Until relatively recently, cell-to-cell communication was rarely considered to constitute a major mechanism for facilitating bacterial adaptation to an environmental challenge. However, it is now clear that diverse bacterial genera communicate using specific, extracellular signal molecules, which facilitate the coordination of gene expression in a multi-cellular fashion. Signalling can be linked to specific environmental or physiological conditions and is employed by bacteria to monitor their cell population density. The term quorum sensing is commonly used to describe the phenomenon whereby the accumulation of a diffusible, low molecular weight signal molecule (sometimes called an ‘autoinducer’) enables individual bacterial cells to sense when the minimum number or quorum of bacteria has been achieved for a concerted response to be initiated (Williams et al., 2000; Swift et al., 2001; Cámara et al., 2002). The term ‘autoinducer’, implies a
positive feedback or autoregulatory mechanism of action. However this is frequently not the case and therefore the term can be misleading and will be avoided here (Cámara et al., 2002). The accumulation of a diffusible signal molecule also indicates the presence of a diffusion barrier, which ensures that more molecules are produced than lost from the micro-habitat (Winzer et al., 2002b; Redfield, 2002). This could be regarded as a type of ‘compartment sensing’, where signal molecule accumulation is both the measure for the degree of compartmentalisation and the means to distribute this information among the entire population. Similarly, diffusion of quorum sensing signal molecules between spatially separated bacterial sub-populations may convey information about their physiological state, their numbers, and the individual environmental conditions encountered.