Environmental Microbiology Reports (2020) 12(2), 136–159 doi:10.1111/1758-2229.12814 Minireview Arsenic and the gastrointestinal tract microbiome Timothy R. McDermott, 1* John F. Stolz2** and Introduction Ronald S. Oremland3 The influence of microbiomes on human health and disease 1Department of Land Resources and Environmental is well established (e.g., the NIH Human Microbiome Project; Sciences, Montana State University, Bozeman, MT, Chatelier et al., 2013), with new studies continuing to demon- 59717, USA. strate the potential for important advancements in our under- 2Department of Biological Sciences and Center for standing of the basis for various human diseases and Environmental Research and Education, Duquesne conditions. One such disease condition concerns arsenic University, Pittsburgh, PA, USA. fi 3United States Geological Survey, Menlo Park, CA, (As), a toxin and carcinogen, ranking rst on the United 94025, USA. States Environmental Protection Agency Priority List of Haz- ardous Substances (Agency for Toxic Substances and Dis- ease Registry, n.d.). Based on hair analyses of Chinchorros Summary culture mummies, excess As exposure dates to prehistoric Arsenic is a toxin, ranking first on the Agency for Toxic populations of present-day Chile (Atacama Desert) (Byrne ’ Substances and Disease Registry and the Environmen- et al., 2010). In today s world more than 200 million people tal Protection Agency Priority List of Hazardous Sub- are exposed to toxic levels of As in drinking water stances. Chronic exposure increases the risk of a broad (Bhattacharjee et al., 2013; Naujokas et al., 2013), with arse- range of human illnesses, most notably cancer; how- nic contamination of drinking water in Bangladesh being ever, there is significant variability in arsenic-induced referred to as the largest mass poisoning event in human his- disease among exposed individuals. Human genetics is tory (Smith et al., 2000). Chronic As exposure has been a known component, but it alone cannot account for the linked to a range of diseases (arsenicosis), including lung, large inter-individual variability in the presentation of skin, bladder and liver cancers (Liu et al., 2008; Faita et al., arsenicosis symptoms. Each part of the gastrointestinal 2013). Interestingly, symptoms vary among individuals shar- tract (GIT) may be considered as a unique environment ing similar exposure (Naujokas et al., 2013; Cubadda et al., with characteristic pH, oxygen concentration, and 2015). Some variability is attributed to host genetics and other microbiome. Given the well-established arsenic redox factors (e.g., diet), but other causes remain to be determined. transformation activities of microorganisms, it is rea- Primary As exposure is via ingestion of either food or, sonable to imagine how the GIT microbiome composi- more commonly, As-contaminated water. In every environ- tion variability among individuals could play a ment thus far studied, microbial activities signifi cantly influ- significant role in determining the fate, mobility and tox- ence As redox speciation (trivalent vs. pentavalent), which icity of arsenic, whether inhaled or ingested. This is a dictates toxicity and mobility. Therefore, it is reasonable to relatively new field of research that would benefitfrom predict that the initial fate of As in the gastrointestinal tract early dialogue aimed at summarizing what is known and (GIT) will likewise be influenced by microbiome As metabo- identifying reasonable research targets and concepts. lism. Since microbiome composition and diversity varies Herein, we strive to initiate this dialogue by reviewing between individuals, there is potential for functional linkages – known aspects of microbe arsenic interactions and between GIT microbiome As metabolism, host exposure, fl placing it in the context of potential for in uencing host relative toxicity and disease likelihood, and thus a factor to fi exposure and health risks. We nish by considering explain symptom variability among individuals. future experimental approaches that might be of value. Much of the basics regarding microbe–As interactions originally derived from work using Escherichia coli or Staphylococcus aureus as models. This was followed by near countless studies using environmental isolates (soil, Received 27 September, 2019; accepted 25 November, 2019. For correspondence. *E-mail [email protected]; **E-mail stolz@ marine, rhizosphere, etc.). Together, these studies have duq.edu been invaluable for understanding what is possible © 2019 Society for Applied Microbiology and John Wiley & Sons Ltd Arsenic and the gastrointestinal tract microbiome 137 elsewhere, including the GIT, and establish a framework in nature. As(V) normally dominates in well-aerated environ- for future work in the GIT microbiome arena. It is worth ments and most likely to be consumed. As(III) is both more noting that the first As-GIT microbiome study was con- toxic and mobile than As(V), making As(III) more problem- ducted by an environmental microbiology group that atic, especially in the context of drinking water from sub-oxic ® developed SHIME (Simulator of the Human Intestinal aquifers. In all environments where arsenic and microbes Microbial Ecosystem), which is a modular bioreactor gut coexist, microbes are the principal drivers of this speciation model that mimics the entire GIT, incorporating the stom- and thus are an integral part of understanding arsenic ach, small intestine and different colon regions as a cycling (Stolz and Oremland, 1999; Inskeep et al., 2001; human intestinal microbial ecosystem (Van de Wiele Oremland and Stolz, 2005; Huang et al., 2014). In simplest et al., 2004; Van de Wiele et al., 2015). It was first used terms, microbial catalysed arsenic transformations can be to examine arsenic metabolism in the context of the GIT summarized as being either oxidation, reduction, and/or (Van de Wiele et al., 2010), exposing a colon microbial (de)methylation. From the perspective of the microbe, all community to either inorganic As (iAs) or As- these reactions are ‘motivated’ by self-interest, such as contaminated soil, recording significant levels of As meth- detoxification of a poison, or in some cases the generation ylation and thiolation. of cellular energy. Table 1 and Figs 2 and 3 are provided as Arsenic research in the GIT microbiome is in a relative reference material for the different transformation functions state of infancy and can benefit from an early exchange discussed below. of ideas, concepts and views. Chi et al. (2018) review how human genetics, lifestyle and diet will influence indi- vidual susceptibility to As-related diseases. We offer to Arsenate resistance and transformations continue this dialogue but explicitly from the perspective ars gene-based resistance of the GIT microbiome. There are now growing GIT microbiome databases for healthy or diseased individ- Microbes reduce As(V) as part of a resistance mecha- uals, including the oral cavity (e.g., nasal, buccal, gingiva nism and/or for energy generation (Stolz and Oremland, and tongue), stomach, small intestine and large intestine 1999; Oremland and Stolz, 2005). While genetically dis- (Fig. 1). We consider each as a unique environment with tinct (ars genes encode resistance; arr genes encode respect to pH and oxygen concentration, as well as the respiratory activity), these processes are not necessarily residence time (i.e., how quickly ingested arsenic moves mutually exclusive and can occur in the same organism through the GIT and hence influencing potential host (reviewed by Andres and Bertin, 2016). They are most exposure). often found in the genome, but the literature documents We begin by summarizing how and why microbes react numerous instances where they are also found on plas- to As to establish a context with respect to the range of mids (e.g., San Francisco et al., 1990; Bruhn et al., 1996; microbial biochemistries that may be occurring in the Uhrynowski et al., 2019). GIT. We discuss how these activities may reduce or Arsenic resistance is encoded by the ars genes enhance toxicity and host exposure, and highlight the (Table 1) expressed in response to As exposure. The ars complexities of microbial arsenic transformations in operon is comprised of at least three genes arsRBC regards to sorting out cause and effect, as well as poten- (Rosen et al., 1992), typically expressed as one transcrip- tial influence(s) on the host. This is followed by the tional unit. ArsR is a repressor that exerts on/off control results of our GIT metagenomes surveys and a review of of the ars operon. ArsC is a reductase that converts the GIT microbiome As metabolism. We finish by As(V) to As(III) (Fig. 2A) and ArsB facilitates As(III) extru- suggesting lines of investigation that may provide greater sion (Figs 2A and 3). Acr3 is another extrusion mecha- insight into how the GIT microbiome contributes to nism and its encoding gene is actually found more pathology as well as potential preventative health frequently than arsB in ars operons (Yang et al., 2015). strategies. Just recently, Shi et al. (2018) characterized ArsK, which also facilitates extrusion of As(III) as well as other triva- lent metalloids (Fig. 3). Importantly, ars gene/operon Microbe–arsenic interactions expression is independent of redox conditions. Crucial factors affecting As cycling in any environment and When As(V) enters the GIT, a bacterium will take it up human exposure (i.e., toxicity, bioavailability and bio- via a phosphate transporter, with two recognized fates. It accumulation) are directly related to its chemical speciation can be reduced by background levels of ArsC to form (Stolz and Oremland, 1999; Inskeep et al., 2001; Oremland As(III), which is the inducer that triggers up-regulation of and Stolz, 2005). The oxyanions arsenite [As(OH)3,hereaf- the ars genes resulting in considerably enhanced ter referred to as As(III)] and its oxidized counterpart arse- As(V) reductase activity and the As(III) efflux pumping -(3-n) nate [HnAsO4 , As(V)] are the prevalent forms of arsenic mechanism (ArsB, ArsK or Acr3). Thus, somewhat © 2019 Society for Applied Microbiology and John Wiley & Sons Ltd, Environmental Microbiology Reports, 12, 136–159 138 T.