Geology Reference
In-Depth Information
that cause diseases in humans (such as
Pseudomonas aeruginosa)
as well as nitrogen-
fixing bacteria in the soil.
Another famous example of quorum sensing (this time by means of a molecule other
than AHL) involves
Myxococcus xanthus
, a rod-shaped soil bacterium that forms flat
gliding colonies in decaying vegetation. When things go well and there is enough food,
Myxococcus
cells divide and move along slimy trails in a tight pack, secreting prey-di-
gesting enzymes in a stunning display of complex social predation described as “wolf-
like” by one key researcher in the field. But when starvation threatens, things change
dramatically. Under these circumstances the cells release a signalling molecule, which,
above a threshold concentration, causes them all to converge into a great hump-shaped
mound as they retrace their steps along their slime trails in a series of beautifully co-
ordinated pulsing waves. Once in the mound, most of the cells commit suicide. The dy-
ing cells release nutrients which are used by a few survivors to make resistant spores
able to sit things out until favourable conditions return. Here we witness 'mere' bacteria
consulting with each other as a group. Each cell receives messages from all the others,
reads its own internal state in relation to these messages, and then contributes its own
response to the community pool of responses.
So far we have looked at how bacteria use a single signalling molecule for quorum
sensing, but in fact most use several such molecules, and many are sensitive to signalling
molecules emanating from species other than their own.
Vibrio fischeri
has a different
signalling pathway which allows it to emit light when there are lots of other species in
the light organ, so it must be sensitive to a whole host of signalling molecules. Further-
more, most bacteria use chemical signals to detect mutants that have become harmful to
the colony. When this happens, the colony activates previously dormant genes, and be-
comes invisible to the mutants by, in effect, spontaneously switching to a different 'lan-
guage'. One prominent investigator has said that these experiences with mutants help
the colony to hone its “social skills”, enabling it to improve its cooperation.
Communication amongst different species of bacteria allows them to form mixed
species colonies which are able to accomplish tasks which a single species alone could
never achieve. A startling example lurks in our own mouths, in the plaque that we so
assiduously brush and floss away every day. Hundreds of bacterial species live in the
plaque, their quorum sensing communication networks dwarfing the combined com-
plexity of all our human communication systems. Next time you brush your teeth, give a
thought to the sophisticated bacterial intelligence that you are so nonchalantly sweeping
away into oblivion.
Bacterial chemical communication is of such startling complexity that it resembles
the basic grammatical structures of human language, so much so that scientists are now
talking about bacterial syntax and even about bacterial social intelligence. This sophist-
