by Bridget O’Brien
If you press a thumbtack into a balloon, it pops. In a similarly destructive manner, many winged-insects have evolved nanoscale thumbtack-like structures on the surface of their wings that ‘pop’ bacteria upon contact. When bacteria settle on a wing surface, these structures, termed “nanopillars”, penetrate the cell, causing the cellular membrane to stretch and the cell to rupture. The antibacterial properties of insect wings provide resistance against pathogens (disease-causing organisms like bacteria), protecting the insect against infections (1).
Since discovering the antimicrobial properties of nanopillars in 2012, a great deal of research has been devoted to fabricating surfaces which mimic those found in nature. One area of interest to scientists is the potential application of nanopillar-like surfaces in medical devices such as implants. If successful in duplicating the same antimicrobial properties found in nature, medical engineers may be able to reduce patients’ dependency on antibiotics as well as the frequency of postoperative infections (2).
Another potential application of nanopillars is in food packaging, where the presence of pathogenic bacteria is a constant threat to human health (3). Nanopillared packaging for common food products like dairy and meat could potentially reduce the frequency of surface bacterial contamination and thus improve food safety.
However, to effectively design nanopillared biomaterial, a better understanding of the mechanisms by which nanopillars kill bacteria is required. While it was once thought that the penetration of nanopillars into bacterial cells was the sole mechanism of cell death, recent research has widened the scope of how nanopillars confer antibacterial protection. One recent study from the University of Bristol suggests that nanopillars’ bactericidal action does not only rely on cell rupture, but may partially be mediated by physiological changes within the bacterial cell itself (4).
Researchers began by fabricating artificial nanopillar arrays mimicking the protrusions found on dragonfly wings. As expected, researchers found that the nanopillars would trap bacteria on the surface and cause cell penetration and deformation. However, the frequency of such events was low and could not account for the large decreases in viable bacteria recorded. Instead, researchers believed that in addition to mechanically ‘popping’ bacterial cells, the nanopillars were able to incite a physiological response in bacteria, resulting in altered gene expression and protein production.
By measuring the gene expression of bacteria in contact with nanopillars, researchers noted an increased expression of unstable molecules, termed reactive oxygen species (ROS). An increase in ROS causes oxidative stress on cells, leading to a disruption of cellular components including lipids, proteins, and DNA, ultimately resulting in cell death (5). While bacteria can produce protective proteins to counteract the effects of ROS, if the stress is too severe, they can use ROS to self-destruct (6). These findings led researchers to propose an alternative mechanism — one which turned their previous conceptions of nanopillars, and the source of their bactericidal mechanism, on its head.
In this newly proposed explanation, when bacteria contact a nanopillared surface, some rupture while others remain in a high state of stress and self-destruct. The study at the University of Bristol complements what scientists already knew about nanopillar’s antimicrobial properties, yet provides a more nuanced explanation of their mechanism of action, suggesting that physiological stress may play an equally important role as mechanical stress. These findings could allow researchers to optimize fabricated nanopillared surfaces for future applications.
Given the increasing rate of antimicrobial resistance, the application of nanopillars as an antibacterial biomaterial provides a promising solution to a growing concern to human health (7). While scientists are just beginning to understand the antibacterial properties of nanopillars and how to effectively apply them to prevent infections in humans, such findings beg the question: What other nanoscopic secrets may be found in the most unlikely of places?