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

One of the most important mechanisms for bacterial cell-to-cell communication and behaviour 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 ") (Camara 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 behavioural 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 bacterial 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). In Gram-negative bacteria, some of the most studied signal molecules are the N-acylhomoserine lactones (AHLs) (Fig. 1A). During the growth of a bacterial population, signal molecules either diffuse or are exported out of the cell into the surrounding environment; their concentration increases and they then act on neighbouring bacterial cells. Achievement of a critical threshold concentration results in: (i) activation of a sensor/response regulator, responsible for signal transduction (T), which in turn triggers the expression of multiple genes and (ii) activation of a positive autoinductive feedback loop to amplify QS signal molecule generation (Figure 1B)