Quorum Sensing
Introduction
Quorum sensing is a system of stimulus and response correlated to population density. Many species of bacteria use quorum sensing to coordinate gene expression according to the density of their local population. In similar fashion, some social insects use quorum sensing to determine where to nest. In addition to its function in biological systems, quorum sensing has several practical applications for computing, robotics, and future technologies.
Mechanism
Quorum sensing can function as a decision-making process in any decentralized system, as long as individual components have: (a) a means of assessing the number of other components they interact with and (b) a standard response once a threshold number of components is detected. For example, bacteria express certain genes, such as those for virulence factors, only when they are at high cell densities. To detect cell density, bacteria use small molecules called autoinducers that increase in concentration as a function of cell density. A small secreted molecule, an autoinducer or pheromone, is produced by bacteria and diffuses freely across the cell membrane. When the external concentration of the autoinducer reaches a threshold level, the autoinducer binds a receptor located on the bacterial surface or within the bacterial cytoplasm, and triggers gene expression.
Quorum Sensing in Bacteria
In bacteria, quorum sensing typically involves the production, release, and subsequent detection of chemical signal molecules called autoinducers, or quorum sensing molecules. Quorum sensing allows bacteria to regulate gene expression collectively, and thus behave like a multicellular organism. It is believed that, via quorum sensing, bacteria can coordinate their behavior and collectively perform tasks which are best carried out en masse, such as changing the composition of a biofilm, creating virulence factors, or swarming motility.
Quorum Sensing in Social Insects
In social insects, quorum sensing is a mechanism that helps them decide on a new nest site. When the insects are disturbed and the colony must relocate, the workers explore the environment and return to the colony. If an insect has found a suitable new nest site, it will return to the colony more rapidly, and thus have a greater likelihood of encountering and 'convincing' other workers to investigate the site. The number of insects visiting the nest site rapidly reaches a quorum, and this then stimulates the entire colony to move to the new site.
Applications
Quorum sensing has several practical applications. It can be used in computing and robotics for swarm intelligence, and for determining consensus in a distributed network like blockchains in cryptocurrencies. In medicine, disrupting quorum sensing molecules has been proposed as a novel way to control bacterial infections.