Original source: Physics World

Natural cell growth — which is essential for bacterial cell division — appears to favour the destruction of E. coli adhered to cicada-wing-like nano-pillars, report researchers in Germany. By understanding the process in more detail, scientists hope to find new solutions for fighting the build-up of bacteria around medical implants, to assist current drug-based treatments (Biomed. Phys. Eng. Express 4 055002).

“Innovative anti-bacterial implant surfaces, which are effective by their topography, might be a possible strategy,” says team member Manfred Köller from BG University Hospital Bergmannsheil in Bochum. “For example, in orthopaedics, such approaches could help to combat bacterial adherence, which is the first step in the formation of unwanted biofilms.”

The group’s study is inspired by observations made elsewhere that nano-pillar surfaces on cicada wings exert bactericidal effects on certain adherent bacteria. The result, reported back in 2012, continues to fascinate the biomedical engineering community, and researchers are keen to determine exactly how such nano-topographies are able to hinder bacterial colonization and growth.

To find out more, the team — which also includes scientists based at Ruhr University Bochum’s Institute for Materials — fabricated test surfaces (5 x 5 mm) covered with titanium nanopillars to mimic the insect wings. Gram-negative E. coli bacteria were allowed to adhere and proliferate on the nanostructured samples for three hours at 37 °C. The researchers incubated one batch under optimal cell growth conditions (brain heart infusion (BHI) medium), and placed the other in limited growth conditions (RPMI1640 medium). This led to an interesting discovery.

As the bacteria grew — a process that involves cell elongation of their rod-like structure — the titanium nano-pillars appeared to represent more of a threat to the micro-organisms. And the stronger the cell growth, the greater the antibacterial effect of the textured sample surface, as measured by the ratio of adherent dead E. coli to adherent living E. coli.

“Our results show that the bacterial growth of gram-negative micro-organisms adherent to nano-pillar-like structures is somehow involved in the phenomenon known as the cicada wing effect,” Köller comments

The team point out the likelihood that cylindrical elongation during growth induces additional mechanical strain on the cell when the bacterium is entangled by nano-pillars. And it follows that a disruption in the cell wall remodelling processes could lead to a loss of pressure inside the micro-organism, putting the E. coli at risk.

Electron microscopy images taken by the scientists show cell bodies collapsed and laid down on, but not punctured by (at least when viewed from above), the spike-covered test specimens used in the study.

The theory that cell growth plays a role in the overall mechanism is bolstered by previous work by the researchers where they observed a delay in the occurrence of bactericidal effects – a result that can be explained by the time gap between attachment and subsequent cell division.

Next steps

Back in the lab, the group is working on ways to increase the potency of the nanostructures to keep surfaces sterile for longer, which would further benefit implant applications. This involves decorating the nano-pillars with silver-iridium caps, which enhance the antibacterial activity of silver via an electrochemical process. Properties of the system include protection against gram-positive bacteria.

“The combination of both the cicada wing effect and such a sacrificial anode system is the next step forward,” Köller predicts.