Arsinothricin, an arsenic-containing non-proteinogenic amino acid analog of glutamate, is a broad-spectrum antibiotic

Microbial arsenic methylation in soil-water systems and its environmental significance

In this review, we have summarized the current knowledge about the environmental importance, relevance, and consequences of microbial arsenic (As) methylation in various ecosystems. In this regard, we have presented As biomethylation in terrestrial and aquatic ecosystems particularly in rice paddy soils and wetlands. The functions of As biomethylation by microbial consortia in anaerobic and aerobic conditions are extensively discussed. In addition, we have tried to explain the interconnections between As transformation and carbon (C), such as microbial degradation of organic compounds and methane (CH4) emission. These processes can cause As release because of the reduction of arsenate (As(V)) to the more mobile arsenite (As(III)) as well as As methylation and the formation of toxic trivalent methylated As species in anaerobic conditions. Furthermore, the sulfur (S) transformation can form highly toxic thiolated As species owing to its interference with As biomethylation. Besides, we have focused on many other mutual interlinks that remain elusive between As and C, including As biomethylation, thiolation, and CH4 emission, in the soil-water systems. Recent developments have clarified the significant and complex interactions between the coupled microbial process in anoxic and submerged soils. These processes, performed by little-known/unknown microbial taxa or well-known members of microbial communities with unrecognized metabolic pathways, conducted several concurrent reactions that contributed to global warming on our planet and have unfavorable impacts on water quality and human food resources. Finally, some environmental implications in rice production and arsenic removal from soil-water systems are discussed. Generally, our understanding of the ecological and metabolic evidence for the coupling and synchronous processes of As, C, and S are involved in environmental contamination-caused toxicity in human food, including high As content in rice grain, water resources, and global warming through methanogenesis elucidate combating global rice safety, drinking water, and climate changes.

A Sb(III)-specific efflux transporter from Ensifer adhaerens E-60

Antimony is pervasive environmental toxic substance, and numerous genes encoding mechanisms to resist, transform and extrude the toxic metalloid antimony have been discovered in various microorganisms. Here we identified a major facilitator superfamily (MFS) transporter, AntB, on the chromosome of the arsenite-oxidizing bacterium Ensifer adhaerens E-60 that confers resistance to Sb(III) and Sb(V). The antB gene is adjacent to gene encoding a LysR family transcriptional regulator termed LysRars, which is an As(III)/Sb(III)-responsive transcriptional repressor that is predicted to control expression of antB. Similar antB and lysRars genes are found in related arsenic-resistant bacteria, especially strains of Ensifer adhaerens, and the lysRars gene adjacent to antB encodes a member of a divergent subgroup of putative LysR-type regulators. Closely related AntB and LysRars orthologs contain three conserved cysteine residues, which are Cys17, Cys99, and Cys350 in AntB and Cys81, Cys289 and Cys294 in LysRars, respectively. Expression of antB is induced by As(III), Sb(III), Sb(V) and Rox(III) (4-hydroxy-3-nitrophenyl arsenite). Heterologous expression of antB in E. coli AW3110 (Δars) conferred resistance to Sb(III) and Sb(V) and reduced the intracellular concentration of Sb(III). The discovery of the Sb(III) efflux transporter AntB enriches our knowledge of the role of the efflux transporter in the antimony biogeochemical cycle.

Arsinothricin Biosynthesis Involving a Radical SAM Enzyme for Noncanonical SAM Cleavage and C-As Bond Formation

Arsinothricin is a potent antibiotic secreted by soil bacteria. The biosynthesis of arsinothricin was proposed to involve a C-As bond formation between trivalent As and the 3-amino-3-carboxypropyl (ACP) group of S-adenosyl-l-methionine (SAM), which is catalyzed by the protein ArsL. However, ArsL has not been characterized in detail. Interestingly, ArsL contains a CxxxCxxC motif and thus belongs to the radical SAM enzyme superfamily, the members of which cleave SAM and generate a 5'-deoxyadenosyl radical. Here, we found that ArsL cleaves the Cγ,Met-S bond of SAM and generates an ACP radical that resembles Dph2, a noncanonical radical SAM enzyme involved in diphthamid biosynthesis. As Dph2 does not contain the CxxxCxxC motif, ArsL is a unique radical SAM enzyme that contains this motif but generates a noncanonical ACP radical. Together with the methyltransferase ArsM, we successfully reconstituted arsinothricin biosynthesis in vitro. ArsL has a conserved RCCLKC motif in the C-terminal sequence and belongs to the RCCLKC-tail radical SAM protein subfamily. By truncation and mutagenesis, we showed that this motif plays an important role in binding to the substrate arsenite and is highly important for its activity. Our results suggested that ArsL has a canonical radical SAM enzyme motif but catalyzes a noncanonical radical SAM reaction, implying that more noncanonical radical SAM chemistry may exist within the radical SAM enzyme superfamily.

Anticancer Effects of the Trivalent Organoarsenical 2-Amino-4-(dihydroxyarsinoyl) Butanoate

According to the National Cancer Institute, breast cancer is a leading cause of death in women. The lack of progesterone and estrogen receptors in triple-negative breast cancer (TNBC) cells results in a lack of response to hormonal, monoclonal, or tyrosine kinase inhibitor therapies. Despite intensive drug discovery, there is still no approved targeted treatment for TNBC. The metalloid arsenic has been used in herbal medicines, antibiotics, and chemotherapeutic drugs for centuries. This paper demonstrates that a trivalent arsenic-containing, nonproteogenic amino acid, R-AST-OH (2-amino-4-(dihydroxyarsinoyl) butanoate), inhibits kidney-type glutaminase (KGA), the enzyme that controls glutamine metabolism and is correlated with tumor malignancy. Cell-based assays using the TNBC MDA-MB-231 and HCC1569 cell lines showed that R-AST-OH kills TNBC cells and is not cytotoxic to a control cell line. The results of in silico molecular docking predictions indicate that R-AST-OH binds to the glutamine binding site and forms a covalent bond with an active site cysteine residue. We hypothesize that R-AST-OH is a single warhead for KGA that irreversibly binds to KGA through the formation of an As-S bond. We propose that R-AST-OH is a promising lead compound for the design of new drugs for the treatment of TNBC.

4-[(E)-2-(1-Pyrenyl)Vinyl]Pyridine Complexes: How to Modulate the Toxicity of Heavy Metal Ions to Target Microbial Infections

Pyrene derivatives are regularly proposed for use in biochemistry as dyes due to their photochemical characteristics. Their antibacterial properties are, however, much less well understood. New complexes based on 4-[(E)-2-(1-pyrenyl)vinyl]pyridine (PyPe) have been synthesized with metal ions that are known to possess antimicrobial properties, such as zinc(II), cadmium(II), and mercury(II). The metal ion salts, free ligand, combinations thereof, and the coordination compounds themselves were tested for their antibacterial properties through microdilution assays. We found that the ligand is able to modulate the antibacterial properties of transition metal ions, depending on the complex stability, the distance between the ligand and the metal ions, and the metal ions themselves. The coordination by the ligand weakened the antibacterial properties of heavy metal ions (Cd(II), Hg(II), Bi(III)), allowing the bacteria to survive higher concentrations thereof. Mixing the ligand and the metal ion salts without forming the complex beforehand enhanced the antibacterial properties of the cations. Being non-cytotoxic itself, the ligand therefore balances the biological consequences of heavy metal ions between toxicity and therapeutic weapons, depending on its use as a coordinating ligand or simple adjuvant.

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.

Nanofibrous Dressing with Nanocomposite Monoporous Microspheres for Chemodynamic Antibacterial Therapy and Wound Healing

The excessive use of antibiotics and consequent bacterial resistance have emerged as crucial public safety challenges for humanity. As a promising antibacterial treatment, using reactive oxygen species (ROS) can effectively address this problem and has the advantages of being highly efficient and having low toxicity. Herein, electrospinning and electrospraying were employed to fabricate magnesium oxide (MgO)-based nanoparticle composited polycaprolactone (PCL) nanofibrous dressings for the chemodynamic treatment of bacteria-infected wounds. By utilizing electrospraying, erythrocyte-like monoporous PCL microspheres incorporating silver (Ag)- and copper (Cu)-doped MgO nanoparticles were generated, and the unique microsphere-filament structure enabled efficient anchoring on nanofibers. The composite dressings produced high levels of ROS, as confirmed by the 2,7-dichloriflurescin fluorescent probe. The sustained generation of ROS resulted in efficient glutathione oxidation and a remarkable bacterial killing rate of approximately 99% against Staphylococcus aureus (S. aureus). These dressings were found to be effective at treating externally infected wounds. The unique properties of these composite nanofibrous dressings suggest great potential for their use in the medical treatment of bacteria-infected injuries.

Insights into Arsenic Secondary Metabolism in Actinomycetes from the Structure and Biosynthesis of Bisenarsan

The unique bioactivities of arsenic-containing secondary metabolites have been revealed recently, but studies on arsenic secondary metabolism in microorganisms have been extremely limited. Here, we focused on the organoarsenic metabolite with an unknown chemical structure, named bisenarsan, produced by well-studied model actinomycetes and elucidated its structure by combining feeding of the putative biosynthetic precursor (2-hydroxyethyl)arsonic acid to Streptomyces lividans 1326 and detailed NMR analyses. Bisenarsan is the first characterized actinomycete-derived arsenic secondary metabolite and may function as a prototoxin form of an antibacterial agent or be a detoxification product of inorganic arsenic species. We also verified the previously proposed genes responsible for bisenarsan biosynthesis, especially the (2-hydroxyethyl)arsonic acid moiety. Notably, we suggest that a C-As bond in bisenarsan is formed by the intramolecular rearrangement of a pentavalent arsenic species (arsenoenolpyruvate) by the cofactor-independent phosphoglycerate mutase homologue BsnN, that is entirely distinct from the conventional biological C-As bond formation through As-alkylation of trivalent arsenic species by S-adenosylmethionine-dependent enzymes. Our findings will speed up the development of arsenic natural product biosynthesis.

Identification and characterization of a phosphinothricin N-acetyltransferase from Enterobacter LSJC7

Phosphinothricin (PPT) is a widely used and non-selective herbicide. PPT-resistance genes, especially PPT N-acetyltransferase genes, have been used in the development of transgenic PPT-resistant crops. However, there are only a limited number of available PPT-resistance genes for use in plant biotechnology. In this study, we found that Enterobacter LSJC7 is highly resistant to PPT and can acetylate PPT to N-acetyl phosphinothricin (Ac-PPT). Furthermore, a novel PPT N-acetyltransferase gene, named LsarsN, was identified from LSJC7. When LsarsN was expressed in E. coli AW3110, it confered resistance to PPT. Ac-PPT was detected in both the culture medium and cells of AW3110 expressing the LsarsN-pET22b plasmid. The purified LsArsN protein also showed strong N-acetylation ability in vitro, and its enzymatic kinetic curve was fitted with the Michaelis-Mentan equation. Compared with wild-type LsArsN, both R72A and R74A mutants showed significantly lower PPT N-acetylation ability. In summary, our results systematically characterized LsArsN with strong ability for PPT N-acetylation, which lays the groundwork for future research into the use of this novel gene, LsarsN, to create PPT-resistant crops.

Arsinothricin Inhibits Plasmodium falciparum Proliferation in Blood and Blocks Parasite Transmission to Mosquitoes

Malaria, caused by Plasmodium protozoal parasites, remains a leading cause of morbidity and mortality. The Plasmodium parasite has a complex life cycle, with asexual and sexual forms in humans and Anopheles mosquitoes. Most antimalarials target only the symptomatic asexual blood stage. However, to ensure malaria eradication, new drugs with efficacy at multiple stages of the life cycle are necessary. We previously demonstrated that arsinothricin (AST), a newly discovered organoarsenical natural product, is a potent broad-spectrum antibiotic that inhibits the growth of various prokaryotic pathogens. Here, we report that AST is an effective multi-stage antimalarial. AST is a nonproteinogenic amino acid analog of glutamate that inhibits prokaryotic glutamine synthetase (GS). Phylogenetic analysis shows that Plasmodium GS, which is expressed throughout all stages of the parasite life cycle, is more closely related to prokaryotic GS than eukaryotic GS. AST potently inhibits Plasmodium GS, while it is less effective on human GS. Notably, AST effectively inhibits both Plasmodium erythrocytic proliferation and parasite transmission to mosquitoes. In contrast, AST is relatively nontoxic to a number of human cell lines, suggesting that AST is selective against malaria pathogens, with little negative effect on the human host. We propose that AST is a promising lead compound for developing a new class of multi-stage antimalarials.

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.

Challenges and opportunities in bringing nonbiological atoms to life with synthetic metabolism

The relatively narrow spectrum of chemical elements within the microbial 'biochemical palate' limits the reach of biotechnology, because several added-value compounds can only be produced with traditional organic chemistry. Synthetic biology offers enabling tools to tackle this issue by facilitating 'biologization' of non-canonical chemical atoms. The interplay between xenobiology and synthetic metabolism multiplies routes for incorporating nonbiological atoms into engineered microbes. In this review, we survey natural assimilation routes for elements beyond the essential biology atoms [i.e., carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S)], discussing how these mechanisms could be repurposed for biotechnology. Furthermore, we propose a computational framework to identify chemical elements amenable to biologization, ranking reactions suitable to build synthetic metabolism. When combined and deployed in robust microbial hosts, these approaches will offer sustainable alternatives for smart chemical production.

Selective Methylation by an ArsM S-Adenosylmethionine Methyltransferase from Burkholderia gladioli GSRB05 Enhances Antibiotic Production

Arsenic methylation contributes to the formation and diversity of environmental organoarsenicals, an important process in the arsenic biogeochemical cycle. The arsM gene encoding an arsenite (As(III)) S-adenosylmethionine (SAM) methyltransferase is widely distributed in members of every kingdom. A number of ArsM enzymes have been shown to have different patterns of methylation. When incubated with inorganic As(III), Burkholderia gladioli GSRB05 has been shown to synthesize the organoarsenical antibiotic arsinothricin (AST) but does not produce either methylarsenate (MAs(V)) or dimethylarsenate (DMAs(V)). Here, we show that cells of B. gladioli GSRB05 synthesize DMAs(V) when cultured with either MAs(III) or MAs(V). Heterologous expression of the BgarsM gene in Escherichia coli conferred resistance to MAs(III) but not As(III). The cells methylate MAs(III) and the AST precursor, reduced trivalent hydroxyarsinothricin (R-AST-OH) but do not methylate inorganic As(III). Similar results were obtained with purified BgArsM. Compared with ArsM orthologs, BgArsM has an additional 37 amino acid residues in a linker region between domains. Deletion of the additional 37 residues restored As(III) methylation activity. Cells of E. coli co-expressing the BgarsL gene encoding the noncanonical radical SAM enzyme that catalyzes the synthesis of R-AST-OH together with the BgarsM gene produce much more of the antibiotic AST compared with E. coli cells co-expressing BgarsL together with the CrarsM gene from Chlamydomonas reinhardtii, which lacks the sequence for additional 37 residues. We propose that the presence of the insertion reduces the fitness of B. gladioli because it cannot detoxify inorganic arsenic but concomitantly confers an evolutionary advantage by increasing the ability to produce AST.

Isolation of a Novel Endophytic Bacillus Strain Capable of Transforming Pentachlorophenol and Structure Determination of Pentachlorophenol Phosphate Using Single-Crystal X-ray Diffraction

A novel approach for the remediation of upland soils contaminated with pentachlorophenol (C₆HCl₅O; PCP) (1), a fungicide, wood perservative, and herbicide, through the exploitation of plant-endophytic bacteria may overcome the existing issues in bioaugmentaion and phytoremidiation. In this study, we isolated the endophytic Bacillus sp. strain PCP15 and determined its metabolite of PCP (1). This strain degraded 8.03 μmol L^-1 PCP (1) within 24 h and generated the novel metabolite PCP phosphate (3). The PCP15 strain showed nearly complete growth inhibition of 20 μmol L^-1 PCP (1). In contrast, PCP15 showed resistance to PCP phosphate (3), indicating that the phosphorylation of PCP, which has never previously been reported in organisms, contributed to the detoxification of PCP (1) in bacterial cells. Our results show the potential for practical application of this strain in hybrid remediation of PCP (1)-contaminated soils and reveal a novel PCP (1) detoxification mechanism in organisms.

Nanotechnology as a Novel Approach in Combating Microbes Providing an Alternative to Antibiotics

The emergence of infectious diseases promises to be one of the leading mortality factors in the healthcare sector. Although several drugs are available on the market, newly found microorganisms carrying multidrug resistance (MDR) against which existing drugs cannot function effectively, giving rise to escalated antibiotic dosage therapies and the need to develop novel drugs, which require time, money, and manpower. Thus, the exploitation of antimicrobials has led to the production of MDR bacteria, and their prevalence and growth are a major concern. Novel approaches to prevent antimicrobial drug resistance are in practice. Nanotechnology-based innovation provides physicians and patients the opportunity to overcome the crisis of drug resistance. Nanoparticles have promising potential in the healthcare sector. Recently, nanoparticles have been designed to address pathogenic microorganisms. A multitude of processes that can vary with various traits, including size, morphology, electrical charge, and surface coatings, allow researchers to develop novel composite antimicrobial substances for use in different applications performing antimicrobial activities. The antimicrobial activity of inorganic and carbon-based nanoparticles can be applied to various research, medical, and industrial uses in the future and offer a solution to the crisis of antimicrobial resistance to traditional approaches. Metal-based nanoparticles have also been extensively studied for many biomedical applications. In addition to reduced size and selectivity for bacteria, metal-based nanoparticles have proven effective against pathogens listed as a priority, according to the World Health Organization (WHO). Moreover, antimicrobial studies of nanoparticles were carried out not only in vitro but in vivo as well in order to investigate their efficacy. In addition, nanomaterials provide numerous opportunities for infection prevention, diagnosis, treatment, and biofilm control. This study emphasizes the antimicrobial effects of nanoparticles and contrasts nanoparticles' with antibiotics' role in the fight against pathogenic microorganisms. Future prospects revolve around developing new strategies and products to prevent, control, and treat microbial infections in humans and other animals, including viral infections seen in the current pandemic scenarios.

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.

Interactions with Arsenic: Mechanisms of Toxicity and Cellular Resistance in Eukaryotic Microorganisms

Arsenic (As) is quite an abundant metalloid, with ancient origin and ubiquitous distribution, which represents a severe environmental risk and a global problem for public health. Microbial exposure to As compounds in the environment has happened since the beginning of time. Selective pressure has induced the evolution of various genetic systems conferring useful capacities in many microorganisms to detoxify and even use arsenic, as an energy source. This review summarizes the microbial impact of the As biogeochemical cycle. Moreover, the poorly known adverse effects of this element on eukaryotic microbes, as well as the As uptake and detoxification mechanisms developed by yeast and protists, are discussed. Finally, an outlook of As microbial remediation makes evident the knowledge gaps and the necessity of new approaches to mitigate this environmental challenge.

Identification of the Biosynthetic Gene Cluster for the Organoarsenical Antibiotic Arsinothricin

The soil bacterium Burkholderia gladioli GSRB05 produces the natural compound arsinothricin [2-amino-4-(hydroxymethylarsinoyl) butanoate] (AST), which has been demonstrated to be a broad-spectrum antibiotic. To identify the genes responsible for AST biosynthesis, a draft genome sequence of B. gladioli GSRB05 was constructed. Three genes, arsQML, in an arsenic resistance operon were found to be a biosynthetic gene cluster responsible for synthesis of AST and its precursor, hydroxyarsinothricin [2-amino-4-(dihydroxyarsinoyl) butanoate] (AST-OH). The arsL gene product is a noncanonical radical S-adenosylmethionine (SAM) enzyme that is predicted to transfer the 3-amino-3-carboxypropyl (ACP) group from SAM to the arsenic atom in inorganic arsenite, forming AST-OH, which is methylated by the arsM gene product, a SAM methyltransferase, to produce AST. Finally, the arsQ gene product is an efflux permease that extrudes AST from the cells, a common final step in antibiotic-producing bacteria. Elucidation of the biosynthetic gene cluster for this novel arsenic-containing antibiotic adds an important new tool for continuation of the antibiotic era. IMPORTANCE Antimicrobial resistance is an emerging global public health crisis, calling for urgent development of novel potent antibiotics. We propose that arsinothricin and related arsenic-containing compounds may be the progenitors of a new class of antibiotics to extend our antibiotic era. Here, we report identification of the biosynthetic gene cluster for arsinothricin and demonstrate that only three genes, two of which are novel, are required for the biosynthesis and transport of arsinothricin, in contrast to the phosphonate counterpart, phosphinothricin, which requires over 20 genes. Our discoveries will provide insight for the development of more effective organoarsenical antibiotics and illustrate the previously unknown complexity of the arsenic biogeochemical cycle, as well as bring new perspective to environmental arsenic biochemistry.

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.

Genetic Determinants of Antibiotic Resistance in Francisella

Tularemia, caused by Francisella tularensis, is endemic to the northern hemisphere. This zoonotic organism has historically been developed into a biological weapon. For this Tier 1, Category A select agent, it is important to expand our understanding of its mechanisms of antibiotic resistance (AMR). Francisella is unlike many Gram-negative organisms in that it does not have significant plasmid mobility, and does not express AMR mechanisms on plasmids; thus plasmid-mediated resistance does not occur naturally. It is possible to artificially introduce plasmids with AMR markers for cloning and gene expression purposes. In this review, we survey both the experimental research on AMR in Francisella and bioinformatic databases which contain genomic and proteomic data. We explore both the genetic determinants of intrinsic AMR and naturally acquired or engineered antimicrobial resistance as well as phenotypic resistance in Francisella. Herein we survey resistance to beta-lactams, monobactams, carbapenems, aminoglycosides, tetracycline, polymyxins, macrolides, rifampin, fosmidomycin, and fluoroquinolones. We also highlight research about the phenotypic AMR difference between planktonic and biofilm Francisella. We discuss newly developed methods of testing antibiotics against Francisella which involve the intracellular nature of Francisella infection and may better reflect the eventual clinical outcomes for new antibiotic compounds. Understanding the genetically encoded determinants of AMR in Francisella is key to optimizing the treatment of patients and potentially developing new antimicrobials for this dangerous intracellular pathogen.

Microbial Metabolic Genes Crucial for S. aureus Biofilms: An Insight From Re-analysis of Publicly Available Microarray Datasets

Bacterial biofilms are microbial lifestyles found in all environments. Up to 80% of human infections and 60–70% of hospital-acquired infections have a biofilm origin, with Staphylococcus aureus one of the leading causes of these infections. Microorganisms in biofilms exhibit significant antimicrobial resistance which poses important treatment challenges, hence the urgent need to identify novel antibiofilm strategies. Microbes form biofilms in response to various factors, and once these 3-dimentional structures form they are highly recalcitrant to removal. The switch from planktonic lifestyle to the biofilm protected mode of growth results in a phenotypic shift in the behavior of the microorganisms in terms of growth rate and gene expression. Given these changes, investigation of microbial gene expression and their modulation at different stages of biofilm maturation is needed to provide vital insight into the behavior of biofilm cells. In this study, we analyzed publicly available transcriptomic dataset of S. aureus biofilms at different stages of maturation to identify consistently upregulated genes irrespective of the biofilm maturation stage. Our reanalysis identified a total of 6 differentially expressed genes upregulated in both 48 and 144-h old S. aureus biofilms. Functional analysis revealed that these genes encode for proteins which play a role in key microbial metabolic pathways. However, these genes, as yet, are unrelated or fully studied in the context of biofilm. Moreover, the findings of this in silico work, suggest that these genes may represent potential novel targets for the development of more effective antibiofilm strategies against S. aureus biofilm-associated infections.

Chemical synthesis of the organoarsenical antibiotic arsinothricin

We report two routes of chemical synthesis of arsinothricin (AST), the novel organoarsenical antibiotic. One is by condensation of the 2-chloroethyl(methyl)arsinic acid with acetamidomalonate, and the second involves reduction of the N-acetyl protected derivative of hydroxyarsinothricin (AST-OH) and subsequent methylation of a trivalent arsenic intermediate with methyl iodide. The enzyme AST N-acetyltransferase (ArsN1) was utilized to purify l-AST from racemic AST. This chemical synthesis provides a source of this novel antibiotic for future drug development.

Arsinothricin is prepared from 2-chloroethyl(methyl)arsinic acid or by reduction of N-acetyl protected derivative of hydroxyarsinothricin and methylation with methyl iodide.

Semisynthesis of the Organoarsenical Antibiotic Arsinothricin

Arsinothricin [AST (1)], a new broad-spectrum organoarsenical antibiotic, is a nonproteinogenic analogue of glutamate that effectively inhibits glutamine synthetase. We report the chemical synthesis of an intermediate in the pathway to 1, hydroxyarsinothricin [AST-OH (2)], which can be converted to 1 by enzymatic methylation catalyzed by the ArsM As(III) S-adenosylmethionine methyltransferase. This is the first report of semisynthesis of 1, providing a source of this novel antibiotic that will be required for future clinical trials.

Organoarsenicals inhibit bacterial peptidoglycan biosynthesis by targeting the essential enzyme MurA

Trivalent organoarsenicals such as methylarsenite (MAs(III)) are considerably more toxic than inorganic arsenate (As(V)) or arsenite (As(III)). In microbial communities MAs(III) exhibits significant antimicrobial activity. Although MAs(III) and other organoarsenicals contribute to the global arsenic biogeocycle, how they exert antibiotic-like properties is largely unknown. To identify possible targets of MAs(III), a genomic library of the gram-negative bacterium, Shewanella putrefaciens 200, was expressed in Escherichia coli with selection for MAs(III) resistance. One clone contained the S. putrefaciens murA gene (SpmurA), which catalyzes the first committed step in peptidoglycan biosynthesis. Overexpression of SpmurA conferred MAs(III) resistance to E. coli. Purified SpMurA was inhibited by MAs(III), phenylarsenite (PhAs(III)) or the phosphonate antibiotic fosfomycin but not by inorganic As(III). Fosfomycin inhibits MurA by binding to a conserved residue that corresponds to Cys117 in SpMurA. A C117D mutant was resistant to fosfomycin but remained sensitive to MAs(III), indicating that the two compounds have different mechanisms of action. New inhibitors of peptidoglycan biosynthesis are highly sought after as antimicrobial drugs, and organoarsenicals represent a new area for the development of novel compounds for combating the threat of antibiotic resistance.

Nutrient-toxic element mixtures and the early postnatal gut microbiome in a United States longitudinal birth cohort

BACKGROUND

The infant microbiome contributes to health status across the lifespan, but environmental factors affecting microbial communities are poorly understood, particularly when toxic and essential elements interact.

OBJECTIVE

We aimed to identify the associations between a spectrum of other early-postnatal nutrient or toxic elemental exposures measured and the infant gut microbiome.

METHODS

Our analysis included 179 six-week-old infants from the New Hampshire Birth Cohort Study. Eleven elements were measured in infant toenail clippings. The gut microbiome was assessed using 16S rRNA V4-V5 hypervariable region targeted sequencing. Multivariable zero-inflated logistic normal regression (MZILN) was used to model the association between element concentrations and taxon relative abundance. To explore interactive and nonlinear associations between the exposures and specific taxa we employed Bayesian Kernel Machine Regression (BKMR). Effect modification by delivery mode, feeding mode, peripartum antibiotic exposure, and infant sex was assessed with stratified models.

RESULTS

We found a negative association between arsenic and microbial diversity in the full population that was accentuated among infants exposed to peripartum antibiotics. Arsenic, cadmium, copper, iron, lead, manganese, nickel, selenium, tin, and zinc were each associated with differences in at least one taxon in the full study population, with most of the related taxa belonging to the Bacteroides and Lactobacillales. In stratified analyses, mercury, in addition to the other elements, was associated with specific taxa. Bifidobacterium, which associated negatively with zinc in MZILN and BKMR models, had a quadratic association with arsenic concentrations. These associations varied with the concentration of the other element.

CONCLUSIONS

Early postnatal toxic and nutrient elemental exposures are associated with differences in the infant microbiome. Further research is needed to clarify the whether these alterations are a biomarker of exposure or if they have implications for child and lifelong health.

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.

Biosynthetic Pathways to Nonproteinogenic α-Amino Acids

Natural nonproteinogenic amino acids vastly outnumber the well-known 22 proteinogenic amino acids. Such amino acids are generated in specialized metabolic pathways. In these pathways, diverse biosynthetic transformations, ranging from isomerizations to the stereospecific functionalization of C-H bonds, are employed to generate structural diversity. The resulting nonproteinogenic amino acids can be integrated into more complex natural products. Here we review recently discovered biosynthetic routes to freestanding nonproteinogenic α-amino acids, with an emphasis on work reported between 2013 and mid-2019.