Microbial Interactions at Nanobiotechnology Interfaces. Группа авторов
Чтение книги онлайн.

Читать онлайн книгу Microbial Interactions at Nanobiotechnology Interfaces - Группа авторов страница 19

СКАЧАТЬ 12 nm at the same concentration not only limited the growth of the bacteria but also killed them completely. Here, the mechanism of action involved ROS production and the accumulation of nano‐sized particles in the cytoplasm of S. aureus (Raghupathi et al., 2011).

      In another study, size‐dependent antimicrobial activity of cobalt ferrite core/shell NPs was demonstrated. Antimicrobial property of three different sizes of NPs (1.65, 5, and 15 nm) was studied against Saccharomyces cerevisiae and Candida parapsilosis. Notably, against S. cerevisiae 1.65 nm exhibited 12 and 25% higher killing than 5 and 15 nm particles, respectively. Similarly, in the case of C. parapsilosis, the same trend was reported whereas 1.6 nm particles showed 15 and 44% higher killing efficiency in comparison to 5 and 15 nm particles, respectively. The antimicrobial activity of the cobalt ferrite NPs at size of 7–8 nm was suggested to be due to intracellular diffusion with subsequent interaction with cell membrane causing oxidative stress and finally DNA damage. It was also suggested that with decrease in size, the cobalt content of the shell might have increased, which in turn improved the interaction or binding efficiency of particle with bacterial cell (Žalnėravičius et al., 2016).

      Morones et al. (2005) investigated the effect of size of silver NPs in the range of 1–100 nm against four different Gram‐negative bacteria (E. coli, V. cholera, P. aeruginosa, and Scrub typhus). High‐angle annular dark‐field (HAADF) scanning transmission electron microscopy (STEM) technique showed that silver NP in range of 1–10 nm was able to attach to bacterial cell membrane, which altered its permeability and respiration. Further the NPs that have penetrated caused intracellular damage by interacting with sulfur and phosphorous‐containing substances such as proteins and DNA. Through this study the author confirmed that the size of silver NPs does play a crucial role in the antibacterial effect (Morones et al., 2005).

      In a similar study, the effect of size was explained in terms of change in diameter of carbon nanotubes: a well‐known antibacterial material. In this study, the antibacterial activity of single‐walled (SWCNTs) and multi‐walled carbon nanotubes (MWCNTs) with outer diameter of about 0.9 and 30 nm respectively was considered for assessing the effect of size. Scanning electron microscopy studies showed that E. coli cells attached to SWCNTs exhibited higher degree of cellular damage than those attached to MWCNTs. It was also observed that the E. coli cells treated with SWCNTs got inactivated (80 ± 10%) at a higher percentage than those treated with MWCNTs (24 ± 4%). Similarly, a metabolic activity study also suggested that the cells attached to SWCNTs had lesser metabolic activity than the cells with MWCNTs. Further, the measurement of cytoplasmic content efflux and gene expression of stress and DNA‐related products of CNTs‐treated bacterial cells confirmed the superior toxicity of SWCNTs in comparison to MWCNTs. Overall these results clearly suggested that the SWCNTs exhibited a greater antimicrobial property than MWCNTs. The mechanism of action involved the partial penetration of CNTs and subsequent membrane damage. These effects of SWCNTs are attributed to the diameter (size) of the nanotubes where the smaller diameter aided in better penetration of CNTs into bacterial cells. Penetration was followed by membrane damage affecting the metabolic activity and altered stress‐related gene expressions (Kang et al., 2008).

      Zhang et al. (2008) prepared different metallic silver and gold NPs by in situ reduction and stabilized with poly(amidoamine) with terminal dimethylamine groups [HPAMAM‐N(CH3)2]. The size and dispersity of the Ag (7.1–1 nm) and Au (7.7–3.9 nm) NMs can be changed by changing the molar ratio of metal with stabilizer. The antimicrobial property of these series of NMs was tested against Gram‐positive bacteria, Gram‐negative bacteria, and fungi. In these cases, the smallest particles with high surface‐to‐volume ratio exhibited the maximum antimicrobial activity against bacteria and fungi. Along with the size, the cationic terminal groups on surface contributed to a certain amount through interaction with the negative bacterial surface (Zhang et al., 2008).

      In addition to size, surface chemistry, composition, and shape also affect the functionality or activity of NMs. Shape plays a crucial role with regard to the interaction and the toxic effects on bacterial cell. Notably, the shape and size of the NMs dictate the physicochemical characteristics such as optical, electromagnetic, catalytic, and the crucial biological properties of the NMs. Taking the aforementioned factors into consideration, researchers attempted to develop various synthesis processes to gain precise control over the physicochemical factors such as size and shape (Bansal et al., 2010; Mulvaney, 1996; Narayanan & El‐Sayed, 2004). In addition to size, the shape of the NM also determines the surface area of the material where even same materials with the same size will have different surface areas because of a change in shape. Next to shape of NM, crystalline nature of the nanostructures also plays an important role. It is generally defined as the relative abundance of particular crystallographic planes where each of them shows specific properties and reactivity. Conclusively, the shape and crystalline nature are important parameters, next to size, that play a significant role in the nano‐bio interaction. Various studies have documented that the shape and crystallinity of NM have a great influence over the behavior of NMs and their biological activity such as antibacterial activity, and their uptake rate. In an earlier study, it was observed that spherical NPs had higher cellular uptake than nanorods (Chithrani, Ghazani, & Chan, 2006). Yang et al. (2016) showed that gold nanorods of different aspect ratio illustrated a significant variation in the cellular uptake rate. A significant increase in the internalization rate was observed with increase in the aspect ratio from 1 to 2 (AR2) where further increase did decrease the cellular internalization rate (Yang et al., 2016).