Quorum sensing

Quorum sensing is the process by which many bacteria coordinate their gene expression according to the local density of their population producing signaling molecules.

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Consequences

The consequence of quorum sensing is the coordination of 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.

Methods and mechanisms

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. fischeri. 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.^[1] 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.

Examples

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 (or planktonic), 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.

In Pseudomonas aeruginosa

Regulation of gene expression can occur through cell-cell communication or quorum sensing (QS) via the production of small molecules called autoinducers. QS is known to control expression of a number of virulence factors. 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.^[2]

See also

• Cell signaling

References

1. ^ Chen X, Schauder S, Potier N, Van Dorsselaer A, Pelczer I, Bassler B, Hughson F (2002). "PDF Structural identification of a bacterial quorum-sensing signal containing boron". Nature 415 (6871): 545-9. PMID 11823863.
2. ^ Cornelis P (editor). (2008). Pseudomonas: Genomics and Molecular Biology, 1st ed., Caister Academic Press. ISBN 978-1-904455-19-6 .