[Mechanisms of bacterial resistance to heavy metals]

Resistance to toxic heavy metals has been found in bacteria from clinical and environmental origins. The genetic determinants of resistance are frequently located on plasmids or transposons. Several heavy metal resistance genes have been cloned and sequenced. The mechanisms of resistance to heavy metals are commonly based on novel membrane transport systems that expel the toxic ions (including cobalt, nickel, zinc, and probably copper and chromium) from the bacterial cytoplasm. Arsenic and cadmium ions are effluxed from the cells by specific membrane ATPases encoded by resistance plasmids. Reduction of mercuric ions to the volatile metallic form by a plasmid-coded enzyme is responsible for mercury resistance. Studies on other resistance determinants (e.g., antimony, bismuth, boron, lead, silver, tin, tellurium) have been reported but the mechanisms of resistance are still unknown.