Another Layer on the Information Story: Quorum Sensing
I was recently directed to a video lecture on the phenomenon of quorum sensing, the mechanism by which bacteria communicate with one another to establish the population density of micro-organisms of their own kind within their proximal environment. Bonnie Bassler, the lecturer in this video, does a masterful job of portraying fairly technical concepts and ideas to a lay-audience.
The purpose of quorum sensing is essentially to ensure that sufficient cell numbers of a given species are present before initiating a response that requires the population density to be above a certain threshold. A single bacterial cell secreting a toxin into a eukaryotic organism is not likely to do the host any harm and would waste resources. If, however, all of the bacterial cells in a large population co-ordinate the expression of the toxin, the toxin is more likely to have the desired effect.
Each species that employs quorum sensing -- which includes most gram negative bacteria, and also some gram positive bacteria -- synthesises a tiny signalling molecule (technically called an "autoinducer"), which diffuses freely across the cell's membrane. Autoinducers are species-specific, which means that each cell of the same species makes the same molecule. This means that the autoinducer is only present in high concentrations inside the cell when there are many cells of the same species nearby. Inside the cell, the autoinducer binds to an activator protein which is specific for that particular molecule and thus signals the bacteria to begin transcription of specific genes. As noted in the video, there is also evidence of a common autoinducer which is shared between many species of bacteria as a "conventional language."
The embedded video discusses the elucidation of this process and its application in regulating light emission in bioluminescent bacteria (in this case, the marine bacterium Aliivibro fischeri). The light results from the action of the enzyme, luciferase (so aptly named!). An activator protein, called LuxR, is responsible for controlling the lux operons, which are in turn responsible for the transcription of the proteins required for luminescence. These operons are induced when the concentration of the autoinducer specific to Aliivibro fischeri reaches a high enough concentration. This autoinducer is itself synthesised by the enzyme which is encoded by the luxI gene.
Quorum sensing is very wide spread, particularly in gram negative bacteria. Pseudomonas aeruginosa, for example, uses such "population sampling" processes to trigger the expression of a significant number of unrelated genes when the population density reaches a certain threshold. These genes subsequently allow the cells to form a biofilm (which increases the pathogenicity of the organism and prevents the penetration of antibiotics).
What is especially striking about quorum sensing is the species-specificity of the autoinducer, as well as the more common language convention. And one wonders -- naturally -- what kind of Darwinian process may have led to its arisal. We are only beginning to scale the foothills of multicellular developmental biology. But there is also evidence that similar processes of cell-cell signalling are at work in higher taxa in influencing how cells work together and co-operate as part of an interconnected unit.
Is it an over-statement to say that information runs the show in Biology?