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.

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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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 2.7×10 9 cfu/mL could synthesize amorphous nanospheres of size 78 nm at 1.5 mM and crystalline nanorods at above 2.0 mM Na 2 SeO 3 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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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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Background The development of nontoxic methods of synthesizing nanoparticles is a major step in nanotechnology to allow their application in nanomedicine. The present study aims to biosynthesize silver nanoparticles (AgNPs) using a cell-free extract of Acinetobacter spp. and evaluate their antibacterial activity. Methods Eighteen strains of Acinetobacter were screened for AgNP synthesis. AgNPs were characterized using various techniques. Reaction parameters were optimized, and their effect on the morphology of AgNPs was studied. The synergistic potential of AgNPs on 14 antibiotics against seven pathogens was determined by disc-diffusion, broth-microdilution, and minimum bactericidal concentration assays. The efficacy of AgNPs was evaluated as per the minimum inhibitory concentration (MIC) breakpoints of the Clinical and Laboratory Standards Institute (CLSI) guidelines. Results Only A. calcoaceticus LRVP54 produced AgNPs within 24 hours. Monodisperse spherical nanoparticles of 8–12 nm were obtained with 0.7 mM silver nitrate at 70°C. During optimization, a blue-shift in ultraviolet-visible spectra was seen. X-ray diffraction data and lattice fringes ( d =0.23 nm) observed under high-resolution transmission electron microscope confirmed the crystallinity of AgNPs. These AgNPs were found to be more effective against Gram-negative compared with Gram-positive microorganisms. Overall, AgNPs showed the highest synergy with vancomycin in the disc-diffusion assay. For Enterobacter aerogenes , a 3.8-fold increase in inhibition zone area was observed after the addition of AgNPs with vancomycin. Reduction in MIC and minimum bactericidal concentration was observed on exposure of AgNPs with antibiotics. Interestingly, multidrug-resistant A. baumannii was highly sensitized in the presence of AgNPs and became susceptible to antibiotics except cephalosporins. Similarly, the vancomycin-resistant strain of Streptococcus mutans was also found to be susceptible to antibiotic treatment when AgNPs were added. These biogenic AgNPs showed significant synergistic activity on the β-lactam class of antibiotics. Conclusion This is the first report of synthesis of AgNPs using A. calcoaceticus LRVP54 and their significant synergistic activity with antibiotics resulting in increased susceptibility of multidrug-resistant bacteria evaluated as per MIC breakpoints of the CLSI standard.