Section 4:
What are key characteristics of
biofilms?
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Test your
knowledge | Go to Section
Five |
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1. Biofilms are complex, dynamic structures
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Bacterial biofilms are remarkably heterogeneous in
virtually all parameters that can be measured accurately and
reproducibly. These heterogeneities: structural, physiological,
chemical, ecological, electrical, etc., have been implicated as the cause of
many phenomena characteristic of the attached—as opposed to the
planktonic—mode of growth. Pure cultures are virtually non-existent
in the world outside the laboratory. Microorganisms, like other
organisms, exist in communities where a variety of interactions
exist.
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One example of the effects of microbial
colonization on metal surfaces is Microbially Influenced Corrosion
(MIC). The presence of microorganisms modifies deposition and
dissolution rates of minerals, and by this mechanism, influences the
electrochemical properties of the metals or alloys. Pitting
corrosion, as seen at right in this example from a stainless steel
surface, results from this activity.
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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 |
Biofilms are also
dynamic and responsive to their environment. Bacterial cells may
detach from biofilms individually or in clumps. When they detach in
clumps, they retain the reduced susceptibility to antimicrobials
characteristic of biofilms. In the right conditions, biofilms can
also migrate across surfaces over a period of time in a variety of
ways, as illustrated below.
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All materials have certain properties of elastic
solids and viscous fluids. 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.
FOR MORE COMPLETE INFORMATION, SEE MODULE 2:
BIOFILM FORMATION AND GROWTH
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2. Genetic expression is different in biofilm
bacteria when compared to
planktonic bacteria
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The double-stranded helix structure of molecular DNA
(deoxyribonucleic acid), discovered in 1953, has by now 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.
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.
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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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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 of Pseudomonas aeruginosa. 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.
What are the implications of this difference in genetic
expression? One example is in the development of antibiotics. These
drugs have traditionally been developed to kill planktonic bacteria.
We now know, however, that planktonic bacteria are more susceptible
to antimicrobials than biofilm bacteria—and also that many of the
infections plaguing humans are actually caused by bacteria in the
biofilm mode of growth. So traditional antibiotics have been
targeting bacterial cells in their relatively unprotected state. We
will need to develop new classes of antibiotics that target
bacteria without 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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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 species, but by
other microbial species—and perhaps even by more complex organisms
in some cases. |
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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, chemical signals produced by cells are
not concentrated enough to cause changes in genetic expression.
However, in biofilms, the matrix material (EPS) that holds cells in
close proximity allows concentrations of signal molecules to build
up in sufficient quantity to effect 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 will not
produce toxins until they sense that an adequate population has been
established to survive host defenses. This system of population
recognition has been termed "quorum sensing." It was first observed
in the marine bacterium Vibrio fischeri, which can produce
light after a sufficient population 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 a new appreciation of
their survival strategies. There is also good evidence that cell
signaling can regulate the differentiation of cells into
sub-populations that carry out different activities within a
microbial community of a single species. In the late 1990s an
investigation of 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.
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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Many studies have shown that the multicellular
construction of biofilms affords protection for cells. This protection is
the result of intrinsic shifts in genetic expression when
floating bacterial cells 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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Test your
knowledge | Go to Section
Five |
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Section Five: Why is an
interdisciplinary approach a good way to study biofilms? |
