Heavy Metal Resistance Systems in Bacteria

Bacteria have developed several efficient systems for detoxifying metals. These mechanisms can be grouped into five categories: (1) intracellular sequestration; (2) export; (3) reduced permeability; (4) extracellular sequestration, and; (5) extracellular detoxification (Rough et al. 1995). Almost all known bacterial resistance mechanisms are encoded on plasmids and transposons (Silver and Walderhaug 1992), and it is probably by gene transfer or spontaneous mutation that bacteria acquire their resistance to heavy metals (Osborn et al. 1997).

In Gram-negative bacteria (e.g., Ralstonia eutropha), the czc system is responsible for resistance to Cd, Zn, and Co. The czc genes encode for a cation-proton antiporter (CzcABC) that exports Cd, Zn, and Co (Nies 1995). A similar mechanism, called the ncc system, has been found in Alcaligenes xylosoxidans, which is resistant to Ni, Cd, and Co. In contrast, the Cd resistance mechanism in Gram-positive bacteria (e.g., Staphylococcus, Bacillus or Listeria) is a Cd-efflux ATPase. The two most well-studied Cu resistance systems are cop from Pseudomonas syringae and pco from Escherichia coli. The cop genes encode for different Cu-binding proteins that allow the sequestration of Cu in the periplasm or in the outer membrane. In contrast, the pco system is expected to be an ion-dependent Cu antiporter (Kunito et al. 1998).

Bacterial resistance properties can be used for different purposes: in the case of mercury pollution, the insertion of the microbial mercury reductase into a transgenic plant improved significantly the phytoextraction process (Heaton et al. 1998). Another example was the inoculation of heavy metal resistant bacteria into a contaminated soil, which seemed to protect the indigenous, sensitive, ammonia-oxidizing bacteria from metal toxicity (Stephen et al. 1999).

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