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Nanomaterials 2020, 10, 1146 4 of 24 biological enzymes. For example, dead or inactive cells of Corynebacterium glutamicum were used to synthesize AgNPs. A large amount of reduction was found on the surface of the inactive cells resulting in the formation of AgNPs with irregular shape and size of 5 to 50 nm [35]. Zinc oxide NPs are also promising as antimicrobial agents, drug delivery and bioimaging probes in next-generation biological applications. Zinc oxide NPs were synthesized using a bacterium Aeromonas hydrophila in simple and cost-effective method. The crystalline nature of the NPs was observed by atomic force microscopy (AFM), which showed that the NPs were spherical and oval with an average size of 57.72 nm [36]. The conversion of metallic ions into NPs through reduction is dependent on functional groups of biomolecules present in the organisms which induce biomineralization, and other environmental factors, such as pH, media composition, concentration of metallic salts and temperature [33]. The size, shape and composition can be highly determined by these environmental factors [37]. For example, at the optimum growth temperature of 20 ◦C, spherical AgNPs were produced with an average diameter of 2–5 nm using Morganella psychrotolerans, while at 25 ◦C, a mixture of triangular and hexagonal nanoplates along with spherical NPs were obtained [38]. Actinomycetes, a group of filamentous bacteria, are known for their metabolic versatility. These bacteria can survive in stressful environmental conditions by using the bioactive potentials [39]. Actinomycetes consist of a significant composition of the microbial population in soils and produce extracellular enzymes to decompose materials. Their enzymes have received more attention than enzymes from other sources due to their high stability and uncommon substrate specificity. These are found in extreme habitats and produce enzymes with high commercial value [40]. Among the 22,000 discovered microbial secondary metabolites, 70% are from actinomycetes while two-thirds of them are originated from the genus Streptomyces [41]. Both extracellular and intracellular synthesis of NPs can be undertaken, but extracellular synthesis is a popular method and has been used commercially in various fields. Biomass extracts of Streptomyces zaomyceticus Oc-5 and Streptomyces pseudogriseolus Acv-11 were used for synthesis of copper oxide NPs (CuONPs). Green synthesized CuONPs were with surface plasmon resonance absorption band at 400 nm, crystalline, spherical with an average size of 78 nm and 80 nm for strain Oc-5 and Acv-11, respectively [42]. In another study, the free-biomass filtrates with metabolites from three endophytic actinomycetes of Streptomyces capillispiralis Ca-1, Streptomyces zaomyceticus Oc-5, and Streptomyces pseudogriseolus Acv-11 served as biocatalysts for green synthesis of AgNPs [43]. An actinobacteria Rhodococcus sp. was used to reduce aqueous AgNO3 for the green synthesis of AgNPs [44]. The extracellular synthesis of gold (Au) NPs was carried out using culture supernatant of soil isolated Streptomyces griseoruber with a size 5–50 nm [45]. The green synthesized metallic NPs show higher antimicrobial potentials than conventionally synthesized NPs because some biomolecules act as capping and stabilizing agents during synthesis of the NPs [14]. Fungi are excellent sources of many bioactive compounds that can be utilized in various sectors. The microscopic filamentous fungi (ascomycetes and imperfect fungi) and other fungal species are reported to produce about 6400 bioactive compounds [46]. These microorganisms possess tolerance to the heavy metals and are capable of internalizing as well as bioaccumulating the metals. So, these organisms have been used for reduction and stabilization during the synthesis of NPs. Moreover, large-scale cultivation of fungi is very easy and can be used to synthesize NPs with uniform shape and size [47–50]. Fungi are more convenient compared to other microbes due to their production of high quantities of enzymes and proteins for NPs synthesis [51,52]. The fungi mediated synthesis of NPs can be extracellular or intracellular [53]. For extracellular synthesis, the aqueous culture filtrates consisting of biomolecules are added to metal precursor, and free NPs are formed in the dispersion. This is a commonly used method, as no techniques are needed to get cell-free NPs [49,54–56]. During intracellular synthesis, a metal precursor is added to the mycelial culture and internalized in the biomass followed by the extraction of NPs. The extraction of the NPs is performed to disrupt the biomass by chemical treatment, centrifugation, and filtration and then release the synthesized NPs [57–59]. The fungal synthesis of metallic NPs is dependent on culture conditions. In a previous study, the culture conditions of Trichothecium sp. reduced Au ions resulting extracellular NPs synthesis butPDF Image | Plant and Microbe-Based Synthesis of Metallic Nanoparticles
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