heavy metal ions. Eukaryotes are extra susceptible to the harmful effects of metal
toxicity than bacteria. Various tolerance processes are activated in the presence of
damaging metal concentrations, such as the induction of peptides of the metalbinding protein family, like metallothioneins (MTs); the control of the intracellular
level of metals from the cytoplasm to the inside of vacuoles, induction of protein
transporters of ligand metal complexes and entry of metal ions from ion channels
existing in the cell wall. In bacteria, all these processes are often coded by plasmids,
which help their dispersal from cell to cell (Valls and de Lorenzo 2002). For
bioremediating heavy metals industrially, low-budget biomaterials should be used
which can be a derivative or waste product with more elimination power because less
price of this biomass is vital for the economical viability of the process (Volesky and
Holan 1995; Zouboulis et al. 2001). A lot of work has been done to improve the
tolerance and/or the microorganism capacity to collect heavy metal ions as well as
those with different parameters like pH (Naeem et al. 2006), temperature, different
metal concentrations and biomass (Soares et al. 2003; Kim et al. 2005), competition
between ions of diverse elements, microorganism and metal contact period (Kotrba
and Ruml 2000), components of the culture medium (Ghosh et al. 2006);
bioaugmentation/biostimulation (Roane and Pepper 2001; Silva et al. 2004), tolerance to harmful effects of heavy metals of Gram-positive/Gram-negative bacteria
(Samuelson et al. 2002); intracellular/extracellular bioaccumulation, viable/nonviable cells, free/immobilized cells and biological mechanisms through aerobic/
anaerobic microorganisms (Dias et al. 2002; Liu et al. 2005; Tabak et al. 2005;
Wang and Chen 2006). New techniques for removing toxic metals from polluted
sites are mainly centred upon biosorption, which is dependent on the metal binding
potential of different biological materials. The capability of biological materials to
store heavy metals from wastewater by either through metabolic pathways or
through physico-chemical methods is known as biosorption (Fourest and Roux
1992). Known important metal absorbents are algae, bacteria, fungi and yeasts
(Volesky 1987). Many local organisms isolated from heavy metals polluted sites
are resistant to heavy metal toxicity and are natural initiating point for all biotechnological applications. It is therefore essential to isolate bacterial strains with new
metabolic potential and to set up degradation pathways both biochemically and
genetically. Most effective metal biosorbent bacteria are Bacillus (Nakajima and
Tsuruta 2004; Tunali et al. 2006), Pseudomonas (Chang et al. 1997; Uslu and
Tanyol 2006) and Streptomyces (Mameri et al. 1999; Selatnia et al. 2004). In the
biosorption solid-phase sorbent or biosorbent, a biological material and a liquid
phase solvent usually water with dissolved sorbate metal ions are present. The
sorbate is favoured because sorbent has a greater affinity for the sorbate species
and attached itself there through various processes till equilibrium is achieved in
between the total solid-bound sorbate species and its remaining part in the solution.
The extent with which sorbent binds to the sorbate defines its distribution between
solid and liquid phases.
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
323
toxicity than bacteria. Various tolerance processes are activated in the presence of
damaging metal concentrations, such as the induction of peptides of the metalbinding protein family, like metallothioneins (MTs); the control of the intracellular
level of metals from the cytoplasm to the inside of vacuoles, induction of protein
transporters of ligand metal complexes and entry of metal ions from ion channels
existing in the cell wall. In bacteria, all these processes are often coded by plasmids,
which help their dispersal from cell to cell (Valls and de Lorenzo 2002). For
bioremediating heavy metals industrially, low-budget biomaterials should be used
which can be a derivative or waste product with more elimination power because less
price of this biomass is vital for the economical viability of the process (Volesky and
Holan 1995; Zouboulis et al. 2001). A lot of work has been done to improve the
tolerance and/or the microorganism capacity to collect heavy metal ions as well as
those with different parameters like pH (Naeem et al. 2006), temperature, different
metal concentrations and biomass (Soares et al. 2003; Kim et al. 2005), competition
between ions of diverse elements, microorganism and metal contact period (Kotrba
and Ruml 2000), components of the culture medium (Ghosh et al. 2006);
bioaugmentation/biostimulation (Roane and Pepper 2001; Silva et al. 2004), tolerance to harmful effects of heavy metals of Gram-positive/Gram-negative bacteria
(Samuelson et al. 2002); intracellular/extracellular bioaccumulation, viable/nonviable cells, free/immobilized cells and biological mechanisms through aerobic/
anaerobic microorganisms (Dias et al. 2002; Liu et al. 2005; Tabak et al. 2005;
Wang and Chen 2006). New techniques for removing toxic metals from polluted
sites are mainly centred upon biosorption, which is dependent on the metal binding
potential of different biological materials. The capability of biological materials to
store heavy metals from wastewater by either through metabolic pathways or
through physico-chemical methods is known as biosorption (Fourest and Roux
1992). Known important metal absorbents are algae, bacteria, fungi and yeasts
(Volesky 1987). Many local organisms isolated from heavy metals polluted sites
are resistant to heavy metal toxicity and are natural initiating point for all biotechnological applications. It is therefore essential to isolate bacterial strains with new
metabolic potential and to set up degradation pathways both biochemically and
genetically. Most effective metal biosorbent bacteria are Bacillus (Nakajima and
Tsuruta 2004; Tunali et al. 2006), Pseudomonas (Chang et al. 1997; Uslu and
Tanyol 2006) and Streptomyces (Mameri et al. 1999; Selatnia et al. 2004). In the
biosorption solid-phase sorbent or biosorbent, a biological material and a liquid
phase solvent usually water with dissolved sorbate metal ions are present. The
sorbate is favoured because sorbent has a greater affinity for the sorbate species
and attached itself there through various processes till equilibrium is achieved in
between the total solid-bound sorbate species and its remaining part in the solution.
The extent with which sorbent binds to the sorbate defines its distribution between
solid and liquid phases.
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
323