Quorum sensing

Quorum sensing is a type of decision-making process used by decentralized groups to coordinate behavior. Many species of bacteria use quorum sensing to coordinate their gene expression according to the local density of their population. Similarly, some social insects use quorum sensing to make collective decisions about where to nest. In addition to its function in biological systems, quorum sensing has several useful applications for computing and robotics.

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.

Bacteria

Some of the best-known examples of quorum sensing come from studies of bacteria. Bacteria use quorum sensing to coordinate certain behaviors 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. A variety of different molecules can be used as signals.

Mechanism

Bacteria that use quorum sensing constantly produce and secrete certain signaling molecules (called "autoinducers" or "pheromones"). These bacteria also have a receptor that can specifically detect the signaling molecule (inducer). When the inducer binds the receptor, it activates transcription of certain genes, including those for inducer synthesis. There is a low likelihood of a bacterium detecting its own secreted AHL. Thus, in order for gene transcription to be activated, the cell must encounter signaling molecules secreted by other cells in its environment.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. However, as the population grows the concentration of the inducer passes a threshold, causing more inducer to be synthesized. This forms a positive feedback loop, and the receptor becomes fully activated. Activation of the receptor induces the up regulation of other specific genes, causing all of the cells to begin transcription at approximately the same time. This coordinated behavior of bacterial cells can be useful in a variety of situations. For instance, the bioluminescent luciferase produced by "V. fischeri" would not be visible if it were produced by a single cell. By using quorum sensing to limit the production of luciferase to situations when cell populations are large, "V. fischeri" cells are able to avoid wasting energy on the production of useless products.

Examples

"Vibrio fischeri"

Quorum sensing was first observed in "Vibrio fischeri", a bioluminiscent bacterium that lives as a mutualistic symbiont in the photophore (or light-producing organ) of the Hawaiian bobtail squid. When "V. fischeri" cells are free-living (or planktonic), the autoinducer is at low concentration and thus cells do not luminesce. However, when they are highly concentrated in the photophore (about 10¹¹ cells/ml) transcription of luciferase is induced, leading to bioluminescence.

"Escherichia coli"

In the Gram-negative bacteria "Escherichia coli", cell division may be partially regulated by AI-2-mediated quorum sensing. This species uses AI-2, which is produced and processed by the "lsr" operon. Part of it encodes an ABC transporter which imports AI-2 into the cells during the early stationary (latent) phase of growth. AI-2 is then phosphorylated by the LsrK kinase, and the newly produced phospho-AI-2 can either be internalized or used to suppress LsrR, a repressor of the "lsr" operon (thereby activating the operon). Transcription of the "lsr" operon is also thought to be inhibited by dihydroxyacetone phosphate (DHAP) through its competitive binding to LsrR. Glyceraldehyde 3-phosphate has also been shown to inhibit the "lsr" operon through cAMP-CAPK-mediated inhibition. This explains why when grown with glucose "E. coli" will lose the ability to internalize AI-2 (because of catabolite repression). When grown normally, AI-2 presence is transient.

"Pseudomonas aeruginosa"

The opportunistic bacteria "Pseudomonas aeruginosa" uses quorum sensing to coordinate the formation of biofilms, swarming motility, exopolysaccharide production, and cell aggregation [cite journal |author=Lewis Sauer K, Camper A, Ehrlich G, Costerton J, Davies D. |title= Pseudomonas aeruginosa displays multiple phenotypes during development as a biofilm |journal=Journal of Bacteriology |volume=184 |issue=4 |pages=1140–1154 |year=2002 |issn=0021-9193 |doi= 10.1128/jb.184.4.1140-1154.2002 |pmid= 11807075] . These bacteria 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. In this species, AI-2 was found to increase expression of sdiA, a transcriptional regulator of promoters which promote ftsQ, part of the ftsQAZ operon essential for cell division. Another form of gene regulation which allows the bacteria to rapidly adapt to surrounding changes is through environmental signaling. Recent studies have discovered that anaerobiosis can significantly impact the major regulatory circuit of QS. This important link between QS and anaerobiosis has a significant impact on production of virulence factors of this organism. cite book | author = Cornelis P (editor). | title = Pseudomonas: Genomics and Molecular Biology | edition = 1st ed. | publisher = Caister Academic Press | year = 2008 | url=http://www.horizonpress.com/pseudo | id = [http://www.horizonpress.com/pseudo ISBN 978-1-904455-19-6 ] ] It is hoped that the therapeutic enzymatic degradation of the signaling molecules will prevent the formation of such biofilms and possibly weaken established biofilms. Disrupting the signalling process in this way is called "quorum quenching".

Molecules involved in quorum sensing

Three-dimensional structures of proteins involved in quorum-sensing were first published in 2001, when the crystal structures of three LuxS orthologs were determined by X-ray crystallography. [cite journal |author=Lewis HA, Furlong EB, Laubert B, Eroshkina GA, Batiyenko Y, Adams JM, Bergseid MG, Marsh CD, Peat TS, Sanderson WE, Sauder JM, Buchanan SG |title=A structural genomics approach to the study of quorum sensing: Crystal structures of three LuxS orthologs |journal=Structure |volume=9 |issue=6 |pages=527–37 |year=2001 |pmid=11435117 |doi= 10.1016/S0969-2126(01)00613-X] In 2002, the crystal structure of the receptor LuxP of "Vibrio harveyi" with its inducer AI-2 (which is one of the few biomolecules containing boron) bound to it was also determined. [cite journal |author=Chen X, Schauder S, Potier N, Van Dorsselaer A, Pelczer I, Bassler B, Hughson F |title=Structural identification of a bacterial quorum-sensing signal containing boron |journal=Nature |volume=415 |issue=6871 |pages=545–9 |year=2002 |pmid=11823863 |url=http://www.nsls.bnl.gov/newsroom/science/2002/pdfs/2002-09-Hughson.pdf PDF |doi=10.1038/415545a] AI-2 signalling is conserved among many bacterial species, including "E. coli", an enteric bacterium and model organism for Gram-negative bacteria. Although this conservation has suggested that autoinducer-2 could be used for widespread interspecies communication, a comparative genomic and phylogenetic analysis of 138 genomes of bacteria, archaea, and eukaryotes did not support the concept of a multispecies signaling system relying on AI-2 outside "Vibrio" species.cite journal |author=Sun J, Daniel R, Wagner-Döbler I, Zeng AP |title=Is autoinducer-2 a universal signal for interspecies communication: a comparative genomic and phylogenetic analysis of the synthesis and signal transduction pathways |journal=BMC Evol. Biol. |volume=4 |issue= |pages=36 |year=2004 |pmid=15456522 |doi=10.1186/1471-2148-4-36 |url=http://www.biomedcentral.com/1471-2148/4/36]

Controversy

As quorum sensing implies a cooperative behavior, this concept has been challenged by the evolutionary implication of cooperative cheaters. This is circumvented by the concept of "diffusion sensing", which has been an alternative and complementary model to quorum sensing. However, both explanations face the problems of signalling in either complex (multiple species sharing the same space) or simple (one single cell confined to a limited volume) environments where the spatial distribution of the cells can be more important for sensing than the cell population density. A new model, "efficiency sensing", which takes into account both problematics, population density and spatial confinement, has been proposed as an alternative. [cite journal |author=Hense BA, Kuttler C, Müller J, Rothballer M, Hartmann A and Kreft JU |title=Does efficiency sensing unify diffusion and quorum sensing? |journal=Nature Reviews Microbiology |volume=5 | pages=230–239 |year=2007 |pmid=17304251 |url=http://www.nature.com/nrmicro/journal/v5/n3/full/nrmicro1600.html |doi=10.1038/nrmicro1600]

ocial Insects

Social insect colonies are an excellent example of a decentralized system, because no individual is in charge of directing or making decisions for the colony. Several groups of social insects have been shown to use quorum sensing when making collective decisions.

Examples

Ants

Colonies of the ant "Temnothorax albipennis" nest in small crevices between rocks. When the rocks shift and the nest is broken open, these ants must quickly choose a new nest to move into. During the first phase of the decision-making process, a small portion of the workers leave the destroyed nest and search for new crevices. When one of these scout ants finds a potential nest, she assesses the quality of the crevice based on a variety of factors including the size of the interior, the number of openings (based on light level), and the presence or absence of dead ants [cite journal |author= Franks, N. R., A. Dornhaus, et al. |title= Not everything that counts can be counted: ants use multiple metrics for a single nest trait |journal= Proceedings of the Royal Society B-Biological Sciences |volume=273 |issue=1583 |pages=165–169 |year=2006 |issn= |doi= 10.1098/rspb.2005.3312] [cite journal |author= Franks, N. R., J. Hooper, et al.|title= Tomb evaders: house-hunting hygiene in ants. |journal=Biology Letters |volume=1 |issue=2 |pages=190–192 |year=2005|doi= 10.1098/rsbl.2005.0302 ] . The worker then returns to the destroyed nest, where she will wait for a short period before recruiting other workers to follow her to the nest she found using a process called tandem running. The waiting period is inversely related to the quality of the site- for instance a worker that has found a poor site will wait longer than a worker that encountered a good site [cite journal |author= Mallon, E. B., S. C. Pratt, et al. |title= Individual and collective decision-making during nest site selection by the ant "Leptothorax albipennis" |journal= Behavioral Ecology and Sociobiology |volume=50 |issue=4 |pages=352–359 |year=2001 |doi= 10.1007/s002650100377 ] . As the new recruits visit the potential nest site and make their own assessment of its quality, the number of ants visiting the crevice increases. During this stage ants may be visiting many different potential nests. However, because of the differences in the waiting period the number of ants in the best nest will tend to increase at the greatest rate. Eventually, the ants in this nest will sense that the rate at which they encounter other ants has exceeded a particular threshold, indicating that the quorum number has been reached [cite journal |author= Pratt, S. C. |title= Quorum sensing by encounter rates in the ant "Temnothorax albipennis". |journal=Behavioral Ecology |volume=16 |issue=2 |pages=488–496 |year=2005 |doi= 10.1093/beheco/ari020 ] . Once the ants sense a quorum, they return to the destroyed nest and begin rapidly carrying the brood, queen, and fellow workers to the new nest. Scouts that are still tandem-running to other potential sites are also recruited to the new nest and the entire colony moves.Thus although no single worker may have visited and compared all of the available options, quorum sensing enables the colony as a whole to quickly make good decisions about where to move.

Honeybees

Honey bees ("Apis mellifera") also use quorum sensing to make decisions about new nest sites. Large colonies reproduce through a process called budding, in which the queen leaves the hive with a portion of the workers to form a new nest elsewhere. After leaving the nest, the workers form a swarm that hangs from a branch or overhanging structure. This swarm persists during the decision-making phase until a new nest site is chosen.

The quorum sensing process in honey bees is similar to the method used by "Temnothorax" ants in several ways. A small portion of the workers leave the swarm to search out new nest sites, and each worker assesses the quality of the cavity she finds. The worker then returns to the swarm and recruits other workers to her cavity using the honey bee waggle dance. However, instead of using a time delay, the number of dance repetitions the worker performs is dependent on the quality of the site. Workers that found poor nests stop dancing sooner, and can therefore be recruited to the better sites. Once the visitors to a new site sense that a quorum number (usually 10 to 20 bees) has been reached, they return to the swarm and begin using a new recruitment method called piping. This vibration signal causes the swarm to take off and fly to the new nest location. In an experimental test this decision-making process enabled honey bee swarms to choose the best nest site in four out of five trials [cite journal |author= Seeley, T. D. and P. K. Visscher |title= Group decision making in nest-site selection by honey bees |journal=Apidologie |volume=35 |issue=2 |pages=101–116 |year=2004 |doi= 10.1051/apido:2004004 ] [cite journal |author= Seeley, T. D. and P. K. Visscher |title= Group decision making in honey bee swarms |journal=American Scientist |volume=94 |issue=3 |pages=220–229 |year=2006 |doi= 10.1511/2006.3.220 ] .

Computing and robotics

Quorum sensing can be a useful tool for improving the function of self-organizing networks such as the SECOAS (Self-Organizing Collegiate Sensor) environmental monitoring system. In this system, individual nodes sense that there is a population of other nodes with similar data to report. The population then nominates just one node to report the data, resulting in power savings [cite journal|author= Britton, M, Sacks, L.|title= The SECOAS Project—Development of a Self-Organising, Wireless Sensor Network for Environmental Monitoring |journal=SANPA|year=2004 ] . Ad-hoc wireless networks can also benefit from quorum sensing, by allowing the system to detect and respond to network conditions [cite journal|author= Peysakhov, M., Regli, W.|title= Ant inspired server population management in a service based computing environment |journal=Swarm Intelligence Symposium, Proceedings 2005 IEEE|year=2005|pages=357–364|doi= 10.1109/SIS.2005.1501643 ] .

Quorum sensing can also be used to coordinate the behavior of autonomous robot swarms. Using a process similar to that used by "Temnothorax" ants, robots can make rapid group decisions without the direction of a controller [cite journal|author= Sahin E., Franks N.|title= Measurement of Space: From Ants to Robots |journal=Proceedings of WGW 2002: EPSRC/BBSRC International Workshop|year=2002 ] .

ee also

*Cell signaling
*Collective behavior

References

External links

* [http://www.nottingham.ac.uk/quorum The Quorum Sensing Website]
* [http://www.hhmi.org/research/investigators/bassler.html Cell-to-Cell Communication in Bacteria]
* [http://www.cs.kent.ac.uk/projects/secoas/papers/SANPA-2004-Britton.pdf The SECOAS project—Development of a Self-Organising, Wireless Sensor Network for Environmental Monitoring]
* [http://www.media.mit.edu/resenv/classes/MAS965/readings/SwarmBots.pdf Measurement of Space: From Ants to Robots]

Further reading

* [http://publishing.royalsociety.org/bacterial-conversations Dedicated issue of "Philosophical Transactions B" on Quorum Sensing. Some articles are freely available.]