ThelasandrhlQuorum Sensing Systems inPseudomonas aeruginosaForm a Multi-Signal Reciprocal Network Which Can Tune Reactivity to Variations in Physical and Social Environments

AbstractResearchers often view the multi-signal quorum sensing systems ofPseudomonas aeruginosaas a hierarchy, topped by thelassystem which acts as a master regulator. By experimentally controlling the concentration of auto-inducer signals in a signal null strain (PAO1ΔlasIΔrhlI), we show that the two primary quorum sensing systems—lasandrhl—act reciprocally rather than hierarchically. Just as thelassystem’s 3-oxo-C12-HSL can induce increased expression ofrhlI, therhlsystem’s C4-HSL increases the expression level oflasI. We develop a mathematical model to quantify relationships both within and between thelasandrhlquorum 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.