Quorum sensing
From Wikipedia, the free encyclopedia
Quorum sensing is the ability of bacteria to communicate and coordinate behavior via signaling molecules.
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[edit] 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 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.
[edit] Role of quorum sensing in specific organisms
Quorum sensing was first observed 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 quorum 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
[edit] Methods and mechanisms of quorum sensing
Bacteria that use quorum sensing produce and secrete certain signaling compounds (called autoinducers or pheromones), one example of which are N-acyl homoserine lactones (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 genes, including those for inducer synthesis. There is a low likelihood of a bacterium 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 in V. fishcheri. This is useful since a single V. fischeri bacterium that is luminescent would have no evolutionary advantage and would be wasting energy.
In Escherichia coli, AI-2 is produced by the lsr operon, encoding an ABC transporter which imports AI-2 into the cells during the early stationary (latent) phase of growth. AI-2 is then phosphorylated by lsrK and the newly produced phospho-AI-2 can either be internalized or used to suppress 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 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.
[edit] Literature
- Catalina Arévalo-Ferro: "A proteomics view of quorum-sensing regulated and surface induced genes in representative Pseudomonas and Burkholderia species". Dissertation Munich 2004, Technical University Munich microform
- Daniel Martinelli: "Regulation of biofilm growth by bacterial quorum sensing compounds". Diss. math.-nat. Univ. Zürich, 2002.
- The Quorum-Sensing Regulon of Vibriofischeri: Novel Components of the Autoinducer/LuxR Regulatory Circuit". Massachusetts Institute of Technology, 1999. 174 p.
- Rosemary Redfield: "Is quorum sensing a side effect of diffusion sensing?" (2002) Trends in Microbiology 10:365-370
Natural History Magazine, May, 2001, Vol. 110, Number 4 "Tiny Conspiracies: Cell-to-cell Communication Allows Bacteria to Coordinate Their Activity" By Bonnie L. Bossier http://www.findarticles.com/p/articles/mi_m1134/is_4_110/ai_74693738
