Biology Reference
In-Depth Information
This phenomenon was discovered by two independent groups about 40 years ago
(Fuqua and Greenberg 2002). The seemingly intelligent behavior of bacteria is the
natural consequence of the following networks of structures and processes that
interact with one another in an organized manner:
1. Bacteria possess the genes coding for diffusible intercellular messengers
called
autoinducers
.
2. Bacteria possess the genes coding for the cell membrane receptors recognizing
and importing the autoinducers present in the pericellular space.
3. Bacteria possess the genes that code for signal transducing proteins that, when
activated by extracellular autoinducers binding to their target receptors, are able
to search for and express the set of genes encoding the proteins that can
implement quorum-sensitive or quorum-dependent effector functions.
An example of a bionetwork embodying these elements organized in space and
time so as to exhibit quorum sensing is given in Fig.
15.11
, which consists of nine
nodes and seven edges. The network representation in this figure is a simple and
convenient way of keeping track of all the main components and their interactions
involved in effectuating quorum sensing. This bionetwork is best viewed as an
example of
dissipative structures
at resting state and, when the system is activated to
exhibit the phenomenon of quorum sensing, the system can be said to undergo a state
transition from the resting to the activated states. Figure
15.11
is not an equilibrium
structure because most of the structures shown in this figure would disappear upon
cessation of free energy supply to the cell. An activated state of dissipative structures is
characterized by a selective activation of a subset of the nodes of the bionetwork
involved, leading to space- and time-organized physicochemical processes that appear
to us observers as
quorum sensing
. The extracellular or cell-cell communication
components of quorum sensing can be viewed as “intercellular dissipative structure”
and represented as a bionetwork wherein cells themselves serve as nodes and
molecule-mediated interactions among them as edges. Hence quorum sensing can
be viewed as
a bionetwork of bionetworks
, reminiscent of the “renormalization”
phenomenon in bionetworks discussed in Sect.
2.4
. Thus the notion of bionetwork is
one of those concepts in biology that can be applied to multiple levels
recursively
and
hence may be at least partially responsible for the phenomena of
self-similarity
and
power laws
universally found in living systems (Gribbin 2004, Whitfield 2006).
It is surprising to find how simple the set of molecular structures and processes are
that can give rise to the seemingly intelligent behaviors of bacterial quorum sensing.
Quorum sensing in bacteria may be regarded as one of the simplest cases of
cellular
intelligence
and
cellular computing
, and we can now confidently say that we have a
more or less complete understanding of the molecular mechanisms underlying one
of the simplest intelligent behaviors of organisms. That is, there is no mystery about
cellular intelligence
. It simply is yet another example of
self-organizing chemical
reaction-diffusion processes
or
dissipative structures of Prigogine.
Hence, we
may logically refer to the intelligent behaviors of cells as the X-ator following the
naming tradition established in the field of self-organization (Babloyantz 1986),
X indicating the name of the city most closely associated with the pioneering
