Arsenic and the gastrointestinal tract microbiome

Meta-omics analysis reveals the marine arsenic cycle driven by bacteria

Arsenic is a toxic element widely distributed in the Earth's crust and ranked as a class I human carcinogen. Microbial metabolism makes significant contributions to arsenic detoxification, migration and transformation. Nowadays, research on arsenic is primarily in areas affected by arsenic pollution associated with human health activities. However, the biogeochemical traits of arsenic in the global marine ecosystem remain to be explicated. In this study, we revealed that seawater environments were primarily governed by the process of arsenate reduction to arsenite, while arsenite methylation was predominant in marine sediments which may serve as significant sources of arsenic emission into the atmosphere. Significant disparities existed in the distribution patterns of the arsenic cycle between surface and deep seawaters at middle and low latitudes, whereas these situations tend to be similar in the Arctic and Antarctic oceans. Significant variations were also observed in the taxonomic diversity and core microbial community of arsenic cycling across different marine environments. Specifically, γ-proteobacteria played a pivotal role in the arsenic cycle in the whole marine environment. Temperature, dissolved oxygen and phosphate were the crucial factors that related to these differentiations in seawater environments. Overall, our study contributes to a deeper understanding of the marine arsenic cycle.

Diverse mechanisms by which chemical pollutant exposure alters gut microbiota metabolism and inflammation

The human gut microbiome, the host, and the environment are inextricably linked across the life course with significant health impacts. Consisting of trillions of bacteria, fungi, viruses, and other micro-organisms, microbiota living within our gut are particularly dynamic and responsible for digestion and metabolism of diverse classes of ingested chemical pollutants. Exposure to chemical pollutants not only in early life but throughout growth and into adulthood can alter human hosts' ability to absorb and metabolize xenobiotics, nutrients, and other components critical to health and longevity. Inflammation is a common mechanism underlying multiple environmentally related chronic conditions, including cardiovascular disease, multiple cancer types, and mental health. While growing research supports complex interactions between pollutants and the gut microbiome, significant gaps exist. Few reviews provide descriptions of the complex mechanisms by which chemical pollutants interact with the host microbiome through either direct or indirect pathways to alter disease risk, with a particular focus on inflammatory pathways. This review focuses on examples of several classes of pollutants commonly ingested by humans, including (i) heavy metals, (ii) persistent organic pollutants (POPs), and (iii) nitrates. Digestive enzymes and gut microbes are the first line of absorption and metabolism of these chemicals, and gut microbes have been shown to alter compounds from a less to more toxic state influencing subsequent distribution and excretion. In addition, chemical pollutants may interact with or alter the selection of more harmful and less commensal microbiota, leading to gut dysbiosis, and changes in receptor-mediated signaling pathways that alter the integrity and function of the gut intestinal tract. Arsenic, cadmium, and lead (heavy metals), influence the microbiome directly by altering different classes of bacteria, and subsequently driving inflammation through metabolite production and different signaling pathways (LPS/TLR4 or proteoglycan/TLR2 pathways). POPs can alter gut microbial composition either directly or indirectly depending on their ability to activate key signaling pathways within the intestine (e.g., PCB-126 and AHR). Nitrates and nitrites' effect on the gut and host may depend on their ability to be transformed to secondary and tertiary metabolites by gut bacteria. Future research should continue to support foundational research both in vitro, in vivo, and longitudinal population-based research to better identify opportunities for prevention, gain additional mechanistic insights into the complex interactions between environmental pollutants and the microbiome and support additional translational science.

Probiotics Pediococcus acidilactici GR-1 promotes the functional strains and remodels gut microbiota to reduce the Cr(VI) toxicity in a dual-chamber simulated intestinal system

Numerous animal studies have demonstrated the toxicity of hexavalent chromium [Cr(VI)] and the bioremediative effects of probiotics on the composition and functions of gut microbiota. Since the precise mechanisms of Cr(VI) detoxification and its interactions with human gut microbiota were unknown, a novel dual-chamber simulated intestinal (DCSI) system was developed to maintain both the stability of the simulated system and the composition of the gut microbiota. Probiotic GR-1 was found to regulate intestinal gut microbiota, thereby reducing the toxicity of Cr(VI) within the DCSI system. The results indicate that Cr(VI) levels were reduced from 2.260 ± 0.2438 μg/g to 1.7086 ± 0.1950 μg/g in the gut microbiota cell pellet, and Cr(VI) permeability decreased from 0.5521 ± 0.1132 μg/L to 0.3681 ± 0.0178 μg/L after 48 h in simulated gut fluid. Additionally, the removal rate of 1,1-Diphenyl-2-picrylhydrazyl (DPPH), reducibility (Vitamin C), and total antioxidant capacity (T-AOC) increased by 50.83%, 31.70%, and 27.56%, respectively, following probiotic treatment. The increase in antioxidant capacity correlated with total Cr removal (P < 0.05, r from -0.80 to 0.73). 16S rRNA sequencing analysis showed that gut microbiota composition was reshaped by the addition of probiotics, which regulated the recovery of the functional gut microbiota to normal levels, rather than restoring the entire gut microbiota composition for community function. Thus, this study not only demonstrates the feasibility and stability of culturing gut microbiota but also offers a new biotechnological approach to synthesizing functional communities with functional strains for environmental risk management.

The Impact of Environmental Toxins on Cardiovascular Diseases

Environmental toxins pose significant risks to human health and are considered major contributors to chronic diseases, particularly cardiovascular diseases (CVDs). Numerous studies have highlighted the detrimental impact of environmental toxicity on cardiovascular health. The primary sources of environmental toxins include pollutants containing particulate matter and organic substances, such as sulfate, nitrates, ammonium, elemental carbon, crystal elements, arsenic (As), mercury (Hg), cadmium (Cd), and Bisphenol A (BPA). Epidemiological research has closely monitored the link between environmental toxins and CVDs. Notably, aerosols and particulate matter, including PM10 and PM2.5, prevalent in ambient air pollution, have been implicated in various CVDs like ischemic heart disease, myocardial infarction, and dysrhythmia. Additionally, heavy metals such as lead and pesticides from environmental toxins are known to contribute to CVDs, even at low levels of exposure over extended periods. Mercury exposure, even at low concentrations, can adversely affect multiple organs, including the heart, kidneys, nervous system, and immune system. With Pb2+ ions exhibiting Ca2+-like properties, lead disrupts various pathways and can lead to cardiac and vascular lesions and functional impairments when blood lead concentrations exceed 100 µg% in adults and 60 µg% in children. Furthermore, cadmium exposure is higher in smokers, primarily due to tobacco use, and is associated with peripheral artery disease. Arsenic toxicity is well-documented, particularly its cardiotoxic effects, which can result in fatal and irreversible myocardial damage. Bisphenol A (BPA) has also been found in urine samples, underscoring its presence as an environmental toxin impacting human health.

Dietary Galactooligosaccharides Supplementation as a Gut Microbiota-Regulating Approach to Lower Early Life Arsenic Exposure

Prebiotics may stimulate beneficial gut microorganisms. However, it remains unclear whether they can lower the oral bioavailability of early life arsenic (As) exposure via regulating gut microbiota and altering As biotransformation along the gastrointestinal (GI) tract. In this study, weanling mice were exposed to arsenate (iAsV) via diet (7.5 μg As g-1) amended with fructooligosaccharides (FOS), galactooligosaccharides (GOS), and inulin individually at 1% and 5% (w/w). Compared to As exposure control mice, As concentrations in mouse blood, liver, and kidneys and As urinary excretion factor (UEF) were reduced by 43.7%-74.1% when treated with 5% GOS. The decrease corresponded to a significant proliferation of Akkermansia and Psychrobacter, reduced percentage of inorganic arsenite (iAsIII) and iAsV by 47.4% and 65.4%, and increased proportion of DMAV in intestinal contents by 101% in the guts of mice treated with 5% GOS compared to the As control group. In contrast, FOS and inulin either at l% or 5% did not reduce As concentration in mouse blood, liver, and kidneys or As UEF. These results suggest that GOS supplementation may be a gut microbiota-regulating approach to lower early life As exposure via stimulating the growth of Akkermansia and Psychrobacter and enhancing As methylation in the GI tract.

Intervention effects of sulfate glycosaminoglycan from swim bladder against arsenic-induced damage in IEC-6 cells

In this study, a purified macromolecular sulfate glycosaminoglycan whose structural characterization is similar to chondroitin sulfate from the swim bladder of Aristichthys nobilis, named SBSG, was used to explore the intervention effects on arsenic-induced intestinal epithelial cells (IEC-6) damage. Arsenic exposure led to cell membrane rupture, mitochondrial dysfunction, oxidative damage, and down-regulation of tight junction proteins expression. Treatment with SBSG could alleviate arsenic exposure-induced cell damage by decreasing the extracellular lactate dehydrogenase activity and influencing mitochondrial membrane potential, reactive oxygen species level, malondialdehyde content, and anti-oxidative enzyme activity. On the other hand, SBSG could promote nitric oxide production to achieve potential immunoregulation. The Western blot showed that intervention of SBSG mainly could restrain the activation of the JNK signaling pathway and up-regulate the expression of ZO-1 against arsenic-induced cell damage. This study provides a new perspective for understanding the heavy metal detoxification of SBSG on the intestinal and indicates that SBSG could be used as natural antioxidant resistant to heavy metal toxicity.

Assessment of Lactobacillus rhamnosus mediated protection against arsenic-induced toxicity in zebrafish: a qPCR-based analysis of Firmicutes and Bacteroidetes groups and embryonic development

Arsenic poses a significant health risk worldwide, impacting the gut microbiota, reproductive health, and development. To address this issue, a cost-effective method like probiotic supplementation could be beneficial. However, the interplay between arsenic toxicity, probiotics, gut microbiota, and maternal transcript modulation remains unexplored. This study investigates the impact of Lactobacillus rhamnosus (L. rhamnosus) DSM 20021 on the proportions of Firmicutes and Bacteroidetes, as well as its effects on embryonic development in zebrafish induced by arsenic trioxide (As2O3). Adult zebrafish were exposed to both high and environmentally relevant concentrations of As2O3 (10, 50, and 500 ppb) for 1, 6, and 12 weeks. qPCR analysis revealed increased proportions of Firmicutes and Bacteroidetes in all As2O3-exposed and As2O3 + L. rhamnosus-exposed groups, while no significant changes were observed in groups exposed only to L. rhamnosus DSM 20021. The larvae, exposed to 500 ppb of As2O3 for 12 weeks, exhibited low growth, decreased survival rates, and morphological deformities. However, these adverse effects were reversed upon exposure to only L. rhamnosus DSM 20021. Furthermore, the expression of DVR1 and ABCC5, which are involved in defense against xenobiotics and embryo development, decreased significantly in As2O3 (500 ppb) and As2O3 (500 ppb) + L. rhamnosus-exposed groups, whereas ameliorative effects were observed in only L. rhamnosus DSM 20021-exposed groups.

Microbial biochemical pathways of arsenic biotransformation and their application for bioremediation

Arsenic is a ubiquitous toxic metalloid, the concentration of which is beyond WHO safe drinking water standards in many areas of the world, owing to many natural and anthropogenic activities. Long-term exposure to arsenic proves lethal for plants, humans, animals, and even microbial communities in the environment. Various sustainable strategies have been developed to mitigate the harmful effects of arsenic which include several chemical and physical methods, however, bioremediation has proved to be an eco-friendly and inexpensive technique with promising results. Many microbes and plant species are known for arsenic biotransformation and detoxification. Arsenic bioremediation involves different pathways such as uptake, accumulation, reduction, oxidation, methylation, and demethylation. Each of these pathways has a certain set of genes and proteins to carry out the mechanism of arsenic biotransformation. Based on these mechanisms, various studies have been conducted for arsenic detoxification and removal. Genes specific for these pathways have also been cloned in several microorganisms to enhance arsenic bioremediation. This review discusses different biochemical pathways and the associated genes which play important roles in arsenic redox reactions, resistance, methylation/demethylation, and accumulation. Based on these mechanisms, new methods can be developed for effective arsenic bioremediation.

Bacterial Modification of the Association Between Arsenic and Autism-Related Social Behavior Scores

Arsenic is related to neurodevelopmental outcomes and is associated with the composition of the gut microbiome. Data on the modifying role of the microbiome are limited. We probed suggestive relationships between arsenic and social behaviors to quantify the modifying role of the infant gut microbiome. We followed children for whom arsenic concentrations were quantified in 6-week-old toenail clippings. Scores on the Social Responsiveness Scale (SRS-2), which measures autism-related social behaviors, were provided by caregivers when the child was approximately 3 years of age. Metagenomic sequencing was performed on infant stools collected at 6 weeks and 1 year of age. To evaluate modification by the top ten most abundant species and functional pathways, we modeled SRS-2 total T-scores as a function of arsenic concentrations, microbiome features dichotomized at their median, and an interaction between exposure and the microbiome, adjusting for other trace elements and sociodemographic characteristics. As compared to the standardized population (SRS-2 T-scores = 50), participants in our study had lower SRS-2 scores (n = 78, mean = 44, SD = 5).The relative abundances of several functional pathways identified in 6-week stool samples modified the arsenic-SRS-2 association, including the pathways of valine and isoleucine biosynthesis; we observed no association among those with high relative abundance of each pathway [β = - 0.67 (95% CI - 1.46, 0.12)], and an adverse association [β = 1.67 (95% CI 0.3, 3.04), pinteraction= 0.05] among infants with low relative abundance. Our findings indicate the infant gut microbiome may alter neurodevelopmental susceptibility to environmental exposures.

Gut Microbiota Control the Bioavailability and Metabolism of Organoarsenicals of Seaweeds in Mice after Oral Ingestion

Edible seaweed consumption is an essential route of human exposure to complex organoarsenicals, including arsenosugars and arsenosugar phospholipids. However, the effects of gut microbiota on the metabolism and bioavailability of arsenosugars in vivo are unknown. Herein, two nori and two kelp samples with phosphate arsenosugar and sulfonate arsenosugar, respectively, as the predominant arsenic species, were administered to normal mice and gut microbiota-disrupted mice treated with the broad-spectrum antibiotic cefoperazone for 4 weeks. Following exposure, the community structures of the gut microbiota, total arsenic concentrations, and arsenic species in excreta and tissues were analyzed. Total arsenic excreted in feces and urine did not differ significantly between normal and antibiotic-treated mice fed with kelp samples. However, the total urinary arsenic of normal mice fed with nori samples was significantly higher (p < 0.05) (urinary arsenic excretion factor, 34-38 vs 5-7%), and the fecal total arsenic was significantly lower than in antibiotic-treated mice. Arsenic speciation analysis revealed that most phosphate arsenosugars in nori were converted to arsenobetaine (53.5-74.5%) when passing through the gastrointestinal tract, whereas a large portion of sulfonate arsenosugar in kelp was resistant to speciation changes and was excreted in feces intact (64.1-64.5%). Normal mice exhibited greater oral bioavailability of phosphate arsenosugar from nori than sulfonate arsenosugar from kelp (34-38 vs 6-9%). Our work provides insights into organoarsenical metabolism and their bioavailability in the mammalian gut.

Urinary microbiota and metabolic signatures associated with inorganic arsenic-induced early bladder lesions

Inorganic arsenic (iAs) contamination in drinking water is a global public health problem, and exposure to iAs is a known risk factor for bladder cancer. Perturbation of urinary microbiome and metabolome induced by iAs exposure may have a more direct effect on the development of bladder cancer. The aim of this study was to determine the impact of iAs exposure on urinary microbiome and metabolome, and to identify microbiota and metabolic signatures that are associated with iAs-induced bladder lesions. We evaluated and quantified the pathological changes of bladder, and performed 16S rDNA sequencing and mass spectrometry-based metabolomics profiling on urine samples from rats exposed to low (30 mg/L NaAsO₂) or high (100 mg/L NaAsO₂) iAs from early life (in utero and childhood) to puberty. Our results showed that iAs induced pathological bladder lesions, and more severe effects were noticed in the high-iAs group and male rats. Furthermore, six and seven featured urinary bacteria genera were identified in female and male offspring rats, respectively. Several characteristic urinary metabolites, including Menadione, Pilocarpine, N-Acetylornithine, Prostaglandin B1, Deoxyinosine, Biopterin, and 1-Methyluric acid, were identified significantly higher in the high-iAs groups. In addition, the correlation analysis demonstrated that the differential bacteria genera were highly correlated with the featured urinary metabolites. Collectively, these results suggest that exposure to iAs in early life not only causes bladder lesions, but also perturbs urinary microbiome composition and associated metabolic profiles, which shows a strong correlation. Those differential urinary genera and metabolites may contribute to bladder lesions, suggesting a potential for development of urinary biomarkers for iAs-induced bladder cancer.

Role of microbial iron reduction in arsenic metabolism from soil particle size fractions in simulated human gastrointestinal tract

Gut microbiota provides protection against arsenic (As) induced toxicity, and As metabolism is considered an important part of risk assessment associated with soil As exposures. However, little is known about microbial iron(III) reduction and its role in metabolism of soil-bound As in the human gut. Here, we determined the dissolution and transformation of As and Fe from incidental ingestion of contaminated soils as a function of particle size (<250 μm, 100-250 μm, 50-100 μm and < 50 μm). Colon incubation with human gut microbiota yielded a high degree of As reduction and methylation of up to 53.4 and 0.074 μg/(log CFU/mL)/hr, respectively; methylation percentage increased with increasing soil organic matter and decreasing soil pore size. We also found significant microbial Fe(III) reduction and high levels of Fe(II) (48 %-100 % of total soluble Fe) may promote the capacity of As methylation. Although no statistical change in Fe phases was observed with low Fe dissolution and high molar Fe/As ratios, higher As bioaccessibility of colon phase (avg. 29.4 %) was mainly contributed from reductive dissolution of As(V)-bearing Fe(III) (oxy)hydroxides. Our results suggest that As mobility and biotransformation by human gut microbiota (carrying arrA and arsC genes) are strongly controlled by microbial Fe(III) reduction coupled with soil particle size. This will expand our knowledge on oral bioavailability of soil As and health risks from exposure to contaminated soils.

Alterations in Microbial-Associated Fecal Metabolites in Relation to Arsenic Exposure Among Infants

In utero and early life exposure to inorganic arsenic (iAs) alters immune response in experimental animals and is associated with an increased risk of infant infections. iAs exposure is related to differences in the gut microbiota diversity, community structure, and the relative abundance of individual microbial taxa both in laboratory and human studies. Metabolomics permits a direct measure of molecular products of microbial and host metabolic processes. We conducted NMR metabolomics analysis on infant stool samples and quantified the relative concentrations of 34 known microbial-related metabolites. We examined these metabolites in relation to both in utero and infant log2 urinary total arsenic concentrations (utAs, the sum of iAs and iAs metabolites) collected at approximately 6 weeks of age using linear regression models, adjusted for infant sex, age at sample collection, type of delivery (vaginal vs. cesarean section), feeding mode (breast milk vs. any formula), and specific gravity. Increased fecal butyrate (b = 214.24), propionate (b = 518.33), cholate (b = 8.79), tryptophan (b= 14.23), asparagine (b = 28.80), isoleucine (b = 65.58), leucine (b = 95.91), malonate (b = 50.43), and uracil (b = 36.13), concentrations were associated with a doubling of infant utAs concentrations (p< 0.05). These associations were largely among infants who were formula fed. No clear associations were observed with maternal utAs and infant fecal metabolites. Metabolomic analyses of infant stool samples lend further evidence that the infant gut microbiota is sensitive to As exposure, and these effects may have functional consequences.

Contribution of gut bacteria to arsenic metabolism in the first year of life in a prospective birth cohort

Gut bacteria are at the interface of environmental exposures and their impact on human systems, and may alter host absorption, metabolism, and excretion of toxic chemicals. We investigated whether arsenic-metabolizing bacterial gene pathways related to urinary arsenic concentrations. In the New Hampshire Birth Cohort Study, urine and stool samples were obtained at six weeks (n = 186) and one year (n = 190) of age. Inorganic arsenic (iAs), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), and arsenobetaine (AsB) were quantified in infant urine samples using high-performance liquid chromatography with inductively coupled plasma mass spectrometry. Total arsenic exposure (tAs) was summarized as Σ(iAs, MMA, DMA) and log2-transformed. Fecal microbial DNA underwent metagenomic sequencing and the relative abundance of bacterial gene pathways were grouped as KEGG Orthologies (KOs) using BioBakery algorithms. Arsenic metabolism KOs with >80% prevalence were log2-transformed and modeled continuously using linear regression, those with <10% were not evaluated and those with 10-80% prevalence were analyzed dichotomously (detect/non-detect) using logistic regression. In the first set of models, tAs was regressed against KO relative abundance or detection adjusting for age at sample collection and child's sex. Effect modification by delivery mode was assessed in stratified models. In the second set of models, the association between the relative abundance/detection of the KOs and arsenic speciation (%iAs, %MMA, %DMA) was quantified with linear regression. Urinary tAs was associated with the increased relative abundance/detection odds of several arsenic-related KOs, including K16509, an arsenate reductase transcriptional regulator, with stronger associations among six-week-olds than one-year-olds. K16509 was also associated with decreased %MMA and increased %DMA at six weeks and one year. Notably, many associations were stronger among operatively-delivered than vaginally-delivered infants. Our findings suggest associations between arsenic-metabolizing bacteria in the infant gut microbiome and urinary arsenic excretion.

Seminal trace elements and their relationship with sperm parameters

Male reproductive problems may derive from many reasons including the environmental toxicants which may either intaken by occupational exposure, nutrition or bad air quality. The increased exposure to these substances due to rapid industrial development and technology has raised the questions: Is there a relationship between sperm parameters and these substances, and if so, in what extent? Results of studies on the subject reported conflicting results, many of which were not investigated in the seminal plasma. The aim of the current study was to evaluate the relationship between 23 metals and trace elements in human seminal plasma and semen parameters many of which were not investigated before. Levels of 23 metals in human seminal plasma were assessed by inductively coupled plasma mass spectrometry (ICP-MS). We examined the differences between subjects with normal ejaculate (normozoospermia) and pathologic ejaculate (with at least one abnormal semen parameter) according to the WHO criteria. The only significant difference was detected for Se while the other element's difference was not statistically significant. Se was statistically significantly increased in normal semen group suggesting the positive effect of this element on semen parameters.

Arsenic in Caribbean bivalves in the context of Sargassum beachings: A new risk for seafood consumers

Sargassum strandings in the coastal environment can introduce arsenic into food webs. In this context, we assessed the risk of exposure to arsenic for consumers of Caribbean bivalves. In 2019, specimens of Asaphis deflorata and Phacoides pectinatus were collected in an Atlantic coastal zone of Martinique (island) to monitor the presence of arsenic species by LC-ICP-MS. The total arsenic (tAs) concentrations were, on average, 34.4 ± 3.8 and 76.9 ± 22.3 µg.g-1 dry weight for P. pectinatus and A. deflorata, respectively. Seven compounds of arsenic were detected in bivalve soft bodies. In P. pectinatus, monomethylarsonic acid was present at a relatively significant concentration (≈ 29.6%). These results were coupled with survey data collected in 2013 and again in 2019, from the main consumers of bivalves. The tAs intake was up to 6 mg.day-1 for a 240 g (wet weight) meal of bivalves. In addition, we proposed toxicological reference doses also based on detected toxic forms of arsenic and tested their relevance. We concluded that monitoring of total arsenic would be sufficient to ensure the protection of bivalve consumers. Consumption patterns expose consumers to a potential health risk. However, due to a decrease in consumption frequency associated with the depletion of bivalve resources by decomposing Sargassum mats, arsenic exposure has decreased. In the French Caribbean, this is the first study on the risk of human arsenic contamination from the ingestion of bivalves. This study is a contribution to the monitoring of arsenic in the Caribbean coastal environment.

Role of human gut bacteria in arsenic biosorption and biotransformation

There is growing evidence that human gut microbiota can metabolize arsenic (As); however, which bacteria play roles in this metabolism is unclear. In this study, we measured the abilities of 21 human gut bacteria strains from diverse clades to adsorb and transform As using in vitro method with the aim of determining which bacteria play a role in As metabolism. Seven strains showed high biosorption of As, ranging from 20.1 to 29.8%, which was attributed to functional groups on the bacterial surfaces, such as hydroxyl, amino, and carboxyl groups. Moreover, six of these seven strains were versatile, as they also had roles in reducing As(V) to As(III), which is mainly regulated by the arsC gene. Escherichia coli had the strongest tolerance to As and the highest reducing ability, with a value of 71.04-73.13 µM As/h. This study reveals that gut bacteria play essential roles in As biosorption and biotransformation, and provides a better understanding of which strains are involved, which has implications for the regulation of As toxicity-based gut bacteria and provides basic data for regulating arsenic to human health.

Potential Application of Living Microorganisms in the Detoxification of Heavy Metals

Heavy metal (HM) exposure remains a global occupational and environmental problem that creates a hazard to general health. Even low-level exposure to toxic metals contributes to the pathogenesis of various metabolic and immunological diseases, whereas, in this process, the gut microbiota serves as a major target and mediator of HM bioavailability and toxicity. Specifically, a picture is emerging from recent investigations identifying specific probiotic species to counteract the noxious effect of HM within the intestinal tract via a series of HM-resistant mechanisms. More encouragingly, aided by genetic engineering techniques, novel HM-bioremediation strategies using recombinant microorganisms have been fruitful and may provide access to promising biological medicines for HM poisoning. In this review, we summarized the pivotal mutualistic relationship between HM exposure and the gut microbiota, the probiotic-based protective strategies against HM-induced gut dysbiosis, with reference to recent advancements in developing engineered microorganisms for medically alleviating HM toxicity.

Biodegradation of arsenobetaine to inorganic arsenic regulated by specific microorganisms and metabolites in mice

Arsenobetaine (AsB) is a primary arsenic (As) compound found in marine organisms. However, in mammals, the metabolic mechanism of AsB remains indistinct. Therefore, in this study, we investigated the biotransformation and regulatory mechanism of AsB, particularly the biodegradation process, in a mouse model to assess the underlying health hazards of AsB. We studied the biotransformation process of AsB in mice through the food chain [AsB feed-marine fish (Epinephelus fuscoguttatus)-mice (Mus musculus)]. Our results showed the significant bioaccumulation of total As, AsB, and, in particular, arsenate [As(V)] through biodegradation in mice tissues. As the abundance of Staphylococcus and Blautia (phylum, Firmicutes) increased, the expression of aqp7 (absorption) and methyltransferase (as3mt) (methylation) was upregulated. In contrast, the expression of S-adenosyl methionine (sam) (methylation) was downregulated. These findings suggest that demethylation and methylation occurred simultaneously in the intestines, with demethylation capacity being greater than that of methylation. Furthermore, Firmicutes such as Staphylococcus and Blautia showed a significant inverse relationship with arachidonic acid, choline, and sphingosine. Gene, microbiome, and metabolomics analyses indicated that Staphylococcus and Blautia and arachidonic acid, choline, and sphingosine participated in the degradation of AsB to As(V) in mouse intestines. Therefore, long-term AsB ingestion through marine fish consumption could cause potential health hazards in humans.

In situ analysis of variations of arsenicals, microbiome and transcriptome profiles along murine intestinal tract

In situ-based studies on microbiome-host interactions after arsenic exposure are few. In this study, the variations in arsenics, microbiota, and host genes along murine intestinal tracts were determined after arsenic exposure for two months. There was a gradual increase in the concentration of total As (C_tAs) in feces from ileum to colon, whereas C_tAs in the corresponding tissues were relatively stable. Differences in arsenic levels between feces and tissues were significantly different. The proportion of arsenite (iAs^Ⅲ) in feces gradually decreased, however, it gradually increased in tissues. After arsenic exposure, the diversity and abundance of microbial community and networks in each segment were significantly dysregulated. Notably, 328, 579 and 90 differently expressed genes were detected in ileum, cecum, and colon, respectively. In addition, microbiome and transcriptome analyses showed a significant correlation between the abundance of Faecalibaculum and expressions of Plb1, Hspa1b, Areg and Duoxa2 genes. This implies that they may be involved in arsenic biotransformation. In vitro experiments using Biofidobactrium and Lactobacillus showed that probiotics have arsenic transformation abilities. Therefore, gut microbiome may modulate arsenic accumulation, excretion and detoxification along the digestive tract. Moreover, the abundance and diversity of gut microbiome may be related to the changes in host health.

Urolithin A attenuates arsenic-induced gut barrier dysfunction

Environmental chemicals such as inorganic arsenic (iAs) significantly contribute to redox toxicity in the human body by enhancing oxidative stress. Imbalanced oxidative stress rapidly interferes with gut homeostasis and affects variety of cellular processes such as proliferation, apoptosis, and maintenance of intestinal barrier integrity. It has been shown that gut microbiota are essential to protect against iAs3+-induced toxicity. However, the effect of microbial metabolites on iAs3+-induced toxicity and loss of gut barrier integrity has not been investigated. The objectives of the study are to investigate impact of iAs on gut barrier function and determine benefits of gut microbial metabolite, urolithin A (UroA) against iAs3+-induced adversaries on gut epithelium. We have utilized both colon epithelial cells and in a human intestinal 3D organoid model system to investigate iAs3+-induced cell toxicity, oxidative stress, and gut barrier dysfunction in the presence or absence of UroA. Here, we report that treatment with UroA attenuated iAs3+-induced cell toxicity, apoptosis, and oxidative stress in colon epithelial cells. Moreover, our data suggest that UroA significantly reduces iAs3+-induced gut barrier permeability and inflammatory markers in both colon epithelial cells and in a human intestinal 3D organoid model system. Mechanistically, UroA protected against iAs3+-induced disruption of tight junctional proteins in intestinal epithelial cells through blockade of oxidative stress and markers of inflammation. Taken together, our studies for the first time suggest that microbial metabolites such as UroA can potentially be used to protect against environmental hazards by reducing intestinal oxidative stress and by enhancing gut barrier function.

Feed-additive Limosilactobacillus fermentum GR-3 reduces arsenic accumulation in Procambarus clarkii

Procambarus clarkii (crayfish) accumulates a high concentration of Arsenic (As) from the aquatic environment and causes considerable human health risks. In this study, Limosilactobacillus fermentum GR-3 strain was isolated from "Jiangshui" and applied for As(III) adsorption and antioxidant abilities. Strain GR-3 removed 50.67% of 50 mg/L As(III) and exhibited the high antioxidant potential of DPPH (1,1-Diphenyl-2-picrylhydrazyl) (87.63%) and hydroxyl radical (74.51%) scavenging rate in vitro. P. clarkii was feed with strain GR-3, the results showed that As(III) concentration reduced, and residual level in hepatopancreas was decreased by 36%, compared to As(III)-exposed group (control). Gut microbial sequencing showed that strain GR-3 restores gut microbiota dysbiosis caused by As(III) exposure. Further application in the field scale was performed and revealed a decrease in As(III) accumulation and increasing 50% aquaculture production of the total output. In summary, feed-additive probiotic is recommended as a novel strategy to minimize aquaculture foods toxicity and safe human health.

The Need to Unravel Arsenolipid Transformations in Humans

The main source of arsenic exposure to humans worldwide is the diet, in particular, drinking water, rice, and seafood. Although arsenic is often considered toxic, it can exist in food as more than 300 chemical species with different toxicities. This diversity makes it difficult for food safety and health authorities to regulate arsenic levels in food, which are currently based on a few arsenic species. Of particular interest are arsenolipids, a type of arsenic species widely found in seafood. Emerging evidence indicates that there are risks associated with human exposure to arsenolipids (e.g., accumulation in breast milk, ability to cross the blood-brain barrier and accumulate in the brain, and potential development of neurodegenerative disorders). Still, more research is needed to fully understand the impact of arsenolipid exposure, which requires establishing interdisciplinary collaborations.

Sub-chronic low-dose arsenic in rice exposure induces gut microbiome perturbations in mice

Long-term consumption of arsenic-contaminated rice has become a public health issue that urgently needs to be addressed. In this study, mice were exposed to arsenic in rice (low dose, 0.91 mg/kg; medium dose, 9.1 mg/kg) for 30 days and 60 days, respectively, and the effects on pathological structures of spleen and skin, as well as the structure of the fecal microbiome were examined. The findings revealed dose/time cumulative effects on pathological changes, with even a low dose exposure for 30 days causing destruction of splenic follicular structure and thickening of dermal keratinized and epidermal layers. The Firmicutes/Bacteroidetes ratio in the community and the positive/negative ratio in network links were higher in arsenic groups, suggesting that arsenic resulted in a less healthy and unstable microbiome for the host. Thus lifetime consumption of arsenic in rice may have potential health effects on humans and must be carefully assessed to safeguard human health. Furthermore, in arsenic groups, arsenic-resistant bacteria or arsenic hazards remediation bacteria changed to be the dominant bacteria and acted as the core bacteria in the network modules. Some microbial arsenic transforming genes (arsC, arsR, arsA, ACR3, and aoxB) differed, indicating that the gut microbiome changed to withstand arsenic stress. Furthermore, Faecalibaculum, Lachnospiraceae_NK4A136_group, Angelakisella, Ruminiclostridium, and Desulfovibrionaceae are positively associated with arsenic dosage and may be useful in the early detection of arsenicals.

Probiotics and gut microbiome - Prospects and challenges in remediating heavy metal toxicity

The gut microbiome, often referred to as "super organ", comprises up to a hundred trillion microorganisms, and the species diversity may vary from person to person. They perform a decisive role in diverse biological functions related to metabolism, immunity and neurological responses. However, the microbiome is sensitive to environmental pollutants, especially heavy metals. There is continuous interaction between heavy metals and the microbiome. Heavy metal exposure retards the growth and changes the structure of the phyla involved in the gut microbiome. Meanwhile, the gut microbiome tries to detoxify the heavy metals by altering the physiological conditions, intestinal permeability, enhancing enzymes for metabolizing heavy metals. This review summarizes the effect of heavy metals in altering the gut microbiome, the mechanism by which gut microbiota detoxifies heavy metals, diseases developed due to heavy metal-induced dysbiosis of the gut microbiome, and the usage of probiotics along with advancements in developing improved recombinant probiotic strains for the remediation of heavy metal toxicity.

Arsenic: A Global Environmental Challenge

Arsenic is a naturally occurring metalloid and one of the few metals that can be metabolized inside the human body. The pervasive presence of arsenic in nature and anthropogenic sources from agricultural and medical use have perpetuated human exposure to this toxic and carcinogenic element. Highly exposed individuals are susceptible to various illnesses, including skin disorders; cognitive impairment; and cancers of the lung, liver, and kidneys. In fact, across the globe, approximately 200 million people are exposed to potentially toxic levels of arsenic, which has prompted substantial research and mitigation efforts to combat this extensive public health issue. This review provides an up-to-date look at arsenic-related challenges facing the global community, including current sources of arsenic, global disease burden, arsenic resistance, and shortcomings of ongoing mitigation measures, and discusses potential next steps.

The gut microbiome and arsenic-induced disease-iAs metabolism in mice

PURPOSE OF REVIEW

This review summarizes inorganic arsenic (iAs) metabolism and toxicity in mice and the gut microbiome and how iAs and the gut microbiome interact to induce diseases.

RECENT FINDINGS

Recently, a variety of studies have started to reveal the interactions between iAs and the gut microbiome. Evidence shows that gut bacteria can influence iAs biotransformation and disease risks. The gut microbiome can directly metabolize iAs, and it can also indirectly be involved in iAs metabolism through the host, such as altering iAs absorption, cofactors, and genes related to iAs metabolism. Many factors, such as iAs metabolism influenced by the gut microbiome, and microbiome metabolites perturbed by iAs can lead to different disease risks. iAs is a widespread toxic metalloid in environment, and iAs toxicity has become a global health issue. iAs is subject to metabolic reactions after entering the host body, including methylation, demethylation, oxidation, reduction, and thiolation. Different arsenic species, including trivalent and pentavalent forms and inorganic and organic forms, determine their toxicity. iAs poisoning is predominately caused by contaminated drinking water and food, and chronic arsenic toxicity can cause various diseases. Therefore, studies of iAs metabolism are important for understanding iAs associated disease risks.

A Single Microbiome Gene Alters Murine Susceptibility to Acute Arsenic Exposure

Environmental toxicant exposure contributes to morbidity and mortality of many human diseases. With respect to arsenic, microbially driven chemical transformations dictate its toxicity and mobility in virtually every environment yet studied, so a general hypothesis is that the human gut microbiome determines disease outcome following exposure. However, the complex nature of the gut microbiome and the myriad of potential interactions with human cells/tissues make it challenging to quantify the influence of specific arsenic-active functions-a requisite step in developing effective disease prevention and/or clinical intervention strategies. To control both mammalian and microbial function during toxicant exposure, we genetically defined the gut microbiome of mice using only Escherichia coli strain, AW3110 (▵arsRBC), or the same strain carrying a single genome copy of the Fucus vesiculosus metallothionein gene (AW3110::fmt); a cysteine-rich peptide that complexes with arsenite, facilitating bioaccumulation and reducing its toxic effects. AW3110::fmt bioaccumulated significantly more arsenic and gnotobiotic mice colonized by this strain excreted significantly more arsenic in stool and accumulated significantly less arsenic in organs. Moreover, AW3110::fmt gnotobiotic mice were protected from acute toxicity exposure (20 ppm AsIII) relative to controls. This study demonstrates-in a highly controlled fashion-that a single microbiome function (arsenic bioaccumulation) encoded by a single gene in a single human gut microbiome bacterium significantly alters mammalian host arsenic exposure. The experimental model described herein allows for a highly controlled and directed assessment of microbiome functions, and is useful to quantify the influence of specific microbiome-arsenic interactions that help mitigate human disease.

Introducing the ArsR-Regulated Arsenic Stimulon

The microbial ars operon encodes the primary bacterial defense response to the environmental toxicant, arsenic. An important component of this operon is the arsR gene, which encodes ArsR, a member of the family of proteins categorized as DNA-binding transcriptional repressors. As currently documented, ArsR regulates its own expression as well as other genes in the same ars operon. This study examined the roles of four ArsR proteins in the well-developed model Gram-negative bacterium Agrobacterium tumefaciens 5A. RNASeq was used to compare and characterize gene expression profiles in ± arsenite-treated cells of the wild-type strain and in four different arsR mutants. We report that ArsR-controlled transcription regulation is truly global, extending well beyond the current ars operon model, and includes both repression as well as apparent activation effects. Many cellular functions are significantly influenced, including arsenic resistance, phosphate acquisition/metabolism, sugar transport, chemotaxis, copper tolerance, iron homeostasis, and many others. While there is evidence of some regulatory overlap, each ArsR exhibits its own regulatory profile. Furthermore, evidence of a regulatory hierarchy was observed; i.e. ArsR1 represses arsR4, ArsR4 activates arsR2, and ArsR2 represses arsR3. Additionally and unexpectedly, aioB (arsenite oxidase small subunit) expression was shown to be under partial positive control by ArsR2 and ArsR4. Summarizing, this study demonstrates the regulatory portfolio of arsenite-activated ArsR proteins and includes essentially all major cellular functions. The broad bandwidth of arsenic effects on microbial metabolism assists in explaining and understanding the full impact of arsenic in natural ecosystems, including the mammalian gut.

Bioaccumulation of trace elements affects chick body condition and gut microbiome in greater flamingos

Elevated concentrations of trace elements represent a major concern to wetland ecosystems, since river estuaries are geochemical endpoints that accumulate pollution. Although the negative impact of environmental exposure of highly toxic elements such as Pb and Hg has received substantial attention, we still lack a comprehensive understanding of the effects that these and other common trace elements have on natural populations. We used greater flamingos as a study system within three sites that represent a gradient of pollution. Controlling for environmental sediment exposure, we assessed if signatures of bioaccumulation in feathers for ten trace elements (As, Cd, Cr, Cu, Hg, Ni, Pb, Se, Sn and Zn) are associated with two known proxies of health: body condition and the gut bacterial microbiome. We found evidence of an adverse effect of Se, Hg, and Pb bioaccumulation on body condition. Furthermore, bioaccumulation of the elements As, Cu, Se, Pb and Zn influenced different aspects of the gut microbiome. Bioaccumulation of Se led to a shift in the microbiome composition, largely driven by an enrichment of Bacteroides plebeius, which is linked to the breakdown of sulphated polysaccharides of algae. Bacteroides plebeius was negatively associated with chick body condition, suggesting an adverse effect of a microalgae diet rich in Se. Pb bioaccumulation was linked with a decrease in microbial diversity (adjusted-R² = 10.4%) and an increase in heterogeneity of the microbial community (adjusted-R² = 10.5%), an indication of impaired gut homeostasis. As, Cu and Zn had more nuanced effects on gut microbiome heterogeneity according to breeding site and bioaccumulation concentration. Our results therefore suggest that in addition to well-studied elements, bioaccumulation of poorly studied elements also adversely affect health of natural populations.

The mycobiome in murine intestine is more perturbed by food arsenic exposure than in excreted feces

Arsenic is a global pollutant that can accumulate in rice and has been confirmed to disturb the gut microbiome. By contrast, the influence on the gut mycobiome is seldom concerned because fungi comprise a numerically small proportion of the whole gut microcommunity. To expand the detection of the mycobiome in different gut sections of mammals and investigate the influence of food arsenic on the gut mycobiome in the digestive tract, we treated mice with feeds containing different compositions of arsenic species (7.3% sodium arsenate, 72.7% sodium arsenite, 1.0% sodium monomethylarsonate, and 19.0% sodium dimethylarsinate) in rice at a total arsenic dose of 30 mg/kg. After 60 days of exposure, the feces of four different sites, the ileum, cecum, colon, and excreted feces, were collected and analyzed by internal transcribed spacer gene sequencing. Among the samples, the major fungal phyla were Ascomycota, Basidiomycota, and Zygomycota; the top 10 fungal genera were Aspergillus, Verticillium, Penicillium, Cladosporium, Alternaria, Fusarium, Ophiocordyceps, Trametes, Mucor, and Nigrospora. In control mice, along the murine digestive tract, the mycobial richness and composition were significantly changed; Aspergillus and Penicillium possessed the higher ability to be stabilized in the murine gut, and larger proportions of positive correlations were observed among the major fungi. After arsenic exposure, the fungal composition was more disturbed in the intestinal tract than in feces. Along the digestive tract, arsenic can trigger larger mycobial variations, and the sensitivities of major fungi to arsenic were changed. Thus, the murine intestinal spatial mycobiota are more perturbed than excreted fecal mycobiota after food arsenic exposure. Feces are insufficient to be selected as a representative of the gut mycobiota in arsenic exposure studies.

Gut microbiota: A target for heavy metal toxicity and a probiotic protective strategy

There is growing epidemiological evidence that heavy metals (HMs) may contribute to the progression of various metabolic diseases and that the etiology and progression of these diseases is partly due to HM-induced perturbations of the gut microbiota. Importantly, the gut microbiota are the first line of defense against the toxic effects of HMs, and there is a bidirectional relationship between the two. Thus, HM exposure alters the composition and metabolic profile of the gut microbiota at the functional level, and in turn, the gut microbiota alter the uptake and metabolism of HMs by acting as a physical barrier to HM absorption and by altering the pH, oxidative balance, and concentrations of detoxification enzymes or proteins involved in HM metabolism. Moreover, the gut microbiota can affect the integrity of the intestinal barrier, which may also in turn affect the absorption of HMs. Specifically, probiotic have been shown to reduce the absorption of HMs in the intestinal tract via the enhancement of intestinal HM sequestration, detoxification of HMs in the gut, changing the expression of metal transporter proteins, and maintaining the gut barrier function. This review is a summary of the bidirectional relationship between HMs and gut microbiota and of the probiotic-based protective strategies against HM-induced gut dysbiosis, with reference to strategies used in the food industry or for medically alleviating HM toxicity.

Microbial Oxidation of Arsenite: Regulation, Chemotaxis, Phosphate Metabolism and Energy Generation

Arsenic (As) is a metalloid that occurs widely in the environment. The biological oxidation of arsenite [As(III)] to arsenate [As(V)] is considered a strategy to reduce arsenic toxicity and provide energy. In recent years, research interests in microbial As(III) oxidation have been growing, and related new achievements have been revealed. This review focuses on the highlighting of the novel regulatory mechanisms of bacterial As(III) oxidation, the physiological relevance of different arsenic sensing systems and functional relationship between microbial As(III) oxidation and those of chemotaxis, phosphate uptake, carbon metabolism and energy generation. The implication to environmental bioremediation applications of As(III)-oxidizing strains, the knowledge gaps and perspectives are also discussed.