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Contents
I. Introduction
II. Formation & Structure
III. Self Defense
IV. Sewage Treatment Applications
V. Others Places They are Found
VI. Conclusion
Works Cited
Photo & Illustration Credits
Appendix
I. Introduction
Biofilms are communities of bacteria or other single-celled organisms organized in "slime." You can find them in plaque, in many persistent infections, on slippery rocks in streams, and in many other everyday places. Perhaps you might wonder why infections or plaque never go away (or always come back). Those infections are caused by bacterial communities of bacteria within a strong biofilm.
Microbiology is not a very old subject of study. However, Anton van
Leeuwenhoek was curious about things he could see with his primitive microscope,
so he scraped the plaque from his teeth and looked at it. It was then that
he saw the "animalculi" (bacteria) that composed this type of biofilm.
Even though Leeuwenhoek studied this biofilm, no one really paid attention
to them. Then, in the 1970s, scientists began to realize the importance
of bacterial biofilms. They discovered that 99% of bacteria live in sessile
bacterial biofilm communities, but it was not until the 1980s and 1990s
that we began to appreciate the complexity of them. We are still studying
these things with more advanced technology to find out more about them.
II. Formation & Structure Not all bacteria are capable of forming biofilms. In a study of one
species, the Pseudomonas aeruginosa, it was shown that the bacteria with
sad(surfaceattachment defective) genes were the only bacteria of the species thatformed biofilms. Others were not able to adhere to solid surfaces and/or form microcolonies. This is because they did not twitch. The bacteria with
sad genes twitch slightly, allowing them to form bonds with surfaces and other bacteria. These sad planktonic (independent) bacteria form very
strong and stable biofilms as they undergo the process of biofilm
formation. First, the sad bacteria must adhere to a solid surface
(be it bone, plastic, or tooth). Then, when they get settled, they must
spread and reproduce to form a microlayer across the surface. When this
is completed, a microcolony can be formed. After the colony is established,
the mature biofilm (bacteria in a polymeric matrix that they secrete) is
finally formed.
Biofilms are complex bacterial "neighborhoods" that have a definite
structure. For one, the bacteria nearer the interior of the biofilm have
a slower metabolism. When bacteria are far away from the wall of the biofilm,
the channels within the matrix cannot deliver much oxygen to them, hence
their metabolic state. However, the cells nearer the exterior of the biofilm
have a more ample supply of necessary nutrients, so they grow more rapidly.
Do not think that biofilms are limited to just one species of bacteria
(or other organisms). Many biofilms have a community of different bacterial
species, such as plaque.
After the biofilm is formed, other types of bacteria can attach to form
a more complex and diverse biofilm.
III. Self Defense Their self-defense mechanisms are the main reason that biofilms cause
such persistent infections. Ordinarily, if bacteria invaded a host, antibiotics,
antibodies, and phagocytes (bacteria attacking cells, such as white blood
cells) would destroy the bacteria before they could cause harm. However,
biofilms are very resistant to these kinds of attacks. For one thing, their
transfer properties help them. The antibodies and antibiotics cannot penetrate
the polymeric matrix that they create, so they are useless against them.
Chemicals that would destroy planktonic cells (such as chlorine, hydrogen
peroxide, etc.) would only kill surface cells, leaving the cells in the
interior unharmed. Not only that, but increased use of such materials would
cause the bacteria to grow resistant to them, making them useless in the
future. As a result of their decreased metabolism, antimicrobial agents
that rely on the intake of it by the bacteria do not work either.
Even though it may seem as though these bacterial biofilms are completely
resistant to antimicrobial agents, they are not. When phagocytes
attack biofilms, they excrete phagocytic enzymes. These enzymes destroy
everything around it. Because of that, the enzymes penetrate the biofilms
to destroy the outer layer of bacteria. Though this injures the biofilm,
it does not severely harm it, because the cells in the inside are still
healthy. Then the cells in the interior can reproduce to replace the cells
that were killed. If the phagocytes keep on excreting these enzymes, the
biofilm will eventually be killed. However, the phagocytes will eventually
run out of the enzyme and allow the biofilm to regrow. Even so, the release
of these phagocytic enzymes destroys human cells, in addition to biofilm
cells. Therefore, this method of destroying biofilms is not very effective.
Biofilms also release
planktonic cells periodically. This serves two main purposes. One is
that it allows free planktonic cells to colonize elsewhere, and the other
is that it gives away "bait" for antimicrobial agents. The cells released
occupy the antibiotics while the biofilms remains unharmed (at least for
a while).
Different bacteria can handle different kinds of attacks. In the study
of the Pseudomonas aeruginosa, it was found that bacteria of the species
with two different types of sad genes created different biofilms. One produced
a biofilm that was very resistant to antimicrobial agents, and another
did not. If a small difference in genetics such as this created such a
different biofilm, imagine what would happen if you had two different species.
This is a reason why bacterially diverse biofilms are somewhat more stable
than those with only one type of bacteria. (This is similar to an ecosystem;
with more organisms there is more stability.)
IV. Sewage Treatment Applications By now you might think that biofilms are all bad, but they have their
up and down sides. Biofilms actually have some good uses, such as in sewage
treatment plants. They are used in sewage treatment plants to remove excessive
nutrients from the water. They are a great alternative to costly chemical
cleaning. However, they have some drawbacks. They corrode the pipes. They
attach themselves to pipes, and bond metal ions within the polysaccharide
matrix. This causes a pH difference between cells in the biofilm of up
to 1.5 units, causing the metal to move away from the pipe surface to other
areas of the biofilm where there is a higher pH, and thus causing the corrosion
of the pipe. This can be very expensive to fix. The biofilms must be physically
detached from the pipes. This usually results in the scraping the pipes,
which is time consuming as well as costly.
Biofilms are also unwanted for water treatment because they harbor bacteria
that are released into drinking water. This can be hazardous. A very common
biofilm bacteria is the Pseudomonas aeruginosa, which can be dangerous
to the health of people with weakened immune systems.
V. Others Places They are Found Perhaps I forgot to mention the necessity of water for a biofilm to
form. If there is not enough water, the biofilm will not form properly.
This is why they form so well in aquatic environments. They live dwelling
on the water's surface in streams, rivers, and other bodies of water. Many
of them also live on rocks, forming a slimy layer on the rock's surface.
This trait of needing water also applies to the human body. This is one
reason why bacteria can form such strong biofilms in the human body.
There are many infections that involve biofilms, such as dental caries,
musculoskeletal infections, contact lens infections, etc. A major concern
about biofilms comes about when artificial biomaterials (such as artificial
skin, knee replacements, artificial hearts, etc.) are surgically inserted
into the body. It is in the interest of the patient and the doctors that
human cells will colonize the new biomaterials first, so that they can
become integrated with the body. However, biofilms may colonize it first.
If this happens, the body will reject the new implants. Because it is very,
very difficult (or impossible) for the body to destroy the biofilm, the
implants would then have to be removed. This rejection puts the patient
at risk, as well as increasing the need of repeated surgery. This is why
biofilms may be very harmful and annoying to patients with biomaterial
implants.
Not all biofilms have humans as their haven. Some biofilm-forming bacteria
are methanotrophs (methane-utilizing
bacteria). These bacteria produce an unusual biofilm. They spread out to
form a thin layer. They form their biofilms using energy from
methane that comes from the wastes of bacteria that decompose cow droppings,
landfill garbage, as well as other things. The methane the decomposing
bacteria release goes into the ground, where it is used by the methanotroph
biofilms.
VI. Conclusion Biofilms are very interesting. They have unusual properties, such as
their unusual strength and their ability to withstand conditions in which
planktonic cells would die. Many infections that seem impossible to remove
are caused by biofilms, as well as the reduction of methane in the atmosphere.
These unique characteristics make biofilms have many applications, like
water treatment. Perhaps in the future we may harbor bacterial biofilms
to be more efficient in humans' benefit. Who knows? We may even find a
way to kill biofilms within a human without harming the host. Imagine what
results that would bring! At this point in time, research is conducted
by microbiologists to answer these questions and bring forth results. In
the three-hundred years of study of biofilms, the most progress has been
made in the last thirty years of it. Perhaps with increased technology
we might accelerate our rate of discovery to tame these vicious biofilms.
That is our goal. We must find out if we will succeed.
By: Cesar Caro
(References are listed in order of amount of usage.) 1. Costerton, J. W., Stewart, Philip S., and Greenberg, E.P. "Bacterial
Biofilms: A Common Cause of Persistent Infections" Science Vol.
284 21 May 1999: 1318-1322
2. "Biofilms" http://www.bact.wisc.edu/ScienceEd/Biofilms.html
3. "The Microbial World: Biofilms" http://helios.bto.ed.ac.uk/bto/microbes/biofilm.htm
4. "An Introduction to Biofilms" http://www.edstrom.com/Lab/WaterQualityBulletins/biofilm/biofilm01.htm
5. Beuling, Evelien. "Mass Transfer Properties of Biofilms" http://www.ct.utwente.nl/~ospt/minipost96/uva/bpk1.html
6. "Oral Biofilms" http://deep13.ra.utk.edu/ceb/proj10.htm
7. "Introduction to Biofilms" http://utweb.utampa.edu/faculty/kbeach/Mar327Lect/mchapla/BIOFILM1.htm
Photo & Illustration Credits 1. Picture 1, Appendix 1: http://www.personal.psu.edu/faculty/j/e/jel5/biofilms
2. Picture 2, Appendix 1: Science Vol. 284 21 May 1999: 1321
3. Appendix 2: http://deep13.ra.utk.edu/ceb/proj10.htm
4. Appendix 3: Science Vol. 284 21 May 1999: 1320
5. Appendix 4: Science Vol. 284 21 May 1999: 1321
6. Appendix 5: http://helios.bto.ed.ac.uk/bto/microbes/biofilm.htm
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These pictures show how biofilms form and the duration of each step.
This picture depicts oral plaque biofilm as seen under a microscope.
The bacterial diversity is clearly shown.
Shown here is how a biofilm withstands attacks by the human immune system.
This diagram shows two different ways the planktonic cells can be released
from a biofilm.
The two pictures show the methane-utilizing bacteria.
LEFT: Two flasks containing an atmosphere of methane, carbon dioxide,
and oxygen. The films have been disturbed, causing them to fold.
Written April 2000
Works Cited
Appendix
Appendix 2: Plaque
Back
Appendix 3: Attack on Biofilm
Back
Appendix 4: Release of Planktonic Cells
Back
Appendix 5: Methanotrophs
RIGHT: Bacterial biofilm as seen under a microscope. The block-like
patterns show the planes of division.
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Copyright 2000-2001 @ Cesar Caro
cesar314@excite.com
Last updated Wednesday, December 13 2000.
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