Biovolatilization of metal(loid)s by intestinal microorganisms in the simulator of the human intestinal microbial ecosystem

Assessing the role of the gut microbiome at the interface between environmental chemical exposures and human health: Current knowledge and challenges

The explosion of microbiome research over the past decade has shed light on the various ways that external factors interact with the human microbiome to drive health and disease. Each individual is exposed to more than 300 environmental chemicals every day. Accumulating evidence indicates that the microbiome is involved in the early response to environmental toxicants and biologically mediates their adverse effects on human health. However, few review articles to date provided a comprehensive framework for research and translation of the role of the gut microbiome in environmental health science. This review summarizes current evidence on environmental compounds and their effect on the gut microbiome, discusses the involved compound metabolic pathways, and covers environmental pollution-induced gut microbiota disorders and their long-term outcomes on host health. We conclude that the gut microbiota may crucially mediate and modify the disease-causing effects of environmental chemicals. Consequently, gut microbiota needs to be further studied to assess the complete toxicity of environmental exposures. Future research in this field is required to delineate the key interactions between intestinal microbiota and environmental pollutants and further to elucidate the long-term human health effects.

Arsenic: Various species with different effects on cytochrome P450 regulation in humans

Arsenic is well-recognized as one of the most hazardous elements which is characterized by its omnipresence throughout the environment in various chemical forms. From the simple inorganic arsenite (iAsIII) and arsenate (iAsV) molecules, a multitude of more complex organic species are biologically produced through a process of metabolic transformation with biomethylation being the core of this process. Because of their differential toxicity, speciation of arsenic-based compounds is necessary for assessing health risks posed by exposure to individual species or co-exposure to several species. In this regard, exposure assessment is another pivotal factor that includes identification of the potential sources as well as routes of exposure. Identification of arsenic impact on different physiological organ systems, through understanding its behavior in the human body that leads to homeostatic derangements, is the key for developing strategies to mitigate its toxicity. Metabolic machinery is one of the sophisticated body systems targeted by arsenic. The prominent role of cytochrome P450 enzymes (CYPs) in the metabolism of both endobiotics and xenobiotics necessitates paying a great deal of attention to the possible effects of arsenic compounds on this superfamily of enzymes. Here we highlight the toxicologically relevant arsenic species with a detailed description of the different environmental sources as well as the possible routes of human exposure to these species. We also summarize the reported findings of experimental investigations evaluating the influence of various arsenicals on different members of CYP superfamily using human-based models.

Effect of gut microbiota on in vitro bioaccessibility of heavy metals and human health risk assessment from ingestion of contaminated soils

To identify the role of gut microbiota in human health risk assessment, the bioaccessibility of heavy metals in 14 soil samples were determined in simulated gastrointestinal fluids. Compared to the small intestinal phase, the bioaccessibility values of the colon phase varied, either increased by 3.5-fold for As, by 2.2-fold for Cr, and by 1.6-fold for Ni, or reduced by 4.4-fold for Cu, respectively. The colon incubation with adult gut microbiota yielded higher bioaccessibility value of As (1.3 times) and Fe (3.4 times) than that of the child in most soil samples. Colon bioaccessibility was about 60% greater of Cd for the adult and 30% higher of Cr for the child. Congruent data on the bioaccessibility of Cu and Ni was observed. In addition, correlation analysis indicated that in vitro bioaccessibility was primarily related to total concentrations of heavy metals in soils, followed by soil pH and active Fe/Mn oxide. Significantly, risk assessment calculated based on colon bioaccessibility indicated that the target hazard quotient (THQ > 1) of As was presented in 3 soil samples for the adult (1.05-3.35) and in 9 soil samples for the child (1.06-26.93). The hazard index (HI) of the child was 4.00 on average, greater than that of the adult (0.62), primarily due to the contribution of As and Cd. It suggested non-carcinogenic risks are likely to occur in children through typical hand-to-mouth behavior. The adjustment of colon bioaccessibility will result in more accurate risk assessment of human exposure to heavy metals from oral ingestion of contaminated soils.

Overview of the rules of the microbial engagement in the gut microbiome: a step towards microbiome therapeutics

Human gut microbiome is a diversified, resilient, immuno-stabilized, metabolically active and physiologically essential component of the human body. Scientific explorations have been made to seek in-depth information about human gut microbiome establishment, microbiome functioning, microbiome succession, factors influencing microbial community dynamics and the role of gut microbiome in health and diseases. Extensive investigations have proposed the microbiome therapeutics as a futuristic medicine for various physiological and metabolic disorders. A comprehensive outlook of microbial colonization, host-microbe interactions, microbial adaptation, commensal selection and immuno-survivability is still required to catalogue the essential genetic and physiological features for the commensal engagement. Evolution of a structured human gut microbiome relies on the microbial flexibility towards genetic, immunological and physiological adaptation in the human gut. Key features for commensalism could be utilized in developing tailor-made microbiome-based therapy to overcome various physiological and metabolic disorders. This review describes the key genetics and physiological traits required for host-microbe interaction and successful commensalism to institute a human gut microbiome.

In Vitro Assessment of Arsenic Release and Transformation from As(V)-Sorbed Goethite and Jarosite: The Influence of Human Gut Microbiota

The importance of arsenic metabolism by gut microbiota has been evidenced in risk characterization from As exposures. In this study, we evaluated the metabolic potency of human gut microbiota toward As(V)-sorbed goethite and jarosite, presenting different behaviors of As release, and the solid-liquid transformation and partitioning. The release of As occurred mainly in the small intestinal phase for jarosite and in the colon phase for goethite, respectively. We found higher degree of As(V) and Fe(III) reduction by human gut microbiota in the colon digests of goethite than jarosite. Speciation analysis using high-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry and X-ray absorption near-edge spectroscopy, revealed that 43.2% and 8.5% of total As was present as As(III) in the liquid and solid phase, respectively, after goethite incubation, whereas almost all generated As(III) was in the colon digests of jarosite. Therefore, As bioaccessibility in human gastrointestinal tract was predominantly contributed to Fe(III) dissolution in jarosite, and to microbial reduction of Fe(III) and As(V) in goethite. It expanded our knowledge on the role of Fe minerals in human health risk assessment associated with soil As exposures.

Arsenic in Rice Bran Products: In Vitro Oral Bioaccessibility, Arsenic Transformation by Human Gut Microbiota, and Human Health Risk Assessment

Despite rice consumption, rice bran as a byproduct of rice milling contains higher arsenic (As). The present study evaluated the metabolic potency of in vitro cultured human colon microbiota toward As from five rice bran products with 0.471-1.491 mg of As/kg. Arsenic bioaccessibility ranged from 52.8 to 78.8% in the gastric phase, and a 1.2-fold increase (66.0-95.8%) was observed upon the small intestinal phase. Subsequently, a significant decline of As bioaccessibility (11.3-63.6%) and a high methylation percentage of 18.5-79.8% were found in the colon phase. The predominant As species in the solid phase was always As(V) (49.6-63.4%), and As-thiolate complexes increased by 10% at the end of colon incubation. Human gut microbiota could induce As bioaccessibility lowering and As transformation in rice bran, which illustrated the importance of food-bound As metabolism in the human body. This will result in a better understanding of health implications associated with As exposures.

Formation of silver nanoparticles by human gut microbiota

Metal nanoparticles have received global attention and one of the most promising nanotechnologies is the use of microorganisms for synthesizing nanoparticles. The oral exposure study has demonstrated the formation of silver (Ag) nanoparticles with in vivo animal models. However, there is limitation in the effect of in vitro cultured human gut microbiota on silver. Here, the phenomenon of biosynthesis of Ag nanoparticles by human gut microbiota has been reported. The size, shape, and morphology of the nanoparticles were characterized using transmission electron microscopy (TEM). Exposure to aqueous AgNO₃ solution resulted in the intracellular reduction of Ag⁺ ions and the formation of approximately spherical Ag nanoparticles with dimensions of 34 ± 10 nm. Electron microscopy analysis of the gut microbial cells by TEM images of thin sections and scanning electron microscopy (SEM), indicated that most of Ag nanoparticles were located on the cytoplasmic membrane and within the cytoplasm, where abundant silver reductases were present. In addition, some nanoparticles were bound to the cell wall, likely via exocytosis. The oral exposure to Ag⁺ led to in vivo formation of nanoparticles by gut microbiota, and their biological effects and potential applications in human body warrant further investigation.

Nutritional status affects the bioaccessibility and speciation of arsenic from soils in a simulator of the human intestinal microbial ecosystem

Arsenic (As) is a highly toxic contaminant in food and soil. In this study, we investigated the effects of four nutritional states (including a fed state with vitamin C, a fed state with protein powder, a fed state with glucose and a fasted state) on the variability of soil As bioaccessibility and biotransformation using the physiologically based extraction test (PBET) combined with a simulator of the human intestinal microbial ecosystem model (SHIME). The results indicated that the vitamin C and protein powder increased As bioaccessibility in gastric digests. In the colon phase, As bioaccessibility was observably enhanced by protein powder, and it varied under the vitamin C and glucose conditions. Additionally, the order of As methylation percentages in the four nutritional states was protein powder > vitamin C > fasted state > glucose (except S2); As bioaccessibility increased 1.3-13.7% and 15.8-35.4% in treatments of the vitamin C and protein powder, respectively. Meanwhile, large amounts of monomethylarsonic acid (MMA^V) were observed in the colon digest in the protein powder condition. In contrast, As methylation was significantly decreased with the addition of glucose, with a decline of 25.9-45.5%. Additionally, glucose enhanced the reduction of As(V). Therefore, nutritional status is a crucial parameter for the prediction of bioaccessibility and speciation of As when assessing health risks from As following oral exposure.

Antimony exposure and speciation in human biomarkers near an active mining area in Hunan, China

Antimony (Sb) exposure threatens human health. To identify human biomarkers for Sb exposure, we analyzed 480 environmental samples from an active Sb mining area in Hunan, China. Elevated Sb concentrations exceeding the reference level were detected in drinking water (70% of n = 83 total samples), foods (80%, n = 188), urine (95%, n = 63), saliva (44%, n = 48), hair (80%, n = 51) and nails (83%, n = 47). Drinking water contributed 85%-100% of the average daily dose (ADD) of Sb, and the total ADD (11.7 μg/kg bodyweight/day) was up to thirty times higher than the oral reference dose (0.4 μg/kg bodyweight/day) as recommended by USEPA. A positive correlation was found between ADD and Sb content in hair (p = 0.02), but not in urine (p = 0.051), saliva (p = 0.52) or nails (p = 0.85), suggesting that hair is the best non-invasive biomarker. Micro X-ray fluorescence analysis indicated that Sb is distributed in discrete spots in hair and nails, and Sb distribution is correlated with other metals. Methylated Sb species were predominant in urine (46%-100%) and saliva (74%-100%) in collected samples, implying that the human metabolic system adopts methylation as an effective pathway to detoxify and excrete Sb.

Chemical transformation of xenobiotics by the human gut microbiota

The human gut microbiota makes key contributions to the metabolism of ingested compounds (xenobiotics), transforming hundreds of dietary components, industrial chemicals, and pharmaceuticals into metabolites with altered activities, toxicities, and lifetimes within the body. The chemistry of gut microbial xenobiotic metabolism is often distinct from that of host enzymes. Despite their important consequences for human biology, the gut microbes, genes, and enzymes involved in xenobiotic metabolism are poorly understood. Linking these microbial transformations to enzymes and elucidating their biological effects is undoubtedly challenging. However, recent studies demonstrate that integrating traditional and emerging technologies can enable progress toward this goal. Ultimately, a molecular understanding of gut microbial xenobiotic metabolism will guide personalized medicine and nutrition, inform toxicology risk assessment, and improve drug discovery and development.

Investigation of bioaccessibility of Cu, Fe, Mn, and Zn in market vegetables in the colon using PBET combined with SHIME

The in vitro bioaccessibility of trace metals associated with oral ingestion of market vegetables (lettuce, pak choi, cole, and leaf lettuce) of Beijing, China was studied. The physiologically based extraction test (PBET) combined with the Simulator of Human Intestinal Microbial Ecosystem (SHIME) was applied to simulate stomach, small intestine, and colon of human. In the gastro-intestinal phases, the bioaccessibility of Cu, Fe, Mn, and Zn varied within 5.7–75.5%, 17.3–50.4%, 13.3–49.1%, and 19.9–63.7%, respectively. There was no significant difference in the metal bioaccessibility between the gastric and small intestinal phases, except for higher Cu bioaccessibility in the small intestine. Besides, the bioaccessibility of the four trace metals in the colon phase was first ever reported. A significant decline in Cu bioaccessibility (1.8–63.7%) and slight increases in the bioaccessibility of Fe (16.7–56.4%), Mn (21.2–71.6%), and Zn (15.7–69.7%) were revealed, which could mainly be attributed to the effect of colon microbiota. In addition, the estimated daily intakes (EDIs) of Cu, Fe, Mn, and Zn were worked out to be 0.7, 8.8, 2.7, and 4.5 μg kg⁻¹ body weight d⁻¹, based on which the potential influences of these trace metals in vegetables on the health of the local consumers was demonstrated.

Bioaccessibility of selenium from cooked rice as determined in a simulator of the human intestinal tract (SHIME)

BACKGROUND

As an essential but also potentially toxic element, both overexposure and underexposure to selenium (Se) can significantly affect public health. Rice is a common source of Se, especially in Asia. Not all Se may be released from the rice and become available for absorption into the bloodstream upon digestion in the gastrointestinal tract. Therefore, the bioaccessibility of Se in cooked white (polished) rice was assessed in vitro using the static gastrointestinal simulator SHIME (Simulator of the Human Intestinal Microbial Ecosystem).

RESULTS

The common cooking procedure in China prior to consumption [i.e. boiling at low rice:water ratios (1:3) until all of the water is absorbed into the rice] did not change total Se levels in the rice. Gastrointestinal digestion of the cooked rice matrix revealed a Se bioaccessibility of 67-76% of total Se. Subsequent microbial activity in the colon reduced the accessibility of Se in the cooked rice to 51-62%.

CONCLUSION

Not all Se present in cooked white rice should be considered as being bioavailable in the small intestine. A minor part is transferred with the remaining food matrix to the colon, where it is available for the microbial metabolism. © 2017 Society of Chemical Industry.

Xenobiotics: Interaction with the Intestinal Microflora

The human body is host to 100 trillion gut microbes, approximately 10-times more than all human cells. It is estimated that the approximately 500-1000 species residing in the human gut encode 150-fold more unique genes than the human genome. The gut microbiota has important functions in metabolic processing, such as energy production, immune cell development, food digestion, and epithelial homeostasis. It has been increasingly recognized that a dysregulated gut microbiome contributes in a significant way to a variety of diseases, including diabetes, obesity, cardiovascular diseases, allergies, and inflammatory bowel disease. In particular, accumulating evidence indicates that functional interactions between the gut microbiome and xenobiotics play a role in mediating chemical toxicity and causing or exacerbating human disease. This review summarizes emerging evidence that illustrates how xenobiotics can affect the gut microbiome structure, create functional changes to the gut microbiome, and become biotransformed by the gut microbiome.

High-Resolution Metabolomics: Review of the Field and Implications for Nursing Science and the Study of Preterm Birth

Most complex health conditions do not have a single etiology but rather develop from exposure to multiple risk factors that interact to influence individual susceptibility. In this review, we discuss the emerging field of metabolomics as a means by which metabolic pathways underlying a disease etiology can be exposed and specific metabolites can be identified and linked, ultimately providing biomarkers for early detection of disease onset and new strategies for intervention. We present the theoretical foundation of metabolomics research, the current methods employed in its conduct, and the overlap of metabolomics research with other "omic" approaches. As an exemplar, we discuss the potential of metabolomics research in the context of deciphering the complex interactions of the maternal-fetal exposures that underlie the risk of preterm birth, a condition that accounts for substantial portions of infant morbidity and mortality and whose etiology and pathophysiology remain incompletely defined. We conclude by providing strategies for including metabolomics research in future nursing studies for the advancement of nursing science.

Advances in Instrumental Analysis of Brominated Flame Retardants: Current Status and Future Perspectives

This review aims to highlight the recent advances and methodological improvements in instrumental techniques applied for the analysis of different brominated flame retardants (BFRs). The literature search strategy was based on the recent analytical reviews published on BFRs. The main selection criteria involved the successful development and application of analytical methods for determination of the target compounds in various environmental matrices. Different factors affecting chromatographic separation and mass spectrometric detection of brominated analytes were evaluated and discussed. Techniques using advanced instrumentation to achieve outstanding results in quantification of different BFRs and their metabolites/degradation products were highlighted. Finally, research gaps in the field of BFR analysis were identified and recommendations for future research were proposed.

Arsenic speciation in total contents and bioaccessible fractions in atmospheric particles related to human intakes

Speciation of inorganic trivalent arsenicals (iAs(III)), inorganic pentavalent arsenicals (iAs(V)), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in total arsenic (As) content and its bioaccessible fractions contained in road dust, household air-conditioning (AC) filter dust and PM2.5 was investigated. Inorganic As, especially iAs(V), was observed as the dominant species. Physiologically based extraction test (PBET), an in-vitro gastrointestinal method, was used to estimate the oral As bioaccessibility in coarse particles and the species present in the oral bioaccessible fraction. A composite lung simulating serum was used to mimic the pulmonary condition to extract the respiratory bioaccessible As and its species in PM2.5. Reduction of iAs(V) to iAs(III) occurred in both in-vitro gastrointestinal and lung simulating extraction models. The inorganic As species was the exclusive species for absorption through ingestion and inhalation of atmospheric particles, which was an important exposure route to inorganic As, in addition to drinking water and food consumption.

A review on completing arsenic biogeochemical cycle: microbial volatilization of arsines in environment

Arsenic (As) is ubiquitous in the environment in the carcinogenic inorganic forms, posing risks to human health in many parts of the world. Many microorganisms have evolved a series of mechanisms to cope with inorganic arsenic in their growth media such as transforming As compounds into volatile derivatives. Bio-volatilization of As has been suggested to play an important role in global As biogeochemical cycling, and can also be explored as a potential method for arsenic bioremediation. This review aims to provide an overview of the quality and quantity of As volatilization by fungi, bacteria, microalga and protozoans. Arsenic bio-volatilization is influenced by both biotic and abiotic factors that can be manipulated/elucidated for the purpose of As bioremediation. Since As bio-volatilization is a resurgent topic for both biogeochemistry and environmental health, our review serves as a concept paper for future research directions.

Arsenic undergoes significant speciation changes upon incubation of contaminated rice with human colon micro biota

Cellular and animal studies involving MMA(III) (monomethyl arsonous acid) and DMA(III) (dimethyl arsinous acid) have indicated that their toxicities meet or exceed that of iAs. Thiolated arsenic metabolites were observed in urine after oral exposure of inorganic arsenic in some studies. For these species, the toxicological profile was not yet fully characterized in human cells. Some studies revealed that trivalent organoarsenic species are well absorbed in the intestine compared to iAs. However, other studies also indicated that a significant amount of rice-bound As reaches the colon, which may be attributed to the fibre-rich nature of the rice. Studies have revealed that microorganisms from the gut environment are important contributors to arsenic speciation changes. We aimed to study how the gut microbial metabolism affects As in different rice matrices. This was done in vitro using colon suspension from the Simulator of the Human Intestinal Microbial Ecosystem (SHIME system). Significant amounts of MMA(III), DMA(III) and MMMTA(V) were formed due to microbial metabolic processes like methylation and thiolation. These results suggested that presystemic metabolism by human gut micro biota should not be neglected in risk assessment studies. In this context, also toxicity and absorption of thiolated species by mammalian cells should be further investigated.

Normal neonatal microbiome variation in relation to environmental factors, infection and allergy

PURPOSE OF REVIEW

Bacterial colonization of the infant intestinal tract begins at birth. We are at the forefront of understanding complex relationships between bacteria and multiple parameters of health of the developing infant. Moreover, the establishment of the microbiome in the critical neonatal period is potentially foundational for lifelong health and disease susceptibility. Recent studies utilizing state-of-the-art culture-independent technologies have begun to increase our knowledge about the gut microbiome in infancy, the impact of multiple exposures, and its effects on immune response and clinical outcomes such as allergy and infection.

RECENT FINDINGS

Postnatal exposures play a central role in the complex interactions between the nearly blank canvas of the neonatal intestine, whereas genetic factors do not appear to be a major factor. Infant microbial colonization is affected by delivery mode, dietary exposures, antibiotic exposure, and environmental toxicants. Successive microbiome acquisition in infancy is likely a determinant of early immune programming, subsequent infection, and allergy risk.

SUMMARY

The novel investigation of the neonatal microbiome is beginning to unearth substantial information, with a focus on immune programming that coevolves with the developing microbiome early in life. Several exposures common to neonatal and infant populations could exert pressure on the development of the microbiome and major diseases including allergy and infection in large populations.

Connection between multimetal(loid) methylation in methanoarchaea and central intermediates of methanogenesis

In spite of the significant impact of biomethylation on the mobility and toxicity of metals and metalloids in the environment, little is known about the biological formation of these methylated metal(loid) compounds. While element-specific methyltransferases have been isolated for arsenic, the striking versatility of methanoarchaea to methylate numerous metal(loid)s, including rare elements like bismuth, is still not understood. Here, we demonstrate that the same metal(loid)s (arsenic, selenium, antimony, tellurium, and bismuth) that are methylated by Methanosarcina mazei in vivo are also methylated by in vitro assays with purified recombinant MtaA, a methyltransferase catalyzing the methyl transfer from methylcobalamin [CH₃Cob(III)] to 2-mercaptoethanesulfonic acid (CoM) in methylotrophic methanogenesis. Detailed studies revealed that cob(I)alamin [Cob(I)], formed by MtaA-catalyzed demethylation of CH₃Cob(III), is the causative agent for the multimetal(loid) methylation observed. Moreover, Cob(I) is also capable of metal(loid) hydride generation. Global transcriptome profiling of M. mazei cultures exposed to bismuth did not reveal induced methyltransferase systems but upregulated regeneration of methanogenic cofactors in the presence of bismuth. Thus, we conclude that the multimetal(loid) methylation in vivo is attributed to side reactions of CH₃Cob(III) with reduced cofactors formed in methanogenesis. The close connection between metal(loid) methylation and methanogenesis explains the general capability of methanoarchaea to methylate metal(loid)s.

Toxicity of volatile methylated species of bismuth, arsenic, tin, and mercury in Mammalian cells in vitro

The biochemical transformation of mercury, tin, arsenic and bismuth through formation of volatile alkylated species performs a fundamental role in determining the environmental processing of these elements. While the toxicity of inorganic forms of most of these compounds are well documented (e.g., arsenic, mercury) and some of them are of relatively low toxicity (e.g., tin, bismuth), the more lipid-soluble organometals can be highly toxic. In the present study we investigated the cyto- and genotoxicity of five volatile metal(loid) compounds: trimethylbismuth, dimethylarsenic iodide, trimethylarsine, tetramethyltin, and dimethylmercury. As far as we know, this is the first study investigating the toxicity of volatile metal(loid) compounds in vitro. Our results showed that dimethylmercury was most toxic to all three used cell lines (CHO-9 cells, CaCo, Hep-G2) followed by dimethylarsenic iodide. Tetramethyltin was the least toxic compound; however, the toxicity was also dependend upon the cell type. Human colon cells (CaCo) were most susceptible to the toxicity of the volatile compounds compared to the other cell lines. We conclude from our study that volatile metal(loid) compounds can be toxic to mammalian cells already at very low concentrations but the toxicity depends upon the metal(loid) species and the exposed cell type.

Investigation of biomethylation of arsenic and tellurium during composting

Though the process of composting features a high microbiological activity, its potential to methylate metals and metalloids has been little investigated so far in spite of the high impact of this process on metal(loid) toxicity and mobility. Here, we studied the biotransformation of arsenic, tellurium, antimony, tin and germanium during composting. Time resolved investigation revealed a highly dynamic process during self-heated composting with markedly differing time patterns for arsenic and tellurium species. Extraordinary high concentrations of up to 150 mg kg(-1) methylated arsenic species as well as conversion rates up to 50% for arsenic and 5% for tellurium were observed. In contrast, little to no conversion was observed for antimony, tin and germanium. In addition to experiments with metal(loid) salts, composting of arsenic hyperaccumulating ferns Pteris vittata and P. cretica grown on As-amended soils was studied. Arsenic accumulated in the fronds was efficiently methylated resulting in up to 8 mg kg(-1) methylated arsenic species. Overall, these studies indicate that metal(loid)s can undergo intensive biomethylation during composting. Due to the high mobility of methylated species this process needs to be considered in organic waste treatment of metal(loid) contaminated waste materials.

Arsenic metabolism by human gut microbiota upon in vitro digestion of contaminated soils

BACKGROUND

Speciation analysis is essential when evaluating risks from arsenic (As) exposure. In an oral exposure scenario, the importance of presystemic metabolism by gut microorganisms has been evidenced with in vivo animal models and in vitro experiments with animal microbiota. However, it is unclear whether human microbiota display similar As metabolism, especially when present in a contaminated matrix.

OBJECTIVES

We evaluated the metabolic potency of in vitro cultured human colon microbiota toward inorganic As (iAs) and As-contaminated soils.

METHODS

A colon microbial community was cultured in a dynamic model of the human gut. These colon microbiota were incubated with iAs and with As-contaminated urban soils. We determined As speciation analysis using high-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry.

RESULTS

We found a high degree of methylation for colon digests both of iAs (10 microg methylarsenical/g biomass/hr) and of As-contaminated soils (up to 28 microg/g biomass/hr). Besides the formation of monomethylarsonic acid (MMA(V)), we detected the highly toxic monomethylarsonous acid (MMA(III)). Moreover, this is the first description of microbial thiolation leading to monomethylmonothioarsonic acid (MMMTA(V)). MMMTA(V), the toxicokinetic properties of which are not well known, was in many cases a major metabolite.

CONCLUSIONS

Presystemic As metabolism is a significant process in the human body. Toxicokinetic studies aiming to completely elucidate the As metabolic pathway would therefore benefit from incorporating the metabolic potency of human gut microbiota. This will result in more accurate risk characterization associated with As exposures.