PROBING BACTERIAL ADHESION USING FORCE SPECTROSCOPY - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 281

When the AFM probe is very far away from the sample, there is no delection
of the cantilever since the probe-sample separation distance is too large to
experience any attractive or repulsive van der Waals or electrostatic forces.
As the probe continues to approach the sample, the cantilever bends towards
the surface, sometimes jumping into contact if there is an attractive force
from the sample. After contact is made, the cantilever bends until it reaches
the speciied force limit that is to be applied, and this region is known as the
constant compliance region. After applying the desired force to the cantilever
while the probe is in contact with the sample, the process is reversed and the
second half of the force cycle takes place, which is known as the retraction
portion. As the probe continues to be retracted from the surface, the force
exerted by the bending of the cantilever overcomes the adhesion force
between the probe and the sample, and the probe “snaps off ” the substrate.
Once the probe has retracted more than a few hundred nanometres, the
cantilever returns to its original position where there is no delection and the
force cycle is complete and ready to start the next measurement.
The large peak observed in the retraction portion of the cycle can
provide valuable information since it is possible to determine the rupture
force required to break the bond of adhesion between two substrates. If we
were studying surfaces such as bacteria or other cells, it is possible to obtain
more than one peak in the retraction curve since there could be multiple
polymers attaching to the AFM probe during the force cycle. As each one
of these polymers detaches from the probe, a new peak will appear on the
force proile.
13.2 ELASTICITY OF BACTERIAL POLYMERS
A bacterial surface is composed of many molecules, including
lipopolysaccharides (LPS, only for Gram-negative bacteria), proteins and the
less well-deined extracellular polymeric substances (EPS), which can include
capsules or vesicles. The LPS extend away from the outer membrane and are
often the irst contact between a bacterium and a substrate ( Fig. 13.1 ) . In
addition, some bacteria can produce specialized structures such as imbriae,
which recognize receptors and bind to mammalian cells, or lagella, which
allow the bacteria to be motile. AFM provides an eficient tool to measure
the adhesive interactions of bacterial polymers at the nano- and pico-Newton
levels, since interactions between bacterial cells and either modiied or
unmodiied AFM probes can be measured in a variety of solutions. Surface-
modiied probes can be helpful for studying hydrophobic or hydrophilic
interactions, for probing the nature of the electrostatic interaction between