Perturbation of the Gut Microbiome by Arsenic

Since the primary route of human exposure to arsenic in our body is through ingestion of contaminated drinking water or food, intestinal microbiota are the most vulnerable to arsenic- mediated toxicity and were found to play a role in arsenic-related diseases (Monachese et al., 2012; Dong et al., 2017; Roggenbeck et al., 2019). In fact, composition of the gut microbial community is determined by arsenic dose and duration of exposure (Coryell et al., 2019), which was demonstrated in both lab animals and humans.

The normal murine colon microbiota is a complex microbial community dominated primarily by Firmicutes (e.g., Clostridium, Coprococcus, Ruminococcus, Lactobacillus) and Bacteroidetes (e.g., Bacteroides, Parabacteroides) but also harbors populations of Deferribacter, Acinetobacter, Enterobacter, and Bifidobacterium. A significant reduction of Firmicutes was shown following an exposure of arsenic through drinking water in mice (Lu et al., 2014, 2015; Richardson et al., 2018). Other studies, however, found increased abundance of Firmicutes associated with decreased population of Bacteroidetes (Guo et al., 2014). In human, arsenic exposure in US infants showed a positive association of Firmicutes phylum and a negative association of Bacteroides and Bifidobacterium (Hoen et al., 2018). In Bangladeshi children (4-6years of age), high levels of arsenic in-home drinking water were associated with a greater abundance of Gammaproteobacteria in the microbi- ome, more specifically, members of the Enterobacteriaceae family (Dong et al., 2017). However, in the US infants, there was a negative interaction between Enterobacteriaceae and urinary arsenic (Hoen et ah, 2018).

Influence of Probiotics and Prebiotics on Gut Microbiome Dysbiosis-Induced by Arsenic and Mercury

Probiotic in food is a safe and inexpensive way to fight exposure to toxic HMs. Both prebi- otic and probiotic preserve intestinal barrier integrity by promoting commensal bacterial growth. Bifidobacterium is an important probiotic which is essential for intestinal microbial homeostasis, gut barrier, and LPS reduction (Li et ah, 2017; Belizario et ah, 2018). Other probiotic especially lactic acid bacteria can contribute to the elimination of HM, thanks to their high affinity for HM. Yogurt containing Lactobacillus rhamnosus provided beneficial effects on pregnant women and children by reducing bioaccumulation of mercury and arsenic (Gokulan et ah, 2018). Study on Tanzanian pregnant women showed probiotic yogurt consumption had a protective effect against arsenic (Bisanz et ah, 2014). In vitro, Lactobacillus acidophilus and Lactobacillus crispatus DSM20584 were able to remove arsenic (50-1,000 ppb) from contaminated water within 4h of exposure in a concentration-dependent manner (Monachese et ah, 2012).

Prebiotics have a protective effect against HMs. Dietary supplementation of galactooligosac- charide (GOS) can be considered as a potentially protective prebiotic that can benefit the host by regulating the microenvironment of the gut to maintain gut microbiota homeostasis (Zhai et ah, 2019). Prebiotic such as oligofructose has a protective role against arsenic-induced alteration in the gastrointestinal microbiome (Starr, 2011). The combination of prebiotic and probiotic could therefore play important roles in controlling the modulation of gut microbiome by HMs.

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