Testing for bacterial resistance to arsenic in monitoring well water by the direct viable counting method.

Direct viable counting of metal-resistant bacteria (DVCMR) has been found to be useful in both enumerating and differentiating metal-resistant and metal-sensitive strains of bacteria. The DVCMR bioassay was used to detect effects of low and high concentrations of arsenic and arsenicals on bacterial populations in groundwater. The level of resistance of the bacterial populations to arsenate was determined by the DVCMR bioassay, and the results showed a linear correlation with the total arsenic concentrations in the monitoring well water samples; no correlation was observed by culture methods with the methods employed. Bacteria resistant to 2,000 micrograms of arsenate per ml were isolated from all monitoring well water samples studied. Strains showed similar antibiotic and heavy-metal profiles, suggesting that the arsenic was not a highly selective pressure for arsenic alone. The monitoring well water samples were amended with arsenate and nutrients to determine the biotransformation mechanisms involved. Preliminary results suggest that bacteria indigenous to the monitoring well water samples did not directly transform, i.e., precipitate or volatilize, dissolved arsenic. It was concluded that arsenic contamination of the groundwater can be monitored by the DVCMR bioassay.     

Arsenic resistance and removal by marine and non-marine bacteria

Arsenic resistance and removal was evaluated in nine bacterial strains of marine and non-marine origins. Of the strains tested, Marinomonas communis exhibited the second-highest arsenic resistance with median effective concentration (EC(50)) value of 510 mg As l(-1), and was capable of removing arsenic from culture medium amended with arsenate. Arsenic accumulation in cells amounted to 2290 microg As g(-1) (dry weight) when incubated on medium containing 5 mg As l(-1) of arsenate. More than half of the arsenic removed was related to metabolic activity: 45% of the arsenic was incorporated into the cytosol fraction and 10% was found in the lipid-bound fraction of the membrane, with the remaining arsenic considered to be adsorbed onto the cell surface. Potential arsenic resistance and removal were also examined in six marine and non-marine environmental water samples. Of the total bacterial colony counts, 28-100% of bacteria showed arsenic resistance. Some of the bacterial consortia, especially those from seawater enriched with arsenate, exhibited higher accumulated levels of arsenic than M. communis under the same condition. These results showed that arsenic resistant and/or accumulating bacteria are widespread in the aquatic environment, and that arsenic-accumulating bacteria such as M. communis are potential candidates for bioremediation of arsenic contaminated water.     

Characterization of arsenite-oxidizing bacteria to decipher their role in arsenic bioremediation

High arsenic groundwater contamination causes serious health risks in many developing countries, particularly in India and Bangladesh. The arsenic fluxes in aquifers are primarily controlled by bacterial populations through biogeochemical cycle. In this present study, two gram-positive rod-shaped bacteria were isolated from shallow aquifers of Bhojpur district in Bihar during the early winter season, able to withstand arsenite (As³⁺) concentration upto 70?mM and 1000?mM of arsenate (As⁵⁺) concentration. They showed high resistance to heavy metals up to 30?mM and utilized some complex sugars along with different carbon sources. Growth at wide range of temperature, pH and salinity were observed. Both these isolates showed high efficiency in converting As³⁺ into less toxic concentrations of As⁵⁺ respectively from arsenic enriched culture media. Along with superior arsenic transformation and arsenic resistance abilities, the isolates showed a wide variety of metabolic capacity in terms of utilizing a variety of carbon sources under aerobic conditions, respectively. This study reports the potential As³⁺-oxidizing bacteria that can play an important role in subsurface arsenic transformation that will aid in designing future bioremediation strategy for the arsenic affected areas.     

Isolation and Characterization of Arsenate-Reducing Bacteria from Arsenic-Contaminated Sites in New Zealand

Two environmental sites in New Zealand were sampled (e.g., water and sediment) for bacterial isolates that could use either arsenite as an electron donor or arsenate as an electron acceptor under aerobic and anaerobic growth conditions, respectively. These two sites were subjected to widespread arsenic contamination from mine tailings generated from historic gold mining activities or from geothermal effluent. No bacteria were isolated from these sites that could utilize arsenite or arsenate under the respective growth conditions tested, but a number of chemoheterotrophic bacteria were isolated that could grow in the presence of high concentrations of arsenic species. In total, 17 morphologically distinct arsenic-resistant heterotrophic bacteria isolates were enriched from the sediment samples, and analysis of the 16S rRNA gene sequence of these bacteria revealed them to be members of the genera Exiguobacterium, Aeromonas, Bacillus, Pseudomonas, Escherichia, and Acinetobacter. Two isolates, Exiguobacterium sp. WK6 and Aeromonas sp. CA1, were of particular interest because they appeared to gain metabolic energy from arsenate under aerobic growth conditions, as demonstrated by an increase in cellular growth yield and growth rate in the presence of arsenate. Both bacteria were capable of reducing arsenate to arsenite via a non-respiratory mechanism. Strain WK6 was positive for arsB, but the pathway of arsenate reduction for isolate CA1 was via a hitherto unknown mechanism. These isolates were not gaining an energetic advantage from arsenate or arsenite utilization, but were instead detoxifying arsenate to arsenite. As a subsidiary process to arsenate reduction, the external pH of the growth medium increased (i.e., became more alkaline), allowing these bacteria to grow for extended periods of time.     

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

Arsenic is a toxic element that exists in two major inorganic forms, arsenate and arsenite. A number of bacteria have been shown to resist arsenic exposure, and even more bacteria appear to possess the genes for arsenic resistance. In this study, the numbers of culturable arsenate-resistant bacteria present in water at three coastal sites in the Lake Pontchartrain estuary, Louisiana, was determined. Despite insignificant (less than 1.33 μM) levels of arsenic in this system, 20–50% of the viable count of bacteria showed appreciable arsenate resistance, suggesting that arsenic-resistant bacteria are common and widespread. A diverse array of arsenate-resistant isolates was obtained, with 16S rRNA sequence analysis indicating 37 different bacterial strains, representing six major bacterial groups. Many of these isolates were affiliated with groups of bacteria that have been poorly characterized in terms of arsenic resistance, such as the Betaproteobacteria or Flavobacteria. Some isolates were capable of tolerating very high (>100 mM) levels of arsenate, although arsenite resistance was generally much lower. The results suggest that arsenic-resistant bacteria are common, even in environments with insignificant arsenic contamination, and that many different groups of aquatic bacteria show appreciable arsenic resistance.     

Effect of Arsenic on Bacterial Growth and Plasma Membrane ATPase Activity

The effects of arsenic in the forms of arsenite and arsenate on bacterial growth and plasma membranes' ATPase activity was studied. Correlation of the rate of ATP hydrolysis was found to be correlated with bacterial resistance to toxic arsenic ions. Detoxification of arsenate by resistant cultures of bacteria was suggested to be related to an increase in bacterial ATPase activity and the degree of ATPase mobilization.     

Arsenic uptake and accumulation in rice (Oryza sativa L.) irrigated with contaminated water

Long-term use of arsenic contaminated groundwater to irrigate crops, especially paddy rice (Oryza sativaL.) has resulted in elevated soil arsenic levels in Bangladesh. There is, therefore, concern regarding accumulation of arsenic in rice grown on these soils. A greenhouse pot experiment was conducted to evaluate the impact of arsenic-contaminated irrigation water on the growth and uptake of arsenic into rice grain, husk, straw and root. There were altogether 10 treatments which were a combination of five arsenate irrigation water concentrations (0–8 mg As l^–1) and two soil phosphate amendments. Use of arsenate containing irrigation water reduced plant height, decreased rice yield and affected development of root growth. Arsenic concentrations in all plant parts increased with increasing arsenate concentration in irrigation water. However, arsenic concentration in rice grain did not exceed the maximum permissible limit of 1.0 mg As kg^–1. Arsenic accumulation in rice straw at very high levels indicates that feeding cattle with such contaminated straw could be a direct threat for their health and also, indirectly, to human health via presumably contaminated bovine meat and milk. Phosphate application neither showed any significant difference in plant growth and development, nor in As concentrations in plant parts.     

Bacterial Communities in Bangladesh Aquifers Differing in Aqueous Arsenic Concentration

At Titas, Bangladesh, two aquifers of different arsenic concentrations and redox conditions were investigated to link variations in geochemistry to in situ bacterial diversity characterized by T-RFLP (terminal restriction fragment length polymorphism) and clone library analysis. While the shallow aquifer was characterized by reduced gray sediments with a higher share of easily mobilized sedimentary arsenic (2.6% was easily mobilized from 18 mg/kg of total arsenic available in sediments) and higher aqueous arsenic concentrations of 120 ± 6 μg/L (45% arsenite), the deeper aquifer consisted of brown oxidized sediments with lower aqueous arsenic concentrations, predominantly as arsenate (60 ± 6 μg/L; 3% arsenite) and a higher share of tightly bound arsenic (only 0.6% of 53 mg/kg total sorbed arsenic was easily mobilized). The bacterial communities of both aquifers were dominated by putative aerobic or denitrifying populations of Pseudomonas, Elizabethkingia and Pantoea. The shallow aquifer was more diverse in bacterial populations of aerobic, facultative and anaerobic bacteria, an observation which may be correlated to more variable geochemical conditions resulting in arsenic mobilization and re-sorption. The deeper aquifer showed higher abundance of aerobic bacterial populations including the presence of iron-oxidizing Sideroxydans possibly of importance for the sorption of arsenic on oxidized iron hydroxides. From the arsenic-affected shallow aquifer, As(III) oxidizing isolates of Comamonas and Microbacterium were obtained, which may provide information on suitable conditions for arsenic immobilization useful for future bioremediation efforts. Supplemental materials are available for this article. Go to the publisher’s online edition of Geomicrobiology Journal to view the free supplemental file.     

Exploring the application of biostimulation strategy for bacteria in the bioremediation of industrial effluent

The purpose of this study was to isolate and characterise toxic element-resistant bacteria from acid mine drainage water and to apply them in the bioremediation of industrial effluent, as well as to identify optimal effluent:nutrient concentration for onsite biostimulation strategy. Wastewater samples were collected from acid mine drainage and industry. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was employed for elemental composition analysis. Isolated bacterial strains were characterised using molecular methods. Bioremediation assays were employed to determine the extent of bacterial tolerance and removal of toxic elements using a biostimulation strategy employing minimal salt medium (MSM) at varied concentrations and positive and negative controls of only MSM and industrial effluent, respectively. Two bacterial strains demonstrated resistance to toxic elements, Bacillus sp. MGI101 and Lysinibacillus sp. MGI102 both isolated from the AMD sites. However, no observable growth of toxic metal-resistant bacteria was obtained from the industrial effluents. Bacterial strains MGI101 and MGI102 demonstrated high resistance to target toxic elements during the screening and tolerance assays. Remarkably, Bacillus sp. MGI101 demonstrated greater ability to remove toxic elements including arsenic, chromium, zinc, copper and aluminium in undiluted solutions of the industrial effluent, with its highest removal capacity observed at > 60% for arsenic and aluminium. Both Bacillus sp.MGI101 and Lysinibacillus sp. MGI102 demonstrated varied abilities for the removal of toxic elements from dilution concentration of effluent mixed with MSM. However, the optimal dilution ratio observed in this experiment was 5:15 (effluent:MSM). Overall results demonstrated that isolated bacterial strains have the potential to be employed in bioremediation programmes of acid mine drainages and multi-element contaminated water.

     

Effect of arsenic on bacterial growth and plasma membrane atpase activity

The effects of arsenic in the forms of arsenite and arsenate on bacterial growth and plasma membranes ATPase activity of was studied. Correlation of The rate of ATP hydrolysis was found to be correlated with bacterial resistance to toxic arsenic ions. Detoxification of arsenate by resistant cultures of bacteria was suggested to be related with an increase in bacterial ATPase activity and the degree of ATPase mobilization.     

Molecular Characterization of the Total Bacteria and Dissimilatory Arsenate-Reducing Bacteria in Core Sediments of the Jianghan Plain,Central China

Arsenic contamination in groundwater has been reported in the Jianghan Plain of China since 2005, yet little is known about the microbial communities involved in As mobilization in this area, especially the dissimilatory arsenate-reducing bacteria (DARB) communities. Here, we conducted a cultivation-independent investigation on core sediments collected from a region with arsenic-contaminated groundwater in the Jianghan Plain to reveal the total bacteria and DARB community structures. Highly diverse As-resistant bacteria communities were found from sediment samples via high-throughput sequencing of 16S rRNA genes. Notably, we identified 27 unique arrA gene (encoding the alpha subunit of dissimilatory arsenate reductase) phylotypes, none of which was related to any previously described arrA gene sequence. This suggests a novel and unique DARB community in the sediments of the Jianghan Plain and expands our knowledge about the distribution and diversity of this group of bacteria in natural environments. Moreover, RDA and CCA demonstrated that total bacterial communities and specific functional groups are controlled by different environmental factors. Specifically, sediment pH, NH₄⁺, total nitrogen, total Fe, total organic carbon and total phosphorus were the key factors driving total bacterial community compositions, while As significantly shaped DARB community structures. This report is the first to describe DARB communities and their correlation with environmental factors in Jianghan Plain sediments, which could give us clues about the origin of the arsenic contamination of groundwater in this region.     

On line biomonitors used as a tool for toxicity reduction evaluation of in situ groundwater remediation techniques

Success of groundwater remediation is typically controlled via snapshot analysis of selected chemical substances or physical parameters. Biological parameters, i.e. ecotoxicological assays, are rarely employed. Hence the aim of the study was to develop a bioassay tool, which allows an on line monitoring of contaminated groundwater, as well as a toxicity reduction evaluation (TRE) of different remediation techniques in parallel and may furthermore be used as an additional tool for process control to supervise remediation techniques in a real time mode. Parallel testing of groundwater remediation techniques was accomplished for short and long time periods, by using the energy dependent luminescence of the bacterium Vibrio fischeri as biological monitoring parameter. One data point every hour for each remediation technique was generated by an automated biomonitor. The bacteria proved to be highly sensitive to the contaminated groundwater and the biomonitor showed a long standing time despite the highly corrosive groundwater present in Bitterfeld, Germany. The bacterial biomonitor is demonstrated to be a valuable tool for remediation success evaluation. Dose response relationships were generated for the six quantitatively dominant groundwater contaminants (2-chlortoluene, 1,2- and 1,4-dichlorobenzene, monochlorobenzene, ethylenbenzene and benzene). The concentrations of individual volatile organic chemicals (VOCs) could not explain the observed effects in the bacteria. An expected mixture toxicity was calculated for the six components using the concept of concentration addition. The calculated EC₅₀ for the mixture was still one order of magnitude lower than the observed EC₅₀ of the actual groundwater. The results pointed out that chemical analysis of the six most quantitative substances alone was not able to explain the effects observed with the bacteria. Thus chemical analysis alone may not be an adequate tool for remediation success evaluation in terms of toxicity reduction.     

Arsenic(V) Reduction in Relation to Iron(III) Transformation and Molecular Characterization of the Structural and Functional Microbial Community in Sediments of a Basin-Fill Aquifer in Northern Utah

Basin-fill aquifers of the Southwestern United States are associated with elevated concentrations of arsenic (As) in groundwater. Many private domestic wells in the Cache Valley Basin, UT, have As concentrations in excess of the U.S. EPA drinking water limit. Thirteen sediment cores were collected from the center of the valley at the depth of the shallow groundwater and were sectioned into layers based on redoxmorphic features. Three of the layers, two from redox transition zones and one from a depletion zone, were used to establish microcosms. Microcosms were treated with groundwater (GW) or groundwater plus glucose (GW+G) to investigate the extent of As reduction in relation to iron (Fe) transformation and characterize the microbial community structure and function by sequencing 16S rRNA and arsenate dissimilatory reductase (arrA) genes. Under the carbon-limited conditions of the GW treatment, As reduction was independent of Fe reduction, despite the abundance of sequences related to Geobacter and Shewanella, genera that include a variety of dissimilatory iron-reducing bacteria. The addition of glucose, an electron donor and carbon source, caused substantial shifts toward domination of the bacterial community by Clostridium-related organisms, and As reduction was correlated with Fe reduction for the sediments from the redox transition zone. The arrA gene sequencing from microcosms at day 54 of incubation showed the presence of 14 unique phylotypes, none of which were related to any previously described arrA gene sequence, suggesting a unique community of dissimilatory arsenate-respiring bacteria in the Cache Valley Basin.     

Identification and quantification of arsC genes in environmental samples by using real-time PCR

The arsC gene is responsible for the first step in arsenate biotransformation encoding the enzyme arsenate reductase. The quantitative real-time PCR method was developed to quantify the abundance of the arsC genes in environmental samples contaminated with arsenic. Two sets of primers that showed high specificity for the target arsC gene were designed based on consensus sequences from 13 bacterial species. The arsC gene was used as an external standard instead of total DNA in the calibration curve for real-time PCR, which was linear over six orders of magnitude and the detection limit was estimated to be about three copies of the gene. Soil samples from arsenic contaminated sites were screened for arsC genes by using PCR and showed the presence of this gene. The copy numbers of the gene ranging from 0.88 x 10(4) to 1.56 x 10(5) per ng total DNA were found in eight arsenic contaminated samples. Soil samples from a bioreactor containing pulp mill biomass and high concentration of arsenate showed a tenfold higher count of arsC gene copies than soil samples collected underground from an arsenic-rich gold mine.     

Structure of bacterial communities in diverse freshwater habitats

The structures and dynamics of bacterial communities from raw source water, groundwater, and drinking water before and after filtration were studied in four seasons of a year, with culture-independent methods. Genomic DNA from water samples was analyzed by the polymerase chain reaction?- denaturing gradient gel electrophoresis system and by cloning of the 16S rRNA gene. Water samples exhibited complex denaturing gradient gel electrophoresis genetic profiles composed of many bands, corresponding to a great variety of bacterial taxa. The bacterial communities of different seasons from the four sampling sites clustered into two major groups: (i) water before and after filtration, and (ii) source water and groundwater. Phylogenetic analyses of the clones from the autumn sampling revealed 13 phyla, 19 classes, and 155 operational taxonomic units. Of the clones, 66% showed less than 97% similarities to known bacterial species. Representatives of the phyla Proteobacteria, Bacteroidetes, and Actinobacteria were found at all four sampling sites. Species belonging to the phylum Firmicutes were an important component of the microbial community in filtered water. Representatives of Enterobacteriaceae were not detected, indicating the absence of fecal pollution in the drinking water. Differences were found in the bacterial populations that were sampled from the same sites in different seasons. Each water habitat had a unique bacterial profile. Drinking water harbors diverse and dynamic microbial communities, part of which may be active and resilient to chlorine disinfection. This study provides, for the first time, basic data for uncultivable drinking water bacteria in Israel.     

Bacterial respiration of arsenic and selenium

Oxyanions of arsenic and selenium can be used in microbial anaerobic respiration as terminal electron acceptors. The detection of arsenate and selenate respiring bacteria in numerous pristine and contaminated environments and their rapid appearance in enrichment culture suggest that they are widespread and metabolically active in nature. Although the bacterial species that have been isolated and characterized are still few in number, they are scattered throughout the bacterial domain and include Gram-positive bacteria, beta, gamma and epsilon Proteobacteria and the sole member of a deeply branching lineage of the bacteria, Chrysiogenes arsenatus. The oxidation of a number of organic substrates (i.e. acetate, lactate, pyruvate, glycerol, ethanol) or hydrogen can be coupled to the reduction of arsenate and selenate, but the actual donor used varies from species to species. Both periplasmic and membrane-associated arsenate and selenate reductases have been characterized. Although the number of subunits and molecular masses differs, they all contain molybdenum. The extent of the environmental impact on the transformation and mobilization of arsenic and selenium by microbial dissimilatory processes is only now being fully appreciated.     

Role of Aspergillus niger acrA in arsenic resistance and its use as the basis for an arsenic biosensor

Arsenic contamination of groundwater sources is a major issue worldwide, since exposure to high levels of arsenic has been linked to a variety of health problems. Effective methods of detection are thus greatly needed as preventive measures. In an effort to develop a fungal biosensor for arsenic, we first identified seven putative arsenic metabolism and transport genes in Aspergillus niger, a widely used industrial organism that is generally regarded as safe (GRAS). Among the genes tested for RNA expression in response to arsenate, acrA, encoding a putative plasma membrane arsenite efflux pump, displayed an over 200-fold increase in gene expression in response to arsenate. We characterized the function of this A. niger protein in arsenic efflux by gene knockout and confirmed that AcrA was located at the cell membrane using an enhanced green fluorescent protein (eGFP) fusion construct. Based on our observations, we developed a putative biosensor strain containing a construct of the native promoter of acrA fused with egfp. We analyzed the fluorescence of this biosensor strain in the presence of arsenic using confocal microscopy and spectrofluorimetry. The biosensor strain reliably detected both arsenite and arsenate in the range of 1.8 to 180 μg/liter, which encompasses the threshold concentrations for drinking water set by the World Health Organization (10 and 50 μg/liter).     

Decreased serum concentrations of nitric oxide metabolites among Chinese in an endemic area of chronic arsenic poisoning in inner Mongolia

Prolonged exposure to arsenic results in peripheral and cardiovascular manifestations, as does impaired production of endothelial nitric oxide (NO). In vitro studies have indicated that endothelial cells undergo damage by arsenic. However, no information has been available on the relationship between NO synthesis and chronic arsenic poisoning in humans. The present study was designed to reveal this question. The subjects were 33 habitants who continued to drink well water containing high concentrations of inorganic arsenic (mean value = 0.41 microg/ml) for about 18 years in Inner Mongolia, China, and 10 other people who lived in this area but exposed to minimal concentrations of arsenic (mean value = 0.02 microg/ml) were employed as controls. Mean blood concentration of total arsenic was six times higher in exposed subjects than controls; 42.1 vs. 7.3 ng/ml, p <.001. Mean serum concentration of nitrite/nitrate, stable metabolites of endogenous NO, was lower in arsenic-exposed subjects than in controls: 24.7 vs. 51.6 microM, p<.001. In total samples, an inverse correlation with serum nitrite/nitrate levels was strong for blood inorganic arsenic (r = -0.52, p <.001) and less strong for its metabolites, monomethyl arsenic (r = -0.45, p<.005) and dimethyl arsenic (r = -0.37, p<.05). Furthermore, serum nitrite/nitrate concentration was significantly correlated with nonprotein sulfhydryl level in whole blood (r = 0.58, p<.001). In an in vitro study, we demonstrated that inorganic arsenite or arsenate suppresses the activity of endothelial NO synthase in human umbilical vein endothelial cells. These results suggest that long-term exposure to arsenic by drinking well water possibly reduces NO production in endothelial cells, resulting in a decrease in reduced nitrite/nitrate concentrations. Peripheral vascular disorders caused by arsenic may be attributable in part to impairment of NO production in vivo.     

Brevibacterium sp. strain CS2: A potential candidate for arsenic bioremediation from industrial wastewater

A multiple metal-resistant Brevibacterium sp. strain CS2, isolated from an industrial wastewater, resisted arsenate and arsenate upto 280 and 40 mM. The order of resistance against multiple metals was Arsenate > Arsenite > Selenium = Cobalt > Lead = Nickel > Cadmium = Chromium = Mercury. The bacterium was characterized as per morphological and biochemical characteristics at optimum conditions (37 ℃ and 7 pH). The appearance of brownish color precipitation was due to the interaction of silver nitrate confirming its oxidizing ability against arsenic. The strain showed arsenic processing ability at different temperatures, pH, and initial arsenic concentration which was 37% after 72 h and 48% after 96 h of incubation at optimum conditions with arsenite 250 mM/L (initial arsenic concentration). The maximum arsenic removal ability of strain CS2 was determined for 8 days, which was 32 and 46% in wastewater and distilled water, respectively. The heat-inactivated cells of the isolated strain showed a bioremediation efficiency (E) of 96% after 10 h. Genes cluster (9.6 kb) related to arsenite oxidation was found in Brevibacterium sp. strain CS2 after the genome analysis of isolated bacteria through illumine and nanopore sequencing technology. The arsenite oxidizing gene smaller subunit (aioB) on chromosomal DNA locus (Prokka_01508) was identified which plays a role in arsenite oxidation for energy metabolism. The presence of arsenic oxidizing genes and an efficient arsenic oxidizing potential of Brevibacterium sp. strain CS2 make it a potential candidate for green chemistry to eradicate arsenic from arsenic-contaminated wastewater.     

Bacterial Diversity and Community Structure in High Arsenic Aquifers in Hetao Plain of Inner Mongolia,China

The bacterial diversity and community structure of high arsenic (As) aquifers was investigated using an integrated approach adopting both geochemistry and molecular biology (polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) and 16S rRNA gene clone library analyses). Nine borehole sediments and one groundwater sample from the living place of a villager (affected by arseniasis) and 12 sediments from a control borehole in Hetao Plain were investigated. The As concentrations ranged from 33.6 to 77.6 mg/kg in high As borehole sediments and 1.5 to 5.8 mg/kg in those samples from the control. The As concentration in the groundwater was 744.8 μg/L. Ratios between As(III) and total As in high As sediments increased gradually with depth and ranged from 0.02 to 0.34. Similarly, the Fe(II)/total Fe presented the same increasing trend with depth. The correlation between TOC contents and total As was positive. High concentrations of total As, S, Fe and TOC were found in clay and low in sand samples. Phylogenetic analysis showed significantly different bacterial communities among high As sediments, control sediments and the high As groundwater. Both DGGE and 16S rRNA gene clone library results showed that the high As sediments were dominated by Thiobacillus, Pseudomonas, Brevundimonas, and Hydrogenophaga, with Thiobacillus being distinctly dominant (63.5%). Whereas the low As sediments were dominated by some other genera including Psychrobacter, Massilia and Desulfotalea. The bacterial populations in the high As groundwater mainly included Pseudomonas, Acinetobacter and Aquabacterium. These results improve our understanding of the bacterial diversity in high As aquifers in Hetao Plain and suggest how specific bacterial populations help mediate the mobilization of As into high As groundwaters.