Detection of mercury(II) by DNA templated gold nanoclusters based on forming thymidine–Hg2+–thymidine duplexes

We have successfully synthesized gold nanoclusters (AuNCs) templated with DNA (5′‐CCCCCCCCCCCCTTTTTT‐3′), and subsequently employed the fluorescent DNA‐AuNCs as a novel probe for sensitive detections of mercury ions (Hg²⁺). Basically, the procedure is due to the formation of thymidine–Hg²⁺–thymidine duplexes between DNA‐AuNCs and Hg²⁺, thus leading to aggregations of DNA‐AuNCs described here occurring, and facilitating their fluorescence decrease. Significantly, this decrease of fluorescent signals permitted sensitive detection of Hg²⁺ in a linear range of 0.1–100 µmol L^?1, with a detection limit of 0.083 µmol L^?1 at a signal‐to‐noise ratio of 3. Additionally, the practicality of this probe for assaying Hg²⁺ in human urine and lake water samples was further validated, and showed various advantages including simplicity, selectivity, sensitivity and low cost, demonstrating its potential to broaden ways for assaying Hg²⁺ in real samples. Copyright © 2014 John Wiley & Sons, Ltd.     

Structural and functional characterization of mercuric reductase from <Emphasis Type="Italic">Lysinibacillus sphaericus</Emphasis> strain G1

In response to the widespread presence of inorganic Hg in the environment, bacteria have evolved resistance systems with mercuric reductase (MerA) as the key enzyme. MerA enzymes have still not been well characterized from gram positive bacteria. Current study reports physico-chemical, kinetic and structural characterization of MerA from a multiple heavy metal resistant strain of Lysinibacillus sphaericus, and discusses its implications in bioremediation application. The enzyme was homodimeric with subunit molecular weight of about 60 kDa. The K_m and V_max were found to be 32 µM of HgCl₂ and 18 units/mg respectively. The enzyme activity was enhanced by β-mercaptoethanol and NaCl up to concentrations of 500 µM and 100 mM respectively, followed by inhibition at higher concentrations. The enzyme showed maximum activity in the pH range of 7–7.5 and temperature range of 25–50 °C, with melting temperature of 67 °C. Cu²⁺ exhibited pronounced inhibition of the enzyme with mixed inhibition pattern. The enzyme contained FAD as the prosthetic group and used NADPH as the preferred electron donor, but it showed slight activity with NADH as well. Structural characterization was carried out by circular dichroism spectrophotometry and X-ray crystallography. X-ray confirmed the homodimeric structure of enzyme and gave an insight on the residues involved in catalytic binding. In conclusion, the investigated enzyme showed higher catalytic efficiency, temperature stability and salt tolerance as compared to MerA enzymes from other mesophiles. Therefore, it is proposed to be a promising candidate for Hg²⁺ bioremediation.     

Characterization of mercury(II)-induced inhibition of photochemistry in the reaction center of photosynthetic bacteria

Mercuric contamination of aqueous cultures results in impairment of viability of photosynthetic bacteria primarily by inhibition of the photochemistry of the reaction center (RC) protein. Isolated reaction centers (RCs) from Rhodobacter sphaeroides were exposed to Hg²⁺ ions up to saturation concentration (~?10³ [Hg²⁺]/[RC]) and the gradual time- and concentration-dependent loss of the photochemical activity was monitored. The vast majority of Hg²⁺ ions (about 500 [Hg²⁺]/[RC]) had low affinity for the RC [binding constant K_b?~?5 mM^?1] and only a few (~?1 [Hg²⁺]/[RC]) exhibited strong binding (K_b?~?50 μM^?1). Neither type of binding site had specific and harmful effects on the photochemistry of the RC. The primary charge separation was preserved even at saturation mercury(II) concentration, but essential further steps of stabilization and utilization were blocked already in the 5 < [Hg²⁺]/[RC]?<?50 range whose locations were revealed. (1) The proton gate at the cytoplasmic site had the highest affinity for Hg²⁺ binding (K_b?~?0.2 μM^?1) and blocked the proton uptake. (2) Reduced affinity (K_b?~?0.05 μM^?1) was measured for the mercury(II)-binding site close to the secondary quinone that resulted in inhibition of the interquinone electron transfer. (3) A similar affinity was observed close to the bacteriochlorophyll dimer causing slight energetic changes as evidenced by a?~?30 nm blue shift of the red absorption band, a 47 meV increase in the redox midpoint potential, and a?~?20 meV drop in free energy gap of the primary charge pair. The primary quinone was not perturbed upon mercury(II) treatment. Although the Hg²⁺ ions attack the RC in large number, the exertion of the harmful effect on photochemistry is not through mass action but rather a couple of well-defined targets. Bound to these sites, the Hg²⁺ ions can destroy H-bond structures, inhibit protein dynamics, block conformational gating mechanisms, and modify electrostatic profiles essential for electron and proton transfer.     

The Role of Ca<Superscript>2+</Superscript> Imbalance in the Induction of Acute Oxidative Stress and Cytotoxicity in Cultured Rat Cerebellar Granule Cells Challenged with Tetrabromobisphenol A

Using primary cultures of rat cerebellar granule cells (CGC) we examined the role of calcium transients induced by tetrabromobisphenol A (TBBPA) in triggering oxidative stress and cytotoxicity. CGC were exposed for 30 min to 10 or 25 µM TBBPA. Changes in intracellular calcium concentration ([Ca²⁺]_i), in the production of reactive oxygen species (ROS), and in the potential of mitochondria (?Ψm) were measured fluorometrically during the exposure. The intracellular glutathione (GSH) and catalase activity were determined after the incubation; cell viability was evaluated 24 h later. TBBPA concentration-dependently increased [Ca²⁺]_i and ROS production, and reduced GSH content, catalase activity, ?Ψm and neuronal viability. The combination of NMDA and ryanodine receptor antagonists, MK-801 and bastadin 12 with ryanodine, respectively, prevented Ca²⁺ transients and partially reduced cytotoxicity induced by TBBPA at both concentrations. The antagonists also completely inhibited oxidative stress and depolarization of mitochondria evoked by 10 µM TBBPA, whereas these effects were only partially reduced in the 25 µM TBBPA treatment. Free radical scavengers prevented TBBPA-induced development of oxidative stress and improved CGC viability without having any effect on the rises in Ca²⁺ and drop in ?Ψm. The co-administration of scavengers with NMDA and ryanodine receptor antagonists provided almost complete neuroprotection. These results indicate that Ca²⁺ imbalance and oxidative stress both mediate acute toxicity of TBBPA in CGC. At 10 µM TBBPA Ca²⁺ imbalance is a primary event, inducing oxidative stress, depolarization of mitochondria and cytotoxicity, whilst at a concentration of 25 µM TBBPA an additional Ca²⁺-independent portion of oxidative stress and cytotoxicity emerges.     

Copper and mercury induced oxidative stresses and antioxidant responses of Spirodela polyrhiza (L.) Schleid

Duckweed is recognized as a phytoremediation aquatic plant due to the production of large biomass and a high level of tolerance in stressed conditions. A laboratory experiment was conducted to investigate antioxidant response and mechanism of copper and mercury tolerance of S. polyrhiza (L.) Schleid. To understand the changes in chlorophyll content, MDA, proline, and activities of ROS-scavenging enzymes (SOD, CAT, GPOD) during the accumulation of Cu⁺² and Hg⁺², S. polyrhiza were exposed to various concentrations of Cu⁺² (0.0–40 μM) and Hg⁺² (0.0–0.4 μM). antioxidant activity initially indicated enhancing trend with application of 10 μM Cu⁺²; 0.2 μM Hg⁺² (SOD), of 20 μM Cu⁺²; 0.2 μM Hg⁺² (CAT) and of 10 μM Cu⁺²;0.2 μM Hg⁺² (GPOD) and then decreased consistently up to 40 μM Cu⁺² and 0.4 μM Hg⁺². In the experiment chlorophyll and frond multiplication initially showed increasing tendency and decreased gradually with the application of increased metal concentration. Application of heavy metal has constantly enhanced proline and MDA content while the maximum increase was observed with the application of 40 μM Cu; 0.4 μM Hg for proline and MDA respectively. The upregulation of antioxidant enzymes and proline reveals that S. polyrhiza has strong biochemical strategies to deal with the heavy metal toxicity induced by the accumulation of Cu⁺² and Hg⁺².     

Subcellular distribution and chemical forms of Co<Superscript>2+</Superscript> in three barley genotypes under different Co<Superscript>2+</Superscript> levels

The toxicity of many heavy metals in plants is closely associated with its subcellular distribution and chemical forms. The subcellular distribution and chemical forms of cobalt (Co²⁺) were investigated using 3 barley genotypes differing in Co²⁺ toxicity resistance, namely Yan66 (resistant), Ea 52 (sensitive), and Humai 4 (moderate), under two Co²⁺ levels (25 and 100 µM). Higher Co²⁺ level in cultural solution significantly increased Co²⁺ accumulation in all subcellular fractions, with vacuole and cell wall having higher concentration. In comparison with 25 µM Co²⁺, 100 µM Co²⁺ treatment caused significant increase of Co²⁺ concentration in the forms of F-NaCl (extracted with 1 M NaCl), F-Ac (extracted with 2% HAc), F-HCl (extracted by 0.6 M HCl), and F-residue (residue forms) in both shoots and roots. There was a significant difference among genotypes in Co²⁺ subcellular distribution and chemical forms, with Ea52 accumulating more Co²⁺ in organelles and Yan66 accumulating more Co²⁺ in vacuole and cell wall. Moreover, the inorganic form of Co²⁺ extracted with 80% ethanol (F-ethanol) and water-soluble form (F-H₂O) were significantly increased in Ea52, while Yan66 accumulated more Co²⁺ in the forms of low-bioavailable molecules (F-NaCl, F-HAc, and F-HCl). The results suggest that the vacuolar sequestration and cell wall deposition of Co²⁺ is a key resistant mechanism for genotype Yan66.     

DNA‐based chemiluminescent nanoprobes for highly sensitive and selective detection of mercury(II) ion

A simple and sensitive DNA‐stablized gold nanoparticle (AuNP)‐based chemiluminescent (CL) probe for detecting mercury ion (Hg²⁺) in aqueous solution has been developed. The CL strategy relies upon the catalytic activity of unmodified AuNPs on the luminol–H₂O₂ CL reaction, and the interaction of unmodified AuNPs with DNA. The unmodified AuNPs can effectively differentiate unstructured and folded DNA. The DNA desorbs from AuNPs in the presence of Hg²⁺, leading to the increase in CL signal. By rationally varying the number of thymine in single‐strand oligonucleotides, the detection range could be tuned. Employing single‐strand oligonucleotides with 14 thymine in the detecting system, a sensitive linear range for Hg²⁺ ions from 5.0 × 10^–10 to 1.0 × 10^–7 mol/L and a detection limit of 2.1 × 10^–10 mol/L are obtained. Changing the number of thymine to 10 and 6, it leads to a narrow detection range but a high sensitivity. Besides, DNA‐based CL nanoprobes exhibit a remarkable selectivity for Hg²⁺ ions over a variety of competing metal ions. Copyright © 2012 John Wiley & Sons, Ltd.     

Genotype-dependent effects of phosphorus supply on physiological and biochemical responses to Al-stress in cultivated and Tibetan wild barley

Aluminium (Al) toxicity and phosphorus (P) deficiency often co-exist in acidic soils and limit plant growth and crop production. To investigate the alleviating effects of different levels of phosphorus on Al stress, greenhouse hydroponic experiments were conducted using two contrasting Tibetan wild barley genotypes XZ16 and XZ61 of Al tolerant and sensitive, respectively, and Al tolerant cv. Dayton. The results showed that Al stress induced reduction in P accumulation in plants; and stem and leaf P concentrations of the three genotypes, except of XZ16 under HP + Al (100 µM Al with high level of 360 µM P) which was close to the control level. XZ16 recorded significantly higher P accumulation in plants, compared with XZ61 and Dayton, and P concentrations in leaves under Al stress, and in stems under NP + Al (100 µM Al with normal level of 180 µM P) and HP + Al. Meanwhile, H⁺-, Ca²⁺Mg²⁺-, and Total- ATPase activities in XZ16 and Dayton under Al stress were markedly higher than in XZ61. Normal or high level of P under Al stress could relieve Al stress as enhanced plant biomass, with increased photosystem II photochemistry (Fv/Fm) and P content, relative to the low level of 90 µM P. Compared with XZ61, addition of high P concentration for XZ16 significantly increased the values of Gs and Tr, with higher root GPX and H⁺-ATPase activities, and such nutrient elements as P, Mg and Ca in stems and leaves, and induced more malate secretion, but less MDA accumulation.     

Dose-dependent effects of developmental mercury exposure on C-start escape responses of larval zebrafish Danio rerio

Zebrafish Danio rerio embryos were exposed to 0, 25, 50 or 75 ppb Hg²⁺ from 0 to 24 h post‐fertilization (hpf) then placed into Hg²⁺‐free water. Inductively coupled plasma‐mass spectrophotometer analysis of whole embryo Hg²⁺ content at 24 hpf showed a positive correlation with exposure regime (Pearson's one‐tailed, r²= 0·698, P < 0·01); at 5 days post‐hatch (dph), whole larval Hg²⁺ content was not detectable. Hg²⁺‐induced behavioural deficits in larvae were, therefore, due to changes during embryogenesis and not to residual Hg²⁺ in the larvae. At 5 dph, larvae were tested for responses to different frequencies but equal intensities of vibrational stimuli generated by a remotely controlled plastic hammer. Data were recorded by high‐speed videography and computer‐analysed for latency of response (ms), amplitude of the response as measured by maximum initial velocity [normalized as body (standard) lengths s^?1; V_max] and duration of behaviour from initial head movement to cessation of caudal tail movement (ms). A single mechanical stimulus resulted in behavioural outcomes that were related to embryonic Hg²⁺ uptake. Response latency increased with exposure level and displayed an increase of ×1·5–2·5 over control values (ANOVA, P < 0·01). The V_max decreased with exposure level to a low of 71% of control at the highest Hg²⁺ concentration (ANOVA, P < 0·01). Duration of behaviour displayed a biphasic response pattern in which exposure to 0, 50 or 75 ppb Hg²⁺ did not result in a significantly different response yet exposure to 25 ppb Hg²⁺ caused a significantly longer time of active response (ANOVA, P < 0·01). Repeated stimulation (1, 2 or 4 hits s^?1) resulted in a concentration‐dependent increase in response failures. Regardless of stimulation frequency, larvae exposed to 0 or 25 ppb Hg²⁺ as embryos maintained higher V_max levels for longer intervals during the testing period than those exposed as embryos to either 50 or 75 ppb Hg²⁺.     

A Michaelis–Menten type equation for describing methylmercury dependence on inorganic mercury in aquatic sediments

Methylation of mercury (Hg) is the crucial process that controls Hg biomagnification along the aquatic food chains. Aquatic sediments are of particular interest because they constitute an essential reservoir where inorganic divalent Hg (Hg^II) is methylated. Methylmercury (MeHg) concentrations in sediments mainly result from the balance between methylation and demethylation reactions, two opposite natural processes primarily mediated by aquatic microorganisms. Thus, Hg availability and the activity of methylating microbial communities control the MeHg abundance in sediments. Consistently, some studies have reported a significant positive correlation between MeHg and Hg^II or total Hg (Hg_T), taken as a proxy for Hg^II, in aquatic sediments using enzyme-catalyzed methylation/demethylation mechanisms. By compiling 1,442 published and unpublished Hg_T–MeHg couples from lacustrine, riverine, estuarine and marine sediments covering various environmental conditions, from deep pristine abyssal to heavily contaminated riverine sediments, we show that a Michaelis–Menten type relationship is an appropriate model to relate the two parameters: MeHg = aHg_T/(K _m  + Hg_T), with a = 0.277 ± 0.011 and K _m  = 188 ± 15 (R ² = 0.70, p < 0.001). From K _m variations, which depend on the various encountered environmental conditions, it appears that MeHg formation and accumulation are favoured in marine sediments compared to freshwater ones, and under oxic/suboxic conditions compared to anoxic ones, with redox potential and organic matter lability being the governing factors.     

A chromosomally based luminescent bioassay for mercury detection in red soil of China

A luminescent reporter gene system was constructed by fusing the mercury-inducible promoter, P _merT , and its regulatory gene, merR, with a promoterless reporter gene EGFP. A stable and nonantibiotic whole-cell reporter (BMB-ME) was created by introducing the system cassette into the chromosome of Pseudomonas putida strain and then applied it for mercury detection in the red soil of China. Spiked with 10 and 100 μg g⁻¹ Hg²⁺ and after 15 and 30 days incubation, soil samples were extracted and evaluated water soluble, bioavailable, organic matter bound, and residual fractions of mercury by both BMB-ME and chemical way. The expression of EGFP was confirmed in soil extraction, and fluorescence intensity was quantified by luminescence spectrometer. The sensor strain BMB-ME appeared to have a detection range similar to that of reversed-phase high-performance liquid chromatography method. The optimal temperature for EGFP expression was 35°C and the lowest detectable concentration of Hg²⁺ 200 nM. Cu²⁺, Fe²⁺, Mn²⁺, Sn²⁺, Zn²⁺, Co²⁺, Ag⁺, Ba²⁺, Mg²⁺, and Pb²⁺ ions at nanomolar level did not interfere with the measurement. These results showed that the BMB-ME constitute an adaptable system for easy sensing of small amounts of mercury in the red soil of China.     

Characterization of the metabolically modified heavy metal-resistant Cupriavidus metallidurans strain MSR33 generated for mercury bioremediation

Background

Mercury-polluted environments are often contaminated with other heavy metals. Therefore, bacteria with resistance to several heavy metals may be useful for bioremediation. Cupriavidus metallidurans CH34 is a model heavy metal-resistant bacterium, but possesses a low resistance to mercury compounds.

Methodology/Principal Findings

To improve inorganic and organic mercury resistance of strain CH34, the IncP-1β plasmid pTP6 that provides novel merB, merG genes and additional other mer genes was introduced into the bacterium by biparental mating. The transconjugant Cupriavidus metallidurans strain MSR33 was genetically and biochemically characterized. Strain MSR33 maintained stably the plasmid pTP6 over 70 generations under non-selective conditions. The organomercurial lyase protein MerB and the mercuric reductase MerA of strain MSR33 were synthesized in presence of Hg²⁺. The minimum inhibitory concentrations (mM) for strain MSR33 were: Hg²⁺, 0.12 and CH₃Hg⁺, 0.08. The addition of Hg²⁺ (0.04 mM) at exponential phase had not an effect on the growth rate of strain MSR33. In contrast, after Hg²⁺ addition at exponential phase the parental strain CH34 showed an immediate cessation of cell growth. During exposure to Hg²⁺ no effects in the morphology of MSR33 cells were observed, whereas CH34 cells exposed to Hg²⁺ showed a fuzzy outer membrane. Bioremediation with strain MSR33 of two mercury-contaminated aqueous solutions was evaluated. Hg²⁺ (0.10 and 0.15 mM) was completely volatilized by strain MSR33 from the polluted waters in presence of thioglycolate (5 mM) after 2 h.

Conclusions/Significance

A broad-spectrum mercury-resistant strain MSR33 was generated by incorporation of plasmid pTP6 that was directly isolated from the environment into C. metallidurans CH34. Strain MSR33 is capable to remove mercury from polluted waters. This is the first study to use an IncP-1β plasmid directly isolated from the environment, to generate a novel and stable bacterial strain useful for mercury bioremediation.     

Cell Surface Display of MerR on <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> for Biosorption of Mercury

The metalloregulatory protein MerR which plays important roles in mer operon system exhibits high affinity and selectivity toward mercury (II) (Hg²⁺). In order to improve the adsorption ability of Saccharomyces cerevisiae for Hg²⁺, MerR was displayed on the surface of S. cerevisiae for the first time with an α-agglutinin-based display system in this study. The merR gene was synthesized after being optimized and added restriction endonuclease sites EcoR I and Mlu I. The display of MerR was indirectly confirmed by the enhanced adsorption ability of S. cerevisiae for Hg²⁺ and colony PCR. The hydride generation atomic absorption spectrometry was applied to measure the Hg²⁺ content in water. The engineered yeast strain not only showed higher tolerance to Hg, but also their adsorption ability was much higher than that of origin and control strains. The engineered yeast could adsorb Hg²⁺ under a wide range of pH levels, and it could also adsorb Hg²⁺ effectively with Cd²⁺ and Cu²⁺ coexistence. Furthermore, the engineered yeast strain could adsorb ultra-trace Hg²⁺ effectively. The results above showed that the surface-engineered yeast strain could adsorb Hg²⁺ under complex environmental conditions and could be used for the biosorption and bioremediation of environmental Hg contaminants.     

A new proposal regarding the transport mechanism of mercury in biological membranes

The interactions of mercury (Hg²⁺) with biological membranes have been investigated. The experimental results indicate that Hg²⁺ induces a rapid alkalinization in energized Lysosomes from rat liver. The interpretation of the process is that the mercury enters the Lysosomes as a Hg(OH)₂ electroneutral compound, thus inducing alkalinization in the matrix.     

Construction of recombinant mercury resistant Acidithiobacillus caldus

A mercury-resistant plasmid of pTMJ212 which was able to shuttle between Acidithiobacillus caldus and Escherichia coli was constructed by inserting the mercury resistant determinants, the mer operon of Acidithiobacillus ferrooxidans, into the IncQ plasmid of pJRD215. pTMJ212 was transferred from Escherichia coli into Acidithiobacillus caldus through conjugation. Furthermore, pTMJ212 was transferred back from Acidithiobacillus caldus into Escherichia coli, thereby confirming the initial transfer of pTMJ212 from Escherichia coli to Acidithiobacillus caldus. Compared to the control, the cell growth of the recombinant Acidithiobacillus caldus increased markedly under mercury (Hg²⁺) stress especially at Hg²⁺ concentrations ranging from 2.0 to 4.5 μg/ml.     

Cation Modulation of Hemoglobin Interaction with Sodium n-Dodecyl Sulfate (SDS). III: Calcium Interaction with R- and Mixed Spin States of Hemoglobin S at pH 5.0: The Musical Chair Paradox

We investigate the interaction of Ca²⁺ (0–500 µM) and a membrane mimic (0.60 mM SDS) with both the R- and mixed spin states hemoglobin S (HbS) as a function of time. These interactions were carried out at pH 5.0. We aim at ascertaining if there is or are differences in the UV–Visible spectra of such interactions to account for the dynamics of calcium ion concentrations [Ca²⁺] in initiating structures which may ultimately suggest HbS polymerization and or resistance to Plasmodium attack. From our results, we conclude that (a) simultaneous interaction of 40 µM Ca²⁺ and 0.60 mM SDS with the R state protein would promote structural formations that can “lock up” the protein for nucleation on the membranes and or become cytotoxic to the parasite; (b) simultaneous R state HbS-SDS or R state HbS-Ca²⁺ would lead to enhanced hemin formation and less deoxyHb species. This condition is unlikely to precipitate polymerization in the HbS but the resulting hemin would poison the parasite; (c) the mixed spin state HbS-SDS and 40 µM Ca²⁺ interaction yields more toxic products to that of the interaction of the mixed spin HbS-SDS with 500 µM Ca²⁺ thus suggesting why the 40 µM Ca²⁺ is important in parasite Hb proteolysis; and (d) pronounced structural changes on interaction with SDS and Ca²⁺ are more in the R state to the mixed spin state.     

Mercury detoxifying enzymes within endospores of a broad-spectrum mercury resistantBacillus pasteurii strain DR2

Bacillus pasteurii DR₂, a broad-spectrum Hg-resistant bacterial strain, exhibited delayed sporulation and less mercury volatilization in the presence of mercury compounds. However, Hg-sensitiveBacillus subtilis sporulated quickly in the presence of HgCl₂ and volatilized no mercury. Levels of Hg²⁺-reductase and organomercurial lyase in the endospores ofBacillus pasteurii DR₂ were lower than those in vegetative cells     

Detoxification of mercury and organomercurials by nitrogen-fixing soil bacteria

Minimal inhibitory concentration values of HgCl₂ and 5 organomercurials were determined against 24 mercury-resistant N₂-fixing soil bacteria previously isolated from soil and identified in our laboratory. These bacterial strains also displayed multiple antibiotic resistant properties. Typical growth pattern of a highly mercury-resistantBeijerinckia sp (KDr₂) was studied in liquid broth supplemented with toxic levels of mercury compounds. Four bacterial strains were selected for determining their ability to volatilize mercury and their Hg-volatilizing capacity was different. Cell-free extracts prepared from overnight mercury-induced cells catalyzed Hg²⁺-induced NADPH oxidation. Specific activities of Hg²⁺-reductase which is capable of catalyzing conversion of Hg²⁺ →Hg(o) of 10 Hg-resistant bacterial strains are also reported.