Quorum sensing

From Freepedia

Quorum sensing is the ability of bacteria to communicate and coordinate behavior via signaling molecules.

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.

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.

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.

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.

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).

Streptococcus pneumoniae uses quorum sensing to become competent.

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.