Hi,
A while ago I posted a draft of a paper I wrote summerizing a bit of
what is know about communication in bacteria. I connected that communication
to thoughts about the Net. I am posting below my final version. I wonder
if anyone could suggest a paper or online biology journal that might be
willing to publish my piece.
Thanks.
Jay
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The Role of Communication in the Survival of Bacteria
Jay Hauben
Ours is a time of rapid advance of communication technology. Histori-
cally, 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 something at its early stages is fruitful
because at the early stages the principles are clearest. 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 abundant
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 molecu-
lar 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.
For C. crescentus, the differentiation into two morphologically dif-
ferent daughter cells requires no communication among cells nor between a
cell and its environment. 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 osmosis 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 differently de-
pending 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 mole
cules that are sensitive 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 concen-
tration 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 nourishing 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 great
er efficiency in breaking down and utilizing the nutrients (Kaiser, 1984).
Again, cell division is symmetrical and all cells function in a similar
fashion under normal conditions. With a secretion similar to autoinducer,
the M. xanthus cells coordinate their motion in order 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 per
haps one hundred thousand in a mound perhaps one tenth of a 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. cre-
scentus may be the exception but it survives by asymmetric differentiation
leading to the adventurous migration of one half of its progeny. For most
bacteria their edge in survival comes from an encoded strategy 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 nutri-
ent supply is dwindling. When enough of them do the signalling, the colony
as a whole being in danger of starvation takes collective action. The in
dividual 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 in
formation during DNA replication arms each generation of bacteria with all
the other survival lessons its species has accumulated during its evolution.
Without the lessons of its species history each generation would be vulner-
able to extinction. But also, communication among unicellular species pro-
vides 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 an indication of the importance of communication for all
species.
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References
Cooper, S. 1991. Bacterial Growth and Division. Academic Press, San
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
Spring, NY., pp. 1-26.
Kaiser, D. 1984. Regulation of muticellular devevolopment in
myxcobacteria. In R. Losick and L. Shapiro. Microbial
Development. Cold Spring Harbor Laboratory, Cold Spring, NY.,
pp. 197-218.
Kaiser, D. and R. Losick. 1993. How and why bacteria talk to each
other. Cell 73: 873-885.
Losick, R. and D. Kaiser. 1997. Why and how bacteria communicate.
Scientific American 276 2: 68-73.
Parkinson, J. S. 1993. Signal transduction schemes in bacteria. Cell
73, 875-871.
Pierce, J. R. 1972. Communication. In Communication. W. H. Freeman, San
Diego, pp. 3-13.
Poindexter, J.S. 1964. Biological properties and classification
of the Caulobacter group. Bacteriol. Rev. 28: 231-295.
Stent, G. S. 1972. Cellular communication. In Communication. W. H.
Freeman, San Diego, pp. 17-25.
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4/29/97
