The las and rhl Quorum Sensing Systems in Pseudomonas aeruginosa Form a Multi-Signal Reciprocal Network Which Can Tune Reactivity to Variations in Physical and Social Environments

Abstract

Researchers often view the multi-signal quorum sensing systems of Pseudomonas aeruginosa as a hierarchy, topped by the las system which acts as a master regulator. By experimentally controlling the concentration of auto-inducer signals in a signal null strain (PAO1{Delta}lasI{Delta}rhlI), we show that the two primary quorum sensing systems--las and rhl--act reciprocally rather than hierarchically. Just as the las systems 3-oxo-C12-HSL can induce increased expression of rhlI, the rhl systems C4-HSL increases the expression level of lasI. We develop a mathematical model to quantify relationships both within and between the las and rhl quorum sensing systems and the downstream genes they influence. The results show that not only do the systems interact reciprocally, but they do so cooperatively and nonlinearly, with the combination of C4-HSL and 3-oxo-C12-HSL increasing expression level far more than the sum of their individual effects. We computationally assess how our parameterized model responds to variation in social (population density) and physical (mass transfer) environment and demonstrate that a reciprocal architecture is more responsive to density and more robust to mass transfer than a strict hierarchy.