cell membrane eventually leading to loss of plasma membrane (Farouk et al. 2018;

Leroueil et al. 2007; Niskanen et al. 2010).

11.3.2 Production of Toxic Ions by Metallic NPs

Recently a tremendous zeal has been witnessed, where a large-scale development

and application of metallic nanoparticles are taking place for eradicating the grave

issue of bacterial infections. It has been deciphered that on coming in direct contact

with the bacterial proteins the metallic NPs results in the formation of sparingly

soluble metal ions, viz., Ag²⁺, Zn²⁺, and Cd²⁺ (Farouk et al. 2018). These metal ions

are considered competent enough to evoke a toxic response in bacterial strains. This

can be explained by taking an example of silver NPs where lodging of these NPs into

the bacterial cellular periphery results in the precipitation of AgCl
in the cytoplasm,

thus resulting in an inhibited respiration and ultimately apoptosis (Farouk et al. 2018;

Niskanen et al. 2010). Degradation of the metallic NPs results in a gradual and

consistent release of metal ions, which are readily absorbed by the bacterial cells.

These absorbed ions establish a bridging with the functional groups (viz., amino

(-NH), mercapto (-SH), carboxylic (-COOH)) of proteins and nucleic acids present

in the cellular organelles (Wang et al. 2017). Disturbed enzymatic activity, altered

cellular compartmental morphology, inhibited physiological and metabolic pro-

cesses, and diminished survival rate are some of the utmost consequences, which

are faced by the bacterial cell, which have encountered such metallic ions (Wang

et al. 2017).

11.3.3 Oxidative Stress (ROS Generation)

Amid all known anti-oxidizing agents, oxygen is considered the most powerful one.

Repeatedly, it has been demonstrated that during the process of respiration it acts as

an efficient electron acceptor and hence can prove to be a critical factor in governing

the survival rate of bacteria (Farouk et al. 2018). Oxygen can exist in varied states

such as singlet, doublet, or triplet; however, it has been shown that both singlet (O2)

and triplet (3O2) oxygen can prove to be toxic for cells and bacteria, respectively

(Farouk et al. 2018). Peroxidation of lipidic bilayer membrane and precipitation of

intra/intercellular proteins are one of the most significant effects that are produced on

the generation of singlet oxygen. This finally results in the disruption of bacterial

cellular compartments and ultimately killing of bacteria (Bronshtein et al. 2006;

Farouk et al. 2018). Oxygen in the singlet state is the major deriving source for

catalyzing several detrimental and unstructured oxidation processes taking place

inside the bacterial cell.

However, during the respiratory cycle, consumption of singlet oxygen molecules

by the bacterial cells results in the formation of free radicals (hydrogen peroxide

activity). These generated free radicals exert oxidative stress on the nucleic acids,

proteins, and lipidic bilayer membrane, thus making it difficult for the bacteria to

164

A. Parmar and S. Sharma