Quorum sensing in bacterial species that use degenerate autoinducers can be tuned by using structurally identical non-native ligands.

Many bacteria use quorum sensing (QS) to regulate cell-density dependent phenotypes that play critical roles in the maintenance of their associations with eukaryotic hosts. In Gram-negative bacteria, QS is primarily controlled by N-acylated L-homoserine lactone (AHL) signals and their cognate LuxR-type receptors. AHL-LuxR-type receptor binding regulates the expression of target genes necessary for QS phenotypes. We recently identified a series of non-native AHLs capable of intercepting AHL-LuxR binding in the marine symbiont Vibrio fischeri, and thereby strongly promoting or inhibiting QS in this organism. V. fischeri utilizes N-(3-oxo)-hexanoyl L-HL (OHHL) as its primary QS signal, and OHHL is also used by several other bacterial species for QS. Such signal degeneracy is common among bacteria, and we sought to determine if our non-native LuxR agonists and antagonists, which are active in V. fischeri, would also modulate QS phenotypes in other bacteria that use OHHL. Herein, we report investigations into the activity of a set of synthetic LuxR modulators in the plant pathogen Pectobacterium carotovora subsp. carotovora Ecc71. This pathogen uses OHHL and two closely related LuxR-type receptors, ExpR1 and ExpR2, to control virulence, and we evaluated their responses to synthetic ligands by quantifying virulence factor production. Our results suggest an overall conservation in the activity trends of the ligands between the ExpR receptors in P. carotovora Ecc71 and LuxR in V. fischeri, and indicate that these compounds could be used as tools to study QS in an expanded set of bacteria. Notable differences in activity were apparent for certain compounds, however, and suggest that it might be possible to selectively regulate QS in bacteria that utilize degenerate AHLs.