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

Abstract

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.

The dynamic response of quorum sensing to density is robust to signal supplementation and individual signal synthase knockouts

“…Gene knockout and signal complementation experiments as a function of environmental variation. A reaction-norm approach [18–20, 48, 69, 70] introduces more metrics for the impacts of knockout/complementation experiments, including changes in reaction-norm slope and average expression level across densities. Black=wild-type per-capita gene expression.…”

“…In contrast to the conventional complementation approach focused on expression in a single environment ( Fig. 2a ), we turn to an examination of how synthase knockouts and signal complementation modify reaction norms [ 48 ], i.e. the reactivity of QS controlled lasB to changes in the social environment ( Fig.…”