Quorum Sensing

Quorum sensing is the process by which many bacteria coordinate their gene expression according to the local density of their population by producing signaling molecules.
Consequences
Quorum sensing coordinates certain behavior or actions between bacteria, based on the local density of the bacterial population. Quorum sensing can occur within a single bacterial species as well as between diverse species, and can regulate a host of different processes, essentially serving as a simple communication network.
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 therapeutic enzymatic degradation of the signalling molecules will prevent the formation of such biofilms and possibly weaken established biofilms. Disrupting the signalling process in this way is called quorum quenching.