Gut Dysbiosis in Animals Due to Environmental Chemical Exposures

Abstract

The gut microbiome consists of over 10³-10⁴ microorganism inhabitants that together possess 150 times more genes that the human genome and thus should be considered an "organ" in of itself. Such communities of bacteria are in dynamic flux and susceptible to changes in host environment and body condition. In turn, gut microbiome disturbances can affect health status of the host. Gut dysbiosis might result in obesity, diabetes, gastrointestinal, immunological, and neurobehavioral disorders. Such host diseases can originate due to shifts in microbiota favoring more pathogenic species that produce various virulence factors, such as lipopolysaccharide. Bacterial virulence factors and metabolites may be transmitted to distal target sites, including the brain. Other potential mechanisms by which gut dysbiosis can affect the host include bacterial-produced metabolites, production of hormones and factors that mimic those produced by the host, and epimutations. All animals, including humans, are exposed daily to various environmental chemicals that can influence the gut microbiome. Exposure to such chemicals might lead to downstream systemic effects that occur secondary to gut microbiome disturbances. Increasing reports have shown that environmental chemical exposures can target both host and the resident gut microbiome. In this review, we will first consider the current knowledge of how endocrine disrupting chemicals (EDCs), heavy metals, air pollution, and nanoparticles can influence the gut microbiome. The second part of the review will consider how potential environmental chemical-induced gut microbiome changes might subsequently induce pathophysiological responses in the host, although definitive evidence for such effects is still lacking. By understanding how these chemicals result in gut dysbiosis, it may open up new remediation strategies in animals, including humans, exposed to such chemicals.

Keywords: air pollution; arsenic; endocrine disrupting chemicals; gastrointestinal system; gut-microbiome-brain axis; heavy metals; lead; nanoparticles.

Figure 1

A triad relationship exists between environmental chemical exposure(s), host genetic/epigenetic/phenotypic background, and gut microbiome effects. Environmental chemical-induced host phenotypic changes may result in gut microbiome alterations. Examples of such host changes include hormonal imbalances, obesity, inappetence, gastrointestinal disease, or growth. Environmental toxicants might also directly result in gut dysbiosis that could in turn affect the host, such as neurobehavioral (further detailed in Figure 2), immunological, and metabolic responses. The host genetic/epigenetic/phenotypic status and/or gut microbiome could change the pharmokinetic dynamics of environmental chemicals, including absorption, distribution, metabolism, and/or excretion, which could alter host vulnerability to certain environmental toxicants.

Figure 2

Mechanisms by which gut dysbiosis may result in neurobehavioral disorders. Diagram has been drawn based on Figures in http://sitn.hms.harvard.edu/flash/2016/second-brain-microbes-gut-may-affect-body-mind/; Borre et al., .