With the rise of antibiotic resistance, understanding how germs are spread in hospitals is vital if hospital acquired infections are to be reduced.
Researchers at Saarland University have now conducted studies in both experimental and theoretical physics to understand how germs stick to surfaces in order to improve antibacterial surfaces. This research can help to inform how we can control the spread of hospital acquired infections.
The results have been published in the journal Nanoscale.
Understanding bacterial mechanisms
Staphylococcus aureus (Staph) bacteria are one of the most common causes of infections in hospital and are also particularly problematic due to their ability to form robust biofilms on both natural and artificial surfaces. These bacteria are also very difficult to remove from surfaces, making it vital to try and prevent these biofilms from forming. Karin Jacobs, Professor of Experimental Physics at Saarland University, said: “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 colonise medical implants as they can then cause serious post-operative infections.”
In order to influence biofilm growth, the researchers had to understand the mechanisms by which the bacteria stick to different materials.
The researchers utilised a scanning atomic force microscope by pressing the bacterial cells onto different types of surfaces and then determining the force needed to lift the adhered cells from the surface, which allowed them 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,” explains Jacobs.
“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.”.
As well as the magnitude of the forces differing between the two surface types, so too do the shapes of the force-distance curves.
“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,” Jacobs added.
Monte Carlo simulations
Led by Professor Ludger Santen, Professor of Theoretical Physics at Saarland University, the team of researchers modelled the dynamics of these complex systems using Monte Carlo simulations.
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.
Santen commented: “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.”
On a hydrophilic surface, however, fewer cell wall proteins are involved in tethering the bacterium to the surface, meaning the bacteria are not stuck as well to the surface, and the shape of the force-distance curve is less uniform.
Erik Maikranz, who carried out the Monte Carlo simulations as part of his doctoral research work, commented: “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.”
Dr Christian Spengler, who performed the experiments in the study, concluded: “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.”