Fluorescence Intensity of Liposomes and E. coli Attached to Nanopillar Arrays: Implications for Bacterial Death on Nanostructures

Abstract

Bacterial death on nanostructures has sparked great interest because the principle behind this phenomenon completely differs from that of conventional chemical antimicrobial agents. Physical interactions between the bacterial envelope and nanostructures generate forces that deform the cell membrane. Eventually, the cell envelope breaks, causing the cytoplasm to leak out. However, the events occurring between these stages are unclear. In this work, we used liposomes as pseudocells because they are composed of a lipid bilayer, which is similar to a cytoplasmic membrane. We monitored time-dependent changes in the fluorescence intensities of rhodamine 6G (R6G)-containing liposomes and mCherry-expressing Escherichia coli cells after adhering to Au nanostructure arrays (pitch; height; diameter = 500, 300, 283 nm) and compared the variations in the fluorescence intensities of the pseudo and E. coli cells over time. Electrochemical impedance spectroscopy was then performed to monitor the cell and liposome attachment. Ratio of change in Rct before and after the liposome injection was about 1.7, which means that Rct increased drastically due to the attachment of liposome. Similarly, ratio of change in Rct before and after the E. coli injection was about 1.15 due to the attachment of cell. The fluorescence intensity of mCherry quickly decreased after cell adhesion within 25 min, whereas that of R6G did not even at 60 min. This finding means that the liposomes were not broken down despite their mechanical strength being equal to or weaker than that of E. coli. The time constant (T) of the decrease in fluorescence intensity of E. coli was calculated by fitting the data to an exponential function and found to be −0.13 min–1, which is in good agreement with our previous results. But we could not calculate the T of liposomes. The results indicate that the death of bacteria on nanostructures likely involves an autolytic mechanism.