Kinetic Modelling of Selenite Reduction by Enterococcus Spp in Batch Reactors

Selenite (SeO32-) is a toxic selenium oxyanion which readily bio-accumulates in the food chain. Enterococcus species were found to reduce SeO32- to elemental selenium (Se0) more rapidly (in 1 h) as compared to other known selenite reducing bacteria. The kinetics of the reduction by the Enterococcus species was investigated in aerobic batch reactors. The data fitted to the kinetic models was obtained from the reduction of various SeO32- concentrations under established optimum conditions (3.5 h, 35±2 °C, pH = 8). The results of these biological experiments were modelled and the biokinetic parameters were estimated with a first order kinetic model for selenite reduction and elemental selenium accumulation. The AQUASIM software for the simulation of aquatic systems was used for generating the models. The estimated parameters for selenite reduction and elemental selenium formation by the biotic system were the reaction rate constant of k= 0.562 h-1 and yield coefficient, YSe0/Se4+ = 0.761. The yield coefficient for the amount of glucose removed per mM of SeO32- , YGlc/Se4+, was not constant and depended of the concentration of selenite being reduced. The model presented in this work was able to fit the experimental data but more work still has to be done in order to develop a more concise model

Optimisation of selenite reduction using Enterococcus spp. under anaerobic conditions

With the drastic increase in selenium-releasing anthropogenic activities, the bioaccumulation of toxic selenium oxyanion in the environment has increased significantly, posing deleterious effects to living organisms. The bio-removal of selenite by Enterococcus spp. under both aerobic and anaerobic batch conditions was examined using batch reactors containing Tryptone Soy Broth (TSB) laced with sodium selenite. The optimal conditions for bacterial growth and selenite reduction were determined under anoxic conditions. Enterococcus spp. reduced selenite more effectively under anaerobic conditions, with 77 % selenite reduction after 20 h. The bacteria also reduced selenite under aerobic conditions, but the process was much slower, with approximately 68 % reduction after 72 h. This correlated with the more rapid bacterial growth seen in the anaerobic batch reactor. Optimal growth conditions for Enterococcus spp. were obtained at a temperature of 25°C, pH of 6 and initial selenite concentration of 30 mg/L. Optimal selenite removal was at temperature of 35°C, pH of 7, and initial selenite concentrations of 30 mg/L. A red precipitate was formed during the selenite removal experiments, a probable sign that the selenite was being reduced to form elemental selenium. The results of this study demonstrated the capability of Enterococcus spp. to bio-transform toxic selenite oxyanions under both oxic and anoxic conditions.http://www.aidic.it/cetam2023Chemical Engineerin

Performance Evaluation of Selenite (SeO32−) Reduction by Enterococcus spp.

Lactic acid bacteria (LAB) such as Enterococcus spp. have an advantage over several bacteria because of their ability to easily adapt to extreme conditions which include high temperatures, highly acidic or alkaline conditions and toxic metals. Although many microorganisms have been shown to reduce selenite (SeO32−) to elemental selenium (Se0), not much work has been done on the combined effect of Enterococcus spp. In this study, aerobic batch reduction of different selenite concentrations (1, 3 and 5 mM) was conducted using Enterococcus hermanniensis sp. and Enterococcus gallinarum sp. (3.5 h, 35 ± 2 °C, starting pH > 8.5). Results from the experiments showed that the average reductions rates were 0.608, 1.921 and 3.238 mmol·(L·h)−1, for the 1, 3 and 5 mM SeO32− concentrations respectively. In addition, more selenite was reduced for the 5 mM concentration compared to the 1 and 3 mM concentrations albeit constant biomass being used for all experiments. Other parameters which were monitored were the glucose consumption rate, protein variation, pH and ORP (oxidation reduction potential). TEM analysis was also conducted and it showed the location of electron-dense selenium nanoparticles (SeNPs). From the results obtained in this study, the authors concluded that Enterococcus species’s high adaptability makes it suitable for rapid selenium reduction and biosynthesis of elemental selenium

Harnessing selenium nanoparticles (SeNPs) for enhancing growth and germination, and mitigating oxidative stress in Pisum sativum L.

Abstract Selenium, an essential micronutrient for plants and animals, can cause selenium toxicity as an oxyanion or at elevated doses. However, the toxic selenite (SeO3 2−) oxyanion, can be converted into less harmful elemental nano-selenium (Se0), with various practical applications. This research aimed to investigate two methods for reducing SeO3 2−: abiotic reduction using cell-free extract from Enterococcus spp. (abiotic-SeNPs) and chemical reduction involving L-ascorbic acid (chemical-SeNPs). Analysis with XPS confirmed the presence of Se0, while FTIR analysis identified surface functional groups on all SeNPs. The study evaluated the effects of SeO3 2−, abiotic-SeNPs, and chemical-SeNPs at different concentrations on the growth and germination of Pisum sativum L. seeds. SeO3 2− demonstrated detrimental effects on germination at concentrations of 1 ppm (germination index (GI) = 0.3). Conversely, both abiotic- and chemical-SeNPs had positive impacts on germination, with GI > 120 at 10 ppm. Through the DPPH assay, it was discovered that SeNPs exhibited superior antioxidant capabilities at 80 ppm, achieving over 70% inhibition, compared to SeO3 2− (less than 20% inhibition), therefore evidencing significant antioxidant properties. This demonstrates that SeNPs have the potential to be utilized as an agricultural fertilizer additive, benefiting seedling germination and development, while also protecting against oxidative stress

Enterococcus spp. Cell-Free Extract: An Abiotic Route for Synthesis of Selenium Nanoparticles (SeNPs), Their Characterisation and Inhibition of Escherichia coli

Selenite (SeO32−), the most toxic and most reactive selenium (Se) oxyanion, can be reduced to elemental selenium (Se0) nanoparticles by a variety of bacteria, including Enterococcus spp. Previously, the orthodox view held that the reduction of SeO32− to Se0 by a wide range of bacteria was solely accomplished by biological processes; however, recent studies have shown that various bacterial strains secrete metal-reducing metabolites, thereby indirectly catalysing the reduction of these metal species. In the current study, selenium nanoparticles were synthesised from the abiotic reduction of selenite with the use of Enterococcus spp. cell-free extract. Once separated from the cell-free extract, the particles were analysed using Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Transmission electron microscopy (TEM) and a Zetasizer. The results revealed that the SeNPs were spherical in shape, containing both amorphous and crystalline properties, and the sizes with the highest frequency ranged close to 200 nm. Additionally, the obtained nanoparticles exhibited antimicrobial properties by directly inhibiting the viability of an E. coli bacterial strain. The results demonstrate not only the potential of abiotic production of SeNPs, but also the potential for these particles as microbial inhibitors in medical or similar fields