154
nanoparticles could be the modification of viral proteins by destroying their disulfide linkages. On similar lines, the copper iodide nanoparticles were also found to
possess high antiviral activity against feline calcivirus (Shionoiri et al. 2012). This
was possibly due to reactive oxygen species-mediated oxidation of capsid proteins.
Hence, nanoparticles have been proven to work in a systematic manner to curb the
growth of drug-resistant microbes and viruses by targeting various types of proteins.
(iii) Antioxidative Potential
The production of reactive oxygen species under the influence of nanoparticles
becomes yet another major mechanism for their inhibitory effect on bacteria. One
of the commonest modes of action of silver nanoparticles is their inhibition of respiratory enzymes leading to the generation of reactive oxygen species (Khalandi et al.
2017). Zinc oxide nanoparticles have been known to generate reactive oxygen species such as OH
−
and H 2 O 2 (Soren et al. 2018). H 2 O 2 in particular gets penetrated
via the bacterial membrane and leads to growth inhibition. The other types of reactive oxygen species including superoxides and OH
−
do not have the ability to enter
the bacterial membrane and hence their mode of action differs from H 2 O 2 . Besides
silver and zinc oxide nanoparticles, Fe 3 O 4 has also been studied with the aim to find
its mechanism of inhibition and the possible generation of reactive oxygen species
like superoxide radicals (O 2
−
), singlet oxygen (
1
O 2 ), hydroxyl radicals (OH), and
H 2 O 2 (Auffan et al. 2008). Very recently, titanium nanoparticles have been proven to
induce the formation of reactive oxygen species in E. coli. This study was conducted by using 2,7-dichlorofluorescein diacetate which fluoresces under oxidative
conditions. The reactive oxygen species estimated to be produced by disturbance in
the electron transport chain was primarily responsible for depolarization of the cell
membrane leading to damage of the cell wall (Ranjan and Ramalingam 2016).
Some studies even suggest that the oxidation state of the metal nanoparticles is
also a major factor in determining their bactericidal effect (Meghana et al. 2015).
For example, copper dioxide nanoparticles are more toxic than copper oxide
nanoparticles since they lead to increased levels of reactive oxygen species like OH
−
and H 2 O 2 . However, it is not very clear whether the damage caused by nanoparticlemediated reactive oxygen species formation is the primary or the secondary
mechanism of disturbance/toxicity (Meghana et al. 2015). An increasing number of
studies have also shown that reactive oxygen species exert oxidative stress by interaction with DNA and proteins in bacterial cells. They have been found to attack the
essential periplasmic proteins/enzymes which are required for maintaining the various physiological cell processes (Jahnke et al. 2016). Nanoparticle-induced reactive
oxygen species have also been shown to enhance oxidative stress by regulating the
expression of proteins paving the way for cell apoptosis.
The exact mechanism of nanoparticle-induced reactive oxygen species production has been a topic of great interest among many other studies. The first and the
well-studied mechanism is through photocatalytic means where light induces the
formation of highly reactive reactants in the nanoparticles (He et al. 2013). For
example, H 2 O 2 or OH
−
, after interaction with zinc oxide nanoparticles, gets oxidized to OH
•
(hydroxyl radical). This free radical after interaction with O 2 and zinc
R. Sinha et al.
nanoparticles could be the modification of viral proteins by destroying their disulfide linkages. On similar lines, the copper iodide nanoparticles were also found to
possess high antiviral activity against feline calcivirus (Shionoiri et al. 2012). This
was possibly due to reactive oxygen species-mediated oxidation of capsid proteins.
Hence, nanoparticles have been proven to work in a systematic manner to curb the
growth of drug-resistant microbes and viruses by targeting various types of proteins.
(iii) Antioxidative Potential
The production of reactive oxygen species under the influence of nanoparticles
becomes yet another major mechanism for their inhibitory effect on bacteria. One
of the commonest modes of action of silver nanoparticles is their inhibition of respiratory enzymes leading to the generation of reactive oxygen species (Khalandi et al.
2017). Zinc oxide nanoparticles have been known to generate reactive oxygen species such as OH
−
and H 2 O 2 (Soren et al. 2018). H 2 O 2 in particular gets penetrated
via the bacterial membrane and leads to growth inhibition. The other types of reactive oxygen species including superoxides and OH
−
do not have the ability to enter
the bacterial membrane and hence their mode of action differs from H 2 O 2 . Besides
silver and zinc oxide nanoparticles, Fe 3 O 4 has also been studied with the aim to find
its mechanism of inhibition and the possible generation of reactive oxygen species
like superoxide radicals (O 2
−
), singlet oxygen (
1
O 2 ), hydroxyl radicals (OH), and
H 2 O 2 (Auffan et al. 2008). Very recently, titanium nanoparticles have been proven to
induce the formation of reactive oxygen species in E. coli. This study was conducted by using 2,7-dichlorofluorescein diacetate which fluoresces under oxidative
conditions. The reactive oxygen species estimated to be produced by disturbance in
the electron transport chain was primarily responsible for depolarization of the cell
membrane leading to damage of the cell wall (Ranjan and Ramalingam 2016).
Some studies even suggest that the oxidation state of the metal nanoparticles is
also a major factor in determining their bactericidal effect (Meghana et al. 2015).
For example, copper dioxide nanoparticles are more toxic than copper oxide
nanoparticles since they lead to increased levels of reactive oxygen species like OH
−
and H 2 O 2 . However, it is not very clear whether the damage caused by nanoparticlemediated reactive oxygen species formation is the primary or the secondary
mechanism of disturbance/toxicity (Meghana et al. 2015). An increasing number of
studies have also shown that reactive oxygen species exert oxidative stress by interaction with DNA and proteins in bacterial cells. They have been found to attack the
essential periplasmic proteins/enzymes which are required for maintaining the various physiological cell processes (Jahnke et al. 2016). Nanoparticle-induced reactive
oxygen species have also been shown to enhance oxidative stress by regulating the
expression of proteins paving the way for cell apoptosis.
The exact mechanism of nanoparticle-induced reactive oxygen species production has been a topic of great interest among many other studies. The first and the
well-studied mechanism is through photocatalytic means where light induces the
formation of highly reactive reactants in the nanoparticles (He et al. 2013). For
example, H 2 O 2 or OH
−
, after interaction with zinc oxide nanoparticles, gets oxidized to OH
•
(hydroxyl radical). This free radical after interaction with O 2 and zinc
R. Sinha et al.