reductase activity (Polti et al. 2010), Amycolatopsis tucumanensis exhibits cupric
reductase activity under different culture conditions (Dávila Costa et al. 2011a), and
Streptomyces sp. M7 has a chromate reductase whose expression was six times
higher in the presence of chromium (Davila Costa, personal communication). This
background makes these strains potential large NPs producers. In fact, experiments
are currently conducted about this subject and preliminary results are very relevant.
6.4 Nanoparticles Toxicity
The main characteristic of NPs is their size, which falls in the zone between atoms or
molecules and the corresponding bulk material. As the size of particle decreases, its
surface area increases and also allows a greater proportion of its atoms or molecules
to be displayed on the surface rather that the interior of the material. The increase in
surface area determines the potential number of reactive groups on the particle
surface. Thus, AgNPs show efficient cytotoxic properties compared to other salts
due to their extremely large surface area, which provides better contact with microorganisms (Kim et al. 2007).
Nanoparticles, unlike antibiotics, do not target specific traits of bacteria, and
antibiotic resistance is often just modification of these traits. There are multiple
possible routes for the antimicrobial properties of NPs. Perhaps most importantly,
the additional surface-reactive groups exposed in NPs might act as active sites for
interactions with molecular dioxygen, which may lead to formation of reactive
oxygen species (ROS) including superoxide radical (O2À), hydrogen peroxide
(H 2 O 2 ), and hydroxyl radical (OHÁ) leading to oxidative stress (Ivask et al. 2010).
For instance, Dasgupta and Ramalingam (2016) studied the release of intracellular
ROS in E. coli treated with different concentrations of engineered AgNP. The
authors found that the ROS generation was dose-dependent and affects the integrity
of bacterial membrane so that cytoplasmic constituents were released. The generality
of these mechanisms allows for NPs to bypass the acquired resistance to antibiotics
that resistant strains contain.
It is believed that one of the mechanisms by which the NPs present antimicrobial
capacity is due to the use of the NPs’ negatively charged ions which bind to the
microorganism cell wall and break it. Another mechanism that explains NP
antimicrobial properties is the passage of smaller NPs through the cell to cause a
direct damage to DNA (El-Deeb et al. 2013) (Fig. 6.2). It has been observed that NPs
are able to attach to the bacterial cell membrane and produce unrest in its normal
functioning. Nanoparticles could be accumulated in the cytoplasm or in the
periplasmic space producing the cell membrane disruption and consequently the
release of the cell contents (Golinska et al. 2017). The alteration of cell membranes
involves the binding of NPs to sulfur-containing proteins present in the plasmatic
membrane (Brayner et al. 2006). Similarly, sulfur content of intracellular enzymes
and DNA make these molecules the target of the NPs (Raghupathi et al. 2011). In
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D. Costa et al.
reductase activity under different culture conditions (Dávila Costa et al. 2011a), and
Streptomyces sp. M7 has a chromate reductase whose expression was six times
higher in the presence of chromium (Davila Costa, personal communication). This
background makes these strains potential large NPs producers. In fact, experiments
are currently conducted about this subject and preliminary results are very relevant.
6.4 Nanoparticles Toxicity
The main characteristic of NPs is their size, which falls in the zone between atoms or
molecules and the corresponding bulk material. As the size of particle decreases, its
surface area increases and also allows a greater proportion of its atoms or molecules
to be displayed on the surface rather that the interior of the material. The increase in
surface area determines the potential number of reactive groups on the particle
surface. Thus, AgNPs show efficient cytotoxic properties compared to other salts
due to their extremely large surface area, which provides better contact with microorganisms (Kim et al. 2007).
Nanoparticles, unlike antibiotics, do not target specific traits of bacteria, and
antibiotic resistance is often just modification of these traits. There are multiple
possible routes for the antimicrobial properties of NPs. Perhaps most importantly,
the additional surface-reactive groups exposed in NPs might act as active sites for
interactions with molecular dioxygen, which may lead to formation of reactive
oxygen species (ROS) including superoxide radical (O2À), hydrogen peroxide
(H 2 O 2 ), and hydroxyl radical (OHÁ) leading to oxidative stress (Ivask et al. 2010).
For instance, Dasgupta and Ramalingam (2016) studied the release of intracellular
ROS in E. coli treated with different concentrations of engineered AgNP. The
authors found that the ROS generation was dose-dependent and affects the integrity
of bacterial membrane so that cytoplasmic constituents were released. The generality
of these mechanisms allows for NPs to bypass the acquired resistance to antibiotics
that resistant strains contain.
It is believed that one of the mechanisms by which the NPs present antimicrobial
capacity is due to the use of the NPs’ negatively charged ions which bind to the
microorganism cell wall and break it. Another mechanism that explains NP
antimicrobial properties is the passage of smaller NPs through the cell to cause a
direct damage to DNA (El-Deeb et al. 2013) (Fig. 6.2). It has been observed that NPs
are able to attach to the bacterial cell membrane and produce unrest in its normal
functioning. Nanoparticles could be accumulated in the cytoplasm or in the
periplasmic space producing the cell membrane disruption and consequently the
release of the cell contents (Golinska et al. 2017). The alteration of cell membranes
involves the binding of NPs to sulfur-containing proteins present in the plasmatic
membrane (Brayner et al. 2006). Similarly, sulfur content of intracellular enzymes
and DNA make these molecules the target of the NPs (Raghupathi et al. 2011). In
202
D. Costa et al.