Screening and safety evaluation of lactic acid bacteria with selenium adsorption capacity

Selenium, one of the essential trace elements in humans and animals, is involved in various biochemical and physiological functions, such as improving immunity and antioxidant properties. Lactic acid bacteria, recognized as "generally recognized as safe (GRAS)" in the world, have the ability to convert inorganic selenium into organic form and have plenty of outstanding probiotic properties, including regulation of gut microbiota, increase of antioxidant capacity, and decrease of intestinal inflammation and so on. Therefore, using them as a safe and efficient selenium supplementation carrier is increasingly becoming a hotspot in recent research. In order to obtain a strain of lactic acid bacteria with good selenium adsorption capacity, 43 strains of Lactobacillus of different species were screened by determining the adsorption rate of selenium in vitro. The adsorption conditions were as follows: The initial selenium concentration was 50 mg/L, the initial cell concentration was 1 g/L, and the adsorption time was 1 h. The selenium adsorption rate and selenium adsorption capacity of the strains were determined by measuring the selenium content of the dried bacteria after nitrolysis. The selenium levels were determinated by inductively coupled plasma mass spectrometer (ICP-MS). At the same time, the adsorption characteristics of the strain were studied by changing the initial pH, initial cell concentration, initial selenium concentration and adsorption time, and acid and bile salt tolerance were determined to confirm whether it can still maintain high activity in the gastrointestinal tract. Then the safety of the strain was evaluate by the acute toxicity test and the 30-d feeding experiment, as well as antibiotic resistance of the strain. The results showed that the Lactobacillus plantarum RS7-1 had the most excellent selenium adsorption ability among the 43 strains, and its adsorption rate for selenium was up to 40.79%±1.27%. The optimum pH of the strain was 5.0, and the optimal initial bacterial concentration was 2 g/L. The maximum adsorption capacity was reached when the initial selenium concentration was 20 mg/L. The adsorption of selenium can be divided into two stages, rapid adsorption stage and adsorption equilibrium stage. The equilibrium time was reached when the adsorption time was 60 min. At the same time, the strain has outstanding acid and bile salt tolerance, without species-specific antibiotic tolerance, and after safety evaluation, the mice fed with the strain shows no significant difference in the food consumption, body weight, biochemical index, hematological indexes of blood compared with mice in blank group, and no organ and tissue lesions were detected. In conclusion, Lactobacillus plantarum RS7-1 is a strain of lactic acid bacteria with superb selenium adsorption capacity, great acid and bile salt tolerance, without toxicity and side effect. This study provides an idea for the selenium supplement with probiotics as the carrier, and the selenium adsorption capacity can be further improved by optimization of adsorption conditions, mutation breeding and genetic engineering, thereby increasing the application value of selenium-enriched probiotic products in the future.