In vitro biosynthesis of organic selenium by Lactobacillus casei from inorganic selenium forms

Abstract

The bioconversion of selenium in the form of sodium selenite (Na2 SeO3 ) (SeIV) or sodium selenate (Na2 SeO4 ) (SeVI) to organic form by Lactobacillus casei (L. casei) was investigated in vitro. MRS media was supplemented with 1, 2, 5, 10 or 20 ppm of Se, inoculated with 2% starter culture with about 105 cfu/ ml of L. casei and then incubated up to 24 hrs at 37o C. Increasing of selenite “Se(IV)” concentrations in the media markedly reduced the bacterial growth compared to selenate “Se(VI)” indicating cytotoxic effect of selenite. The media supplemented with 5 ppm or more of Se(IV) became reddish after 24 hr of incubation as a result of the formation of 100-200 nanometer particles of selenium (SeNPs). Se speciation of the cultured media supernatants and its corresponding cell fractions was carried out by HPLC-ICP-MS technique. The bioconversion rate of Se to organic form by L. casei was extremely higher in Se(IV) than Se(VI) in both fractions, however the media supernatant contained the highest content. Increasing the media Se content resulted in gradual increase of organic Se concentration in both cells and supernatant fractions. The medium supplemented with 1 ppm Se(IV) was completely depleted from the inorganic Se as it completely converted to organic form. Although the cell fractions from all Se(VI) supplemented media contained only organic Se, the media supernatant contained significant residual amount of the inorganic form. Our results demonstrate the ability of L. casei to convert Se(IV) or Se(VI) up to 20 ppm to organic form(s) either in the cultured media or inside the bacterial cells. However, Se(IV) but not Se(VI), at a limit concentration of 1 ppm, was completely converted and accumulated in an organic form in the cell fraction and the cultured medium.