What is Quorum Sensing

One of the most important mechanisms for bacterial cell-to-cell communication and behaviour coordination under changing environments is often referred to as ‘quorum sensing’.

The discovery that bacteria are able to communicate with each other changed our general perception of many single, simple organisms inhabiting our world. Instead of language, bacteria use signalling molecules which are released into the environment. As well as releasing the signalling molecules, bacteria are also able to measure the number (concentration) of the molecules within a population.

Nowadays we use the term ‘Quorum Sensing’ (QS) to describe the phenomenon whereby the accumulation of signalling molecules enable a single cell to sense the number of bacteria (cell density). In the natural environment, there are many different bacteria living together which use different classes of signalling molecules.

As they employ different languages they cannot necessarily talk to all other bacteria. Today, several quorum sensing systems are intensively studied in various organisms such as marine bacteria and several pathogenic bacteria.

Cell to cell communication

How do Bacteria Talk to each other?

Research has shown that disease causing bacteria operate in two modes, a ‘quiet’ mode when they are present but do not show much activity and an ‘active’ mode in which they attack the host system. It is theorized that when bacteria are present in low numbers they remain quiet, and only turn virulent when they are present in large enough numbers to overwhelm the host’ immune system response.

Each bacterium secretes a small amount of a chemical for each it has surface receptors. The larger the number of bacteria present, the higher the level of chemical, and this is detected by the surface receptors. When they detect a sufficient numbers are present (a “quorum”) they start attacking their host.

One of the most studies bacteria is Vibrio fischeri. This luminescent bacterium can be found in small amounts in the ocean and in large amounts in isolated areas such as the light organs of the squid. When in small concentration of cells, Vibrio fischeri does not give off light, but in high cell density these bacteria emit a blue green light.

Squid emitting light as a result of quorum sensing activity by bacteria Vibrio fischeri inside it. Picture Source : http://www.tumblr.com/tagged/hawaiian

Why do Bacteria talk to each Other?

As environment condition often change, bacteria need to react quickly in order to survive and sustain. These responses include

• Adaptation to availability of nutrients
• Defence against other microorganisms which may compete for the same nutrient
• Avoidance of toxic compounds potentially dangerous for the bacteria
• To coordinate their virulence in order to escape the immune response of the host to be able to create an infection.

Quorum sensing — a type of bacterial communication

Can Bacteria Communicate between Different species?

Yes, bacteria can communicate between different species and this is reffered to as quorum sensing cross talk.

What are the benefits of quorum sensing research?

Quorum Sensing research has many potential applications, most of these involve controlling bacteria by interfering with their signalling systems. For example many bacteria rely on Quorum Sensing to control the expression of the genes, which cause disease. If we can block the Quorum Sensing systems we may be able to prevent these bacteria from being dangerous and causing disease.

References

1. Bluth, Brian J., Frew, Sarah E. and McNally, Brian. 1997. “Cell-cell communication of Lux operon in Vibiro fisheri”,
2. The quorum-sensing site: The home of bacterial cell-cell communication on the Web
3. Small talk:A Cell-to-cell communication in bacteria. (A lecture at the meeting of the American     Physical Society on Jan 31, 2004 given by Prof. Bassler)