Table 1. Human exposure to pollutants and their interaction with the GI microbiota.
| Chemical | Source | Human exposure | Metabolism by microbiota | Effect on microbiota | References |
|---|---|---|---|---|---|
| PAHs | Air and food pollutants resulting from incomplete combustion of fossil fuel, tobacco | Mean total intake of 3.12 mg per day (97% through food, 1.6% air, 0.2% water, 0.4% soil) | In vitro: hydroxylation; in vivo: deconjugation of liver metabolites, involved in the formation of CH3S-metabolites | 32,104,105 | |
| Nitro-PAHs | Air and food pollutants, derivatives of PAHs | Diesel exhaust identified as main source of exposure. 2NF: range from 0 to 92 ng/m3 | Reduction to amine metabolites | 36,106 | |
| Nitrotoluenes | Intermediates in the manufacture of dyes, chemicals, explosives | Mainly occupational. 2-nitrotoluene: 0.35–0.7 mg/m3 through air; 420 mg per day through skin | Reduction to amine metabolite and hydolysis of glucuronide conjugates | 39,40,107 | |
| Pesticides | Pollutants in air and food | Chlorpyrifos: mainly through diet 0.01 to 0.14 μg/kg bw per day; DDT: through diet 0.29 μg/kg bw per day | Dechlorination of organochlorides. Deconjugation of propachlor in vivo | Perinatal exposure to chlorpyrifos (1 mg/kg bw per day) induced dysbiosis at adulthood (rat) | 45,92,108 |
| PCBs | Industrial chemicals now prohibited but persistent in water sediments and soils | Mainly through diet DL-PCBs: 0.29 pg TEQ WHO98/kg bw per day; NDL-PCBs: 2.71 ng/kg bw per day | Bacterial C–S-lyase plays an important role in formation of methyl sulfone (MeSO2)-metabolites in vivo | Mixture of PCBs (150 μM/kg for 2 days) decreased the abundance of many bacteria (mainly Proteobacteria) | 55,56,109,110 |
| Metals | Ubiquitous environmental contaminants | Mainly through diet: arsenic 0.78 μg/kg bw per day; lead 0.2 μg/kg bw per day; cadmium 0.16 μg/kg bw per day | Involved in demethylation of mercury, methylation of arsenic and bismuth | Cadmium (20–50 mg/kg bw/d for 45 days); lead (100 or 500 mg/l) or arsenic (10 p.p.m. for 4 weeks) induced dysbiosis (mouse) | 93,94,96,109 |
| Azo dyes | Food colourants | Mainly through diet | Azoreduction of the azo bound to produce aromatic amines | 74,111 | |
| Melamine | Widely used in plastics, illegal food contaminant | TDI: 0.2 mg/kg bw (EU) | Metabolised to cyanuric acid | 75 | |
| Artificial sweeteners | Food additives | ADI (FDA, US): Aspartame: 50 mg/kg bw; saccharin: 15 mg/kg bw | Cyclamate metabolised to cyclohexamine | Aspartame (5–7 mg/kg/d), sucralose and saccharin (5 mg/kg per day) induce dysbiosis in animals with potential deleterious metabolic effect for the host (mouse and human) | 80,98,102,103 |
| Other POPs (e.g., PCDFs) | Pollutants formed during industrial processes | Mainly through diet: PCDD/Fs 0.176 pg TEQ WHO98/kg bw per day | 2,3,7,8 TCDF (24 μg/kg) induced dysbiosis and affected the faecal metabolic profiles (mouse) | 97,109 |
Abbreviations: ADI, acceptable daily intake; DL-PCBs, dioxin-like PCBs; EU, European Union; FDA, Food and Drug Administration; NDL-PCBs, Non-dioxin-like PCBs; PCBs, polychlorobiphenyls; PAHs, polycyclic aromatic hydrocarbons; POPs, persistent organic pollutants; TEQ, toxic equivalency; TDI, tolerable daily intake.