A density-dependent mechanism for communication and coordination among populations of bacteria in which the individual bacteria secrete and detect substances called autoinducers. These accumulate in the environment depending on the density of the population. Upon reaching a certain threshold concentration, the autoinducer triggers a specific response. Such mechanisms can regulate a wide range of bacterial activities, including bioluminescence, sporulation, virulence, and biofilm formation. Some autoinducers are used for intraspecific communication, whereas others affect the behaviour of other species.