Quorum Sensing Signal Selectivity and the Potential for Interspecies Cross Talk

Many species of proteobacteria communicate with kin and coordinate group behaviors through a form of cell-cell signaling called acyl-homoserine lactone (AHL) quorum sensing (QS). Most AHL receptors are thought to be specific for their cognate signal, ensuring that bacteria cooperate and share resources only with closely related kin cells. Although specificity is considered fundamental to QS, there are reports of "promiscuous" receptors that respond broadly to nonself signals. These promiscuous responses expand the function of QS systems to include interspecies interactions and have been implicated in both interspecies competition and cooperation. Because bacteria are frequently members of polymicrobial communities, AHL cross talk between species could have profound impacts. To better understand the prevalence of QS promiscuity, we measured the activity of seven QS receptors in their native host organisms. To facilitate comparison of our results to previous studies, we also measured receptor activity using heterologous expression in Escherichia coli We found that the standard E. coli methods consistently overestimate receptor promiscuity and sensitivity and that overexpression of the receptors is sufficient to account for the discrepancy between native and E. coli reporters. Additionally, receptor overexpression resulted in AHL-independent activity in Pseudomonas aeruginosa Using our activation data, we developed a quantitative score of receptor selectivity. We find that the receptors display a wide range of selectivity and that most receptors respond sensitively and strongly to at least one nonself signal, suggesting a broad potential for cross talk between QS systems.IMPORTANCE Specific recognition of cognate signals is considered fundamental to cell signaling circuits as it creates fidelity in the communication system. In bacterial quorum sensing (QS), receptor specificity ensures that bacteria cooperate only with kin. There are examples, however, of QS receptors that respond promiscuously to multiple signals. "Eavesdropping" by these promiscuous receptors can be beneficial in both interspecies competition and cooperation. Despite their potential significance, we know little about the prevalence of promiscuous QS receptors. Further, many studies rely on methods requiring receptor overexpression, which is known to increase apparent promiscuity. By systematically studying QS receptors in their natural parent strains, we find that the receptors display a wide range of selectivity and that there is potential for significant cross talk between QS systems. Our results provide a basis for hypotheses about the evolution and function of promiscuous signal receptors and for predictions about interspecies interactions in complex microbial communities.