Nutritional Status and Gastrointestinal Microbes Affect Arsenic Bioaccessibility from Soils and Mine Tailings in the Simulator of the Human Intestinal Microbial Ecosystem

Abstract

In vitro gastrointestinal models, used to measure the metal(loid) bioaccessibility for site specific risk assessment, are typically operated under fasted conditions. We evaluated the hypothesis that fed conditions increase arsenic bioaccessibility on three reference soils (NIST 2711, NIST 2709, and BGS 102) and the bulk and <38 mum size fractions of a mine tailing. The three nutritional states included a fed state with a carbohydrate mixture, a second fed state with homogenized crowberries (Empetrum nigrum), and a fasted state. The carbohydrate mixture increased arsenic bioaccessibility from four of five samples in the simulator of the human intestinal microbial ecosystem (SHIME) stomach but only three of five samples in the SHIME small intestine and colon. In contrast, crowberries increased arsenic bioaccessibility from four of five samples in the SHIME small intestine but had variable affects in the SHIME stomach and colon. The effect of nutritional status on arsenic bioaccessibility was potentially mediated via ligand-promoted dissolution in the SHIME stomach and small intestine. The displacement of arsenic with phosphate was potentially present in the SHIME small intestine but not the SHIME stomach. Microbial activity increased arsenic bioaccessibility relative to sterile conditions from four of five samples under fasted conditions and three of the five samples under fed conditions, which may suggest that in vitro gastrointestinal (GI) models operated under fed conditions and with microbes provide a more conservative estimate of in vitro bioaccessibility. However, for some samples, the arsenic bioaccessibility in the SHIME colon (with microbial activity) was equivalent to values observed in a separate physiologically based extraction test under small intestinal conditions (without microbial activity). These results suggest that the incorporation of microbial activity into in vitro GI models does not necessarily make estimates of arsenic bioaccessibility more protective than those generated using in vitro models that do not include microbial activity.