Mushroom Extract–Reduced Metal Nanoparticles: An Effective Approach Against Food Pathogenic Bacteria

Introduction

Nanoparticles (NPs) are ultra-fine particles with distinct chemistry and morphologic dimensions (Chawla et al., 2020). NPs constitute of physiochemical and optoelectronic features, therefore, can keep the electrons within the particles that are smaller than delocalized electrons present in large amount (Sudha et ah, 2018; Chawla et ah, 2018). The synthesis of NPs results in the formation of functional NPs; these functional NPs are extensively used for different purposes like diagnosis, therapeutic, catalysis, electronics, and photonics (Lu et ah, 2007). Formerly, NPs were synthesized by mainly two techniques physical and wet chemistry; however, alternate to these two processes, green synthesis techniques that include the use of eco-friendly and nontoxic reagents are in great demand (Chung et ah, 2016). In this context, extract prepared from various plants is majorly used for the synthesis of metal oxide NPs, in addition to plant-extract microorganisms such as bacteria, fungi, and algae, which are extensively used to carry out the process of green synthesis (Chaudhari et ah, 2016; Chawla et ah, 2019). Living organisms used for the synthesis of metal oxide NPs have an advantage over chemically synthesized metal oxide NPs as they constitute special traits that provide resistance mechanisms against heavy metals due to their exposure to xenobiotic compounds (Banerjee and Rai, 2018). The resistance mechanism could be due to the presence of primary and secondary metabolites, as well as extracellular macromolecules. All these factors work together intracellularly or extracellularly in an accompanying manner to achieve the detoxification process, and that leads to the process of biosynthesis of NPs (Siddiqi and Husen, 2016; Agarwal et ah, 2020). The extracellular compounds responsible for resistance mechanisms in living cells are diverse and, hence, can be utilized for the synthesis of NPs that have different morphology, size dimensions, and functional properties (Sirelkhatim et ah, 2015). The macromolecules, including vitamins, enzymes, and polysaccharides produced extracellularly or intracellularly, have a high reduction potential and thus synthesize NPs by reducing metal ions or semiconductor ions present in them in an effective manner (Ravindran et ah, 2013; Pathania et ah, 2018). The microorganism includes both prokaryotes and eukaryotes that have complexity in their tissues and organs which are being utilized for the benignant biosynthesis process. Due to complexity, these organisms are responsible for obtaining different types of NPs. Prokaryotes, including both bacteria and actinomycetes, have been extensively used for the synthesis of NPs. Likewise, both unicellular and multicellular eukaryotic organisms devoid of hierarchy in their organ structure also shown excellent results in the synthesis of NPs (Kalia and Kaur, 2018). Among eukaryotes, both macrofungi and microfungi consist of peculiar characteristics of secretion of enzymes extracellularly and therefore show promising results in the synthesis of NPs. The synthesis of NPs by extracellular enzymes is mediated by proteins. These proteins act as reducing and capping agents and provide a reduction in downstream isolation and purification process during the synthesis of NPs (Sinha et al„ 2015). Fungi generate a huge mycelial biomass that offers a large surface area for the generation of dissolved ions. These dissolved ions when generated provide an interface for the synthesis of NPs (Kalia and Kaur, 2018). Synthesis of NPs via mushrooms gained intense interest among researchers in the recent few years as they constitute a wide range of bioactive compounds (Roy et ah, 2019). These bioactive compounds provide a huge potential for the synthesis of NPs. In addition to bioactive compounds, both edible and non-edible mushrooms consist of numerous proteins and polysaccharides which synthesize both organic and inorganic NPs through the intracellular and extracellular pathway (Owaid, 2019). The NPs which are synthesized via fungi are highly stable, are water-soluble, and have good dispersion properties which have proved to be beneficial (Iravani et ah, 2014; Dhull et ah, 2019). NPs synthesized using mushrooms, therefore, is a promising technology for synthesis of NPs is stable, eco-friendly, and non-toxic nature (Sharma et ah, 2019).

Green Synthesis of NPs

The synthesis of NPs has gained importance in the field of nanotechnology in the recent era. The synthesis of NPs is done to obtain NPs of the desired shape, size, and functionalities and to carry out this process top-down and top-up methods are used (Wang et ah, 2015). These two methods are the fundamental principles usually used in the existing literature. The top-down method includes the preparation of NPs through different approaches like lithographic, ball milling, etching, and sputtering. The bottom-up approach includes two methods for the synthesis of NPs; the first one includes chemical deposition, the sol-gel process, spray and laser pyrolysis, molecular condensation, and aerosol process; the latter includes the process of green synthesis. Among these two methods of the bottom-up approach, green synthesis of nanometal, ions have become a common and effective approach in the recent era. This method utilizes the principle that includes the removal and use of hazardous chemical compounds in the manufacture of NPs (Singh et al., 2018). The NPs thus synthesized are highly stable, are small in size, and have a high surface area-to-vol- ume ratio, surface modifiability, excellent magnetic properties, and biocompatibility (Sen et ah, 2013). In green chemistry, biological materials, including green plants, are microorganisms that are incorporated to carry out the synthesis of NPs. The microorganisms used include bacteria, fungi, and algae (Malik et ah, 2014; Gurunathan et ah, 2014). The use of microorganisms for the synthesis of nanometal ions is preferred over plant extract as the microorganisms can be cultured and preserved for constant use (Gurunathan et ah, 2015) The microorganisms have diverse nature and among this diversity mushrooms, the macrofungi are found abundant in nature. Mushrooms have been reported as one of the rich sources of bioactive compounds bearing distinct biological activities (Mohanta et ah, 2018). Mushrooms secret extracellular compounds, proteins, and polysaccharides in addition to the bioactive compounds. These compounds can reduce metal ions under optimized

FIGURE 2.1 Synthesis of nanoparticles from mushroom (Mycosynthesis).

conditions and therefore are highly stable small in size and non-toxic in nature. The NPs thus obtained by mycosynthesis are referred to as biogenic NPs (Figure 2.1).