Both cells to undertake tasks that would only

Gram-negative and Gram-positive bacteria use a form of chemical communication,
known as quorum sensing, to sense and respond to variations in cell density in
their environment, enabling them to synchronize gene expression and coordinate behaviours.
This phenomenon is critical for bacterial survival as it allows individual
cells to undertake tasks that would only be effective in a social context, such
as biofilm formation or bioluminescence. Not only are cells able to detect presence
and quantity of neighbouring cells but they are also able to differentiate
between kin and non kin and thus discern species variety in their environment. This
allows them to discriminate against competing non kin by, for example, triggering
collective inhibition of production of public goods so that the competitors
will not take advantage of them.

mechanism of quorum sensing is almost analogous across all species of bacteria,
with slight variations in the nature of the signalling molecules and receptors
used to elicit a response. In general, small molecules, known as autoinducers,
are produced within individual cells and exported. As cell density increases,
the production and release of autoinducers, by individual cells, grows, causing
their extracellular concentration to rise. Once this concentration has passed a
certain threshold, the autoinducer will bind to its receptor which, by a
variety of pathways, activates heterogeneous changes in gene expression
patterns, triggering a specific response. This enables cells to collectively react
to changes in cell density.  

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must be able to respond to a variety of autoinducers in order to adapt to
changing environments and quorum sensing network architectures provide the
means by which this can be achieved. The existence of these communication
networks allows for more flexible signal-response dynamics and improved signal
fidelity, as well as providing adaptability to deal with fluctuations in
temperature, pH and the presence of non kin species. The details of these
fundamental steps in Gram-positive and Gram-negative bacteria will be discussed
in further detail, with emphasis on the network architectures of the quorum
sensing circuits.


and collective behaviours mediated by quorum sensing were first observed and
understood in Gram-negative bacteria. The most distinctive feature in their
quorum sensing mechanism is the use of derivatives of S-adenosylmethionine
(SAM) as autoinducers, most commonly, acyl homoserine lactones (AHL). These are
small molecules that are synthesized intracellularly (e.g. by a LuxI synthase)
and released passively outside the cell. They have an N-acylated
homoserine-lactone ring and a variable 4/18 carbon acyl chain, derived from
fatty acid synthesis, the length of which contributes to stability and
therefore longevity of elicited response. AHLs will accumulate as cell density
increases and diffuse back into cells, through the cell membrane, where they
will bind to their cognate receptor. It is common for multiple AHLs to be
present in the environment at once and thus a cellular response may be based on
the blend of autoinducers which it detects. To accommodate this, complex quorum
sensing networks exist within cells, containing a variety of receptors, each of
which must be highly specific for their cognate AHL. Amongst other feedback
systems that are present in these networks, autoinduction provides a feed
forward loop within which quorum sensing, which has been activated by an
autoinducer, itself induces the production of more secreted autoinducer in
order to amplify the signal.  


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