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Bacteria employ sophisticated cell-to-cell communication or ‘quorum sensing’ (QS) systems for promoting collective behaviours that depend on the actions of one or more chemically distinct diffusible signal molecules. As determinants of cell population density, multiple QS systems are often integrated with each other and within global regulatory networks and subject to the prevailing environmental conditions as well as the presence and activities of other organisms. QS signal molecules, although largely considered as effectors of QS-dependent gene expression are also emerging as multifunctional molecules that influence life, development and death in single and mixed microbial populations and impact significantly the outcome of host–pathogen interactions.