Title : Selenium biotransformation into Nano-Se by probiotics
Abstract:
This study investigated the participation of probiotic microorganisms in the synthesis of selenium nanoparticles (Nano-Se) and compared certain aspects of different mechanisms of Nano-Se synthesis by L. plantarum and B. subtilis.
To obtain the biogenic Nano-Se with the help of probiotic microorganisms, the nutrient medium was enriched with sodium selenite in different concentrations from 1 to 30 ppm for Se and cultivated in aerobic conditions using a shaker and compared with the corresponding samples that were cultivated in static conditions. The growth of bacteria was evaluated by Viable cell count, the formation of Nano-Se was evidenced by the change and intensity of red color, then confirmed by transmission (TEM) microscopy. The green synthesized Nano-Se from probiotic microorganisms were characterized by UV-visible spectroscopy. It was found that the number of viable L. plantarum IMB B-7679 during cultivation in the presence of 30 ppm Se in the composition of Na2SeO3 decreased by 1.5 times compared to the control and the appearance pink colors of the cultural environment were observed. According to TEM data, the intracellular pathway of Nano-Se formation by L. plantarum was proven. Visualized by TEM, Nano-Se synthesized by L. plantarum IMB B-7679 had a spherical shape and their size ranged from 150-180 nm. The same time B. Subtilis IMB B-7392 and IMB B-7393 displayed the tolerance against Na2SeO3 is due to the extracellular formations of Nano-Se, which were confirmed by TEM. Analysis TEM revealed the presence of electron-dense Nano-Se particles, and the formation of nanoagglomerates, which were localized extracellularly and had the average size of 120 ± 20 nm for B. subtilis IMB B-7392 and 370 ± 92.4 nm (±SD) for IMB B-7393. It should be noted that the extracellular biosynthesis of Nano-Se is preferable to the intracellular synthesis of Nano-Se, as it occurs outside the bacterial cell and it is cheaper to obtain Nano-Se.
The formation of Nano-Se was confirmed by UV-Vis absorption spectra, which showed absorption bands for Nano-Se produced by L. plantarum IMB B-7679 at 240 nm and Nano-Se formed by B. subtilis IMB B-7392, B-7393, respectively λ 230 and 251 nm. So, bacterial reduction of Na2SeO3 to Nano-Se is a promising method for obtaining selenium-enriched probiotics.
The capacity for intracellular Nano-Se production requires the maintenance of intracellular redox homeostasis to protect cells from oxidative damage. There is an assumption that glutathione is involved in the reduction of selenite in certain strains of lactobacillus [1; 2].
Meanwhile, Bacillus subtilis is a probiotic bacterium that has a different aerobic extracellular mechanism to reduce Se(IV) to Nano-Se. It is known that, cultures of B. Subtilis do not have the enzyme glutathione but they do have bacillithiol [3]. In addition, the reduction of selenite to biogenic Nano-Se by B. Subtilis occurs with the participation of thioredoxin reductase and reduced thiols, which are contained in microbial cells and can catalyze the reduction of selenite [4; 5; 6].
But the increase in the intensity of the red color of the medium culture of B. Subtilis IMB B-7393 under aerobic conditions, makes it possible to predict the involvement of additional enzymes to explain the biological reduction of SeO32−.
So, aeration is an essential parameter for the growth of Bacillus subtilis and L.plantarum cultures, selenium is involved in biotransformation processes under aerobic conditions, but the localization of Nano-Se are different, which may be related to different ways of its biotransformation. The inexpensive and ecological method of obtaining Nano-Se with the help of probiotics can be an alternative to chemical and physical methods of obtaining nanoforms.
