and efficiency (Tai et al., 2007). Routinely employed metal oxides include: magnetite (Sun & Zeng, 2002), iron oxide, aluminum oxide (Mukherjee et al., 2011), silicon dioxide, titanium dioxide, zinc oxide (Sharma, Jandaik, Kumar, Chitkara, & Sandhu, 2016), cerium oxide, and copper oxide (Ren et al., 2009).
1.3.2.3.3 Composite‐Based NMs
In general, composite materials are described as materials with two or more different materials combined to blend the properties of all the constituent materials. In the same way, the composite‐based NM is a multiphase material with at least one of the dimensions in nanoscale, which is obtained by combining one NM with other or blending one NM with bulk or larger material to form a NM (Vollath, 2013). Bones and the eggshells are the best examples of naturally occurring composite NMs. These NMs generally possess highly improved physical, chemical, mechanical, and biological properties in comparison to their constituent materials. Nanocomposite materials can be of different combinations such as metal/metal, metal/ceramic, carbon/carbon, and ceramic/ceramic (Jeevanandam et al., 2018).
1.3.3 Classification Based on Origin
Based on their origin, NMs are classified as natural and synthetic NMs. The NMs that are produced by biological species or anthropogenic activities in nature without human intervention are called natural NMs. The NMs formed in nature are present throughout earth's atmosphere, hydrosphere, and lithosphere. This may include the NMs present in whole troposphere, oceans, sea, rivers, lake, groundwater, rocks, lava, soils, even microorganism, and higher organisms (Hochella, Spencer, & Jones, 2015; Sharma et al., 2015). Synthetic NMs are the NMs that are synthesized through physical, chemical, biological, or hybrid methods besides the materials that are produced from engine exhaust, smoke, and mechanical grinding (Wagner et al., 2014). Even though synthetic NMs are more advantageous as aforementioned, the major problem is predicting the fate and behavior of the materials in the environment. Currently there are a lot of strategies to perform the risk assessment of the synthetic NP in various environmental conditions. Still extrapolating the behavior of synthetic NMs from existing knowledge is a major challenge.
1.4 Application of NMs
NMs have found broad applications in various fields such as nanofluids, medical sectors, in cutting tools, automotive sector, wear, and corrosion‐resistant coatings. The engineering of NMs to form lighter, as well as extremely stronger materials has found its application in making hard and strong surface coating over material as resistive coatings, faster acting switches, medicines, storage devices with enhanced storage capacity and in building materials.
1.4.1 Advanced Application of NMs as Antimicrobial Agents
Apart from the above applications, NMs have been employed in the medical field as both theragnostic and diagnostic agents. Gold NPs are well‐known for their application in the medical field whereas silver NPs have found applications as antimicrobial materials. Apart from silver, in recent times, a number of NMs have gained attention as antimicrobial agents in the healthcare sector because of the development of resistant bacteria caused by the uncontrolled usage of antibiotics. Antimicrobial‐resistance is considered one of the critical issues that need an immediate solution. In this regard NMs with unique features and specific functionality have gained interest in combating antimicrobial resistance. In the following sections, we will briefly explain bacterial resistance and the role of NMs in bacterial resistance.
1.5 Bacterial Resistance to Antibiotics
The most serious threat to public health are infectious diseases and mortalities that have resulted from chronic infections. The common causative agents for most infectious diseases are bacteria. Before the discovery of antibiotics, the old treatment modalities involved the use of synthetic compounds such as sulfa drugs, quinolones, and salvarsan as chemotherapeutic agents (Aminov, 2010). Later on, in the twentieth century antibiotics emerged as wonder drugs. However, the wild use of antibiotics with uncontrolled measures led to the emergence of antibiotic‐resistant pathogens and the foremost dangerous multidrug‐resistant strains.
The first antibiotic resistance was reported with the enzyme called penicillinase produced from pathogenic Escherichia coli (Abraham & Chain, 1940). In nature, the organism that produces antibiotics has self‐resistance against its own antibiotic. Most of them have more than one simultaneous mechanism to protect the cells completely from their own bioactive molecules. The most common mechanism of self‐resistance involves antibiotic modification or degradation, antibiotic efflux, antibiotic sequestration, and target modification. In the producer organisms, the genetic code for the self‐resistances are clustered with the antibiotic synthesis gene and hence their expression is co‐regulated. The widespread use of antibiotics and coexistence of antibiotic producer organism with nonproducers led to the origin of antibiotic resistance (Kaur & Peterson, 2018). Since NMs have shown potential to deal with antibiotic resistance, a brief discussion on the mechanism of antibiotic resistance is included in this section.
1.5.1 Mechanism of Antibiotic Resistance
The mechanism of bacterial antibiotic resistance can be categorized into intrinsic and extrinsic. The antibiotic resistance mechanism that fixed in the genetic core of the organisms is an intrinsic mechanism encoded in chromosomes. This may include the enzyme system which inactivates antibiotics, nonspecific efflux pump systems, and permeability barrier mechanisms (Cox & Wright, 2013; Fajardo et al., 2008). AcrAB/TolC efflux pump in E. coli is one of the well‐studied intrinsic resistance systems. These efflux systems are generally very nonspecific and help in exporting different antibiotics, detergents, dyes, and disinfectants (Nikaido & Takatsuka, 2009). Similarly, a mechanism involving permeability barrier in E. coli and other Gram‐negative bacteria for vancomycin is also an intrinsic resistance system where the outer membrane acts as a permeability barrier (Arthur & Courvalin, 1993). On the other hand, the resistance system that is obtained from other organisms such as producers by horizontal gene transfer is called the acquired resistance system. Unlike the intrinsic resistance system, the resistance elements of the acquired systems are generally embedded in plasmids and transposons. Acquired resistance system includes the plasmid‐encoded specific efflux pumps and enzymes that can alter or modify the antibiotics or the target of antibiotics (Bismuth et al., 1990).
According to Wang, Hu, and Shao (2017), the resistance mechanism can be categorized into different subdivisions on the basis of the biochemistry at the protein level target alterations, passive or inactive enzyme generation, active efflux pumps, permeability barrier, biofilm formation, elimination and emergence of certain specific protein. It has been noted that in the same bacterium there may exist two or more simultaneous mechanisms from the aforementioned categories as resistance mechanism such as antagonist induction through metabolic pathway and production of competitive inhibitor to counteract the antibiotics. In general, the molecular mechanisms of antibiotic resistance are divided into three types: (i) antibiotic modification, (ii) antibiotic efflux, and (iii) target modification or bypass or protection mechanisms (Wang et al., 2017).
1.5.1.1 Antibiotics Modification
Antibiotics modification is the common resistance mechanism of pathogenic bacteria against antibiotics of aminoglycosides class. So far, multiple types of aminoglycosides modifying enzymes (AMEs) have been identified in both Gram‐negative and Gram‐positive bacteria (Ramirez and Tolmasky, 2010; Schwarz et al., 2004). The genetic code for these systems is embedded in the mobile genetic elements (MGEs) of pathogenic or resistant bacteria (Ramirez & Tolmasky, 2010). The chromosomal determinants of the aminoglycosides modifying enzymes have been found in the large number of bacteria present in the environment such as Acinetobacter
