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11/13/2001
Disrupting Bacterial Communications
Brian Trumbore just dropped off an interesting article on head
lice. It seems that perhaps one in every five children in Britain
have these creatures roaming around in their hair. Getting rid of
them is no easy task and there is a good deal of controversy over
the best procedures. As widespread as it is, the lice problem
seems rather low on the scale of problems that need solutions,
especially in these times of anthrax and other deadly diseases
that terrorists might employ. I found an article in the July 2001
issue of Scientific American that seems more relevant now than
it was back in July.
The article, by J. W. Costerton and Philip S. Stewart, is titled
"Battling Biofilms". The opening paragraph notes the similarity
between the Pentagon''s "information warfare" and a developing
approach by researchers fighting harmful bacteria. As we''ve
now seen in Afghanistan, a major objective of the Pentagon''s
strategy has been to destroy the Taliban''s ability to communicate.
Within the past decade or so, communication among bacteria has
also been recognized as playing an important role in spreading
disease.
We''re all familiar with the work of those who culture bacteria in
dishes and then add an antibiotic or some other drug to see if the
drug kills the bacteria. We often hear of some promising drug
that seems to be the solution to curing a certain disease, based on
such experiments. Yet, years later we don''t seem to hear
anything more about that drug. What''s the problem? The drug
doesn''t work the same way in the human body. Sometimes the
drug is unacceptable because of serious side effects. However,
scientists now realize that there''s another problem - biofilms.
What do your teeth, contact lenses, metal piping in steam-driven
electric power plants and the Washington, D.C. water supply
have in common? All present surfaces for the growth of
biofilms. Dental plaque is a biofilm, as is the growth on your
lens that could cause corneal infection. Biofilms in power plants
speed up corrosion, the cause of half their unscheduled outages.
Biofilms resistant to chlorination have apparently on several
occasions degraded the quality of water supplies below the
Federal standards in Washington.
What is a biofilm? Ever since the germ theory of disease was
proved, virtually everybody looked at these bacteria swimming
around in various fluids as single cells. That''s how they are
studied in those cultures, as single cells in a watery liquid.
Overlooked was all that slimy stuff that is found all around us.
Or, in the case of dental plaque, inside us. What the slimes were
showing us was something we''ve known for thousands of years.
The solitary life isn''t as good as a life in a family or a
community. Bacteria are no different. They like to get together
to form a biofilm.
When the individual bacterium plops down on a surface, it first
grabs onto the surface, then begins making and sending out
signaling molecules. If there are other bacteria in the region,
they''re also sending out these signaling molecules. When the
concentration of these molecules gets high enough, it''s a signal
that there are enough bacteria in the area to form a "quorum".
It''s time for the bacteria to come together and form a little
community by clumping together. If enough bacteria are around
there will be a number of these little communities and the
bacteria begin making a gooey, liquidy stuff that keeps the
clumps together. When things are just right, the cells start
multiplying and this biofilm grows, like a mushroom.
Now we have a network of these clumps of bacteria held together
in this porous gooey stuff, with channels in it that allow water
and nutrients to flow. But, while the outer clumps get a full share
of nutrients, the clumps deeper in the biofilm don''t get the same
share. As a result, the bacteria within the biofilm are in different
stages. The well-fed bacteria may be very active and multiplying
while the undernourished inner bacteria are sluggish or even
dormant. Sometimes, other kinds of bacteria passing by decide
the biofilm looks attractive and they jump in to join in the fun.
This can prove to be a beneficial immigration for the original
bacteria, which may thrive on the waste products of the new
arrivals or vice versa. So, now you have a multicultural biofilm
with different "races" and different stages of the same bacteria.
This diversity within the biofilm is not good if you''re trying to
kill the bacteria. For example, some drugs such as penicillin go
after reproducing bacteria but may be ineffective against the
dormant bacteria. Thus you might kill the outer bacteria but the
inner dormant ones are still unaffected and ready to spring into
action to rebuild the biofilm when the time is ripe. Those
bacteria that have become resistant to penicillin may have the
ability to manufacture a chemical that fights the penicillin. A
single cell, however, may still succumb to the penicillin but a
biofilm colony of the same bacteria will produce enough of the
chemical to mount an effective defense.
You may have an example of this benefit to a bacterial
community in a biofilm in that slimy stuff that forms in your
shower. You may reach for the chlorine bleach and a brush to
scrub the area clean. But that biofilm has stuff in it that tends to
neutralize the bleach and you may think you''ve killed the
buggers. However, the inner bacteria are still quite alive, if
dormant. You may find it takes a lot more scrubbing and a lot
more bleach to do the job than you had thought.
As with any community, there are some individuals who prefer
to strike out on their own and as the biofilm ages, some of the
bacteria revert to their single cell behavior and wander off to
establish new biofilms of their own.
Now that medical researchers know that the real challenge posed
by bacteria is in biofilms, they are starting to target various
stages in the biofilm''s development. We''ve seen that a key to
forming the biofilm is those signaling molecules. Disrupting the
communications is one approach. Accordingly, efforts are being
made to find drugs that will inhibit the signaling molecules from
being formed or released or even being sensed. Workers are also
trying to find compounds to apply to medical implants that will
stop the production of that gooey stuff that holds the biofilm
together. There is also the possibility of finding something that
will inhibit the bacteria from sticking to the surface in the first
place. All sorts of things can now be tried.
Biofilms have now been implicated in a number of maladies as
varied as periodontal disease, prostate infections, tuberculosis
and Legionnaire''s disease. Wouldn''t it be great if something
could be found that would either prevent the formation of the
hurtful biofilms or break them up after they formed? A few
years ago, workers by the names of Kjelleberg and Steinberg in
an Australian university made a discovery that has prompted a
new field of work on just such a compound. They found that the
fronds of a certain red alga in the waters of Botany Bay hardly
ever were covered with biofilms. Yet other types of algae in the
same waters harbored biofilms routinely.
Kjelleberg and Steinberg found that the "clean" red alga
produced a chemical in the class known as furanones. I have no
idea what furanones are - let''s call then Fury for short. The other
algae did not have Fury associated with them. Obviously, it was
hoped that it was Fury that was preventing or killing the
biofilms. Remember those signaling molecules I mentioned
earlier? It turns out that Fury is similar to the molecules used by
bacteria in signaling and detecting when they have the "quorum"
to begin a biofilm. Speculation is that, because of the similarity,
Fury grabs onto the sites on the bacteria where the signaling
molecules normally attach. By beating those signaling molecules
to the punch, Fury prevents the bacteria from finding out there''s a
quorum and the biofilm doesn''t form.
Experiments have shown that Fury can prevent or even break up
biofilms in mice. However, it doesn''t appear that that the Fury
compounds tested so far are safe for human use. Nevertheless,
Fury has stimulated a major effort to find other, safer compounds
that can mimic those signaling molecules and carry out the same
strategy as the Pentagon -disrupt the communications. Let''s hope
both the military and the biological strategies prove wildly
successful!
Allen F. Bortrum
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