Arsenic is both a product from natural sources and of human activities, and is widely distributed in the environment, essentially in 3 different oxidation states: As (-III) (arsine), As (+III) (arsenite) and As (+V) (arseniate). The soluble and toxic forms, arsenite and arseniate are frequently reported as contaminants of water and arsenic has been identified as a major risk factor for human health in many places in the world. The most dramatic effects have recently been described in India and Bangladesh, where it is estimated to present a risk for close to 50 million people. Even in France, pollution of water by arsenic has been detected in numerous regions. Acute arsenic poisoning is characterized by oesophageal and abdominal pains and hemorrhagic diarrhea. Prolonged exposure may be the cause of various cancers as well as cutaneous changes such as hyperkeratosis.

The ecology of this metalloid is strongly dependent on microbial transformations which affect the mobility and bioavailability as well as the toxicity of arsenic in the environment. Bacteria implicated in the processes of biotransformation such as oxidation, reduction and methylation have been described. To date, the best-studied mechanism of transformation is the reduction of arseniate (AsV) to arsenite (AsIII). On the other hand, the physiology, enzymology and genetics of the oxidation of arsenite as well as the underlying regulatory processes remain largely unknown at present.

The beta-proteobacterium Thiomonas spp. is a heterotrophic bacterium capable of reducing and oxidizing arsenic with the objective of detoxification. It will be interesting to study the implication of this metabolism in processes involving energetics of resistance and detoxification using comparative, functional and structural genomics approaches. A better understanding of the mechanisms involved may be profitable in bioremediation procedures.