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PURPOSE OF QUORUM SENSING 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''. ROLE OF QUORUM SENSING IN SPECIFIC ORGANISMS 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 Symbiont in the light-producing organ of the Hawaiian Bobtail Squid . When ''V. fischeri'' cells are free-living, the autoinducer is at low concentration and thus cells do not luminesce. In the light organ of the squid ( Photophore ), they are highly concentrated (about 1011 cells/ml) and transcription of Luciferase is induced, leading to Bioluminescence . Processes possibly regulated or partially regulated by AI-2-mediated quroum sensing in '' E. Coli '' include Cell Division . In other species such as '' Pseudomonas Aeruginosa '' quorum-related processes include biofilm development, exopolysaccharide production, and cell aggregation. 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. '' Streptococcus Pneumoniae '' uses quorum sensing to become Competent . METHODS AND MECHANISMS OF QUORUM SENSING Bacteria that use quorum sensing produce and secrete certain signaling Compounds (called ''autoinducers'' or '' Pheromone s''), one example of which are ''N''-acyl Homoserine Lactone s (AHL). These bacteria also have a Receptor that can specifically detect the AHL ( Inducer ). When the inducer binds the receptor, it activates Transcription of certain Gene s, including those for inducer synthesis. There is a low likelihood of a bacteria detecting its own secreted AHL. 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 . This is helpful since one bacteria that is luminescent provides no evolutionary advantage and wastes energy. In '' E. Coli '', AI-2 is produced by the lsr Operon , encoding an ABC Transporter which imports AI-2 into the cells during the early stationary phase of growth. AI-2 is then phosphorylated by lsrK and the newly produced phospho-AI-2 can either be internalized or used to supress lsrR, an inhibitor of the lsr operon (thereby activating the operon). 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 cells ''E. coli'' will lose the ability to internalize AI-2 (because of catabolite repression). When grown normally, AI-2 presence is transient. 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 ). Autoinducer-2 is conserved among many bacterial species, including ''Escherichia coli'', an enteric bacterium and model organism for Gram negative bacteria. Autoinducer-2 appears to be used for interspecies communication because of this conservation. LITERATURE
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