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

Abstract

Quorum sensing (QS) is a widespread mechanism of environment sensing and behavioural coordination in bacteria. At its core, QS is based on the production, sensing and response to small signalling molecules. Previous work withPseudomonas aeruginosashows that QS can be used to achievequantitativeresolution and deliver a dosed response to the bacteria’s density environment, implying a sophisticated mechanism of control. To shed light on how the mechanistic signal components contribute to graded responses to density, we assess the impact of genetic (AHL signal synthase deletion) and/or signal supplementation (exogenous AHL addition) perturbations onlasBreaction-norms to changes in density. Our approach condenses data from 2000 timeseries (over 74 000 individual observations) into a comprehensive view of QS-controlled gene expression across variation in genetic, environmental and signal determinants oflasBexpression. We first confirm that deleting either (∆lasI, ∆rhlI) or both (∆lasIrhlI) AHL signal synthase gene attenuates QS response to density. In the∆rhlIbackground we show persistent yet attenuated density-dependentlasBexpression due to native 3-oxo-C12-HSL signalling. We then test if density-independentquantities of AHL signal (3-oxo-C12-HSL, C4-HSL) added to the WT either flatten or increase responsiveness to density and find that the WT response is robust to all tested concentrations of signal, alone or in combination. We then move to progressively supplementing the genetic knockouts and find that cognate signal supplementation of a single AHL signal (∆lasI+3-oxo-C12-HSL, ∆rhlI+C4HSL) is sufficient to restore the ability to respond in a density-dependent manner to increasing density. We also find that dual signal supplementation of the double AHL synthase knockout restores the ability to produce a graded response to increasing density, despite adding a density-independentamount of signal. Only the addition of high concentrations of both AHLs and PQS can force maximallasBexpression and ablate responsiveness to density. Our results show that density-dependent control oflasBexpression is robust to multiple combinations of QS gene deletion and density-independent signal supplementation. Our work develops a modular approach to query the robustness and mechanistic bases of the central environmentalsensingphenotype of quorum sensing.