INFLUENCE OF HIERARCHICAL QUORUM SENSING SYSTEMS ON BACTERIAL DYNAMICS

Quorum sensing (QS) is a form of cell-cell communication where bacteria secrete a diffusible signal that binds to a specific receptor, in order to alter gene expression in a density-dependent manner. Although a single QS system is sufficient, in principle, to elicit a density-dependent response, many bacteria encode for multiple QS systems, which show partial redundancy in the regulation of gene expression. Hierarchical regulation of QS systems, as in the pathogen Pseudomonas aeruginosa, is a commonly observed architecture in which one QS system controls the expression of another. The potential benefit of this hierarchy is yet to be determined. To study this network design, we combine mathematical modeling with the experimental analysis of a synthetic hierarchical system, which was constructed by introducing an exogenous QS system as a regulator of the naturally non-hierarchical ComQXP QS system of the model bacterium Bacillus subtilis.

By combining a theoretical model and preliminary experimental results, we conclude that facultative cheating plays a part in the evolution of a hierarchical QS architecture. When a subpopulation of bacteria possessing the hierarchical QS system is a minority, a delay in the quorum response leads it to exploit the majority coding for a single QS system. Unlike obligate cheaters, cooperative activity is regained with the increase in the relative frequency of the subpopulation.

Furthermore, working with a synthetic hierarchy offers us the means to explore and quantify the roles of different components in the system, such as the threshold of each system and its effect on the delay between QS responses of the two systems.