Abstract
Lactic acid bacteria (LAB) such as Enterococcus spp. have an advantage over several bacteria because of their ability to easily adapt to extreme conditions which include high temperatures, highly acidic or alkaline conditions and toxic metals. Although many microorganisms have been shown to reduce selenite (SeO32−) to elemental selenium (Se0), not much work has been done on the combined effect of Enterococcus spp. In this study, aerobic batch reduction of different selenite concentrations (1, 3 and 5 mM) was conducted using Enterococcus hermanniensis sp. and Enterococcus gallinarum sp. (3.5 h, 35 ± 2 °C, starting pH > 8.5). Results from the experiments showed that the average reductions rates were 0.608, 1.921 and 3.238 mmol·(L·h)−1, for the 1, 3 and 5 mM SeO32− concentrations respectively. In addition, more selenite was reduced for the 5 mM concentration compared to the 1 and 3 mM concentrations albeit constant biomass being used for all experiments. Other parameters which were monitored were the glucose consumption rate, protein variation, pH and ORP (oxidation reduction potential). TEM analysis was also conducted and it showed the location of electron-dense selenium nanoparticles (SeNPs). From the results obtained in this study, the authors concluded that Enterococcus species’s high adaptability makes it suitable for rapid selenium reduction and biosynthesis of elemental selenium.
Highlights
Selenium (Se) is an essential micronutrient for both plants and animals
In order to study the mechanism of SeO32− to Se0 reduction, bacteria biomass was used to reduce various selenite concentrations (1, 3 and 5 mM), which were added as Na2SeO3
This study established that the Enterococcus spp. were capable of rapidly reducing selenite with a very high recovery of elemental selenium
Summary
Selenium (Se) is an essential micronutrient for both plants and animals. Selenium is termed a double-edged element because of its narrow range of acceptable concentrations. Selenate (SeO42−) and selenite (SeO32−) are the dominant selenium species Both oxyanions are known to be toxic, soluble in water and bio-accumulate in the food chain, even at low concentrations [3,4]. Literature suggests that SeO32− reduction can occur aerobically via detoxification [1,6], via Painter-type reactions (Equation (1)) [13,14] or with the aid of siderophores [15,16], as a form of self-defence Painter type reactions such as the one shown above are thermodynamically favourable reactions, and these involve selenium oxyanions and the reduced thiol groups of proteins that result in the formation of selenotrisulphides (RS-Se-SR) [17]. Some Enterococcus strains have a competitive edge over other microbial species in ecological niches because they harbour many bacteriocin-related genes simultaneously [25] These features are useful in food applications against spoilage and pathogenic organism contamination [26]. Enterococcus spp. was used in the aerobic batch reduction of SeO32− to Se0 and the elemental selenium was recovered and quantified
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