ENMs. Soo and co-workers (2020) have prepared Ag-doped TiO2 nanofibers using

calcination and electrospinning methods and evaluated the antibacterial property of

AgTiO2 against S. aureus and S. albany bacteria, which showed the enhancement in

the antibacterial property of AgTiO2 as compare to pure TiO2 nanofibers.

27.4.6 Calcium (Ca)- and Magnesium (Mg)-Based ENMs

Among the widely explored metal oxides as antimicrobial agents, MgO and CaO are

of particular interest because they are stable under harsh process conditions and

generally regarded as safe materials to human beings. Moreover, they have antimi-

crobial activity without photoactivation, compared to TiO2 which requires

photoactivation. It has been verified that the antibacterial mechanism of CaO and

MgO ENMs is driven by the generation of superoxide on the surface of these

particles and also an increase in pH value by the hydration of CaO and MgO with

water.

Nguyen et al. (2018) investigated the antimicrobial properties of MgO NPs against

prevalent infectious bacteria (E. coli, P. aeruginosa, S. epidermidis, S. aureus, and

MRSA) and yeasts (C. albicans, C. albicans, C. glabrata, and C. glabrata). The MgO

NMs was reported to be more effective against GN bacteria than GP bacteria owing to

differences in the structures of cell wall and membrane. The interactions of MgO NMs

with cell wall and/or membrane were suggested to be the key mechanism for the lethal

effects of MgO against planktonic bacteria. In another study, Yamamoto and

colleagues (2010) fabricated the CaCO3 (grain) and nanoscaled MgO (20 nm fine

crystallite) based composite powder by thermal decomposition of dolomite for oral

hygiene application. Gedda et al. (2015) fabricated CaO nanoplates with a length

ranging 40–130 nm and breadth ranging 30–100 nm by using shrimp shells as the

source. As-fabricated NMs were reported to possess effective antimicrobial activity

against GP (E. coli) and GN (S. aureus) bacteria. The NPs were observed to accumu-

late around the cell membranes and inside the bacterial cells, thereby suggesting that

NPs must have infiltrated the cells by destroying the cell membranes, which

demonstrates the mode of bactericidal action of the CaO NPs.

According to the reports, MgO NPs damage the cell membrane and then cause the

leakage of intracellular contents which in turn lead to death of the bacterial cells.

Hewitt et al. (2001) reported that MgO initiated the some degree of cytoplasmic

membrane de-polarisation changes in E. coli bacteria. Leung et al. (2014) described

that strong antibacterial activity of the MgO NPs could be observed in the absence of

any ROS production. They declared that the mechanism of antimicrobial activity

might be due to the cell membrane damage. MgO NPs showed the bactericidal

activity against both GP and GN bacteria. Sawai et al. (2000) investigated

antibacterial activity of MgO against E. coli and S. aureus. They suggested that

the presence of active oxygen, such as superoxide, on the surfaces of MgO NPs was

one of the primary factors that affect their antibacterial activity.

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