Omics Reflection on the Bacterial Escape from the Toxic Trap of Metal(loid)s

Abstract

Soil and water contamination with toxic metal(loid)s (TMs) has increased globally in the present era due to rapid industrialization, improper waste disposal, application of chemical fertilizers and pesticides, mining, and other anthropogenic activities. Most of them are toxic even at low concentrations, are persistent in the environment, and cannot be degraded easily through biological, physical, and chemical means to an innocuous by-product. On the other hand, some TMs are known to play vital roles in metabolic processes of microorganisms, either directly or indirectly. Metals such as copper (Cu), zinc (Zn), and nickel (Ni) are essential micronutrients, but show toxicity at high concentrations. Sometimes, they may be exposed to non-essential metal(loid)s, including mercury (Hg), cadmium (Cd), lead (Pb), and arsenic (As). Since microorganisms present in the field contaminated with TMs are continuously exposed to metal(loid)s stress condition, they can over time develop adaptation and resistance to those TMs. Essentially, all bacteria have genes for resistance against different toxic metal ions, including AsO2 −, https://www.w3.org/1998/Math/MathML"> AsO 4 3 − https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003247883/20bd2417-86b6-418a-998c-d8e500f29915/content/C016_equ_0001.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> , Cd2+, Co2+, https://www.w3.org/1998/Math/MathML"> CrO 4 2 − https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003247883/20bd2417-86b6-418a-998c-d8e500f29915/content/C016_equ_0002.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> , Cu2+, Hg2+, Ni2+, and Zn2+. The largest group of resistance systems functions by energy-dependent efflux of toxic ions. Fewer involve enzymatic transformations (oxidation, reduction, methylation, and demethylation) or metal-binding proteins (metallothionein SmtA and chaperone CopZ). Some of the efflux resistance systems are ATPases, and others are chemiosmotic ion/proton exchangers. Moreover, resistance to other toxic metal ions involves a series of metal-binding and membrane transport proteins and enzymes, which convert more toxic to fewer toxic forms. Bacteria that exhibit resistance properties against metal(loid)s have the inherent ability to grow in high concentrations of those metal(loid)s in contaminated environment. This chapter presents microbial interactions with metal(loid)s, which have significant implications for the environment and can play important roles in detoxification, bioremediation, bioleaching, plant growth promotion, and phytoremediation of TMs. Furthermore, this chapter provides a glimpse of omics technologies and their importance to understand the microbial mechanisms involved in the bioremediation of TMs.