In Vitro Method To Assess Soil Arsenic Metabolism by Human Gut Microbiota: Arsenic Speciation and Distribution

Abstract

Arsenic (As) speciation and distribution are two important factors in assessing human health risk from As-contaminated soil. In this study, we used the combination of physiologically based extraction test (PBET) and Simulator of Human Intestinal Microbial Ecosystem (SHIME) to determine soil As metabolism by human gut microbiota. The results showed that the percentage of soil arsenate [As(V)] transformation reached 22.1−38.2%, while that of arsenite [As(III)] attained 66.5−92.0%; 30.1−56.4% of As(V) transformed was attached to the soil solid phase. In comparison to sequential extraction results, almost all amorphous Fe/Al-oxide-bound As was liberated in the colon phase. An X-ray absorption nearedge structure (XANES) showed that the As(III) percentage in the soil solid phase reached 16.6−26.9% and reached 73.4% (soil 1) in the colon phase. Additionally, plenty of As(III) and different extents of methylation were also observed in colon extraction solution. As bioaccessibility in the colon phase was 1.8−2.8 times that in the small intestinal phase. Our results indicated that human gut microbiota increased As bioaccessibility, and large amounts of As(III) were adsorbed onto the soil solid phase as a result of microbial reduction. Determining As speciation and distribution in extraction solution and soil solid phases will allow for an accurate assessment of the risk to human health upon soil As exposure. ■ INTRODUCTIONArsenic (As) is a common toxic contaminant that is extensively distributed in the environment. 1 Human exposure to As in soils is a major public concern and is associated with serious risks to human health because As species are known carcinogens. 2,3 In most cases, human exposure to soil As occurs via inhalation, inadvertent oral ingestion, and dermal contact. 4,5 Inadvertent oral ingestion has been considered an important exposure route for soil As, especially ingestion through outdoor hand-to-mouth activities by children. 6 The estimation of As bioaccessibility (gastric and small intestinal phases) in contaminated soils through in vitro methods has been used for human health risk assessment. On the basis of previous studies, 7−9 simple, fast, and inexpensive in vitro methods can effectively estimate in vivo relative As bioavailability. Generally, bioaccessibility is defined as the fraction of As that is soluble in the gastrointestinal environment of humans and available for absorption, whereas bioavailability is determined by the fraction of As that is absorbed into the systemic circulation. 7,8 Currently, As bioaccessibility in the gastric and small intestinal phases may underestimate its harm to human health. 10 Studies have clearly illustrated that gut microbiota significantly affect As metabolism by, for example, reduction, methylation, and thiolation. 11 As metabolism in contaminated soil by human gut microbiota can affect As bioaccessibility and speciation. 10,12 Therefore, As biotransformation must be considered an essential part of the risk assessment of oral soil As exposure using i...

Variability of arsenic bioaccessibility and metabolism in soils by human gut microbiota using different in vitro methods combined with SHIME

“…1). An increase in the As bioaccessibility in PBET-SHIME-C compared to PBET-I has previously been reported (Yin et al, 2015b). In addition, the DIN-SHIME-C As bioaccessibility was greater than the DIN-I values for four of the six soils evaluated (Fig.…”

“…After 3-4 weeks of adaptation, stable microbial communities were obtained in the respective colon compartments. The distal colon microbial fermentation activity (short chain fatty acid production and ammonium production) and community composition were investigated, and found to be consistent with those of previous SHIME runs and the in vivo situation (Molly et al, 1994;Yin et al, 2015b). For the dynamic SHIME, the feed solution contained (in grams/liter) arabinogalactan (1.0), pectin (2.0), xylan (1.0), starch (4.0), glucose (0.4), yeast extract (3.0), peptone (3.0), mucin (1.0), and cystein (0.5).…”

“…In our previous study, Yin et al (2015b) determined the transformation of As species by human gut microbiota using a combination of PBET and SHIME in five contaminated soils from mining and farmland sites in China with high As concentrations ranging from 110.9 to 802.5 mg kg − 1 . In this study, six soils including four previous soils, one Chinese reference material (GSS-5, National Institute of Metrology, China) and one reference material (NIST 2710a), were used to evaluate As bioaccessibility and speciation using a combination of different commonly used in vitro methods and SHIME.…”

“…In brief, soils were added to polypropylene conical centrifuge tubes with the gastric and small intestinal digests at specific soil/solution (s/s) ratios. Following the small intestinal phase, these digests were transferred into 100 mL anaerobic serum bottles and corresponding volumes of colon solution from the colon compartment of the dynamic SHIME system were added with double ratios (s/s) of the small intestinal phase (Van de Wiele et al, 2010;Yin et al, 2015b). The bottles were capped with butyl rubber stoppers and immediately flushed with nitrogen gas for 20-30 min to obtain anaerobic conditions.…”

“…The bottles were capped with butyl rubber stoppers and immediately flushed with nitrogen gas for 20-30 min to obtain anaerobic conditions. The digests were shaken at 150 rpm and incubated at 37°C for 48 h Yin et al, 2015b). The experiments were conducted in triplicate.…”