Heavy Metals, Bacterial Resistance

Abstract

Essentially all bacteria with 2000 or more genes have genes for toxic metal ion resistances that include those for Ag+, AsO2−, AsO43−, Cd2+, Co2+, CrO42−, Cu2+, Hg2+, Ni2+, Pb2+, TeO32−, Tl+, and Zn2+. The largest group of resistance systems functions by energy-dependent efflux of toxic ions. Others involve enzymatic transformations (reduction, oxidation, methylation, and demethylation) or metal-binding proteins. Some of the efflux resistance systems are ATPases and others are chemiosmotic ion/proton exchangers. Resistance to inorganic Hg2+ and to organomercurials (such as CH3Hg+ and phenylmercury) involves a series of metal-binding and membrane transport proteins that pass Hg2+ to the enzymes mercuric reductase and organomercurial lyase, which overall convert them from more toxic to less toxic forms. Arsenic resistance and metabolizing systems occur in three patterns: the widely found ars operon that is present in most bacterial genomes, the arr genes for the periplasmic arsenate reductase that functions in anaerobic respiration as a terminal electron acceptor, and the aox genes for the periplasmic arsenite oxidase that functions as an initial electron donor in aerobic resistance to arsenite.