Delayed formation of zero-valent selenium nanoparticles by Bacillus mycoides SeITE01 as a consequence of selenite reduction under aerobic conditions

Abstract

BackgroundSelenite (SeO32−) oxyanion shows severe toxicity to biota. Different bacterial strains exist that are capable of reducing SeO32− to non-toxic elemental selenium (Se0), with the formation of Se nanoparticles (SeNPs). These SeNPs might be exploited for technological applications due to their physico-chemical and biological characteristics. The present paper discusses the reduction of selenite to SeNPs by a strain of Bacillus sp., SeITE01, isolated from the rhizosphere of the Se-hyperaccumulator legume Astragalus bisulcatus.ResultsUse of 16S rRNA and GyrB gene sequence analysis positioned SeITE01 phylogenetically close to B. mycoides. On agarized medium, this strain showed rhizoid growth whilst, in liquid cultures, it was capable of reducing 0.5 and 2.0 mM SeO32− within 12 and 24 hours, respectively. The resultant Se0 aggregated to form nanoparticles and the amount of Se0 measured was equivalent to the amount of selenium originally added as selenite to the growth medium. A delay of more than 24 hours was observed between the depletion of SeO32 and the detection of SeNPs. Nearly spherical-shaped SeNPs were mostly found in the extracellular environment whilst rarely in the cytoplasmic compartment. Size of SeNPs ranged from 50 to 400 nm in diameter, with dimensions greatly influenced by the incubation times. Different SeITE01 protein fractions were assayed for SeO32− reductase capability, revealing that enzymatic activity was mainly associated with the membrane fraction. Reduction of SeO32− was also detected in the supernatant of bacterial cultures upon NADH addition.ConclusionsThe selenite reducing bacterial strain SeITE01 was attributed to the species Bacillus mycoides on the basis of phenotypic and molecular traits. Under aerobic conditions, the formation of SeNPs were observed both extracellularly or intracellullarly. Possible mechanisms of Se0 precipitation and SeNPs assembly are suggested. SeO32− is proposed to be enzimatically reduced to Se0 through redox reactions by proteins released from bacterial cells. Sulfhydryl groups on peptides excreted outside the cells may also react directly with selenite. Furthermore, membrane reductases and the intracellular synthesis of low molecular weight thiols such as bacillithiols may also play a role in SeO32− reduction. Formation of SeNPs seems to be the result of an Ostwald ripening mechanism.

Bacillus cereus, selenite-reducing bacterium from contaminated lake of an industrial area: a renewable nanofactory for the synthesis of selenium nanoparticles

“…In the current study, the selenite reduction was noted only in the membrane protein fraction and supernatant. Thus, the results confirm that the extracellular proteins released by the bacterial cells and the proteins associated with membrane/cell wall are involved in the metalloid reduction (Lampis et al 2014). These proteins probably act as oxidation-reduction enzymes or proton antitransporters (Zhang et al 2011).…”

“…4 Time-dependent UV-Vis absorption spectra of selenium nanoparticles synthesized by strain AJK3 at various selenite concentrations: a 0.25 mM, b 0.5 mM, and c 1.0 mM comparison with the anaerobic bacteria, the phenomenon is mostly cited in aerobic, Gram +ve Bacillus species such as B. cereus (Dhanjal and Cameotra 2010), Bacillus sp. MSh-1 (Shakibaie et al 2015), B. megaterium (Mishra et al 2011), B. mycoides SeITE01 (Lampis et al 2014), Bacillus sp. E5 (Durán et al 2015), B. licheniformis (Khiralla and El-Deeb 2015), B. subtilis (Wang et al 2010), etc.…”

“…The selenite reductase activity was carried out at 37 °C for 24 h in 400 μL of supernatant or nutrient broth containing 100 μg of protein and 1 mM sodium selenite. The reaction mixtures without supernatant or protein fractions served as controls (Dhanjal and Cameotra 2010;Lampis et al 2014).…”

“…In addition, from the soil of a antimony mine from Lengshuijiang, southern China (Zheng et al 2014), soil of a magnesite mine from Salem, India (Ramya et al 2015), soil of a coal mine from West Bengal, India (Dhanjal and Cameotra 2010), selenium laden agricultural soil of North-East Punjab, India (Bajaj et al 2012), soil of mangrove forest from Bhitarkanika, Orissa, India (Mishra et al 2011), rhizosphere soil of a selenium hyperaccumulator legume grown in seleniferous mine from Sardina, Italy (Lampis et al 2014), rhizosphere of wheat grown in herbicide contaminated soil (Dwivedi et al 2013), and rhizosphere of cereal plants grown in ash-derived volcanic soil of southern Chile (Durán et al 2015). Others include rock fragments of black oil shale from Haenam, Korea (Tam et al 2010), food wastes collected from local market of Giza, Egypt (Khiralla and El-Deeb 2015), sub gingival dental plaque (Pearce et al 2008) etc.…”

Anticancer activity of biostabilized selenium nanorods synthesized by Streptomyces bikiniensis strain Ess_amA-1

“…[6][7][8][9] SNMs also exhibit unique physical, chemical, and biological properties compared to that of Se compounds. 10 Various types of SNMs, which are stabilized and modified with different kinds of biological macromolecules, are reported to possess excellent anticancer activities. 1,[11][12][13] Based on this, some researchers suggest that biological macromolecules stabilized and modified may have potential applications as anticancer agents for the killing of human cancer cells.…”