merA Gene Research Articles

Characterization of mercury-resistant clinical Aeromonas species

Mercury-resistant Aeromonas strains isolated from diarrhea were studied. Resistance occurs via mercuric ion reduction but merA and merR genes were only detected in some strains using PCR and dot hybridization. Results indicate a high variability in mer operons in Aeromonas. To our knowledge, this is the first report of mercury-resistant clinical Aeromonas strains.

Interactions between snow chemistry, mercury inputs and microbial population dynamics in an Arctic snowpack.

We investigated the interactions between snowpack chemistry, mercury (Hg) contamination and microbial community structure and function in Arctic snow. Snowpack chemistry (inorganic and organic ions) including mercury (Hg) speciation was studied in samples collected during a two-month field study in a high Arctic site, Svalbard, Norway (79°N). Shifts in microbial community structure were determined by using a 16S rRNA gene phylogenetic microarray. We linked snowpack and meltwater chemistry to changes in microbial community structure by using co-inertia analyses (CIA) and explored changes in community function due to Hg contamination by q-PCR quantification of Hg-resistance genes in metagenomic samples. Based on the CIA, chemical and microbial data were linked (p = 0.006) with bioavailable Hg (BioHg) and methylmercury (MeHg) contributing significantly to the ordination of samples. Mercury was shown to influence community function with increases in merA gene copy numbers at low BioHg levels. Our results show that snowpacks can be considered as dynamic habitats with microbial and chemical components responding rapidly to environmental changes.

Mercuric reductase activity of multiple heavy metal‐resistant Lysinibacillus sphaericus G1

A culture was isolated from an industrial mercuric salt-contaminated soil, which could tolerate Cd, Co, Zn, Cr, and Hg up to 190, 525, 350, 935, and 370 μM, respectively. The isolate was identified as Lysinibacillus sphaericus by 16S rRNA gene sequencing. It bioaccumulated Cd, Co, and Zn, and reductively detoxified Cr and Hg. Chromate reductase and mercuric reductase (MerA) activities in the cell extract were 2.4 and 0.13 units mg(-1) protein, respectively. The study also describes designing of broad-specificity primers based on firmicute merA genes. These primers were successfully used to amplify a 440 bp merA fragment from the current isolate. Based on the partial sequence, complete merA ORF of 1641 bp was amplified. It showed 99% similarity to a putative merA gene from distantly related Streptococcus agalactiae, but only 72% identity with the well-characterized merA from a more closely related Bacillus cereus RC607. The gene sequence possessed all the features required for the functioning of MerA enzyme, and its function was confirmed by recombinant expression in E. coli. To the best of our knowledge, this is the first report of full length merA gene from L. sphaericus.

Characterization and potential application in mercury bioremediation of highly mercury-resistant marine bacterium Bacillus thuringiensis PW-05

Bacillus thuringiensis PW-05 was isolated from the Odisha coast and was found to resist 50 ppm of Hg as HgCl2 as well as higher concentrations of CdCl2, ZnSO4, PbNO3 and Na2HAsO4. Resistance towards several antibiotics, viz amoxycillin, ampicillin, methicillin, azithromycin and cephradine (CV) was also observed. The mer operon possessed by most of the mercury-resistant bacteria was also found in this isolate. Atomic absorption spectroscopy revealed that the isolate can volatilize >90 % of inorganic mercury. It showed biofilm formation in the presence of 50 ppm HgCl2 and can produce exopolysaccharide under same conditions. The isolate was found to volatilize mercury efficiently under a wide range of environmental parameters, i.e. pH (7 to 8), temperature (25 °C to 40 °C) and salinity (5 to 25 ppt). merA gene expression has been confirmed by real-time reverse transcriptase PCR study. Fourier transform infrared study revealed that -SH and -COOH groups play a major role in the process of adaptation to Hg. Hence, this isolate B. thuringiensis PW-05 shows an interesting potential for bioremediation of mercury.

Mercury induced community tolerance in microbial biofilms is related to pollution gradients in a long-term polluted river

The net toxicity of different forms of mercury, in the long-term during their transformation processes, leads to the selection of resistant bacterial cells and this result in community tolerance which is pollution induced. Accordingly, based on profiles of a bacterial community structure, analysis of Hg resistant culturable bacteria and quantification of merA genes, we assessed development of pollution induced community tolerance in a mercury-polluted gradient in the Idrijca River. TTGE analysis did not show effects of mercury pollution to bacterial community diversity, while quantification of merA genes showed that merA genes can be correlated precisely (R2=0.83) with the total concentration of mercury in the biofilm microbial communities in the pollution gradient.

Simultaneous pyrosequencing of the 16S rRNA, IncP-1 trfA, and merA genes

The use of amplicon pyrosequencing makes it possible to produce thousands of sequences of the same gene at relatively low costs. Here we show that it is possible to simultaneously sequence the 16S rRNA gene, IncP-1 trfA gene and mercury reductase gene (merA) as a way for screening the diversity of several genes in the same samples. As a proof-of-concept two different soil samples and a wastewater sample were screened. Multiplexing identifiers (MIDs) and sequencing adapters were added to amplicons using a tailed PCR approach and the universal overhangs U1 and U2 for this approach were redesigned. Furthermore, this is the first time the IncP-1 plasmid diversity was studied by amplicon pyrosequencing and for this purpose a clustering threshold of 89% nucleotide sequence similarity was determined to differentiate the IncP-1 subgroups.

Mercuric reductase genes (merA) and mercury resistance plasmids in High Arctic snow, freshwater and sea-ice brine

Bacterial reduction in Hg(2+) to Hg(0) , mediated by the mercuric reductase (MerA), is important in the biogeochemical cycling of Hg in temperate environments. Little is known about the occurrence and diversity of merA in the Arctic. Seven merA determinants were identified among bacterial isolates from High Arctic snow, freshwater and sea-ice brine. Three determinants in Bacteriodetes, Firmicutes and Actinobacteria showed < 92% (amino acid) sequence similarity to known merA, while one merA homologue in Alphaproteobacteria and 3 homologues from Betaproteobacteria and Gammaproteobacteria were > 99% similar to known merA's. Phylogenetic analysis showed the Bacteroidetes merA to be part of an early lineage in the mer phylogeny, whereas the Betaproteobacteria and Gammaproteobacteria merA appeared to have evolved recently. Several isolates, in which merA was not detected, were able to reduce Hg(2+) , suggesting presence of unidentified merA genes. About 25% of the isolates contained plasmids, two of which encoded mer operons. One plasmid was a broad host-range IncP-α plasmid. No known incompatibility group could be assigned to the others. The presence of conjugative plasmids, and an incongruent distribution of merA within the taxonomic groups, suggests horizontal transfer of merA as a likely mechanism for High Arctic microbial communities to adapt to changing mercury concentration.

Isolation and molecular identification of mercury resistant bacteria and detection of Escherichia coli mercuric reductase gene from wastewater of Khowr-e-Musa, Iran

Mercuric compounds are extending over large natural environment as a result of industrial pollution. Resistance to these compounds have been found in a wide range of bacterial species isolated from various environment. The aim of this study was to investigate of mercury resistant bacteria on the seashore wastewater of Khowr-e-Musa in Mahshahr area, in the south west of Iran, which one of the most important petrochemical chlor-alkali unit is located there. For this purpose, water samples were taken from wastewater of three stations. Amount of total mercury in the samples was measured using cold vapor atomic absorption spectrophotometery. Two approaches namely, conventional biochemical test and modern molecular approaches were used for identification. Mercury toxicity was measured via minimal inhibitory concentration method. Bacillus cereus, E.coli and Staphylococcus aureus were isolated and identified based on 16S rRNA gene homology, and resistance to mercuric chloride was at 400, 450 and 75 ppm, respectively. The location of E.coli mer operon was determined by plasmid curing. E.coli showed the presence of a plasmid DNA which is carries mer operon, and 1695 bp of merA gene was amplified by PCR method. The results exhibited that isolated bacteria in present study were resistant and could grow on high concentration of mercury. * Corresponding Author: Farshid Kafilzadeh  kafilzadeh@jia.ac.ir

Mercurial-resistance determinants in Pseudomonas strain K-62 plasmid pMR68

We report the complete nucleotide sequence of plasmid pMR68, isolated from Pseudomonas strain K-62, two plasmids contribute to broad-spectrum mercury resistance and that the mer operon from one of them (pMR26) has been previously characterized. The plasmid was 71,020 bp in length and contained 75 coding regions. Three mer gene clusters were identified. The first comprised merR-orf4-orf5-merT1-merP1-merF-merA-merB1, which confers bacterial resistance to mercuric ions and organomercury. The second and third clusters comprised merT2-merP2, which encodes a mercury transport system, and merB2, which encodes an organomercurial lyase, respectively. The deduced amino acid sequences for the proteins encoded by each of the mer genes identified in pMR68 bore greater similarity to sequences from Methylobacterium extorquens AM1 than to those from pMR26, a second mercury-resistance plasmid from Pseudomonas strain K-62. Escherichia coli cells carrying pMKY12 (containing merR-orf4-orf5-merT1-merP1-merF-merA-merB1 cloned from pMR68) and cells carrying pMRA114 (containing merR-merT-merP-merA-merG-merB1 cloned from plasmid pMR26) were more resistant to, and volatilized more, mercury from mercuric ions and phenylmercury than the control cells. The present results, together with our earlier findings, indicate that the high phenylmercury resistance noted for Pseudomonas strain K-62 seems to be achieved by multiple genes, particularly by the multiple merB encoding organomercurial lyase and one merG encoding cellular permeability to phenylmercury. The novel mer gene identified in pMR68 may help us to design new strategies aimed at the bioremediation of mercurials.

Diversity of Mercury Resistant Escherichia coli Strains Isolated from Aquatic Systems in Rio de Janeiro, Brazil

Escherichia coli may harbor genetic mercury resistance markers which makes this bacterial species a promising alternative for bioremediation processes. The objective of this study was to investigate phenotypic and genetic characteristics related to diversity and mercury resistance among 178 Escherichia coli strains isolated from residential, industrial, agricultural, and hospital wastewaters and recreational waters at Rio de Janeiro city. Genetic and conventional methods were carried out in order to determine mercury resistance. Random amplification of polymorphic DNA (RAPD-PCR) and denaturing gradient gel electrophoresis (DGGE) were used to investigate genetic variability. RAPD data revealed a high degree of polymorphism among E. coli mercury resistant strains and showed reproducibility and good discriminative results. DGGE typing detected diversity within the merA gene fragment. Our findings represent an improvement in epidemiological studies of HgR E. coli and support the evidence of nonclonal nature of mercury resistant E. coli strains circulating in rural and urban aquatic systems in Rio de Janeiro city.

Mercury detoxification using genetic engineered Nicotiana tabacum

Phytoremediation is a low cost alternative solution to soil contamination compared with traditional removal and/or disposal techniques. One of the phytoremediation technologies is the phytovolatilization, whereby the contaminant is not primarily accumulated in above-ground tissues, but is instead transformed by the plant into the atmosphere. The detoxification of highly toxic organmercurial compounds and subsequent volatilization of elemental mercury is a unique example for the successful phytoremediation based on genetic engineering approach. For mercury removal techniques we constructed a dicistronic construct containing the bacterial mercury detoxification genes merA and merB under the control of the Arabidopsis Actin2 promoter and terminator. The resulted construct was introduced to Agrobacterium competent cells using heat shock transformation method. In the mean time, we germinated tobacco (Nicotiana tabacum var. Gold leaf) seeds on MS media and infected leaf discs of 6-8 weeks tobacco seedlings with Agrobacterium containing Ti plasmid harboring merA/merB dicistronic construct. The results showed that about 90% of tobacco seedlings were carrying the mer genes. Tobacco seeds were collected from wild type and transgenic lines and tested for mercury resistance. The results showed that transgenic plants are resistant to both Phenyl Mercuric Acetate (PMA) and HgCl2. The root length and dry weight of wild and transgenic seedlings growing on both media amended with mercury compounds and media without mercury (control) were scored. The results showed that the root lengths and dry weight of the transgenic lines are significantly higher by 60 and 17-folds, respectively, compared to wild type. The results showed clear evidence that the transgenic plants are resistant to both organic and inorganic mercury compounds and can be used to clean up mercury contaminated sites.

Manila clams from Hg polluted sediments of Marano and Grado lagoons (Italy) harbor detoxifying Hg resistant bacteria in soft tissues

A mechanism of mercury detoxification has been suggested by a previous study on Hg bioaccumulation in Manila clams (Ruditapes philippinarum) in the polluted Marano and Grado lagoons and in this study we demonstrate that this event could be partly related to the detoxifying activities of Hg-resistant bacteria (MRB) harbored in clam soft tissues. Therefore, natural clams were collected in six stations during two different periods (winter and spring) from Marano and Grado Lagoons. Siphons, gills and hepatopancreas from acclimatized clams were sterile dissected to isolate MRB. These anatomical parts were glass homogenized or used for whole, and they were lying on a solid medium containing 5mgl−1 HgCl2 and incubated at 30°C. A total of fourteen bacterial strains were isolated and were identified by 16S rDNA sequencing and analysis, revealing that strains were representative of eight bacterial genera, four of which were Gram-positive (Enterococcus, Bacillus, Jeotgalicoccus and Staphylococcus) and other four were Gram-negative (Stenotrophomonas, Vibrio, Raoultella and Enterobacter). Plasmids and merA genes were found and their sequences determined. Fluorescence in situ hybridization (FISH) technique shows the presence of Firmicutes, Actinobacteria and Gammaproteobacteria by using different molecular probes in siphon and gills. Bacterial clumps inside clam flesh were observed and even a Gram-negative endosymbiont was disclosed by transmission electronic microscope inside clam cells. Bacteria harbored in cavities of soft tissue have mercury detoxifying activity. This feature was confirmed by the determination of mercuric reductase in glass-homogenized siphons and gills.

Bacterial strains resistant to inorganic and organic forms of mercury isolated from polluted sediments of the Orbetello Lagoon, Italy, and their possible use in bioremediation processes

Bacteria are able to adapt to heavy metals in contaminated environments, by developing specific mechanisms of resistance. A mercury (Hg)-resistant bacterial community was isolated from polluted sediments of the Orbetello Lagoon, Italy. The members of the Hg-resistant bacterial community showed high levels of resistance both to the inorganic and to the organic forms of Hg. 16S rRNA gene sequencing showed the presence of different genera, and a bacterial strain resistant to Hg belonging to the genus Luteimonas was evidenced for the first time. The merA gene coding for mercury-reductase conferring resistance to inorganic Hg, and the merB gene coding for mercury-lyase enzymes, for resistance to the organic forms of Hg, were detected in the tested bacterial strains. The community showed the presence of bacteria belonging to the genera Pseudomonas and Psychrobacter that highlighted the capability to reduce Hg2+ to the volatile form of Hg0 . Experiments carried out with immobilized cells of the Hg-resistant strains removed 96% of Hg in sediments leachates in a bioremediation laboratory scale pilot plant. A methylating activity of the sulphate-reducing bacteria of the same polluted sediments was moreover evidenced. These results evidenced the presence of microbial communities highly adapted to the presence of Hg into the sediments of the lagoon. The use of the isolated autochthonous bacterial strains for bioremediation of the native sediments contaminated by Hg is suggested.

The IncF plasmid pRSB225 isolated from a municipal wastewater treatment plant’s on-site preflooder combining antibiotic resistance and putative virulence functions is highly related to virulence plasmids identified in pathogenic E. coli isolates

The IncF antibiotic resistance and virulence plasmid pRSB225, isolated from an unknown bacterium released with the purified wastewater from a municipal sewage treatment plant into the environment has been analysed at the genomic level by pyrosequencing. The 164,550bp plasmid comprises 210 coding sequences (cds). It is composed of three replicons (RepFIA, RepFIB, and RepFII) and encodes further plasmid-specific functions for stable maintenance and inheritance and conjugative plasmid transfer. The plasmid is self-transmissible and shows a narrow host range limited to the family Enterobacteriaceae. The accessory modules of the plasmid mainly comprise genes conferring resistance to ampicillin (blaTEM-1b), chloramphenicol (catA1), erythromycin (mphA), kanamycin and neomycin (aphA1), streptomycin (strAB), sulphonamides (sul2), tetracycline (tetA(B)) and trimethoprim (dfrA14), as well as mercuric ions (mer genes). In addition, putative virulence-associated genes coding for iron uptake (iutA/iucABCD, sitABCD, and a putative high-affinity Fe2+ uptake system) and for a toxin/antitoxin system (vagCD) were identified on the plasmid. All antibiotic and heavy metal resistance genes are located either on class 1 (Tn10-remnant, Tn4352B) and class 2 transposons (Tn2-remnant, Tn21, Tn402-remnant) or a class 1 integron, whereas almost all putative virulence genes are associated with IS elements (IS1, IS26), indicating that transposition and/or recombination events were responsible for acquisition of the accessory pRSB225 modules. Particular modules of plasmid pRSB225 are related to corresponding segments of different virulence plasmids harboured by pathogenic Escherichia coli strains. Moreover, pRSB225 modules were also detected in entero-aggregative-haemorrhagic E. coli (EAHEC) draft genome sequences suggesting that IncF plasmids related to pRSB225 mediated gene transfer into pathogenic E. coli derivatives.

Impact of mercury on denitrification and denitrifying microbial communities in nitrate enrichments of subsurface sediments

The contamination of groundwater with mercury (Hg) is an increasing problem worldwide. Yet, little is known about the interactions of Hg with microorganisms and their processes in subsurface environments. We tested the impact of Hg on denitrification in nitrate reducing enrichment cultures derived from subsurface sediments from the Oak Ridge Integrated Field Research Challenge site, where nitrate is a major contaminant and where bioremediation efforts are in progress. We observed an inverse relationship between Hg concentrations and onset and rates of denitrification in nitrate enrichment cultures containing between 53 and 1.1 μM of inorganic Hg; higher Hg concentrations increasingly extended the time to onset of denitrification and inhibited denitrification rates. Microbial community complexity, as indicated by terminal restriction fragment length polymorphism (tRFLP) analysis of the 16S rRNA genes, declined with increasing Hg concentrations; at the 312 nM Hg treatment, a single tRFLP peak was detected representing a culture of Bradyrhizobium sp. that possessed the merA gene indicating a potential for Hg reduction. A culture identified as Bradyrhizobium sp. strain FRC01 with an identical 16S rRNA sequence to that of the enriched peak in the tRFLP patterns, reduced Hg(II) to Hg(0) and carried merA whose amino acid sequence has 97 % identity to merA from the Proteobacteria and Firmicutes. This study demonstrates that in subsurface sediment incubations, Hg may inhibit denitrification and that inhibition may be alleviated when Hg resistant denitrifying Bradyrhizobium spp. detoxify Hg by its reduction to the volatile elemental form.

Chlor-alkali plant contamination of Aussa River sediments induced a large Hg-resistant bacterial community

A closed chlor-alkali plant (CAP) discharged Hg for decades into the Aussa River, which flows into Marano Lagoon, resulting in the large-scale pollution of the lagoon. In order to get information on the role of bacteria as mercury detoxifying agents, analyses of anions in the superficial part (0–1 cm) of sediments were conducted at four stations in the Aussa River. In addition, measurements of biopolymeric carbon (BPC) as a sum of the carbon equivalent of proteins (PRT), lipids (LIP), and carbohydrates (CHO) were performed to correlate with bacterial biomass such as the number of aerobic heterotrophic cultivable bacteria and their percentage of Hg-resistant bacteria. All these parameters were used to assess the bioavailable Hg fraction in sediments and the potential detoxification activity of bacteria. In addition, fifteen isolates were characterized by a combination of molecular techniques, which permitted their assignment into six different genera. Four out of fifteen were Gram negative with two strains of Stenotrophomonas maltophilia, one Enterobacter sp., and one strain of Brevibacterium frigoritolerans. The remaining strains (11) were Gram positive belonging to the genera Bacillus and Staphylococcus. We found merA genes in only a few isolates. Mercury volatilization from added HgCl2 and the presence of plasmids with the merA gene were also used to confirm Hg reductase activity. We found the highest number of aerobic heterotrophic Hg-resistant bacteria (one order magnitude higher) and the highest number of Hg-resistant species (11 species out of 15) at the confluence of the River Aussa and Banduzzi's channel, which transport Hg from the CAP, suggesting that Hg is strongly detoxified [reduced to Hg(0)] at this location.

Seasonal mercury transformation and surficial sediment detoxification by bacteria of Marano and Grado lagoons

Marano and Grado lagoons are polluted by mercury from the Isonzo River and a chlor-alkali plant, yet despite this contamination, clam cultivation is one of the main activities in the region. Four stations (MA, MB, MC and GD) were chosen for clam seeding and surficial sediments were monitored in autumn, winter and summer to determine the Hg detoxifying role of bacteria. Biotransformation of Hg species in surficial sediments of Marano and Grado lagoons was investigated while taking into consideration the speciation of organic matter in the biochemical classes of PRT (proteins), CHO (carbohydrates) and LIP (lipids), water-washed cations and anions, bacterial biomass, Hg-resistant bacteria, some specific microbial activities such as sulfate reduction rates, Hg methylation rates, Hg-demethylation rates, and enzymatic ionic Hg reduction. MeHg in sediments was well correlated with PRT content, whereas total Hg in sediments correlated with numbers of Hg-resistant bacteria. Correlations of the latter with Hg-demethylation rates in autumn and winter suggested a direct role Hg-resistant bacteria in Hg detoxification by producing elemental Hg (Hg0) from ionic Hg and probably also from MeHg. MeHg-demethylation rates were ∼10 times higher than Hg methylation rates, were highest in summer and correlated with high sulfate reduction rates indicating that MeHg was probably degraded in summer by sulfate-reducing bacteria via an oxidative pathway. During the summer period, aerobic heterotrophic Hg-resistant bacteria decreased to <2% compared to 53% in winter. Four Hg-resistant bacterial strains were isolated, two Gram-positive (Staphylococcus and Bacillus) and two Gram-negative (Stenotrophomonas and Pseudomonas). Two were able to produce Hg0, but just one contained a merA gene; while other two strains did not produce Hg0 even though they were able to grow at 5μgml of HgCl2. Lagoon sediments support a strong sulfur cycle in summer that controls Hg methylation and demethylation. However, during winter, Hg-resistant bacteria that are capable of degrading MeHg via the mer-catalyzed reductive pathway increase in importance.

Chromate-resistance genes in plasmids from antibiotic-resistant nosocomial enterobacterial isolates

The presence of chromate-resistance genes in enterobacteria was evaluated in a collection of 109 antibiotic-resistant nosocomial isolates from nine major cities in México. Results were compared with the presence of mercury-resistance genes. Susceptibility tests showed that 21% of the isolates were resistant to chromate (Cr(R)), whereas 36% were resistant to mercury (Hg(R)). Cr(R) levels were high in Klebsiella pneumoniae (61%), low in Enterobacter cloacae (12%) and Escherichia coli (4%), and null in Salmonella sp. isolates. Colony hybridization demonstrated that the majority of metal-resistant isolates hybridized with chrA gene (87% of Cr(R) isolates), encoding a CHR transporter homologue, and merA gene (74% of Hg(R) isolates), encoding MerA mercuric reductase, suggesting that most isolates expressed these widespread metal-resistance systems. Southern blot hybridization of Cr(R) isolates showed that plasmids of 80, 85, and 95 kb from K. pneumoniae isolates, and of 100 kb from an E. cloacae isolate, contained chrA-related sequences. These plasmids belonged to IncN or IncP incompatibility groups, and conferred Cr(R), as well as multiple antibiotic resistance, when transferred by conjugation to an E. coli standard strain. These data indicated that Cr(R) genes may be distributed among clinical enterobacteria via conjugative plasmids, probably by coselection with antibiotic-resistant genes.

Effects of long‐term fertilization on the diversity of bacterial mercuric reductase gene in a Chinese upland soil

Soil mercury (Hg) pollution has received considerable attention due to its neurotoxin effects and its potential risk to food safety. The microbial transformation of Hg plays a key role in reducing Hg toxicity by the mercuric reductase (MerA) conferred by genes arranged in the mer operon. This study investigated the effects of long-term fertilization on the diversity of bacterial mercuric reductase gene (merA), which specify the reduction of ionic Hg²⁺ to the volatile elemental form Hg⁰, in an agricultural soil with relatively high Hg content. The soil samples were collected from different treatments, including control without fertilizer (CK), fertilizer nitrogen (N), combined fertilizers (NPK) of N, phosphorus (P) and potassium (K), and NPK plus organic manure (NPK + OM). The merA gene diversity patterns were analyzed based on the merA clone libraries and sequencing measurements. Results showed that the merA gene diversity was influenced by soil variables depending on the fertilization practices. In particular, NH₄⁺ and NO₃⁻ contents had strong effect on the merA gene diversity pattern both in the N and NPK treatments, whereas the merA gene diversity pattern in NPK + OM treatment was distinctly influenced by the contents of organic matter, available P and K. These results suggested that long-term fertilization had significant influences on merA gene diversity, which could be helpful to understand the Hg reduction process and potentially serve microbial remediation of Hg contaminated soil.

Bacterial mer operon-mediated detoxification of mercurial compounds: a short review

Mercury pollution has emerged as a major problem in industrialized zones and presents a serious threat to environment and health of local communities. Effectiveness and wide distribution of mer operon by horizontal and vertical gene transfer in its various forms among large community of microbe reflect importance and compatibility of this mechanism in nature. This review specifically describes mer operon and its generic molecular mechanism with reference to the central role played by merA gene and its related gene products. The combinatorial action of merA and merB together maintains broad spectrum mercury detoxification system for substantial detoxification of mercurial compounds. Feasibility of mer operon to coexist with antibiotic resistance gene (ampr, kanr, tetr) clusters enables extensive adaptation of bacterial species to adverse environment. Flexibility of the mer genes to exist as intricate part of chromosome, plasmids, transposons, and integrons enables high distribution of these genes in wider microbial gene pool. Unique ability of this system to manipulate oligodynamic property of mercurial compounds for volatilization of mercuric ions (Hg2+) makes it possible for a wide range of microbes to tolerate mercury-mediated toxicity.