**8. Nanoparticles linked to silver**

Metallic nanoparticles have different functions, like the following: (i) the marking of a particular stretch of DNA; (ii) the increase in resistance of metals and in the case of nanoparticles linked to silver; (iii) the antimicrobial action (both against Gram-negative bacteria, which have a thin layer of peptidoglycan and against Gram-positive, whose layer is thicker); and (iv) fungicide, which makes these particles a special nanostructured material to be incorporated into the control of such pathogens [72, 73].

However, there are no general consensuses about the mechanisms that can explain the action of silver nanoparticles in the inhibition of microbial growth. Some researchers claim that silver reacts with the thiol group of some vital enzymes to microorganisms and inactive them. Others claim dimerization of the pyrimidines of DNA, thus preventing the replication and thus their growth [74]. Another hypothesis is that the silver nanoparticle causes a change in the cell membrane, causing the output of reducing sugars of the membrane and thus causing cell death [75].

It is known that silver is a toxic metal, for both humans at high concentrations, as for most microorganisms, it is the preferred substance for inhibition thereof, when compared to gold

Antibiotic Resistance among *Escherichia coli*: Isolates and Novel Approaches to the Control of *E. coli* Infections

http://dx.doi.org/10.5772/67400

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The nanocomposites efficiency, containing silver, for the silver ion is much higher than the single metal species, as it has been proved in experiments [78]. It is not completely understood yet, but it is believed that connecting silver to other nanoparticles, such as silica and lignin, can inhibit the growth of microorganisms and these nanoparticles contribute to the destabiliz-

Propolis is a natural resinous substance collected from the leaf buds of different tree species by honeybees and known for its biological properties (antibacterial, antifungal, and antioxi-

Some authors [80] evaluated the antimicrobial activity of propolis nanoparticles in comparison with ethanol-propolis extract against *E. coli*. Ethanol-propolis extract was obtained from green propolis resin, in absolute ethanol under agitation during 15 days. To obtain the propolis nanoparticles, ethanol-propolis extract at 13.75% (w/v) was mixed with polyvinyl-alcohol solution at 0.1% (w/v). The size of the nanoparticles was determined by dynamic light scatter-

Antimicrobial activity of propolis nanoparticles and ethanol-propolis extract was tested against *E. coli*. Qualitative minimum inhibitory concentrations (MIC) of both solutions were evaluated by agar-well diffusion method, as shown in **Figure 3**. The result was 3.44% (w/v)

**Figure 3.** Determination of qualitative MIC of propolis nanoparticles against *E. coli* by agar-well diffusion method in

ing (DLS), atomic force microscopy (AFM), and it was about 70 nm in average [80].

for ethanol-propolis extract and 1.15% (w/v) for propolis nanoparticles.

nanoparticles, zinc, and magnesium titanium [78].

**9. Bioactivity of propolis nanoparticles against** *E. coli*

ing effect of the cell membrane.

dant) [79].

plate dish [80].

A study conducted by Xu et al. [76] concluded that reactive oxygen species (ROS) played a very important role in the mechanism of AgNPs antibacterial activity, because in anaerobic conditions the efficiency was significantly lower.

Recently [77], α-Ag<sup>2</sup> WO<sup>4</sup> microcrystals were synthetized and tested for antimicrobial activity against *E. coli*. The successful of the inactivation was directly related to the presence of specific defects in crystal surfaces. This interaction crystal-bacteria leads to a production of OH\* and O2 H\* radicals that interact with several components of bacteria such as peptidoglycan, DNA, cell wall, proteins, and other bacterial structures (**Figure 2**) [77].

**Figure 2.** α-Ag<sup>2</sup> WO<sup>4</sup> microcrystals in FE-SEM images (a, c, e, and g) and, respectively crystal shape (b, d, f, and h). The crystals were synthetized by MH method. The points highlighted in different colors corresponds each to its respective crystallographic planes [17].

It is known that silver is a toxic metal, for both humans at high concentrations, as for most microorganisms, it is the preferred substance for inhibition thereof, when compared to gold nanoparticles, zinc, and magnesium titanium [78].

The nanocomposites efficiency, containing silver, for the silver ion is much higher than the single metal species, as it has been proved in experiments [78]. It is not completely understood yet, but it is believed that connecting silver to other nanoparticles, such as silica and lignin, can inhibit the growth of microorganisms and these nanoparticles contribute to the destabilizing effect of the cell membrane.
