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

Abstract

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.

1. INTRODUCTION

The burgeoning field of exposomics, or the examination of both chemical pollutant and non-chemical stressors and their biological impacts across the life course on health and disease, points to the need for an increased understanding of biological pathways and mechanisms by which external perturbations alter host regulatory mechanisms and common disease pathologies (1). Vertebrate gut microbiota include trillions of viruses, fungi, and bacteria found within the gastrointestinal tract and are increasingly acknowledged as a critical component in host physiology and health. Gut microbiota perform various beneficial tasks within the human body and are increasingly considered a unique organ system due to their fundamental role in regulating host metabolism and immune system regulation (2-10).

Natural and manmade chemical pollutants are often ingested, and gut microbiota are a first line of defense in their metabolism and detoxification . Emerging animal and human evidence supports the notion that the ingestion of chemical pollutants can directly interact with microbiota, increase the co-selection of harmful microbes, or directly interact with gut mucosal and intestinal barrier integrity through a number of signaling and receptor-mediated pathways with direct and indirect effects on systemic inflammation. Chemical pollutants may also alter the composition or interact with numerous commensal microbes involved in producing secondary bile acids, short-chain fatty acids (SCFAs), carbohydrates, and other metabolites, some of which are absorbed via the intestinal tract for energy and immune defense. Microbial metabolism can also change the conformation of chemical pollutants to forms that support their excretion and or lead to the bioactivation of more toxic forms.

Over the last two decades, technological advances in genomic sequencing have led to an increased understanding of the composition and metabolic function of gut microbiota (11,12). Gut microbiota are responsible for metabolizing numerous polysaccharides that produce several beneficial metabolites, including vitamins and SCFAs (butyrate, acetate, and propionate). The Human Gut Microbiome Project was among the first to demonstrate that Bacteroidetes and Firmicutes make up approximately 90% of the human gut microbiota, with Actinomycetota, Verrucomicrobiota, Proteobacteria, and Fusobacteriota also recognized as important phyla (13-17). Each of these phyla contains a number of commensal microbial species responsible for normal gut functions. Many pathogenic bacteria are found in the Bacteroidetes phylum, with one of the most well-known examples being E. coli (family Enterobacteriaceae) (14,18). Other phyla that contain pathogenic microbes include members of Proteobacteria, such as the Burkholderiales order, members of Thermodesulfobacteriota, such as the Desulfovibrionaceae family, and members of Actinomycetota, such as Collinsela (17,19,20). These bacteria can promote dysbiosis and damage the intestinal lining by producing toxins that kill other bacteria or disrupt intestinal lining cells or induce an inflammatory response that can further contribute to dysbiosis and host health (14,18,20-24). Many beneficial bacteria in the Firmicutes phylum, such as the Ruminococcaceae and Lachnospiraceae families, produce anti-inflammatory and energy-providing SCFAs (25,26). Other important bacteria include members of Verrucomicrobiota such as Akkermansia muciniphila, which degrades mucus in addition to producing SCFAs (27).

The introduction of chemical pollutants into this complex gut ecosystem may alter normal gut function with both local and systemic effects (Figure 1). Changes in gut bacterial composition can lead to microbial byproducts including bacterial toxins and bacteria being released directly into the bloodstream and/or changing the permeability of the intestinal lining, increasing the potential for acute infections and immune system dysregulation (28). Beyond these direct influences on infection risk, longer-term gut dysbiosis can alter normal microbial metabolic pathways, including the production of beneficial metabolites. For example, SCFAs regulate the gut immune system by inhibiting the production of pro-inflammatory mediators by intestinal epithelial cells, particularly those regulated via MAPK and NF-κB pathways (29). A decrease in SCFA-producing bacteria is associated with not only decreased anti-inflammatory response but also decreases in gut integrity (e.g., increased gut permeability, decreased mucin production, and gut motility) as well as certain gastrointestinal disorders and diseases (29). Other beneficial metabolic biotransformations include deconjugation, oxidation and epimerization of hydrophilic bile acids important in cholesterol regulation and vitamin absorption. Other metabolites interact with other important signaling receptors necessary for gut and immune homeostasis (30) such as Indole-3 carbinol, a metabolite of tryptophan and agonist of the aryl hydrocarbon receptor (AHR) (30). In addition to these beneficial metabolites, some metabolic processes, such as amino acid fermentation, can lead to the production of toxic metabolites, including cresol.

Figure 1.

Overview of the diverse mechanisms by which chemical pollutants interact with gut microbial pathways and host immune response pathways associated with inflammation related disease outcomes.

While there has been increasing evidence to support that chemical pollutant exposures contribute to systemic inflammation, to date, the literature lacks a summary of the foundational evidence from both animal and human models that illustrates the diverse and common mechanisms by which chemical pollutant exposures frequently found in the human exposome intersect with gut microbiota contributing to altered host metabolism and inflammation (31). 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 either directly or indirectly to alter disease risk, with a particular focus on inflammatory pathways. Select chemical pollutants and pollutant classes were chosen because ingestion is a primary route of exposures including: (i) heavy metals, (ii) persistent organic pollutants (POPs), and (iii) nitrates. A summary of diverse mechanisms by which chemical pollutants affect gut microbiota metabolism and inflammation can be found in Table 1. We use foundational evidence from select animal models, in vitro studies, and human studies to illustrate the multiple mechanisms by which chemical pollutants can have local influence in altering gut microbial composition and intestinal inflammation with more systemic impacts on host metabolism and immune system regulation.

Table 1.

Summary of diverse mechanisms by which chemical pollutant exposure alters gut microbiota metabolism and inflammation

Chemical Pollutant	Human Exposure Pathways	Chemical Pollutant Exposure Links to Gut Dysbiosis		Select Studies
		Interactions with gut microbiota, contributing to altered metabolite with implications for systemic inflammation	Inflammation in Intestinal Lining of Gut
Lead	Ingestion of contaminated food, water, and dust from lead paint	Decrease in abundance of beneficial bacteria (Ruminococcaceae, Lachnospiraceae, Akkermansia) and Firmicutes:Bacteriodes ratio in animal models. Increase in abundance of opportunistic/pathogenic bacteria in animal/human models (Parabacteriodes, Burkholdirales). Alters levels of key metabolites along glycolysis/TCA cycle in gut microbes.	Increase in inflammatory cytokines and associated proteins associated with alterations to gut physiology (IL-6, TNF-α, and NF-κB) in animal models. Increase in changes to gut physiology (gut permeability, inflammation) in animal models. May be regulated by the LPS/TLR4 pathway.	(12,45,47,49,93,179,180)
Arsenic	Organic arsenic: ingestion of shellfish. Inorganic arsenic: ingestion of contaminated well water, agricultural and industrial products. Inorganic arsenic is more toxic and most studied in these assays.	Certain gut bacteria (e.g., Bacilli) can metabolize inorganic arsenic, and are increased upon exposure in animal models. SCFA producing (e.g., Ruminococcae) are reduced in animal models. In human models, exposure associated with increase in inflammatory/pathogenic bacteria (e.g., Collinsela).	Many arsenic upregulated bacteria are Gram negative and contain lipopolysaccharide (LPS), associated with inflammatory response in host. Animal models found increase in NF-κB and inflammatory cytokines in gut upon exposure. Inflammation may be further altered by lack of SCFA producing bacteria.	(60,61,71)
Cadmium	Non-smoking: ingestion of contaminated food (e.g., rice), contact with contaminated household items and construction materials. Smokers: Inhalation of cigarette smoke.	Reduces SCFA producing bacteria (e.g., Ruminococcae, Bifidobacteria) and increases Gram- positive, pathogenic bacteria (e.g., Erysipelotrichaceae) in animal models.	Decreased mucosal lining production, increased damage to villi and wall thinning in animal models. Increase in TNF-α in ileal cells post exposure in animal models. Inflammation may be further altered by lack of SCFA producing bacteria.	(28,90-92)
PCB-126	Ingestion of contaminated fatty foods, breast milk exposure in infants.	Decreased gut richness and diversity in mice. Increase in pathogenic bacteria (e.g., Aeromonas) in zebrafish. Sex-specific changes in zebrafish microbial composition. Modulates bacterial metabolism and cell function in mice.	Exposure leads to AHR activation in intestinal epithelial cells (IECs) and immune cells in the epithelium. Increases expression of AHR response genes Cyp1a1 and Cyp1b1 and inflammatory markers in animal models.	(114,119,120)
PCB-153	Ingestion of contaminated fatty foods, breast milk exposure in infants.	Decreased commensal bacteria abundance, growth rate (e.g., Bifidobacterium longum), and increase in certain bacterial metabolic processes (e.g., LPS formation) in animal models (mice and zebrafish).	Exposure associated with increase in inflammatory cytokine concentration and the NF-κB pathway via ATM/NEMO activation from DNA damage (mice and intestinal cell culture).	(122,125,127,128)
PFOS	Ingestion of contaminated food or water (e.g., fish), occupational (e.g., firefighting).	Altered Bacteriodes:Firmicutes ratio. Potential sex specific bacteria composition changes in mice (e.g., male mice had decreases in certain beneficial bacteria, while females had increases in certain beneficial bacteria). Decrease in beta diversity in exposed human infants.	Exposure may induce an inflammatory response via proteoglycan/TLR2/TNF-α and/or NF-κB, increase in pro-inflammatory cytokines (IL-1B, IL-6, TNF- α, interferon-γ (IFN-γ)).	(144,146,149)
PFOA	Ingestion of contaminated food or water (e.g., fish), occupational (e.g., firefighting).	Alters Bacteriodes:Firmicutes ratio, decreases several SCFA producing bacteria (e.g., Bifidobacterium, Lactobacillus) and their metabolic activity (e.g., SCFA production). In human infants, exposure associated with decrease in Lactobacillus, and increase in Enterococcus.	Exposure may induce an inflammatory response via proteoglycan/TLR2/TNF-α and/orNF-κB, increase in pro-inflammatory cytokines (IL-1B, IL-6, TNF- α, interferon-γ (IFN-γ)). Decrease in SCFA production may be further influencing inflammatory response.	(147-149)
Nitrates	Ingestion of contaminated water, food.	Dietary nitrate can be used in nitrogen respiration by some bacteria (e.g., Lactobacillus casei rhamnosus)., can be converted to N-nitroso compounds (NOCs) by some gut bacteria (e.g., Enterococcus).	NOCs may induce DNA damage/oxidative stress in the intestinal lining and increase inflammatory markers in animal models. Dietary nitrate associated with higher risk of colorectal cancer development in certain human populations.	(162,164,168,169,181-184)

2. HEAVY METALS

Heavy metals are both essential (e.g. iron, copper, zinc and others) and toxic(e.g. lead, cadmium, arsenic); toxic metals are more likely considered chemical pollutants and nonessential. Both essential and non-essential metals interact with gut microbiota to influence host metabolism and immune system regulation, which contribute to both local and systemic inflammation. Iron, for example, is a common essential nutrient found in food sources such as dark leafy greens, and is known for its important role in hemoglobin production. With respect to gut-microbe interactions, iron consumption has been shown to reduce harmful gut bacteria and enhance several important metabolic pathways including homone synthesis, cell-cycle regulation, anti-oxidation, electron transfer, oxygen storage and transport (32). In contrast, exposure to toxic levels of non-essential heavy metals can lead to cellular and organ damage due to their capacity to induce oxidative stress and inhibit the activity of specific enzymes and increasing evidence supports that much of this originates in the gut (33). Prolonged exposure to toxic levels of heavy metals can drive a range of pathological consequences, from neurological damage to carcinogenesis (33-35), and there may be a link between heavy metals and neurologic toxicity via the gut-brain axis (36). Lead, arsenic, and cadmium were chosen as three examples of ubiquitous heavy metals found in diet and drinking water, each of which has unique effects on the human gut microbiome and intestinal system.

2.1. Lead

Lead Ingestion through household dust, contaminated soil, and drinking water

Lead (Pb) is among the most well-known toxic heavy metals. Lead is naturally occurring and, throughout the centuries, has been used in numerous industrial processes, gasoline, and household products. Exposure has decreased in the general population due to restrictions on lead usage, but it still presents a significant risk to those who live in lead-contaminated areas, particularly children who are more likely to ingest lead-based paint and dust. Lead in soil can also contaminate food sources and lead exposure via drinking water infrastructure is also of continued concern for many communities and large urban centers with lead service lines and pipes (37,38). Lead exposure is associated with developmental and neurological toxicity in children and neonates and reproductive toxicity in adult with no established safe levels of exposure (37,39-42). Lead exposure is also associated with genotoxicity in some animal models, reducing antioxidant production and increasing reactive oxygen species (43). Chronic and/or high lead exposures are also associated with changes to gut physiology consistent with gut dysbiosis leading to changes in the immune system (12).

Animal Evidence of Lead Altering Gut Bacterial Composition Linked to Metabolism and Inflammation

Studies of lead exposure at low level doses consistent with lower level population based estimates of exposure have been shown to negatively affect gut microbiota composition in adult mice and humans (44,45). Adult male mice fed low concentrations of lead for 8 weeks had decreased abundance of beneficial bacterial families Ruminococcaceae and Lachnospiraceae, and the mucin-producing bacteria Akkermansia (44). Long-term subchronic exposure (0.03-0.1 mg/L) in adult male mice found decreases in the Firmicutes to Bacteroidetes ratio and genera Dehalobacterium, and an increase in the abundance of the opportunistic gram-negative pathogen Parabacteroides in the cecum (45). The same study found that lead exposure altered key metabolites along the glycolysis/TCA cycle pathways of gut bacteria, suggesting that lead may affect key microbial metabolic pathways (45). Lead exposure has also been shown to increase levels of gram-negative bacteria in the gut, which produce the bacterial toxin lipopolysaccharide (LPS) that binds TLR4 receptor on immune cells (46). Stimulation of the LPS/TLR4 pathway can lead to systemic inflammation characterized by increased pro-inflammatory cytokines such as IL-6 and TNF-α. LPS is also associated with the development of inflammatory gut diseases such as irritable bowel disease (IBD) (77,78 81, 82).

Another mechanism by which lead induces gut inflammation is through the decrease of beneficial bacteria that produce metabolites that maintain gut health. Lead exposure is associated with a decrease in Lachnospiraceae and Ruminococcaceae, two SCFA producing bacterial families. In addition, lead may alter bacteria that support intestinal integrity. One study found that chronic lead exposure in mice led to the downregulation of mRNA for tight junction proteins, which was associated with an increase in gut permeability (44). A diagram of the potential mechanisms of action of lead in the gut are illustrated in Figure 2.

Figure 2.

Differential pathways by which select chemical pollutants commonly ingested by humans including Metals, POPs, PFAS, and Nitrates interact with the gut microbial community, contributing to gut dysbiosis, altered metabolism, chemical toxicity, and changes within the gut intestinal barrier

Human Evidence of Lead Altering Bacterial Composition in the Gut linked to Metabolism and Inflammation

Evidence from human studies appears to corroborate the effects of lead exposure on the gut microbiome observed in mice. A Survey of the Health of Wisconsin (SHOW) study found that urinary lead concentrations were associated with increased abundances of the phylum Proteobacteria and order Burkholderiales (47). Proteobacteria, which includes Burkholderiales, is Gram-negative and associated with pathogenic activity (Figure 2) (17,22). Heavy metals, including lead, have been linked to immune system dysregulation and function, with implications for both acute response to infections. The association between lead and the immune system may also potentially influence many chronic conditions, including cancer, cardiovascular and autoimmune disease (48). However, fewer studies have examined the role that heavy metals like lead and gut microbiota interactions play in altering these pathways in humans. A small number of population-based studies have shown that lead exposure can increase antibiotic-resistant strains of bacteria in the gut based on stool measurements (49). In addition, analysis of the National Health and Nutrition Examination Survey found increasing levels of nasal methylene-resistant staphylococcus aureus (MRSA) with increasing exposure to lead, again suggesting selective pressure for antibiotic resistance. Chronic exposure to these bacteria can have acute implications for clinical treatment and immune system regulation.

Arsenic

Arsenic Ingestion through Diet and Drinking Water

Arsenic, similar to lead, is an element that occurs naturally in the environment, largely in soil and groundwater and exists in both organic and inorganic forms. Organic arsenic is most commonly found in food such as shellfish, rice, and certain meats (50). Inorganic arsenic is found in water through natural geological formations, which can then enter drinking water. Indirect exposure to inorganic arsenic in the general human population occurs through contaminated water in agriculture and through uptake in certain foods (50,51)(53,54). Inorganic arsenic is more commonly associated with toxic risk (52). Inorganic arsenic is most commonly found in the human body in its trivalent form, arsenite (As(III)), or a pentavalent form, arsenate (As(V)), both of which have documented negative effects on the gut microbiome and/or host (55,56).

Arsenic metabolism, reduction of proinflammatory bacteria, altered metabolites and immune response in mice

Evidence from animal models demonstrates multiple mechanisms by which arsenic alters human gut homeostasis with both local and systemic effects. Arsenic directly interacts with gut microbiota, which metabolize organic arsenic to inorganic arsenic, increasing its toxicity (56). Some bacteria, such as E. coli (family Enterobacteriaceae) and C. butyricum (family Clostridiaceae) contain arsenic resistance (Ars) genes, which can either metabolize 1) trivalent arsenite to pentavalent arsenate, 2) pentavalent arsenate to trivalent arsenite, or 3) trivalent arsenite or pentavalent arsenate to methylated compounds. (ref)? These resistance genes also encode for efflux transporter proteins to extrude these metabolites from the bacteria. Trivalent arsenite and methylated arsenic compounds are toxic to many gut microbiota, and previous studies have observed bacteria utilizing these compounds as bioloigical weapons to eliminate competition, altering the local microbial community in the process (51,57-59). Arsenic exposure also increases the presence of several pro-inflammatory bacteria such as the Desulfovibrionaceae family, Deferribacteriales order, and Collinsella and Bilophila genera (56,60,61). Deferribacteriales are notable, as they are normally found at low levels in the human gut and are indicative of intestinal inflammation when enriched in immunocompromised individuals (62). Similar to lead, another proposed mechanism by which arsenic can induce inflammation is through LPS activation of the TLR4 pathway (65). Several bacteria enriched during arsenic exposure (Desulfovibrio, Bilophila) are gram-negative and, therefore, contain LPS in their outer membranes (46). To further support this link, mice and human intestinal cell lines exposed to arsenite had higher levels of inflammatory cytokines associated with IBD, such as TNF-α and IL-8 (61,66). Arsenic has also been shown to directly activate NF-κB in the host cells via oxidative DNA damage (67). Thus, a host-dependent mechanism may also be at play and may even interact with the microbe-dependent mechanism. If these changes are associated with dysbiosis and consequent LPS levels, the LPS/TLR4/NF-κB pathway could lead to subsequent inflammation and other negative health effects for the host (68,69).

Both chronic and acute changes in gut microbial composition were observed in animal studies of arsenic exposure, and these impacts seem to be age dependent. In one study, mice given a single dose of arsenite (0.05mg/kg/day) had an increase in Clostridium sulfatireducens after 24 hours. The observation that this level returned to normal after 48 hours highlights the need to evaluate temporal changes in the microbiome after acute or changing exposures (61). Interestingly, the gut microbiome of juvenile mice exposed to arsenic at post natal day 21 was different than adult mice, exhibiting a predominance of bacteria related to the Clostridia class, and Eubacteriales and Deferribacteriales orders, the first of which contains genera that can methylate arsenic (59).

Similar to lead, arsenic’s effect on gut inflammation may be influenced by the associated decrease in SCFA producing bacteria. Chronic arsenic exposure induced gut dysbiosis and inflammation in several animal studies through several mechanisms (56,70). In mice, long-term exposure to arsenite at 10 ppb for ten weeks and 250 ppb for five and ten weeks in drinking water significantly increased the class Bacilli, which can metabolize arsenate and arsenite (60). Arsenic exposure also decreased beneficial bacteria Lachnospiraceae and Ruminococcaceae that produce SCFAs (71-73). In another study, a dose of ~200 g/kg/day arsenite for 8 days in adult female mice increased Bilophila and Alistipes massiliensis. Exposure also increased Lactobacillus johnsonii, a probiotic that can methylate arsenite (61,74). The lack of SCFA producing bacteria and increased gram-negative bacteria induce metabolic changes and inflammation that can lead to the development of gut diseases (75-77). A diagram of the potential mechanisms of action of arsenic in the gut are illustrated in Figure 2.

Evidence of Altered Gut Microbiome from Arsenic Exposure in Human Populations

Like lead, arsenic-induced microbiome dysbiosis has also been observed in human populations (60). For example, acute arsenic exposure via groundwater (up to 500 ug/L) promoted changes in the gut microbiota in two Nepalese communities, Mahuawa and Ghanashyampur (60). Both communities had increases in pathogenic bacteria and/or arsenic metabolizing bacteria. Those with the highest concentrations of urinary arsenic in the Ghanashyampur community had increases in Collinsella and Bilophila, pathogenic arsenic metabolizing bacteria associated with inflammation (60,63,78). Those with the highest concentrations of urinary arsenic in the Mahuawa community had increases in bacteria within the Desulfovibrionaceae family, which can metabolize arsenic and are associated with gut inflammation in high abundances (23,60). They also had increases in Bacillaceae, a family of bacteria that contain several pathogenic species that can also metabolize arsenic (60,79,80). Additionally, the Mahuawa community had decreases in bacterial families that contain beneficial bacteria, such as Clostridiaceae and Ruminococcus that were negatively correlated with urinary arsenic concentration. The authors determined the differences between the communities to be due to geographic differences and differences in urinary arsenic concentrations (25,60,81).

2.2. Cadmium

Cadmium ingestion via soil contamination, green leafy vegetables, and smoking

Cadmium (Cd) is a non-essential heavy metal that is found naturally in the soil. Plants can take up cadmium from the soil, allowing it to enter the food chain (82,83). Cadmium also enters the environment as a result of its use in cadmium-nickel batteries, the manufacturing of certain household items, and construction materials (82). Cadmium is also a major component of tobacco, with cigarettes containing 0.5-1 μg Cd/cigarette (84). As a result, smokers can have up to four to five times more cadmium in their blood than nonsmokers (84). Exposure in the general, nonsmoking population occurs via ingestion of food from cadmium contaminated soil, especially from green leafy vegetables and cereal crops such as rice (82,83). In mammalian systems, cadmium has long been known to inhibit antioxidant enzyme function and contribute to the production of reactive oxygen species (85,86). It is only recently that the potential effects of cadmium on gut microbiome have been explored.

Cadmium exposures alters commensual and inflammatory related bacteria composition and intestinal barrier

Cadmium can be tolerated or resisted by bacteria that encode cad or znt genes, although to date, this effect has largely been studied in the environment rather than the in gut microbiota (87). Some examples include members of phyla Actinobacteria, Verrucomicrobia, and Proteobacteria, as well as species Streptococcus thermophilus and Bacillus cereus (87). Many of these bacteria are gram-positive. Cadmium influence on the gut in mammalian models is in concordance with increases in cadmium-resistant bacteria and decreases in beneficial bacteria, similar to arsenic. Adult male mice exposed to 20 mg/kg and 100 mg/kg cadmium chloride (CdCl₂) in drinking water over 20 weeks displayed increases in Erysipelotrichia, a Gram-positive bacteria class. Interestingly, cadmium exposure was also associated with decreases in class Clostridia, and families Lachnospiraceae and Ruminococcaceae. While Erysipelotrichia is commonly associated with gut inflammation, Lachnospiraceae and Ruminococcaceae are commensal bacteria associated with beneficial short-chain fatty acid (SCFA) production (88-90). Decreases to the latter two bacteria were also associated with arsenic exposure, suggesting a similar mechanism of action between the two metals on bacteria. Interestingly, this study found that CdCl₂ directly inhibited certain microbial metabolic activities, including amino acid synthesis and oxidative phosphorylation (90). Similarly, female mice exposed to CdCl₂ (5, 20, and 100ppm in drinking water) for 5 days displayed decreases in Lachnospiraceae and Lactobacillaceae, and increases in Erysipelotrichaceae (91). At 3 weeks of CdCl₂ exposure, alterations in the Firmicutes : Bacteriodes ratio and decreases in beneficial bacteria Bifidobacteria and Lactobacilli were observed.

Cadmium studies also suggest the importance of both microbe and host dependent mechanisms, as well as their interplay in microbiome induced gut dysbiosis and health via inflammatory mechanisms. Chronic CdCl₂ exposure to adult rats after 20 weeks led to a decrease in the protective mucous lining of the intestinal wall, leading to damage to the villi and wall thinning (92). Interestingly, acute exposure for 24 hours is also associated with an increase in Verrucomicrobia, a phylum that is resistant to cadmium and produces mucin (92). Previously mentioned studies also report an association between CdCl₂ exposure and ileal expression of TNF-α (90,91). As the majority of observed microbial changes associated with cadmium exposure involve Gram-positive and/or cadmium resistant bacteria, it is possible that increased expression of proinflammatory cytokines occurs through TLR2 binding of peptidoglycan versus the LPS/TLR4 cascade. While the experimental approaches to examine cadmium exposure and its effects on gut health/microbiome are quite varied (e.g., dose, acute vs chronic, etc.), the collective results suggest that cadmium toxicity can accumulate and may act through changes in the microbiome, changes in the host’s inflammatory state, and interactions between the two (67,91). For example, cadmium may directly act to increase Gram-positive and/or cadmium resistant bacteria, thus inducing gut inflammation, which in turn alters the gut microbiome further through immunological mechanisms. This mechanism may be further driven by the decrease in SCFA producing bacteria associated with cadmium exposure, such as Lachnospiraceae and Ruminococcaceae (29,90). A diagram of the potential mechanisms of action of cadmium in the gut are illustrated in Figure 2.

Potential Influences of Cadmium Exposure on Immune System Regulation

Mounting animal evidence and other population-based research has shown that exposure to heavy metals can alter gut microbial composition with numerous implications for immune system regulation and systemic inflammation. However, no known human population-based studies have examined cadmium and gut microbiome alterations with inflammatory mechanisms to date (28,93). Research that examines mixtures of metals, including lead and cadmium, on the prevalence of antibiotic genes found lead but not cadmium to increase the prevalence of nasal MRSA, although findings are limited as they are not directly associated with gut microbes. Additional research beyond these limited population-based studies is needed to support animal evidence that cadmium does lead to both altered microbiota composition and metabolic function and host systemic inflammation through interactions with the intestine.

3. PERSISTENT ORGANIC POLLUTANTS (POPs)

Persistent organic pollutants (POPs) are a broad class of chemical pollutants that are lipophilic, have long-half lives, and are widely dispersed in the environment. Exposure to POPs has generated numerous negative health outcomes in animals and humans, including cancer, cardiovascular disease, and endocrine disruption (94,95). While many POPs have been phased out of production, new ones, such as perfluorinated compounds, are being introduced regularly into the environment through new manufacturing, product development and use. Human exposure to POPs often occurs via the consumption of contaminated food and water. As POPs are released into waterways by various industrial processes, their lipophilicity allows them to be uptaken from the soil to crops and in fish and wildlife, where they accumulate (96). Given their recalcitrance to both environmental and microbial degradation, often due to their lipophilicity and extensive halogenation, the extended half-lives of these compounds suggest that their influence on human health may be cumulative and important for decades to come (97).

3.1. Polychlorinated Biphenyls (PCBs)

PCB ingestion occurs via industrial pollution, contaminated animal products

As a class, PCBs are among the most well-known and widespread POPs (98). These halogenated aromatic compounds were commonly used in flame retardants, coolants, and lubricants, but are no longer produced in the USA (99). Today, PCBs are still released from hazardous waste sites or from municipal dumps that contain discarded equipment or improperly discarded industrial chemicals (100). Individual PCB congeners are named in relationship to the position and number of chlorine atoms on the biphenyl rings, with over 200 different PCB congeners possible (101,102). These compounds can be further divided into two subclasses, coplanar and noncoplanar, based on the orientation of their biphenyl rings. Those PCBs that do not contain chlorine atoms on the ortho positions are coplanar and often enact their toxicity in the human body through the aryl hydrocarbon receptor (AHR) (103). Those PCBs that have chlorines at ortho positions are non-coplanar, and do not typically activate the AHR, interacting with other signaling molecules to enact toxicity (e.g., pregnane-x) (104). In humans, PCB exposure often occurs via ingestion of contaminated food products, especially those with high fat content, as PCBs readily partition into lipids (105,106). This route may be particularly important for neonates as they can be exposed to PCBs through breastmilk, which is high in lipid content (107). PCB exposure has been associated with endocrine disruption, neurotoxicity, and genotoxicity (107-110). The interactions of PCBs with the gut microbiome have also been recently explored.

PCB-126 exposure modulates the gut via the AHR pathway, alters bacterial community composition and function

Among the most studied mechanism linking PCBs with gut microbiome and host inflammation is through the AHR pathway (111-113). Exposure to the coplanar PCB congener, PCB-126 (3,3',4,4',5-Pentachlorobiphenyl), has been shown to directly cause microbiome dysbiosis in animal models, leading to decreases in Clostridiales, Bifidobacterium, Lactobacillus, Ruminococcus, and Oscillospira and increases in Akkermansia and Helicobacter (114,115). More indirectly, the AHR has been implicated in the physiology of a wide range of immune cells. It has been suggested that AHR activation in resident immune cells, such an innate lymphoid cells (ILCs) or intraepithelial lymphocytes (IELs), influences and leads to changes in the composition of the microbial communities (116-118). In support of this idea, PCB-126 exposure increases gut transit time in wild type, but not Ahr-null mice, supporting the concept that PCBs have functional impacts on gut physiology through the AHR (119). PCB-126 exposure also resulted in higher levels of inflammatory markers (e.g., C─X─C motif chemokine 10/interferon γ-induced protein 10 (CXCL10/IP-10)) and the AHR response gene Cyp1a1 (114). Similarly, exposure to PCB-126 in zebrafish led to significant increases in CYP1a1 and CYP1b1 (120).

Beyond the AHR pathway, metagenomic analysis of gut microbial populations in mice reveal that PCB-126 exposure modulates a number of important microbial functions (e.g., carbohydrate metabolism, quorum sensing, bacterial chemotaxis, and flagellar assembly) (119). Chronic exposure to PCB-126 (50 μg/kg weekly for 8 weeks, p.o.), leads to decreased microbial richness and diversity within the cecum of wild type male mice, as well as decreased levels of Proteobacteria, both of which are associated with IBD (17,119,121). Similar studies in zebrafish support the overall influence of PCB-126 on the gut microbiome. In female zebrafish, PCB-126 exposure increased the abundance of Aeromonas, a genus of Proteobacteria, in the intestines (122). A more recent zebrafish study further corroborates this point, as PCB-126 exposure was associated with sex-specific changes in beneficial gut bacteria (120). In female fish, exposure decreased Propionibacterium, a SCFA producing bacteria, and Sphingomonas, whose members degrade pollutants such as polycyclic aromatic hydrocarbons (PAHs), dibenzodioxin (DBD), and dibenzofuran (DBF) (123,124). In male fish, PCB-126 exposure led to decreases in Propionibacterium and Cetobacterium, the latter of which aids in complex carbohydrate and peptide fermentation and vitamin B12 production. A diagram of PCB-126’s potential mechanisms of action in the gut is illustrated in Figure 2.

PCB-153 exposure alters bacterial growth and community composition, potentially via inflammation related mechanisms

An exposure study examining a spectrum of POPs, including the noncoplanar congener, PCB-153 (2,2',4,4',5,5'-hexachlorobiphenyl), a known human carcinogen, provides further evidence that this class of compounds can also influence the gut microbiome directly. A comparison of PCB-153, PCB-126, as well as two classic AHR agonists, TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), and TCDF (2,3,7,8-tetrachlorodibenzofuran), on isolated mouse cecum microbiota revealed that exposure to these POPs at levels as low as 0.6 μM leads to the reduced growth of the beneficial, Gram-positive bacteria Bifidobacterium longum. Decreases in B. longum growth after 18 hours and increases in Pi and DiBAC4-stained cells (indicating damage to cells) were also observed (125). Exposure to 6 μM PCB-153 led to significant alterations in bacterial transcription within several key taxa of mouse gut bacteria (e.g., Bacteroides, Clostridium, Lactobacillus, Bifidobacterium, and Fusobacterium). It also led to significant enrichment in bacterial transcript levels for genes involved in a number of important bacterial processes (e.g., fatty acid biosynthesis, LPS biosynthesis, vitamin B6 metabolism) (125). Supporting experiments in zebrafish demonstrate PCB-153 exposure is associated with decreased Streptococcus and Capnocytophaga abundance (122).

PCB-153 mediated gut dysbiosis and subsequent gut inflammation are presumed to be independent of the AHR, as this receptor has been shown to be relatively nonresponsive to noncoplanar ligands (126). However, evidence does suggest that compounds like PCB-153 may exert primarily indirect effects on the microbiome through a host mediated mechanism by as yet uncharacterized signaling pathways (127,128). In one study, male C57BL/6 mice exposed to 300 μmol/kg/day of PCB-153 via gavage for two days revealed that intestinal epithelial cells (IECs) of the small intestine exhibited increases in intestinal permeability, as well as significant increases in inflammatory cytokine transcription (e.g., IL-6, TNF-α) and NF-κB (128). Moreover, application of an NF-κB inhibitor, pyrrolidine dithiocarbamate (PDTC), which significantly reduced both IL-6 and TNF-α post PCB-153 exposure, provides preliminary evidence that some PCBs may act through NF-κB. To further investigate how NF-κB was activated, the same study exposed human intestinal cells (SW480) to 10 μM, 50 μM, or 100 μM PCB-153. They found that after exposure to all levels of PCB-153, cells exhibited increases in markers of DNA damage. Cells were then measured for intermediates between DNA damage and NF-κB activation, phosphorylated ATM and NEMO. Both were found to be significantly increased post-PCB 153 activation. The authors suggested that PCB-153 activates NF-κB through ATM/NEMO (128). There is additional evidence that PCB-153 may also be contributing to gut inflammation through altering gut microbes directly, primarily in the elimination of SCFA producing Bifidobacterium, as well as the potential increase in Gram-negative bacteria through the increased presence of LPS associated with PCB-153 exposure (29,125). A diagram of PCB-153’s potential mechanisms of action in the gut are illustrated in Figure 2.

To our knowledge, no known human population-based studies have examined PCB and gut microbiome alterations with inflammatory mechanisms. Given the existing regulations banning the use of most PCBs, human research may be limited to older adult populations and regions of the world for which persistent organic pollutant levels remain high. While no direct, human population-based studies have been identified, a strong connection between persistent organic pollutant exposure and cardiovascular, obesity, and cancer combined with animal evidence summarized above suggests chemical pollutant-gut microbiome–and-intestinal interactions contribute to immune system dysregulation underlying these common conditions.

3.2. Per- and Polyfluoroalkyl Substances (PFAS)

PFAS ingestion occurs through environmental contamination of food and water sources, household products

PFAS are made up of many thousands of chemicals, with differing acidic functional groups and carbon chain lengths (129). Historically, the most well known PFAS manufactured include PFOS and PFOA display long environmental half-lives that result from their lipophilicity and extensive halogenation (129). New replacement products have shorter half lives with only emerging health impacts. Historically, these compounds including PFOA and PFOS were produced to make fluoropolymer coatings and linings for products such as Teflon^™ (polytetrafluoroethylene), personal care products, water resistant clothing, electrical wiring, firefighting foam, anionic surfactants and components of plastic polymers (122,123). Certain PFAS has also been used in stain-resistant textiles, leather, upholstery, and carpet manufacturing (132,133). Both PFOS and POSF were banned from production in the USA in 2000, but are still produced in other countries (134,135).

PFASs enter the environment as runoff from foams or as byproducts of manufacturing, where it can contaminate water, sediment and ultimately the food chain (131,136). In general, human exposure can occur through occupational exposure or consumption of contaminated food and water (131,136,137). Both PFOS and PFOA have long biological half-lives and are carcinogenic and known endocrine disruptors, affecting thyroid responses, in both animal and human studies (138,139). PFAS have been found to be hepatotoxic in humans, while animal studies have shown that PFAS can exert hepatotoxic and immunotoxic effects, the latter of which could play an important role in host-directed gut microbiome dysbiosis and pathology (140-143). Many of the newer PFAS lack toxicity information-however, PFOS and PFOA have also been studied for their influence on gut microbiome and intestinal health (132,142,144).

PFOS exposure alters microbial community composition in a sex-specific manner

Several animal studies report toxic effects of PFOS on gut microbiota composition, as well as gastrointestinal/hepatic physiology. Dietary administration of PFOS (0.003%) to male mice significantly altered the Bacteriodetes : Firmicutes ratio, indicative of dysbiosis and inflammatory pathology (145,146). PFOS exposure also altered the liver metabolome, including increases in markers of oxidative stress, altered fatty acid composition, and upregulation of PPARα transcriptional targets along with increases in liver damage biomarkers and altered liver metabolism (146). Another study found that there were sex-specific differences between gut bacterial composition in male and female mice exposed to PFOS (144). Exposed males had significantly lowered concentrations of Lactobacillus and Enterococcus, and significantly higher concentrations of Allobaculum. Femaleshad decreases in Akkermansia and increases in Bifidobacterium, Allobaculum, and Turicibacter. Interestingly, these effects were reduced in PFOS exposed mice that received an antibiotic and fecal transplant of live L. reuteri, E. faecalis, and Akk. Muciniphila (144). Findings suggest a possible mechanism linking PFOS exposure to liver damage and other toxic endpoints is through inflammatory and metabolic mechanisms mediated by gut microbiota.

PFOA exposure alters microbial composition and function at subacute and subchronic levels

Exposure to PFOA has also been shown to alter the gut microbiome in subacute and subchronic exposure studies in mice (147). Under multiple dosing conditions, PFOA increased liver weight, liver injury, oxidative stress markers, and changes in cytokine profile compared to controls (147). In addition to these classic measures of toxicity, PFOA exposure also led to reductions in microbial SCFA synthesis (e.g., propionic acid, butyric acid), slight decreases in lipid metabolism, and slight increases in glycolysis, as well as changes to other cell functions. Interestingly, they also found that PFOA exposure increased α-diversity in the subacute treated animals, yet differences in β-diversity were noted in both subacute and subchronic exposures (147). Among the changes, subacute exposure increased Parabacteroides, Dorea, and AF12 (antifungal taxon) and decreaed Bacteroides (147). Subchronic exposure was associated with a decrease in the Firmicutes : Bacteroidetes ratio as well as decreases in Bifidobacterium and Lactobacillus (147). In another study, a 1 ppm PFOA exposure to mice resulted in changes in both fecal metabolites and microbiome composition (148). The effects of PFOA on the gut microbiome may also be sex-specific. Male and female adult mice exposed to PFOA both exhibited changes in their gut microbiome, but only female mice had significant changes in α-diversity (117).

Evidence of Altered Gut Microbiome from PFAS Exposure in Human Populations

Evidence from human population-based studies supports the idea that PFAS affect the gut microbiome. For example, increasing PFOS exposure via breastmilk in 1 month old infants is associated with a lower abundance of the commensal bacteria Lactobacillus, and a higher abundance of Enterococcus (149), a pathogenic bacterium associated with ulcerative colitis development (150). It was also found to be associated with a 5% decrease in bacterial diversity for every 63 ng/L increase in PFOS (149).

Potential inflammatory mechanisms of PFAS exposure’s effect on the gut

Though the evidence suggests that PFAS affect the gut microbiome, there is less evidence to suggest PFAS directly induce gut pathology. A recent study has found that PFOS and PFOA may induce an inflammatory response in the hepatic system via NF-κB (151). Male black spotted frogs exposed to PFOS and PFOA had upregulated key inflammatory cytokine transcriptions (e.g., IL-1β, IL-6, TNF- α, IFN-γ), significantly higher levels of TNF-α in the liver (as compared to controls) and upregulated NF-κB pathway, which activates cytokine transcription and release. Moreover, PFOS and PFOA were found to directly activate NF-kB in the liver by binding to subunits NF-kB1 and NF-kB2. Because the hepatic system is closely linked to the gut microbiome, this study may be relevant for discerning host-induced mechanisms in the gut. Another hypothesized mechanism is the proteoglycan/TLR2/TNF-α signaling pathway, as many of the bacteria that are upregulated by PFOS and PFOA are Gram-positive. These pathways may be further influenced by changes in metabolite production, particularly for PFOA exposure, since SCFA levels were decreased post-exposure. This may lead to an increase in inflammatory response and gut disease , as well as a decrease in gut function (148,152). A diagram of PFAS potential mechanisms of action on the gut are illustrated with PFOA in Figure 2.

4. Nitrates and Nitrites

Nitrates and Nitrites are ingested through groundwater, crop residues, processed meats

Nitrates and nitrites are found in the environment and food sources in inorganic and organic forms. Inorganic nitrates and nitrites are a class of agricultural chemicals, often used in combination with fertilizer and pesticides, whose metabolites can be either beneficial or harmful chemicals (153-155). Common exposure pathways to nitrates and nitrites are primarily through contaminated food and water (153,156). While still an emerging area of research, there is preliminary evidence to suggest that exposure to nitrates can alter gut mucosal integrity and/or the gut microbiome (157).

Organic nitrates and nitrites are synthetic compounds mostly used in medical applications, or inorganic forms, which occur naturally in food and the environment (158). They are also known food preservatives, particularly found in processed and cured meats (153,159). Inorganic nitrates and nitrites are used in agricultural settings to promote crop growth, they are often found to correlate with pesticide application. These nitrates/nitrites are often salts of potassium or sodium (160). Use in agriculture contributes to nitrate and nitrite exposure through groundwater used as drinking water, as well as through residues on soil and crops. Dietary nitrate and nitrite primarily function as regulators of the cardiovascular and immune systems, performing functions such as lowering blood pressure via vasodilation and platelet aggregation reduction, blood vessel protection, and preventing inflammation by raising neutrophil levels. Their presence reduces cardiac disease and metabolic disorders in animal models and humans (154). However, certain metabolites of nitrates/nitrites can be harmful to the human body, and potentially the gut. One example of these is N-nitroso compounds (NOCs), which are formed when nitrite reacts with a secondary amide or amine (153). While nitrates and nitrites are a part of normal biology, metabolism of exogenous nitrites poses a toxicological risk as they can be metabolized into carcinogenic metabolites (155,161,162). There is also evidence that NOCs may influence gut inflammation and contribute to gut pathology (e.g., intestinal cancer) (155,163).

Nitrates and nitrites may be metabolized by gut microbiota to form genotoxic componds

Nitrate/nitrites interaction with gut microbes appears to be akin to arsenic’s, where gut microbes metabolize nitrates/nitrites to either beneficial or toxic compounds, which can then affect intestinal health. Rather than the gut microbiome being affected by exposure to nitrates, the gut microbiome may determine the toxicity of nitrates, and affect intestinal and host health. Gut bacteria that contain nitrate reductase and/or narG, such as Lactobacillus casei rhamnosus, reduce nitrates to nitrites via nitrate respiration (164). Nitrites can also be converted to NOCs via anaerobic metabolism by Enterobacteria, which form nitrosamines from nitrate and secondary amines (159,165). Certain NOCs are known to induce DNA damage and cause oxidative stress in animal models. In turn, these can lead to pro-inflammatory cytokine activation (TNF-α, IL-1β) and tumor formation, including gastrointestinal tumors (155,161,162,166,167). A diagram of dietary nitrates/nitrites potential mechanisms of action in the gut are illustrated in Figure 2.

Evidence of Altered Gut Microbiome from Nitrates and Nitrites Exposure in Human Populations

While no direct observations have been studied, recent epidemiological evidence suggests that dietary nitrate exposure may be inducing gut pathology via NOCs. A rural Wisconsin female cohort surveyed for both nitrate exposure and colorectal cancer risk was found to have a significant association between nitrate exposure and proximal colon cancer (168). More recently, Denmark citizens exposed to dietary nitrate levels in drinking water above 3.87 mg/L (limit is 50 mg/L) had a significantly increased risk of developing colorectal cancer (169). As nitrates and nitrites have also been found beneficial to gut bacteria and the host, further research is needed to elucidate the potential association between nitrate metabolites and gut pathology, how diet may modify this association, and the purported mechanism of action.

5. CONCLUSIONS AND FUTURE PERSPECTIVES

The interplay between the chemical environment, the gut microbiome, and the host is inextricably linked and influences homeostasis and health. As we move into the era of precision medicine, it is critical we understand the relationship between the exposome, the gut microbiome, and human health in order to prevent and treat diseases. The exposome was first defined by Christopher Wild in 2005 as the totality of exposures and their biological impacts across the life course (1). While it is not feasible to provide a comprehensive review of the exposome, in this review, we provide evidence from the literature of how select prototype environmental toxicants can alter the normal gut microbiome, promote inflammation, and cause disease. The mechanisms linking ingestible chemical pollutants exposures to chronic disease are diverse and still emerging. Therefore, it is critically important to consider longitudinal studies of gut microbiome composition in humans. As one considers multiple influences of gut microbiome on other omic outputs, including metabolic characterization of the exposome, it is clear that the role of the gut microbial ecosystem is critically important and can not be ignored.

We are in a nascent state of understanding the interactions between the exposome and the microbiome but we identified – several common mechanisms linking chemical pollutant exposures interaction with gut microbial communities and host health via the inflammatory pathways. Within various common classes of ingestible chemical pollutants, we have outlined several broad mechanisms of how xenobiotics affect the gut microbiome with specific examples within each different class, showing the diversity of response and variety of mechanisms of their biological effects on metabolism and immune function.

The gut microbial ecosystem is among the first line of defense for metabolizing foreign and often toxic chemicals. As such, the composition of gut microbiota can influence chemical pollutant metabolism, transforming them from a less toxic to a more toxic form, which can trigger a cascade of interacting effects. Chemical pollutant and their metabolites can interact with the gut mucosal lining, and trigger receptor-mediated responses both in the gut and systemically. We have shown here how these all have some relationship with diverse inflammatory pathways and response. For example, arsenic and cadmium are both heavy metals, yet they influence the gut microbiome by distinctly different mechanisms (arsenic associated with increases in primarily Gram-negative bacteria, and cadmium associated with increases in primarily Gram-positive bacteria). These can drive pro-inflammatory changes via the LPS/TLR4 or proteoglycan/TLR2 pathways that are further supported by changes in metabolite production, such as a decrease in SCFAs. Another critical point to note is the importance of male-female differences in the microbiome response to chemical pollutants. Future research should continue to explore the importance of sex-dependent microbiome response due to implications for future clinical research and prevention in high risk populations.

Heavy metals are likely to play a significant role in gut health through their interactions with both the host and its microbiome. The effects of exposure to heavy metals and interactions with the gut support complex interactions between chemical pollutant-gut microbiota-host immune response. However, a comparison of different non-essential metals suggests multiple mechanisms at play. As an illustration of this idea, we review evidence that lead, arsenic, and cadmium have negative impacts on the gut microbiome, induce inflammatory immune responses, and elicit some form of pathology in either human or animal models (45,47,55,60,90,91,170). Arsenic and lead exposure increases the abundance of predominantly Gram-negative and/or arsenic-resistant bacteria in animals and humans, while cadmium increases the abundance of predominantly Gram-positive and/or cadmium-resistant bacteria (45,47,55,60,87,90,91). Thus, different heavy metals may drive gut inflammation and related disease (e.g., IBD) by distinctive pathways, which may be further enforced by changes in metabolite production (29,65,152,171). Moreover, these metals provide an example of how complex the relationship between xenobiotics and the gut microbiome might be, as multiple steps likely occur, each with a distinctive effect on the gut microbiome.

Animal and human exposure to POPs may affect both the host and gut microbiome through altering bacterial metabolism and activating several signaling pathways. Studies to date suggest that the AHR signaling pathway is central to the mechanisms of how PCB-126 alters the gut microbiome, although in vitro studies do suggest that PCB-126 may be able to directly affect the microbiome (114,119,120). Further, PCB-153 seems to act inversely to PCB-126. As a noncoplanar PCB congener, it is not surprising that PCB-153 acts in an AHR independent manner and there is evidence that this PCB, in part, acts directly on gut microbes, in addition to inflammatory effects. Additionally, PFAS are known endocrine disruptors and it is possible that they could affect the gut microbiome (172). It is possible, if not likely, that POPs can dually act on the host and microorganisms, as suggested by the PFOS and PCB data (114,125,128,144,146). PFOA exposure appears to alter metabolic functions such as SCFA synthesis, suggesting that SCFAs may be contributing to disease development alongside gut dysbiosis and inflammation (148). This is further corroborated by the available epidemiological data, which suggests that POP exposure can alter the gut microbiome composition and metabolic function (149).

While overall evidence of nitrates/nitrites influence on gut pathology is limited, there are several common mechanisms of interaction between nitrates and other chemical pollutants with the potential for both local and system influences in host metabolism and immune system response. Like arsenic, the gut can alter the metabolism of these classes of chemical pollutants, changing their toxicity (49). In addition, competition within the gut for antibiotic-resistance genes for lead, arsenic and nitrates is a common pathology across these chemical pollutants.

Future Directions

While animal and human studies to date provide important insights into potential mechanisms, several gaps exist, and there is an increasing need to examine additional causal mechanisms and translation to human studies. Existing data is currently highly dependent on the use of animal models as diverse as zebrafish and mice, both of which have widely different gut microbiome compositions shaped by multiple environmental factors (173). There is also limited information to harmonize dose response relationships across studies. Some of the most useful experiments may be the in vitro microbiome exposure to xenobiotics to isolate potential microbiome-driven effects. Future experiments may more clearly define the no observed effect level (NOEL) and the consequences of chronic exposure for many of these xenobiotic-microbiome relationships in altering immune system homeostasis. In addition, it is well established that microbial communities vary across species. Therefore, another consideration towards increased understanding of the extent to which animal model findings can be effectively translated to human outcomes because, for example, the PCB-126 influenced Aeromonas is a common commensal bacteria genus in freshwater and marine environments, but it is not common in the human gut microbiome, mainly only found in immunocompromised individuals (174).

Future studies comparing gut microbiome changes in animal models to human health and pathology may consider focusing on locating bacteria that 1) can be found across multiple animal models as well as the human population and 2) have similar functions in the models’ gut as to humans. For example, Lactobacillus reuteri is a probiotic commonly found in vertebrates that produces the SCFA lactic acid and protects against inflammation and lipid metabolism (175-177). Observing the abundance of this bacterium change in response to xenobiotic exposure may be helpful in determining if the xenobiotic affects gut composition and pathology. Another method to more easily compare animal gut microbiome changes to human health may be to utilize human fecal transplants to mimic the human microbiome in an animal model (178). The studies reviewed here provide important data on how xenobiotic exposure affects both the gut microbiome and the host, but due to their interconnectedness, understanding the order in which sequelae occur to induce pathology is challenging to elucidate. For example, while POPs appear to all have distinct effects on the gut microbiome composition, only PCB-126 appears to initially affect the host via AHR. In contrast, it is unclear if PCB-153 and PFAS first affect the host or bacterial composition to cause subsequent inflammation and dysbiosis. Utilizing knockouts of certain genes related to inflammatory response (e.g., ATM/NEMO) or removal of key bacteria that may cause host response (e.g., pathogenic bacteria) could be helpful to determine whether interactions on the host or microbiome are direct.

Finally, there is an increased need to expand longitudinal population-based studies of gut microbiome and health. Several chronic diseases and cancers, including early-onset colorectal cancer and increasing incidence of inflammatory bowel disease, are of concern, as our other inflammatory-related chronic conditions, including liver disease. Each of these has been shown to be associated in some way with a chemical pollutant-gut microbiota-host inflammatory mechanism. One limitation of existing research is understanding how phenotypes also intersect to influence gut microbiome composition, something beyond the scope of the current review. With increasing technological advances, research should also begin to explore causal mechanisms, much of the observational data today is not able to capture specific mechanisms and pathways that are potentially modifiable. Thus, additional research is needed to understand both the timing of exposure to chemical pollutants and the response related to gut microbiome changes to identify modifiable windows of exposure and opportunities for the prevention of inflammatory-related disease pathologies. Finally, these studies designed to examine mixtures of exposure to these common, ingestible chemical pollutants and their interaction with host microbiota and intestinal integrity are needed to further advance human health risk assessments, including increased understanding of absorption, distribution, metabolism, and excretion, and dose-response relationships.