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

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]. 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

114 *Escherichia coli* Escherichia coli - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications

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

H\* radicals that interact with several components of bacteria such as peptidoglycan, DNA,

microcrystals were synthetized and tested for antimicrobial activity

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

conditions the efficiency was significantly lower.

cell wall, proteins, and other bacterial structures (**Figure 2**) [77].

WO<sup>4</sup>

Recently [77], α-Ag<sup>2</sup>

O2

**Figure 2.** α-Ag<sup>2</sup>

WO<sup>4</sup>

crystallographic planes [17].

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 antioxidant) [79].

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

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

**Figure 3.** Determination of qualitative MIC of propolis nanoparticles against *E. coli* by agar-well diffusion method in plate dish [80].

The shown antimicrobial activity of propolis nanoparticles is of potential interest for direct applications or in film formulations, for example. Therefore, results obtained in this study, set the bases for future studies, using films as support for propolis nanoparticles, and for application in many products.

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