Objectives
The objectives of this section are
to provide you with
an understanding of some of the key characteristics of biofilms;
a recognition of how microorganisms that are part of a biofilm are
different from those same microorganisms in isolation.
Outcomes
Upon completion of this section, you will be able
to summarize some
of the key characteristics of biofilms;
to discuss some of the ways in which microorganisms which are part
of a biofilm differ from the same microorganisms that exist in
isolation;
to describe why microorganisms that exist in a biofilm are harder to
destroy than the same microorganisms in isolation;
to discuss how
microorganisms in a biofilm can communicate with each other.
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Section 4:
What are key characteristics of
biofilms?
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Test your
knowledge |
Go to Section
Five |
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About Section 4
In this section we introduce you to some of the key
characteristics of biofilms. When we as scientists and engineers
begin to learn about a new life system in order to exploit it for
good or to destroy it if it is harmful, we need to understand as
much as possible about it. What are its characteristics, and how
will this knowledge help us accomplish what we need to do? The more
we know about how a system functions, the more we know about how to
deal with it. Quite a few things are now known about biofilms, but
there is a lot left to uncover. We discuss a few of the things we
have learned about biofilms here that help give us insight into why
traditional forms of treatment do not seem to work well on biofilms
and how we might develop more effective treatments. This overview
will prepare you for later modules in which some of these things are
explored in depth.
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1. Biofilms are complex, dynamic structures
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Biofilms are remarkably
heterogeneous. Many measurements and observations have been made of
various biofilms; they all point to the diversity of individual
biofilm colonies. As we have mentioned before, in typical, naturally
occurring biofilms (as opposed to some that are grown in a
laboratory for experimental reasons) there are nearly always a large
number of different kinds of microorganisms living together. In
addition, different biofilms seem to exhibit different internal
structures, different chemical properties, different electrical
properties, and, indeed, different properties of just about any
other measurement or observation that can be made. Each of
these properties seems to contribute to the characteristics of the
biofilm as a whole that make it different (e.g., hard to kill)
compared to dealing with each of the microorganisms in isolation (not in a biofilm, but in a planktonic
environment). |
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So here is a big question. If there is such a wide
diversity of properties in different biofilms, how can we expect to
find characteristics that apply to all biofilms? We are glad you
ask. What has been discovered is that in spite of their wide
diversity, biofilms do seem to have some common attributes, such as
their ability to grow on virtually any surface, how they attach to a
surface, their mode of growth, their ability to spread, how they are
nourished, how they maintain themselves as a colony, and so forth.
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Pitting corrosion on 316S stainless steel, an example of
microbially influenced corrosion. Image, courtesy of Z. Lewandowski and W. Dickinson, MSU-CBE |
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For example, the image at the right shows pitting
and corrosion of a stainless steel surface. This was caused by a
biofilm, whose presence influenced how and how fast minerals were
deposited on the surface. This, in turn, modified the
electrochemical properties of the stainless steel, which caused the
pitting corrosion of this seemingly impervious metal. |
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Can we extend what
we learn about this kind of biofilm to other sorts of biofilms, such
as plaque on teeth? Apparently so, as discussed in the rest of this
section.
Here are some of the more evident characteristics
common to all observed biofilms:
Biofilms appear to show aspects of both
solids and liquids—much like slug slime—and fall into a category
called "viscoelastic." However, as biofilms collect sediment, or
become scaled with rust or calcium deposits, they become less fluid
and more like a brittle solid.
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FOR MORE COMPLETE INFORMATION, SEE MODULE 2:
BIOFILM FORMATION AND GROWTH, to
come.
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2. Genetic expression is different in biofilm
bacteria when compared to
planktonic bacteria
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Here is a somewhat startling characteristic of bacteria in a biofilm
as observed by biofilm scientists and engineers. The same kind of
bacteria are different when they are in a biofilm than when they are
isolated in planktonic form (that is, floating as single cells in
water). Let's think about this for a moment. This is one of those
scientific discoveries that seems counterintuitive. It might seem so
obvious that a bacteria cell is a bacteria cell is a bacteria cell
that one might not even think to check whether a particular
bacterium is different when it is found in different environments.
The details of how this is determined is an advanced topic, but you
might find it interesting to hear how it is done.
The double-stranded helix structure of molecular DNA
(deoxyribonucleic acid), discovered in 1953 (by Watson and Crick),
has become a familiar image. DNA molecules, composed of units called
genes, carry the "instructions" that determine characteristics of
living organisms and comprise the genetic material passed along to
offspring through reproduction. |
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SDS PAGE preparation of the outer membrane proteins (OMPs) of
Pseudomonas aeruginosa cells in planktonic and biofilm states.
Courtesy, Hongwei Yu |
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The genes that form DNA molecules also play a crucial role in
cellular activities. Simple cells like bacteria control their
internal functions using various parts of their genetic code to
initiate chemical activities. So, for instance, consuming nutrients
and getting rid of waste products are processes that are carried out
under the influence of genetic instructions. When genes are
activated to make chemical products (amino acids and proteins), they
are said to be upregulated; when the genes are de-activated, they
are downregulated. The proteins made by activated genes constitute
about half of the material inside a cell, and are responsible for
numerous activities that keep a cell viable.
Since not all of the genes in a cell are activated to make
proteins all of the time, we can get a picture of cellular activity
by examining the proteins produced by cells at a particular time.
One way to get this kind of protein "snapshot" is by a technique
called SDS-PAGE (for "Sodium Dodecyl Sulfate" and "PolyAcrylamide
Gel Electrophoresis"). This technique allows scientists to see large
(nearer the top) and small (nearer the bottom) cellular proteins as
dark bands in an array of columns. In the SDS PAGE gel above, we see
proteins from the outer membranes of planktonic (outlined in blue,
Lanes 1-4 and 6) and biofilm (outlined in red, Lane 5) bacteria, of
a single strain. The bands of proteins are strikingly different,
telling us that the planktonic and biofilm forms of a single species
are expressing different genes, and therefore carrying out different
activities.
So what? Beyond the intellectual interest this holds for biofilm
scientists and engineers, what practical use does this knowledge
have? One example is in the development of antibiotics. These drugs
traditionally have been developed to kill planktonic bacteria under
the assumption that they would kill the same bacteria
wherever they were found. We now know, however, that
1. planktonic bacteria are more susceptible to antimicrobial
chemicals designed to kill them than are biofilm bacteria, and
2. many of the infections plaguing humans are actually caused by
bacteria in the biofilm mode of growth, not the planktonic mode of
growth.
Put these two things together with the fact that traditional
antibiotics have been designed for and tested on bacterial cells in
their relatively unprotected, planktonic state and we can begin to
understand why it is that antibiotics don't work well on these same
bacteria when they exist in a biofilm—the same bacterium is
different in the biofilm state than in the planktonic state for
which the antibiotic was designed and tested!
This presents scientists and engineers with a new challenge,
namely the development of new classes of antibiotics that target
bacteria that exist in the biofilm state. Understanding the genetic
activity of biofilm bacteria will help us to find new ways to target
these cells and disrupt their functions.
FOR MORE COMPLETE
INFORMATION, SEE MODULE 3: GENETICS AND MOLECULAR BIOLOGY, to
come.
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3. Biofilm cells can coordinate behavior via intercellular
"communication" using biochemical signaling molecules
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Another characteristic of cells found in a biofilm is that they
can communicate with each other. Really, in order for any community
to succeed, there must be good communication among its members.
Biofilm communities appear to be no different. Now how, you might
ask, can single-cell microorganisms, such as bacteria, communicate
with each other? One of the fascinating aspects of bacterial
community living is that it provides a setting for bacteria to
communicate using chemical signals. There is evidence that some of
these chemical signals, produced by cells and passed through their
outer membranes, may be interpreted not just by members of the same
cell species, but by other microbial species that are part of the
same biofilm community — and perhaps even by more complex organisms
in some cases. The sensing of these chemical signals by neighboring
cells in the biofilm can cause the neighboring cells to behave
differently. How? By causing different genetic expression to occur
in those cells, as described in the account in subsection 2 above. |
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In the cartoon above, various species of
bacteria are represented by different colors. Bacteria can produce
chemical signals ("talk") and other bacteria can respond to them
("listen") in a process commonly known as cell-cell communication or
cell-cell signaling. This communication can result in coordinated
behavior of microbial populations. Courtesy, MSU-CBE.
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In planktonic populations of these same kinds of cells, chemical
signals produced by the cells are simply not concentrated enough
when passed through the water to cause changes in genetic
expression. However, in biofilms, the matrix (glue) material (EPS)
that holds cells close together allows concentrations of
cell-produced chemical signal molecules to build up in sufficient
quantity to cause changes in cellular behavior. Bacterial
populations will activate some genes only when they are able to
sense, via cell signaling, that their population is numerous enough
to make it advantageous and/or "safe" to initiate that genetic
activity.
For example, some bacterial pathogens (the bad-guy,
disease-causing bacteria) will not produce toxins until they sense
that an adequate population of themselves has been established to
survive host defenses (e.g., antibodies, produced by a host human or
animal, that can kill the bacteria). This system of population
recognition has been termed "quorum sensing" (you've got it right;
this comes from the same term used in a committee when enough
members are present to legally take some action). It was first
observed in the marine bacterium Vibrio fischeri, which can
produce light after a sufficient population of this bacterium has
developed.
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Though planktonic
cells secrete chemical signals (HSLs, for homoserine lactones), the
low concentration of signal molecules does not change genetic
expression. Biofilm cells are held together in dense populations, so
the secreted HSLs attain higher concentrations. HSL molecules then
re-cross the cell membranes and trigger changes in genetic activity.
Courtesy, MSU-CBE.
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The discovery that simple cells are
capable of coordinated behavior has given us an entire, new
appreciation of their survival strategies. There is also good
evidence that cell signaling can cause cells of the same variety to
form sub-populations that carry out different activities. For
example, in the late 1990s an investigation of a biofilm community
the marine bacterium Pseudoalteromonas revealed two physiologically
distinct subpopulations. In effect there was a cellular division of
labor: one group stayed attached to the surface and made nutrient
available to the the second group, which reproduced and released
daughter cells to the surrounding water.
In summary, the life of a simple, single-cell microorganism, such as
a bacterium, is not so simple after all! And when these
microorganisms are found in a biofilm colony their complexity
increases tremendously. In order to treat and/or make beneficial use
of biofilms, we must continue to identify and exploit the
characteristics that are exhibited by microorganisms that form a
biofilm.
FOR MORE COMPLETE
INFORMATION, SEE MODULE 3: GENETICS AND MOLECULAR BIOLOGY, to
come.
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4. Biofilms are less susceptible to antimicrobial agents
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Another final characteristic of biofilms that we explore
in this section—one that we have hinted at numerous times in this
module—is that the microorganisms in a biofilm are much less susceptible
to antimicrobial agents (chemicals designed to kill those microorganisms)
than are the same microorganisms found in a planktonic state. Many studies
have shown that the multicellular construction of biofilms affords
protection for the cells that are part of these biofilms. This protection
is the result of intrinsic shifts in the way these cells behave (through
different genetic expression as described in subsection 2 above) once they
attach to surfaces and begin to form biofilms. Some of the hypothesized
mechanisms of protection from antimicrobial agents are pictured in the
diagram below. |
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A.
Free-floating cells utilize nutrients, but do not have sufficient
metabolic activity to deplete substrates from the neighborhood of
the cells.
In contrast, the collective metabolic activity of groups of cells in
the biofilm leads to substrate concentration gradients and localized
chemical microenvironments. Reduced metabolic activity may result in
less susceptibility to antimicrobials. |
B.
Free-floating cells carry the genetic code for numerous protective
stress responses. Planktonic cells, however, are readily overwhelmed
by a strong antimicrobial challenge. These cells die before stress
responses can be activated.
In contrast, stress responses are effectively implemented in some of
the cells in a biofilm at the expense of other cells which are
sacrificed. |
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C.
Free-floating cells neutralize the antimicrobial agent. The capacity
of a lone cell, however, is insufficient to draw down the
antimicrobial concentration in the neighborhood of the cell.
In contrast, the collective neutralizing power of groups of cells
leads to slow or incomplete penetration of the antimicrobial in the
biofilm. |
D.
Free-floating cells spawn protected persister cells. But under
permissive growth conditions in a planktonic culture, persisters
rapidly revert to a susceptible state.
In contrast, persister cells accumulate in biofilms because they
revert less readily and are physically retained by the biofilm
matrix. |
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Back to top |
Test your
knowledge |
Go to Section
Five |
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Section Five: Why is an
interdisciplinary approach a good way to study biofilms? |
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Module 1 Intro page
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