INFORMATIONAL HANDOUT:
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What is Cell Signaling?
When we use this phrase, we are talking about the way cells communicate - using molecules that
are produced, dispersed and then 'received.' In order to be the recipient of a chemical message,
a cel must have specific receptors for the signaling molecule. The sensor may sit on the cell
membrane - or it can be internal. In other cases, signals target specific sequences in the DNA
where they bind (this is how some hormonal signals are received in your body). A signaling
molecule binds to it's receptor by the old 'lock and key' method. The two molecules fit together in
a way that allows them to tightly bind to one another. Other molecules don't fit, so they don't bind.
Once a signal molecule is bound, it alters the shape or blocks some site on the receptor - which
causes things to happen in the cell. A typical consequence is that a new protein gets transcribed
or translated - or the transcription or translation of a protein is halted.
Molecules of all shapes
and sizes external to the
cell. Some are signals,
some are not.
Those that are signals
will target specific
receptor molecules either
on cell surface (red),
inside cell (blue), or they
may bind to specific
sequences in DNA.
Once bound to the
target location, the
receptor/signal
complex causes
some important
change to occur.
What is Quorum sensing and how do bacteria talk to each other?
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 signaling molecules which are released into the environment. As well as releasing the
signaling 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 signaling 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 various classes of signaling 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.
Why do bacteria talk to each other?
QS enables bacteria to co-ordinate their behavior. As environmental conditions often change
rapidly, bacteria need to respond quickly in order to survive. These responses include adaptation
to availability of nutrients, defense against other microorganisms that may compete for the same
Biology 215, Ruth A. Gyure
Western CT State University
nutrients and the avoidance of toxic compounds that are potentially dangerous for the bacteria. It
is very important for pathogenic bacteria during infection of a host (e.g. humans, other animals or
plants) to co-ordinate their virulence in order to escape the immune response of the host in order
to be able to establish a successful infection.
Do all bacteria use the same signal molecules?
Different bacterial species use different molecules to communicate. There are several different
classes of signaling molecule (see examples). Within each class there are also minor variations
such as length of side chains etc. In some cases a single bacterial species can have more than
one QS system and therefore use more than one signal molecule. The bacterium may respond to
each molecule in a different way. In this sense the signal molecules can be thought of as words
within a language, each having a different meaning.
Can bacteria from one species communicate with those from another species?
There is evidence that interspecies communication via QS can occur. This is referred to as
quorum sensing cross talk. Cross talk has implications in many areas of microbiology as in nature
bacteria almost always exist in mixed species populations such as biofilms.
What are the benefits of quorum sensing research?
QS research has many potential applications, most of these involve controlling bacteria by
interfering with their signalling systems. For example many bacteria rely on QS to control the
expression of the genes which cause disease. If we can block the QS systems we may be able to
prevent these bacteria from being dangerous.
Quorum sensing in Vibrio fischeri
In the late 1960s, the marine bioluminescent bacteria Vibrio fischeri was being grown in liquid
cultures and it was observed that the cultures produced light only when large numbers of bacteria
were present (Greenberg, 1997). At first, the behavior was misunderstood. It was later shown that
the luminescence was initiated by the accumulation of an activator molecule or "autoinducer"
(Nealson et al, 1970, Eberhard, 1972). This molecule is made by the bacteria and activates
luminescence when it has accumulated to a high enough concentration. The bacteria are able to
sense their cell density by monitoring the autoinducer concentration. This mechanism of cell
density sensing was termed quorum sensing (QS). The molecule produced by V. fischeri was first
isolated and characterised in 1981 by Eberhard et al. and identified as something we refer to as an acyl-homoserine lactone (AHL). Analysis of the genes involved in QS in V. fischeri was first
carried out by Engebrecht et al (1983), and this led to the basic model for quorum sensing in V.
fischeri which is now the model for other similar quorum sensing systems.
For many years following this, it was thought that AHL-based QS was limited to marine bacteria
such as V. fischeri and V. harveyi. Research into antibiotic synthesis carried out at Nottingham
and Warwick led to the discovery that QS was far more widespread than previously thought.
Staphylococcus aureus and Pseudomonas aeruginosa, for example, both important human
pathogens, have similar quorum sensing systems.
ACKNOWLEDGEMENTS: University of Nottingham Quorum Sensing Site
http://www.nottingham.ac.uk/quorum/introduction.htm
Biology 215, Ruth A. Gyure
Western CT State University