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:
Figure 1.4 Schematic of the antibacterial mechanism of NMs.
Cell membrane disruption: Interaction of the NMs with the surface of the microbes causes mechanical damage to cell wall, which in turn leads to cell wall disruption followed by leakage of the cytoplasmic content and subsequent cell death (Pal, Tak, & Song, 2007).
DNA damage: NMs or metal ions from NMs diffuse through the cell wall and effectively interact with DNA and affect or modify the morphology of the DNA. This, in turn, interrupts the duplication or replication of DNA, leading to cell death (Feng et al., 2000).
Release of metal ions: Metal ions released from the NM diffuse into the cells and bind to thiol‐containing proteins including enzymes and compromise their function. In addition to that, metal ions generally function as cofactors for a number of enzymes. Hence homeostasis of metal ions is very important for the survival of the microorganism. Metal ions released from the NM diffuse in excess into bacterial cells, affecting the homeostasis leading to dysfunction of proteins and enzymes (Feng et al., 2000).
Interrupted transmembrane electron transport: Often the interaction of NM with cell wall damages the cell wall and hampers the electron transport chain leading to the disruption of cellular respiration and eventual cell death (Lemire, Harrison, & Turner, 2013).
Oxidative stress: Few of the NMs such as metals and metal oxides prominently induce production of ROS, which includes hydroxyl free radicals, superoxides, and hydrogen peroxide. The produced free radicals further oxidize cell wall components causing their disruption and act on the internal components of cells. In few cases, the oxidation continues till the entire cell is oxidized into CO2 and water. In some specific NMs such as carbon NMs, oxidative stress is induced without the involvement of ROS, which acts by electron transfer mechanism (Jacoby et al., 1998). Schematic of the antibacterial mechanism of NMs is given in the Figure 1.4 as described by Hajipour et al. (2012).
1.10 Factors Affecting the Antibacterial Activity of NMs
As discussed earlier, the activity of any NM system depends on its physicochemical properties such as size, shape, zeta potential, crystal structure, charge, and other factors. The physicochemical factors' influence on the surface area, surface energy, and atomic ligand deficiency dictates the behavior of NMs, which in turn affects the activity of NMs. Hence it is very important to study the effect of these factors on the activity of the NMs. Schematic of the factors influencing the antimicrobial activity of the NMs is given the Figure 1.5 as detailed by Daima and Bansal (2015).
1.10.1 Size
A key advantage of any NM system is its higher surface area‐to‐volume ratio in comparison to the micro and macro structures. Practically it is possible to contain a significantly high number of smaller NMs in comparison to bigger particles in the same volume. High surface