Enumeration and characterization of culturable arsenate resistant bacteria in a large estuary

Siderophore-producing Bacillus amyloliquefaciens BM3 mitigate arsenic contamination and suppress Fusarium wilt in brinjal plants

AIM

Arsenic contamination in agricultural soils poses a serious health risk for humans. Bacteria that produce siderophores, primarily for iron acquisition, can be relevant in combating arsenic toxicity in agricultural soils and simultaneously act as biocontrol agents against plant diseases. We evaluated the arsenic bioremediation and biocontrol potential of the rhizosphere isolate Bacillus amyloliquefaciens BM3 and studied the interaction between the purified siderophore bacillibactin and arsenic.

METHODS AND RESULTS

BM3 showed high arsenic resistance [MIC value 475 and 24 mM against As(V) and As(III), respectively] and broad spectrum in-vitro antagonism against several phytopathogenic fungi. BM3 was identified by biochemical characterization and 16S rRNA gene sequencing. Scanning electron microscopy (SEM) analysis revealed increased cell size of BM3 when grown in presence of sub-lethal arsenic concentrations. Bioremediation assays showed a 74% and 88.1% reduction in As(V) and As(III) concentrations, respectively. Genetic determinants for arsenic resistance (arsC and aoxB) and antifungal traits (bacAB and chiA) were detected by PCR. Arsenic chelating ability of bacillibactin, the siderophore purified from culture filtrate of BM3 and identified through spectroscopic data analysis, was observed in CAS assay and fluorescence spectrometry. In-vivo application of talc-based formulation of BM3 in brinjal seedlings showed significant reduction in Fusarium wilt disease.

CONCLUSION

Strain B. amyloliquefaciens BM3 may be useful in arsenic bioremediation and may be considered for large field trials as an alternative to chemical fungicides by inhibiting soil borne pathogens.

How arsenic contamination influences downslope wetland plant and microbial community structure and function

Mine tailings are prevalent worldwide and can adversely impact adjacent ecosystems, including wetlands. This study investigated the impact of gold (Au) mine tailings contamination on peatland soil and pore water geochemistry, vegetation and microbial communities, and microbial carbon (C) cycling. Maximum arsenic (As) concentrations in peat and pore water reached 20,137 mg kg^-1 and 16,730 μg L^-1, respectively, but decreased by two orders of magnitude along a 128 m gradient extending from the tailings into the wetland. Carbon and other macronutrient (N, P, K) concentrations in peat and pore water significantly increased with distance from contamination. Relative percent cover and species richness of vascular and non-vascular plants significantly increased with distance into the wetland, with higher non-vascular richness being found at intermediate distances before transitioning to a vascular plant dominated community. Bacterial and archaeal community composition exhibited a decreased proportion of members of the phylum Acidobacteria (notably of the order Acidobacteriales) and increased diversity and richness of methanogens across a larger range of orders farther from the tailings source, an indication of microbial C-cycling potential. Consistent with changes in microbial communities, in vitro microbial CH₄ production potential significantly increased with distance from the contaminant source. This study demonstrates both the profound negative impact that metalliferous tailings contamination can have on above and belowground communities in peatlands, and the value of wetland preservation and restoration.

Use of Microbial Consortia in Bioremediation of Metalloid Polluted Environments

Metalloids are released into the environment due to the erosion of the rocks or anthropogenic activities, causing problems for human health in different world regions. Meanwhile, microorganisms with different mechanisms to tolerate and detoxify metalloid contaminants have an essential role in reducing risks. In this review, we first define metalloids and bioremediation methods and examine the ecology and biodiversity of microorganisms in areas contaminated with these metalloids. Then we studied the genes and proteins involved in the tolerance, transport, uptake, and reduction of these metalloids. Most of these studies focused on a single metalloid and co-contamination of multiple pollutants were poorly discussed in the literature. Furthermore, microbial communication within consortia was rarely explored. Finally, we summarized the microbial relationships between microorganisms in consortia and biofilms to remove one or more contaminants. Therefore, this review article contains valuable information about microbial consortia and their mechanisms in the bioremediation of metalloids.

Characteristics and Comparative Genomic Analysis of a Novel Virus, VarioGold, the First Bacteriophage of Variovorax

Variovorax represents a widespread and ecologically significant genus of soil bacteria. Despite the ecological importance of these bacteria, our knowledge about the viruses infecting Variovorax spp. is quite poor. This study describes the isolation and characterization of the mitomycin-induced phage, named VarioGold. To the best of our knowledge, VarioGold represents the first characterized virus for this genus. Comparative genomic analyses suggested that VarioGold is distinct from currently known bacteriophages at both the nucleotide and protein levels; thus, it could be considered a new virus genus. In addition, another 37 prophages were distinguished in silico within the complete genomic sequences of Variovorax spp. that are available in public databases. The similarity networking analysis highlighted their general high diversity, which, despite clustering with previously described phages, shows their unique genetic load. Therefore, the novelty of Variovorax phages warrants the great enrichment of databases, which could, in turn, improve bioinformatic strategies for finding (pro)phages.

Assessing the Diversity and Metabolic Potential of Psychrotolerant Arsenic-Metabolizing Microorganisms From a Subarctic Peatland Used for Treatment of Mining-Affected Waters by Culture-Dependent and -Independent Techniques

Arsenic contamination in water by natural causes or industrial activities is a major environmental concern, and treatment of contaminated waters is needed to protect water resources and minimize the risk for human health. In mining environments, treatment peatlands are used in the polishing phase of water treatment to remove arsenic (among other contaminants), and peat microorganisms play a crucial role in arsenic removal. The present study assessed culture-independent diversity obtained through metagenomic and metatranscriptomic sequencing and culture-dependent diversity obtained by isolating psychrotolerant arsenic-tolerant, arsenite-oxidizing, and arsenate-respiring microorganisms from a peatland treating mine effluent waters of a gold mine in Finnish Lapland using a dilution-to-extinction technique. Low diversity enrichments obtained after several transfers were dominated by the genera Pseudomonas, Polaromonas, Aeromonas, Brevundimonas, Ancylobacter, and Rhodoferax. Even though maximal growth and physiological activity (i.e., arsenite oxidation or arsenate reduction) were observed at temperatures between 20 and 28°C, most enrichments also showed substantial growth/activity at 2–5°C, indicating the successful enrichments of psychrotolerant microorganisms. After additional purification, eight arsenic-tolerant, five arsenite-oxidizing, and three arsenate-respiring strains were obtained in pure culture and identified as Pseudomonas, Rhodococcus, Microbacterium, and Cadophora. Some of the enriched and isolated genera are not known to metabolize arsenic, and valuable insights on arsenic turnover pathways may be gained by their further characterization. Comparison with phylogenetic and functional data from the metagenome indicated that the enriched and isolated strains did not belong to the most abundant genera, indicating that culture-dependent and -independent methods capture different fractions of the microbial community involved in arsenic turnover. Rare biosphere microorganisms that are present in low abundance often play an important role in ecosystem functioning, and the enriched/isolated strains might thus contribute substantially to arsenic turnover in the treatment peatland. Psychrotolerant pure cultures of arsenic-metabolizing microorganisms from peatlands are needed to close the knowledge gaps pertaining to microbial arsenic turnover in peatlands located in cold climate regions, and the isolates and enrichments obtained in this study are a good starting point to establish model systems. Improved understanding of their metabolism could moreover lead to their use in biotechnological applications intended for bioremediation of arsenic-contaminated waters.

Investigation of arsenic-resistant, arsenite-oxidizing bacteria for plant growth promoting traits isolated from arsenic contaminated soils

The problem of arsenic (As) pollution being severe warrants opting for low-cost microbial remediation strategies. The present study of identifying suitable bacterial strains led to the isolation of eleven As-tolerant strains from the As-contaminated rhizosphere soils of West Bengal, India. They were found to oxidize/reduce 55-31.6% of 5 mM As(III) and 73-37.6% of 5 mM As(V) within 12 h. The four isolates (BcAl-1, JN 73, LAR-2, and AR-30) had a high level of As(III) oxidase activity along with a higher level of As(V) and As(III) resistance. The agar diffusion assay of the isolates further confirmed their ability to endure As stress. The presence of aoxB gene was observed in these four As(III) oxidizing isolates. Evaluation of plant growth-promoting characteristics revealed that BcAl-1 (Burkholderia cepacia), JN 73 (Burkholderia metallica), AR-30 (Burkholderia cenocepacia), and LAR-2 (Burkholderia sp.) had significant plant growth-promoting characteristics (PGP), including the ability to solubilize phosphate, siderophore production, indole acetic acid-like molecules production, ACC deaminase production, and nodule formation under As stressed condition. BcAl-1 and JN 73 emerged as the most promising traits in As removal as well as plant growth promotion.

Indigenous soil bacteria and the hyperaccumulator Pteris vittata mediate phytoremediation of soil contaminated with arsenic species

Arsenic (As) is a pollutant of major concern worldwide, posing as a threat to both human health and the environment. Phytoremediation has been proposed as a viable mechanism to remediate As-contaminated soil environments. Pot experiments were performed to evaluate the phytoextraction efficiency of As by Pteris vittata, a known As hyperaccumulating fern, from soil amended with different concentrations of arsenate [As(V)] and arsenite [As(III)], the more common, inorganic As forms in soil. The greatest accumulation of As (13.3 ± 0.36 g/kg Dwt) was found in fronds of plants grown in soil spiked with 1.0 g As(V)/kg. The maximum As-bioaccumulation factor (27.3 ± 1.9) was achieved by plants grown in soil amended with 0.05 g As(V)/kg. A total of 864 bacterial cultures were isolated and examined for their ability to enhance phytoremediation of As-contaminated soils. Traits examined included tolerance to As (III and V), production of siderophores, and/or ability to solubilize calcium phosphate and indole acetic acid (IAA) production. A culture-based survey shows greater numbers of viable and As-resistant bacteria were found in the rhizosphere of As-grown plants compared to bulk and unplanted soils. The percentage of bacteria resistant to As(V) was greater (P < 0.0001) than those resistant to As(III) in culture medium containing 0.5, 1, 1.5, and 2 g As/L. Higher (P < 0.0001) percentages of siderophore producing (77%) and phosphate solubilizing (61%) bacteria were observed among cultures isolated from unplanted soil. About 5% (44 of 864) of the isolates were highly resistant to both As (III) and As (V) (2 g/L), and were examined for their As-transformation ability and IAA production. A great proportion of the isolates produced IAA (82%) and promoted As (V)-reduction (95%) or As(III)-oxidation (73%), and 71% exhibited dual capacity for both As(V) reduction and As(III) oxidation. Phylogenetic analysis indicated that 67, 23, and 10% of these isolates belonged to Proteobacteria, Actinobacteria, and Firmicutes, respectively. Analysis of the 16S rRNA gene sequences confirmed that these isolates were closely related to 12 genera and 25 species of bacteria and were dominated by members of the genus Pseudomonas (39%). These results show that these isolates could potentially be developed as inocula for enhancing plant uptake during large scale phytoremediation of As-impacted soils.

Characterisation of hyper tolerant Bacillus firmus L-148 for arsenic oxidation

Groundwater arsenic pollution causes millions of deaths worldwide. Long term natural and anthropogenic activities have increased arsenic levels in groundwater causing higher threats of arsenic exposure. Arsenic hyper-tolerant Firmicute Bacillus firmus L-148 was isolated from arsenic limiting Lonar lake soil, which tolerated more than 3 M arsenic and could oxidize 75 mM arsenite [As(III)] in 14 days. It oxidized As(III) in presence of heavy metals and had unusual pH optima at 9.2. B. firmus L-148 was studied at the biochemical, protein, genomic and transcript level for understanding its arsenic oxidizing machinery. The proteomic and transcript analysis exhibited the presence of ars and aio operon and supported the inducible nature of ars operon. Robust, hyper-tolerant, fast As(III) oxidizing, least nutrient requiring and multi-metal resistance qualities of the strain were used in microcosm studies for bioremediation. Artificial groundwater mimicking microcosm with 75 mM As(III) was developed. Modulation of carbon source, iron and multi metals affected growth and As(III) oxidation rate. The As(III) oxidation was recorded to be 77% in 15 days in presence of sodium acetate and Fe ions. This microcosm study can be explored for bioremediation of arsenic contaminated water and followed by precipitation using other methods.

Isolation and characterization of bacteria from a brazilian gold mining area with a capacity of arsenic bioaccumulation

In Paracatu, a city in Minas Gerais State (Brazil), the gold mineral extraction produces wastes that contribute to environmental contamination by arsenic. This work describes the evaluation of arsenic concentration from soil of a gold mining area in Paracatu and the selection of arsenic resistant bacteria. In the process of culturing enrichment, 38 bacterial strains were isolated and the minimum inhibitory concentration (MIC) was determined in solid medium for each strain. Three bacterial strains named P1C1Ib, P2Ic and P2IIB were resistant to 3000 mg L^-1 of arsenite. Analysis of 16S rDNA gene sequences revealed that these bacteria belong to Bacillus cereus and Lysinibacillus boronitolerans species. After cultivation of the strains P1C1Ib, P2Ic and P2IIIb, 69.38%-71.88% of arsenite and 82.39%-85.72% of arsenate concentrations were reduced from the culture medium, suggesting the potential application of theses strains in bioremediation processes.

Biotechnological remediation of arsenate from aqueous solution using a novel bacterial strain: Isotherm, kinetics and thermodynamic studies

Arsenic is a global environmental contaminant that imposes a big health threat which requires an immediate attention to clean-up the contaminated areas. This study examined the biosorption ability of a novel Bacillus strain for the removal of arsenate (pentavalent arsenic) from aqueous solution. The optimum biosorption condition was studied as a function of biomass dosage, contact time and pH. Dubinin-Radushkevich (D-R), Freundlich, and Langmuir models were applied in describing the biosorption isotherm. The maximal biosorption capacity (92%) was obtained at 25 °C, biomass concentration 2000 mg/L at pH value of 4 and contact period of 50 min. Strain 139SI act as an admirable host to the arsenate. Thermodynamic assessment (ΔG⁰, ΔH⁰, and ΔS⁰) also suggested the chemisorption and feasible process of As(V) biosorption. The reuse study illustrated the highest recovery of 93% using 1 M HCl, and a decrease of 25% in recovery of As(V) ions after 10 times desorption process.

ArxA From Azoarcus sp. CIB, an Anaerobic Arsenite Oxidase From an Obligate Heterotrophic and Mesophilic Bacterium

Arsenic is a toxic element widely distributed in nature, but numerous bacteria are able to resist its toxicity mainly through the ars genes encoding an arsenate reductase and an arsenite efflux pump. Some “arsenotrophic” bacteria are also able to use arsenite as energy supplier during autotrophic growth by coupling anaerobic arsenite oxidation via the arx gene products to nitrate respiration or photosynthesis. Here, we have demonstrated that Azoarcus sp. CIB, a facultative anaerobic β-proteobacterium, is able to resist arsenic oxyanions both under aerobic and anaerobic conditions. Genome mining, gene expression, and mutagenesis studies revealed the presence of a genomic island that harbors the ars and arx clusters involved in arsenic resistance in strain CIB. Orthologous ars clusters are widely distributed in the genomes of sequenced Azoarcus strains. Interestingly, genetic and metabolic approaches showed that the arx cluster of the CIB strain encodes an anaerobic arsenite oxidase also involved in the use of arsenite as energy source. Hence, Azoarcus sp. CIB represents the prototype of an obligate heterotrophic bacterium able to use arsenite as an extra-energy source for anaerobic cell growth. The arsenic island of strain CIB supports the notion that metabolic and energetic skills can be gained by genetic mobile elements.

Arsenic respiration and detoxification by purple non-sulphur bacteria under anaerobic conditions

Two arsenic-resistant purple non-sulphur bacteria (PNSB), Q3B and Q3C, were isolated (from industrial contaminated site and paddy fields) and identified by SSU rRNA gene sequencing as Rhodospirillum and Rhodospirillaceae species, respectively. Maximum arsenic reduction by these PNSB was observed in anaerobic conditions. Rhodospirillum sp. Q3B showed 74.92% (v/v) arsenic reduction while Rhodospirillaceae sp. Q3C reduced arsenic up to 76.67% (v/v) in anaerobic conditions. Rhodospirillaceae sp. Q3C was found to contain highest carotenoid content up to 5.6mg·g^-1. Under anaerobic conditions, the isolates were able to respire arsenic in the presence of lactate, citrate, and oxalate. Rhodospirillum sp. Q3B and Rhodospirillaceae sp. Q3C were also found to produce hydrogen gas. Such diverse bacteria can be useful tools for bioremediation purposes. These bacteria can be further exploited and optimized to treat wastewater containing arsenic along with bio-hydrogen production.

Environmental Concentrations of Copper, Alone or in Mixture With Arsenic, Can Impact River Sediment Microbial Community Structure and Functions

In many aquatic ecosystems, sediments are an essential compartment, which supports high levels of specific and functional biodiversity thus contributing to ecological functioning. Sediments are exposed to inputs from ground or surface waters and from surrounding watershed that can lead to the accumulation of toxic and persistent contaminants potentially harmful for benthic sediment-living communities, including microbial assemblages. As benthic microbial communities play crucial roles in ecological processes such as organic matter recycling and biomass production, we performed a 21-day laboratory channel experiment to assess the structural and functional impact of metals on natural microbial communities chronically exposed to sediments spiked with copper (Cu) and/or arsenic (As) alone or mixed at environmentally relevant concentrations (40 mg kg-1 for each metal). Heterotrophic microbial community responses to metals were evaluated both in terms of genetic structure (using ARISA analysis) and functional potential (using exoenzymatic, metabolic and functional genes analyses). Exposure to Cu had rapid marked effects on the structure and most of the functions of the exposed communities. Exposure to As had almost undetectable effects, possibly due to both lack of As bioavailability or toxicity toward the exposed communities. However, when the two metals were combined, certain functional responses suggested a possible interaction between Cu and As toxicity on heterotrophic communities. We also observed temporal dynamics in the functional response of sediment communities to chronic Cu exposure, alone or in mixture, with some functions being resilient and others being impacted throughout the experiment or only after several weeks of exposure. Taken together, these findings reveal that metal contamination of sediment could impact both the genetic structure and the functional potential of chronically exposed microbial communities. Given their functional role in aquatic ecosystems, it poses an ecological risk as it may impact ecosystem functioning.

Influence of the Chemical Form of Antimony on Soil Microbial Community Structure and Arsenite Oxidation Activity

In the present study, the influence of the co-contamination with various chemical forms of antimony (Sb) with arsenite (As[III]) on soil microbial communities was investigated. The oxidation of As(III) to As(V) was monitored in soil columns amended with As(III) and three different chemical forms of Sb: antimony potassium tartrate (Sb[III]-tar), antimony(III) oxide (Sb₂O₃), and potassium antimonate (Sb[V]). Soil microbial communities were examined qualitatively and quantitatively using 16S rDNA- and arsenite oxidase gene (aioA)-targeted analyses. Microbial As(III) oxidation was detected in all soil columns and 90–100% of added As(III) (200 μmol L⁻¹) was oxidized to As(V) in 9 d, except in the Sb(III)-tar co-amendments that only oxidized 30%. 16S rDNA- and aioA-targeted analyses showed that the presence of different Sb chemical forms significantly affected the selection of distinct As(III)-oxidizing bacterial populations. Most of the 16S rRNA genes detected in soil columns belonged to Betaproteobacteria and Gammaproteobacteria, and some sequences were closely related to those of known As(III) oxidizers. Co-amendments with Sb(III)-tar and high concentrations of Sb₂O₃ significantly increased the ratios of aioA-possessing bacterial populations, indicating the enrichment of As(III) oxidizers resistant to As and Sb toxicity. Under Sb co-amendment conditions, there was no correlation between aioA gene abundance and the rates of As(III) oxidation. Collectively, these results demonstrated that the presence of different Sb chemical forms imposed a strong selective pressure on the soil bacterial community and, thus, the co-existing metalloid is an important factor affecting the redox transformation of arsenic in natural environments.

Genetic diversity and characterization of arsenic-resistant endophytic bacteria isolated from Pteris vittata, an arsenic hyperaccumulator

Background

Alleviating arsenic (As) contamination is a high-priority environmental issue. Hyperaccumulator plants may harbor endophytic bacteria able to detoxify As. Therefore, we investigated the distribution, diversity, As (III) resistance levels, and resistance-related functional genes of arsenite-resistant bacterial endophytes in Pteris vittata L. growing in a lead-zinc mining area with different As contamination levels.

Results

A total of 116 arsenite-resistant bacteria were isolated from roots of P. vittata with different As concentrations. Based on the 16S rRNA gene sequence analysis of representative isolates, the isolates belonged to Proteobacteria, Actinobacteria, and Firmicutes. Major genera found were Agrobacterium, Stenotrophomonas, Pseudomonas, Rhodococcus, and Bacillus. The most highly arsenite-resistant bacteria (minimum inhibitory concentration > 45 mM) were isolated from P. vittata with high As concentrations and belonged to the genera Agrobacterium and Bacillus. The strains with high As tolerance also showed high levels of indole-3-acetic acid (IAA) production and carried arsB/ACR3(2) genes. The arsB and ACR3(2) were most likely horizontally transferred among the strains.

Conclusion

The results of this study suggest that P. vittata plants with high As concentrations may select diverse arsenite-resistant bacteria; this diversity might, at least partly, be a result of horizontal gene transfer. These diverse endophytic bacteria are potential candidates to enhance phytoremediation techniques.

Assessment of arsenic oxidation potential of Microvirga indica S-MI1b sp. nov. in heavy metal polluted environment

Arsenic oxidizing α-proteobacterial strain Microvirga indica S-MI1b sp. nov. was isolated from metal industry soil and has the ability to oxidize 15 mM of arsenite [As(III)] completely in 39 h. The strain S-MI1b resists to different heavy metals and it oxidizes arsenite in presence of Li, Pb, Hg, Sb(III), Cd, Cr(VI), Ni, and exhibited growth inhibitory effect in presence of Hg, Cu, and Cd at higher concentration. The morphology of Microvirga indica S-MI1b changed in presence of heavy metals however there was no accumulation of As(III) in the cells. The study showed that Microvirga indica S-MI1b can oxidize arsenite at broad pH ranges from 4.0 to 9.0 with optimum at pH 7.0. The kinetic studies of arsenite oxidation by strain S-MI1b signified that it has greater affinity towards As(III). The arsenite oxidase activity of cells grown in presence of Li and Cr(VI) supported the cell culture studies. This is first report on biotransformation of arsenite by Microvirga genus and also arsenite oxidation in presence of heavy metals.

Taxonomically-linked growth phenotypes during arsenic stress among arsenic resistant bacteria isolated from soils overlying the Centralia coal seam fire

Arsenic (As), a toxic element, has impacted life since early Earth. Thus, microorganisms have evolved many As resistance and tolerance mechanisms to improve their survival outcomes given As exposure. We isolated As resistant bacteria from Centralia, PA, the site of an underground coal seam fire that has been burning since 1962. From a 57.4°C soil collected from a vent above the fire, we isolated 25 unique aerobic As resistant bacterial strains spanning seven genera. We examined their diversity, resistance gene content, transformation abilities, inhibitory concentrations, and growth phenotypes. Although As concentrations were low at the time of soil collection (2.58 ppm), isolates had high minimum inhibitory concentrations (MICs) of arsenate and arsenite (>300 mM and 20 mM respectively), and most isolates were capable of arsenate reduction. We screened isolates (PCR and sequencing) using 12 published primer sets for six As resistance genes (AsRGs). Genes encoding arsenate reductase (arsC) and arsenite efflux pumps (arsB, ACR3(2)) were present, and phylogenetic incongruence between 16S rRNA genes and AsRGs provided evidence for horizontal gene transfer. A detailed investigation of differences in isolate growth phenotypes across As concentrations (lag time to exponential growth, maximum growth rate, and maximum OD590) showed a relationship with taxonomy, providing information that could help to predict an isolate's performance given As exposure in situ. Our results suggest that microbiological management and remediation of environmental As could be informed by taxonomically-linked As tolerance, potential for resistance gene transferability, and the rare biosphere.

Determination of physiological, taxonomic, and molecular characteristics of a cultivable arsenic-resistant bacterial community

A collection of 219 bacterial arsenic-resistant isolates was constituted from neutral arsenic mine drainage sediments. Isolates were grown aerobically or anaerobically during 21 days on solid DR2A medium using agar or gelan gum as gelling agent, with 7 mM As(III) or 20 mM As(V) as selective pressure. Interestingly, the sum of the different incubation conditions used (arsenic form, gelling agent, oxygen pressure) results in an overall increase of the isolate diversity. Isolated strains mainly belonged to Proteobacteria (63%), Actinobacteria (25%), and Bacteroidetes (10%). The most representative genera were Pseudomonas (20%), Acinetobacter (8%), and Serratia (15%) among the Proteobacteria; Rhodococcus (13%) and Microbacterium (5%) among Actinobacteria; and Flavobacterium (13%) among the Bacteroidetes. Isolates were screened for the presence of arsenic-related genes (arsB, ACR3(1), ACR3(2), aioA, arsM, and arrA). In this way, 106 ACR3(1)-, 74 arsB-, 22 aioA-, 14 ACR3(2)-, and one arsM-positive PCR products were obtained and sequenced. Analysis of isolate sensitivity toward metalloids (arsenite, arsenate, and antimonite) revealed correlations between taxonomy, sensitivity, and genotype. Antimonite sensitivity correlated with the presence of ACR3(1) mainly present in Bacteroidetes and Actinobacteria, and arsenite or antimonite resistance correlated with arsB gene presence. The presence of either aioA gene or several different arsenite carrier genes did not ensure a high level of arsenic resistance in the tested conditions.

Arsenite oxidation by Pseudomonas arsenicoxydans immobilized on zeolite and its potential biotechnological application

Pseudomonas arsenicoxydans has been recently described as a new arsenite oxidizing bacterial species. Arsenite detoxification activity by this species was determined by HPLC/HG/AAS. P. arsenicoxydans showed a high rate of As(III) conversion, particularly when immobilized (it oxidizes 100 % of 500 μg arsenite present in the medium after 48 of incubation). Arsenite oxidizing activity, mediated by a constitutive periplasmic enzyme, was determined following the transfer of reducing equivalents from arsenite to 2,4-dichlorophenolindophenol (DCIP) showing that approximately 75 % (0.173 µmol DCIP min(-1) mg(-1)) of the total activity (0.231 µmol DCIP min(-1) mg(-1)) was detected in the periplasmic fraction. Using PCR with primers specific for arsenite oxidase gene showed the presence of a gene encoding for arsenite oxidase in P. arsenicoxydans. Results show the potential biotechnological application of P. arsenicoxydans as a candidate for detoxification of As(III).

Implementation of zero-valent iron (ZVI) into drinking water supply - role of the ZVI and biological processes

Arsenic in drinking water is concerning millions of people around the world, even though many solutions to the problem have come up in recent years. One of the promising solutions for removing arsenic from water is by implementation of a zero-valent iron (ZVI) in the drinking water production. The purpose of this work was to study a treatment of As pollution based on the ZVI, aeration and sand filtration that was monitored for period of 45 months. In applied configuration and conditions ZVI was not able to remove arsenic alone, but it worked as a source of ferrous ions that during its oxidation enabled to co-precipitate arsenic compounds in the sand filter. The results show that after a lag phase of about 6 months, it was possible to achieve water production with an As content from 20 μg L(-1) to below 5 μg L(-1). The treatment also enabled to remove phosphates that were present in groundwater and affected As uptake by hindering its co-precipitation with Fe compounds. Determination of colony forming units on As amended agar helped to find arsenic resistant bacteria at each stage of treatment and also in the sand filter backwash sludge. Bacterial communities found in groundwater, containing low concentration of As, were found to have high As resistance. The results also indicate that the lag phase might have been also needed to initiate Fe ions release by corrosion from elemental Fe by help of microbial activity.

Removal of arsenic from groundwater by using a native isolated arsenite-oxidizing bacterium

Arsenic (As) contamination of groundwater is a significant public health concern. In this study, the removal of arsenic from groundwater using biological processes was investigated. The efficiency of arsenite (As(III)) bacterial oxidation and subsequent arsenate (As(V)) removal from contaminated groundwater using bacterial biomass was examined. A novel As(III)-oxidizing bacterium (As7325) was isolated from the aquifer in the blackfoot disease (BFD) endemic area in Taiwan. As7325 oxidized 2300μg/l As(III) using in situ As(III)-contaminated groundwater under aerobic conditions within 1d. After the oxidation of As(III) to As(V), As(V) removal was further examined using As7325 cell pellets. The results showed that As(V) could be adsorbed efficiently by lyophilized As7325 cell pellets, the efficiency of which was related to lyophilized cell pellet concentration. Our study conducted the examination of an alternative technology for the removal of As(III) and As(V) from groundwater, indicating that the oxidation of As(III)-contaminated groundwater by native isolated bacterium, followed by As(V) removal using bacterial biomass is a potentially effective technology for the treatment of As(III)-contaminated groundwater.

Influence of inoculation of arsenic-resistant Staphylococcus arlettae on growth and arsenic uptake in Brassica juncea (L.) Czern. Var. R-46

An arsenic hypertolerant bacterium was isolated from arsenic contaminated site of West Bengal, India. The bacteria was identified as Staphylococcus arlettae strain NBRIEAG-6, based on 16S rDNA analysis. S. arlettae was able to remove arsenic from liquid media and possesses arsC gene, gene responsible for arsenate reductase activity. The biochemical profiling of the isolated strain showed that it had the capacity of producing indole acetic acid (IAA), siderophores and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase. Furthermore, an experiment was conducted to test the effect of S. arlettae inoculation on concurrent plant growth promotion and arsenic uptake in Indian mustard plant [Brassica juncea (L.) Czern. Var. R-46] when grown in arsenic spiked (5, 10 and 15 mg kg(-1)) soil. The microbial inoculation significantly (p<0.05) increased biomass, protein, chlorophyll and carotenoids contents in test plant. Moreover, as compared to the non-inoculated control, the As concentration in shoot and root of inoculated plants were increased from 3.73 to 34.16% and 87.35 to 99.93%, respectively. The experimental results show that the plant growth promoting bacteria NBRIEAG-6 has the ability to help B. juncea to accumulate As maximally in plant root, and therefore it can be accounted as a new bacteria for As phytostabilization.

Arsenic-tolerant, arsenite-oxidising bacterial strains in the contaminated soils of West Bengal, India

As biological agents represent an affordable alternative to costly metal decontamination technologies, we isolated arsenic (As) oxidising bacteria from the As-contaminated soils of West Bengal, India. These strains were closely related to various species of Bacillus and Geobacillus based on their 16S rRNA gene sequences. They were found to be hyper-resistant to both As(V) (167-400 mM) and As(III) (16-47 mM). Elevated rates of As(III) oxidation (278-1250 μM h(-1)) and arsenite oxidase activity (2.1-12.5 nM min(-1) mg(-1) protein) were observed in these isolates. Screening identified four strains as superior As-oxidisers. Among them, AMO-10 completely (100%) oxidised 30 mM of As(III) within 24 h. The presence of the aoxB gene was confirmed in the screened isolates. Phylogenetic tree construction based on the aoxB sequence revealed that two strains, AGO-S5 and AGH-02, clustered with Achromobacter and Variovorax, whereas the other two (AMO-10 and ADP-25) remained unclustered. The increased rate of As(III) oxidation by these native strains might be exploited for the remediation of As in contaminated environments. Notably, this study presents the first correlation regarding the presence of the aoxB gene and As(III) oxidation ability in Geobacillus stearothermophilus.

Characterization and transcription of arsenic respiration and resistance genes during in situ uranium bioremediation

The possibility of arsenic release and the potential role of Geobacter in arsenic biogeochemistry during in situ uranium bioremediation was investigated because increased availability of organic matter has been associated with substantial releases of arsenic in other subsurface environments. In a field experiment conducted at the Rifle, CO study site, groundwater arsenic concentrations increased when acetate was added. The number of transcripts from arrA, which codes for the α-subunit of dissimilatory As(V) reductase, and acr3, which codes for the arsenic pump protein Acr3, were determined with quantitative reverse transcription-PCR. Most of the arrA (>60%) and acr3-1 (>90%) sequences that were recovered were most similar to Geobacter species, while the majority of acr3-2 (>50%) sequences were most closely related to Rhodoferax ferrireducens. Analysis of transcript abundance demonstrated that transcription of acr3-1 by the subsurface Geobacter community was correlated with arsenic concentrations in the groundwater. In contrast, Geobacter arrA transcript numbers lagged behind the major arsenic release and remained high even after arsenic concentrations declined. This suggested that factors other than As(V) availability regulated the transcription of arrA in situ, even though the presence of As(V) increased the transcription of arrA in cultures of Geobacter lovleyi, which was capable of As(V) reduction. These results demonstrate that subsurface Geobacter species can tightly regulate their physiological response to changes in groundwater arsenic concentrations. The transcriptomic approach developed here should be useful for the study of a diversity of other environments in which Geobacter species are considered to have an important influence on arsenic biogeochemistry.

Microbial interactions in the arsenic cycle: adoptive strategies and applications in environmental management

Arsenic (As) is a nonessential element that is often present in plants and in other organisms. However, it is one of the most hazardous of toxic elements globally. In many parts of the world, arsenic contamination in groundwater is a serious and continuing threat to human health. Microbes play an important role in regulating the environmental fate of arsenic. Different microbial processes influence the biogeochemical cycling of arsenic in ways that affect the accumulation of different arsenic species in various ecosystem compartments. For example, in soil, there are bacteria that methylate arsenite to trimethylarsine gas, thereby releasing arsenic to the atmosphere.In marine ecosystems, microbes exist that can convert inorganic arsenicals to organic arsenicals (e.g., di- and tri-methylated arsenic derivatives, arsenocholine,arsenobetaine, arsenosugars, arsenolipids). The organo arsenicals are further metabolized to complete the arsenic cycle.Microbes have developed various strategies that enable them to tolerate arsenic and to survive in arsenic-rich environments. Such strategies include As exclusion from cells by establishing permeability barrier, intra- and extracellular sequestration,active efflux pumps, enzymatic reduction, and reduction in the sensitivity of cellular targets. These strategies are used either singly or in combination. In bacteria,the genes for arsenic resistance/detoxification are encoded by the arsenic resistance operons (ars operon).In this review, we have addressed and emphasized the impact of different microbial processes (e.g., arsenite oxidation, cytoplasmic arsenate reduction, respiratory arsenate reduction, arsenite methylation) on the arsenic cycle. Microbes are the only life forms reported to exist in heavy arsenic-contaminated environments. Therefore,an understanding of the strategies adopted by microbes to cope with arsenic stress is important in managing such arsenic-contaminated sites. Further future insights into the different microbial genes/proteins that are involved in arsenic resistance may also be useful for developing arsenic resistant crop plants.

The exposition of a calcareous Mediterranean soil to toxic concentrations of Cr, Cd and Pb produces changes in the microbiota mainly related to differential metal bioavailability

The involvement of the bacterial community of an agricultural Mediterranean calcareous soil in relation to several heavy metals has been studied in microcosms under controlled laboratory conditions. Soil samples were artificially polluted with Cr(VI), Cd(II) and Pb(II) at concentrations ranging from 0.1 to 5000 mg kg(-1) and incubated along 28 d. The lowest concentrations with significant effects in soil respirometry were 10 mg kg(-1) Cr and 1000 mg kg(-1) Cd and Pb. However, only treatments showing more than 40% inhibition of respirometric activity led to significant changes in bacterial composition, as indicated by PCR-DGGE analyses. Presumable Cr- and Cd-resistant bacteria were detected in polluted microcosms, but development of the microbiota was severely impaired at the highest amendments of both metals. Results also showed that bioavailability is an important factor determining the impact of the heavy metals assayed, and even an inverted potential toxicity ranking could be achieved if their soluble fraction is considered instead of the total concentration. Moreover, multiresistant bacteria were isolated from Cr-polluted soil microcosms, some of them showing the capacity to reduce Cr(VI) concentrations between 26% and 84% of the initial value. Potentially useful strains for bioremediation were related to Arthrobacter crystallopoietes, Stenotrophomonas maltophilia and several species of Bacillus.

Indigenous soil bacteria with the combined potential for hydrocarbon consumption and heavy metal resistance

INTRODUCTION

Transconjugant bacteria with combined potential for hydrocarbon utilization and heavy metal resistance were suggested by earlier investigators for bioremediation of soils co-contaminated with hydrocarbons and heavy metals. The purpose of this study was to offer evidence that such microorganisms are already part of the indigenous soil microflora.

METHODS

Microorganisms in pristine and oily soils were counted on nutrient agar and a mineral medium with oil as a sole carbon source, in the absence and presence of either sodium arsenate (As V), sodium arsenite (As III) or cadmium sulfate, and characterized via 16S rRNA gene sequencing. The hydrocarbon-consumption potential of individual strains in the presence and absence of heavy metal salts was measured.

RESULTS

Pristine and oil-contaminated soil samples harbored indigenous bacteria with the combined potential for hydrocarbon utilization and As and Cd resistance in numbers up to 4 × 10⁵ CFU g⁻¹. Unicellular bacteria were affiliated to the following species arranged in decreasing order of predominance: Bacillus subtilis, Corynebacterium pseudotuberculosis, Brevibacterium linens, Alcaligenes faecalis, Enterobacter aerogenes, and Chromobacterium orangum. Filamentous forms were affiliated to Nocardia corallina, Streptomyces flavovirens, Micromonospora chalcea, and Nocardia paraffinea. All these isolates could grow on a wide range of pure aliphatic and aromatic hydrocarbons, as sole sources of carbon and energy, and could consume oil and pure hydrocarbons in batch cultures. Low As concentrations, and to a lesser extent Cd concentrations, enhanced the hydrocarbon-consumption potential by the individual isolates.

CONCLUSION

There is no need for molecularly designing microorganisms with the combined potential for hydrocarbon utilization and heavy metal resistance, because they are already a part of the indigenous soil microflora.

Isolation and characterization of aerobic culturable arsenic-resistant bacteria from surfacewater and groundwater of Rautahat District, Nepal

Arsenic (As) contamination of groundwater is a serious Environmental Health Management issue of drinking water sources especially in Terai region of Nepal. Many studies have reported that due to natural abundance of arsenic in the environment, various bacteria have developed different resistance mechanisms for arsenic compound. In this study, the culturable arsenic-resistant bacteria indigenous to surfacewater as well as groundwater from Rautahat District of Nepal were randomly isolated by standard plate count method on the basis of viable growth on plate count agar amended with arsenate ranging from 0, 0.5, 10, 40, 80 to 160 milligram per liter (mg/l). With respect to the morphological and biochemical tests, nine morphologically distinct potent arsenate tolerant bacteria showed relatedness with Micrococcus varians, Micrococcus roseus, Micrococcus luteus, Pseudomonas maltophilia, Pseudomonas sp., Vibrio parahaemolyticus, Bacillus cereus, Bacillus smithii 1 and Bacillus smithii 2. The isolates were capable of tolerating more than 1000 mg/l of arsenate and 749 mg/l of arsenite. Likewise, bioaccumulation capability was highest with M. roseus (85.61%) and the least with B. smithii (47.88%) indicating the potential of the organisms in arsenic resistance and most probably in bioremediation.

Molecular identification of arsenic-resistant estuarine bacteria and characterization of their ars genotype

In the present study, 44 arsenic-resistant bacteria were isolated through serial dilutions on agar plate with concentrations ≥0.05 mM of sodium arsenite and ≥10 mM of sodium arsenate from Mandovi and Zuari--estuarine water systems. The ars genotype characterization in 36 bacterial isolates (resistant to 100 mM of sodium arsenate) revealed that only 17 isolates harboured the arsA (ATPase), B (arsenite permease) and C (arsenate reductase) genes on the plasmid DNA. The arsA, B and C genes were individually detected using PCR in 16, 9 and 13 bacterial isolates respectively. Molecular identification of the 17 isolates bearing the ars genotype was carried using 16S rDNA sequencing. A 1300 bp full length arsB gene encoding arsenite efflux pump and a 409 bp fragment of arsC gene coding for arsenate reductase were isolated from the genera Halomonas and Acinetobacter. Phylogenetic analysis of arsB and arsC genes indicated their close genetic relationship with plasmid borne ars genes of E. coli and arsenate reductase of plant origin. The putative arsenate reductase gene isolated from Acinetobacter species complemented arsenate resistance in E. coli WC3110 and JM109 validating its function. This study dealing with isolation of native arsenic-resistant bacteria and characterization of their ars genes might be useful to develop efficient arsenic detoxification strategies for arsenic contaminated aquifers.

Diversity and community structure of culturable arsenic-resistant bacteria across a soil arsenic gradient at an abandoned tungsten-tin mining area

We studied the bacterial diversity at a single location (the Terrubias mine; Salamanca province, Spain) with a gradient of soil As contamination to test if increasing levels of As would (1) change the preponderant groups of arsenic-resistant bacteria and (2) increase the tolerance thresholds to arsenite [As(III)] and arsenate [As(V)] of such bacteria. We studied the genetic and taxonomic diversity of culturable arsenic-resistant bacteria by PCR fingerprinting techniques and 16S rRNA gene sequencing. Then, the tolerance thresholds to As(III) and As(V) were determined for representative strains and mathematically analyzed to determine relationships between tolerances to As(III) and As(V), as well as these tolerances with the soil contamination level. The diversity of the bacterial community was, as expected, inversely related to the soil As content. The overall preponderant arsenic-resistant bacteria were Firmicutes (mainly Bacillus spp.) followed by γ-Proteobacteria (mainly Pseudomonas spp.), with increasing relative frequencies of the former as the soil arsenic concentration increased. Moreover, a strain of the species Rahnella aquatilis (γ-Proteobacteria class) exhibited strong endurance to arsenic, being described for the first time in literature such a phenotype within this bacterial species. Tolerances of the isolates to As(III) and As(V) were correlated but not with their origin (soil contamination level). Most of the strains (64%) showed relatively low tolerances to As(III) and As(V), but the second most numerous group of isolates (19%) showed increased tolerance to As(III) rather than to As(V), even though the As(V) anion is the prevalent arsenic species in soil solution at this location. To our knowledge, this is the first study to report a shift towards preponderance of Gram-positive bacteria (Firmicutes) related to high concentrations of soil arsenic. It was also shown that, under aerobic conditions, strains with relatively enhanced tolerance to As(III) predominated over the most As(V)-tolerant ones.

Arsenic mobilization by epilithic bacterial communities associated with volcanic rocks from Camarones River, Atacama Desert, northern Chile

The arsenic biogeochemical cycle is greatly dependent on microbial transformations that affect both the distribution and mobility of arsenic species in the environment. In this study, a microbial biofilm from volcanic rocks was characterized on the basis of its bacterial composition and ability to mobilize arsenic under circumneutral pH. Biofilm microstructure was analyzed by scanning electron microscopy (SEM)-energy-dispersive spectroscopy (EDS). Strains were isolated from biofilms and identified by 16S rDNA sequences analysis. Arsenic oxidation and reduction capacity was assayed with high-performance liquid chromatography coupled to gaseous formation performing the detection by atomic absortion in a quartz bucket (HPLC/HG/QAAS), and polymerase chain reaction was used to detect aox and ars genes. Bacterial communities associated with volcanic rocks were studied by denaturing gradient gel electrophoresis (DGGE). The SEM-EDS studies showed the presence of biofilm after 45 days of incubation. The relative closest GenBank matches of the DNA sequences, of isolated arsenic-resistant strains, showed the existence of four different genus: Burkholderia, Pseudomonas, Erwinia, and Pantoea. Four arsenite-resistant strains were isolates, and only three strains were able to oxidize >97% of the As(III) present (500 uM). All arsenate-resistant isolates were able to reduce between 69 and 86% of total As(V) (1000 uM). Analysis of 16S rDNA sequences obtained by DGGE showed the presence of four bacterial groups (∝-proteobacteria, γ-proteobacteria, Firmicutes, and Actinobacteria). Experiments demonstrate that epilithic bacterial communities play a key role in the mobilization of arsenic and metalloids speciation.

Arsenite-oxidizing and arsenate-reducing bacteria associated with arsenic-rich groundwater in Taiwan

Drinking highly arsenic-contaminated groundwater is a likely cause of blackfoot disease in Taiwan, but microorganisms that potentially control arsenic mobility in the subsurface remain unstudied. The objective of this study was to investigate the relevant arsenite-oxidizing and arsenate-reducing microbial community that exists in highly arsenic-contaminated groundwater in Taiwan. We cultured and identified arsenic-transforming bacteria, analyzed arsenic resistance and transformation, and determined the presence of genetic markers for arsenic transformation. In total, 11 arsenic-transforming bacterial strains with different colony morphologies and varying arsenic transformation abilities were isolated, including 10 facultative anaerobic arsenate-reducing bacteria and one strictly aerobic arsenite-oxidizing bacterium. All of the isolates exhibited high levels of arsenic resistance with minimum inhibitory concentrations of arsenic ranging from 2 to 200 mM. Strain AR-11 was able to rapidly oxidize arsenite to arsenate at concentrations relevant to environmental groundwater samples without the addition of any electron donors or acceptors. We provide evidence that arsenic-reduction activity may be conferred by the ars operon(s) that were not amplified by the designed primers currently in use. The 16S rRNA sequence analysis grouped the isolates into the following genera: Pseudomonas, Bacillus, Psychrobacter, Vibrio, Citrobacter, Enterobacter, and Bosea. Among these genera, we present the first report of the genus Psychrobacter being involved in arsenic reduction. Our results further support the hypothesis that bacteria capable of either oxidizing arsenite or reducing arsenate coexist and are ubiquitous in arsenic-contaminated groundwater.

Antibiotic susceptibility and arsenic tolerance of urinary bacteria isolated from arsenic-exposed people in Nepal

Community-based information on antibiotic susceptibility of urinary bacteria is useful in clinical practice, including empiric therapy. Owing to evidence of coselection of metal and antibiotic resistance, there is growing concern on testing of such selective pressure in clinical as well as environmental bacterial isolates. We examined arsenic tolerance and antibiotic resistance in urinary isolates and their possible coselection among arsenic-exposed subjects. Urinary arsenic levels were assessed by atomic absorption spectrophotometer. Antibiotic resistance and arsenic tolerance of urinary bacteria were observed by modified Kirby-Bauer and minimum inhibitory concentration methods, respectively. The percentage of one, two, and multidrug-resistant urinary isolates were 30.4%, 37%, and 30.4%, respectively. Isolates showed variable tolerance to arsenic species. Gram-negative isolates were more tolerant to arsenic species than Gram positive. Although statistically insignificant, arsenic tolerance tended to increase from total susceptible to two-drug resistance. However, multiple drug resistance was not induced by the urinary arsenic (p>0.05). We observed moderately positive correlation between urinary arsenic level to arsenic tolerance of isolates (p<0.05). Although tolerance significantly correlated to urinary arsenic level, coselection/coresistance of arsenic to the antibiotic resistance in urinary isolates is inconclusive and remains to be further elucidated.

Effect of acute and chronic arsenic exposure on growth, structure and virulence of Aeromonas hydrophila isolated from fish

Aeromonas hydrophila being a ubiquitous bacterium is prone to arsenic exposure. The present study was designed to determine the role of arsenic on growth and virulence of A. hydrophila. Exposure to arsenic (1 mg L(-1) and 2 mg L(-1)) had no effect on growth but significantly inhibited the hemolytic and cytotoxic potential of exposed bacteria. Transmission electron microscopy revealed loss of membrane integrity and presence of condensed cytoplasm suggestive of acute stress in bacteria exposed to arsenic. Arsenic-adapted bacteria were developed by repeated sub-culturing in presence of arsenic. Arsenic-adaptation led to significant recovery in hemolytic and cytotoxic potential. The arsenic-adapted bacteria exhibited normal membrane integrity, decreased cytoplasmic condensation and possessed scattered polysome like structures in the cytoplasm. A positive correlation was observed between arsenic tolerance and resistance to several antimicrobials. Arsenic-adaptation failed to confer cross-protection to mercury and cadmium stress. SDS-PAGE analysis revealed the expression of two new proteins of approximately 85 kDa and 79 kDa respectively in arsenic-adapted A. hydrophila. Plasmid-curing and transformation studies clearly indicate plasmid has no role on arsenic resistance trait of the bacteria. Our study, for the first time, reports a structure and function relationship of xenobiotics on bacteria.

Pentavalent arsenate reductase activity in cytosolic fractions of Pseudomonas sp., isolated from arsenic-contaminated sites of Tezpur, Assam

Pentavalent arsenate reductase activity was localized and characterized in vitro in the cytosolic fraction of a newly isolated bacterial strain from arsenic-contaminated sites. The bacterium was gram negative, rod-shaped, nonmotile, non-spore-forming, and noncapsulated, and the strain was identified as Pseudomonas sp. DRBS1 following biochemical and molecular approaches. The strain Pseudomonas sp. DRBS1 exhibited enzymatic machinery for reduction of arsenate(V) to arsenite(III). The suspended culture of the bacterium reduced more than 97% of As(V) (40-100 mM) to As(III) in 48 h. The growth rate and total cellular yield decreased in the presence of higher concentration of arsenate. The suspended culture repeatedly reduced 10 mM As(V) within 5 h up to five consecutive inputs. The cell-free extracts reduced 86% of 100 microM As(V) in 40 min. The specific activity of arsenate reductase enzyme in the presence of 100 microM arsenate is 6.68 micromol/min per milligram protein. The arsenate reductase activity is maximum at 30 degrees C and at pH 5.2. The arsenate reductase activity increased in the presence of electron donors like citrate, glucose, and galactose and metal ions like Cd(+2), Cu(+2), Ca(+2), and Fe(+2). Selenate as an electron donor also supports the growth of strain DRBS1 and significantly increased the arsenate reduction.

Arsenic resistant bacteria isolated from arsenic contaminated river in the Atacama Desert (Chile)

In this study, arsenic resistant bacteria were isolated from sediments of an arsenic contaminated river. Arsenic tolerance of bacteria isolated was carried out by serial dilution on agar plate. Redox abilities were investigated using KMnO4. arsC and aox genes were detected by PCR and RT-PCR, respectively. Bacterial populations were identified by RapID system. Forty nine bacterial strains were isolated, of these, 55 % corresponded to the reducing bacteria, 4% to oxidizing bacteria, 8% presented both activities and in 33% of the bacteria none activity was detected. arsC gene was detected in 11 strains and aox genes were not detected. The activity of arsenic transforming microorganisms in river sediment has significant implications for the behavior of the metalloid.

Diversity of arsenate reductase genes (arsC Genes) from arsenic-resistant environmental isolates of E. coli

A polymerase chain reaction (PCR) approach was used to assess the occurrence and diversity of arsenate reductase gene (arsC gene) in arsenic-resistant environmental E. coli strains. For this purpose, two different sets of primers were designed for the specific amplification of approximately 370-bp fragments from the arsC gene. These primers were used to screen a collection of 25 environmental arsenic-resistant strains isolated from different geographical regions of India, as well as Bangladesh. The PCR results showed that 17 out of the 25 environmental isolates (68%) contained a gene related to the arsC family. Phylogenetic analysis of the protein sequences deduced from the amplicons indicated a prevalence of arsC genes in the isolated strains. A significant divergence in the DNA sequence was found in the arsC genes among As-resistant environmental E. coli strains from this study, and arsenic resistance, a genetic character, arose from a common ancestral background.