[Bacterial systems for expelling toxic metals]

Bacteria have developed diverse resistance strategies towards toxic metals with which they interact in the environment. The mechanisms of tolerance include extracellular precipitation, sequestration by cell envelopes, intracellular accumulation, redox transformations and membrane efflux systems. Genes responsible for these processes may be encoded by the chromosome or by plasmids. Since some toxic metals are also essential micronutrients (i.e. copper, cobalt, zinc, nickel), bacteria must precisely adjust the functioning of uptake and efflux systems to maintain their adequate intracellular levels. In the case of metals with no biological function (i.e. cadmium, silver), transport systems must be oriented only to the extrusion of the toxic ions. In the last few years, several bacterial systems devoted to the efflux of toxic metals were analyzed at the molecular level resulting in a detailed understanding of the biochemical mechanisms of resistance. Among these are the membrane pathways that extrude cations derived from copper, cadmium, zinc, nickel, cobalt and silver. Two general mechanisms have been found: those involving P-type ATPases, and some using proton antiporter systems.