74

5 Bioremediation of Heavy Metals

Table 5.4

The environmental heavy metal pollution and the responsible genes conferring

the metal resistance.

Heavy metal

Resistance operon

References

Arsenic

arsRBC or arsRDABC

[1, 6]

Cadmium

cadCA

[25]

Chromium

chrBACF

[26]

Mercury

mer operons

[27]

Lead

pbr operon

[28]

Nickel

ncc operon

[29]

able to accumulate Cu in the form of copper sulfide and was also able to accumulate

Ni in the form of phosphide salts. Metallothionein, a cysteine-rich soluble protein,

has been reported to help in the accumulation of Cd by Pseudomonas putida. This

bioaccumulation strategy can be exploited for heavy metal removal or recovery pro-

cess from contaminated water or soil samples.

5.5.1.1

Genetic Circuitry Involved in Microbial Bioremediation

Environmental heavy metal pollution became a serious health hazard globally

in contemporary times [1, 5]. The metal-resistant phenotype of microorganisms

conferred by the presence of microbial operons is well evident in scientific literature.

Although the resistance genes were originally discovered on plasmids, they have also

been found on the chromosomes of a diverse group of organisms. Metal-resistant

genes are common in microbial communities growing in contaminated environ-

ments (Table 5.4). Heavy metals exert a strong selective pressure on microorganisms,

resulting in major changes in the structure and diversity of the microbial community.

The resistance genes conferring heavy metal resistance often arranged in oper-

ons [1]. Microbial arsenic resistance genes are organized as ars operons, which may

involve three genes constituting the arsRBC or five genes forming arsRDABC operon.

Cells expressing the five genes arsRDABC are more resistant to arsenic than those

expressing only the arsRBC genes. The cadmium resistance operon cadCA has been

isolated and characterized previously in Staphylococcus sp.

5.5.1.2

Different Heavy Metal–Resistant Mechanisms

Microbes exert myriad different heavy metal–resistant mechanisms.

Intracellular Accumulation Transport of the metal across the cell membrane yields

intracellular accumulation, which is a metabolism-dependent process. Many bacte-

ria have the ability of sequestering metals from the environment. After entering into

the cell by this energy-dependent process, heavy metals may be compartmentalized

and/or converted to innocuous form by binding with cellular metabolic components

such as carbide, sulfide, phosphide or hydroxide.