Jump-start and push-start: Mutual activation through crosstalk in coupled quorum sensing pathways

Many bacterial species are able to coordinate population-wide phenotypic responses through the exchange of diffusible chemical signals, a behavior known as quorum sensing. A quorum sensing bacterium may employ multiple types of chemical signals and detect them using inter-connected pathways that crosstalk with each other. While there are many hypotheses for the advantages of sensing multiple signals, the prevalence and functional significance of crosstalk between the sensing pathways are much less understood. Here we explore the effect of intra-cellular signal crosstalk on a simple model of a quorum sensing circuit. The model captures key aspects of typical quorum sensing pathways, including detection of multiple signals that crosstalk at the receptor and promoter levels, positive feedback, and hierarchical positioning of sensing pathways. We find that a variety of behaviors can be tuned by modifying crosstalk and feedback strengths. These include activation or inhibition of one output by the non-cognate signal, broadening of dynamic range of the outputs, and the ability of either the upstream or downstream branch to modulate the feedback circuit of the other branch. Our findings show how crosstalk between quorum sensing pathways can be viewed not solely as a detriment to the flow of information but also as a mechanism that enhances the functional range of the full regulatory system: When positive feedback systems are coupled through crosstalk, several new modes of activation or deactivation become possible.