Salt-induced osmotic stress stimulates selenium biotransformation in Lactobacillus rhamnosus ATCC 53103

Lactic acid bacteria(LAB)have certain selenium(Se)-enriched effect, and biotransform inorganic selenium into organic selenium and Se0. Osmotic stress can promote selenium-enriched effect to some extent. The accumulation of selenium by Lactobacillus rhamnosus ATCC 53103 under osmotic stress at different cultivation conditions, including salt concentration, pH, temperature (°C), was investigated in this work. The morphology of Se-enriched bacteria was observed by transmission electron microscopy (TEM). The Design-Expert software for response surface methodology on a basis of single-factor experiment design was used to select the optimal condition that could improve Se enrichment efficiency. The results indicated that the optimum condition was as follows: 8% salt concentration, culture at pH 6.5 and 37 °C. Furthermore, under the optimal conditions, the ratio of Se enrichment was 65.35%, which increased by 14.7% compared with the unstressed group. In addition, the black microparticles inside and outside of the selenium-enriched L. rhamnosus were proved to be selenium element by energy spectrum, as observed by TEM. Analysis of proteins of cells by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) revealed that selenium was present in the proteins ranging from 23 to 36 kDa and 70 kDa. The selenium content increased after salt stress, which further proved that salt stress promoted the selenium enrichment of L. rhamnosus.