Beyond Thresholds: Quorum-Sensing as Quantitatively Varying Reaction Norms to Multiple Environmental Dimensions

Many types of bacteria engage in complex collective behaviors, often controlled by a form of cell-cell communication mediated by diffusible signal molecules called quorum sensing (QS). Each individual produces and responds to these diffusible signaling molecules, inferring properties of their environment, and modulating the regulation of hundreds of downstream genes in response. A threshold concept is ingrained in the QS literature following the use of the legal 'quorum' analogy, where populations of bacteria are synchronously sub-quorate below a threshold population size or synchronously quorate above a threshold population size. However, a growing body of literature highlights that QS can produce more nuanced, graded responses to environmental variation, implying there is no simple threshold 'quorum' on either the single cell or population scale. In this commentary, we discuss reaction norms as a tool to quantify and compare QS-controlled behaviors in an environmental context.