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Bacteria that multiply on surfaces are a major headache in health care when they gain a foothold on, for example, implants or in catheters. Researchers at Chalmers University of Technology in Sweden have found a new weapon to fight these hotbeds of bacterial growth—one that does not rely on antibiotics or toxic metals.

The key lies in a completely new application of this year’s Nobel Prize-winning material: metal-organic frameworks. These materials can physically impale, puncture and kill bacteria before they have time to attach to the surface.

Because once bacteria attach to a surface, they start to multiply while encasing themselves in what is known as a biofilm—a viscous, slimy coating that protects the bacteria and makes them difficult to kill. Biofilms thrive particularly well in humid environments and can pose serious challenges in health care.

For example, bacteria can attach to medical devices such as catheters, hip replacements and dental implants, and lead to hospital-acquired infections (HAI), also known as nosocomial infections—a widespread problem worldwide that causes great suffering and high health care costs, and an increased risk of the development of antibiotic resistance.

Biofilms can also form on ship hulls, where they can lead to troublesome algal biofouling and barnacle growth, slowing down the ship while increasing its fuel consumption.

Furthermore, antifouling paints containing toxic biocides are often used on ship hulls to combat this problem, with an associated risk of harmful substances leaching into the marine environment. Biofilms in industrial piping systems are also a widespread problem that can cause corrosion, clog the systems, reduce their efficiency, and increase energy consumption, for example.

New way to use metal-organic frameworks

Researchers at Chalmers University of Technology have now found a new way to attack biofilms by coating surfaces with nanostructures—metal-organic frameworks—that kill bacteria mechanically.

The study, published in the journal Advanced Science, was carried out in a collaboration between two teams of researchers at the University: Professor Ivan Mijakovic’s and Professor Lars Öhrström’s.

“Our study shows that these nanostructures can act like tiny spikes that physically injure the bacteria, quite simply puncturing them so that they die. It’s a completely new way of using such metal-organic frameworks,” says the study’s lead author, Zhejian Cao, Ph.D. in Materials Engineering and researcher at Chalmers.

The coating is constructed in a way that allows it to be applied to a variety of surfaces and integrated into other materials. A major advantage of the method is that it prevents or reduces biofilm formation without the need to use antibiotics or toxic metals.

“It fights a major global problem, as it eliminates the risk that controlling bacteria will lead to antibiotic resistance,” says Cao.

Metal-organic frameworks (MOFs) are a new class of materials with exceptional properties where metal ions are interlinked into three-dimensional structures with large cavities and channels in the material.

The Chalmers researchers explored a completely different function for MOFs in their study.

“There have been previous attempts to use metal-organic frameworks for antibacterial purposes, but in those cases the bacteria were killed by toxic metal ions or antimicrobial agents released by the MOFs. Instead, we have grown one MOF on top of another, which results in the formation of sharp nanotips that can puncture and kill the bacteria when they approach,” says Cao. The nanotips were created by controlling the crystalline growth in the material, and a major challenge was finding the right distance between the nanotips to maximize their effect.

“If the distance between the nanotips is too large, bacteria can slip through and attach to the surface. If the distance is too small, however, the mechanical stress exerted by the nanotips on the bacterial cell capsule may be reduced so that the bacteria survive—the same principle that allows you to lie on a bed of nails without getting hurt,” says Cao.

Possible to achieve large-scale production

Lars Öhrström is a co-author of the study and has worked with metal-organic frameworks for 30 years. He emphasizes that there are numerous practical advantages to using MOF coatings for controlling bacteria on surfaces compared to other solutions.

“These coatings can be produced at much lower temperatures than, for example, the graphene arrays previously developed at Chalmers. This facilitates large-scale production and makes it possible to apply the coatings to temperature-sensitive materials such as the plastics used in medical implants. In addition, the organic polymers in metal-organic frameworks can be created from recycled plastics, having the potential to contribute to a circular economy,” says Lars Öhrström.

The study was conducted in Professor Lars Öhrström’s research group at the Department of Chemistry and Chemical Engineering, and in Professor Ivan Mijakovic’s group at the Department of Life Sciences.