Quorum Sensing: A Cell-Cell Signalling Mechanism Used to Coordinate Behavioral Changes in Bacterial Populations

Abstract

One of the most important mechanisms for bacterial cell-to-cell communication and behavior coordination under changing environments is often referred to as “quorum sensing” (QS). QS relies on the activation of a sensor kinase or response regulator protein by, in many cases, a diffusible, low molecular weight signal molecule (a “pheromone” or “autoinducer”) (Cámara et al., 2002). Consequently, in QS, the concentration of the signal molecule reflects the number of bacterial cells in a particular niche and perception of a threshold concentration of that signal molecule indicates that the population is “quorated”, i.e. ready to make a behavioral decision. Bacteria cell-to-cell communication is perhaps the most important tool in the battle for survival; they employ communication to trigger transcriptional regulation resulting in sexual exchange and niche protection in some cases, to battle host’ defences and coordinate population migration. Ultimately, bacteria cell-to-cell communication is used to effect phenotypic change. The importance of coordinated gene-expression (and hence phenotypic change) in bacteria can be understood if one realizes that only by pooling together the activity of a quorum of cells can a bacterium be successful. It is increasingly apparent that, in nature, bacteria function less as individuals and more as coherent groups that are able to inhabit multiple ecological niches (Lazdunski et al., 2004). Within quorum sensing process several key elements must be considered: (i) the gene(s) involved in signal synthesis, (ii) the gene(s) involved in signal transduction, and (iii) the QS signal molecule(s).