Table 1.2 Bandgap energy and activation wavelength for various metal oxide NMs (Gardini et al., 2018).
| S. No | Material | Bandgap energy (eV) | Activation wavelength (nm) |
|---|---|---|---|
| 1 | CdO | 2.1 | 590 |
| 2 | Fe2O3 | 2.2 | 565 |
| 3 | WO3 | 2.8 | 443 |
| 4 | TiO2 | 3.2 | 387 |
| 5 | ZnO | 3.2 | 390 |
Figure 1.2 Schematic representation of photoactivated ROS generation and antimicrobial property of NMs.
In a similar way, ZnO, a semiconductor with larger bandgap energy, is applied in coatings, paints, and sunscreens. Upon exposure of UV light, photocatalysis induces the ROS production, which is responsible for its antimicrobial effect. Here, the roughness of the system depends on the surface defects. Efficacy of the ZnO NP system increases with decrease in size where the roughness of the particle along with its high surface area causes the disruption of the microbial cell wall (Padmavathy & Vijayaraghavan, 2008). ZnO NMs have also been reported to interact with some disease target proteins. Chatterjee et al. (2010) studied the effect of ZnO NPs over periplasmic domain structure of ToxR protein of Vibrio cholerae. ToxR protein plays a critical role in the regulation of expression of many virulence factors of the bacteria. It was observed that the binding of the protein ToxR to ZnO NPs' surface reduced the stability of protein where it was more susceptible to denaturation. Further, significant change in the structure of the protein was also observed (Chatterjee et al., 2010).
1.7.3 Carbonaceous NMs
At the nanoscale, carbon forms different allotropes, which include graphene, carbon nanotubes, fullerenes, and nanodiamonds where each of them exerts inherent unique properties. The antimicrobial effect of carbon NMs such as graphene, graphene oxide, reduced graphene oxide, and carbon nanotubes depends on the level of cell wall disruption and amount of ROS induced in the microbial cells (Liu et al., 2011). Carbon materials hold the advantages of commercial viability and environmental safety in comparison to conventional NPs such as silver and other metal NPs.
1.7.4 Cationic Polymer NMs
Polymers with an inherent positive surface charge or added with positively charged moieties are called cationic polymer NMs. The most commonly used cationic polymer in antimicrobial application is chitosan (Rekha Deka, Kumar Sharma, & Kumar, 2015). Chitosan is a cationic polysaccharide derived from chitin by partial deacetylation. The positively charged surface moieties aid in the interaction of NM with negatively charged bacterial cell wall, which causes the rupture of membrane and subsequent cytoplasmic content leakage (Qi et al., 2004). The efficacy of the system relies on the pH, degree of deacetylation, molecular weight, and presence of other substances such as proteins, lipids, and metal ions. The major disadvantage of chitosan‐based nanosystems is their poor solubility at neutral pH causing them to precipitate in culture medium.
In general, the efficacy of the antimicrobial NMs depends on the interaction of the material with bacteria and the mechanism of action. Further, the interaction of NMs with bacteria depends on a few crucial factors such as electrostatic attraction, van der Waals forces, receptor–ligand interaction, and hydrophobic interaction. Understanding the basics of the NMs' interaction with the microbial cell is likely to pave way for the design of novel antimicrobial agents with crucial insight into their toxicity and mechanism of action. In the following sections, we discuss the interaction of NMs with microbial cells and the mode of action.
1.8 Interaction of NMs with Bacteria
Based on the cell wall structure, bacteria are divided into two categories as Gram‐positive bacteria and Gram‐negative bacteria. Gram‐positive bacteria have a thick peptidoglycan layer ranging from 15 to 100 nm. Further, they contain a phosphate‐containing polymeric chain of teichoic acid that is responsible for the negative charge of the bacteria (Neu, 1992). In the case of Gram‐negative bacteria, an extra hydrophobic lipid bilayer is present over a thin peptidoglycan layer (20–50 nm). The presence of an extra lipid layer limits the permeability of several hydrophilic antimicrobial agents, which is the reason for the high resistant nature of Gram‐negative bacteria (Gupta, Landis, & Rotello, 2016). The negative surface charge of bacteria is due to the lipids and carbohydrates of the lipopolysaccharide layer. Hence, the structure of the bacterial cell wall determines the interaction of bacteria with NMs. Schematics of cell wall structure of Gram‐positive and Gram‐negative bacteria are given in Figure 1.3 as detailed by Hajipour et al. (2012)
In an earlier study, a homogenous distribution of cetyltrimethylammonium bromide (CTAB) coated gold NPs was observed over the Bacillus cereus. This was explained on the basis of electrostatic interaction between negatively charged teichoic acid moieties on the bacteria and positively charged gold NPs (Berry et al., 2005). In another study, it was reported that mannose functionalized gold NPs bound to the pili of Gram‐negative E. coli. It was attributed to the receptor–ligand interaction between mannose and lectin containing pili (Lin et al., 2002). Later, Hayden et al. (2012) suggested that the positively charged NMs exhibited high toxicity over bacteria. This electrostatic interaction might be a plausible reason for the spatialized aggregation of cationic and hydrophobic gold NPs on the negatively charged bacterial membrane (Hayden et al., 2012). The interaction of NPs with the membrane generally effects in membrane blebbing, tubule formation, and other membrane damage.
Figure 1.3 Bacterial cell wall structure of (a) Gram‐positive bacteria (b) Gram‐negative bacteria.
1.9 Antibacterial Mechanism of NMs
NMs exert different mechanisms of action against microbes as represented in Figure 1.4. Most of the time, the antimicrobial systems employ multiple mechanisms to take over the multiple resistance mechanisms developed by microorganisms, thereby increasing the antimicrobial efficiency. In brief, different mechanisms of action of NMs are given below:
