Bacteria were for a long time believed to exist as individual cells that sought primarily to find nutrients and multiply. The discovery of intercellular communication among bacteria has led to the realization that bacteria are capable of coordinated activity that was once believed to be restricted to multicellular organisms. The capacity to behave collectively as a group has obvious advantages, for example, the ability to migrate to a more suitable environment/better nutrient supply and to adopt new modes of growth, such as sporulation or biofilm formation, which may afford protection from deleterious environments. The "language" used for this intercellular communication is based on small, self-generated signal molecules called autoinducers. Through the use of autoinducers, bacteria can regulate their behavior according to population density. The phenomenon of quorum sensing, or cell-to-cell communication, relies on the principle that when a single bacterium releases autoinducers (AIs) into the environment, their concentration is too low to be detected. However, when sufficient bacteria are present, autoinducer concentrations reach a threshold level that allows the bacteria to sense a critical cell mass and, in response, to activate or repress target genes. Most of the bacteria thus far identified that utilize quorum-sensing systems are associated in some way with plants or animals. The nature of these relationships can be either amicable, as characterized by symbiotic bacteria, or adversarial, as seen with pathogenic bacteria. There are numerous bacteria that have components of a quorum-sensing system for which the phenotype regulated remains an enigma. Similarly, there are bacteria known to regulate a specific phenotype via quorum sensing for which one or more of the regulatory components have thus far eluded identification. In this review we give examples of pathogenic relationships, focusing on organisms for which many of the facets of their quorum-sensing systems have been elucidated.

Communication among bacterial cells through quorum-sensing (QS) systems is used to regulate ecologically and medically important traits, including virulence to hosts. QS is widespread in bacteria; it has been demonstrated experimentally in diverse phylogenetic groups, and homologs to the implicated genes have been discovered in a large proportion of sequenced bacterial genomes. The widespread distribution of the underlying gene families (LuxI/R and LuxS) raises the questions of how often QS genes have been transferred among bacterial lineages and the extent to which genes in the same QS system exchange partners or coevolve. Phylogenetic analyses of the relevant gene families show that the genes annotated as LuxI/R inducer and receptor elements comprise two families with virtually no homology between them and with one family restricted to the {gamma}-Proteobacteria and the other more widely distributed. Within bacterial phyla, trees for the LuxS and the two LuxI/R families show broad agreement with the ribosomal RNA tree, suggesting that these systems have been continually present during the evolution of groups such as the Proteobacteria and the Firmicutes. However, lateral transfer can be inferred for some genes (e.g., from Firmicutes to some distantly related lineages for LuxS). In general, the inducer/receptor elements in the LuxI/R systems have evolved together with little exchange of partners, although loss or replacement of partners has occurred in several lineages of {gamma}-Proteobacteria, the group for which sampling is most intensive in current databases. For instance, in Pseudomonas aeruginosa, a transferred QS system has been incorporated into the pathway of a native one. Gene phylogenies for the main LuxI/R family in Pseudomonas species imply a complex history of lateral transfer, ancestral duplication, and gene loss within the genus.