Zinc Oxide Nanoparticles

ZnONPs have been proved to be toxic for gram-positive and gram-negative bacteria, as well as for the spores, which are resistant to elevated pressure and temperature (Ma et ah, 2013). Alternatively, zinc oxidepossesses the minimal outcome of body cells, which makes it secure for utilization as the antimicrobial mediator (Espitia et ah, 2012). A certain survey presented the antimicrobial capacity of ZnONPs is based on the selective size and concentration of the particles (Ma et ah, 2013). Along with this, the mechanism accounted for the antibacterial capacity of ZnONPs, and the production of hydrogen peroxide (H₂0₂) from the zinc oxide surface is considered more efficient for the inhibition of growth of bacteria. The inhibitory outcome of hydrogen by alive cells stimulates the catalase enzyme; it stimulates the dissolution of H₂0₂ to H₂0 and 0₂. It also attracts electrons from bacterial cell walls and damages the basic molecular structure of the lipid layer and cell proteins on the cell surface. The cell walls are oxidized and destroyed or break completely. Synthesis of ZnONPs can be done using diverse sources (Madhumitha et ah, 2016). They are harmless with high-quality photocatalysis and high lucidity. Synthesis of ZnONPs can also be made from different plant parts, such as bark, flowers, leaves, rhizomes, roots, and fruits (Ahmed et al„ 2017). ZnONPs demonstrate a probable antibacterial capacity (Bhuyan et ah, 2015) and a good deprivation of the photo and have applications in the medicine delivery (Ali et ah, 2016) and anticancer properties (Vimala et ah, 2014). ZnONPs are broadly experienced metallic NPs for antimicrobials. The broad- spectrum gram-positive and gram-negative bacteria, for example L. monocytogenes, S. aureus, and E. coli, has verified sensitivity against ZnONPs (Jones et ah, 2008; Liu et ah, 2009). The management of ZnONPs microbial cells undergoes ROS, membrane destruction of reducing sugars, lipid peroxidation, reducing sugars membrane, proteins, cell viability, and DNA (Tiwari et ah, 2018). ZnONPs generate ROS for instance hydrogen peroxide and superoxide anion in the cells (Kumar et ah, 2011; Horie et ah, 2012). ROS cause membrane leakage of nucleic acids and proteins by enhancing lipid peroxidation. Zn⁺² ions released by NPs also injure the plasma membrane and cooperate with intracellular machinery (McDevitt et ah, 2011; Li et ah, 2011). ZnONPs was shown to slow down the growth of carbapenem-resistant A. baumannii by producing ROS and causing damage of membrane, signifying that ZnONPs residential as an option to carbapenems (Beta-lactam; Tiwari et ah, 2018).

Titanium Dioxide Nanoparticles

TiO.NPs have drawn considerable interest because they show' unique and enhanced properties compared with their counterparts of bulk material. These NPs demonstrate quantum size effects, in w'hich materialistic effects such as physical and chemical properties depend heavily on particle size (Othman et ah, 2014). NPs made from (TiO.NPs) are photocatalytic. Ti0₂NPs decompose organic compounds when exposed to non-lethal UV light at a wavelength of less than 385 nm by the formation and continuous release of superoxide ions and hydroxyl radicals (Beyth et ah, 2015). The TiO.NPs of a nanometerscale can further boost titanium oxide antimicrobial activity (Othman et ah, 2014). TiO.NPs possess interesting catalytic, optical, dielectric, and antibacterial properties and, thus, can be used in various industries (Vamathevan et ah, 2002), sensors (Varghese et ah, 2003), biosensors (Zhou et ah, 2005), solar cells (Zhang et ah, 2014), and medical diagnostics as image-contrast agents (Kirillin et ah, 2009).

TiO.NPs w'ith various morphologies such as nanotubes and nanorods are usually synthesized using various stabilizing and reducing agents (Chen and Mao, 2007). Hydrothermal preparation is an alternative solution because of its low efficacy and easiness (Mali et ah, 2011), but green paths are necessary to be built to provide a consistent source in adequate quantities with no harmful environmental effects. TiO.NPs are synthesized from plants (Sankar et ah, 2015) and fungi also (Rajakumar et ah, 2012; Hunagund et ah, 2016). TiO.NPs often demonstrate antimicrobial effects through multiple mechanisms indicating that the possibility of microbial cells developing resistance to these NPs is extremely low. TiO.NPs have been stated to have a bactericidal effect against S. aureus, Escherichia coli. Pseudomonas aeruginosa, and E.faecium (Foster et ah, 2011; Li et ah, 2012). TiO.NPs destroy microorganisms by producing ROS w'ith the exposure of UV radiation (Li et ah, 2012). The produced ROS interfere with the oxidative phosphorylation of the cell membrane, which results in cell death. A recent study suggested that exposing cells to TiO, photocatalysis

TABLE 6.1

Nanoparticle Activity Against Pathogens and Mode of Action

would rapidly inactivate the regulatory signalling stage, efficiently decreases the coenzyme-independent respiratory chains. They decrease the ability to absorb and hold iron and phosphorous and lowers the ability of biosynthesis and degrade heme (Fe-S cluster) groups (Foster et al., 2011).

Magnesium Oxide Nanoparticles

In the human body, after potassium, magnesium is the second common intracellular cation and has a total 25% mineral. As compared with magnesium oxide (MgO) NPs, TiCLNPs. AgNPs, and CuNPs can be synthesized from economical and available precursors. The main function of MgONPs in bacteria is to induce ROS and inhibit essential microbial enzymes. The alkaline effect of MgONPs is considered as another primary aspect in the antibacterial activity (Li et ah, 2012). According to Tang and Lv (2014), MgO can incapacitate the microorganisms by generating ROS; however, the mechanism of adsorption and direct particle penetration of the cell membrane may be other potential forms of pathogen inactivation, such alternatives must be considered for further aspects. In contrast, Hossain et ah (2014) explored the virulence of three separate MgONPs in E. coli and documented the absence of ROS formation for two MgONPs. The authors proposed that proteomics results clearly showed the absence of oxidative stress and indicated that the primary mechanism of cell death is associated with damage to the cell membrane which does not appear to be related to lipid peroxidation. Like other NPs, the MgONPs also contain the ROS, which is the leading mechanism behind their antimicrobial activity (Hossain et ah, 2014). MgONPs physically interact with the surface of the cell and disrupt the membrane integrity, leading to membrane leakage (Blecher et ah, 201 la). Additionally, they damage the cells by irreversible intracellular biomolecular oxidation. Other research, however, showed that MgONPs exhibit greater antibacterial activity without ROS and lipid peroxidation. The authors recommended that the antibacterial activity of MgONPs is related to the interface of NPs with the plasma membrane of microbes, pH change, and release of Mg⁺² ions (Leung et ah, 2014). The antimicrobial action of MgONPs has also been demonstrated because of the adsorption of halogen molecules on the surface of MgO (Blecher et ah, 2011a). Over the ancient years, NPs, especially nano-silver coating, have served as an antibacterial measure in dental implants, bone prostheses, and surgical instruments and as a coating for wound dressing to fight the microorganisms in lesions (Correa et ah, 2015; Burdusel et ah, 2018). These NPs target the bacterial cells and disturb the crucial function of cell membranes such as membrane respiration and membrane permeability (Dakal et ah, 2016; Slavin et ah, 2017). They also react with intracellular components such as nucleic acids and polypeptides inhibit the divisions of cells and transfer of genes (Guzman et ah, 2012a; Azam et ah, 2012). Many reports show the bactericidal properties of numerous NPs, mainly zinc, magnesium, gold, copper, titanium, and silver (Vimbela et ah, 2017; Hoseinnejad et ah, 2018).

The mode of action of NPs and antibiotics seems identical in the instance of involvement of proteins, RNA, and DNA, as well as membrane disruption (Correa et ah, 2015). However, these metallic NPs exhibit antimicrobial activity through multiple mechanisms, which decrease the possibility of resistance in microorganisms to grow against them (Slavin et al., 2017). Microbial cells will need to acquire various simultaneous gene mutations to establish resistance against these NPs, which is rare. Also, the composite of these NPs by a green synthesis mode will result in the formation of polysaccharides, small biologically active compounds, and proteins. Their interface with the NPs, thus further enhance their antimicrobial action against the MDR microorganisms. In this chapter, we have addressed some metallic NPs which are synthesized by green method(s) and their mechanism of action in contrast to several pathogenic microorganisms.