A Study in Balance: How Microbiomes are Changing the Shape of Environmental Health

Kellyn S. Betts

Disclosures

Environ Health Perspect. 2011;119(8):a340-a346. 

In This Article

Microbiomes and Metals

Scientists have known for decades that certain bacteria in the environment alter metal compounds in ways that make the metals more bioaccessible to humans. The evidence that certain microbes in the human gut also can transform metals has been steadily accumulating, although only recently have environ-mental health scientists begun to grasp the implications.

One of the catalysts for this shift in thinking was the 2010 publication of a study in which Van de Wiele and his team used a "human gastrointestinal simulator" to analyze how human intestinal bacteria metabolize inorganic arsenic from contaminated soils. Different arsenic species vary widely in their toxicity, so it matters a great deal how arsenic is transformed within the body. Van de Wiele's team showed the bacteria transformed inorganic arsenic to methylated arsenicals and thioarsenicals, including the first known observation of metabolically derived monomethylmonothioarsonic acid, whose toxicokinetic properties are unclear.[18] When considered together with other recent research, these results suggest interindividual differences among human microbiomes may make a significant difference in the toxicity of metals and their contributions to chronic diseases associated with these metals, such as cardiovascular disease and type 2 diabetes mellitus.

Scientists agree that re-examining environ-mental exposures through the lens of the microbiome is likely to yield more insights into bacterial impacts. For instance, the ability of intestinal bacteria to demethylate methyl-mercury[19] is important because the process could result in unexpected exposure to toxic inorganic mercury. "It is possible that [many people] may be internally exposed to inorganic mercury much more than we have ever calculated because of demethylation of the mercury we take in through fish consumption," Silbergeld says.

Many people worldwide are already regularly exposed to inorganic mercury during gold-mining operations.[20] A major percentage of Americans also may be exposed to high levels of the element via silver amalgam dental fillings in their teeth, according to an analysis by Mark Richardson, a risk assessment specialist at consulting firm SNC Lavalin.[21,22] On the basis of an analysis he presented at a 14–15 December 2010 meeting of the U.S. Food and Drug Administration Dental Products Panel, Richardson estimated that one-quarter to one-third of Americans may be regularly exposed to concentrations of inorganic mercury from amalgam fillings that exceed recommended exposure limits set by the U.S. Environmental Protection Agency (EPA) and the California EPA.

Thousands of studies have shown that mercury affects many metabolic processes and organ systems in humans and experimental animals. Silbergeld says research by her laboratory and others indicates inorganic mercury can also impact the mucosal immune system, for instance by increasing the production of proinflammatory cytokines and serum levels of biomarkers of immune alteration related to autoimmunity.[23,24,25,26] On top of that, contact between mucosal cells of the immune system and the intestinal microbiome means each will affect the other, she says. To Silbergeld, this suggests interactions between environmental contaminants and the microbiome may be bidirectional.

The implications also further strengthen the argument for including immunology and infectious disease under the umbrella of environ-mental health.[27] Linda Birnbaum, director of the National Institute of the Environ-mental Health Sciences (NIEHS), says the institute's research is repeatedly uncovering new interactions between the immune system and diseases she says clearly have environmental components, such as autism and breast cancer. "We know there are numerous and complex relationships between the microbiome and our immune system," Birnbaum says.

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