Quorum sensing (QS) refers to the ability of microbial cells to communicate with one another to determine their densities. This communication is usually facilitated by the production and extracellular export of a small signaling molecule which other bacteria can then detect. The concentration of the small molecule is tied to transcriptional regulation in members of the quorum sensing bacterial population. The more QS bacteria producing the small signaling molecule in a given area, the higher the concentration of that molecule. When the small molecule concentration reaches a certain amount, the bacteria are at 'quorum' and all participating QS bacteria undergo changes in gene expression. In natural environments, this change often involves increases in virulence, biofilm formation, bioluminescence, sporulation, or competence. In synthetic environments, these signals can be used as input for genetic circuits. QS is currently an area of intense research for its implications in human health, microbial ecology, and more recently, synthetic biology.
The LuxR/I System
The first characterized QS system, LuxI/R controls the bioluminescent _lux_CDABE operon in _V. fischeri_. These bacteria generally inhibit the light organs of various aquatic creatures and upon reaching a high density of 10E11 cell/mL, trigger the generation of luminescence. This is facilitated by a two component system, LuxI and LuxR. LuxI produces a homoserine lactone (HSL) 3-oxo-C6-HSL and LuxR binds the HSL (also referred to as an autoinducer) and facilitates activation of the _lux_ operon. The low basal expression of the HSL by LuxI is not enough by itself to activate cytoplasmic LuxR and the rapid diffusion of the small molecule out of the bacteria ensures that it will not accumulate without input from other cells. When the concentration of bacteria crosses a threshold and the HSL concentration is high enough, the HSL-bound LuxR actiavtes the _lux_ operon and triggers bioluminescence in the bacterial population.
LasR/I and RhlR/I
AIP Mediated Systems
Applications of Quorum Sensing in Synthetic Biology