Meenal Kowshik

Extracellular synthesis of silver nanoparticles by a silver-tolerant yeast strain MKY3

Silver nanoparticles in the size range of 2-5 nm were synthesized extracellularly by a silver-tolerant yeast strain MKY3, when challenged with 1 mM soluble silver in the log phase of growth. The nanoparticles were separated from dilute suspension by devising a new method based on differential thawing of the sample. Optical absorption, transmission electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy investigations confirmed that metallic (elemental) silver nanoparticles were formed. Extracellular synthesis of nanoparticles could be highly advantageous from the point of view of synthesis in large quantities and easy downstream processing.

Microbial Synthesis of Semiconductor PbS Nanocrystallites

The use of microbes as producers of semiconductor nanocrystals is demonstrated. When torulopsis yeast is challenged with lead, it builds intracellular spherical crystallites of PbS, 2-5 nm in diameter (see Figure for an HR-TEM image) and pure by X-ray diffraction. The crystals, which can be isolated by freeze-thawing, show a sharp absorption maximum at 330 nm, corresponding to a bandgap of 3.75 eV.

Green synthesis of lead sulfide nanoparticles by the lead resistant marine yeast, Rhodosporidium diobovatum

Biosynthesis of nanoparticles using microorganisms has attracted a lot of attention in recent years as this route has the potential to lead to synthesis of monodisperse nanoparticles. Here, we report the intracellular synthesis of stable lead sulfide nanoparticles by a marine yeast, Rhodosporidium diobovatum. The PbS nanoparticles were characterized by UV‐visible absorption spectroscopy, X‐ray diffraction (XRD) and energy dispersive atomic spectroscopy (EDAX). UV‐visible absorption scan revealed a peak at 320 nm, a characteristic of the nanosize range. XRD confirmed the presence of PbS nanoparticles of cubic structure. Crystallite size as determined from transmission electron microscopy was found to be in the range of 2–5 nm. Elemental analysis by EDAX revealed the presence of particles composed of lead and sulfur in a 1:2 ratio indicating that PbS nanoparticles were capped by a sulfur‐rich peptide. A quantitative study of lead uptake through atomic absorption spectrometry revealed that 55% of lead in the medium was accumulated in the exponential phase, whereas a further 35% was accumulated in the stationary phase; thus, the overall recovery of PbS nanoparticles was 90%. The lead‐exposed yeast displayed a marked increase (280% over the control) in nonprotein thiols in the stationary phase.

Biosynthesis of silver nanoparticles by marine bacterium, Idiomarina sp. PR58-8

Metal-tolerant microorganisms have been exploited in recent years to synthesize nanoparticles due to their potential to offer better size control through peptide binding and compartmentalization. In this paper, we report the intracellular synthesis of silver nanoparticles (SNPs) by the highly silver-tolerant marine bacterium, Idiomarina sp. PR58-8 on exposure to 5 mM silver nitrate. SNPs were characterized by UV-visible spectrophotometry, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). UV-visible absorption scan of a 48 h culture exposed to 5 mM silver nitrate revealed a broad peak at 450 nm indicative of the surface plasmon resonance of SNPs. XRD analysis confirmed the presence of elemental silver and the crystallite size was calculated to be 25 nm using Scherrer formula. The average particle size as per TEM analysis was found to be 26 nm. Metal stress is known to induce the production of non-protein thiols (NP–SHs) which sequester metal ions. In this study, the production of NP–SHs was followed from 6–48 h, wherein it was observed that the NP–SH levels in the silver-exposed culture were consistently higher (261% on an average) than in the unexposed culture.

Anti-biofilm efficacy of low temperature processed AgCl-TiO2 nanocomposite coating.

Biofilms are a major concern in the medical settings and food industries due to their high tolerance to antibiotics, biocides and mechanical stress. Currently, the development of novel methods to control biofilm formation is being actively pursued. In the present study, sol-gel coatings of AgCl-TiO2 nanoparticles are presented as potential anti-biofilm agents, wherein TiO2 acts as a good supporting matrix to prevent aggregation of silver and facilitates its controlled release. Low-temperature processed AgCl-TiO2 nanocomposite coatings inhibit biofilm formation by Escherichia coli, Staphylococcus epidermidis and Pseudomonas aeruginosa. In vitro biofilm assay experiments demonstrated that AgCl-TiO2 nanocomposite coated surfaces, inhibited the development of biofilms over a period of 10days as confirmed by scanning electron microscopy. The silver release kinetics exhibited an initial high release, followed by a slow and sustained release. The anti-biofilm efficacy of the coatings could be attributed to the release of silver, which prevents the initial bacterial adhesion required for biofilm formation.

Anti-quorum sensing activity of AgCl-TiO2 nanoparticles with potential use as active food packaging material.

To study the anti‐quorum sensing (anti‐QS) activity of AgCl‐TiO2 nanoparticles (ATNPs) and its mechanism.

Mechanisms of Metal Resistance and Homeostasis in Haloarchaea

Haloarchaea are the predominant microflora of hypersaline econiches such as solar salterns, soda lakes, and estuaries where the salinity ranges from 35 to 400 ppt. Econiches like estuaries and solar crystallizer ponds may contain high concentrations of metals since they serve as ecological sinks for metal pollution and also as effective traps for river borne metals. The availability of metals in these econiches is determined by the type of metal complexes formed and the solubility of the metal species at such high salinity. Haloarchaea have developed specialized mechanisms for the uptake of metals required for various key physiological processes and are not readily available at high salinity, beside evolving resistance mechanisms for metals with high solubility. The present paper seeks to give an overview of the main molecular mechanisms involved in metal tolerance in haloarchaea and focuses on factors such as salinity and metal speciation that affect the bioavailability of metals to haloarchaea. Global transcriptomic analysis during metal stress in these organisms will help in determining the various factors differentially regulated and essential for metal physiology.

In vivo synthesis of selenium nanoparticles by Halococcus salifodinae BK18 and their anti‐proliferative properties against HeLa cell line

Nanoparticles synthesis by bacteria and yeasts has been widely reported, however, synthesis using halophilic archaea is still in a nascent stage. This study aimed at the intracellular synthesis of selenium nanoparticles (SeNPs) by the haloarchaeon Halococcus salifodinae BK18 when grown in the presence of sodium selenite. Crystallographic characterization of SeNPs by X‐ray diffraction, Selected area electron diffraction, and transmission electron microscopy exhibited rod shaped nanoparticles with hexagonal crystal lattice, a crystallite domain size of 28 nm and an aspect ratio (length:diameter) of 13:1. Energy disruptive analysis of X‐ray analysis confirmed the presence of selenium in the nano‐preparation. The nitrate reductase enzyme assay and the inhibitor studies indicated the involvement of NADH‐dependent nitrate reductase in SeNPs synthesis and metal tolerance. The SeNPs exhibited good anti‐proliferative properties against HeLa cell lines while being non‐cytotoxic to normal cell line model HaCat, suggesting the use of these SeNPs as cancer chemotherapeutic agent. This is the first study on selenium nanoparticles synthesis by haloarchaea.

Anti-neoplastic selenium nanoparticles from Idiomarina sp. PR58-8

Selenium nanoparticles (SeNPs) with novel biological activities, cancer cell selectivity, and low toxicity towards normal cells have gained attention for chemo-therapeutic and chemo-preventive applications. These nanoparticles may be synthesized using micro-organisms, which is the green alternative of nanofabrication. Here we report the intracellular synthesis of SeNPs by the moderate halophilic bacterium, Idiomarina sp. PR58-8 using sodium selenite as the precursor. Characterization of SeNPs by XRD exhibited the characteristic Braggs peak of hexagonal selenium with a crystallite domain size of 34nm. Morphological characterization by TEM exhibited spherical nanoparticles with a size distribution of 150-350nm. The non-protein thiols were found to be involved in resistance/reduction of sodium selenite. The SeNPs exhibited selectivity in exerting cytotoxicity towards human cervical cancer cell line, HeLa, while being non-toxic towards model normal cell line, HaCaT. The SeNPs induced a caspase-dependent apoptosis in HeLa cell lines as exhibited by the ROS assay, apoptotic index assay, and western blot analysis. These results suggest the application of SeNPs synthesized by Idiomarina sp. PR58-8 as potential anti-neoplastic agents.

Laser scanning photothermal microscopy: fast detection and imaging of gold nanoparticles.

We report on the design and construction of a laser scanning photothermal microscope and present images of gold nanoparticles of size as small as 5 nm. Laser scanning method allows fast image acquisition at 80 μs pixel dwell time so that a 500 × 500 pixel image is acquired in 20 s. Photothermal imaging at fast time scales can have potential applications in variety of fields including tracking of biomolecular transport processes.