their antibacterial effect through synergistic effect. It was reported earlier that the introduction of antibiotics such as kanamycin, erythromycin, ampicillin, and chloramphenicol along with silver NPs has enhanced the antibacterial property of silver NPs (Fayaz et al., 2010). The ampicillin silver NP complex has shown the highest antimicrobial effect over other complexes. Here, the strong van der Waals attraction force caused the interaction of NP with the bacterial cell surface. This interaction led to the lysis of cell wall and subsequent penetration of NP into cell where it intervened with the DNA unwinding and effected in the death of bacterial cell (Fayaz et al., 2010). In another study, Jaiswal and Mishra (2018) showed that the functionalization of silver NPs with curcumin improved the antimicrobial properties and also reduced the cytotoxicity of the silver NPs against human keratinocytes (Jaiswal & Mishra, 2018). However, in recent years a lot of work is going on with functionalization of NMs and the basic understanding of the interaction of the bacterial system with functionalized NPs. Better understanding of mechanism of functionalized NMs along with suitable nano‐bio interface phenomenon will guide us to develop more standard design criteria to develop advanced materials with peculiar and desired properties.
1.14 Conclusion and Future Perspectives
This section delineates an overview of NMs, their role in microbial resistance, and the effect of physicochemical factors on their antimicrobial property. The development of microbial resistance to antibiotics and other common disinfectants has driven researchers to look for novel strategies to treat infections. Pertaining to this issue, NMs have emerged as a promising solution to microbial resistance due to their broad spectrum of antimicrobial property along with ease of integration with other products for diverse applications. It is clear from the literature that the antimicrobial property of the material depends on certain crucial physicochemical properties such as size, shape, and surface chemistry. Further, the antimicrobial property of the system can be tuned by controlling those crucial physicochemical properties of nanostructures. Understanding of this phenomenon can be exploited to tailor NMs of interest with reduced nonspecific toxicity where the NMs can be engineered to be specifically active against microbial cells rather than mammalian cell systems. The discussion over the mechanism of action of metal and metal oxide NMs suggested that the mechanisms of action of these NMs are not merely dependent on the release of metal ions from the NMs but also depend on the nanostructures (size and shape) of the NMs, which contribute directly to the antimicrobial property of the NMs. In the present scenario, the toxic effect of NM systems and the mechanism of action over human systems are not clearly understood. All these issues can be addressed if we develop standardized testing protocols and define the NMs' properties on an international level and enforce it. Most importantly, future research in this field should be directed to further understand the complex relationship between the various physicochemical factors over the antimicrobial property and mechanism of action of the NMs.
Questions and Answers
1 Why the study of the nano‐bio interface is necessary?Bio–nano interface hosts “the dynamic physicochemical interactions, kinetics and thermodynamic exchanges between nanomaterial surfaces and the surfaces of biological components.” In the last three decades, there has been an exponential increase in the application of nanomaterials in various fields including the health sector. This is leading toward a long‐term co‐existence of such nanomaterials with living systems which may result in adverse toxicological effects to the living bodies. In this regard, it is necessary to study the effect of these materials on the biological entities such proteins, DNA, RNA, cell membrane, cell organelles, cells, tissues, and organs.
2 Do nanomaterials occur in nature?Yes, nanomaterials do occur in nature and are called “natural nanomaterials.” They are produced by biological species or anthropogenic activities in nature without human intervention. The nanomaterials formed in nature are present throughout the earth's atmosphere, hydrosphere, and lithosphere, such as in volcanic ash, sea spray, and smoke.
3 Explain the terms “antibiotic resistance” and “post‐antibiotic era”?“Antibiotic resistance” is the ability of microbes such as bacteria to resist the killing effects or overcoming the actions of the antibiotics. In recent times, a rapid increase in the level of microbial resistance to antibiotics is leading to an era called as “Post‐antibiotic era” where the mortality rate caused because of microbial infections will be higher than that of cancer as stated by Centre for Disease Control and Prevention.
4 How can nanomaterials combat antibiotic resistance and what are the different nanomaterials used as antimicrobial agents?The traditional antibiotics exhibit their antimicrobial activity by one of these mechanisms: (i) inhibition of cell wall synthesis, (ii) inhibition of fatty acid biosynthesis, (iii) inhibition of protein synthesis, and (iv) compromising the cell membrane functions. Application of either one of the simple mechanisms by the traditional antibiotics is the main reason for the occurrence of bacterial resistance. On the other hand, nanomaterials have different mechanisms of action such as: (i) disputing cell membranes, (ii) causing DNA damage, (iii) interrupting the transmembrane electron transport, and (iv) inducing oxidative stress. Further, nanomaterials can be designed to have multiple mechanisms that act simultaneously against microbes. Hence, it becomes difficult for microbes to develop resistance against nanomaterials as they are unlikely to have many mutated genes. The most commonly exploited antibacterial nanomaterials include nanometals (silver, copper); metal oxides (Zinc oxide, titanium dioxide); carbonaceous materials (graphene, graphene oxide, carbon nanotubes); and cationic polymers (chitosan).
5 Among Gram‐negative and Gram‐positive bacteria which has higher resistance over hydrophilic drugs and state the reason?Among these two groups of bacteria, Gram‐negative bacteria have higher resistance over hydrophilic drugs in comparison to Gram‐positive since Gram‐negative bacteria have an extra hydrophobic lipid bilayer over a thin peptidoglycan layer (20–50 nm). The presence of such an extra lipid layer limits the permeability of several hydrophilic antimicrobial agents, which is one of the reasons for high resistance of Gram‐negative bacteria.
6 In general, smaller sized nanomaterials exhibit higher antimicrobial property in comparison to the bigger sized particles of the same material. State the reason.Generally, smaller sized nanomaterials have higher surface area and thus high chance of prolonged interaction with the microbial system and diffuse through the cell membrane in comparison to bigger nanomaterials with smaller surface area. Additionally, it is possible to contain a significantly higher number of smaller NMs in comparison to bigger particles in the same volume. The higher surface area‐to‐volume ratio owing to increased number of particles results in exposure of greater numbers of atoms. Exposure of a greater number of atoms on the surface results in increased interaction of nanomaterial surface with bacteria increasing the number of reactive oxygen species at faster rate followed by inhibition or elimination of the bacteria.
7 Does the shape of the nanomaterial affect the antimicrobial property? If yes explain why?Yes, the shape is also one of the crucial factors that affect the structural behavior and antimicrobial activity of the nanomaterials. Apart from size, shape of the nanomaterials also has influence over the surface area where even same materials with the same size will have different surface area because of a change in shape. The surface area of the nanomaterials strongly determines the level of interaction of the nanomaterial with the microbial surface. Thus, shape plays a crucial role with regard to interaction and the toxic effects on bacterial cell where nanomaterials of different shapes would cause varying degrees of bacterial cell damage and antibacterial property.
References
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3 Acharya, D., Singha, K. M., Pandey, P., Mohanta, B., Rajkumari, J., & Singha, L. P. (2018). Shape dependent physical mutilation and lethal effects of silver nanoparticles on bacteria. Scientific Reports, 8(1), 201.
4 Adams,