To comprehend the antibacterial mechanism of Ag NPs, Li et al. (2010) studied

the effect of Ag NPs on the permeability and the membrane structure of E. coli cells,

as a model for GN bacterium. Results showed that Ag NPs seemingly enhanced the

permeability of membrane and inactivated the activity of respiratory chain dehydro-

genase enzyme, resulting in the leakage of the cellular materials and inhibition of

respiration. In contrast, Kim et al. (2007) suggested that the antimicrobial activity of

Ag NPs is related with the formation of free radicals and subsequent free radical-

induced membrane damage.

27.4.2 Gold (Au)-Based ENMs

In the field of nanotechnology, Au-based ENMs have been widely analysed for

various applications. Besides many applications, because of its various inherent

properties like non-cytotoxicity, biocompatibility, inertness, high stability, polyva-

lent effects, ease of identification, photothermal activity, and easy functionalization,

the antimicrobial property of Au-based ENMs have been largely exploited.

Studies have shown that ultra-small-sized Au nanoclusters (NCs) possess com-

paratively broad spectrum of antimicrobial activity than their larger counterparts. For

instance, Zheng et al. (2017) found that the wide-spectrum antimicrobial properties

can possibly be imparted to Au NPs (>2 nm) via precise control of Au size down to

ultra-small NCs dimension (i.e. <2 nm). Au NCs was observed to kill both GP

(S. aureus, S. epidermidis and B. subtilis) and GN bacteria, (E. coli and

P. aeruginosa). This wide-spectrum antimicrobial activity of Au NCs has attributed

their ultra-small size, which allowed them to better interact with bacteria. These

interactions were reported to create a metabolic imbalance in cells, leading to about

two- to threefold increase of intracellular ROS production that kills bacteria conse-

quently. Overall, results suggested that the internalization of Au NCs, modulation of

cell metabolism, and intracellular ROS generation were the determining factors for

the high antimicrobial efficiency of Au NCs.

Unlike other metals, Au is generally considered as an inert and biocompatible

material; therefore, several efforts have been performed to craft desired antimicrobial

chemicals such as peptides, cationic ligands, and other antibiotics on the surface of

Au NPs. Recently, Li et al. (2020) demonstrated that positively charged Au NCs

stabilized with cationic ligand ((11-mercaptoundecyl)-N,N,N trimethylammonium

bromide) hold a great potential to be used as an antimicrobial agent against

MDR bacteria. Ultra-small size and positive charge on the surface were the reasons

for the effective antimicrobial activity of Au NCs through a combined mechanism

that includes destruction of cell membrane, DNA damage, and generation of ROS.

Tiwari et al. (2011) investigated the antibacterial and antifungal activities of the Au

NPs functionalized with 5-fluorouracil against M. luteus, S. aureus, P. aeruginosa,

E. coli, A. fumigates, and A. niger microbes. The authors claimed that Au NPs were

more effective against GN bacteria than GP bacteria due to their easy internalization

into the GN bacterial cell membrane.

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