MOLECULAR SIGNALS. “Quorum sensing depends on the synthesis of small molecules (… pheromones or autoinducers) that diffuse in and out of bacterial cells. As the bacterial population density increases, so does the synthesis of quorum sensing signal molecules … Once a critical threshold concentration has been reached, a target sensor kinase or response regulator is activated (or repressed) so facilitating the expression of quorum sensing-dependent genes.”
http://rstb.royalsocietypublishing.org/content/362/1483/1119