Bactericidal activity of self-assembled palmitic and stearic fatty acid crystals on highly ordered pyrolytic graphite

Elena P. Ivanova, Song Ha Nguyen, Yachong Guo, Vladimir A. Baulin, Hayden K. Webb, Vi Khanh Truong, Jason V. Wandiyanto, Christopher J. Garvey, Peter J. Mahon, David E. Mainwaring, Russell J. Crawford

The wings of insects such as cicadas and dragonflies have been found to exhibit nanostructure arrays that are assembled from fatty acids, and that physically interact with and significantly disrupt bacterial cell membranes. Such mechanobactericidal surfaces are of significant interest, as they can kill bacteria without the need for antibacterial chemicals. Here, we report on the bactericidal effect of two main lipid components of insect wings epicuticle, palmitic (C16) and stearic (C18) fatty acid films after re-crystallisation on the surface of highly ordered pyrolytic graphite. It appeared that the presence of two additional CH2 groups resulted in the formation of different surface structures. Scanning electron microscopy and atomic force microscopy showed that palmitic acid microcrystallites were more asymmetric than those of stearic acid where the palmitic acid microcrystallites were observed as an angular abutment in the scanning electron micrographs. The principal differences between the two types of long-chain saturated fatty acids crystallites were the larger density of peaks in the upper contact plain of the palmitic acid crystallites as well as their greater proportion of asymmetrical shapes, in comparison to stearic acid. These two parameters might contribute to higher bactericidal activity on surfaces derived from palmitic acid. Both palmitic and stearic acid crystallised surfaces displayed bactericidal activity against Gram-negative, rod-shaped Pseudomonas aeruginosa and Gram-positive, spherical Staphylococcus aureus cells. These microcrystallite interfaces might be a useful tool in fabricating effective bactericidal nanocoatings. Nanostructured cicada and dragonfly wing surfaces have been discovered to be able physically kill bacterial cells. Here, we report on the successful fabrication of bactericidal three-dimensional structures of two main lipid components of insect wings epicuticule, palmitic (C16) and stearic (C18) fatty acids. Both palmitic and stearic acids after crystallisation on highly ordered pyrolytic graphite surfaces displayed bactericidal activity against both Gram-negative Pseudomonas aeruginosa and Gram-positive Staphylococcus aureus cells. The simplicity of the production of these microcrystallite interfaces suggests that a fabrication technique based on solution deposition could be effectively applied to form bactericidal nanocoatings.