Model
of the adhesion mechanism by which the bacterium Staphylococcus aureus binds to
hydrophobic ('low-energy') surfaces (left) compared with hydrophilic
('high-energy') surfaces (right). On the left, a large number of cell wall
molecules (shown here as tiny compressible springs) are involved in binding the
cell to the hydrophobic surface. On the hydrophilic surface shown on the right,
far fewer molecules are involved. The results were obtained by a team of
experimental and theoretical physicists at Saarland University who performed
computational Monte Carlo simulations of force-distance data from atomic force
microscopy experiments. Courtesy: Saarland University
Staphylococcus
aureus bacteria are one of the most common causes of infections acquired by
patients during a stay in hospital. These pathogens are particularly
problematic because they can form robust biofilms on both natural and
artificial surfaces from which they are very difficult to remove. "The
individual bacteria within these biofilms are effectively protected from attack
by antibiotics or by the human immune system. That's why it can be so dangerous
when these bacteria colonize medical implants as they can then cause serious
post-operative infections," explains Karin Jacobs, Professor of
Experimental Physics at Saarland University. It is therefore crucial to try and
prevent these biofilms from forming in the first place.
However,
to be able to influence biofilm growth, the researchers had to understand the
mechanisms by which the bacteria adhere to different materials. Using a
scanning atomic force microscope, they pressed the minute bacterial cells onto
different types of surfaces and then determined the force needed to lift the
adhered cells from the surface. This experimental configuration allowed the
researchers to record what are known as force-distance curves. "We used
extremely smooth silicon surfaces as model surfaces. In one set of experiments,
the silicon surfaces were prepared so that they had high water-wettability; in
another set of experiments they were treated to be highly hydrophobic. We were
able to show that the bacterial cells adhered far more strongly to the
hydrophobic surfaces, from which water simply rolled off, than on the
hydrophilic (water-wettable) surfaces," explains Karin Jacobs.
But it is
not just the magnitude of the forces that differ between the two surface types,
so too do the shapes of the force-distance curves (see figure). "On the
hydrophobic surfaces, we see very smooth curves with a characteristic cup shape.
On the hydrophilic surfaces, in contrast, we observe force-distance curves with
a very jagged profile," says Professor Jacobs.
In order
to understand these results, the dynamics of these complex systems were modeled
using Monte Carlo simulations that were carried out in the research group led
by Professor Ludger Santen, Professor of Theoretical Physics at Saarland
University. The model treats the bacterial cell as a rigid sphere and the
molecules in the cell wall that tether the cell to the surface as minute
springs. "It turns out that in order to reproduce the experimental
results, the role played by the random (stochastic) nature of the molecular
binding process is more important than trying to increase the complexity of the
model. We have now uncovered why the bacteria cells behave so differently on
different types of surfaces. On hydrophobic surfaces, a large number of the
cell wall proteins adhere to the surface, which results in a strong binding
force and yields a smooth force-distance curve," explains Ludger Santen.
In
contrast, on a hydrophilic surface, far fewer cell wall proteins are involved
in tethering the bacterium to the surface. As a result, the bacteria are held
less strongly on the surface and the shape of the force-distance curve is less
uniform. "The jagged shape of the curves that we see with hydrophilic
surfaces is caused by a few individual cell wall molecules as they are pulled
from the surface. Because fewer cell wall proteins are involved, the bacteria
bind less strongly to hydrophilic surfaces," says Erik Maikranz, who
carried out the Monte Carlo simulations as part of his doctoral research work.
Due to the
different shapes of the force-distance curves, the physicists suppose that on a
hydrophilic surface fewer cell wall proteins are involved in the binding
process because these molecules first have to overcome a potential barrier,
which effectively reduces the number of protein macromolecules that can tether
the cell to the surface. "The potential barrier to adhesion on hydrophilic
surfaces is relatively high, so only a few of the cell wall proteins are able
to overcome this energy barrier in a particular time. On hydrophobic surfaces,
however, the barrier is negligibly small, so that many cell wall proteins can
adhere directly to the surface," explains Dr. Christian Spengler, who
performed the experiments in the study.