communication and bacteria

Jay Robert Hauben jrh29 at ciao.cc.columbia.edu
Thu Jul 17 20:29:50 EST 1997


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.



         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 
Cooper, S. 1991. Bacterial Growth and Division. Academic Press, San 
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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