RESEARCH REVIEW Corynebacterium glutamicum as a model bacterium for the bioremediation of arsenic Arsenic: chemical properties and uses

Luís Mateos, Efrén Ordóñez, Michal Letek, José Gil, J Gil
Arsenic is an extremely toxic metalloid that, when present in high concentrations, severely threatens the biota and human health. Arsenic contamination of soil, water, and air is a global growing environmental problem due to leach-ing from geological formations, the burning of fossil fuels, wastes generated by the gold mining industry present in uncontrolled landfills, and improper agriculture or medical uses. Unlike organic contaminants, which are degraded into harmless chemical species,
more » ... and metalloids cannot be destroyed, but they can be immobilized or transformed into less toxic forms. The ubiquity of arsenic in the environment has led to the evolution in microbes of arsenic defense mechanisms. The most common of these mechanisms is based on the presence of the arsenic resistance operon (ars), which codes for: (i) a regulatory protein, ArsR; (ii) an arsenite permease, ArsB; and (iii) an enzyme involved in arsenate reduction, ArsC. Corynebacterium glutamicum, which is used for the industrial production of amino acids and nucleotides, is one of the most arsenic-resistant microorganisms described to date (up to 12 mM arsenite and >400 mM arseniate). Analysis of the C. glutamicum genome revealed the presence of two complete ars operons (ars1 and ars2) comprising the typical three-gene structure arsRBC, with an extra arsC1íocatedarsC1íocated downstream from arsC1 (ars1 operon), and two orphan genes (arsB3 and arsC4). The involvement of both ars operons in arsenic resistance in C. glutam-icum was confirmed by disruption and amplification of those genes. The strains obtained were resistant to up to 60 mM arsenite, one of the highest levels of bacterial resistance to arsenite so far described. Using tools for the genetic manipulation of C. glutamicum that were developed in our laboratory, we are attempting to obtain C. glutamicum mutant strains able to remove arsenic from contaminated water. [Int Microbiol 2006; 9(3):207-215] The metalloid arsenic (As) is a member of group V of the periodic table of elements and is thus classified as a heavy metal [42]. Arsenic occurs in nature in four oxidation states (+5, +3, 0, and-3), with pentavalent arsenate [+5, As(V)], and trivalent arsenite [+3, As(III)] being the most common forms. Elemental arsenic occurs only rarely in nature, but it can be found as microcrystalline masses in Siberia, Germany, France, Italy, Romania, and the USA. Most arsenic is found in conjunction with sulfur in minerals such as arsenopyrite (FeAsS), realgar (AsS), orpiment (As 2 S 3), and enargite (Cu 3 AsS 4). Arsenic enters water, air, and land through wind-blown dust and water runoff. In most cases, arsenic occurs natural