Resistance mechanisms to arsenicals and antimonials

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).

Transcriptome analysis of an arsenite-/antimonite-oxidizer, Bosea sp. AS-1 reveals the importance of the type 4 secretion system in antimony resistance

Bosea sp. AS-1 is an arsenite [As(III)] and antimonite [Sb(III)] oxidizer previously isolated by our group from the Xikuangshan Antimony (Sb) Mine area. Our previous study showed that Bosea sp. AS-1 had a preference for oxidizing As(III) or Sb(III) with different carbon sources, which suggested that different metabolic mechanisms may be utilized by the bacteria to survive in As(III)- or Sb(III)-contaminated environments. Here, we conducted whole-genome and transcriptome sequencing to reveal the molecular mechanisms utilized by Bosea sp. AS-1 to resist As(III) or Sb(III). We discovered that AS-1 acquired various As- and Sb-resistant genes in its genome and might resist As(III) or Sb(III) through the regulation of multiple pathways, such as As and Sb metabolism, the bacterial secretion system, oxidative phosphorylation, the TCA cycle and bacterial flagellar motility. Interestingly, we discovered that genes of the type IV secretion system (T4SS) were activated in response to Sb(III), and inhibiting T4SS activity in AS-1 dramatically reduced its oxidation efficiency and tolerance to Sb(III). To our knowledge, this is the first study showing the activation of T4SS genes by Sb and a direct involvement of T4SS in bacterial Sb resistance. Our findings establish the T4SS as an important Sb resistance factor in bacteria and may help us understand the spread of Sb resistance genes in the environment.

A Bifan Motif Shaped by ArsR1, ArsR2, and Their Cognate Promoters Frames Arsenic Tolerance of Pseudomonas putida

Prokaryotic tolerance to inorganic arsenic is a widespread trait habitually determined by operons encoding an As (III)-responsive repressor (ArsR), an As (V)-reductase (ArsC), and an As (III)-export pump (ArsB), often accompanied by other complementary genes. Enigmatically, the genomes of many environmental bacteria typically contain two or more copies of this basic genetic device arsRBC. To shed some light on the logic of such apparently unnecessary duplication(s) we have inspected the regulation—together and by separate—of the two ars clusters borne by the soil bacterium Pseudomonas putida strain KT2440, in particular the cross talk between the two repressors ArsR1/ArsR2 and the respective promoters. DNase I footprinting and gel retardation analyses of Pars1 and Pars2 with their matching regulators revealed non-identical binding sequences and interaction patterns for each of the systems. However, in vitro transcription experiments exposed that the repressors could downregulate each other’s promoters, albeit within a different set of parameters. The regulatory frame that emerges from these data corresponds to a particular type of bifan motif where all key interactions have a negative sign. The distinct regulatory architecture that stems from coexistence of various ArsR variants in the same cells could enter an adaptive advantage that favors the maintenance of the two proteins as separate repressors.

Introducing the ArsR-Regulated Arsenic Stimulon

The microbial ars operon encodes the primary bacterial defense response to the environmental toxicant, arsenic. An important component of this operon is the arsR gene, which encodes ArsR, a member of the family of proteins categorized as DNA-binding transcriptional repressors. As currently documented, ArsR regulates its own expression as well as other genes in the same ars operon. This study examined the roles of four ArsR proteins in the well-developed model Gram-negative bacterium Agrobacterium tumefaciens 5A. RNASeq was used to compare and characterize gene expression profiles in ± arsenite-treated cells of the wild-type strain and in four different arsR mutants. We report that ArsR-controlled transcription regulation is truly global, extending well beyond the current ars operon model, and includes both repression as well as apparent activation effects. Many cellular functions are significantly influenced, including arsenic resistance, phosphate acquisition/metabolism, sugar transport, chemotaxis, copper tolerance, iron homeostasis, and many others. While there is evidence of some regulatory overlap, each ArsR exhibits its own regulatory profile. Furthermore, evidence of a regulatory hierarchy was observed; i.e. ArsR1 represses arsR4, ArsR4 activates arsR2, and ArsR2 represses arsR3. Additionally and unexpectedly, aioB (arsenite oxidase small subunit) expression was shown to be under partial positive control by ArsR2 and ArsR4. Summarizing, this study demonstrates the regulatory portfolio of arsenite-activated ArsR proteins and includes essentially all major cellular functions. The broad bandwidth of arsenic effects on microbial metabolism assists in explaining and understanding the full impact of arsenic in natural ecosystems, including the mammalian gut.

Integrated Metabolomics and Targeted Gene Transcription Analysis Reveal Global Bacterial Antimonite Resistance Mechanisms

Antimony (Sb)-resistant bacteria have potential applications in the remediation of Sb-contaminated sites. However, the effect of Sb(III) exposure on whole-cell metabolic change has not been studied. Herein, we combined untargeted metabolomics with a previous proteomics dataset and confirmatory gene transcription analysis to identify metabolic responses to Sb(III) exposure in Agrobacterium tumefaciens GW4. Dynamic changes in metabolism between control and Sb(III)-exposed groups were clearly shown. KEGG pathway analysis suggested that with Sb(III) exposure: (1) the branching pathway of gluconeogenesis is down-regulated, resulting in the up-regulation of pentose phosphate pathway to provide precursors of anabolism and NADPH; (2) glycerophospholipid and arachidonic acid metabolisms are down-regulated, resulting in more acetyl-CoA entry into the TCA cycle and increased capacity to produce energy and macromolecular synthesis; (3) nucleotide and fatty acid synthesis pathways are all increased perhaps to protect cells from DNA and lipid peroxidation; (4) nicotinate metabolism increases which likely leads to increased production of co-enzymes (e.g., NAD⁺ and NADP⁺) for the maintenance of cellular redox and Sb(III) oxidation. Expectedly, the total NADP⁺/NADPH content, total glutathione, and reduced glutathione contents were all increased after Sb(III) exposure in strain GW4, which contribute to maintaining the reduced state of the cytoplasm. Our results provide novel information regarding global bacterial responses to Sb(III) exposure from a single gene level to the entire metabolome and provide specific hypotheses regarding the metabolic change to be addressed in future research.

Comprehensive genomic and proteomic profiling reveal Acinetobacter johnsonii JH7 responses to Sb(III) toxicity

Antimony (Sb) pollution poses a severe health threat to ecosystems. However, the toxic effects of Sb on biota are far from being elucidated. One of the unresolved questions is the molecular signal pathways underlying microbial adaptation to excess antimonite or Sb(III) exposure. The response of a Sb(III)-resistant bacterium Acinetobacter. johnsonii JH7 to Sb(III) stress was investigated using genomic and proteomic profiling. Sb(III) induced the formation of reactive oxygen species thereby leading to oxidative stress and the up-regulation of antioxidant enzyme activities. In addition, two important operons (ars and pst) playing critical roles in this cellular response were identified. The ars proteins functioned cooperatively to expel Sb(III) thereby decreasing antimonite toxicity. Downregulation of the phosphate-specific transporter might reduce the uptake of Sb(V) while hindering phosphorus assimilation. Interaction of Sb(III) with JH7 strain cells also affected peptide syntheses and folding, energy conversion, and stability of the cellular envelope. The present study provides for the first time a global map of cellular adaptation to excess Sb(III). Such information is potentially useful to future Sb pollution remediation strategies.

Anaerobic Bacterial Immobilization and Removal of Toxic Sb(III) Coupled With Fe(II)/Sb(III) Oxidation and Denitrification

Antimony (Sb) pollution is a worldwide problem. In some anoxic sites, such as Sb mine drainage and groundwater sediment, the Sb concentration is extremely elevated. Therefore, effective Sb remediation strategies are urgently needed. In contrast to microbial aerobic antimonite [Sb(III)] oxidation, the mechanism of microbial anaerobic Sb(III) oxidation and the effects of nitrate and Fe(II) on the fate of Sb remain unknown. In this study, we discovered the mechanism of anaerobic Sb(III) oxidation coupled with Fe(II) oxidation and denitrification in the facultative anaerobic Sb(III) oxidizer Sinorhizobium sp. GW3. We observed the following: (1) under anoxic conditions with nitrate as the electron acceptor, strain GW3 was able to oxidize both Fe(II) and Sb(III) during cultivation; (2) in the presence of Fe(II), nitrate and Sb(III), the anaerobic Sb(III) oxidation rate was remarkably enhanced, and Fe(III)-containing minerals were produced during Fe(II) and Sb(III) oxidation; (3) qRT-PCR, gene knock-out and complementation analyses indicated that the arsenite oxidase gene product AioA plays an important role in anaerobic Sb(III) oxidation, in contrast to aerobic Sb(III) oxidation; and (4) energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (XRD) analyses revealed that the microbially produced Fe(III) minerals were an effective chemical oxidant responsible for abiotic anaerobic Sb(III) oxidation, and the generated Sb(V) was adsorbed or coprecipitated on the Fe(III) minerals. This process included biotic and abiotic factors, which efficiently immobilize and remove soluble Sb(III) under anoxic conditions. The findings revealed a significantly novel development for understanding the biogeochemical Sb cycle. Microbial Sb(III) and Fe(II) oxidation coupled with denitrification has great potential for bioremediation in anoxic Sb-contaminated environments.

The AbaR antibiotic resistance islands found in Acinetobacter baumannii global clone 1 - Structure, origin and evolution

In multiply resistant Acinetobacter baumannii, complex transposons located in the chromosomal comM gene carry antibiotic and heavy metal resistance determinants. For one type, known collectively as AbaR, the ancestral form, AbaR0, entered a member of global clone 1 (GC1) in the mid 1970s and continued to evolve in situ forming many variants. In AbaR0, antibiotic and mercuric ion resistance genes are located between copies of a cadmium-zinc resistance transposon, Tn6018, and this composite transposon is in a class III transposon, Tn6019, carrying arsenate/arsenite resistance genes and five tni transposition genes. The antibiotic resistance genes in the AbaR0 and derived AbaR3 configurations are aphA1b, bla_TEM, catA1, sul1, tetA(A), and cassette-associated aacC1 and aadA1 genes. These genes are in a specific arrangement of fragments from well-known transposons, e.g. Tn1, Tn1721, Tn1696 and Tn2670, that arose in an IncM1 plasmid. All known GC1 lineage 1 isolates carry AbaR0 or AbaR3, which arose around 1990, or a variant derived from one of them. Variants arose via deletions caused by one of three internal IS26s, by recombination between duplicate copies of sul1 or Tn6018, or by gene cassette addition or replacement. A few GC2 isolates also carry an AbaR island with different cassette-associated genes, aacA4 and oxa20.

Effects upon metabolic pathways and energy production by Sb(III) and As(III)/Sb(III)-oxidase gene aioA in Agrobacterium tumefaciens GW4

Agrobacterium tumefaciens GW4 is a heterotrophic arsenite [As(III)]/antimonite [Sb(III)]-oxidizing strain. The As(III) oxidase AioAB is responsible for As(III) oxidation in the periplasm and it is also involved in Sb(III) oxidation in Agrobacterium tumefaciens 5A. In addition, Sb(III) oxidase AnoA and cellular H₂O₂ are also responsible for Sb(III) oxidation in strain GW4. However, the deletion of aioA increased the Sb(III) oxidation efficiency in strain GW4. In the present study, we found that the cell mobility to Sb(III), ATP and NADH contents and heat release were also increased by Sb(III) and more significantly in the aioA mutant. Proteomics and transcriptional analyses showed that proteins/genes involved in Sb(III) oxidation and resistance, stress responses, carbon metabolism, cell mobility, phosphonate and phosphinate metabolism, and amino acid and nucleotide metabolism were induced by Sb(III) and were more significantly induced in the aioA mutant. The results suggested that Sb(III) oxidation may produce energy. In addition, without periplasmic AioAB, more Sb(III) would enter bacterial cells, however, the cytoplasmic AnoA and the oxidative stress response proteins were significantly up-regulated, which may contribute to the increased Sb(III) oxidation efficiency. Moreover, the carbon metabolism was also activated to generate more energy against Sb(III) stress. The generated energy may be used in Sb transportation, DNA repair, amino acid synthesis, and cell mobility, and may be released in the form of heat.

Effect of Trace and Toxic Elements of Different Brands of Cigarettes on the Essential Elemental Status of Irish Referent and Diabetic Mellitus Consumers

Cigarette smoking interferes with the metal homeostasis of the human body, which plays a crucial role for maintaining the health. A significant flux of heavy metals, among other toxins, reaches the lungs through smoking. In the present study, the relationship between toxic element (TE) exposure via cigarette smoking and diabetic mellitus incidence in population living in Dublin, Ireland is investigated. The trace [zinc (Zn) and selenium (Se)] and toxic elements arsenic (As), aluminum (Al), cadmium (Cd), nickel (Ni), mercury (Hg), and lead (Pb) were determined in biological (scalp hair and blood) samples of patients diagnosed with diabetic mellitus, who are smokers living in Dublin, Ireland. These results were compared with age- and sex-matched healthy, nonsmokers controls. The different brands of cigarette (filler tobacco, filter, and ash) consumed by the studied population were also analyzed for As, Al, Cd, Ni, Hg, and Pb. The concentrations of TEs in biological samples and different components of cigarette were measured by inductively coupled plasma atomic emission spectrophotometer after microwave-assisted acid digestion. The validity and accuracy of the methodology were checked using certified reference materials (CRM). The recovery of all the studied elements was found to be in the range of 96.4-99.7% in certified reference materials. The filler tobacco of different branded cigarettes contains Hg, As, Al, Cd, Ni, and Pb concentrations in the ranges of 9.55-12.4 ng/cigarette, 0.432-0.727 μg/cigarette, 360-496 μg/cigarette, 1.70-2.12 μg/cigarette, 0.715-1.52 μg/cigarette, and 0.378-1.16 μg/cigarette, respectively. The results of this study showed that the mean values of Al, As, Cd, Hg, Ni, and Pb were significantly higher in scalp hair and blood samples of diabetic mellitus patients in relation to healthy controls, while the difference was significant in the case of smoker patients (p < 0.001). The levels of all six toxic elements were twofolds to threefolds higher in scalp hair and blood samples of nondiabetic mellitus smoker subjects as compared to nonsmoker controls. The high exposure of toxic metals as a result of cigarette smoking may be synergistic with risk factors associated with diabetic mellitus.

Regulation of arsenite oxidation by the phosphate two-component system PhoBR in Halomonas sp. HAL1

Previously, the expression of arsenite [As(III)] oxidase genes aioBA was reported to be regulated by a three-component regulatory system, AioXSR, in a number of As(III)-oxidizing bacterial strains. However, the regulation mechanism is still unknown when aioXSR genes are absent in some As(III)-oxidizing bacterial genomes, such as in Halomonas sp. HAL1. In this study, transposon mutagenesis and gene knock-out mutation were performed, and two mutants, HAL1-phoR₉₃₁ and HAL1-▵phoB, were obtained in strain HAL1. The phoR and phoB constitute a two-component system which is responsible for phosphate (Pi) acquisition and assimilation. Both of the mutants showed negative As(III)-oxidation phenotypes in low Pi condition (0.1 mM) but not under normal Pi condition (1 mM). The phoBR complementation strain HAL1-▵phoB-C reversed the mutants' null phenotypes back to wild type status. Meanwhile, lacZ reporter fusions using pCM-lacZ showed that the expression of phoBR and aioBA were both induced by As(III) but were not induced in HAL1-phoR₉₃₁ and HAL1-▵phoB. Using 15 consensus Pho box sequences, a putative Pho box was found in the aioBA regulation region. PhoB was able to bind to the putative Pho box in vivo (bacterial one-hybrid detection) and in vitro (electrophoretic mobility gel shift assay), and an 18-bp binding sequence containing nine conserved bases were determined. This study provided the evidence that PhoBR regulates the expression of aioBA in Halomonas sp. HAL1 under low Pi condition. The new regulation model further implies the close metabolic connection between As and Pi.

Genome-wide association mapping identifies a new arsenate reductase enzyme critical for limiting arsenic accumulation in plants

Inorganic arsenic is a carcinogen, and its ingestion through foods such as rice presents a significant risk to human health. Plants chemically reduce arsenate to arsenite. Using genome-wide association (GWA) mapping of loci controlling natural variation in arsenic accumulation in Arabidopsis thaliana allowed us to identify the arsenate reductase required for this reduction, which we named High Arsenic Content 1 (HAC1). Complementation verified the identity of HAC1, and expression in Escherichia coli lacking a functional arsenate reductase confirmed the arsenate reductase activity of HAC1. The HAC1 protein accumulates in the epidermis, the outer cell layer of the root, and also in the pericycle cells surrounding the central vascular tissue. Plants lacking HAC1 lose their ability to efflux arsenite from roots, leading to both increased transport of arsenic into the central vascular tissue and on into the shoot. HAC1 therefore functions to reduce arsenate to arsenite in the outer cell layer of the root, facilitating efflux of arsenic as arsenite back into the soil to limit both its accumulation in the root and transport to the shoot. Arsenate reduction by HAC1 in the pericycle may play a role in limiting arsenic loading into the xylem. Loss of HAC1-encoded arsenic reduction leads to a significant increase in arsenic accumulation in shoots, causing an increased sensitivity to arsenate toxicity. We also confirmed the previous observation that the ACR2 arsenate reductase in A. thaliana plays no detectable role in arsenic metabolism. Furthermore, ACR2 does not interact epistatically with HAC1, since arsenic metabolism in the acr2 hac1 double mutant is disrupted in an identical manner to that described for the hac1 single mutant. Our identification of HAC1 and its associated natural variation provides an important new resource for the development of low arsenic-containing food such as rice.

Synergic role of the two ars operons in arsenic tolerance in Pseudomonas putida KT2440

The chromosome of Pseudomonas putida KT2440 carries two clusters of genes, denoted ars1 and ars2, that are annotated as putative arsenic resistance operons. In this work, we present evidence that both operons encode functional arsenic-response regulatory genes as well as arsenic extrusion systems that confer resistance to both arsenite [As(III)] and arsenate [As(V)]. Transcriptional fusions of P(ars1) and P(ars2) to lacZ revealed that expression of both operons was induced by arsenite and arsenate. We generated single mutants in ars1 and ars2, which showed lower resistance to arsenic than the wild-type strain. A double ars1/ars2 was found to be highly sensitive to arsenic. Minimum inhibitory concentrations (MICs) for single mutants decreased two- to fourfold with respect to the parental strain, while in the double mutant the MIC decreased 128-fold for arsenite and 32-fold for arsenate. Bioinformatic analysis revealed that the ars2 resistance operon is part of the core genome of P. putida, while the ars1 operon appears to only occur in the KT2440 strain, suggesting that ars1 was acquired by horizontal gene transfer. The presence of ars1 in KT2440 may explain why it exhibits higher resistance to arsenic than other P. putida strains, which bear only the ars2 operon.

Functional coexistence of twin arsenic resistance systems in Pseudomonas putida KT2440

The genome of the soil bacterium Pseudomonas putida KT2440 bears two virtually identical arsRBCH operons putatively encoding resistance to inorganic arsenic species. Single and double chromosomal deletions in each of these ars clusters of this bacterium were tested for arsenic sensitivity and found that the contribution of each operon to the resistance to the metalloid was not additive, as either cluster sufficed to endow cells with high-level resistance. However, otherwise identical traits linked to each of the ars sites diverged when temperature was decreased. Growth of the various mutants at 15°C (instead of the standard 30°C for P. putida) uncovered that ars2 affords a much higher resistance to As (III) than the ars1 counterpart. Reverse transcription polymerase chain reaction of arsB1 and arsB2 genes as well as lacZ fusions to the Pars1 and Pars2 promoters traced the difference to variations in transcription of the corresponding gene sets at each temperature. Functional redundancy may thus be selected as a stable condition - rather than just as transient state - if it affords one key activity to be expressed under a wider range of physicochemical settings. This seems to provide a straightforward solution to regulatory problems in environmental bacteria that thrive under changing scenarios.

Tunable reporter signal production in feedback-uncoupled arsenic bioreporters

Escherichia coli-based bioreporters for arsenic detection are typically based on the natural feedback loop that controls ars operon transcription. Feedback loops are known to show a wide range linear response to the detriment of the overall amplification of the incoming signal. While being a favourable feature in controlling arsenic detoxification for the cell, a feedback loop is not necessarily the most optimal for obtaining highest sensitivity and response in a designed cellular reporter for arsenic detection. Here we systematically explore the effects of uncoupling the topology of arsenic sensing circuitry on the developed reporter signal as a function of arsenite concentration input. A model was developed to describe relative ArsR and GFP levels in feedback and uncoupled circuitry, which was used to explore new ArsR-based synthetic circuits. The expression of arsR was then placed under the control of a series of constitutive promoters, which differed in promoter strength, and which could be further modulated by TetR repression. Expression of the reporter gene was maintained under the ArsR-controlled Pars promoter. ArsR expression in the systems was measured by using ArsR-mCherry fusion proteins. We find that stronger constitutive ArsR production decreases arsenite-dependent EGFP output from Pars and vice versa. This leads to a tunable series of arsenite-dependent EGFP outputs in a variety of systematically characterized circuitries. The higher expression levels and sensitivities of the response curves in the uncoupled circuits may be useful for improving field-test assays using arsenic bioreporters.

Antimony resistance and environment: Elusive links to explore during Leishmania life cycle

Leishmania drug resistance and particularly antimony resistance still continues to emerge in different part of the world. Because visceral and cutaneous leishmaniasis are transmitted in foci with zoonotic or anthroponotic life-cycles, the link between chemotherapeutic resistance and the selection for drug resistance, through drug consumption, cannot be as obvious for all forms of leishmaniasis. The underlying factors that trigger the selection of antimony resistant parasites are poorly studied in regard to environmental aspects. Recently, a correlation between the emergence of antimony unresponsiveness in India and water arsenic contamination has been raised. The presence of some yet unidentified environmental factors driving the selection of antimony resistant Leishmania populations in a zoonotic context of leishmaniasis is also currently questioned. The identification of key molecules involved in the selection of antimony resistance and their importance in the selective process have to be re-evaluated in light of the environment were all the hosts of Leishmania (mammalian and arthropod) evolved. These new insights will help to (i) address the risk of therapeutic failure associated with the emergence of drug-resistance and (ii) propose new therapeutic protocols to aim at reducing the risk of resistance in endemic areas.

Microevolution of Cryptococcus neoformans driven by massive tandem gene amplification

The subtelomeric regions of organisms ranging from protists to fungi undergo a much higher rate of rearrangement than is observed in the rest of the genome. While characterizing these ~40-kb regions of the human fungal pathogen Cryptococcus neoformans, we have identified a recent gene amplification event near the right telomere of chromosome 3 that involves a gene encoding an arsenite efflux transporter (ARR3). The 3,177-bp amplicon exists in a tandem array of 2-15 copies and is present exclusively in strains with the C. neoformans var. grubii subclade VNI A5 MLST profile. Strains bearing the amplification display dramatically enhanced resistance to arsenite that correlates with the copy number of the repeat; the origin of increased resistance was verified as transport-related by functional complementation of an arsenite transporter mutant of Saccharomyces cerevisiae. Subsequent experimental evolution in the presence of increasing concentrations of arsenite yielded highly resistant strains with the ARR3 amplicon further amplified to over 50 copies, accounting for up to ~1% of the whole genome and making the copy number of this repeat as high as that seen for the ribosomal DNA. The example described here therefore represents a rare evolutionary intermediate-an array that is currently in a state of dynamic flux, in dramatic contrast to relatively common, static relics of past tandem duplications that are unable to further amplify due to nucleotide divergence. Beyond identifying and engineering fungal isolates that are highly resistant to arsenite and describing the first reported instance of microevolution via massive gene amplification in C. neoformans, these results suggest that adaptation through gene amplification may be an important mechanism that C. neoformans employs in response to environmental stresses, perhaps including those encountered during infection. More importantly, the ARR3 array will serve as an ideal model for further molecular genetic analyses of how tandem gene duplications arise and expand.

Case-control study of male cancer patients exposed to arsenic-contaminated drinking water and tobacco smoke with relation to non-exposed cancer patients

The investigated data indicated that inorganic arsenic in drinking water is associated with increased mortality from different types of cancers. In the present study, biological samples (blood and scalp hair) of male subjects having lung and bladder cancers and non-cancerous subjects belonging to arsenic (As)-exposed area of southern parts of Pakistan were analysed for As contents. The As levels in drinking water of understudy area showed that sections of understudy population are exposed to arsenic concentrations, which was 3-15-fold higher than the permissible level (<10 μg/L). For comparative purposes the biological samples of matched male cancer patient, as referent patients belonging to big city (Hyderabad) who had used municipal treated water with low arsenic levels <10 μg/L, were also collected. The exposed cancer patients have 2-3-fold higher level of As in both biological samples compared to non-exposed case-matched cancerous male subjects. This study is compelling evidence in support of positive associations between arsenic-contaminated water, food and cigarette with different types of risks of cancer.

Solid phase biosensors for arsenic or cadmium composed of A trans factor and cis element complex

The presence of toxic metals in drinking water has hazardous effects on human health. This study was conducted to develop GFP-based-metal-binding biosensors for on-site assay of toxic metal ions. GFP-tagged ArsR and CadC proteins bound to a cis element, and lost the capability of binding to it in their As- and Cd-binding conformational states, respectively. Water samples containing toxic metals were incubated on a complex of GFP-tagged ArsR or CadC and cis element which was immobilized on a solid surface. Metal concentrations were quantified with fluorescence intensity of the metal-binding states released from the cis element. Fluorescence intensity obtained with the assay significantly increased with increasing concentrations of toxic metals. Detection limits of 1 μg/L for Cd(II) and 5 μg/L for As(III) in purified water and 10 µg/L for Cd(II) and As(III) in tap water and bottled mineral water were achieved by measurement with a battery-powered portable fluorometer after 15-min and 30-min incubation, respectively. A complex of freeze dried GFP-tagged ArsR or CadC binding to cis element was stable at 4 °C and responded to 5 μg/L As(III) or Cd(II). The solid phase biosensors are sensitive, less time-consuming, portable, and could offer a protocol for on-site evaluation of the toxic metals in drinking water.

Respiratory effects in people exposed to arsenic via the drinking water and tobacco smoking in southern part of Pakistan

In this study, a survey has been conducted during 2005-2007 on surface and groundwater arsenic (As) contamination and its impact on the health of local population, of villages located on the banks of Manchar lake, southern part of Sindh, Pakistan. We have also assessed the relationship between arsenic exposure through respiratory disorders in male subjects with drinking water and smoking cigarettes made from tobacco grown in agricultural land irrigated with As contaminated lake water. The biological samples (blood and scalp hair) were collected from As exposed subjects (100% smokers) and age matched healthy male subjects (40.2% smoker and 59.8% non smokers) belong to unexposed areas for comparison purposes. The As concentration in drinking water (surface and underground water), agricultural soil, cigarette tobacco and biological samples were determined by electrothermal atomic absorption spectrometry. The range of As concentrations in lake water was 35.2-158 microg/L (average 97.5 microg/L), which is 3-15 folds higher than permissible limit of World Health Organization (WHO, 2004). While the As level in local cigarette tobacco was found to be 3-6 folds higher than branded cigarettes (0.37-0.79 microg/g). Arsenic exposed subjects (with and without RD) had significantly elevated levels of As in their biological samples as compared to referent male subject of unexposed area. These respiratory effects were more pronounced in individuals who had also As induced skin lesions. The linear regressions showed good correlations between As concentrations in water versus hair and blood samples of exposed subjects with and without respiratory problems.

Determination of toxic elements in different brands of cigarette by atomic absorption spectrometry using ultrasonic assisted acid digestion

In this paper, a sample preparation method based on ultrasonic assisted acid extraction (UAE) of Al, As, Cd, Ni and Pb from filler tobacco (FT) of different local branded (LBCs) and imported branded cigarettes (IBCs) consumed in Pakistan. Acid extracts obtained upon sonication were determined by atomic absorption spectrometry. Parameters influencing extraction such as pre-sonication time 10-15 min, sonication time 2-10 min, mass of sample 0.1-0.5 g, particle size <35-125 microm and, extractant (nitric acid) concentrations (2-10%) were fully optimized to get quantitative recovery of toxic elements (TEs). The proposed method was validated with a certified reference material (CRM) Virginia tobacco leaves (ICHTJ-cta-VTL-2), and also applied a conventional acid digestion method (CDM) on same CRM, for comparative purpose. The average relative standard deviation of UAE method of CRM, ranged as 1.64-8.42% for N = 6, depending on the analytes. Extraction efficiencies for CRM were close to 97.1, 97.5, 98.5, 97.8 and 96.7 for Al, As, Cd, Ni and Pb, respectively. The concentrations of Al, As, Cd, Ni and Pb in cigarettes were present in all branded cigarettes, there is no any significance difference among LBCs and IBCs, except in few cases of LBCs, have high level of Cd and Al. Our results were compared with other worldwide studies.

Development of bacteria-based bioassays for arsenic detection in natural waters

Arsenic contamination of natural waters is a worldwide concern, as the drinking water supplies for large populations can have high concentrations of arsenic. Traditional techniques to detect arsenic in natural water samples can be costly and time-consuming; therefore, robust and inexpensive methods to detect arsenic in water are highly desirable. Additionally, methods for detecting arsenic in the field have been greatly sought after. This article focuses on the use of bacteria-based assays as an emerging method that is both robust and inexpensive for the detection of arsenic in groundwater both in the field and in the laboratory. The arsenic detection elements in bacteria-based bioassays are biosensor-reporter strains; genetically modified strains of, e.g., Escherichia coli, Bacillus subtilis, Staphylococcus aureus, and Rhodopseudomonas palustris. In response to the presence of arsenic, such bacteria produce a reporter protein, the amount or activity of which is measured in the bioassay. Some of these bacterial biosensor-reporters have been successfully utilized for comparative in-field analyses through the use of simple solution-based assays, but future methods may concentrate on miniaturization using fiberoptics or microfluidics platforms. Additionally, there are other potential emerging bioassays for the detection of arsenic in natural waters including nematodes and clams.

Mechanism of metalloid-induced death in Leishmania spp.: role of iron, reactive oxygen species, Ca2+, and glutathione

There is growing evidence that metalloid-induced cell death in protozoan parasites is due to oxidative injury; however, the biochemical changes related to this event are not fully understood. Leishmania spp. demonstrated cross-resistance to two related metalloids, arsenic and antimony, and both metalloids induced cell death accompanied by cell shrinkage and DNA fragmentation that was preceded by an increase in reactive oxygen species. Both drugs caused mitochondrial dysfunction in terms of loss of membrane potential and a drop in ATP levels. Arsenic treatment resulted in an elevation of intracellular Ca2+ levels that did not occur with antimony exposure. Cellular glutathione level was reduced after antimony treatment but arsenic did not affect glutathione. Inhibition of Ca2+ influx during arsenic treatment reduced cell death, whereas supplementation of glutathione during antimony treatment rescued cell loss. Under iron-depleted conditions, the cytotoxic effects of arsenic and antimony did not occur and cell survival increased; in contrast, the presence of excess iron resulted in higher cell death. Therefore, this study provides a new possibility that iron can potentiate parasite death induced by metalloids like arsenic and antimony. In addition, an important observation is that the two similar metalloids produce toxicity by very different mechanisms.

Development of a set of simple bacterial biosensors for quantitative and rapid measurements of arsenite and arsenate in potable water

Testing for arsenic pollution is commonly performed with chemical test kits of unsatisfying accuracy. Bacterial biosensors are an interesting alternative as they are easily produced, simple, and highly accurate devices. Here, we describe the development of a set of bacterial biosensors based on a nonpathogenic laboratory strain of Escherichia coli, the natural resistance mechanism of E. coli against arsenite and arsenate, and three reporter proteins: bacterial luciferase, beta-galactosidase and Green Fluorescent Protein (GFP). The biosensors were genetically optimized to reduce background expression in the absence of arsenic. In calibration experiments with the biosensors and arsenite-amended potable water, arsenite concentrations at 4 microg of As/L (0.05 microM) were routinely and accurately measured. The currently most quantitative system expressed the bacterial luciferase as reporter protein, responding proportional with a concentration range between 8 and 80 microg of As/L. Sensor cells could be stored as frozen batches, resuspended in plain media, and exposed to the aqueous test sample, and light emission was measured after 30-min incubation. Field testing for arsenite was achieved with a system that contained beta-galactosidase, producing a visible blue color at arsenite concentrations above 8 microg/L. For this sensor, a protocol was developed in which the sensor cells were dried on a paper strip and placed in the aqueous test solution for 30 min after which time color development was allowed to take place. The GFP sensor showed good potential for continuous rather than end point measurements. In all cases, growth of the biosensors and production of the strip test was achieved by very simple means with common growth media, and quality control of the sensors was performed by isolating the respective plasmids with the genetic constructs according to simple standard genetic technologies. Therefore, the biosensor cells and protocols may offer a realistic alternative for measuring arsenic contamination in potable water.

Genomic dissection reveals locus response to stress for mammalian acetylcholinesterase

The mammalian acetylcholinesterase (ACHE) locus was investigated using computational predictive methods and experiments of reverse transcription polymerase chain reaction (RT-PCR). Computational analysis identified two genes downstream to ACHE, an inversely oriented arsenite resistance gene homologue (ARS), and a novel previously unidentified gene (PIX), co-oriented with ACHE. Experimental evidence shows coregulation of murine ACHE and ARS following confined swim, indicating coordinated locus response to stress, that is possibly mediated by altered cholinergic neurotransmission.

Novel Intracellular SbV reducing activity correlates with antimony susceptibility in Leishmania donovani

The standard treatment of human visceral leishmaniasis involves the use of pentavalent antimony (Sb(V)). Its mechanism of action is unknown because of the limited information available about intracellular antimony metabolism and about the genes that regulate these processes. Herein, flow injection-inductively coupled plasma mass spectrometry (ICP-MS), flow injection hydride generation ICP-MS, and ion chromatography ICP-MS were used to measure antimony accumulation and intracellular metabolism in the human protozoan parasite Leishmania donovani. Amastigotes (the intracellular form) and promastigotes (the extracellular form) accumulate Sb(V) and Sb(III) via separate transport systems. Stage-specific intracellular Sb(V) reducing activity was apparent in amastigotes, which reduced the negligibly toxic Sb(V) to highly toxic Sb(III). This amastigote-specific reducing activity was deficient in the Pentostam-resistant mutant L. donovani Ld1S.20. These data indicate that parasite susceptibility to Sb(V) correlates with its level of Sb(V) reducing activity. Also, in promastigotes of both wild-type L. donovani and the Pentostam-resistant mutant L. donovani Ld1S.20, Sb(V) inhibited the toxicity of Sb(III) but not of As(III). Both Sb(V) and Sb(III) were toxic to wild-type amastigotes. However, as observed in promastigotes, in mutant amastigotes Sb(V) inhibits Sb(III) but not As(III) activity. Anion exchange chromatography showed that intracellular antimony metabolism occurred in both promastigotes and amastigotes. These data demonstrate that the interaction between the two antimony oxidation states occurs intracellularly, within the parasite. The results also indicate that Sb(V) anti-leishmanial activity is dependent on its reduction to Sb(III). The mechanism of this novel intracellular Sb(V) reduction has yet to be identified, and it may or may not be enzymatic. This is the first description of intracellular Sb(V) reducing activity in Leishmania as well as in any prokaryotic or eukaryotic cell.

Elevated levels of polyamines and trypanothione resulting from overexpression of the ornithine decarboxylase gene in arsenite-resistant Leishmania

The levels of trypanothione, a glutathione-spermidine conjugate, are increased in the protozoan parasite Leishmania selected for resistance to the heavy metal arsenite. The levels of putrescine and spermidine were increased in resistant mutants. This increase is mediated by overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis. Gene overexpression is generally mediated by gene amplification in Leishmania but, here, the mRNA and the enzymatic activity of ODC are increased without gene amplification. This RNA overexpression is stable when cells are grown in the absence of the drug and does not result from gene rearrangements or from an increased rate of RNA synthesis. Transient transfections suggest that mutations in the revertant cells contribute to these elevated levels of RNA. Stable transfection of the ODC gene increases the level of trypanothione, which can contribute to arsenite resistance. In addition to ODC overexpression, the gene for the ABC transporter PGPA is amplified in the mutants. The co-transfection of the ODC and PGPA genes confers resistance in a synergistic fashion in partial revertants, also suggesting that PGPA recognizes metals conjugated to trypanothione.

ABC transporters in Leishmania and their role in drug resistance

ABC transporters have been found in several parasitic protozoa including Leishmania. At least two Leishmania ABC transporters are involved in drug resistance. One is PgpA, which is involved in resistance to arsenic and antimony-containing compounds. Antimonials are the drug of choice against Leishmania infections. Transfection and biochemical studies suggest that PgpA recognizes metals conjugated to thiols. The second ABC transporter is closely related to mammalian P-glycoproteins and confers resistance to anticancer drugs by a mechanism that remains to be elucidated. Additional ABC transporters are likely to be present in Leishmania and these are discussed in relation to the phenomenon of antimony resistance.