Many indigenous organisms isolated from sites contaminated with heavy metals have tolerance to heavy metal toxicity and these microbial activities have always been the natural starting point for all biotechnological applications. But, before releasing a GMO into the environment, the researchers should emphasize the ethical responsibilities to be considered before using such novel strategies for bioremediation. al., 2007; Chen & Wang, 2008). Such suicide mechanisms are based on the controlled expression in the gene host that encodes proteins that are lethal to it. The main heavy metals associated with environmental contamination, and which offer potential danger to the ecosystem, are copper (Cu), zinc (Zn), silver (Ag), lead (Pb), mercury (Hg), arsenic (As), cadmium (Cd), chromium (Cr), strontium (Sr), cesium (Cs), cobalt (Co), nickel (Ni), thallium (Tl), tin (Sn) and vanadium (V) (Wang & Chen, 2006). al., 2005), competitiveness of ions of different elements; microorganism-metal contact time (Kotrba & Ruml, 2000), composition of the culture medium (Ghosh et. al., 2003; Kim et. Most heavy metals are cations and this determines their sorption to negatively charged functional groups that are present in: cell surfaces, which are generally anionic at a pH of between 4 and 8; surfaces with residual hydroxides (OH-) or thiol (SH-) and anionic salts, such as PO4- and SO4-, humic acid, and clay minerals (Roane & Pepper, 2000). al., 2003), etc. al., 1997). al., 2005). al. Copyright © 2020 Elsevier B.V. or its licensors or contributors. Among the different contaminants, heavy metals have received special attention due to their strength and persistence in accumulating in ecosystems, where they cause damage by moving up the food chain to finally accrue in human beings, who are at the top of this chain (Figure 1) (Volesky, 2001; Ahluwalia & Goyal, 2007; Machado et. Thus, in general it appears that the potential impact of introduction of GMOs on native microbial populations is not uniform and therefore must be evaluated on a case-by-case basis. Available from: Progress in Molecular and Environmental Bioengineering, From Analysis and Modeling to Technology Applications, From Emergent Biotechnology to Clinical Practice, New developments in organisms capable of enhanced bioremediation, Creative Commons Attribution-NonCommercial-ShareAlike-3.0 License. Bioremediation involves the use of plants or microorganisms, viable or not, natural or genetically engineered to treat environments contaminated with organic molecules that are difficult to break down (xenobiotics) and to mitigate toxic heavy metals, by transforming them into substances with little or no toxicity, hence forming innocuous products (Dobson & Burgess, 2007; Li & Li, 2011). According to these characteristics, protein synthesis is only induced by metals. Since then, there has been an intensive effort to investigate the binding properties of heavy metals to different types of biomass (Chen & Wang, 2008). Although the utilization of GMOs in the field has been limited due to possible risks involved in the horizontal transfer of genetic material, the results that have been obtained are nevertheless important in assessing the benefits and obstacles associated with their applications in bioremediation. The process continues until equilibrium is established between the amount of solid-bound sorbate species and its portion remaining in the solution. Cell walls of microbial biomass, which consists mostly of polysaccharides, proteins and lipids, have abundant metal binding groups such as carboxyl, sulphate, phosphate and amino groups. al., 2004a). al., 1990; Nourbaksh et. The aim was to increase the capacity these special microorganisms in adsorption processes involving heavy metal ions in comparison with non-recombinant microorganisms (Bae et. al., 2007). al., 2010; Kuroda & Ueda, 2011). Important parameters in this regard are growth rate, inoculum size, environmental conditions, including spatial distribution, and the presence of competing microorganisms. Many bacteria, such as Actinomycetes Azotobacter and those of the genus Pseudomonas, synthesize these substances to capture the iron ions they require for their metabolic activity, or for biosorption (Pattus & Abdallah, 2000; Gazsó, 2001). Metallothioneins (MTs): Metallothioneins (MTs) are a group of well-preserved structures of proteins that act as antioxidants, and they are distributed among all living organisms.