Actinomycetes‐mediated biogenic synthesis of metal and metal oxide nanoparticles: progress and challenges

Actinomycetes‐mediated biogenic synthesis of metal nanoparticles and their antimicrobial activities are well documented. Actinomycetes facilitate both intracellular and extracellular metal nanoparticles synthesis and are efficient candidates for the production of polydispersed, stable and ultra‐small size metal nanoparticles. Secondary metabolites and new chemical entities derived from Actinomycetes have not been extensively studied for the synthesis of metal/metal oxide nanoparticles. The present review focuses on biogenic synthesis of metal nanoparticles from Actinomycetes and the scope for exploring Actinomycetes‐derived compounds (enzymes, organics acids and bioactive compounds) as metal and metal oxide reducing agents for the synthesis of desired nanoparticles. This review also focuses on challenges faced in the applications of nanoparticles and the methods to synthesize biogenic metal nanoparticles with desired physiochemical properties such as ultra‐small size, large surface to mass ratio, high reactivity etc. Methods to evade their toxicity and unique interactions with biological systems to improve their chance as an alternative therapeutic agent in medical and pharmaceutical industry are also discussed.     

Antimicrobial silver: uses, toxicity and potential for resistance

This review gives a comprehensive overview of the widespread use and toxicity of silver compounds in many biological applications. Moreover, the bacterial silver resistance mechanisms and their spread in the environment are discussed. This study shows that it is important to understand in detail how silver and silver nanoparticles exert their toxicity and to understand how bacteria acquire silver resistance. Silver ions have shown to possess strong antimicrobial properties but cause no immediate and serious risk for human health, which led to an extensive use of silver-based products in many applications. However, the risk of silver nanoparticles is not yet clarified and their widespread use could increase silver release in the environment, which can have negative impacts on ecosystems. Moreover, it is shown that silver resistance determinants are widely spread among environmental and clinically relevant bacteria. These resistance determinants are often located on mobile genetic elements, facilitating their spread. Therefore, detailed knowledge of the silver toxicity and resistance mechanisms can improve its applications and lead to a better understanding of the impact on human health and ecosystems.     

The antibacterial activity of biogenic silver and its mode of action

In a previous study, biogenic silver nanoparticles were produced by Lactobacillus fermentum which served as a matrix preventing aggregation. In this study the antibacterial activity of this biogenic silver was compared to ionic silver and chemically produced nanosilver. The minimal inhibitory concentration (MIC) was tested on Gram-positive and Gram-negative bacteria and was comparable for biogenic silver and ionic silver ranging from 12.5 to 50 mg/L. In contrast, chemically produced nanosilver had a much higher MIC of at least 500 mg/L, due to aggregation upon application. The minimal bactericidal concentration (MBC) in drinking water varied from 0.1 to 0.5 mg/L for biogenic silver and ionic silver, but for chemically produced nanosilver concentrations, up to 12.5 mg/L was needed. The presence of salts and organic matter decreased the antimicrobial activity of all types of silver resulting in a higher MBC and a slower inactivation of the bacteria. The mode of action of biogenic silver was mainly attributed to the release of silver ions due to the high concentration of free silver ions measured and the resemblance in performance between biogenic silver and ionic silver. Radical formation by biogenic silver and direct contact were found to contribute little to the antibacterial activity. In conclusion, biogenic nanosilver exhibited equal antimicrobial activity compared to ionic silver and can be a valuable alternative for chemically produced nanosilver.     

Employment of Cassia angustifolia leaf extract for zinc nanoparticles fabrication and their antibacterial and cytotoxicity

The plant Cassia angustifolia belongs to Saudi Arabia, which is one of the native places and now cultured throughout the global countries. Medical care in the Arab world is an essential outlet for medicinal plants, both because they are crucial elements for prophetic medicine and due to their lengthy background in the Middle East. C.angustifolia is one of the medicinal plants used in the Saudi Arabia. The usage of plant extracts for synthesizing nanoparticles is conducive to other biological material, since it avoids the lengthy phase of cell culture maintenance. Silver nanoparticles attract further attention due to their strong conductivity, stability and antimicrobial activity across different metal nanoparticles. The present study was designed in the Saudi C. angustifolia leaves with the zinc synthesis of nanoparticles and its antibacterial ability. The plant extracts of C. angustifolia was used for synthesis of zinc nanoparticles, antimicrobial activities against bacterial strains have been tested along with transmission electron microscope (TEM), UV spectroscopy and antimicrobial activities have been conducted. This study showed that silver ions may be transferred from the plant extract to silver nanoparticles. AgNPs biogenic capacity to antibacterial with lovo cell with IC₅₀ ranged from 33.5 ± 0.2 μg/mL demonstrated strong antibacterial capacity to antibody. The overall absorption value for the extract was between 420 and 440 nm and the color transition to green was the plasma absorption of the AgNPs. TEM results was showed in 200,000 magnification. The uniqueness of the current study is that Cassia angustifolia leaf extract from Saudi Arabia was used to prepare the metallic nanoparticles. Additionally, ZnCl₂ may also be used as nanoparticles of mineral salt and zinc, which, since their application has been confirmed, are antimicrobial.     

Biogenic nanoparticles: copper, copper oxides, copper sulphides, complex copper nanostructures and their applications

Copper nanoparticles have been the focus of intensive study due to their potential applications in diverse fields including biomedicine, electronics, and optics. Copper-based nanostructured materials have been used in conductive films, lubrification, nanofluids, catalysis, and also as potent antimicrobial agent. The biogenic synthesis of metallic nanostructured nanoparticles is considered to be a green and eco-friendly technology since neither harmful chemicals nor high temperatures are involved in the process. The present review discusses the synthesis of copper nanostructured nanoparticles by bacteria, fungi, and plant extracts, showing that biogenic synthesis is an economically feasible, simple and non-polluting process. Applications for biogenic copper nanoparticles are also discussed.     

Mycoendophytes as efficient synthesizers of bionanoparticles: nanoantimicrobials,mechanism, and cytotoxicity

Mycoendophytes are the fungi that occur inside the plant tissues without exerting any negative impact on the host plant. They are most frequently isolated endophytes from the leaf, stem, and root tissues of various plants. Among all fungi, the mycoendophytes as biosynthesizer of noble metal nanoparticles (NPs) are less known. However, some reports showing efficient synthesis of metal nanoparticles, mainly silver nanoparticles and its remarkable antimicrobial activity against bacterial and fungal pathogens of humans and plants. The nanoparticles synthesized from mycoendophytes present stability, polydispersity, and biocompatibility. These are non-toxic to humans and environment, can be gained in an easy and cost-effective manner, have wide applicability and could be explored as promising candidates for a variety of biomedical, pharmaceutical, and agricultural applications. Mycogenic silver nanoparticles have also demonstrated cytotoxic activity against cancer cell lines and may prove to be a promising anticancer agent. The present review focuses on the biological synthesis of metal nanoparticles from mycoendophytes and their application in medicine. In addition, different mechanisms of biosynthesis and activity of nanoparticles on microbial cells, as well as toxicity of these mycogenic metal nanoparticles, have also been discussed.     

Low-shear-modeled microgravity-grown Penicillium chrysogenum -mediated biosynthesis of silver nanoparticles with enhanced antimicrobial activity and its anticancer effect in human liver cancer and fibroblast cells

Gravitational force and shear forces induce various changes in gene expression and metabolite production of microorganisms. Previous reports have shown that there are differences in the expression of different sets of proteins and enzymes under microgravity conditions compared to normal gravity. The aim of this study is to utilize culture filtrates of Penicillium chrysogenum grown under microgravity and normal conditions to synthesize silver nanoparticles and to examine whether there is any difference between their physiochemical and biological function. Synthesized nanoparticles were characterized using UV–Vis spectroscopy, FTIR, XRD, and TEM. Biological functional studies such as antimicrobial activity, cytotoxic studies, and anticancer activity were carried out. Antimicrobial activity was tested using antibiotic susceptibility testing by Kirby–Bauer method and cytotoxicity tests were carried out using 3T3-L1 normal fibroblasts cells and Hep-G2 cancer cell lines. Interestingly, our results indicated that microgravity-synthesized silver nanoparticles possess enhanced antibacterial activity and cytotoxic effect against cancer cells compared to normal gravity-synthesized silver nanoparticle. This work highlighted the importance of gravitational vector on the fungal enzyme profiles and their role in silver nanoparticle synthesis with enhanced biological activity.     

Broad-spectrum bioactivities of silver nanoparticles: the emerging trends and future prospects

There are alarming reports of growing microbial resistance to all classes of antimicrobial agents used against different infections. Also the existing classes of anticancer drugs used against different tumours warrant the urgent search for more effective alternative agents for treatment. Broad-spectrum bioactivities of silver nanoparticles indicate their potential to solve many microbial resistance problems up to a certain extent. The antibacterial, antifungal, antiviral, antiprotozoal, acaricidal, larvicidal, lousicidal and anticancer activities of silver nanoparticles have recently attracted the attention of scientists all over the world. The aim of the present review is to discuss broad-spectrum multifunctional activities of silver nanoparticles and stress their therapeutic potential as smart nanomedicine. Much emphasis has been dedicated to the antimicrobial and anticancer potential of silver nanoparticles showing their promising characteristics for treatment, prophylaxis and control of infections, as well as for diagnosis and treatment of different cancer types.     

Aqueous extract of gum olibanum (Boswellia serrata): A reductant and stabilizer for the biosynthesis of antibacterial silver nanoparticles

A simple and ecofriendly biosynthetic process has been developed for silver nanoparticles using the aqueous extract of gum olibanum (Boswellia serrata), a renewable natural plant biopolymer. The water soluble compounds in the gum serve as dual functional reducing and stabilizing agents. The effect of concentration of gum and silver nitrate; and reaction time on nanoparticle synthesis was studied. The UV–visible spectroscopy, transmission electron microscopy and X-ray diffraction techniques were used to characterize the synthesized nanoparticles. By tuning the reaction conditions, size controlled spherical nanoparticles of around 7.5 ± 3.8 nm was achieved. Using Fourier transform infrared spectroscopy and Raman spectroscopy, a probable mechanism involved in reduction and stabilization of nanoparticles has been explained. The produced silver nanoparticles exhibited substantial antibacterial activity on both the Gram classes of bacteria. By virtue of being biogenic and encapsulated with proteins, these surface functionalized nanoparticles can be easily integrated for various biological applications.     

Functionalization of biomolecules on nanoparticles: specialized for antibacterial applications

Biological efficiency of existing antimicrobial agents is still inadequate to ensure optimal therapeutic index. Developing biocompatible advanced functional materials with antimicrobial properties could be promising for environmentally benign applications. Nanoparticles and other nanoscale materials are of great interest due to their multiple potential applications in material science, medicine, and industry. Nanomaterials possess well renowned antimicrobial activity against several microorganisms; however, it has some non-specific toxicity. Biofunctionalization of nanomaterials is one such topic to address this issue. Rational selection of therapeutically active biomolecules for design of nanoparticles will certainly increase the biological applicability. The present paper describes the current status of different types of biofunctionalized nanoparticles and their antibacterial applications. Key principles such as strategies involved at bio-/nanointerface, the structural activity relationship, and mechanism of action involved in the antibacterial activity of functionalized nanoparticles are briefly discussed. This knowledge is important from the objective of generation of advanced functional nanomaterials with antimicrobial properties.     

Antioxidant,cytotoxic and antibacterial potentials of biosynthesized silver nanoparticles using bee’s honey from two different floral sources in Saudi Arabia

Nowadays, the innovative study of silver nanoparticles (AgNPs) is excessive since they have incredible biomedical applications. The current study aimed to find out the potential of honey from two different floral sources (Ziziphus spina-christi and Acacia gerrardii) as biogenic mediators to synthesize AgNPs and to evaluate their antioxidant, cytotoxic and antimicrobial abilities. Biogenic AgNPs were studied for particle characterizations and the expected biomolecules helped in the reduction process of silver (Ag) ions to AgNPs. Results demonstrated different sizes (50–98 nm) and potentials −42 and −40 for AgNPs prepared using different biological materials, therefore different 1,1-Diphenyl-2-picrylhydrazyl (DPPH) scavenging free radicals were observed. Cytotoxic effect in a dose-dependent manner was detected against HepG2 ca cells for biogenic AgNPs resulted from cell apoptosis that detected by caspase 3/7 activation and AO/EB staining in the treated cells compared to their corresponding controls. Furthermore, biogenic AgNPs suppressed the growth of Methicillin-resistant bacteria Staphylococcus aureus (Gram-positive) besides Escherichia coli and Peseudomonas aeruginosa (Gram-negative). The IC₅₀ of AgNPs was between 15.8 and 14.1 μg/mL and the antibacterial capability was between 22.8 ± 1.2 and 17.0 ± 0.1 mm. Bacterial membrane disturbance was evident in the current study when treated bacteria were studied by field emission scanning electron microscopy (FE-SEM) in relation to untreated controls. Overall, the present findings indicated the possibility of simple green synthesis of AgNPs using bee’s honey, which are effective agents in some biomedical applications. Detailed future work is needed to further validate the results.     

Formation of silver nanoparticles in water samples from Antarctic Lake Untersee

Comparative integral assessment of the biological characteristics of the water samples of ice-covered Antarctic Lake Untersee was carried out using a new nanobiotechnological approach based on registration of biogenic nanoparticles of reduced silver Ag⁰. Formation of reduced silver nanoparticles occurred in all samples containing aboriginal microorganisms, while nanoparticles were not formed in the samples from which bacterial cells were removed. Size distribution of biogenic silver nanoparticles varied in the samples from five water horizons. The method proposed provides for rapid detection of live microbiological objects in the samples by detection of formation of biogenic nanoparticles of reduced silver. The method was termed OBNG (Observation of Biogenic Nanoparticles Growth).     

Antimicrobial activity of chemically and biologically synthesized silver nanoparticles against some fish pathogens

Pathogens isolated from fish appear to possess considerable antimicrobial resistance and represent a problem for the economy and public health. Natural antimicrobial substitutes to traditional antibiotics represent an essential tool in the fight against antibiotic resistance. Nanotechnology has shown considerable potential in different research fields, and the antimicrobial properties of silver nanoparticles are known. Silver has been used for medical purposes since ancient times because of its bactericidal properties, and the highly reactive surfaces of silver nanoparticles (AgNPs) indicate that they might have a function in antimicrobial applications. This work aimed to study the antimicrobial properties of biologically produced AgNPs from Origanum vulgare leaves compared to chemically produced AgNPs. Both types were characterized by UV–vis spectrophotometry, TEM, and dynamic light scattering and tested against three bacterial strains (Streptococcus agalactiae, and Aeromonas hydrophila, both isolated from Nile tilapia and Vibrio alginolyticus, isolated from sea bass) and three fungal strains (Aspergillus flavus, Fusarium moniliforme, and Candida albicans, all isolated from Nile tilapia). Disk diffusion test and evaluation of ultrastructure changes of tested microorganisms treated with AgNPs by transmission electron microscopy were performed. Moreover, the hemolytic properties of AgNPs were studied on chicken and goat red blood cells. The results obtained declare that the green biological production of silver nanoparticles is safer and more effective than the chemical one; moreover, AgNPs have interesting dose-dependent antimicrobial properties, with better results for biologically produced ones; their effectiveness against tested bacterial and fungal strains opens the way to their use to limit fish diseases, increase economy and improve human health.     

Biogenic synthesis of silver nanoparticles using Brazilian propolis

Biological methods have been used to synthesize silver nanoparticles through materials such as bacteria, fungi, plants, and propolis due to their reducing properties, stabilizer role and environmentally friendly characteristic. Considering the antimicrobial activity of propolis as well as the broad-spectrum antibacterial effects of silver nanoparticles, this study aim to describe the use of Brazilian propolis to synthesize silver nanoparticles (AgNP-P) and investigate its antimicrobial activity. The synthesis was optimized by factorial design, choosing the best conditions for smaller size particles. AgNP-P demonstrated a maximum absorbance at 412 nm in ultraviolet-visible spectra, which indicated a spherical format and its formation. Dynamic light scattering demonstrated a hydrodynamic size of 109 nm and polydispersity index less than 0.3, showing a good size distribution and stability. After its purification via centrifugation, microscopy analysis corroborates the format and showed the presence of propolis around silver nanoparticle. X-ray diffraction peaks were attributed to the main planes of the metallic silver crystalline structure; meanwhile infrared spectroscopy demonstrated the main groups responsible for silver reduction, represented by ∼22% of AgNP-P indicates by thermal analysis. Our product revealed an important antimicrobial activity indicating a synergism between propolis and silver nanoparticles as expected and promising to be an effective antimicrobial product to be used in infections.     

Bacteria and nanosilver: the quest for optimal production

Silver nanoparticles (AgNPs) have potential uses in many applications, but current chemical production methods are challenged by scalability, limited particle stability, and the use of hazardous chemicals. The biological processes present in bacteria to mitigate metallic contaminants in their environment present a potential solution to these challenges. Before commercial exploitation of this technology can be achieved, the quality of bacteriogenic AgNPs needs to be improved for certain applications. While the colloidal and morphological stabilities of biogenic AgNPs are widely regarded as superior to chemogenic particles, little control over the synthesis of particle morphologies has been achieved in biological systems. This article reviews a range of biosynthetic reaction conditions and how they affect AgNP formation in bacteria to understand which are most influential. While there remains uncertainty, some general trends are emerging: higher Ag⁺ concentrations result in higher AgNP production, up to a point at which the toxic effects begin to dominate; the optimal temperature appears to be heavily species-dependent and linked to the optimal growth temperature of the organism. However, hotter conditions generally favor higher production rates, while colder environments typically give greater shape diversity. Little attention has been paid to other potentially important growth conditions including halide concentrations, oxygen exposure, and irradiation with light. To fully exploit biosynthetic production routes as alternatives to chemical methods, hurdles remain with controlling particle morphologies and require further work to elucidate and harness them. By better understanding the factors influencing AgNP production, a foundation can be laid from which shape-controlled production can be achieved.     

Silver nanoparticles as a new generation of antimicrobials

Silver has been in use since time immemorial in the form of metallic silver, silver nitrate, silver sulfadiazine for the treatment of burns, wounds and several bacterial infections. But due to the emergence of several antibiotics the use of these silver compounds has been declined remarkably. Nanotechnology is gaining tremendous impetus in the present century due to its capability of modulating metals into their nanosize, which drastically changes the chemical, physical and optical properties of metals. Metallic silver in the form of silver nanoparticles has made a remarkable comeback as a potential antimicrobial agent. The use of silver nanoparticles is also important, as several pathogenic bacteria have developed resistance against various antibiotics. Hence, silver nanoparticles have emerged up with diverse medical applications ranging from silver based dressings, silver coated medicinal devices, such as nanogels, nanolotions, etc.     

The effect of temperature on antibacterial activity of biosynthesized silver nanoparticles

The purpose of this study was the evaluation of two different temperatures on antibacterial activity of the biosynthesized silver nanoparticles. 38 silver nanoparticles-producing bacteria were isolated from soil and identified. Biosynthesis of silver nanoparticles by these bacteria was verified through visible light spectrophotometry. Two strains were relatively active for production of silver nanoparticles. These strains were subjected for molecular identification and recognized as Bacillus sp. and Acinetobacter schindleri. In the present study, the effect of temperatures was evaluated on structure and antimicrobial properties of the silver nanoparrticles by transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis and antimicrobial Agar well diffusion methods. The silver nanoparticles showed antibacterial activity against all the pathogenic bacteria; however, this property was lost after treatment of the silver nanoparticles by high temperatures (100 and 300 °C). TEM images showed that the average sizes of heated silver nanoparticles were >100 nm. However, these were <100 nm for non-heated silver nanoparticles. Although, XRD patterns showed the crystalline structure of heated silver nanoparticles, their antibacterial activities were less. This was possible because of the sizes and accordingly less penetration of the particles into the bacterial cells. In addition, elimination of the capping agents by heat might be considered another reason.     

Synthesis,characterization, and antibacterial properties of silver nanoparticles-graphene and graphene oxide composites

In the field of nanotechnology, silver nanoparticles have been considered a promising antibacterial material for a century. The potential applications of graphene-based materials are increasingly recognized for their special physico-chemical and biological properties. In particular, graphene and graphene oxide as the foundation of nanocomposites have garnered much interest among researchers in many fields. In this review, we concentrate on different aspects of silver nanoparticle composites with graphene and graphene oxide, focusing on their synthesis methods, special characteristics, and antibacterial properties; we also briefly discuss limitations and future research.     

Green synthesis of silver nanoparticles by Conocarpus Lancifolius plant extract and their antimicrobial and anticancer activities

Due to drug addiction and the emergence of antibiotic resistance in pathogens, the disease load and medication intake have risen worldwide. The alternative treatment for drug-resistant infections is Nano formulation-based antimicrobial agents. The plant extract of Conocarpus Lancifolius fruits was used to synthesize silver nanoparticles in the current study, and it was further employed as an antimicrobial and anticancer agent. Nanoparticles have been characterized by UV–visible spectrometer revealed the notable peak of λ_max = 410–442 nm, which confirms the reduction of silver ion to elemental silver nanoparticles, and the biological moieties in the synthesis were further confirmed by FTIR analysis. The stability and crystalline nature of materials were approved by XRD analysis and expected the size of the nanomaterials of 21 to 173 nm analyzed by a nanophox particle-size analyzer. In vitro, synthesized materials act as an antibacterial agent against Streptococcus pneumonia and Staphylococcus aureus. The inhibition zones of 18 and 24 mm have been estimated to be antibacterial activity against both bacteria. The potency of up to 100% of AgNPs for bacterial strains was incubated overnight at 60 μg/ml. Based on our results, biogenic AgNPs reveal significant activity against fungal pathogen Rhizopusus stolonifera and Aspergillus flavus that cause leading infectious diseases. Additionally, nanomaterials were biocompatible and demonstrated the potential anticancer activities against MDA MB-231 cells after 24-hour exposure.     

Mechanistic aspects in the biogenic synthesis of extracellular metal nanoparticles by peptides,bacteria, fungi,and plants

Metal nanoparticles have been studied and applied in many areas including the biomedical, agricultural, electronic fields, etc. Several products of colloidal silver are already on the market. Research on new, eco-friendly and cheaper methods has been initiated. Biological production of metal nanoparticles has been studied by many researchers due to the convenience of the method that produces small particles stabilized by protein. However, the mechanism involved in this production has not yet been elucidated although hypothetical mechanisms have been proposed in the literature. Thus, this review discusses the various mechanisms provided for the biological synthesis of metal nanoparticles by peptides, bacteria, fungi, and plants. One thing that is clear is that the mechanistic aspects in some of the biological systems need more detailed studies.