Gut bacteria transforms arsenic, increases its toxicity.

Van de Wiele, T, CM Gallawa, KM Kubachka, JT Creed, N Basta, EA Dayton, S Whitacre, GD Laing and K Bradham. 2010. Arsenic metabolism by human gut microbiota upon in vitro digestion of contaminated soils. Environmental Health Perspectives http://dx.doi.org/10.1289/ehp.0901794.

Bacteria living in human intestines can change arsenic's chemistry, in some cases producing a more toxic form that is linked to cancer. 

This is the first report of arsenic becoming more harmful as it passes through the human digestive track. The results parallel those found in animal studies and suggest that regulators may need to take into account the way exposures occur when determining the health risks associated with arsenic.

Arsenic pollution is a serious global health problem. It is driven by exposure to naturally-occurring sources – especially groundwater and food – and human activities associated with smelting and mining that create dust or liberate the metal from the soil.

High levels of arsenic occur naturally in India, Pakistan, parts of the United States and other hot spots around the world. Exposure to excess arsenic can lead to cancers of the lung, liver, bladder and kidney.

In this study, bacteria from a human intestine were cultured in a specialized system that simulates the digestive system, including the stomach, small intestine and colon. Researchers added either inorganic arsenic or four types of soils with arsenic naturally present. After incubation, the resulting mixtures were analyzed to determine what types of arsenic were present.

They found that the bacteria altered a significant portion of the initial inorganic arsenic. The microbes changed the arsenic from a less toxic form to a more toxic variety. Further changes turned some of this arsenic into two even more toxic types – called monomethylarsonous acid and monomethylmonothioarsonic acid – after methyl groups were added.

The toxic varieties were formed from both pure inorganic arsenic and from inorganic arsenic present in soils, suggesting that this process occurs even with arsenic that adheres to soil particles.

These unexpected results highlight concern about increasing arsenic's toxicity when it is transformed in the gut.