Factors Affecting the Antimicrobial Activity of Nanoparticles’ Concentration and Size
In NPs, the role of size and concentration has been analysed. At a low' concentration of 0.01 ppm, silver NPs with different sizes has been investigated for activity (Poole 2002; Kim et al., 2007). To kill and destroy bacteria, the smallest AgNPs w'ere more effective in comparison to larger ones. The small-sized NPs discharged more silver cations because of their high surface-to-volume ratio; therefore, they are more effective in comparison to large-sized NPs (Torres et al., 2013).
6.7.1 Chemical Composition
The base of NPs is their chemical composition, which determines the differences in their activities. The NPs are observed to produce ROS (titanium oxide [TiO,], silicon oxide [Si02] and zinc oxide [Zn02]) against B. subtilis and E. coli. The ascending order of the biocidal activity of these compounds was SiO, to TiO, to ZnO. By 10-ppm concentration of ZnONPs the growth of B. subtilis was 90% inhibited, while 1000 and 2000 ppm of TiO, and SiO,, respectively, arerequired to inhibit the development of B. subtilis to 90%. Although, the inhibition effect of NPs on E. coli was partly at 10 ppm of ZnONPs and 500 ppm of TiO:NPs and SiO,NPs (Huang et al., 2008). Additionally, it was explained that light or dark conditions do not affect the bactericidal activity, but the growth inhibition includes mechanism-based ROS production.
6.7.2 The Shape of Nanoparticles
Studies showed that various shapes (elongated rod, truncated triangular, and spherical) of AgNPs have different biocidal activity intensity. The difference in the colony-forming unit count of £ coli depends on the media inoculated with different shapes of NPs. The morphology of NPs seems to stimulate the activity of NPs. The shape-determined activity was dependent on the term of facets. The spherical NPs primarily had 100 facets, rod-shaped NPs had 111 facets on the side surface and 100 on the end, and truncated triangular NPs have 111 facets on top basal planes. The 111 facets are of high atom density that aid the antibacterial reactivity of NPs (Liu et al., 2010).
6.7.3 Target Microorganisms
It is reported in many studies that NPs showed greater biocidal activity against gramnegative rod-shaped bacteria as compared to gram-positive bacteria. E. coli and 5. aureus were used to analyse the impact of AgNPs and results showed way more activity against E. coli (MIC 3.3-3.6) than 5. aureus (MIC more than 33 nm). The higher concentration of peptidoglycans is present in the cell wall of gram-positive bacteria (5. aureus) which depicts the difference in the results (Wang et al., 2010). Huang et al. revealed the activity of ZnONPs against both the gram-positive (5. aureus) and gram-negative (£. coli) bacteria (Huang et al., 2008). However, in another study, ZnONPs were found to be most effective against 5. aureus than £. coli and P. aeruginosa (Mao et al., 2004).
6.7.4 Photoactivation
Ti02NPs showed good activity against £. coli, which noticeably increases with UV radiations. Additionally, it was observed that Ti02NPs without photoactivation have negligible activity, that is it shows about 20 % of growth inhibition of S. aureus. On the other side, ZnONPs show escalating activity after photoactivation by UV-visual radiation (Shankar et al., 2003; Philip 2009). ROS production can be enhanced with photoactivation with blue light, which further enhances the activities of these NPs (Lipovsky et al., 2011). However, it is also a well-known fact that no surviving microorganisms have shown any effect because of bacterial incubation with NPs in the dark (Huang et al., 2008).
Biogenic Metallic Nanoparticles for Antibacterial Applications
Silver, copper oxide, gold, titanium oxide, magnesium oxide, and zinc oxide are the important metal NPs used as antimicrobial agents because their potent antibacterial and antifungal effects are well known (Beyth et al., 2015; Zhang, 2015). Recently, several investigations reported that different metal oxide NPs showed biocidal action against several pathogenic bacteria. The antimicrobial potential of NPs is well known to be the purpose of the surface area in contact with the microbes (Ravishankar and Jamuna, 2011; Franci et al., 2015; Chiriac et al., 2016). The bactericidal action of metal NPs has been accredited to their smaller size and high surface to volume ratio, which enables them to interact closely with the outer membranes of microbes and is not only because of the discharge of metal ions in the solution (Ruparelia et al., 2008). Specific metal ions are necessary for the development of all organisms, and their privation can cause damage to the structure of plasma membranes and nucleic acids. However, the excess of these ions or the presence of other unnecessary ions can be fatal to the cells, which was often caused by protein dysfunction, oxidative stress, or membrane damage (Lemire et al., 2013). Some metal NPs are effective antimicrobial agents against several pathogenic microbes. This antimicrobial activity is responsible for water treatment, biomedical and surgical devices, synthetic textiles, and food processing or packaging (Ravishankar and Jamuna, 2011).
