*3.1.1 Formation of reactive oxygen species (ROS)*

They are a group of reactive molecules produced in some metabolic processes in which oxygen participates: the superoxide anion O2<sup>−</sup> which is a powerful oxidizing agent very reactive with water. Hydrogen peroxide H2O2 and the hydroxyl radical (•OH) which is the most reactive, since accepting one more electron, gives rise to a water molecule. Metal oxides NPs are capable of producing different reactive oxygen species may participate in different types of reactions in which they can undergo oxidation or reduction processes. ROS produce disruption of DNA, damage by oxidation of polyunsaturated fatty acids and amino acids. The alteration of the balance in the mechanisms of production and elimination of ROS, in favor of production, originates the state of oxidative stress in the bacteria cell. In the case of O2 and H2O2 cause less acute stress reactions and can be neutralized by endogenous antioxidants, such as superoxide and catalase enzymes, while OH<sup>−</sup> and O2 can lead to acute microbial death.

**17**

*In vitro Antimicrobial Activity Evaluation of Metal Oxide Nanoparticles*

*3.1.2 Damage to the wall-cell membrane due to electrostatic interaction and* 

The electronegative groups of the polysaccharides in the bacterial membrane have an attraction sites by metal cations. The difference in charge between bacterial membranes and the NPs of metal oxides leads to electrostatic attraction and thus accumulates on the bacteria surface, altering the structure and permeability of the cell membrane. Gram-negative bacteria have a higher negative charge than Gram-positive bacteria and therefore the electrostatic interaction will be stronger in Gram-negative strains. The pores of the membranes are in the order of nanometers, therefore the smaller the particle size and the greater the surface area, the greater the efficiency of the metal oxide nanoparticles. In the same way, the cations extracted from the NPs of the metal oxides and their accumulation in the cell wall, create pits in it, leading to a change in permeability due to the sustained release of lipopolysaccharides, membrane proteins and intracellular factors. In addition, this mechanism has been linked to the interruption of the replication of adenosine triphosphate (ATP) and the deoxyribonucleic acid (DNA) of the bacterium, leading to its death. One study indicates that the action of NPs depends on the components and structure of the bacterial cell. The unique components of Gram-negative bacteria, such as LPS, can prevent the adhesion of metal oxides NPs to the barrier of bacterial cells and regulate the flow of ions in and out of the bacterial cell

The balance of metallic elements is essential for microbial survival, since it regulates metabolic functions by helping coenzymes, cofactors and catalysts. When the bacteria have an excess of metals or metal ions, there will be a disorder in the metabolic functions. Metal ions bind with DNA and alter the helical nature by cross-linking between and within the DNA strands. The metal ions neutralize the charges in LPS and increase the permeabilization of the outer membrane. The ions of metal oxides might also cause the decomposition of bacterial cells due to the diffusion of metal ions by generating large amounts of hydroxyl radicals and diffusion in bacterial cells. Other studies indicate that NPs of metal oxides slowly release metal ions through adsorption, dissolution and hydrolysis; they are toxic

Protein dysfunction is another mode of antibacterial activity exhibited by NPs of metal oxides. The metal ions catalyze the oxidation of the side chains of amino acids resulting in carbonyls bound to proteins. The carboxylation levels within the protein molecule serve as a marker for the oxidative damage of the protein. This carboxylation of proteins will lead to the loss of catalytic activity in the case of

Electrical properties of metal oxide NPs interact with nucleic acids inducing suppress of cell division by altering processes of replication of the chromosomal DNA and the plasmid in microorganism. It is known that signal transduction in bacteria is affected by NPs of metal oxide. Phosphotyrosine is an essential component of mechanism of signal transduction in bacteria. NPs dephosphorylate

*DOI: http://dx.doi.org/10.5772/intechopen.84369*

*accumulation*

membrane.

*3.1.3 Loss of homeostasis by metal ions*

and abrasive to bacteria and, therefore, lyse the cells.

enzymes, which finally triggers the degradation of proteins.

*3.1.4 Dysfunction of proteins and enzymes*

*3.1.5 Inhibition of the transduction signal*

*In vitro Antimicrobial Activity Evaluation of Metal Oxide Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.84369*

*Nanoemulsions - Properties, Fabrications and Applications*

There are findings about the potential mechanisms of action, where it attempts to explain the bactericidal effect of metal oxide NPs [10, 24–27]. Some of these include the action of reactive oxygen species (ROS), the electrostatic interaction, accumulation, ions delivered and contact by itself of NPs, that induce a several effects from outside and into the bacteria, and that it will be described below

They are a group of reactive molecules produced in some metabolic processes in which oxygen participates: the superoxide anion O2<sup>−</sup> which is a powerful oxidizing agent very reactive with water. Hydrogen peroxide H2O2 and the hydroxyl radical (•OH) which is the most reactive, since accepting one more electron, gives rise to a water molecule. Metal oxides NPs are capable of producing different reactive oxygen species may participate in different types of reactions in which they can undergo oxidation or reduction processes. ROS produce disruption of DNA, damage by oxidation of polyunsaturated fatty acids and amino acids. The alteration of the balance in the mechanisms of production and elimination of ROS, in favor of production, originates the state of oxidative stress in the bacteria cell. In the case of O2 and H2O2 cause less acute stress reactions and can be neutralized by endogenous antioxidants, such as superoxide and catalase enzymes, while OH<sup>−</sup>

**3.1 Mechanisms of antimicrobial activity**

*3.1.1 Formation of reactive oxygen species (ROS)*

and O2 can lead to acute microbial death.

*Mechanisms of action of the bactericidal effect from metal oxide nanoparticles.*

(**Figure 1**).

**16**

**Figure 1.**
