Quorum sensing

In a previous post on chemotaxis I briefly discussed the way that bacteria move in response to chemical gradients. This may be described in a rather anthropomorphic way by saying that an individual bacterium has particular goals which it pursues actively. On a similar level of description it turns out that bacteria cooperate with each other within a population. An example of this is the phenomenon known as quorum sensing. It plays a role in the formation of biofilms. These consist of colonies of bacteria surrounded by solid material which they have secreted. Quorum sensing is interesting on a purely theoretical level but it may also have practical consequences. Instead of fighting bacterial infection by attacking individual organisms using antibiotics an alternative might be to interfere with the social life of the bacteria by influencing quorum sensing. Under these circumstances individual bacteria could still survive but the multiplication of their population which leads to disease could be prevented. A useful source of information on quorum sensing is the The quorum sensing site. It should be noted that this is not just a phenomenon which occurs in exceptional cases – it is apparently a widespread phenomenon in bacterial colonies.

In many cases the signalling molecules involved in quorum sensing have been identified. There is also rather detailed information about how the bacteria react to the signal. There has been some discussion as to whether it is not better to talk about diffusion sensing in some cases. This means that the concentration of a molecule produced by a bacterium is sensed but that the presence of other bacteria is not necessary. In this way the organism can assess the volume available for diffusion of the chemical rather than the number of other organisms present. For more information on this see this article. There is already a considerable literature on mathematical models for quorum sensing, using ODE and/or reaction diffusion type models. See for example a paper of Dockery and Keener (Bull. Math. Biol. 63, 91). I do not yet have a good overview of the field.

The first organism in which quorum sensing was observed was Vibrio fischeri. This bacterium is responsible for the bioluminescence of certain marine animals and the production of light is regulated by quorum sensing. When I hear the name Vibrio I automatically think of the sinister member of the genus, Vibrio cholerae. This bacterium, the cause of cholera, has been reported to use quorum sensing but in a negative sense, i.e. the presence of more bacteria can lead to a suppression of their activity. This organism has had a notable career in history and also played a major role in establishing the fame of Robert Koch who identified it as the cause of an epidemic in Alexandria in 1883, in competition with associates of his rival Louis Pasteur.

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