In biology, an autoinducer is a signaling molecule that enables detection and response to changes in the population density of bacterial cells. Synthesized when a bacterium reproduces, autoinducers pass outside the bacterium and into the surrounding medium.[1] They are a key component of the phenomenon of quorum sensing: as the density of quorum-sensing bacterial cells increases, so does the concentration of the autoinducer. A bacterium’s detection of an autoinducer above some minimum threshold triggers altered gene expression.[2][3]
Performed by both Gram-negative and Gram-positive bacteria, detection of autoinducers allows them to sense one another and to regulate a wide variety of physiological activities, including symbiosis, virulence, motility, production of antibiotics, and formation of biofilms.[4]
Autoinducers take a number of different forms depending on the species of bacteria, but their effect is in many cases similar. They allow bacteria to communicate both within and between species, and thus 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.