Quorum sensing is the regulation of gene expression in response tofluctuations in cell-population density. Quorum sensing bacteria produce andrelease chemical signal molecules called autoinducers that increase inconcentration as a function of cell density. The detection of a minimalthreshold stimulatory concentration of an autoinducer leads to an alteration ingene expression. Gram-positive and Gram-negative bacteria use quorum sensingcommunication circuits to regulate a diverse array of physiological activities.These processes include symbiosis, virulence, competence, conjugation,antibiotic production, motility, sporulation, and biofilm formation. Ingeneral, Gram-negative bacteria use acylated homoserine lactones asautoinducers, and Gram-positive bacteria use processed oligo-peptides tocommunicate. Recent advances in the field indicate that cell-cell communicationvia autoinducers occurs both within and between bacterial species. Furthermore,there is mounting data suggesting that bacterial autoinducers elicit specificresponses from host organisms. Although the nature of the chemical signals, thesignal relay mechanisms, and the target genes controlled by bacterial quorumsensing systems differ, in every case the ability to communicate with oneanother allows bacteria to coordinate the gene expression, and therefore thebehavior, of the entire community. Presumably, this process bestows uponbacteria some of the qualities of higher organisms. The evolution of quorumsensing systems in bacteria could, therefore, have been one of the early stepsin the development of multicellularity.