5.5 Bioremediation – The Emerging Sustainable Strategy

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Table 5.3

Adaptive mechanisms in microorganisms resulting in metal resistance

physiology.

Adaptations

Features

References

Extrusion system

Metals are pushed out through the cells using

mechanisms such as chromosomal or

plasmid-mediated events

[4]

Biotransformation

Microorganisms convert the toxic metal to

non-toxic forms

[6]

Degradation

enzymes

Using enzymes such as oxidases and reductases:

microbes produce these enzymes to convert

pollutants to metabolic products

[20]

Exopolysaccharides

(EPS)

Microorganisms get adapted to the contaminated

surrounding by secreting EPS, which develops as

an outer hydrophobic cell membrane comprising

efflux pumps against the cell membrane

disrupting contaminants (e.g. solvents)

[4, 21]

Metallothioneins

The metal-binding proteins to which metals form

a complex

[22]

Metal ions bind to the bacterial cell surface via different interactions such as cova-

lent bonding and electrostatic and van der Waals forces. Microorganisms that act as

metal accumulators possess an inherent property of converting toxic form of metal

contaminants to non-toxic or less toxic form. The life cycle of microorganisms are

intricately associated with the biochemical cycle of different heavy metals, which

also influences the process of redox transformations of environmental heavy metal

leading to different oxidation states with different solubility and mobility, therefore

influencing the toxicity factor.

Certain microorganisms in nature have evolved genetic machinery that encodes

cellular circuitry that orchestrates to ensure heavy metal resistance for the

metal-contaminated ecological niche (Table 5.3). Scientific studies have previously

reported myriad microorganisms having heavy metal remediation capabilities.

Efficient Ni removal was observed with Escherichia coli AS21 previously [23].

Arsenic remediation was observed in Micrococcus sp. isolated from the paddy field

of West Bengal, India [6]. Earlier studies have also reported Cd, Cr, Hg, and Pb

decontamination in a microbe-assisted way in Bacillus subtilis 38 (B38) [24].

An array of resistance strategies have been reported in microorganisms that could

resist high metal concentrations, for instance, extracellular sequestration, alteration

in cell morphology, altered permeability, precipitation of heavy metals, and biosorp-

tion of heavy metals [1]. Microbes could accumulate heavy metals within the cell

by utilizing different metabolic pathways that have been extensively studied and

observed in a wide range of microbes [6]. Both Gram-positive and Gram-negative

bacteria have some cellular components such as teichoic acid, polypeptide, and pro-

tein, such as metallothionein, which helps in cellular accumulation and conversion

to less toxic form. Earlier studies have reported that Pseudomonas aeruginosa was