with no critical cell mass or quorum at which behavioral responses are activated on either the individual cell or population scale. In an infection context, our results indicate there is not a hard threshold separating sub-quorate stealth mode and a quorate attack mode.