Physiological changes induced by selenium in a probiotic Lactobacillus reuteri strain: a proteomic study.

Selenium (Se) is an essential dietary trace element for humans and microorganisms. It generally may be considered an anticarcinogenic and antioxidant agent. Se deficiency is associated with various chronic diseases such as oxidative stress, cardiovascular diseases and some kinds of cancer. The use of Se-enriched yeasts and Lactobacilli is an interesting approach to solve selenium-defi- ciency: these microorganisms, grown in presence of inorganic forms of selenium, are able to convert it into more bio-available organic forms, introducing it into proteins (selenoproteins); so, if added to ice-creams or yogurt, organic Se forms are released at host gut level. In this study we focused our attention on Lactobacillus reuteri LB2 BM, isolated from human fecal samples, able to accumulate 10 fold more selenium than other strains reported in the literature. Our purpose was to understand, by means of a comparative proteomic approach, if selenium addition could modify protein expression causing variations in the metabolism of the strain. Two-dimensional electrophoresis followed by MALDI-TOF/TOF mass spectrometry was performed on L. reuteri grown in a MRS medium fortified with sodium selenite (4.38 mg/l) and in a control condition (same medium without selenium). An overall picture has been obtained analyzing the bacterial proteome in various cellular district (in toto proteome, membrane-cell wall enriched fraction, extracellular proteome) and in different pI range (4-7 and 6-11). Moreover, in order to understand the moment of selenium uptake and to establish the maximal toxic concentration determining a growth inhibition, a microwave assisted digestion procedure, used for the dissolution of biological samples, followed by ICP-MS, has been performed on samples collected at different times of the growth curve. This methodology is a powerful tool to evaluate quantitative changes of the metal level in the medium.