DEFINITIONS OF QUORUM SENSING

Bacteria modulate their phenotype in response to physiochemical changes in their environment. The changes may result from the movement of the bacteria from one location to another, the actions of the bacteria modifying their environment or the responses of a host to the presence of the bacteria. The responses of the bacterium to these changes in its environment are often at the level of the regulation of gene expression. In the study of bacterial pathogenesis it is necessary to ascertain which environmental parameters can signal the virulent phenotype and how the bacterium perceives and responds to these environmental parameters. Some factors are based upon the properties of the host, e.g., temperature (19, 37), iron restriction (86) and the prevailing ionic environment (3, 31, 38, 115). One important parameter is the size of the bacterial population, where changes are regulated by bacterium-to-bacterium signalling in a process that perceives the population cell density, known as quorum sensing (see reviews in 27, 29, 63, 98, 108, 110). It is the combined response to all the sensory input that the bacterium receives that determines the final phenotype, but for many examples quorum sensing provides a dominant signal (see 113 for a review).

In this review the basic concepts of bacterium-to-bacterium signalling are introduced and the mechanisms by which this signalling contributes to the control of bacterial virulence, with special notice taken of the role of signalling in the switching of bacteria from a benign state to a pathogenic state, are described.