Biology Reference
In-Depth Information
functionalize AFM cantilevers with cells or biologically relevant molecules.
Scientists interested in biology who choose AFM as part of their research
can also more easily combine AFM with other forms of microscopy, such as
side-view luorescence microscopy
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or total internal relection luorescence
microscopy.
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In addition to direct integration, AFM can be used side by side
with complementary techniques, such as confocal laser scanning microscopy
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and quartz crystal microbalance with dissipation monitoring.
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One of the key challenges that remain is in the ability to separate biological
interaction forces from effects due to the compliance of the bacterium. There
are still some cases for which the “origin” of the force curve made on a
bacterial cell can be dificult to determine. To briely summarize, the irst and
most widely accepted approach to set the origin of a displacement curve was
speciied as the point when the cantilever delection is linear with respect
to sample displacement at high force.
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Although this method is simple, it
was tested with silica spheres and is generally valid on hard surfaces. Force
measurements on bacteria often show that cantilever delection is a non-
linear function of displacement. The Hertz theory can be used to estimate
the point of zero,
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provided that there are no adhesion forces observed. This
is often not valid because AFM experiments with bacteria are very likely
to show adhesion between the bacterium and substrate. Speciically when
attraction is observed in the approach curve, it was suggested that the point
at which the probe contacts the surface can be considered the position of
zero separation.
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Again, this method may not work since attraction may not
be observed. An alternate method proposed was to consider the origin by
taking the derivative of the force with respect to the distance, and inding the
point at which the derivative becomes non-zero.
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An issue with this method
is that the typical amount of data scatter makes it hard to unambiguously
determine the zero point. More studies on model systems can help clarify this
issue for future AFM users.
Another challenge lies in the ability to apply quantitative models to AFM
force proiles on bacteria. It has been very attractive to use DLVO-type models
to describe bacterial interaction forces, but thus far, such models have failed
to capture the behaviour of AFM experiments.
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A recent study has reported
better agreement between extended DLVO predictions and AFM force proiles
for the interactions of hydrophobic bacteria
Acinetobacter venetianus
RAG1
and
20S-E1-c with alkanethiol-functionalized
(hydrophobic) AFM probes (gold-coated silicon).
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The authors simply added
a term for steric interactions to the typical DLVO interactions. For this speciic
case of two hydrophobic surfaces interacting with one another, the modelling
was consistent with experimental measurements from the AFM. However,
retraction force data were not shown so we are unable to evaluate the extent
Rhodococcus erythropolis
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