The Role of Communication in Bacteria
Ours is a time of rapid advance of communication technology.
Historically, major changes in communication technology, like the
invention of spoken language, the invention of writing, and the
invention of movable type and the printing press have had profound
effects upon human society. But also communication of genetic
information from generation to generation during DNA replication, of
metabolic information from cell to cell during development and
everyday life and of information between organisms plays a
fundamental role at all levels of life (Pierce, 1972 and Stent,
1972). Aristotle taught that to study a thing at its early stages
is fruitful because at the early stages the principles of a thing
are clearest. In order to understand the importance of the current
telecommunications revolution to human society it is worthwhile
therefore to investigate the role of communication in one of the
oldest forms of life, bacteria.
Bacteria are among the oldest, most wide spread and most
abundent organisms on our planet (Losick and Kaiser, 1997). How have
they been able to survive and spread over such long time and
distances? Even under electon microscopic magnification they appear
as little more than rigid vessels filled with strands of DNA and an
amorphous cytoplasm, surrounded by a cell membrane which might
include flagella or pilli (Losick and Kaiser, 1997). A typical
Caulobacter crescentus, for example, appears to be a cigar shaped
cell with a stalk extending out from one pole. In the process of
cell division, as two daughters are forming, a fagellum can be seen
at the opposite pole from the stalk (Poindexter, 1964 and Ely and
Shapiro, 1984). Modern laboratory techniques, however, allow us to
know that at the molecular level even C. crescentus cells have a
fairly complex structure (Ely and Shapiro, 1984).
The stalk of a C. crescentus cell has an adhesive at its far
end so it can hold fast to solid surfaces (Poindexter, 1964). Its
cell division is assymetrical, one daughter cell retains the
original stalk and remains in place to repeat the reproductive
cycle, while the other daughter cell (the swarmer) uses its
flagellum to swim away. Since the stalked daughter likely remains
where its mother was, it cannot avoid cell threatening conditions
there. But the swarmer cell guided by chemical gradients can seek
favorable conditions. The swarmer eventually releases its flagellum.
Developing at the same pole a stalk, it begins the cycle again. By
having half its progeny as swarmers, C. crescentus has enhanced the
possibility of its survival as a species. But in so doing, it
sacrificies any advantage that might have arisen from collective
action among its daughter cells.
There is no communication among C. crescentus cells nor between
a cell and its environment that determines the differentiation into
two morphologically different daughter cells. The plan for this
differentiation is contained in the DNA of C. crescentus cells and is
replicated in each daughter cell. Such a survival strategy is the
result of the evolutionary history of the species. DNA replication
achieves the communication from parent to offspring of the
accumulated trail and error survival lessons of that species. The
DNA also contains genes, the expression of which, for example, leads
to the synthesis of the proteins that are used to construct the
flagellum. How this is achieved is not fully understood but the non-
random placement of the flagellum requires control that must
ultimately come from the DNA (Cooper, 1991). The whole life of the
cell is a constant osmossis of molecules from the environment as
nutrients or contact with molecules from the environment as signals.
In response to these nutrients and signals, various genes are
expressed that control the enfolding of plans encoded in the DNA.
So, even without communication with other cells, each C.
crescentus cell experiences constant internal communication and
interaction with its environment.
For most bacteria, cell division is symmetrical yielding two
identical daughter cells each with a complete copy of its parent's
DNA. But sometimes in the course of their lives such identical cells
behave differnetly depending upon signals they get not from the DNA
but from each other. For example, each cell of Vibrio fischeri has a
mechanism encoded in its DNA for producing light (Losick and Kaiser,
1997). If isolated cells produce light they would not achieve any
benefit worth the energy expended to produce the light and might
thereby be more easily targeted by preditors sensitive to light. But
isolated cells do not produce light. Instead, there is a squid,
Euprymna scopes, that has developed a mechanism for concentrating V.
fischeri into a part of its body called a light organ. V. fischeri
cells continuously synthesize and secrete a molecule, so-called
autoinducer (Kaiser and Losick, 1993). In their cell membranes there
exist other molecules that are sensitve to the relative presence of
autoinducer. When the V. fischeri live freely in the ocean, the
concentration of the bacteria and whatever they cast off is
extremely low. When a Euprymna scopes succeeds in concentrating the
V. fischeri cells in its light organ, however, the concentration of
autoinducer will increase. When the bacteria sense autoinducer at or
above threshold concentration, production of light is triggered. Now
safe in large numbers within the light organ, the production of
light does not endanger the cells. In fact, in exchange for the
light which the squid needs, a nurishing sheltered haven is provided
(Losick and Kaiser, 1997). Autoinducer is a relatively small
molecule called a homoserine lactone, one of a family of molecules
bacteria use to measure their own cell density in conjunction with
triggering collective behavior. Casting off and sensing autoinducer
is a V. fischeri cell's mechanism for communicating its presence
and sensing the presence of numbers of other cells.
Myxococcus xanthus is another species for which density
information is important (Kaiser, 1984). These rod shaped bacteria
inhabit cultivated soil, breaking down water insoluable organic
material as they glide about or swarm onto a particularly rich
nutrient source. A dense population yields a greater efficiency in
breaking down and utilizing the nutrients (Kaiser, 1984). Again,
cell division is symmetrical and each cell functions in a similar
fashion under normal conditions. With a secretion similar to
autoinducer, the M. xanthus cells direct their motion to gather to
feed together at the sites of abundent nutrient. But sometimes when
nurtients become scarce there is a problem. No individual cell can
move fast enough or far enough to insure finding a new source of
food or a source it could deal with alone. Even if a cell could form
a spore or in another way try to survive, its chance of success is
low.
To avoid the consequences of depleted nutrients, when an M.
xanthus cell senses decreasing available nutrients, it synthesizes
and casts off a molecule (of the homoserine lactone family) called
factor A (Kaiser, 1984). Low density of factor A does not influence
other nearby M. xanthus cells. If however approaching starvation is
sensed and responded to by significant numbers of cells, a threshold
concentration of factor A is reached (kaiser and Losick, 1993). The
cells detecting this concentration of factor A cease functioning
normally. Instead they join, via signals back and forth, in a
coordinated motion with other such cells culminating in the piling
of perhaps one hundred thousand in a mound perhaps one tenth
millimeter high (Losick and Kaiser, 1997).
When the mound is complete, the M. xanthus cells continue to
move but now with a motion that reorients them with respect to each
other. They secrete another molecule, so-called factor C, that
adheres to their cell membranes. Factor C somehow signals proper
orientation to interior cells for optimal dense packing (Kaiser and
Losick, 1993). Surface cells move until by some signal they are side
by side in small groups which are dislocated with respect to each
other (Kaiser, 1984). When these motions are complete, the
concentration of factor C signals previously identical cells to
begin to behave differently (Kaiser and Losick, 1993). Depending
where they are in the mound, they either lyse or form spores. The
resulting structure, called a fruiting body protrudes from the
surface. More than half the cells have sacrificed themselves to form
a stalk and shell to hold the other cells which have morphogenicized
into spores resistent to heat, to desiccation, etc. The value to the
colony and the species is that such a densely packed fruiting body,
protruding from the surface has a much enhanced chance of being
carried elsewhere by the wind or an animal so as to at some time and
place encounter enough nutrient and with enough numbers to resume
the normal life cycle of M. xanthus. (Losick and Kaiser, 1997)
The survival strategies of bacteria encoded in their DNA are
quite varied and often complex. The notion that bacteria have
survived because they are rugged individuals proves untrue for
almost all species. C. crescentus may be the exception but it
survives by assymetric differentiation leading to the adventurous
migration of one half of its progeny. For most bacteria their edge
in survival comes from an encoded stradegy that includes cooperation
among large numbers of individual cells. There can be no such social
behavior without communication among the cells. V. fischeri signal
each other that there are enough of themselves present that they are
safe to produce light. All members of the community benefit from
the broadcast messages from each individual. M. xanthus signal each
other that the nurtient supply is dwindling. When enough of them do
the signalling, the colony as a whole being in danger of starvation
takes collective action. The individual cells coordinate their
motions to form a mound. Then some cells sacrifice themselves to
create a supporting structure while others become encased, dorment
spores. The resulting fruiting body is a densely packed package of
spores held high enough off the surface to have a chance for motion
as a whole to a new location where nutrient is available.
Besides making social behavior possible, communication of
genetic information during DNA replication arms each generation with
all the other survival lessons the species has accumulated during
its evolution. Without the lessons of its species history each
generation would be vulnerable to extinction. But also,
communication among unicellular species provides a suggestive
glimpse of the direction of evolution from unicellular to
muticelluar species and of the possible current direction of
evolution for all species from less communication to more, from less
cooperation to more. The fundamental role communication plays in
survival strategies of bacteria can be taken as the basis for
optimism that the enhanced communication among all the people of the
world made possible by new technology will lead to a greater ability
of the human species to solve its problems and increase its chances
of meaningful survival.
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References
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Diego.
Ely, B. and L. Shapiro. 1984. Regulation of cell differentiation in
Caulorbacter crescentus. In R. Losick and L. Shapiro.
Microbial Development. Cold Spring Harbor Laboratory, Cold
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Kaiser, D. 1984. Regulation of muticellular devevolopment in
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Development. Cold Spring Harbor Laboratory, Cold Spring, NY.,
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Kaiser, D. and R. Losick. 1993. How and why bacteria talk to each
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Losick, R. and D. Kaiser. 1997. Why and how bacteria communicate.
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Jay Hauben 4/27/97