The widespread use of antibiotics has led to the emergence of multidrug-resistant strains of bacteria and thus a current concern for food safety and human health. Interest in new antimicrobials has focused on metal oxide nanoparticles. Specifically, titanium dioxide (TiO2) has been recognized as an attractive antimicrobial compound due to its photocatalytic nature and its recognition as chemically stable, non-toxic, cheap and Generally Safe (GRAS).
Various studies have revealed that this metal oxide has excellent antifungal and antibacterial properties against a wide range of both Gram-positive and Gram-negative bacteria. These properties are significantly enhanced by the synthesis of titanium dioxide nanoparticles (TiO2 NPs). Recent developments in the synthesis pathways of TiO2 NPs and the antimicrobial activity of these nanostructures are presented. In addition, TiO2 NPs support the inactivation of microorganisms due to their strong oxidizing power with free radical generation such as hydroxyl and superoxide anion radicals, which show growth reduction against various microorganisms such as Escherichia coli and Staphylococcus aureus.
Understanding the main mechanisms of the antimicrobial action of these nanoparticles was the second main objective of this chapter. Such as Escherichia coli and Staphylococcus aureus. Understanding the main mechanisms of the antimicrobial action of these nanoparticles is the second main goal of the research. Such as Escherichia coli and Staphylococcus aureus. Understanding the main mechanisms of the antimicrobial action of these nanoparticles is the second main objective of this section.
The potential health impact and toxicity of nanoparticles (NP) on the environment is an important issue that needs to be addressed at this time. Various studies have confirmed that metal oxide NPs traditionally synthesized using chemical methods such as sol-gel synthesis and chemical vapor deposition show different levels of toxicity to test organisms. In recent years, researchers have emphasized the development of nanoparticles promoted through processes characterized by environmental sustainability and an ecological view, mild reaction conditions and non-toxic precursors.
Due to this increased sensitivity to green chemistry and biological processes, ecological processes are currently being investigated for the synthesis of non-toxic nanoparticles. These biological methods are considered to be safe, cost-effective, biocompatible, non-toxic, sustainable and environmentally friendly processes. Moreover, chemically synthesized NPs exhibit less stability and more agglomeration, as a result of which biologically synthesized NPs are more dispersed, It has been explained that it results in stable sized and less energy consuming processes.
These biosynthetic methods, also called “green synthesis”, use various biological resources found in nature for living plant products, plant extracts, algae, fungi, yeast, bacteria and viruses. (Synthesis of NPs) Among these methods, processes using plant-based materials are considered most suitable for large-scale green synthesis of NPs for their convenience and safety. On the other hand, the reduction rate of metal ions in the presence of plant extract is much faster than microorganisms and provides stable particles.
It contains biomolecules that are highly studied by researchers, such as plants, phenols, nitrogen compounds, terpenoids, and other metabolites. It is well known that the hydroxyl and carboxylic groups present in these biological compounds act as stabilizers and reducing agents due to their high antioxidant activities. Therefore, plant extracts have been studied as one of the best green alternatives for metal oxide nanoparticle synthesis. In recent years, TiO 2 nanoparticles have been obtained using different plant extracts, but not all of them have been studied for their antimicrobial activity.
Synthesis of TiO 2 NPs Using Plant Extracts
Different factors need to be evaluated in this research area in order to obtain TiO 2 NPs with better properties and maintain their biocompatibility. It has been shown that nanoparticles derived from green synthesis can have a better morphology and size translated into better antimicrobial activity. Mobeen and Sundaram obtained TiO 2 NPs from titanium tetrachloride precursor by chemical and green synthesis method. In the chemical-based method, sulfuric acid and ammonium hydroxide were used and in the green synthesis these chemical reagents were replaced with orange peel extract.
The nanoparticles obtained using the natural extract presented a well-defined and smaller crystal structure (about 17.30 nm) compared to nanoparticles synthesized by the chemical method (21.61 nm). Both methods resulted in anatase crystal structures and when evaluating antimicrobial activity, more environmentally friendly NPs revealed higher bactericidal activity against Gram positive and Gram negative bacteria compared to chemically synthesized nanoparticles.
Bavanilatha et al. The synthesis of TiO 2 NPs green is also detailed in Glycyrrhiza glabra root extract. Antibacterial activity against Staphylococcus aureus and Klebsiella pneumonia was investigated and in vivo toxicity tests were also performed using the zebrafish embryonic model (Danio rerio). The results showed their biocompatibility because no distinctive malformations were observed at each embryonic stage relative to different variations of NP in healthy embryos of adult fish and embryonic controls.
Subhapriya and Gomathipriya obtained biosynthesized TiO 2 NPs using a Trigonella foenum-graecum leaf extract, obtained spherical NPs and varied in size from 20 to 90 nm, and their antimicrobial activities were evaluated by the standard disk diffusion method. NPs to Yersinia enterocolitica (10.6 mm), Escherichia coli (10.8 mm), Staphylococcus aureus (11.2 mm), Enterococcus faecalis (11.4 mm) and Streptococcus faecalis (11.6 mm) showed significant antimicrobial activity against. Results confirm that TiO 2 has been developed NPs as an effective antimicrobial drug that can lead to the advancement of new antimicrobial drugs.
Spherical TiO 2 NPs were synthesized from plants, in particular by applying a Morinda citrifolia leaf extract and by the advanced hydrothermal method. TiO 2 developed NPs showed a size between 15 and 19 nm in a perfect hemispherical shape. Additionally, their antimicrobial activities were tested against human pathogens such as Staphylococcus aureus, Escherichia coli, Bacillus subtilis, Pseudomonas aeruginosa, Candida albicans and Aspergillus niger. TiO 2 NPs exhibited interesting antimicrobial activity mainly against Gram-positive bacteria.
In addition to plants, other organisms can produce inorganic compounds at the intracellular or extracellular level. The synthesis of TiO 2 NPs through microorganisms including bacteria, fungi and yeasts also meets the requirements for environmentally friendly strategies and exponentially increasing technological demand by avoiding the use of toxic chemicals in synthesis and protocols. The metabolites produced by the microorganism offer biological reduction, capping and stabilizing properties that improve the synthesis performance of NPs.
Jayaseelan et al. They cited glycyl-L-proline, one of the most abundant metabolites of Aeromonas hydrophilia bacteria, as the main compound acting as a capping and stabilizing agent during TiO 2. Green synthesis of NPs moreover, interest in fungi in the green synthesis of metal oxide nanoparticles has increased in recent years. Fungal enzymes or metabolites also inherently offer the potential to obtain elemental or ionic state metals from their corresponding salts. Different studies have been conducted based on the green synthesis of TiO 2 NPs from bacteria and fungi. Some of them have been synthesized for antimicrobial and antifungal purposes and their target microorganisms have also been declared.
Two important factors affecting NP synthesis are the type and sources of microorganisms. Some microorganisms commonly used in the food industry are Lactobacillus, a bacterium used in dairy products and as a probiotic supplement, and Saccharomyces cerevisiae, a yeast widely used in bakery. Jha et al. He investigated the efficacy of both microorganisms to synthesize TiO 2 NPs. A comparison between synthesis by Lactobacillus from yogurt and probiotic tablets resulted in different NP sizes: a particle size of 15–70 nm for yogurt and 10–25 nm for tablets. This difference is due to the purity of the bacteria. In general, the synthesis of NPs by TiO 2 Microorganisms did not provide stable dimensions as it was not industrially scalable compared to the synthesis of nanoparticles from plants.
Antimicrobial Activity of TiO 2 NPs
Bacteria such as Staphylococcus aureus, Burkholderia cepacia, Pseudomonas aeruginosa, Clostridium difficile, Klebsiella pneumoniae, Escherichia coli, Acinetobacter baumannii, Mycobacterium tuberculosis and Neisseria gonorrhoeae are seriously harmful bacteria seen in humans after 40 years. In this sense, titanium dioxide nanoparticles are one of the antimicrobial NPs whose studies have attracted attention in recent years. The basic solution is the use of antibiotics, antimicrobial and antifungal agents. However, there has been an increase in the resistance of several bacterial strains to these substances in recent years, and therefore there is currently a great interest in the search for new antimicrobial agents. Antimicrobial nanoparticles have been studied for their high activity, especially metal oxide nanoparticles. TiO 2 is a thermally stable and biocompatible chemical compound with high photocatalytic activity and has shown good results against bacterial contamination.
The main factors that differentiate antimicrobial activity among TiO 2 are NP morphology, crystal structure, and size. López de Dicastillo et al. Hollow TiO NPs increased when irradiated with UV-A light due to the photocatalytic nature of this oxide. Irradiation time ranged from 20 minutes to 3 hours. 2 nanotubes showed interesting antimicrobial reduction thanks to the improvement of the specific surface area. This fact can be explained by the nature of titanium dioxide, and one of the main mechanisms of its action is that reactive oxygen species (ROS) are generated on its surface during the photocatalysis process when exposed to light of a suitable wavelength. It is important to emphasize that some research studies have proven TiO’s antimicrobial activity.
Author: Ozlem Guvenc Agaoglu