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Bacteriogenic synthesis of metal nanoparticles is ecofriendly and greatly influenced by physico-chemical reaction parameters with respect to shape and size. Thus, present work aimed to synthesize and optimization of bacteriogenic gold nanoparticles (AuNPs) and study their antioxidant activity. Acinetobacter sp. cells were able to synthesize AuNPs, when challenged with tetra-chloroauric acid (HAuCl4). By physicochemical optimization, maximum synthesis was obtained with 72 h old culture using 2.1 × 109 CFU/ml cell density. Whereas, pH-7 is suitable for AuNPs synthesis. HAuCl4 concentration (0.5 mM) enhanced the formation of monodispersed and spherical nanoparticles (15 ± 10 nm). At 37°C temperature, Acinetobacter sp. released nanoparticles in supernatant. From characterization, AuNPs were found to be crystalline in nature with negative surface charge. AuNPs showed up to 86% different radical scavenging ability, exhibiting antioxidant activity. In conclusion, spherical AuNPs can be synthesized using Acinetobacter sp. through physicochemical optimization. This is the first report of antioxidant activity exhibited by monodispersed bacteriogenic AuNPs synthesized using Acinetobacter sp.

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Acinetobacter baumannii has emerged as one of the major nosocomial pathogens implicated in variety of severe infections and mortality. It is rapidly developing multi-drug resistance and also possesses surface colonization ability, which make it most difficult to treat through traditional antibiotics. This is an extensive study to describe the antibacterial activity of bacteriagenic silver nanoparticles (AgNPs) against A. baumannii AIIMS 7 in planktonic and biofilm mode. Minimum inhibitory concentration of antibiotics were in the range of 1 to 4096 µg/ml whereas AgNPs inhibited planktonic bacteria at concentration of 16 µg/ml. Fractional inhibitory concentration index revealed the synergistic interaction of AgNPs with doxycycline, tetracycline and erythromycin. Nanoparticles exhibited significant biofilm disruption activity with minimum biofilm eradication concentration of 2 mg/ml. Eradication of mature biofilm was enhanced on exposure to combination of AgNPs and antibiotics. These nanoparticles affected bacterial growth and distorted cellular morphology. Intracellular oxidative stress, induced in presence of AgNPs, also rendered bacteria susceptible to killing by nanoparticles. Besides this, AgNPs were found to interact with thiol-groups, which indicate their potential to interact with cellular proteins to exhibit antimicrobial activity.

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Synthesis of nanoparticles is an enzymatic reduction process in microorganisms. In the present study, a protein, lignin peroxidase has been purified by DEAE-Cellulose anion exchange chromatography and Biogel P-150 gel filtration chromatography from the cell suspension of Acinetobacter sp. SW30 responsible for the synthesis of gold nanoparticles (AuNP) and selenium nanoparticles (SeNP). The purified fraction has a specific activity of 29.4U/mg/min with 959 fold purification. Native and SDS PAGE confirmed that purified lignin peroxidase is monomeric enzyme with 97.4KDa molecular weight. The enzyme synthesized spherical crystalline AuNP (10±2nm) and amorphous SeNP (100±10nm). It has maximum activity at pH 2 and temperature 40°C, with 1.0mMKm value, when n-propanol was used as a substrate. Activity was completely inhibited by sodium thiosulphate and zinc sulphate. This is the first report on association of lignin peroxidase in the synthesis of AuNP and SeNP from Acinetobacter sp. SW30.

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Metals present in environment render the bacteria to attain certain resistance machinery to survive, one of which is transformation of metal ions to nano forms. Various enzymes and proteins have been suggested to play significant role in synthesis of silver nanoparticles (AgNPs) in bacteria. In present study, we have purified lignin peroxidase from secreted enzyme extract of Acinetobacter sp. employing diethyl aminoethyl cellulose ion exchange and Biogel P-150 gel filtration column chromatography. The purified fraction has a specific activity of 1.571 U/mg with substrate n-propanol and 6.5-fold purification. The tetrameric enzyme, with molecular weight of 99 kDa, consisted of dimers of two polypetides of 23.9 and 24.6 kDa as revealed by native and SDS-PAGE. On exposure to purified enzyme, spherical polydispersed AgNPs of ~ 50 nm were obtained as observed under transmission electron microscope. Optimum activity of the purified enzyme was obtained at pH 2 and 60 °C with n-propanol as substrate. This is the first report describing the reduction of extracellular silver ions by lignin peroxidase purified from Acinetobacter sp.

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The aim of this study was to synthesize selenium nanoparticles (SeNPs) using cell suspension and total cell protein of Acinetobacter sp. SW30 and optimize its synthesis by studying the influence of physiological and physicochemical parameters. Also, we aimed to compare its anticancer activity with that of chemically synthesized SeNPs in breast cancer cells. Cell suspension of Acinetobacter sp. SW30 was exposed to various physiological and physicochemical conditions in the presence of sodium selenite to study their effects on the synthesis and morphology of SeNPs. Breast cancer cells (4T1, MCF-7) and noncancer cells (NIH/3T3, HEK293) were exposed to different concentrations of SeNPs. The 18 h grown culture with 2.7×109 cfu/mL could synthesize amorphous nanospheres of size 78 nm at 1.5 mM and crystalline nanorods at above 2.0 mM Na2SeO3 concentration. Polygonal-shaped SeNPs of average size 79 nm were obtained in the supernatant of 4 mg/mL of total cell protein of Acinetobacter sp. SW30. Chemical SeNPs showed more anticancer activity than SeNPs synthesized by Acinetobacter sp. SW30 (BSeNPs), but they were found to be toxic to noncancer cells also. However, BSeNPs were selective against breast cancer cells than chemical ones. Results suggest that BSeNPs are a good choice of selection as anticancer agents.

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Cell biomass and metal salt concentration have great influence on morphology of biosynthesized nanoparticle. The aim of present study was to evaluate the effect of varying cell density and gold salt concentrations on synthesis of nanoparticles and its morphology, which has not been studied in bacteria till now. When cells of Acinetobacter sp. SW30 were incubated with different cell density and gold chloride concentrations, tremendous variation in color of colloidal solution containing gold nanoparticles (AuNP) was observed indicating variation in their size and shapes. Surprisingly, monodispersed spherical AuNP of size ~19 nm were observed at lowest cell density and HAuCl4 salt concentration while increase in cell number resulted in formation of polyhedral AuNP (~39 nm). Significance of this study lays in the fact that the shape and dispersity of AuNP can be customized depending up on the requirement. FTIR spectrum revealed shift from 3221 to 3196 cm-1 indicating the presence and role of amino acids in Au3+ reduction while possible involvement of amide I and II groups in stabilization of AuNP. The rate constant was calculated for cell suspension of 2.1 × 109 cfu/ml challenged with 1.0 mM HAuCl4, incubated at 30 °C and pH 7 using the slopes of initial part of the plot log (Aα - At) versus time as 1.99 × 10-8 M. Also, this is the first study to report the kinetics of gold nanoparticle synthesis by Acinetobacter sp. SW30.

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Selenium nanoparticles (SeNPs) are gaining importance in the field of medicine owing to their antibacterial and anticancer properties. SeNPs are biocompatible and non-toxic compared to the counterparts, selenite (SeO3 (-2)) and selenate (SeO4 (-2)). They can be synthesized by physical, chemical, and biological methods and have distinct bright orange-red color. Biogenic SeNPs are stable and do not aggregate owing to natural coating of the biomolecules. Various hypotheses have been proposed to describe the mechanism of microbial synthesis of SeNPs. It is primarily a two-step reduction process from SeO4 (-2) to SeO3 (-2) to insoluble elemental selenium (Se(0)) catalyzed by selenate and selenite reductases. Phenazine-1-carboxylic acid and glutathione are involved in selenite reduction. Se factor A (SefA) and metalloid reductase Rar A present on the surface of SeNPs confer stability to the nanoparticles. SeNPs act as potent chemopreventive and chemotherapeutic agents. Conjugation with antibiotics enhances their anticancer efficacy. These also have applications in nanobiosensors and environmental remediation.

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Silver nanoparticles (AgNPs) have received tremendous attention due to their significant antimicrobial properties. Large numbers of reports are available on the physical, chemical, and biological syntheses of colloidal AgNPs. Since there is a great need to develop ecofriendly and sustainable methods, biological systems like bacteria, fungi, and plants are being employed to synthesize these nanoparticles. The present review focuses specifically on bacteria-mediated synthesis of AgNPs, its mechanism, and applications. Bacterial synthesis of extra- and intracellular AgNPs has been reported using biomass, supernatant, cell-free extract, and derived components. The extracellular mode of synthesis is preferred over the intracellular mode owing to easy recovery of nanoparticles. Silver-resistant genes, c-type cytochromes, peptides, cellular enzymes like nitrate reductase, and reducing cofactors play significant roles in AgNP synthesis in bacteria. Organic materials released by bacteria act as natural capping and stabilizing agents for AgNPs, thereby preventing their aggregation and providing stability for a longer time. Regulation over reaction conditions has been suggested to control the morphology, dispersion, and yield of nanoparticles. Bacterial AgNPs have anticancer and antioxidant properties. Moreover, the antimicrobial activity of AgNPs in combination with antibiotics signifies their importance in combating the multidrug-resistant pathogenic microorganisms. Multiple microbicidal mechanisms exhibited by AgNPs, depending upon their size and shape, make them very promising as novel nanoantibiotics.

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Resistance among mycobacteria leading to multidrug-resistant and extensively drug-resistant tuberculosis is a major threat. However, nanotechnology has provided new insights in drug delivery and medicine development. This is the first comparative report to determine the activity of chemically and biologically synthesised silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) against mycobacteria. Screening data revealed the high mycobactericidal efficiency of AgNPs, with minimum inhibitory concentrations (MICs) of <3µg/mL, whereas no such activity was exhibited by AuNPs at concentrations up to 100µg/mL. Moreover, in vitro and ex vivo THP-1 infection model assays showed greater efficacy of chemical AgNPs compared with biogenic AgNPs to inhibit active and dormant stage mycobacterial growth. Up to 40% cytotoxicity against human cell lines was observed at a AgNP concentration of 10× MIC (30µg/mL) after 48h. AgNPs were shown to have more specificity towards mycobacteria than towards other Gram-negative and Gram-positive pathogenic bacteria. The selectivity index was found to be in the range of 11-23, indicating the potential of these nanoparticles for use in developing new therapeutics for tuberculosis.

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Gold nanoparticles have enormous applications in cancer treatment, drug delivery and nanobiosensor due to their biocompatibility. Biological route of synthesis of metal nanoparticles are cost effective and eco-friendly. Acinetobacter sp. SW 30 isolated from activated sewage sludge produced cell bound as well as intracellular gold nanoparticles when challenged with HAuCl4 salt solution. We first time report the optimization of various physiological parameters such as age of culture, cell density and physicochemical parameters viz HAuCl4 concentration, temperature and pH which influence the synthesis of gold nanoparticles. Gold nanoparticles thus produced were characterized by various analytical techniques viz. UV-Visible spectroscopy, X-ray diffraction, cyclic voltammetry, transmission electron microscopy, selected area electron diffraction, high resolution transmission electron microscopy, environmental scanning electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy and dynamic light scattering. Polyhedral gold nanoparticles of size 20 ± 10 nm were synthesized by 24 h grown culture of cell density 2.4 × 10(9) cfu/ml at 50 °C and pH 9 in 0.5 mM HAuCl4. It was found that most of the gold nanoparticles were released into solution from bacterial cell surface of Acinetobacter sp. at pH 9 and 50 °C.

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