Abstract
Nanomaterials such as nanoparticles exhibit remarkable antimicrobial activities. Nanoparticles directly disturb the cell membrane or cytoplasmic proteins because they pass through the cell wall. Nanoporous Au (NPG) is another antimicrobial nanomaterial, which cannot pass through the cell wall of bacteria but can still kill bacteria, utilising interactions between the surface of NPG and cell wall of bacteria. The origins of antimicrobial activities without direct interactions are unknown. It is necessary to elucidate these mechanisms to ensure safe usage. Here we show that the antimicrobial mechanism of NPG consists of two interactions: between the surface of NPG and cell wall, and between the cell wall and cell membrane. Fluorescent experiments showed that the cell wall was negatively hyperpolarised by NPG, and molecular dynamics simulations and first-principles calculations suggested that the hyperpolarisation of the cell wall leads to delicate structural changes in the membrane proteins, rendering them bactericidal. Thus, the hyperpolarisation induced by NPG plays a critical role in both interactions. The combination of molecular dynamics simulations and first-principles calculations allows a deeper understanding of the interactions between metallic surfaces and biomolecules, because charge transfer and exchange interactions are calculated exactly.
Highlights
Bacteria such as Escherichia coli (E. coli) and Staphylococcus epidermidis (S. epidermidis) sometimes cause harmful effects to humans: for example, E. coli behaves as an etiologic agent when it intrudes into blood vessels or the urinary system
Nanoporous Au (NPG), which has an open porous structure with pores and ligaments in the nanometre range[23], was found to exhibit antimicrobial properties against E. coli and S. epidermidis[24]. In this case as well, its antimicrobial activity must be related to interactions between nanostructured surfaces and the cell wall, because NPG is too bulky to pass through cell walls of bacteria
Which was clarified by inductively-coupled plasma atomic emission spectroscopy and Merckoquant peroxide test, (2) direct contact between bacteria and NPG substrate was necessary for the antimicrobial efficacy of NPG, because the antimicrobial activity of NPG was almost zero at high relative humidity (RH) (=90%), where the frequency of contact between bacteria and substrate was minimised, (3) the microarray analyses showed that NPG disturbed the function of the cell membrane of E. coli, and (4) the measurement of viable bacteria count showed that a long incubation time of 24 h was needed to cause the antimicrobial activity of NPG
Summary
Bacteria such as Escherichia coli (E. coli) and Staphylococcus epidermidis (S. epidermidis) sometimes cause harmful effects to humans: for example, E. coli behaves as an etiologic agent when it intrudes into blood vessels or the urinary system. Nanostructured substrates cannot pass through the cell wall because of their bulky dimensions, with typical macroscopic lengths larger than millimetres Their antimicrobial mechanisms are presumably related to disorder or denaturation of cell walls. Nanoporous Au (NPG), which has an open porous structure with pores and ligaments in the nanometre range[23], was found to exhibit antimicrobial properties against E. coli and S. epidermidis[24] In this case as well, its antimicrobial activity must be related to interactions between nanostructured surfaces and the cell wall, because NPG is too bulky to pass through cell walls of bacteria. In the present study, the interactions between NPG and E. coli are investigated from the viewpoint of hyperpolarisation and strength of the cell wall. This computational strategy allows a deeper understanding of the role of hyperpolarisation in antimicrobial activity
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