gene cadX which is a transcription regulator gene. High cadmium tolerance is due to
both of these two genes. cadX has 40% similarity with cadC sequence, while the
cadX* gene of pRW001 plasmid is also having similarity with the first 78 nucleotides
of cadX gene sequence. After this, it is truncated due to which its transcription
regulation capacity is lost. The cadX protein also has around 30% homology with
ArsR protein of the ars operon (Yoon and Silver 1991). These are various genetic
adjustments through which bacteria acquire resistance in the cadmium-polluted
environment. Practically, degradation of metals cannot be done; thus, nearly all
biological metal remediation strategies are dependent on detoxification as well as
immobilization of metal to decrease its biological toxicity and also obstruct metal
transfer. Remediation of cadmium through bacteria primarily is done by binding
metal to the bacterial cell wall. Near to cadmium, calcium ions as well as protons are
discharged which shows the cell wall’s competitive binding behaviour. According to
Plette et al. (1996), when bivalent ions surround the cell wall, it becomes positively
charged depending upon pH-dependent charging along with metal binding. Carboxylic as well as phosphatic sites of the bacterial cell wall are employed for powerful
coordination with the cadmium ions which assist in their site remediation. Peptidoglycan, phospholipids and lipopolysaccharides are constituent of the cell wall of
Gram-negative bacteria. Several processes such as ion exchange, complexation,
coordination, adsorption, electrostatic interaction, chelation and microprecipitation
bind metals on the cell coat due to higher anionic character and charged behaviour of
lipopolysaccharides (Vijayaraghavan and Yun 2008). Intracellular cadmium (36%)
sequestration is done by Pseudomonas strain H1, and Bacillus strain H9 to (Roane
and Pepper 2001) lessen cadmium toxicity. These strains produce exopolymers
(EPS) which store cadmium and lessen soluble cadmium by 22% and 11%, respectively. Another option is to use microbial biomass for cadmium bioremediation
which is cheap and eco-friendly. This process is performed by independent extracellular adsorption through surface complexation, ion exchange or electrostatic interaction resulting in intracellular storage by the surface of the cell (Vargas-García et al.
2012). According to Khan et al. (2015), 18.8%, 37%, and 56% Cd
2+ are removed
from the aqueous medium after 48, 96, and 144 h, respectively, while Cd
2+ level
enhanced in the medium after 192 h and intracellular Cd
2+ storage is reduced. It is
because of the activity of efflux system in E. coli for its existence. This shows that
metal accumulation and efflux system in bacteria are interdependent. The CdS
nanoparticle utilization is another approach cadmium remediation which is produced
by functionalized EPS of P. aeruginosa JP-11 to remove 88.66% of cadmium from
aqueous solutions (Raj et al. 2016).
12.7.1 Chromium
Seventh highly abundant metal on the earth is chromium which is found in the
environment in two stable states, i.e. trivalent Cr
3+ and hexavalent Cr
6+ . Chromium
is responsible for oxidative damage and inhibition of sulphate membrane transport in
bacteria. Microbes have developed two processes to tolerate chromium. One of them
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
333
both of these two genes. cadX has 40% similarity with cadC sequence, while the
cadX* gene of pRW001 plasmid is also having similarity with the first 78 nucleotides
of cadX gene sequence. After this, it is truncated due to which its transcription
regulation capacity is lost. The cadX protein also has around 30% homology with
ArsR protein of the ars operon (Yoon and Silver 1991). These are various genetic
adjustments through which bacteria acquire resistance in the cadmium-polluted
environment. Practically, degradation of metals cannot be done; thus, nearly all
biological metal remediation strategies are dependent on detoxification as well as
immobilization of metal to decrease its biological toxicity and also obstruct metal
transfer. Remediation of cadmium through bacteria primarily is done by binding
metal to the bacterial cell wall. Near to cadmium, calcium ions as well as protons are
discharged which shows the cell wall’s competitive binding behaviour. According to
Plette et al. (1996), when bivalent ions surround the cell wall, it becomes positively
charged depending upon pH-dependent charging along with metal binding. Carboxylic as well as phosphatic sites of the bacterial cell wall are employed for powerful
coordination with the cadmium ions which assist in their site remediation. Peptidoglycan, phospholipids and lipopolysaccharides are constituent of the cell wall of
Gram-negative bacteria. Several processes such as ion exchange, complexation,
coordination, adsorption, electrostatic interaction, chelation and microprecipitation
bind metals on the cell coat due to higher anionic character and charged behaviour of
lipopolysaccharides (Vijayaraghavan and Yun 2008). Intracellular cadmium (36%)
sequestration is done by Pseudomonas strain H1, and Bacillus strain H9 to (Roane
and Pepper 2001) lessen cadmium toxicity. These strains produce exopolymers
(EPS) which store cadmium and lessen soluble cadmium by 22% and 11%, respectively. Another option is to use microbial biomass for cadmium bioremediation
which is cheap and eco-friendly. This process is performed by independent extracellular adsorption through surface complexation, ion exchange or electrostatic interaction resulting in intracellular storage by the surface of the cell (Vargas-García et al.
2012). According to Khan et al. (2015), 18.8%, 37%, and 56% Cd
2+ are removed
from the aqueous medium after 48, 96, and 144 h, respectively, while Cd
2+ level
enhanced in the medium after 192 h and intracellular Cd
2+ storage is reduced. It is
because of the activity of efflux system in E. coli for its existence. This shows that
metal accumulation and efflux system in bacteria are interdependent. The CdS
nanoparticle utilization is another approach cadmium remediation which is produced
by functionalized EPS of P. aeruginosa JP-11 to remove 88.66% of cadmium from
aqueous solutions (Raj et al. 2016).
12.7.1 Chromium
Seventh highly abundant metal on the earth is chromium which is found in the
environment in two stable states, i.e. trivalent Cr
3+ and hexavalent Cr
6+ . Chromium
is responsible for oxidative damage and inhibition of sulphate membrane transport in
bacteria. Microbes have developed two processes to tolerate chromium. One of them
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
333