The molecular mechanism of action of bactericidal gold nanoparticles on Escherichia coli

Abstract

This work examines the molecular mechanism of action of a class of bactericidal gold nanoparticles (NPs) which show potent antibacterial activities against multidrug-resistant Gram-negative bacteria by transcriptomic and proteomic approaches. Gold NPs exert their antibacterial activities mainly by two ways: one is to collapse membrane potential, inhibiting ATPase activities to decrease the ATP level; the other is to inhibit the subunit of ribosome from binding tRNA. Gold NPs enhance chemotaxis in the early-phase reaction. The action of gold NPs did not include reactive oxygen species (ROS)-related mechanism, the cause for cellular death induced by most bactericidal antibiotics and nanomaterials. Our investigation would allow the development of antibacterial agents that target the energy-metabolism and transcription of bacteria without triggering the ROS reaction, which may be at the same time harmful for the host when killing bacteria.

Introduction

Multidrug-resistant (MDR) strains of pathogenic bacteria emerge with increasing frequency in hospitals and the general community alike [1]. The need for effective antibacterial agents is undisputed. The rapid growth of nanotechnology has provided many types of materials for biomedical applications, including in the field of antimicrobes [2], cancer imaging and therapeutics [3], [4], persistent biofouling [5], and so forth. Many metal- and carbon-based nanomaterials, such as Ag, TiO₂, ZnO, fullerene derivatives, carbon nanotubes (CNT), and graphene show antimicrobial properties [6], [7], [8]. The antibacterial mechanisms of Ag NPs include damaging the membrane, binding to and inactivating proteins, inhibiting the replication of DNA and so forth [9]. TiO₂ and ZnO NPs mainly kill bacteria via ROS-production under UV irradiation [10], [11]. Carbon-based materials may exert oxidant or mechanical damage on bacteria [12], [13]. Gold NPs may become useful in the development of antibacterial strategies because of their non-toxicity, versatility in surface modification, polyvalent effects and photothermal effects [14], [15], [16], [17], [18], [19]. We have developed a strategy to fight against MDR bacteria via presenting inactive small organic molecules, such as 4, 6-diamino-2-pyrimidinethiol on gold NPs (Au_DAPT NPs), which act on Escherichia coli and Pseudomonas aeruginosa via multiple means, such as disorganizing cell membranes, binding to nucleic acids, and inhibiting protein synthesis [20], [21]. To better understand the fundamental biological mechanisms of gold NPs to design NP-based antibacterial agents, we explore the changes of bacteria that take place on both gene and protein levels.

Transcriptomic and proteomic analysis has been widely used in studying the molecular mechanisms of the action of antibacterial agents [22], [23], [24] and other fields, such as mechanisms of cytotoxicity of biomaterials [25], [26]. Using these methods, researchers found that CNTs up-regulate stress-related sigma factor, genes involved in soxRS and oxyR systems, and genes related to glycolysis, fatty acid beta-oxidation, and fatty acid biosynthesis pathways [12]. Ag NPs dissipate the proton motive force to accumulate envelope protein precursors in the cytoplasm of E. coli. This accumulation prevents these proteins from properly functioning in the membrane. Ag NPs could also elevate formate acetyltransferase, implying an anaerobic condition, and reduce recombinase A related to DNA repair in Staphylococcus aureus [27], [28].

Here, we investigate the mode of antibacterial action of gold NPs on E. coli using gene expression microarrays and two-dimensional polyacrylamide gel electrophoresis (2D PAGE), supported with biochemical studies, focusing on oxidative phosphorylation, ribosome, chemotaxis pathways, and ROS-related processes.