Figure 27.3 gave the schematic representation of the possible mechanism of

bacterial cell damage by the ENMs. Since, these ENMs have followed many

mechanisms for its antibacterial property and due to this reason, there is a very

rare chance of getting bacterial resistance towards these metal-based NPs in near

future. For bacteria to develop resistance against ENMs, the microbial cells have to

undertake several gene mutations that are not feasible instantaneously. In literature,

the researchers have reported various greener methods of ENMs fabrication that

enhance in antibacterial property against different types of bacteria.

27.4

Antimicrobial Effect of Some Common Metal-Based ENMs

Since the notable bacterial effects of reactive metal-based ENMs have been reported

by Klabunde and colleagues (Stoimenov et al. 2002), there has been significant

interest in other inorganic ENMs as antibacterial materials. Various metal-based

ENMs and their corresponding oxides have been thoroughly studied for their distinct

antimicrobial properties (see Table 27.1).

27.4.1 Silver (Ag)-Based ENMs

Ag and its compounds have been used for many millennia to prevent microbial

infections, even before the realization that microbes were the agents of infection. In

ancient times, the Greeks, Romans, Egyptians, and others used Ag to preserve water

and

food.

Compared

with

other

metals,

Ag

exhibits

higher

toxicity

to

microorganisms and lower toxicity to mammalian cells. Besides, it has been suc-

cessfully used to treat multiple infectious diseases against a wide variety of microbes

including bacteria, fungi, and viruses as well as non-infectious diseases, often with

remarkable effectiveness. In many cases, with several microbial species growing

resistant to antimicrobial drugs today, Ag-based ENMs are efficiently used to

disinfect as well as coat medical equipment such as external fixation pins, heart

valves, endotracheal tubes, cardiac, urinary catheters, etc. to reduce infections. Some

permanent implants like mega-endo-prostheses which are implanted after the

removal of bone tumours are coated with Ag.

Many studies have been conducted on the Ag-based ENMs to support its use in

biological applications. The mechanism of the inhibitory effects of silver (Ag⁺) ions

on microorganisms is only partially known so far. Sondi and Salopek-Sondi (2004)

first reported the antibacterial activity of elementary Ag NPs against E. coli. The

results revealed that the increase in the permeability of the cell wall via incorporation

of Ag NPs in the cellular membrane and formation of pits on its surface caused the

cell death. Hassan and a co-worker (2018) fabricated the Ag NP-based scaffold

which showed 100% antibacterial efficacy against E. hirae and E. coli. There were

many green approaches to fabricate these ENMs which reduce toxic concerns and

enhance the antibacterial property of the material (Roy et al. 2019).

27

Antimicrobial Applications of Engineered Metal-Based Nanomaterials

505