Metal removal by metallothionein and an acid phosphatase PhoN, surface-displayed on the cells of the extremophile, Deinococcus radiodurans

Synthetic bacteria designed using ars operons: a promising solution for arsenic biosensing and bioremediation

The global concern over arsenic contamination in water due to its natural occurrence and human activities has led to the development of innovative solutions for its detection and remediation. Microbial metabolism and mobilization play crucial roles in the global cycle of arsenic. Many microbial arsenic-resistance systems, especially the ars operons, prevalent in bacterial plasmids and genomes, play vital roles in arsenic resistance and are utilized as templates for designing synthetic bacteria. This review novelty focuses on the use of these tailored bacteria, engineered with ars operons, for arsenic biosensing and bioremediation. We discuss the advantages and disadvantages of using synthetic bacteria in arsenic pollution treatment. We highlight the importance of genetic circuit design, reporter development, and chassis cell optimization to improve biosensors' performance. Bacterial arsenic resistances involving several processes, such as uptake, transformation, and methylation, engineered in customized bacteria have been summarized for arsenic bioaccumulation, detoxification, and biosorption. In this review, we present recent insights on the use of synthetic bacteria designed with ars operons for developing tailored bacteria for controlling arsenic pollution, offering a promising avenue for future research and application in environmental protection.

Potential applications of extremophilic bacteria in the bioremediation of extreme environments contaminated with heavy metals

Protecting the environment from harmful pollutants has become increasingly difficult in recent decades. The presence of heavy metal (HM) pollution poses a serious environmental hazard that requires intricate attention on a worldwide scale. Even at low concentrations, HMs have the potential to induce deleterious health effects in both humans and other living organisms. Therefore, various strategies have been proposed to address this issue, with extremophiles being a promising solution. Bacteria that exhibit resistance to metals are preferred for applications involving metal removal due to their capacity for rapid multiplication and growth. Extremophiles are a special group of microorganisms that are capable of surviving under extreme conditions such as extreme temperatures, pH levels, and high salt concentrations where other organisms cannot. Due to their unique enzymes and adaptive capabilities, extremophiles are well suited as catalysts for environmental biotechnology applications, including the bioremediation of HMs through various strategies. The mechanisms of resistance to HMs by extremophilic bacteria encompass: (i) metal exclusion by permeability barrier; (ii) extracellular metal sequestration by protein/chelator binding; (iii) intracellular sequestration of the metal by protein/chelator binding; (iv) enzymatic detoxification of a metal to a less toxic form; (v) active transport of HMs; (vi) passive tolerance; (vii) reduced metal sensitivity of cellular targets to metal ions; and (viii) morphological change of cells. This review provides comprehensive information on extremophilic bacteria and their potential roles for bioremediation, particularly in environments contaminated with HMs, which pose a threat due to their stability and persistence. Genetic engineering of extremophilic bacteria in stressed environments could help in the bioremediation of contaminated sites. Due to their unique characteristics, these organisms and their enzymes are expected to bridge the gap between biological and chemical industrial processes. However, the structure and biochemical properties of extremophilic bacteria, along with any possible long-term effects of their applications, need to be investigated further.

Ag+-stabilized DNA triplex coupled with catalytic hairpin assembly and CRISPR/Cas12a amplifications for sensitive metallothionein assay

Metallothionein (MT) is a protein biomarker secreted by liver in response to the treatment for heavy metal toxicity and oncological diseases. On the basis of a new Ag+-stabilized DNA triplex probe (Ag+-SDTP), we establish a fluorescent biosensing system for high sensitivity detection of MT by combining catalytic hairpin assembly (CHA) and the CRISPR/Cas12a signal enhancements. The MT analyte complexes with Ag+ in Ag+-SDTP to disrupt the triplex structure and to release the ssDNA strands, which trigger subsequent CHA formation of many protospacer adjacent motif (PAM)-containing dsDNAs from two hairpins. Cas12a/crRNA further recognizes these PAM sequences to activate its trans-catalytic activity to cyclically cleave the fluorescently quenched ssDNA reporters to recovery drastically amplified fluorescence for detecting MT down to 0.34 nM within the dynamic range of 1∼800 nM. Moreover, the sensing method is able to selectively discriminate MT from other non-specific molecules and can realize low level detection of MT in diluted human serums, manifesting its potentiality for monitoring the disease-specific MT biomarker at trace levels.

Bioremediation potential of cadmium by recombinant Escherichia coli surface expressing metallothionein MTT5 from Tetrahymenathermophila

Cadmium (Cd) is a common heavy metal contaminant in industrial wastewater that causes many diseases in humans. Metallothionein (MT), a cysteine-rich metal-binding protein, is well known in chelate-heavy metals. In this study, we expressed MTT5 of Tetrahymena thermophila fused with Lpp-OmpA in the outer membrane of Escherichia coli to determine its ability to accumulate and adsorb Cd. Our results revealed that our recombinant E. coli had a 4.9-fold greater Cd adsorption compared to wild E. coli. Adsorption isothermic analysis demonstrated that the adsorption behavior for Cd in our recombinant bacteria was better fitted into the Freundlich isotherm model than Langmuir isotherm model. Fourier-transform infrared spectroscopy indicated that phosphate and organic phosphate groups were involved in the interaction between Cd and the bacterial surface. Using quantitative reverse transcription polymerase chain reaction, we further showed that the expression of metal-resistance genes (dnaK and clpB) was downregulated due to surface MTT5 protected our recombinant bacteria from Cd²⁺ adsorption. Furthermore, we showed that our recombinant bacteria could adsorb Cd from the contaminated wastewater containing other metals and were suggested to be applied in the field study.

Molecular Mechanisms Underlying Bacterial Uranium Resistance

Environmental uranium pollution due to industries producing naturally occurring radioactive material or nuclear accidents and releases is a global concern. Uranium is hazardous for ecosystems as well as for humans when accumulated through the food chain, through contaminated groundwater and potable water sources, or through inhalation. In particular, uranium pollution pressures microbial communities, which are essential for healthy ecosystems. In turn, microorganisms can influence the mobility and toxicity of uranium through processes like biosorption, bioreduction, biomineralization, and bioaccumulation. These processes were characterized by studying the interaction of different bacteria with uranium. However, most studies unraveling the underlying molecular mechanisms originate from the last decade. Molecular mechanisms help to understand how bacteria interact with radionuclides in the environment. Furthermore, knowledge on these underlying mechanisms could be exploited to improve bioremediation technologies. Here, we review the current knowledge on bacterial uranium resistance and how this could be used for bioremediation applications.

Bioengineered Microbes for Soil Health Restoration - Present Status and Future

According to the United Nations Environment Programme (UNEP), soil health is declining over the decades and it has an adverse impact on human health and food security. Hence, soil health restoration is a need of the hour. It is known that microorganisms play a vital role in remediation of soil pollutants like heavy metals, pesticides, hydrocarbons, etc. However, the indigenous microbes have a limited capacity to degrade these pollutants and it will be a slow process. Genetically modified organisms (GMOs) can catalyze the degradation process as their altered metabolic pathways lead to hypersecretions of various biomolecules that favor the bioremediation process. This review provides an overview on the application of bioengineered microorganisms for the restoration of soil health by degradation of various pollutants. It also sheds light on the challenges of using GMOs in environmental application as their introduction may affect the normal microbial community in soil. Since soil health also refers to the potential of native organisms to survive, the possible changes in the native microbial community with the introduction of GMOs are also discussed. Finally, the future prospects of using bioengineered microorganisms in environmental engineering applications to make the soil fertile and healthy have been deciphered. With the alarming rates of soil health loss, the treatment of soil and soil health restoration need to be fastened to a greater pace and the combinatorial efforts unifying GMOs, plant growth-promoting rhizobacteria, and other soil amendments will provide an effective solution to soil heath restoration ten years ahead.