text: Autoinducers are signaling molecules that are produced in response to changes in cell-population density.  As the density of quorum sensing bacterial cells increases so does the concentration of the autoinducer.  Detection of signal molecules by bacteria acts as stimulation which leads to altered gene expression once the minimal threshold is reached. Quorum sensing is a phenomenon that allows both Gram-negative and Gram-positive bacteria to sense one another and to regulate a wide variety of physiological activities.  Such activities include symbiosis, virulence, motility, antibiotic production, and biofilm formation.  Autoinducers come in a number of different forms depending on the species, but the effect that they have is similar in many cases.  Autoinducers allow bacteria to communicate both within and between different species.  This communication alters gene expression and allows bacteria to mount coordinated responses to their environments, in a manner that is comparable to behavior and signaling in higher organisms. Not surprisingly, it has been suggested that quorum sensing may have been an important evolutionary milestone that ultimately gave rise to multicellular life forms.