Heavy metal resistance genes and proteins in bacteria and their application.

Introduction Bacteria may carry determinants of resistance to a number of heavy metals, including silver, bismuth, cadmium, cobalt, copper, mercury, lead, nickel, thallium or zinc cations and the oxyanions of arsenic, chromium, antimony, tellurium or tungsten. Such resistance determinants have been reviewed extensively [ 1-41. Bacterial resistance to heavy metals is conferred by specific resistance determinants which are often, but not always, carried on plasmids or transposons. Resistance is specific to one or a few metals, and the mechanisms of resistance include efflux of the metal, modification of the speciation of the metal, sequestration of the metal, or a combination of these mechanisms. Recent progress in understanding the mechanisms of heavy metal resistance has indicated that similar mechanisms for resistance to a single metal may occur across a wide range of bacterial genera, and that related mechanisms of resistance may apply to different heavy metals. Living organisms have been in intimate contact with heavy metals released into the environment by geochemical processes since organisms first evolved, and over geological timescales micro-organisms have evolved to occupy ecological niches containing high concentrations of heavy metals. That similar mechanisms have been selected across different bacterial genera and for different metals is not surprising. The proteins conferring metal resistance, the genetic regulatory mechanisms and the organisms conferring resistance to heavy metals are now beginning to be exploited in bioremediation and biomonitoring strategies for environments which are contaminated with heavy metals.