Quorum sensing is the ability of bacteria to communicate and coordinate behavior via signaling molecules.

Bacteria that use quorum sensing produce and secrete certain signaling compounds (called autoinducers or pheromones), which are normally N-acyl homoserine lactones (AHL). The bacteria also have a receptor that can specifically detect the inducer. When the inducer binds the receptor, it activates transcription of certain genes, including those for inducer synthesis.

When only a few other bacteria of the same kind are in the vicinity, diffusion reduces the concentration of the inducer in the surrounding medium to almost zero, so the bacteria produce little inducer. With many bacteria of the same kind, the concentration of the inducer passes a threshold, whereupon more inducer is synthesised. This forms a positive feedback loop, and the receptor becomes fully activated. This induces the up regulation of other specific genes, such as luciferase.

The purpose of quorum sensing is to coordinate certain behaviour or actions between bacteria of the same kind, depending on their number. For example, opportunistic bacteria, such as Pseudomonas aeruginosa can grow within a host without harming it, until they reach a certain concentration. Then they become aggressive, their numbers sufficient to overcome the host's immune system and form a biofilm, leading to disease. It is hoped that the enzymatic degradation of the signalling molecules will prevent the formation of such biofilms and possibly weaken established biofilms. Distrupting the signalling process in this way is called quorum quenching.

The first organisms in which quorum sensing was observed were Myxobacteria and Streptomyces species. However, the most popular example is the regulation of light production in Vibrio fischeri, a bioluminiscent bacterium that lives as a symbiote in the light-producing organ of deep-sea bobtail squid. When the bacteria are free-living, they are at low concentration and do not luminesce. In the light-producing organ (photophore), they are highly concentrated (about 10¹¹ cells/ml) and transcription of luciferase is induced, leading to bioluminescence.

A first X-ray structure of a receptor (LuxP) was discovered in Vibrio harveyi in 2002, together with its inducer (AI-2), which is one of the few biomolecules containing boron (Nature 415, 545ff PDF).

Streptococcus pneumoniae uses quorum sensing to become competent.