Efflux Transporter ArsK Is Responsible for Bacterial Resistance to Arsenite, Antimonite, Trivalent Roxarsone, and Methylarsenite

Exploring ecological effects of arsenic and cadmium combined exposure on cropland soil: from multilevel organisms to soil functioning by multi-omics coupled with high-throughput quantitative PCR

Arsenic (As) and cadmium (Cd) pose potential ecological threats to cropland soils; however, few studies have investigated their combined effects on multilevel organisms and soil functioning. Here, we used collembolans and soil microbiota as test organisms to examine their responses to soil As and Cd co-contamination at the gene, individual, and community levels, respectively, and further uncovered ecological relationships between pollutants, multilevel organisms, and soil functioning. At the gene level, collembolan transcriptome revealed that elevated As concentrations stimulated As-detoxifying genes AS3MT and GST, whereas the concurrent Cd restrained GST gene expression. At the individual level, collembolan reproduction was sensitive to pollutants while collembolan survival wasn't. At the community level, significant but inconsistent correlations were observed between the biodiversity of different soil keystone microbial clusters and soil As levels. Moreover, soil functioning related to nutrient (e.g., carbon, nitrogen, phosphorus, and sulfur) cycles was inhibited under As and Cd co-exposure only through the mediation of plant pathogens. Overall, these findings suggested multilevel bioindicators (i.e., AS3MT gene expression in collembolans, collembolan reproduction, and biodiversity of soil keystone microbial clusters) in cropland soils co-contaminated with As and Cd, thus improving the understanding of the ecotoxicological impact of heavy metal co-contamination on soil ecosystems.

Adaptation to metal(loid)s in strain Mucilaginibacter rubeus P2 involves novel arsenic resistance genes and mechanisms

Arsenic is a ubiquitous environmental toxi substance that affects human health. Compared to inorganic arsenicals, reduced organoarsenicals are more toxic, and some of them are recognized as antibiotics, such as methylarsenite [MAs(III)] and arsinothricin (2-amino-4-(hydroxymethylarsinoyl)butanoate, or AST). To date, organoarsenicals such as MAs(V) and roxarsone [Rox(V)] are still used in agriculture and animal husbandry. How bacteria deal with both inorganic and organoarsenic species is unclear. Recently, we identified an environmental isolate Mucilaginibacter rubeus P2 that has adapted to high arsenic and antinomy levels by triplicating an arsR-mrarsUBact-arsN-arsC-(arsRhp)-hp-acr3-mrme1Bact-mrme2Bactgene cluster. Heterologous expression of mrarsMBact, mrarsUBact, mrme1Bact and mrme2Bact, encoding putative arsenic resistance determinants, in the arsenic hypersensitive strain Escherichia coli AW3110 conferred resistance to As(III), As(V), MAs(III) or Rox(III). Our data suggest that metalloid exposure promotes plasticity in arsenic resistance systems, enhancing host organism adaptation to metalloid stress.

Widespread Distribution of the arsO Gene Confers Bacterial Resistance to Environmental Antimony

Microbial oxidation of environmental antimonite (Sb(III)) to antimonate (Sb(V)) is an antimony (Sb) detoxification mechanism. Ensifer adhaerens ST2, a bacterial isolate from a Sb-contaminated paddy soil, oxidizes Sb(III) to Sb(V) under oxic conditions by an unknown mechanism. Genomic analysis of ST2 reveals a gene of unknown function in an arsenic resistance (ars) operon that we term arsO. The transcription level of arsO was significantly upregulated by the addition of Sb(III). ArsO is predicted to be a flavoprotein monooxygenase but shows low sequence similarity to other flavoprotein monooxygenases. Expression of arsO in the arsenic-hypersensitive Escherichia coli strain AW3110Δars conferred increased resistance to Sb(III) but not arsenite (As(III)) or methylarsenite (MAs(III)). Purified ArsO catalyzes Sb(III) oxidation to Sb(V) with NADPH or NADH as the electron donor but does not oxidize As(III) or MAs(III). The purified enzyme contains flavin adenine dinucleotide (FAD) at a ratio of 0.62 mol of FAD/mol protein, and enzymatic activity was increased by addition of FAD. Bioinformatic analyses show that arsO genes are widely distributed in metagenomes from different environments and are particularly abundant in environments affected by human activities. This study demonstrates that ArsO is an environmental Sb(III) oxidase that plays a significant role in the detoxification of Sb(III).

Antimony efflux underpins phosphorus cycling and resistance of phosphate-solubilizing bacteria in mining soils

Microorganisms play crucial roles in phosphorus (P) turnover and P bioavailability increases in heavy metal-contaminated soils. However, microbially driven P-cycling processes and mechanisms of their resistance to heavy metal contaminants remain poorly understood. Here, we examined the possible survival strategies of P-cycling microorganisms in horizontal and vertical soil samples from the world's largest antimony (Sb) mining site, which is located in Xikuangshan, China. We found that total soil Sb and pH were the primary factors affecting bacterial community diversity, structure and P-cycling traits. Bacteria with the gcd gene, encoding an enzyme responsible for gluconic acid production, largely correlated with inorganic phosphate (Pi) solubilization and significantly enhanced soil P bioavailability. Among the 106 nearly complete bacterial metagenome-assembled genomes (MAGs) recovered, 60.4% carried the gcd gene. Pi transportation systems encoded by pit or pstSCAB were widely present in gcd-harboring bacteria, and 43.8% of the gcd-harboring bacteria also carried the acr3 gene encoding an Sb efflux pump. Phylogenetic and potential horizontal gene transfer (HGT) analyses of acr3 indicated that Sb efflux could be a dominant resistance mechanism, and two gcd-harboring MAGs appeared to acquire acr3 through HGT. The results indicated that Sb efflux could enhance P cycling and heavy metal resistance in Pi-solubilizing bacteria in mining soils. This study provides novel strategies for managing and remediating heavy metal-contaminated ecosystems.

Toxic effects of cadmium on the physiological and biochemical attributes of plants, and phytoremediation strategies: A review

Anthropogenic activities pose a more significant threat to the environment than natural phenomena by contaminating the environment with heavy metals. Cadmium (Cd), a highly poisonous heavy metal, has a protracted biological half-life and threatens food safety. Plant roots absorb Cd due to its high bioavailability through apoplastic and symplastic pathways and translocate it to shoots through the xylem with the help of transporters and then to the edible parts via the phloem. The uptake and accumulation of Cd in plants pose deleterious effects on plant physiological and biochemical processes, which alter the morphology of vegetative and reproductive parts. In vegetative parts, Cd stunts root and shoot growth, photosynthetic activities, stomatal conductance, and overall plant biomass. Plants' male reproductive parts are more prone to Cd toxicity than female reproductive parts, ultimately affecting their grain/fruit production and survival. To alleviate/avoid/tolerate Cd toxicity, plants activate several defense mechanisms, including enzymatic and non-enzymatic antioxidants, Cd-tolerant gene up-regulations, and phytohormonal secretion. Additionally, plants tolerate Cd through chelating and sequestering as part of the intracellular defensive mechanism with the help of phytochelatins and metallothionein proteins, which help mitigate the harmful effects of Cd. The knowledge on the impact of Cd on plant vegetative and reproductive parts and the plants' physiological and biochemical responses can help selection of the most effective Cd-mitigating/avoiding/tolerating strategy to manage Cd toxicity in plants.

Arsenic in medicine: past, present and future

Arsenicals are one of the oldest treatments for a variety of human disorders. Although infamous for its toxicity, arsenic is paradoxically a therapeutic agent that has been used since ancient times for the treatment of multiple diseases. The use of most arsenic-based drugs was abandoned with the discovery of antibiotics in the 1940s, but a few remained in use such as those for the treatment of trypanosomiasis. In the 1970s, arsenic trioxide, the active ingredient in a traditional Chinese medicine, was shown to produce dramatic remission of acute promyelocytic leukemia similar to the effect of all-trans retinoic acid. Since then, there has been a renewed interest in the clinical use of arsenicals. Here the ancient and modern medicinal uses of inorganic and organic arsenicals are reviewed. Included are antimicrobial, antiviral, antiparasitic and anticancer applications. In the face of increasing antibiotic resistance and the emergence of deadly pathogens such as the severe acute respiratory syndrome coronavirus 2, we propose revisiting arsenicals with proven efficacy to combat emerging pathogens. Current advances in science and technology can be employed to design newer arsenical drugs with high therapeutic index. These novel arsenicals can be used in combination with existing drugs or serve as valuable alternatives in the fight against cancer and emerging pathogens. The discovery of the pentavalent arsenic-containing antibiotic arsinothricin, which is effective against multidrug-resistant pathogens, illustrates the future potential of this new class of organoarsenical antibiotics.

The ArsQ permease and transport of the antibiotic arsinothricin

The pentavalent organoarsenical arsinothricin (AST) is a natural product synthesized by the rhizosphere bacterium Burkholderia gladioli GSRB05. AST is a broad-spectrum antibiotic effective against human pathogens such as carbapenem-resistant Enterobacter cloacae. It is a non-proteogenic amino acid and glutamate mimetic that inhibits bacterial glutamine synthetase. The AST biosynthetic pathway is composed of a three-gene cluster, arsQML. ArsL catalyzes synthesis of reduced trivalent hydroxyarsinothricin (R-AST-OH), which is methylated by ArsM to the reduced trivalent form of AST (R-AST). In the culture medium of B. gladioli, both trivalent species appear as the corresponding pentavalent arsenicals, likely due to oxidation in air. ArsQ is an efflux permease that is proposed to transport AST or related species out of the cells, but the chemical nature of the actual transport substrate is unclear. In this study, B. gladioli arsQ was expressed in Escherichia coli and shown to confer resistance to AST and its derivatives. Cells of E. coli accumulate R-AST, and exponentially growing cells expressing arsQ take up less R-AST. The cells exhibit little transport of their pentavalent forms. Transport was independent of cellular energy and appears to be equilibrative. A homology model of ArsQ suggests that Ser320 is in the substrate binding site. A S320A mutant exhibits reduced R-AST-OH transport, suggesting that it plays a role in ArsQ function. The ArsQ permease is proposed to be an energy-independent uniporter responsible for downhill transport of the trivalent form of AST out of cells, which is oxidized extracellularly to the active form of the antibiotic.

A Coculture of Enterobacter and Comamonas Species Reduces Cadmium Accumulation in Rice

The accumulation of cadmium (Cd) in plants is strongly impacted by soil microbes, but its mechanism remains poorly understood. Here, we report the mechanism of reduced Cd accumulation in rice by coculture of Enterobacter and Comamonas species. In pot experiments, inoculation with the coculture decreased Cd content in rice grain and increased the amount of nonbioavailable Cd in Cd-spiked soils. Fluorescence in situ hybridization and scanning electron microscopy detection showed that the coculture colonized in the rhizosphere and rice root vascular tissue and intercellular space. Soil metagenomics data showed that the coculture increased the abundance of sulfate reduction and biofilm formation genes and related bacterial species. Moreover, the coculture increased the content of organic matter, available nitrogen, and potassium and increased the activities of arylsulfatase, β-galactosidase, phenoloxidase, arylamidase, urease, dehydrogenase, and peroxidase in soils. In subsequent rice transcriptomics assays, we found that the inoculation with coculture activated a hypersensitive response, defense-related induction, and mitogen-activated protein kinase signaling pathway in rice. Heterologous protein expression in yeast confirmed the function of four Cd-binding proteins (HIP28-1, HIP28-4, BCP2, and CID8), a Cd efflux protein (BCP1), and three Cd uptake proteins (COPT4, NRAM5, and HKT6) in rice. Succinic acid and phenylalanine were subsequently proved to inhibit rice divalent Cd [Cd(II)] uptake and activate Cd(II) efflux in rice roots. Thus, we propose a model that the coculture protects rice against Cd stress via Cd immobilization in soils and reducing Cd uptake in rice. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Biochemical, genomic and structural characteristics of the Acr3 pump in Exiguobacterium strains isolated from arsenic-rich Salar de Huasco sediments

Arsenic is a highly toxic metalloid of major concern for public safety. However, microorganisms have several resistance mechanisms, particularly the expression of arsenic pumps is a critical component for bacterial ability to expel it and decrease intracellular toxicity. In this study, we aimed to characterize the biochemical, structural, and genomic characteristics of the Acr3 pump among a group of Exiguobacterium strains isolated from different sites of the arsenic-rich Salar de Huasco (SH) ecosystem. We also determined whether the differences in As(III) resistance levels presented by the strains could be attributed to changes in the sequence or structure of this protein. In this context, we found that based on acr3 sequences the strains isolated from the SH grouped together phylogenetically, even though clustering based on gene sequence identity did not reflect the strain’s geographical origin. Furthermore, we determined the genetic context of the acr3 sequences and found that there are two versions of the organization of acr3 gene clusters, that do not reflect the strain’s origin nor arsenic resistance level. We also contribute to the knowledge regarding structure of the Acr3 protein and its possible implications on the functionality of the pump, finding that although important and conserved components of this family of proteins are present, there are several changes in the amino acidic sequences that may affect the interactions among amino acids in the 3D model, which in fact are evidenced as changes in the structure and residues contacts. Finally, we demonstrated through heterologous expression that the Exiguobacterium Acr3 pump does indeed improve the organisms As resistance level, as evidenced in the complemented E. coli strains. The understanding of arsenic detoxification processes in prokaryotes has vast biotechnological potential and it can also provide a lot of information to understand the processes of evolutionary adaptation.

Methylation of arsenic differs with substrates in Arcticibacter tournemirensis R1 from an As-contaminated paddy soil

Arsenic can be methylated by arsenite (As(III)) S-adenosylmethionine methyltransferases (ArsMs) among various kingdoms of life. The intermediate product methylarsenite (MAs(III)) is highly toxic and can be utilized as an antibiotic by some microbes. ArsM gene is widely distributed in the members of every kingdom from bacteria to humans and displays a high diversity of sequence. Based on arsenic methylating capacity, ArsM proteins can be divided into two phylogenetically distinct clades (Groups 1 and 2). In this study, we show that Arcticibacter tournemirensis R1 isolated from arsenic contaminated paddy soil is resistant to both As(III) and MAs(III), but exhibits different methylation activities for As(III) and MAs(III). The A. tournemirensis R1 shows low As(III) methylation activity and produces an unknown arsenic compound. In contrast, it shows high methylation activity with MAs(III), with the main product of dimethylarsenate (DMAs(V)). An AtarsM gene is found in ars operon of A. tournemirensis R1 genome and is regulated by an atypical transcriptional repressor ArsR. Expressed in Escherichia coli AtArsM confers resistance to As(III) and MAs(III). Both in vivo and in vitro assays show that AtArsM methylates As(III) and MAs(III) to dimethyl- and trimethyl‑arsenicals. AtArsM has four conserved cysteine residues, which are present in most ArsMs and can be classified into phylogenetic group 2 family, producing trimethylated arsenic metabolites. The high arsenic methylation and volatilization activity of AtArsM provides a potential strategy for arsenic bioremediation. The methylation activity differs with As(III) and MAs(III) in A. tournemirensis R1 indicates that there may have different detoxification mechanisms for As(III) and MAs(III), which are worth investigating in the future.

Environmental Behavior and Remediation Methods of Roxarsone

Roxarsone (ROX) is used extensively in the broiler chicken industry, and most is excreted in poultry litter. ROX degradation produces inorganic arsenic, which causes arsenic contamination of soil and aquatic environment. Furthermore, elevated arsenic concentrations are found in livers of chickens fed ROX. Microorganisms, light, and ions are the main factors that promote ROX degradation in the environment. The adsorption of ROX on different substances and its influencing factors have also been studied extensively. Additionally, the remediation method, combining adsorption and degradation, can effectively restore ROX contamination. Based on this, the review reports the ecological hazards, discussed the transformation and adsorption of ROX in environmental systems, documents the biological response to ROX, and summarizes the remediation methods of ROX contamination. Most previous studies of ROX have been focused on identifying the mechanisms involved under theoretical conditions, but more attention should be paid to the behavior of ROX under real environmental conditions, including the fate and transport of ROX in the real environment. ROX remediation methods at real contaminated sites should also be assessed and verified. The summary of previous studies on the environmental behavior and remediation methods of ROX is helpful for further research in the future.

Characterization of Two Highly Arsenic-Resistant Caulobacteraceae Strains of Brevundimonas nasdae: Discovery of a New Arsenic Resistance Determinant

Arsenic (As), distributed widely in the natural environment, is a toxic substance which can severely impair the normal functions in living cells. Research on the genetic determinants conferring functions in arsenic resistance and metabolism is of great importance for remediating arsenic-contaminated environments. Many organisms, including bacteria, have developed various strategies to tolerate arsenic, by either detoxifying this harmful element or utilizing it for energy generation. More and more new arsenic resistance (ars) determinants have been identified to be conferring resistance to diverse arsenic compounds and encoded in ars operons. There is a hazard in mobilizing arsenic during gold-mining activities due to gold- and arsenic-bearing minerals coexisting. In this study, we isolated 8 gold enrichment strains from the Zijin gold and copper mine (Longyan, Fujian Province, China) wastewater treatment site soil, at an altitude of 192 m. We identified two Brevundimonas nasdae strains, Au-Bre29 and Au-Bre30, among these eight strains, having a high minimum inhibitory concentration (MIC) for As(III). These two strains contained the same ars operons but displayed differences regarding secretion of extra-polymeric substances (EPS) upon arsenite (As(III)) stress. B. nasdae Au-Bre29 contained one extra plasmid but without harboring any additional ars genes compared to B. nasdae Au-Bre30. We optimized the growth conditions for strains Au-Bre29 and Au-Bre30. Au-Bre30 was able to tolerate both a lower pH and slightly higher concentrations of NaCl. We also identified folE, a folate synthesis gene, in the ars operon of these two strains. In most organisms, folate synthesis begins with a FolE (GTP-Cyclohydrolase I)-type enzyme, and the corresponding gene is typically designated folE (in bacteria) or gch1 (in mammals). Heterologous expression of folE, cloned from B. nasdae Au-Bre30, in the arsenic-hypersensitive strain Escherichia coli AW3110, conferred resistance to As(III), arsenate (As(V)), trivalent roxarsone (Rox(III)), pentavalent roxarsone (Rox(V)), trivalent antimonite (Sb(III)), and pentavalent antimonate (Sb(V)), indicating that folate biosynthesis is a target of arsenite toxicity and increased production of folate confers increased resistance to oxyanions. Genes encoding Acr3 and ArsH were shown to confer resistance to As(III), Rox(III), Sb(III), and Sb(V), and ArsH also conferred resistance to As(V). Acr3 did not confer resistance to As(V) and Rox(V), while ArsH did not confer resistance to Rox(V).

ArsZ from Ensifer adhaerens ST2 is a novel methylarsenite oxidase

Trivalent methylarsenite (MAs(III)) produced by biomethylation is more toxic than inorganic arsenite (As(III)). Hence, MAs(III) has been proposed to be a primordial antibiotic. Other bacteria evolved mechanisms to detoxify MAs(III). In this study, the molecular mechanisms of MAs(III) resistance of Ensifer adhaerens ST2 were investigated. In the chromosome of E. adhaerens ST2 is a gene encoding a protein of unknown function. Here we show that this gene, designated arsZ, encodes a novel MAs(III) oxidase that confers resistance by oxidizing highly toxic MAs(III) to relatively nontoxic MAs(V). Two other genes, arsRK, are adjacent to arsZ but are divergently encoded in the opposite direction. Heterologous expression of arsZ in Escherichia coli confers resistance to MAs(III) but not to As(III). Purified ArsZ catalyzes thioredoxin- and NAPD⁺ -dependent oxidation of MAs(III). Mutational analysis of ArsZ suggests that Cys59 and Cys123 are involved in oxidation of MAs(III). Expression of arsZ, arsR and arsK genes is induced by MAs(III) and As(III), and is likely controlled by the ArsR transcriptional repressor. These results demonstrate that ArsZ is a novel MAs(III) oxidase that contributes to E. adhaerens tolerance to environmental organoarsenicals. The arsZRK operon is widely present in bacteria within the Rhizobiaceae family. This article is protected by copyright. All rights reserved.

Functional characterization of the methylarsenite-inducible arsRM operon from Noviherbaspirillum denitrificans HC18

Microbial arsenic methylation by arsenite (As(III)) S-adenosylmethionine methyltransferases (ArsMs) can produce the intermediate methylarsenite (MAs(III)), which is highly toxic and is used by some microbes as an antibiotic. Other microbes have evolved mechanisms to detoxify MAs(III). In this study, an arsRM operon was identified in the genome of an MAs(III)-methylation strain Noviherbaspirillum denitrificans HC18. The arsM gene (NdarsM) is located downstream of an open reading frame encoding an MAs(III)-responsive transcriptional regulator (NdArsR). The N. denitrificans arsRM genes are co-transcribed whose expression is significantly induced by MAs(III), likely by alleviating the repressive effect of ArsR on arsRM transcription. Both in vivo and in vitro assays showed that NdArsM methylates MAs(III) to dimethyl- and trimethyl-arsenicals but does not methylate As(III). Heterologous expression of NdarsM in arsenic-sensitive Escherichia coli AW3110 conferred resistance to MAs(III) but not As(III). NdArsM has the four conserved cysteine residues present in most ArsMs, but only two of them are essential for MAs(III) methylation. The ability to methylate MAs(III) by enzymes such as NdArsM may be an evolutionary step originated from enzymes capable of methylating As(III). This finding reveals a mechanism employed by microbes such as N. denitrificans HC18 to detoxify MAs(III) by further methylation.

Organoarsenical tolerance in Sphingobacterium wenxiniae, a bacterium isolated from activated sludge

Organoarsenicals enter the environment from biogenic and anthropogenic sources. Trivalent inorganic arsenite (As(III)) is microbially methylated to more toxic methylarsenite (MAs(III)) and dimethylarsenite (DMAs(III)) that oxidize in air to MAs(V) and DMAs(V). Sources include the herbicide monosodium methylarsenate (MSMA or MAs(V)), which is microbially reduced to MAs(III), and the aromatic arsenical roxarsone (3-nitro-4-hydroxybenzenearsonic acid or Rox), an antimicrobial growth promoter for poultry and swine. Here we show that Sphingobacterium wenxiniae LQY-18^T , isolated from activated sludge, is resistant to trivalent MAs(III) and Rox(III). Sphingobacterium wenxiniae detoxifies MAs(III) and Rox(III) by oxidation to MAs(V) and Rox(V). Sphingobacterium wenxiniae has a novel chromosomal gene, termed arsU1. Expressed in Escherichia coli arsU1 confers resistance to MAs(III) and Rox(III) but not As(III) or pentavalent organoarsenicals. Purified ArsU1 catalyses oxidation of trivalent methylarsenite and roxarsone. ArsU1 has six conserved cysteine residues. The DNA sequence for the three C-terminal cysteines was deleted, and the other three were mutated to serines. Only C45S and C122S lost activity, suggesting that Cys45 and Cys122 play a role in ArsU1 function. ArsU1 requires neither FMN nor FAD for activity. These results demonstrate that ArsU1 is a novel MAs(III) oxidase that contributes to S. wenxiniae tolerance to organoarsenicals.

Oxidation of organoarsenicals and antimonite by a novel flavin monooxygenase widely present in soil bacteria

Arsenic can be biomethylated to form a variety of organic arsenicals differing in toxicity and environmental mobility. Trivalent methylarsenite (MAs(III)) produced in the methylation process is more toxic than inorganic arsenite (As(III)). MAs(III) also serves as a primitive antibiotic and, consequently, some environmental microorganisms have evolved mechanisms to detoxify MAs(III). However, the mechanisms of MAs(III) detoxification are not well understood. In this study, we identified an arsenic resistance (ars) operon consisting of three genes, arsRVK, that contribute to MAs(III) resistance in Ensifer adhaerens ST2. ArsV is annotated as an NADPH-dependent flavin monooxygenase with unknown function. Expression of arsV in the arsenic hypersensitive Escherichia coli strain AW3110Δars conferred resistance to MAs(III) and the ability to oxidize MAs(III) to MAs(V). In the presence of NADPH and either FAD or FMN, purified ArsV protein was able to oxidize both MAs(III) to MAs(V) and Sb(III) to Sb(V). Genes with arsV-like sequences are widely present in soils and environmental bacteria. Metagenomic analysis of five paddy soils showed the abundance of arsV-like sequences of 0.12-0.25 ppm. These results demonstrate that ArsV is a novel enzyme for the detoxification of MAs(III) and Sb(III) and the genes encoding ArsV are widely present in soil bacteria.

Glutathione is involved in the reduction of methylarsenate to generate antibiotic methylarsenite in Enterobacter sp. CZ-1

Methylarsenate (MAs(V)) is a product of microbial arsenic (As) biomethylation and has also been widely used as an herbicide. Some microbes are able to reduce nontoxic MAs(V) to highly toxic methylarsenite (MAs(III)) possibly as an antibiotic. The mechanism of MAs(V) reduction in microbes has not been elucidated. Here, we found that the bacterium Enterobacter sp. CZ-1 isolated from an As-contaminated paddy soil has a strong ability to reduce MAs(V) to MAs(III). Using a MAs(III)-responsive biosensor to detect MAs(V) reduction in E. coli Trans5α transformants of a genomic library of Enterobacter sp. CZ-1, we identified gshA, encoding a glutamate-cysteine ligase, as a key gene involved in MAs(V) reduction. Heterologous expression of gshA increased the biosynthesis of glutathione (GSH) and MAs(V) reduction in E. coli Trans5α. Deletion of gshA in Enterobacter sp. CZ-1 abolished its ability to synthesize GSH and decreased its MAs(V) reduction ability markedly, which could be restored by supplementation of exogenous GSH. In the presence of MAs(V), Enterobacter sp. CZ-1 was able to inhibit the growth of Bacillus subtilis 168; this ability was lost in the gshA-deleted mutant. In addition, deletion of gshA greatly decreased the reduction of arsenate to arsenite. These results indicate that GSH plays an important role in MAs(V) reduction to generate MAs(III) as an antibiotic. IMPORTANCE Arsenic is a ubiquitous environmental toxin. Some microbes detoxify inorganic arsenic through biomethylation, generating relatively nontoxic pentavalent methylated arsenicals, such as methylarsenate. Methylarsenate has also been widely used as an herbicide. Surprisingly, some microbes reduce methylarsenate to highly toxic methylarsenite possibly to use the latter as an antibiotic. How microbes reduce methylarsenate to methylarsenite is unknown. Here, we show that gshA encoding a glutamate-cysteine ligase in the glutathione biosynthesis pathway is involved in methylarsenate reduction in Enterobacter sp. CZ-1. Our study provides new insights into the crucial role of glutathione in the transformation of a common arsenic compound to a natural antibiotic.

Phosphorus-arsenic interaction in the 'soil-plant-microbe' system and its influence on arsenic pollution

Elevated arsenic (As) in soil is of public concern due to the carcinogenicity. Phosphorus (P) strongly influences the adsorption, absorption, transport, and transformation of As in the soil and in organisms due to the similarity of the chemical properties of P and As. In soil, P, particularly inorganic P, can release soil-retained As (mostly arsenate) by competing for adsorption sites. In plant and microbial systems, P usually reduces As (mainly arsenate) uptake and affects As biotransformation by competing for As transporters. The intensity and pattern of PAs interaction are highly dependent on the forms of As and P, and strongly influenced by various biotic and abiotic factors. An understanding of the PAs interaction in 'soil-plant-microbe' systems is of great value to prevent soil As from entering the human food chain. Here, we review PAs interactions and the main influential factors in soil, plant, and microbial subsystems and their effects on the As release, absorption, transformation, and transport in the 'soil-plant-microbe' system. We also analyze the application potential of P fertilization as a control for As pollution and suggest the research directions that need to be followed in the future.

Bacterial adaptive strategies to cope with metal toxicity in the contaminated environment - A review

Heavy metal contamination poses a serious environmental hazard, globally necessitating intricate attention. Heavy metals can cause deleterious health hazards to humans and other living organisms even at low concentrations. Environmental biotechnologists and eco-toxicologists have rigorously assessed a plethora of bioremediation mechanisms that can hamper the toxic outcomes and the molecular basis for rejuvenating the hazardous impacts, optimistically. Environmental impact assessment and restoration of native and positive scenario has compelled biological management in ensuring safety replenishment in polluted realms often hindered by heavy metal toxicity. Copious treatment modalities have been corroborated to mitigate the detrimental effects to remove heavy metals from polluted sites. In particular, Biological-based treatment methods are of great attention in the metal removal sector due to their high efficiency at low metal concentrations, ecofriendly nature, and cost-effectiveness. Due to rapid multiplication and growth rates, bacteria having metal resistance are advocated for metal removal applications. Evolutionary implications of coping with heavy metals toxicity have redressed bacterial adaptive/resistance strategies related to physiological and cross-protective mechanisms. Ample reviews have been reported for the bacterial adaptive strategies to cope with heavy metal toxicity. Nevertheless, a holistic review summarizing the redox reactions that address the cross-reactivity mechanisms between metallothionein synthesis, extracellular polysaccharides production, siderophore production, and efflux systems of metal resistant bacteria are scarce. Molecular dissection of how bacteria adapt themselves to metal toxicity can augment novel and innovative technologies for efficient detoxification, removal, and combat the restorative difficulties for stress alleviations. The present comprehensive compilation addresses the identification of newer methodologies, summarizing the prevailing strategies of adaptive/resistance mechanisms in bacterial bioremediation. Further pitfalls and respective future directions are enumerated in invigorating effective bioremediation technologies including overexpression studies and delivery systems. The analysis will aid in abridging the gap for limitations in heavy metal removal strategies and necessary cross-talk in elucidating the complex cascade of events in better bioremediation protocols.

Identification of a MarR Subfamily That Regulates Arsenic Resistance Genes

In this study, comprehensive analyses were performed to determine the function of an atypical MarR homolog in Achromobacter sp. strain As-55. Genomic analyses of Achromobacter sp. As-55 showed that this marR is located adjacent to an arsV gene. ArsV is a flavin-dependent monooxygenase that confers resistance to the antibiotic methylarsenite [MAs(III)], the organoarsenic compound roxarsone(III) [Rox(III)], and the inorganic antimonite [Sb(III)]. Similar marR genes are widely distributed in arsenic-resistant bacteria. Phylogenetic analyses showed that these MarRs are found in operons predicted to be involved in resistance to inorganic and organic arsenic species, so the subfamily was named MarR_ars. MarR_ars orthologs have three conserved cysteine residues, which are Cys36, Cys37, and Cys157 in Achromobacter sp. As-55, mutation of which compromises the response to MAs(III)/Sb(III). GFP-fluorescent biosensor assays show that AdMarR_ars (MarR protein of Achromobacter deleyi As-55) responds to trivalent As(III) and Sb(III) but not to pentavalent As(V) or Sb(V). The results of RT-qPCR assays show that arsV is expressed constitutively in a marR deletion mutant, indicating that marR represses transcription of arsV. Moreover, electrophoretic mobility shift assays (EMSAs) demonstrate that AdMarR_ars binds to the promoters of both marR and arsV in the absence of ligands and that DNA binding is relieved upon binding of As(III) and Sb(III). Our results demonstrate that AdMarR_ars is a novel As(III)/Sb(III)-responsive transcriptional repressor that controls expression of arsV, which confers resistance to MAs(III), Rox(III), and Sb(III). AdMarR_ars and its orthologs form a subfamily of MarR proteins that regulate genes conferring resistance to arsenic-containing antibiotics. IMPORTANCE In this study, a MarR family member, AdMarR_ars was shown to regulate the arsV gene, which confers resistance to arsenic-containing antibiotics. It is a founding member of a distinct subfamily that we refer to as MarR_ars, regulating genes conferring resistance to arsenic and antimony antibiotic compounds. AdMarR_ars was shown to be a repressor containing conserved cysteine residues that are required to bind As(III) and Sb(III), leading to a conformational change and subsequent derepression. Here we show that members of the MarR family are involved in regulating arsenic-containing compounds.

Genetic and evolutionary characterization of the Major Facilitator Superfamily transporters of the antibacterial, Pantoea Natural Product 3

Pantoea Natural Product 3 (PNP-3) is an antibiotic produced by Pantoea agglomerans that is effective against a broad range of multi-drug resistant bacteria. PNP-3 is encoded by a unique, eight-gene biosynthetic gene cluster composed of predicted enzymes (pnp3b, pnp3e-h), a regulator (pnp3d), and two Major Facilitator Superfamily transporters (pnp3a and pnp3c). To better characterize the role of the transporters, we generated pnp3a and pnp3c mutants and evaluated PNP-3 production. Disruption of pnp3a in Pantoea results in impaired growth and loss of antibiosis, suggesting a role in PNP-3 export and resistance. In contrast, pnp3c mutants display only reduced antibiotic production/export, suggesting a minor role for Pnp3c. Expression of pnp3a in susceptible Erwinia amylovora led to increased PNP-3 tolerance, while co-expression of pnp3a and pnp3e-h resulted in the production and export of PNP-3. Comparative genomic analyses identified pnp3a in 12 other Pantoea strains, eight of which carry a complete or nearly complete PNP-3 biosynthetic cluster. The four other Pantoea strains that carry pnp3a lack most of the PNP-3 cluster; however, they are PNP-3 tolerant. These results suggest Pnp3a plays an essential role in PNP-3 export and resistance in Pantoea.

ArsV and ArsW provide synergistic resistance to the antibiotic methylarsenite

Toxic organoarsenicals enter the environment from biogenic and anthropogenic activities such as microbial methylation of inorganic arsenic and pentavalent herbicides such as monosodium methylarsenate (MSMA or MAs(V)). Trivalent MAs(III) is considerably more toxic than arsenite or arsenate. Microbes have evolved mechanisms to detoxify organoarsenicals. We previously identified ArsV, a flavin-linked monooxygenase and demonstrated that it confers resistance to methylarsenite by oxidation to methylarsenate. The arsV gene is usually in an arsenic resistance (ars) operon controlled by an ArsR repressor and adjacent to a methylarsenite efflux gene, either arsK or a gene for a putative transporter. Here we show that Paracoccus sp. SY oxidizes methylarsenite. It has an ars operon with three genes, arsR, arsV and a transport gene termed arsW. Heterologous expression of arsV in Escherichia coli conferred resistance to MAs(III), while arsW did not. Co-expression of arsV and arsW increased resistance compared with either alone. The cells oxidized methylarsenite and accumulated less methylarsenate. Everted membrane vesicles from E. coli cells expressing arsW-accumulated methylarsenate. We propose that ArsV is a monooxygenase that oxidizes methylarsenite to methylarsenate, which is extruded by ArsW, one of only a few known pentavalent organoarsenical efflux permeases, a novel pathway of organoarsenical resistance.

The Microbiology of Metal Mine Waste: Bioremediation Applications and Implications for Planetary Health

Mine wastes pollute the environment with metals and metalloids in toxic concentrations, causing problems for humans and wildlife. Microorganisms colonize and inhabit mine wastes, and can influence the environmental mobility of metals through metabolic activity, biogeochemical cycling and detoxification mechanisms. In this article we review the microbiology of the metals and metalloids most commonly associated with mine wastes: arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc. We discuss the molecular mechanisms by which bacteria, archaea, and fungi interact with contaminant metals and the consequences for metal fate in the environment, focusing on long-term field studies of metal-impacted mine wastes where possible. Metal contamination can decrease the efficiency of soil functioning and essential element cycling due to the need for microbes to expend energy to maintain and repair cells. However, microbial communities are able to tolerate and adapt to metal contamination, particularly when the contaminant metals are essential elements that are subject to homeostasis or have a close biochemical analog. Stimulating the development of microbially reducing conditions, for example in constructed wetlands, is beneficial for remediating many metals associated with mine wastes. It has been shown to be effective at low pH, circumneutral and high pH conditions in the laboratory and at pilot field-scale. Further demonstration of this technology at full field-scale is required, as is more research to optimize bioremediation and to investigate combined remediation strategies. Microbial activity has the potential to mitigate the impacts of metal mine wastes, and therefore lessen the impact of this pollution on planetary health.

Living to the High Extreme: Unraveling the Composition, Structure, and Functional Insights of Bacterial Communities Thriving in the Arsenic-Rich Salar de Huasco Altiplanic Ecosystem

Microbial communities inhabiting extreme environments such as Salar de Huasco (SH) in northern Chile are adapted to thrive while exposed to several abiotic pressures and the presence of toxic elements such as arsenic (As). Hence, we aimed to uncover the role of As in shaping bacterial composition, structure, and functional potential in five different sites in this altiplanic wetland using a shotgun metagenomic approach. The sites exhibit wide gradients of As (9 to 321 mg/kg), and our results showed highly diverse communities and a clear dominance exerted by the Proteobacteria and Bacteroidetes phyla. Functional potential analyses show broadly convergent patterns, contrasting with their great taxonomic variability. As-related metabolism, as well as other functional categories such as those related to the CH4 and S cycles, differs among the five communities. Particularly, we found that the distribution and abundance of As-related genes increase as the As concentration rises. Approximately 75% of the detected genes for As metabolism belong to expulsion mechanisms; arsJ and arsP pumps are related to sites with higher As concentrations and are present almost exclusively in Proteobacteria. Furthermore, taxonomic diversity and functional potential are reflected in the 12 reconstructed high-quality metagenome assembled genomes (MAGs) belonging to the Bacteroidetes (5), Proteobacteria (5), Cyanobacteria (1), and Gemmatimonadetes (1) phyla. We conclude that SH microbial communities are diverse and possess a broad genetic repertoire to thrive under extreme conditions, including increasing concentrations of highly toxic As. Finally, this environment represents a reservoir of unknown and undescribed microorganisms, with great metabolic versatility, which needs further study. IMPORTANCE As microbial communities inhabiting extreme environments are fundamental for maintaining ecosystems, many studies concerning composition, functionality, and interactions have been carried out. However, much is still unknown. Here, we sampled microbial communities in the Salar de Huasco, an extreme environment subjected to several abiotic stresses (high UV radiation, salinity and arsenic; low pressure and temperatures). We found that although microbes are taxonomically diverse, functional potential seems to have an important degree of convergence, suggesting high levels of adaptation. Particularly, arsenic metabolism showed differences associated with increasing concentrations of the metalloid throughout the area, and it effectively exerts a significant pressure over these organisms. Thus, the significance of this research is that we describe highly specialized communities thriving in little-explored environments subjected to several pressures, considered analogous of early Earth and other planets, that have the potential for unraveling technologies to face the repercussions of climate change in many areas of interest.

Antimicrobial Activity of Metals and Metalloids

Competition shapes evolution. Toxic metals and metalloids have exerted selective pressure on life since the rise of the first organisms on the Earth, which has led to the evolution and acquisition of resistance mechanisms against them, as well as mechanisms to weaponize them. Microorganisms exploit antimicrobial metals and metalloids to gain competitive advantage over other members of microbial communities. This exerts a strong selective pressure that drives evolution of resistance. This review describes, with a focus on arsenic and copper, how microorganisms exploit metals and metalloids for predation and how metal- and metalloid-dependent predation may have been a driving force for evolution of microbial resistance against metals and metalloids. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

NemA Catalyzes Trivalent Organoarsenical Oxidation and Is Regulated by the Trivalent Organoarsenical-Selective Transcriptional Repressor NemR

Synthetic aromatic arsenicals such as roxarsone (Rox(V)) and nitarsone (Nit(V)) have been used as animal growth enhancers and herbicides. Microbes contribute to redox cycling between the relatively less toxic pentavalent and highly toxic trivalent arsenicals. In this study, we report the identification of nemRA operon from Enterobacter sp. Z1 and show that it is involved in trivalent organoarsenical oxidation. Expression of nemA is induced by chromate (Cr(VI)), Rox(III), and Nit(III). Heterologous expression of NemA in Escherichia coli confers resistance to Cr(VI), methylarsenite (MAs(III)), Rox(III), and Nit(III). Purified NemA catalyzes simultaneous Cr(VI) reduction and MAs(III)/Rox(III)/Nit(III) oxidation, and oxidation was enhanced in the presence of Cr(VI). The results of electrophoretic mobility shift assays and fluorescence assays demonstrate that the transcriptional repressor, NemR, binds to either Rox(III) or Nit(III). NemR has three conserved cysteine residues, Cys21, Cys106, and Cys116. Mutation of any of the three resulted in loss of response to Rox(III)/Nit(III), indicating that they form an Rox(III)/Nit(III) binding site. These results show that NemA is a novel trivalent organoarsenical oxidase that is regulated by the trivalent organoarsenical-selective repressor NemR. This discovery expands our knowledge of the molecular mechanisms of organoarsenical oxidation and provides a basis for studying the redox coupling of environmental toxic compounds.

Organoarsenic feed additives in biological wastewater treatment processes: Removal, biotransformation, and associated impacts

Aromatic organoarsenicals are widely used in animal feeding operations and cause arsenic contamination on livestock wastewater and manure, thereby raising the risk of surface water pollution. Biological wastewater treatment processes are often used for livestock wastewater treatment. Organoarsenic removal and biotransformation under aerobic and anaerobic conditions, and the associated impacts have received extensive attention due to the potential threat to water security. The removal efficiency and biotransformation of organoarsenicals in biological treatment processes are reviewed. The underlying mechanisms are discussed in terms of functional microorganisms and genes. The impacts associated with organoarsenicals and their degradation products on microbial activity and performance of bioreactors are also documented. Based on the current research advancement, knowledge gaps and potential research in this field are discussed. Overall, this work delivers a comprehensive understanding on organoarsenic behaviors in biological wastewater treatment processes, and provides valuable information on the control of arsenic contamination from the degradation of organoarsenicals in biological wastewater treatment processes.

Arsenate-Induced Changes in Bacterial Metabolite and Lipid Pools during Phosphate Stress

Agrobacterium tumefaciens GW4 is a heterotrophic arsenite-oxidizing bacterium with a high resistance to arsenic toxicity. It is now a model organism for studying the processes of arsenic detoxification and utilization. Previously, we demonstrated that under low-phosphate conditions, arsenate [As(V)] could enhance bacterial growth and be incorporated into biomolecules, including lipids. While the basic microbial As(V) resistance mechanisms have been characterized, global metabolic responses under low phosphate remain largely unknown. In the present work, the impacts of As(V) and low phosphate on intracellular metabolite and lipid profiles of GW4 were quantified using liquid chromatography-mass spectroscopy (LC-MS) in combination with transcriptional assays and the analysis of intracellular ATP and NADH levels. Metabolite profiling revealed that oxidative stress response pathways were altered and suggested an increase in DNA repair. Changes in metabolite levels in the tricarboxylic acid (TCA) cycle along with increased ATP are consistent with As(V)-enhanced growth of A. tumefaciens GW4. Lipidomics analysis revealed that most glycerophospholipids decreased in abundance when As(V) was available. However, several glycerolipid classes increased, an outcome that is consistent with maximizing growth via a phosphate-sparing phenotype. Differentially regulated lipids included phosphotidylcholine and lysophospholipids, which have not been previously reported in A. tumefaciens The metabolites and lipids identified in this study deepen our understanding of the interplay between phosphate and arsenate on chemical and metabolic levels.IMPORTANCE Arsenic is widespread in the environment and is one of the most ubiquitous environmental pollutants. Parodoxically, the growth of certain bacteria is enhanced by arsenic when phosphate is limited. Arsenate and phosphate are chemically similar, and this behavior is believed to represent a phosphate-sparing phenotype in which arsenate is used in place of phosphate in certain biomolecules. The research presented here uses a global approach to track metabolic changes in an environmentally relevant bacterium during exposure to arsenate when phosphate is low. Our findings are relevant for understanding the environmental fate of arsenic as well as how human-associated microbiomes respond to this common toxin.

Comamonas testosteroni antA encodes an antimonite-translocating P-type ATPase

Antimony, like arsenic, is a toxic metalloid widely distributed in the environment. Microbial detoxification of antimony has recently been identified. Here we describe a novel bacterial P_1B-type antimonite (Sb(III))-translocating ATPase from the antimony-mining bacterium Comamonas testosterone JL40 that confers resistance to Sb(III). In a comparative proteomics analysis of strain JL40, an operon (ant operon) was up-regulated by Sb(III). The ant operon includes three genes, antR, antC and antA. AntR belongs to the ArsR/SmtB family of metalloregulatory proteins that regulates expression of the ant operon. AntA belongs to the P_1B family of the P-type cation-translocating ATPases. It has both similarities to and differences from other members of the P_1B-1 subfamily and appears to be the first identified member of a distinct subfamily that we designate P_1B-8. Expression AntA in E. coli AW3110 (Δars) conferred resistance to Sb(III) and reduced the intracellular concentration of Sb(III) but not As(III) or other metals. Everted membrane vesicles from cells expressing antA accumulated Sb(III) but not As(III), where uptake in everted vesicles reflects efflux from cells. AntC is a small protein with a potential Sb(III) binding site, and co-expression of AntC with AntA increased resistance to Sb(III). We propose that AntC functions as an Sb(III) chaperone to AntA, augmenting Sb(III) efflux. The identification of a novel Sb(III)-translocating ATPase enhances our understanding of the biogeochemical cycling of environmental antimony by bacteria.

Microbial Cd(II) and Cr(VI) resistance mechanisms and application in bioremediation

The heavy metals cadmium (Cd) and chromium (Cr) are extensively used in industry and result in water and soil contamination. The highly toxic Cd(II) and Cr(VI) are the most common soluble forms of Cd and Cr, respectively. They enter the human body through the food chain and drinking water and then cause serious illnesses. Microorganisms can adsorb metals or transform Cd(II) and Cr(VI) into insoluble or less bioavailable forms, and such strategies are applicable in Cd and Cr bioremediation. This review focuses on the highlighting of novel achievements on microbial Cd(II) and Cr(VI) resistance mechanisms and their bioremediation applications. In addition, the knowledge gaps and research perspectives are also discussed in order to build a bridge between the theoretical breakthrough and the resolution of Cd(II) and Cr(VI) contamination problems.

Recent advances in exploring the heavy metal(loid) resistant microbiome

Heavy metal(loid)s exert selective pressure on microbial communities and evolution of metal resistance determinants. Despite increasing knowledge concerning the impact of metal pollution on microbial community and ecological function, it is still a challenge to identify a consistent pattern of microbial community composition along gradients of elevated metal(loid)s in natural environments. Further, our current knowledge of the microbial metal resistome at the community level has been lagging behind compared to the state-of-the-art genetic profiling of bacterial metal resistance mechanisms in a pure culture system. This review provides an overview of the core metal resistant microbiome, development of metal resistance strategies, and potential factors driving the diversity and distribution of metal resistance determinants in natural environments. The impacts of biotic factors regulating the bacterial metal resistome are highlighted. We finally discuss the advances in multiple technologies, research challenges, and future directions to better understand the interface of the environmental microbiome with the metal resistome. This review aims to highlight the diversity and wide distribution of heavy metal(loid)s and their corresponding resistance determinants, helping to better understand the resistance strategy at the community level.

Simultaneous removal of chromate and arsenite by the immobilized Enterobacter bacterium in combination with chemical reagents

Simultaneous chromate [Cr(VI)] reduction and arsenite [As(III)] oxidation is a promising pretreatment process for Cr and As removal. Here, a facultative anaerobic bacterium, Enterobacter sp. Z1, presented capacities of simultaneous Cr(VI) reduction and As(III) oxidation during anoxic cultivation in a wild range of temperature (20-45 °C) and pH (Cerkez et al., 2015; Chen et al., 2015; China Environmental Prote, 1996; Fan et al., 2008, 2019) conditions. Strikingly, strain Z1 could simultaneously contribute up to 92.8% of the reduction of Cr(VI) and 45.8% of the oxidation of As(III) in wastewater. The cells of strain Z1 were embedded with sodium alginate to produce biobeads, and the biobeads exhibited stable ratio of Cr(VI) reduction (91.8%) and As(III) oxidation (29.6%) even in the 5 continuous cycles of wastewater treatment. Moreover, in a process pretreated with the Z1 biobeads followed a precipitation with Ca(OH)₂ and FeCl₃, the removal efficiencies in wastewater were 98.9% and 98.3% for total Cr and As, respectively, which were 44.1% and 9.8% higher than those of using Ca(OH)₂ and FeCl₃, only. The residual amounts of Cr and As met the national standard levels of wastewater discharge. Proteomics analysis showed that cysteine, sulfur and methionine metabolisms, As resistance and oxidoreductase (CysH, CysI, CysJ, NemA and HemF) were induced by Cr(VI) and As(III). Moreover, the addition of cysteine to the medium also significantly improved bacterial Cr(VI) reduction rate. Our results provide a novel microbial pretreatment approach for enhancing remediation of Cr(VI) and As(III) pollution in wastewater, and reveal the evident that cysteine, sulfur and methionine metabolisms, As resistance and oxidoreductases are associated with the redox conversion of Cr(VI) and As(III).

Microbial Oxidation of Arsenite: Regulation, Chemotaxis, Phosphate Metabolism and Energy Generation

Arsenic (As) is a metalloid that occurs widely in the environment. The biological oxidation of arsenite [As(III)] to arsenate [As(V)] is considered a strategy to reduce arsenic toxicity and provide energy. In recent years, research interests in microbial As(III) oxidation have been growing, and related new achievements have been revealed. This review focuses on the highlighting of the novel regulatory mechanisms of bacterial As(III) oxidation, the physiological relevance of different arsenic sensing systems and functional relationship between microbial As(III) oxidation and those of chemotaxis, phosphate uptake, carbon metabolism and energy generation. The implication to environmental bioremediation applications of As(III)-oxidizing strains, the knowledge gaps and perspectives are also discussed.

Bioaccumulation and biotransformation of arsenic in Leptolyngbya boryana

Leptolyngbya boryana (L. boryana) is a typical filamentous cyanobacterium that is widely distributed in aquatic ecosystems and is considered to play an important role in the arsenic biogeochemical cycle. Our results showed that L. boryana resisted arsenite (As(III)) and arsenate (As(V)) concentrations up to 0.25 mM and 5 mM, respectively. When exposed to 100 μM As(III) or As(V) for 4 weeks, L. boryana accumulated as much arsenic as 570.0 mg kg^-1 and 268.5 mg kg^-1, respectively. After the 4-week exposure to As(III) and As(V), organoarsenicals including dimethylarsenate (DMAs(V)) and oxo-arsenosugar-phosphate (Oxo-PO₄) were detected in the cells of L. boryana, while inorganic arsenic, especially As(V), was still the main species in both the cells and medium. Furthermore, arsenic oxidation was observed to be solely caused by L. boryana cells and was considered the dominant detoxification pathway. In conclusion, due to its powerful arsenic accumulation, biotransformation, and detoxification abilities, L. boryana might play an important role in arsenic remediation in aquatic environments.

Genomic Variation and Arsenic Tolerance Emerged as Niche Specific Adaptations by Different Exiguobacterium Strains Isolated From the Extreme Salar de Huasco Environment in Chilean - Altiplano

Polyextremophilic bacteria can thrive in environments with multiple stressors such as the Salar de Huasco (SH). Microbial communities in SH are exposed to low atmospheric pressure, high UV radiation, wide temperature ranges, salinity gradient and the presence of toxic compounds such as arsenic (As). In this work we focus on arsenic stress as one of the main adverse factors in SH and bacteria that belong to the Exiguobacterium genus due to their plasticity and ubiquity. Therefore, our aim was to shed light on the effect of niche conditions pressure (particularly arsenic), on the adaptation and divergence (at genotypic and phenotypic levels) of Exiguobacterium strains from five different SH sites. Also, to capture greater diversity in this genus, we use as outgroup five As(III) sensitive strains isolated from Easter Island (Chile) and The Great Salt Lake (United States). For this, samples were obtained from five different SH sites under an arsenic gradient (9 to 321 mg/kg: sediment) and isolated and sequenced the genomes of 14 Exiguobacterium strains, which had different arsenic tolerance levels. Then, we used comparative genomic analysis to assess the genomic divergence of these strains and their association with phenotypic differences such as arsenic tolerance levels and the ability to resist poly-stress. Phylogenetic analysis showed that SH strains share a common ancestor. Consequently, populations were separated and structured in different SH microenvironments, giving rise to multiple coexisting lineages. Hence, this genotypic variability is also evidenced by the COG (Clusters of Orthologous Groups) composition and the size of their accessory genomes. Interestingly, these observations correlate with physiological traits such as growth patterns, gene expression, and enzyme activity related to arsenic response and/or tolerance. Therefore, Exiguobacterium strains from SH are adapted to physiologically overcome the contrasting environmental conditions, like the arsenic present in their habitat.

Biotransformation of arsenic and toxicological implication of arsenic metabolites

Arsenic is a well-known environmental carcinogen and chronic exposure to arsenic through drinking water has been reported to cause skin, bladder and lung cancers, with arsenic metabolites being implicated in the pathogenesis. In contrast, arsenic trioxide (As₂O₃) is an effective therapeutic agent for the treatment of acute promyelocytic leukemia, in which the binding of arsenite (iAs^III) to promyelocytic leukemia (PML) protein is the proposed initial step. These findings on the two-edged sword characteristics of arsenic suggest that after entry into cells, arsenic reaches the nucleus and triggers various nuclear events. Arsenic is reduced, conjugated with glutathione, and methylated in the cytosol. These biotransformations, including the production of reactive metabolic intermediates, appear to determine the intracellular dynamics, target organs, and biological functions of arsenic.

Arsenic and the gastrointestinal tract microbiome

Arsenic is a toxin, ranking first on the Agency for Toxic Substances and Disease Registry and the Environmental Protection Agency Priority List of Hazardous Substances. Chronic exposure increases the risk of a broad range of human illnesses, most notably cancer; however, there is significant variability in arsenic-induced disease among exposed individuals. Human genetics is a known component, but it alone cannot account for the large inter-individual variability in the presentation of arsenicosis symptoms. Each part of the gastrointestinal tract (GIT) may be considered as a unique environment with characteristic pH, oxygen concentration, and microbiome. Given the well-established arsenic redox transformation activities of microorganisms, it is reasonable to imagine how the GIT microbiome composition variability among individuals could play a significant role in determining the fate, mobility and toxicity of arsenic, whether inhaled or ingested. This is a relatively new field of research that would benefit from early dialogue aimed at summarizing what is known and identifying reasonable research targets and concepts. Herein, we strive to initiate this dialogue by reviewing known aspects of microbe-arsenic interactions and placing it in the context of potential for influencing host exposure and health risks. We finish by considering future experimental approaches that might be of value.

ArsM-mediated arsenite volatilization is limited by efflux catalyzed by As efflux transporters

Arsenic (As) methylation is regarded as an efficient strategy for As contamination remediation by As volatilization. However, most microorganisms display low As volatilization efficiency, which is possibly linked to As efflux transporters competing for cytoplasmic As(III) as a substrate. Here, we developed two types of As biosensors in Escherichia coli to compare the As efflux rate of three efflux transporters and to further investigate the correlation between As efflux rates and As volatilization. The engineered As-sensitive E. coli AW3110 expressing arsB_RP, acr3_RP or arsB_EC displayed a higher As resistance compared to the control. The fluorescence intensity was in a linear correlation in the range of 0-2.0 μmol/L of As(III). The intracellular As(III) concentration was negatively related to As efflux activity of As efflux transporter, which was consistent with the As resistance assays. Moreover, arsM derived from R. palustris CGA009 was subsequently introduced to construct an E. coli AW3110 co-expressing arsB/acr3 and arsM, which exhibited higher As(III) resistance, lower fluorescence intensity and intracellular As concentration compared to the engineered E. coli AW3110 expressing only arsB/acr3. The As volatilization efficiency was negatively related to As efflux activity of efflux transporters, the recombinants without arsB/acr3 displayed the highest rate of As volatilization. This study provided new insights into parameters affecting As volatilization with As efflux being the main limiting factor for As methylation and subsequent volatilization in many microorganisms.

Reduction of Organoarsenical Herbicides and Antimicrobial Growth Promoters by the Legume Symbiont Sinorhizobium meliloti

Massive amounts of methyl [e.g., methylarsenate, MAs(V)] and aromatic arsenicals [e.g., roxarsone (4-hydroxy-3-nitrophenylarsonate, Rox(V)] have been utilized as herbicides for weed control and growth promotors for poultry and swine, respectively. The majority of these organoarsenicals degrade into more toxic inorganic species. Here, we demonstrate that the legume symbiont Sinorhizobium meliloti both reduces MAs(V) to MAs(III) and catalyzes sequential two-step reduction of nitro and arsenate groups in Rox(V), producing the highly toxic trivalent amino aromatic derivative 4-hydroxy-3-aminophenylarsenite (HAPA(III)). The existence of this process suggests that S. meliloti possesses the ability to transform pentavalent methyl and aromatic arsenicals into antibiotics to provide a competitive advantage over other microbes, which would be a critical process for the synthetic aromatic arsenicals to function as antimicrobial growth promoters. The activated trivalent aromatic arsenicals are degraded into less-toxic inorganic species by an MAs(III)-demethylating aerobe, suggesting that environmental aromatic arsenicals also undergo a multiple-step degradation pathway, in analogy with the previously reported demethylation pathway of the methylarsenate herbicide. We further show that an FAD-NADPH-dependent nitroreductase encoded by mdaB gene catalyzes nitroreduction of roxarsone both in vivo and in vitro. Our results demonstrate that environmental organoarsenicals trigger competition between members of microbial communities, resulting in gradual degradation of organoarsenicals and contamination by inorganic arsenic.

The Human Gut Microbiome's Influence on Arsenic Toxicity

PURPOSE OF REVIEW

Arsenic exposure is a public health concern of global proportions with a high degree of interindividual variability in pathologic outcomes. Arsenic metabolism is a key factor underlying toxicity, and the primary purpose of this review is to summarize recent discoveries concerning the influence of the human gut microbiome on the metabolism, bioavailability, and toxicity of ingested arsenic. We review and discuss the current state of knowledge along with relevant methodologies for studying these phenomena.

RECENT FINDINGS

Bacteria in the human gut can biochemically transform arsenic-containing compounds (arsenicals). Recent publications utilizing culture-based approaches combined with analytical biochemistry and molecular genetics have helped identify several arsenical transformations by bacteria that are at least possible in the human gut and are likely to mediate arsenic toxicity to the host. Other studies that directly incubate stool samples in vitro also demonstrate the gut microbiome's potential to alter arsenic speciation and bioavailability. In vivo disruption or elimination of the microbiome has been shown to influence toxicity and body burden of arsenic through altered excretion and biotransformation of arsenicals. Currently, few clinical or epidemiological studies have investigated relationships between the gut microbiome and arsenic-related health outcomes in humans, although current evidence provides strong rationale for this research in the future.

SUMMARY

The human gut microbiome can metabolize arsenic and influence arsenical oxidation state, methylation status, thiolation status, bioavailability, and excretion. We discuss the strength of current evidence and propose that the microbiome be considered in future epidemiologic and toxicologic studies of human arsenic exposure.

Arsenic-resistance mechanisms in bacterium Leclercia adecarboxylata strain As3-1: Biochemical and genomic analyses

Microbial arsenic transformation is important in As biogeochemical cycles in the environment. In this study, a new As-resistant bacterial strain Leclercia adecarboxylata As3-1 was isolated and its associated mechanisms in As resistance and detoxification were evaluated based on genome sequencing and gene annotations. After subjecting strain As3-1 to medium containing arsenate (AsV), AsV reduction occurred and an AsV-enhanced bacterial growth was observed. Strain As3-1 lacked arsenite (AsIII) oxidation ability and displayed lower AsIII resistance than AsV, probably due to its higher AsIII accumulation. Polymerase chain reaction and phylogenetic analysis showed that strain As3-1 harbored a typical AsV reductase gene (arsC) on the plasmids. Genome sequencing and gene annotations identified four operons phoUpstBACS, arsHRBC, arsCRDABC and ttrRSBCA, with 8 additional genes outside the operons that might have involved in As resistance and detoxification in strain As3-1. These included 5 arsC genes explaining why strain As3-1 tolerated high AsV concentrations. Besides ArsC, TtrB, TtrC and TtrA proteins could also be involved in AsV reduction and consequent energy acquisition for bacterial growth. Our data provided a new example of diverse As-regulating systems and AsV-enhanced growth without ArrA in bacteria. The information helps to understand the role of As in selecting microbial systems that can transform and utilize As.

Efflux proteins MacAB confer resistance to arsenite and penicillin/macrolide-type antibiotics in Agrobacterium tumefaciens 5A

Antibiotic and arsenic (As) contaminations are worldwide public health problems. Previously, the bacterial ABC-type efflux protein MacAB reportedly conferred resistance to macrolide-type antibiotics but not to other metal(loid)s. In this study, the roles of MacAB for the co-resistance of different antibiotics and several metal(loid)s were analyzed in Agrobacterium tumefaciens 5A, a strain resistant to arsenite [As(III)] and several types of antibiotics. The macA and macB genes were cotranscribed, and macB was deleted in A. tumefaciens 5A and heterologously expressed in Escherichia coli AW3110 and E. coli S17-1. Compared to the wild-type strain 5A, the macB deletion strain reduced bacterial resistance levels to several macrolide-type and penicillin-type antibiotics but not to cephalosporin-type antibiotics. In addition, the macB deletion strain showed lower resistance to As(III) but not to arsenate [As(V)], antimonite [Sb(III)] and cadmium chloride [Cd(II)]. The mutant strain 5A-ΔmacB cells accumulated more As(III) than the cells of the wild-type. Furthermore, heterologous expression of MacAB in E. coli S17-1 showed that MacAB was essential for resistance to macrolide, several penicillin-type antibiotics and As(III) but not to As(V). Heterologous expression of MacAB in E. coli AW3110 reduced the cellular accumulation of As(III) but not of As(V), indicating that MacAB is responsible for the efflux of As(III). These results demonstrated that, in addition to macrolide-type antibiotics, MacAB also conferred resistance to penicillin-type antibiotics and As(III) by extruding them out of cells. This finding contributes to a better understanding of the bacterial resistance mechanisms of antibiotics and metal(loid)s.

Role of ArsEFG in Roxarsone and Nitarsone Detoxification and Resistance

Organoarsenical biotransformations are important components of the global cycling of arsenic. Roxarsone (3-nitro-4-hydroxybenzenearsenate or Rox(V)) and nitarsone (4-nitrobenzene arsenate or Nit(V)) are synthetic aromatic organoarsenicals used in the poultry industry as additives to prevent coccidiosis and improve feed efficiency. Here, we describe a novel pathway of resistance to roxarsone and nitarsone involving biotransformation of their trivalent forms (Rox(III)) and (Nit(III)) to the trivalent organoarsenicals HAPA(III) and pAsA(III), coupled to active extrusion of the aromatic aminobenezylarsenicals from the cells. The arsE, arsF, and arsG were cloned from the arsenic island in the chromosome of Shewanella putrefaciens 200. When expressed in Escherichia coli together, but not alone, arsEFG conferred resistance to Rox(III) and Nit(III) and decreased the accumulation of both. The cells transformed Rox(III) or Nit(III) to HAPA(III) or pAsA(III) by reducing the nitro group to an amine. Everted membrane vesicles from cells expressing arsG accumulated HAPA(III) or pAsA(III). Our data indicate that ArsE and ArsF together reduce Rox(III) or Nit(III) to HAPA(III) or pAsA(III), which are extruded from the cells by the efflux permease ArsG. Identification of the coupled pathway of ArsE, ArsF, and ArsG catalysis is a molecular description of a novel pathway for resistance to roxarsone and nitarsone.

Pathways of arsenic uptake and efflux

Arsenic is a non-essential, environmentally ubiquitous toxic metalloid. In response to this pervasive environmental challenge, organisms evolved mechanisms to confer resistance to arsenicals. Inorganic pentavalent arsenate is taken into most cells adventitiously by phosphate uptake systems. Similarly, inorganic trivalent arsenite is taken into most cells adventitiously, primarily via aquaglyceroporins or sugar permeases. The most common strategy for tolerance to both inorganic and organic arsenicals is by efflux that extrude them from the cytosol. These efflux transporters span across kingdoms and belong to various families such as aquaglyceroporins, major facilitator superfamily (MFS) transporters, ATP-binding cassette (ABC) transporters and potentially novel, yet to be discovered families. This review will outline the properties and substrates of known arsenic transport systems, the current knowledge gaps in the field, and aims to provide insight into the importance of arsenic transport in the context of the global arsenic biogeocycle and human health.

Efficient arsenate reduction by As-resistant bacterium Bacillus sp. strain PVR-YHB1-1: Characterization and genome analysis

Arsenate (AsV) reduction in bacteria is essential to alleviate their arsenic (As) toxicity. We isolated a Bacillus strain PVR-YHB1-1 from the roots of As-hyperaccumulator Pteris vittata. The strain was efficient in reducing AsV to arsenite (AsIII), but the associated mechanisms were unclear. Here, we investigated its As resistance and reduction behaviors and associated genes at genome level. Results showed that the strain tolerated up to 20 mM AsV. When grown in 1 mM AsV, 96% AsV was reduced to AsIII in 48 h, with its AsV reduction ability being positively correlated to bacterial biomass. Two ars operons arsRacr3arsCDA and arsRKacr3arsC for As metabolisms were identified based on draft genome sequencing and gene annotations. Our data suggested that both operons might have attributed to efficient As resistance and AsV reduction in PVR-YHB1-1, providing clues to better understand As transformation in bacteria and their roles in As transformation in the environment.

The antibiotic action of methylarsenite is an emergent property of microbial communities

Arsenic is the most ubiquitous environmental toxin. Here, we demonstrate that bacteria have evolved the ability to use arsenic to gain a competitive advantage over other bacteria at least twice. Microbes generate toxic methylarsenite (MAs(III)) by methylation of arsenite (As(III)) or reduction of methylarsenate (MAs(V)). MAs(III) is oxidized aerobically to MAs(V), making methylation a detoxification process. MAs(V) is continually re-reduced to MAs(III) by other community members, giving them a competitive advantage over sensitive bacteria. Because generation of a sustained pool of MAs(III) requires microbial communities, these complex interactions are an emergent property. We show that reduction of MAs(V) by Burkholderia sp. MR1 produces toxic MAs(III) that inhibits growth of Escherichia coli in mixed culture. There are three microbial mechanisms for resistance to MAs(III). ArsH oxidizes MAs(III) to MAs(V). ArsI degrades MAs(III) to As(III). ArsP confers resistance by efflux. Cells of E. coli expressing arsI, arsH or arsP grow in mixed culture with Burkholderia sp. MR1 in the presence of MAs(V). Thus MAs(III) has antibiotic properties: a toxic organic compound produced by one microbe to kill off competitors. Our results demonstrate that life has adapted to use environmental arsenic as a weapon in the continuing battle for dominance.