Increased intracellular persulfide levels attenuate HlyU-mediated hemolysin transcriptional activation in Vibrio cholerae

The vertebrate host’s immune system and resident commensal bacteria deploy a range of highly reactive small molecules that provide a barrier against infections by microbial pathogens. Gut pathogens, such as Vibrio cholerae , sense and respond to these stressors by modulating the expression of exotoxins that are crucial for colonization. Here, we employ mass-spectrometry-based profiling, metabolomics, expression assays and biophysical approaches to show that transcriptional activation of the hemolysin gene hlyA in V. cholerae is regulated by intracellular reactive sulfur species (RSS), specifically sulfane sulfur. We first present a comprehensive sequence similarity network analysis of the arsenic repressor (ArsR) superfamily of transcriptional regulators where RSS and reactive oxygen species (ROS) sensors segregate into distinct clusters. We show that HlyU, transcriptional activator of hlyA in V. cholerae , belongs to the RSS-sensing cluster and readily reacts with organic persulfides, showing no reactivity and remaining DNA-bound following treatment with various ROS in vitro, including H 2 O 2 . Surprisingly, in V. cholerae cell cultures, both sulfide and peroxide treatment downregulate HlyU-dependent transcriptional activation of hlyA . However, RSS metabolite profiling shows that both sulfide and peroxide treatment raise the endogenous inorganic sulfide and disulfide levels to a similar extent, accounting for this crosstalk, and confirming that V. cholerae attenuates HlyU-mediated activation of hlyA in a specific response to intracellular RSS. These findings provide new evidence that gut pathogens may harness RSS-sensing as an evolutionary adaptation that allows them to overcome the gut inflammatory response by modulating the expression of exotoxins.

Looking for the mechanism of arsenate respiration of Fusibacter sp. strain 3D3, independent of ArrAB

The literature has reported the isolation of arsenate-dependent growing microorganisms which lack a canonical homolog for respiratory arsenate reductase, ArrAB. We recently isolated an arsenate-dependent growing bacterium from volcanic arsenic-bearing environments in Northern Chile, Fusibacter sp. strain 3D3 (Fas) and studied the arsenic metabolism in this Gram-positive isolate. Features of Fas deduced from genome analysis and comparative analysis with other arsenate-reducing microorganisms revealed the lack of ArrAB coding genes and the occurrence of two arsC genes encoding for putative cytoplasmic arsenate reductases named ArsC-1 and ArsC-2. Interestingly, ArsC-1 and ArsC-2 belong to the thioredoxin-coupled family (because of the redox-active disulfide protein used as reductant), but they conferred differential arsenate resistance to the E. coli WC3110 ΔarsC strain. PCR experiments confirmed the absence of arrAB genes and results obtained using uncouplers revealed that Fas growth is linked to the proton gradient. In addition, Fas harbors ferredoxin-NAD⁺ oxidoreductase (Rnf) and electron transfer flavoprotein (etf) coding genes. These are key molecular markers of a recently discovered flavin-based electron bifurcation mechanism involved in energy conservation, mainly in anaerobic metabolisms regulated by the cellular redox state and mostly associated with cytoplasmic enzyme complexes. At least three electron-bifurcating flavoenzyme complexes were evidenced in Fas, some of them shared in conserved genomic regions by other members of the Fusibacter genus. These physiological and genomic findings permit us to hypothesize the existence of an uncharacterized arsenate-dependent growth metabolism regulated by the cellular redox state in the Fusibacter genus.

Relaxed control of sugar utilization in Parageobacillus thermoglucosidasius DSM 2542

Though carbon catabolite repression (CCR) has been intensively studied in some more characterised organisms, there is a lack of information of CCR in thermophiles. In this work, CCR in the thermophile, Parageobacillus thermoglucosidasius DSM 2542 has been studied during growth on pentose sugars in the presence of glucose. Physiological studies under fermentative conditions revealed a loosely controlled CCR when DSM 2542 was grown in minimal medium supplemented with a mixture of glucose and xylose. This atypical CCR pattern was also confirmed by studying xylose isomerase expression level by qRT-PCR. Fortuitously, the pheB gene, which encodes catechol 2, 3-dioxygenase was found to have a cre site highly similar to the consensus catabolite-responsive element (cre) at its 3' end and was used to confirm that expression of pheB from a plasmid was under stringent CCR control. Bioinformatic analysis suggested that the CCR regulation of xylose metabolism in P. thermoglucosidasius DSM 2542 might occur primarily via control of expression of pentose transporter operons. Relaxed control of sugar utilization might reflect a lower affinity of the CcpA-HPr (Ser46-P) or CcpA-Crh (Ser46-P) complexes to the cre(s) in these operons.

Genomic Insight of Alicyclobacillus mali FL18 Isolated From an Arsenic-Rich Hot Spring

Extreme environments are excellent places to find microorganisms capable of tolerating extreme temperature, pH, salinity pressure, and elevated concentration of heavy metals and other toxic compounds. In the last decades, extremophilic microorganisms have been extensively studied since they can be applied in several fields of biotechnology along with their enzymes. In this context, the characterization of heavy metal resistance determinants in thermophilic microorganisms is the starting point for the development of new biosystems and bioprocesses for environmental monitoring and remediation. This work focuses on the isolation and the genomic exploration of a new arsenic-tolerant microorganism, classified as Alicyclobacillus mali FL18. The bacterium was isolated from a hot mud pool of the solfataric terrains in Pisciarelli, a well-known hydrothermally active zone of the Campi Flegrei volcano near Naples in Italy. A. mali FL18 showed a good tolerance to arsenite (MIC value of 41 mM), as well as to other metals such as nickel (MIC 30 mM), cobalt, and mercury (MIC 3 mM and 17 μM, respectively). Signatures of arsenic resistance genes (one arsenate reductase, one arsenite methyltransferase, and several arsenite exporters) were found interspersed in the genome as well as several multidrug resistance efflux transporters that could be involved in the export of drugs and heavy metal ions. Moreover, the strain showed a high resistance to bacitracin and ciprofloxacin, suggesting that the extreme environment has positively selected multiple resistances to different toxic compounds. This work provides, for the first time, insights into the heavy metal tolerance and antibiotic susceptibility of an Alicyclobacillus strain and highlights its putative molecular determinants.

Micro-colonization of arsenic-resistant Staphylococcus sp. As-3 on arsenopyrite (FeAsS) drives arsenic mobilization under anoxic sub-surface mimicking conditions

We investigated the subsurface biomatrix of the most abundant As-mineral, arsenopyrite (FeAsS), and meticulously studied a potential biogenic arsenic mobilization phenomenon. An arsenic-resistant [up to 7.5 mM As(III) and 200 mM As(V)] and arsenate-reducing bacterial strain (Staphylococcus sp. As-3) was isolated from a sediment core sample taken from the Budai borehole, on the southwestern coast of Taiwan. Isolate As-3 could reduce 5 mM As(V) to 3.04 mM in 96 h, generating 1.6 mM As(III) under anoxic conditions. Isolate As-3, which adsorbed As(V) up to 19.02 mg g^-1 (cdw) and As(III) up to 0.46 mg g^-1 (cdw), demonstrated effective As-bioaccumulating ability, as corroborated by a TEM-EDS analysis. Under anaerobic batch conditions, isolate As-3 micro-colonies could grow on as well as interact with arsenopyrite (FeAsS), mobilizing arsenic into soluble phase as As(III) and As(V). Using synchrotron radiation-based FTIR micro-spectroscopy, various functional group signatures and critical chemical bonds enabling a direct interaction with arsenopyrite were underpinned, such as a potential P-OFe bond involved in facilitating bacteria-mineral interaction. Using atomic force microscopy, we analyzed the scattered bacterial cell arrangement and structure and measured various biomechanical properties of micro-colonized Staphylococcus sp. As-3 cells on arsenopyrite. We suggest that the release of organic acids from As-3 drives soluble arsenic release in the aqueous phase under anoxic conditions through oxidative dissolution. Furthermore, arsC-encoding putative cytoplasmic arsenic reductase sequencing and transcript characterization indicated that arsC plays a possible role in the reduction of moderately soluble As(V) to highly soluble toxic As(III) under anoxic conditions. Thus, we suggest that firmicutes such as Staphylococcus sp. As-3 may play an important role in microbially-mediated arsenic mobilization, leading to arsenic release in the sub-surface niche.

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

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

Distribution of Arsenic Resistance Genes in Prokaryotes

Arsenic is a metalloid that occurs naturally in aquatic and terrestrial environments. The high toxicity of arsenic derivatives converts this element in a serious problem of public health worldwide. There is a global arsenic geocycle in which microbes play a relevant role. Ancient exposure to arsenic derivatives, both inorganic and organic, has represented a selective pressure for microbes to evolve or acquire diverse arsenic resistance genetic systems. In addition, arsenic compounds appear to have been used as a toxin in chemical warfare for a long time selecting for an extended range of arsenic resistance determinants. Arsenic resistance strategies rely mainly on membrane transport pathways that extrude the toxic compounds from the cell cytoplasm. The ars operons, first discovered in bacterial R-factors almost 50 years ago, are the most common microbial arsenic resistance systems. Numerous ars operons, with a variety of genes and different combinations of them, populate the prokaryotic genomes, including their accessory plasmids, transposons, and genomic islands. Besides these canonical, widespread ars gene clusters, which confer resistance to the inorganic forms of arsenic, additional genes have been discovered recently, which broadens the spectrum of arsenic tolerance by detoxifying organic arsenic derivatives often used as toxins. This review summarizes the presence, distribution, organization, and redundance of arsenic resistance genes in prokaryotes.

Analysis of the ars gene cluster from highly arsenic-resistant Burkholderia xenovorans LB400

The Burkholderia xenovorans LB400 multireplicon genome displays a relatively high proportion of redundant genes, including several genes predicted to be related to arsenic resistance. These comprise an ars gene cluster, composed of the arsR3, acr3, arsC1 and arsH genes, as well as two arsB, arsC2, and seven individual arsR genes. The objective of this work was to elucidate the involvement of the ars gene cluster in arsenic resistance by the LB400 strain. Susceptibility tests showed that B. xenovorans LB400 is highly resistant to arsenate and arsenite. Arsenic resistance was induced by prior exposure of LB400 to arsenate or arsenite. reverse transcription-polymerase chain reaction assays using total RNA from LB400 showed arsenite-induced transcription of the arsR3 gene, suggesting that the ars gene cluster constitutes an arsenite-responsive operon. Transfer of cloned LB400 ars genes to heterologous Escherichia coli or Pseudomonas aeruginosa strains demonstrated that the ArsR3 transcriptional repressor, ArsC1 arsenate reductase, and the Acr3 arsenite efflux pump encoded in the LB400 ars gene cluster, are all associated to the arsenic resistance phenotype of this strain. The ars gene cluster from Burkholderia xenovorans LB400 is responsible for the inducible arsenic-resistance phenotype of the bacterium.

In silico identification and characterization of sensory motifs in the transcriptional regulators of the ArsR-SmtB family

The ArsR-SmtB family of proteins displays the greatest diversity among the bacterial metal-binding transcriptional regulators with regard to the variety of metal ions that they can sense. In the presence of increased levels of toxic heavy metals, these proteins dissociate from their cognate DNA upon the direct binding of metal ions to the appropriate sites, designated motifs on the proteins, either at the interface of the dimers or at the intra-subunit locations. In addition to the metal-mediated regulation, some proteins were also found to control transcription via redox reactions. In the present work, we have identified several new sequence motifs and expanded the knowledge base of metal binding sites in the ArsR-SmtB family of transcriptional repressors, and characterized them in terms of the ligands to the metal, distribution among different phyla of bacteria and archaea, amino acid propensities, protein length distributions and evolutionary interrelationships. We built structural models of the motifs to show the importance of specific residues in an individual motif. The wide abundance of these motifs in sequences of bacteria and archaea indicates the importance of these regulators in combating metal-toxicity within and outside of the hosts. We also show that by using residue composition, one can distinguish the ArsR-SmtB proteins from other metalloregulatory families. In addition, we show the importance of horizontal gene transfer in microorganisms, residing in similar habitats, on the evolution of the structural motifs in the family. Knowledge of the diverse metalloregulatory systems in microorganisms could enable us to manipulate specific genes that may result in a toxic metal-free environment.

Metal homeostasis in bacteria: the role of ArsR-SmtB family of transcriptional repressors in combating varying metal concentrations in the environment

Bacterial infections cause severe medical problems worldwide, resulting in considerable death and loss of capital. With the ever-increasing rise of antibiotic-resistant bacteria and the lack of development of new antibiotics, research on metal-based antimicrobial therapy has now gained pace. Metal ions are essential for survival, but can be highly toxic to organisms if their concentrations are not strictly controlled. Through evolution, bacteria have acquired complex metal-management systems that allow them to acquire metals that they need for survival in different challenging environments while evading metal toxicity. Metalloproteins that controls these elaborate systems in the cell, and linked to key virulence factors, are promising targets for the anti-bacterial drug development. Among several metal-sensory transcriptional regulators, the ArsR-SmtB family displays greatest diversity with several distinct metal-binding and nonmetal-binding motifs that have been characterized. These prokaryotic metolloregulatory transcriptional repressors represses the expression of operons linked to stress-inducing concentrations of metal ions by directly binding to the regulatory regions of DNA, while derepression results from direct binding of metal ions by these homodimeric proteins. Many bacteria, e.g., Mycobacterium tuberculosis, Bacillus anthracis, etc., have evolved to acquire multiple metal-sensory motifs which clearly demonstrate the importance of regulating concentrations of multiple metal ions. Here, we discussed the mechanisms of how ArsR-SmtB family regulates the intracellular bioavailability of metal ions both inside and outside of the host. Knowledge of the metal-challenges faced by bacterial pathogens and their survival strategies will enable us to develop the next generation drugs.

Arsenic Detoxification by Geobacter Species

Insight into the mechanisms for arsenic detoxification by Geobacter species is expected to improve the understanding of global cycling of arsenic in iron-rich subsurface sedimentary environments. Analysis of 14 different Geobacter genomes showed that all of these species have genes coding for an arsenic detoxification system (ars operon), and several have genes required for arsenic respiration (arr operon) and methylation (arsM). Genes encoding four arsenic repressor-like proteins were detected in the genome of G. sulfurreducens; however, only one (ArsR1) regulated transcription of the ars operon. Elimination of arsR1 from the G. sulfurreducens chromosome resulted in enhanced transcription of genes coding for the arsenic efflux pump (Acr3) and arsenate reductase (ArsC). When the gene coding for Acr3 was deleted, cells were not able to grow in the presence of either the oxidized or reduced form of arsenic, while arsC deletion mutants could grow in the presence of arsenite but not arsenate. These studies shed light on how Geobacter influences arsenic mobility in anoxic sediments and may help us develop methods to remediate arsenic contamination in the subsurface.

IMPORTANCE

This study examines arsenic transformation mechanisms utilized by Geobacter, a genus of iron-reducing bacteria that are predominant in many anoxic iron-rich subsurface environments. Geobacter species play a major role in microbially mediated arsenic release from metal hydroxides in the subsurface. This release raises arsenic concentrations in drinking water to levels that are high enough to cause major health problems. Therefore, information obtained from studies of Geobacter should shed light on arsenic cycling in iron-rich subsurface sedimentary environments, which may help reduce arsenic-associated illnesses. These studies should also help in the development of biosensors that can be used to detect arsenic contaminants in anoxic subsurface environments. We examined 14 different Geobacter genomes and found that all of these species possess genes coding for an arsenic detoxification system (ars operon), and some also have genes required for arsenic respiration (arr operon) and arsenic methylation (arsM).

Arsenic resistance strategy in Pantoea sp. IMH: Organization, function and evolution of ars genes

Pantoea sp. IMH is the only bacterium found in genus Pantoea with a high As resistance capacity, but its molecular mechanism is unknown. Herein, the organization, function, and evolution of ars genes in IMH are studied starting with analysis of the whole genome. Two ars systems - ars1 (arsR1B1C1H1) and ars2 (arsR2B2C2H2) - with low sequence homology and two arsC-like genes, were found in the IMH genome. Both ars1 and ars2 are involved in the As resistance, where ars1 is the major contributor at 15 °C and ars2 at 30 °C. The difference in the behavior of these two ars systems is attributed to the disparate activities of their arsR promoters at different temperatures. Sequence analysis based on concatenated ArsRBC indicates that ars1 and ars2 clusters may be acquired from Franconibacter helveticus LMG23732 and Serratia marcescens (plasmid R478), respectively, by horizontal gene transfer (HGT). Nevertheless, two arsC-like genes, probably arising from the duplication of arsC, do not contribute to the As resistance. Our results indicate that Pantoea sp. IMH acquired two different As resistance genetic systems by HGT, allowing the colonization of changing ecosystems, and highlighting the flexible adaptation of microorganisms to resist As.

Regulatory Activities of Four ArsR Proteins in Agrobacterium tumefaciens 5A

ArsR is a well-studied transcriptional repressor that regulates microbe-arsenic interactions. Most microorganisms have an arsR gene, but in cases where multiple copies exist, the respective roles or potential functional overlap have not been explored. We examined the repressors encoded by arsR1 and arsR2 (ars1 operon) and by arsR3 and arsR4 (ars2 operon) in Agrobacterium tumefaciens 5A. ArsR1 and ArsR4 are very similar in their primary sequences and diverge phylogenetically from ArsR2 and ArsR3, which are also quite similar to one another. Reporter constructs (lacZ) for arsR1, arsR2, and arsR4 were all inducible by As(III), but expression of arsR3 (monitored by reverse transcriptase PCR) was not influenced by As(III) and appeared to be linked transcriptionally to an upstream lysR-type gene. Experiments using a combination of deletion mutations and additional reporter assays illustrated that the encoded repressors (i) are not all autoregulatory as is typically known for ArsR proteins, (ii) exhibit variable control of each other's encoding genes, and (iii) exert variable control of other genes previously shown to be under the control of ArsR1. Furthermore, ArsR2, ArsR3, and ArsR4 appear to have an activator-like function for some genes otherwise repressed by ArsR1, which deviates from the well-studied repressor role of ArsR proteins. The differential regulatory activities suggest a complex regulatory network not previously observed in ArsR studies. The results indicate that fine-scale ArsR sequence deviations of the reiterated regulatory proteins apparently translate to different regulatory roles.

IMPORTANCE

Given the significance of the ArsR repressor in regulating various aspects of microbe-arsenic interactions, it is important to assess potential regulatory overlap and/or interference when a microorganism carries multiple copies of arsR This study explores this issue and shows that the four arsR genes in A. tumefaciens 5A, associated with two separate ars operons, encode proteins exhibiting various degrees of functional overlap with respect to autoregulation and cross-regulation, as well as control of other functional genes. In some cases, differences in regulatory activity are associated with only limited differences in protein primary structure. The experiments summarized herein also present evidence that ArsR proteins appear to have activator functions, representing novel regulatory activities for ArsR, previously known only to be a repressor.

From deep-sea volcanoes to human pathogens: a conserved quorum-sensing signal in Epsilonproteobacteria

Chemosynthetic Epsilonproteobacteria from deep-sea hydrothermal vents colonize substrates exposed to steep thermal and redox gradients. In many bacteria, substrate attachment, biofilm formation, expression of virulence genes and host colonization are partly controlled via a cell density-dependent mechanism involving signal molecules, known as quorum sensing. Within the Epsilonproteobacteria, quorum sensing has been investigated only in human pathogens that use the luxS/autoinducer-2 (AI-2) mechanism to control the expression of some of these functions. In this study we showed that luxS is conserved in Epsilonproteobacteria and that pathogenic and mesophilic members of this class inherited this gene from a thermophilic ancestor. Furthermore, we provide evidence that the luxS gene is expressed--and a quorum-sensing signal is produced--during growth of Sulfurovum lithotrophicum and Caminibacter mediatlanticus, two Epsilonproteobacteria from deep-sea hydrothermal vents. Finally, we detected luxS transcripts in Epsilonproteobacteria-dominated biofilm communities collected from deep-sea hydrothermal vents. Taken together, our findings indicate that the epsiloproteobacterial lineage of the LuxS enzyme originated in high-temperature geothermal environments and that, in vent Epsilonproteobacteria, luxS expression is linked to the production of AI-2 signals, which are likely produced in situ at deep-sea vents. We conclude that the luxS gene is part of the ancestral epsilonproteobacterial genome and represents an evolutionary link that connects thermophiles to human pathogens.

Screening of plant growth-promoting traits in arsenic-resistant bacteria isolated from agricultural soil and their potential implication for arsenic bioremediation

Twelve arsenic (As)-resistant bacteria (minimum inhibitory concentration ranging from 10 to 30mM and 150 to 320mM for As(III) and As(V), respectively) were isolated from the agricultural soil of the Chianan Plain in southwestern Taiwan using enrichment techniques. Eight isolates capable of oxidizing As(III) (rate of oxidation from 0.029 to 0.059μMh(-1) 10(-9) cell) and exhibiting As(III)-oxidase enzyme activity belong to Pseudomonas, Acinetobacter, Klebsiella and Comamonas genera, whereas four isolates that did not show As(III)-oxidizing activity belong to Geobacillus, Bacillus, Paenibacillus, and Enterobacter genera. Assessment of the parameters of plant growth promotion revealed that Pseudomonas sp. ASR1, ASR2 and ASR3, Geobacillus sp. ASR4, Bacillus sp. ASR5, Paenibacillus sp. ASR6, Enterobacter sp. ASR10 and Comamonas sp. ASR11, and ASR12 possessed some or all of the studied plant growth-promoting traits, including phosphate-solubilization, siderophore, IAA-like molecules and ACC deaminase production. In addition, the ability of As-resistant isolates to grow over wide ranges of pH and temperatures signify their potential application for sustainable bioremediation of As in the environment.

Fate of arsenate following arsenite oxidation in Agrobacterium tumefaciens GW4

The fate of arsenate (As(V) ) generated by microbial arsenite (As(III) ) oxidation is poorly understood. Agrobacterium tumefaciens wild-type strain (GW4) was studied to determine how the cell copes with As(V) generated in batch culture. GW4 grown heterotrophically with mannitol used As(III) as a supplemental energy supply as reflected by enhanced growth and increased cellular levels of NADH and ATP. Under low phosphate (Pi) conditions and presence of As(III) oxidation, up to ∼ 50% of the resulting As(V) was taken up and found associated with the periplasm, membrane or cytoplasm fractions of the cells. Arsenic was found associated with proteins and polar lipids, but not in nucleic acids or sugars. Thin-layer chromatography and gas chromatography-mass spectrometry analysis suggested the presence of arsenolipids in membranes, presumably as part of the bilayer structure of the cell membrane and replacing Pi under Pi-limiting conditions. The potential role of a Pi-binding protein (PstS) for As(V) uptake was assessed with the His-tag purified protein. Intrinsic tryptophan fluorescence spectra analysis suggests that PstS can bind As(V) , but with lower affinity as compared with Pi. In early stationary phase cells, the As(V)  : Pi ratio was approximately 4.3 and accompanied by an altered cell ultrastructure.

Involvement of the Acr3 and DctA anti-porters in arsenite oxidation in Agrobacterium tumefaciens 5A

Microbial arsenite (AsIII) oxidation forms a critical piece of the arsenic cycle in nature, though our understanding of how and why microorganisms oxidize AsIII remains rudimentary. Our model organism Agrobacterium tumefaciens 5A contains two distinct ars operons (ars1 and ars2) that are similar in their coding region content. The ars1 operon is located nearby the aio operon that is essential for AsIII oxidation. The AsIII/H(+) anti-porters encoded by acr3-1 and acr3-2 are required for maximal AsIII and antimonite (SbIII) resistance, but acr3-1 (negatively regulated by ArsR-1) appears more active in this regard and also required for AsIII oxidation and expression of aioBA. A malate-phosphate anti-porter DctA is regulated by RpoN and AsIII, and is required for normal growth with malate as a sole carbon source. Qualitatively, a ΔdctA mutant was normal for AsIII oxidation and AsIII/SbIII resistance at metalloid concentrations inhibitory to the Δacr3-1 mutant; however, aioBA induction kinetics was significantly phase-shift delayed. Acr3 involvement in AsIII/SbIII resistance is reasonably well understood, but the role of Acr3 and DctA anti-porters in AsIII oxidation and its regulation is unexpected, and suggests that controlled AsIII trafficking across the cytoplasmic membrane is important to a process understood to occur in the periplasm.

Distribution of microbial arsenic reduction, oxidation and extrusion genes along a wide range of environmental arsenic concentrations

The presence of the arsenic oxidation, reduction, and extrusion genes arsC, arrA, aioA, and acr3 was explored in a range of natural environments in northern Chile, with arsenic concentrations spanning six orders of magnitude. A combination of primers from the literature and newly designed primers were used to explore the presence of the arsC gene, coding for the reduction of As (V) to As (III) in one of the most common detoxification mechanisms. Enterobacterial related arsC genes appeared only in the environments with the lowest As concentration, while Firmicutes-like genes were present throughout the range of As concentrations. The arrA gene, involved in anaerobic respiration using As (V) as electron acceptor, was found in all the systems studied. The As (III) oxidation gene aioA and the As (III) transport gene acr3 were tracked with two primer sets each and they were also found to be spread through the As concentration gradient. Sediment samples had a higher number of arsenic related genes than water samples. Considering the results of the bacterial community composition available for these samples, the higher microbial phylogenetic diversity of microbes inhabiting the sediments may explain the increased number of genetic resources found to cope with arsenic. Overall, the environmental distribution of arsenic related genes suggests that the occurrence of different ArsC families provides different degrees of protection against arsenic as previously described in laboratory strains, and that the glutaredoxin (Grx)-linked arsenate reductases related to Enterobacteria do not confer enough arsenic resistance to live above certain levels of As concentrations.

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.