Peeling off slimy bacterial biofilms

Researchers have found a new way to completely peel off bacterial biofilms.

By looking at the films from a biological as well as mechanical engineering perspective, Princeton University researchers showed that water penetrating the junction between biofilms and surfaces, coupled with gentle peeling, can result in effective removals.

The work, bridging molecular biology, materials science and mechanical engineering, took advantage of the collaborative research communities between molecular biology and engineering.

The new method is expected to help in thwarting harmful biofilms, as well as controlling the beneficial biofilms increasingly relied on for wastewater treatment, microbial fuel cells and other applications.

The method has been developed by Jing Yan, an associate research scholar working jointly in the Princeton labs of Howard Stone, the Donald R. Dixon ’69 and Elizabeth W. Dixon Professor of Mechanical and Aerospace Engineering; and Bonnie Bassler, the Squibb Professor of Molecular Biology and Howard Hughes Medical Institute Investigator. Yan is the co-lead author of the paper along with Alexis Moreau, who was a visiting student in Stone’s lab and is now back at the University of Montpellier in France. The findings have been published in journal Advanced Materials.

“By investigating and defining the material properties of bacterial biofilms, rather than their biological properties, we have invented a new method for detaching entire biofilms,” said study co-author Bonnie Bassler.

Vibrio cholerae

For their investigation, the Princeton researchers turned to the bacterium Vibrio cholerae, which forms biofilms in seawater and fresh water and in the human intestine. Measurements revealed that the biofilms it produces exhibit mechanical behaviours very similar to hydrogels, which are materials extensively studied in Stone’s lab.

Well-characterised, manipulatable hydrogels have many applications, especially in biomedicine, including wound dressing, drug delivery and tissue engineering. Both biofilms and hydrogels are largely made of water (about 90%). They possess defined structural networks that make them soft, viscous and elastic. Their stretchiness has a limit, however. If disturbed too vigorously, biofilms and hydrogels will break into pieces. This fragility poses a challenge for biofilm removal. It also hinders the intentional transfer of beneficial films between surfaces, for instance in industrial settings, and when running experiments in the lab to better understand biofilms in the first place.

To learn how to avoid such fragmentation, the researchers examined the attachment of the V. cholerae biofilms to a variety of surface types. The researchers saw that the edges of the biofilms were water repellent, while surfaces they adhered to were sometimes water attractive.

Capillary peeling

Based on this insight, the researchers sought to drive a wedge between the biofilm and attached surface by driving water into the space at which the materials meet. This technique, known as capillary peeling, successfully created a lengthening crack that culminated in full separation of the biofilm from the surface. The water-assisted peeling must go slowly to prevent biofilm tears — akin to carefully removing a sticker — but the results showed that the extra time was well worth it. “Our capillary peeling method worked astonishingly well,” Yan said.

One obstacle for deploying the method outside the lab is that many biofilms exist in already-aqueous environments, where capillary peeling would appear to be a non-starter. For those cases, Yan and colleagues have proposed two potential solutions to explore in future research. For biofilms initially grown underwater, the film and its adhered-to object could be removed from solution and dried out before removal attempts. Alternatively, introducing bubbles to the biofilm-substrate interface might deliver the same sort of capillary force.

Multidisciplinary approach

Overall, the new study illustrates the value of a multidisciplinary approach, bridging different fields to make key new insights.

“The interdisciplinary team on this study that combines engineering, theory and biology is indeed perfect for the complex problem of biofilms,” said Shmuel Rubinstein, an associate professor of applied physics at Harvard University who was not involved in the research.

Other authors of the study are Ned Wingreen, the Howard A. Prior Professor of the Life Sciences; Andrej Košmrlj, an assistant professor of mechanical and aerospace engineering; Sepideh Khodaparast, a former research scholar in Stone’s lab now at Imperial College London; associate research scholar Sampriti Mukherjee; postdoctoral researchers Jie Feng, Sheng Mao and Antonio Perazzo; and graduate student Chenyi Fei.

Image credit: Yan et al./Princeton University