Microorganisms can modulate the toxicity of mercury. Methylation increases toxicity, while demethylation (resistance) decreases toxicity. The role of the gut microbiome in mercury transformations and human exposure to toxic methylmercury was investigated. The human gastrointestinal tract is a densely populated ecosystem, and thus may have an important role in mercury and methylmercury cycling and related fetal exposure.
Results allow the identification of risk factors for methylmercury exposure associated with the gut microbiome. Findings showed the human microbiome contributes little to mercury transformations in the gut.
The mechanisms by which gut microbiota contribute to methylmercury exposure in humans remain unclear. The main objective of this pilot study was to investigate potential correlations between biomarkers for mercury methylation and mercury resistance in the human gut microbiome with methylmercury exposure. Metagenomic whole genome shotgun sequencing, which enables rapid identification of microorganisms and genes, was used to identify genes implicated with mercury methylation and mercury resistance in human gut microbiota. The diversity of gut microbiota from a cohort of pregnant women was determined using 16S rRNA gene profiling. Six samples with highest and lowest methylmercury concentrations were searched for homologs of the primary gene implicated with mercury methylation (hgcA) and two genes involved in mercury resistance (merA and merB). Seventeen bacterial genera were significantly correlated (increasing or decreasing) with gut methylmercury, gut inorganic (unmethylated) mercury, or hair total mercury. However, aside from one genus, there was no correlation with biomarker results derived from metagenomic whole genome shotgun sequencing. There were no definitive matches for hgcA or merB, while merA was detected at low concentrations in all six samples. Proportional differences in gut methylmercury levels cannot be attributed to methylation/demethylation by gut microbiota. Exposure to toxic mercury species appears to be unrelated to the presence of hgcA or genes in the mer operon in the gut microbiome.
Rothenberg, S. E., S. Keiser, N. J. Ajami, M. C. Wong, J. Gesell, J. F. Petrosino, and A. Johs. 2016. “The Role of Gut Microbiota in Fetal Methylmercury Exposure: Insights from a Pilot Study.” Toxicology Letters 242: 60-67. DOI: 10.1016/j.toxlet.2015.11.022
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