Using Polymer Coatings to Change how Bacteria Move

Featured Image: Micrograph of Methicillin-Resistant Staphylococcus aureus (MRSA). Courtesy: National Institute of Allergy and Infectious Diseases.

Title: Mechanical Properties and Concentrations of Poly(ethylene glycol) in Hydrogels and Brushes Direct the Surface Transport of Staphylococcus aureus

Authors: K. W. Kolewe, S. Kalasin, M. Shave, J. D. Schiffman, and M. M. Santore

Journal: ACS Applied Materials & Interfaces

Year: 2018

https://dx.doi.org/10.1021/acsami.8b18302

One of the most incredible advances in medicine in the past few decades is the rise of new devices that can be implanted directly into the body. Pacemakers, joint replacements, and all kinds of other devices are helping people live healthier, longer lives all over the world! But any time something is put into the body that isn’t naturally found there, there is always a chance of infection. Bacteria that started outside the body could grow on this new material, making this potentially life-saving device a constant source of infection. Because of this, researchers are researching how bacteria move and stick to materials that are used for these types of applications.

In this research, scientists are trying to fill a gap in the way we understand bacteria sticking to surfaces. A lot of the research in the past has looked at the different chemicals that can be used to coat materials and how the bacteria interact with them. While this is important, there is another important factor: the mechanical properties of the coatings. This research has started to show that how soft or stiff this coating is on the molecular level can change how a bacterial cell interacts with it, even influence whether or not it sticks to a surface it wouldn’t otherwise stick to.

The coating the researchers chose to explore was poly(ethylene glycol), or PEG, which is a long polymer and used in many different applications. In the medical field, it is often used because it is biocompatible (doesn’t hurt living tissue), easy to work with, and doesn’t allow proteins to stick to it. Two different types of coating were used, both applied onto glass surfaces. The first was a hydrogel, where long chains of PEG are linked to each other, forming a network of molecules (Figure 1A). The second was a coating of “brushes,” where many individual PEG molecules were attached directly to the surface, so that the polymer chains all stick straight up (like bristles on a brush) (Figure 1B).

Figure 1: Structures of PEG coating on glass slides, both as a linked hydrogel network (left) and standing brushes (right). Adapted with permission from Kolewe, K. W.;Kalasin, S.; Shave, M.; Schiffman, J. D.; Santore, M. M. ACS Appl. Mater. Interfaces 2019, 11, 1, 320-330. Copyright 2019 American Chemical Society.

To make a variety of surfaces for the bacteria to interact with, the researchers made different versions of each coating as well. The hydrogels were either “soft” or “stiff,” which described how easily they could be compressed. Chemically, the higher the concentration of PEG used to make the hydrogel, the stiffer the material. Two types of brushes were made too, with different lengths of PEG (one with a molecular weight of 2000 Daltons and the other of 5000 Daltons). The higher molecular weight corresponds to longer PEG molecules and a thicker coating. While the stiff hydrogel was the stiffest coating, these brush coatings were stiffer than the “soft” hydrogels.

The researchers had to develop a way to quantify the way the bacterial stuck to the PEG-coated surfaces. Staphylococcus aureus bacteria (the type responsible for MRSA), were flowed across the surface of the coating and watched under the microscope. By measuring the distance the bacteria cells traveled vs. time, the researchers could measure when the cells were “engaged” with the coating instead of just flowing along (Figure 2). The “engagement” periods showed slower movement.

Figure 2: Distance of two S. aureus cells traveled over time on a PEG-coated surface. Regions with shallower slopes correspond to when the cells were engaged with the coating. Adapted with permission from Kolewe, K. W.;Kalasin, S.; Shave, M.; Schiffman, J. D.; Santore, M. M. ACS Appl. Mater. Interfaces 2019, 11, 1, 320-330. Copyright 2019 American Chemical Society.

After they started seeing the bacteria slowing down as they interacted with the coating, the researchers wanted to make sure that this effect was due to something with the bacteria themselves, and not the coating. So, they also flowed across the materials silica microparticles also coated in PEG (so they didn’t stick). Looking at the data, it was clear that the bacteria interacted with the coating longer (had more “residence time”) than the spheres (Figure 3), meaning that something more complicated is happening with the bacteria.

Figure 3: Overview of residence time of both bacteria cells (left, A) and PEG-coated particles (right, B) on surfaces with different PEG coatings. Adapted with permission from Kolewe, K. W.;Kalasin, S.; Shave, M.; Schiffman, J. D.; Santore, M. M. ACS Appl. Mater. Interfaces 2019, 11, 1, 320-330. Copyright 2019 American Chemical Society.

While the exact interactions between the PEG coating and the bacteria are still unknown, the researchers were able to determine that the bacteria interacted more with the “stiffer” coatings (like the stiff hydrogel and the brushes) than the “softer” coatings (like the soft hydrogels) (Figure 3A). This same pattern wasn’t seen in the silica microspheres, again meaning that it was due to something different in the bacteria. The scientists hypothesized that this preference might be due to the stiffer coatings having a higher density of PEG available for the cells to interact with and adhere to.

Even though the specific way these interactions work is still being determined, this research is some of the first to show how the mechanical properties of a coating alone can shape its interaction with bacteria. This is hugely important information for scientists in the future looking to design new coatings that won’t harbor bacterial growth.

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