Heavy metal resistance in pseudomonads

Abstract

The genomes of pseudomonad'species display a wide variety of adaptive genes which allow them to tolerate heavy metal toxicity. Several resistance systems have been directly characterized by genetic and biochemical tests, whereas other may only be inferred by comparison with the systems encoded in the genomes of other bacteria. This review briefly summarizes the mechanisms of resistance to heavy metals in pseudomonads. For this purpose, metals have been divided into three groups: (i) micronutrient cations that show toxicity (copper, cobalt, nickel, zinc); (ii) nonessential toxic cations (cadmium, lead, mercury, siliver); and (iii) toxic oxyanions (derived from arsenic, chromium, selenium and tellurium). Pseudomonads have evolved two main strategies to cope with heavy metal toxicity: membrane transporters able to efflux toxic ions from the cytoplasm, which include members of the major membrane protein families, and enzymatic redox detoxification pathways, which transform metals to less-toxic forms. Most heavy metal resistance systems in pseudomonads are encoded by complex operons, located on chromosomes or on plasmids, commonly involving delicate regulatory switches functioning at the transcriptional level. The abundance and diversity of genetic determinants conferring metal tolerance in the genomes of pseudomonads may be associated with the varied environments that these versatile bacteria inhabit.