QUORUM SENSING

Quorum sensing is a term that refers to the coordinated behavior exhibited by a population of bacteria. The phenomenon involves a communication between the bacterial members of the population and, via a triggering signal, the carrying out of a particular function.

Examples of quorum sensing are the coordinated feeding behavior and the formation of spores that occur in large populations of myxobacteria and actinomycetes. Quorum sensing also occurs in bacterial biofilms, where signals between bacteria can stimulate and repress the production of the extracellular polysaccharide in different regions of the biofilm, and the exodus of portions of the population from the biofilm, in order to establish a new biofilm elsewhere.

Historically, the first indication of quorum sensing was the discovery of the chemical trigger for luminescence in the bacterium Photobacterium fischeri in the 1990s. At high densities of bacteria, luminescence occurs. Light production, however, does not occur at lower numbers or densities of bacteria. The phenomenon was correlated with the production of a compound whose short name is homoserine lactone. The same molecule has since been shown to trigger responses in other quorum sensing systems in other bacteria. Examples of these responses include the production of disease-causing factors by Pseudomonas aeruginosa and cell division in Escherichia coli.

Quorum sensing enables a bacterial population to respond quickly to changing environmental conditions and, in the case of biofilms, to enable regions within the mature biofilm to perform the different functions necessary to sustain the entire community.

In Photobacterium fischeri the relatively hydrophobic ("water-hating") nature of the homoserine lactone molecule drives its diffusion into the cell wall surrounding a bacterium. Once inside the bacterium, the molecule interacts with a protein known as LuxR. The LuxR then induces the transcription of a region the genetic material that contains the genes that code for the luminescent proteins.

The molecular nature of the means by which quorum sensing triggers such homoserine lactone evoke a bacterial response in other bacteria is still unclear. Furthermore, the discovery of several quorum sensing systems in bacteria such as Pseudomonas aeruginosa indicate that multiple sensing pathways are operative, at different times or even simultaneously. For example, within a biofilm, bacteria may be actively manufacturing exopolysaccharide, repressed in the polymer's construction, growing slowly, or resuming the active growth that is the hallmark of free-floating bacteria. Resolving the molecular nature of the spectrum of quorum sensing activities could lead to strategies to disrupt the inter-cellular communication in disease processes.