Plasmid-controlled mercury biotransformation by Clostridium cochlearium T-2

Formation of harmful compounds in biotransformation of lilial by microorganisms isolated from human skin

CONTEXT

The biotransformation of lilial results in an acid that is used in the dairy industry, in perfumery, as an intermediate in the manufacture of pharmaceuticals and cosmetics, and as a food additive for enhancing taste.

OBJECTIVE

This study investigates the biotransformation of lilial by Staphylococcus aureus and Staphylococcus epidermidis, two bacterial species isolated from human skin.

MATERIALS AND METHODS

Both species of Staphylococcus were isolated in samples taken from the skin of individuals living in a rural area of Iran. The pH of the culture medium was optimized, and after culturing the microorganisms, the bacteria were added to a flask containing a nutrient broth and incubated for several hours. The flasks of bacteria were combined with lilial, and various biochemical tests and diagnostics were performed, including Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectrophotometry (UV-Vis), and gas chromatography-mass spectroscopy (GC-MS).

RESULTS

The S. aureus produced isobutyric acid (2-methylpropanoic acid) after 72 h (71% of the total products yielded during biotransformation), whereas the S. epidermidis produced terpenoid alcoholic media after 24 h (90% of total products obtained).

DISCUSSION AND CONCLUSION

The results obtained indicate that biotransformation of lilial by S. aureus is more desirable than by S. epidermidis due to the highly efficient production of a single product. Bourgeonal and liliol were two toxic compounds produced during biotransformation, which indicates that the use of lilial in cosmetics can be harmful to the skin.

Methylmercury Resistance in Desulfovibrio desulfuricans Strains in Relation to Methylmercury Degradation

Two strains of Desulfovibrio desulfuricans, one known to synthesize monomethylmercury from ionic mercury, were grown to determine methylmercury toxicity and for comparison with an anaerobic strain of Clostridium pasteurianum, a H(2) producer, and with the broad-spectrum mercury-resistant Pseudomonas putida strain FB-1, capable of degrading 1 mug of methylmercury to methane and elemental mercury in 2 h. The CH(3)HgCl resistance of D. desulfuricans strains was 10 times that of P. putida FB-1 and 100 times that of C. pasteurianum. The methylmercury resistance of D. desulfuricans was related to the disappearance of methylmercury from cultures by transformation to dimethylmercury, metacinnabar, methane, and traces of ionic mercury. During a 15-day experiment the kinetics of the two volatile compounds dimethylmercury [(CH(3))(2)Hg] and methane were monitored in the liquid by a specific new technique with purge-and-trap gas chromatography in line with Fourier transform infrared spectroscopy and in the headspace by gas chromatography with flame ionization detection. Insoluble metacinnabar (cubic HgS) of biological origin was detected by X-ray diffractometry in the gray precipitate from the insoluble residue of the pellet of a 1-liter culture spiked with 100 mg of CH(3)HgCl. This was compared with a 1-liter culture of D. desulfuricans LS spiked with 100 mg of HgCl(2). In a further experiment, it was demonstrated that insoluble, decomposable, white dimethylmercury sulfide [(CH(3)Hg)(2)S] formed instantly in the reaction of methylmercury with hydrogen sulfide. This organomercurial was extracted with chloroform and identified by gas chromatography in line with mass spectrometry. The D. desulfuricans strains were resistant to high concentrations of methylmercury because they produced insoluble dimethylmercury sulfide, which slowly decomposed under anaerobic conditions to metacinnabar and volatilized to dimethylmercury and methane between pHs 6.2 and 6.5 for high (4.5-g . liter) or low (0.09-g . liter) sulfate contents. Methane was produced from CH(3)HgCl at a lower rate than by the broad-spectrum Hg-resistant P. putida strain FB-1.

Genetic engineering of transgenic tobacco for enhanced uptake and bioaccumulation of mercury

To further enhance the efficiency and potential of plants for phytoremediation of mercury pollution, a genetically engineered tobacco to simultaneously express mercury transporter, mercury transporter (MerT) and mercury chelator, polyphosphate (polyP) was constructed by integrating bacterial merT gene in polyphosphate kinase gene (ppk)-transgenic tobacco, and its ability to phytoremediate mercury was evaluated. Integration of merT gene into ppk-transgenic tobacco did not significantly affect the mercury resistant phenotypes and polyP production. Transgenic expression of MerT in ppk-transgenic tobacco resulted in accelerated and enhanced mercury uptake into tobacco. In addition, tobacco expressing MerT and polyP accumulated significantly more mercury than the ppk-transgenic tobacco from medium containing a wide range of low concentrations of Hg(2+). The combination of accelerated mercury uptake and enhanced mercury accumulation mediated by MerT represents one way for shortening the purification completion time, and for improving tobacco plants to be more suitable for use in phytoremediation of low levels of mercury contamination.

Accumulation of mercury in transgenic tobacco expressing bacterial polyphosphate

The feasibility of transgenic tobacco, engineered to express bacterial polyphosphate (polyP), for phytoremediation of mercury pollution was evaluated. T3 progeny of the transgenic tobacco produced a large amount of polyP in leaves and showed a relatively high resistant phenotype to Hg2+ than its wild-type progenitors. These results suggest that the integrated ppk gene, encoding polyphosphate kinase (PPK), a key enzyme for polyP biosynthesis, is stably conserved in tobacco genome, and translated to active PPK which catalyzed biosynthesis of polyP, and suggest that polyP is capable of reducing the cytotoxicity of Hg2+, probably via chelation formation with polyP. The transgenic tobacco expressing polyP accumulated significantly more mercury than its wild-type progenitors from Hg2+-containing agar medium and simulated soils without taxing the tobacco plants suggesting that the transported Hg2+ was accumulated as a less toxic Hg-polyP complex in the tobacco tissues. Based on the results obtained in the present study, the polyP-mediated accumulation of mercury from mercurial-contaminated soils may provide an ecologically compatible approach for phytoremediation of mercury pollution.

Polyphosphate produced in recombinant Escherichia coli confers mercury resistance

An Escherichia coli strain was generated by fusion of a merA-deleted broad-spectrum mer operon from Pseudomonas K-62 with a bacterial polyphosphate kinase gene (ppk) from Klebsiella aerogenes in vector pUC119. A large amount of the ppk-specified polyphosphate was identified in the mercury-induced bacterium with the fusion plasmid designated pMKB18 but not in the cells without mercury induction. These results suggest that the synthesis of polyphosphate as well as the expression of the mer genes is mercury-inducible and regulated by merR. The E. coli strain with pMKB18 was more resistant to both Hg2+ and C6H5Hg+ than its isogenic strain with cloning vector pUC119. The recombinant strain accumulated more mercury from Hg2+- and C6H5Hg+-contaminated medium. Hg2+ transported into the cytoplasm appeared to be bound by chelation with the polyphosphate produced by the recombinant cells. The transported phenylmercury was degraded to Hg2+ before the chelation since polyphosphate did not directly chelate with C6H5Hg+. These results indicate that polyphosphate is capable of reducing the cytotoxicity of the transported Hg2+ probably via chelation between polyphosphate and Hg2+.

Evaluation of ppk-specified polyphosphate as a mercury remedial tool

To evaluate the utility of polyphosphate kinase gene (ppk)-specified polyphosphate in mercury remediation, a fusion plasmid, pMK27, with ppk from Klebsiella aerogenes and mercury transport genes, merT and merP, from Pseudomonas K-62, was constructed. The transcription and translation of ppk, merT and merP were found to be mercury-inducible. The ppk-specified polyphosphate was identified in cells preinduced by Hg2+, but not in cells without mercury induction, suggesting that the synthesis of polyphosphate is regulated by merR. The hypersensitive phenotype to Hg2+, shown by bacteria with pMRD141, which contains merT and merP, was almost completely restored to its original levels when the ppk was introduced into the plasmid, suggesting that the Hg2+-toxicity was reduced by the polyphosphate, probably via chelation formation. Bacteria with pMK27 accumulated approximately 6-fold more mercury than the bacteria with cloning vector, pUC119. These results clearly demonstrate that the polyphosphate is capable of retaining mercury in the cells without taxing the cells. Based on the results obtained in the present study, the fusion plasmid pMK27 may serve as a strategy for mercury remediation.

Recent advances in the genetics of the clostridia

Several laboratories around the world have started work on genetic analysis of clostridia. Interest in this diverse group of anaerobic organisms has grown with increasing awareness of the benefits that may accrue from their biotechnological exploitation. Research to date has focussed on construction of shuttle vectors containing replicons from clostridial and streptococcal plasmids, development of methods for transferring genes, and molecular cloning of genes--especially those involved in toxigenicity, fermentative metabolism and polysaccharide utilization. In selected species gene transfer by protoplast transformation, electroporation and conjugation has been accomplished and transposable elements have been introduced. It can be anticipated that our understanding of the molecular biology of these interesting organisms will grow rapidly in the future, bringing with it improved prospects for rational biotechnological exploitation.

Screening for plasmids in the genus Clostridium

A plasmid screening was performed on 150 strains out of 75 clostridial species using a modification of the alkaline-lysis procedure. In 26 strains representing 21 species one or more plasmid bands were detected ranging in size from 3 to more than 100 kilobase pairs. Clostridium aceticum proved to contain a single small plasmid (pCA1) of 5.4 kbp as revealed by restriction analysis and electron microscopy. A physical map of pCA1 has been constructed. Spontaneous mutants of C. aceticum defective in autotrophic growth have been isolated. No direct correlation between plasmid content and autotrophy could be found.

Plasmids in Clostridium botulinum and related Clostridium species

Toxigenic Clostridium botulinum and nontoxigenic C. sporogenes, C. subterminale, and C. botulinum-like organisms from a variety of sources were screened for plasmids. Of the 68 toxigenic C. botulinum isolates, 56% carried one or more plasmids, ranging in mass from 2.1 to 81 megadaltons. Within individual groups (based on the type of neurotoxin produced), many strains showed identical plasmid banding patterns on agarose gels. Of the 15 nontoxigenic strains tested, 40% also carried one or more plasmids ranging from 1.7 to 25.0 megadaltons, with both unique and common banding patterns represented. A total of 67 plasmids from both toxigenic and nontoxigenic strains were detected. At this time, no phenotypic functions have been assessed for these plasmids, and they must therefore be considered cryptic. A variety of lysing and extraction techniques were necessary to detect plasmids in the different C. botulinum groups.

Methylation of mercury from dental amalgam and mercuric chloride by oral streptococci in vitro

The capacity of the oral bacteria Streptococcus mitior, S. mutans and S. sanguis to methylate mercury was investigated in vitro. Mercuric chloride and pulverized dental amalgam in distilled water, respectively, were used as sources of mercury. Methylmercury was found in the bacterial cells of all three tested strains. The results indicate that organic mercury compounds may be formed in the oral cavity.

Plasmid detection and isolation in strains of Clostridium acetobutylicum and related species

Twenty-one strains of Clostridium acetobutylicum, C. butylicum and Clostridium saccharoperbutylacetonicum were examined. Seven of them contained extrachromosomal DNA molecules, with a size ranging from 2.6 to more than 50 megadaltons. Strain M1 carries a small plasmid of 2.6 megadaltons, AB10 at least one plasmid of 2.6 megadaltons, AB12 one plasmid of 5.2 megadaltons, AB14 and AB16 a plasmid of about 7 megadaltons and a large one of more than 50 megadaltons, AB17 carries at least one plasmid of 6.7 megadaltons and AB18 two plasmids (4-6 megadaltons and 10-12 megadaltons). All of them are cryptic as at present no function can be correlated with their presence in a bacterial strain.

Involvement of mercury methylation in microbial mercury detoxication

A vitamin B12 requiring strain was isolated from Clostridium cochlearium T-2C which is known to synthesize various types of vitamin B12 including methylcobalamin and has an ability to methylate inorganic mercury. The vitamin B12 auxotroph lacking the mercury-methylating activity showed higher sensitivity to inorganic mercury than its original strain, while the sensitivity of both strains to methylmercury was relative low and essentially the same. These data seem to present affirmative evidence to postulate the physiological role of methylcobalamin-dependent methylation of mercury to be a process of detoxication.

Role of hydrogen sulfide in mercury resistance determined by plasmid of Clostridium cochlearium T-2

Mercury resistance of Clostridium cochlearium T-2P was found to be controlled by a different mechanism from those reported so far since no mercury-reducing activity was detected in this strain. The H2S generating ability as well as the demethylating activity in this bacterium was eliminated by the treatment of the cured acridine dye and recovered by the conjugation of the cured strain with the parent strain. In addition, the strain which lost their abilities to generate H2S and to decompose methylmercury, showed higher sensitivity to mercurials than the parent strain. From these results, the genes conferring both the activities seemed to reside on the plasmid and the mechanism of mercury resistance was probably based on a detoxification mechanism involving methylmercury decomposition and inactivation of the inorganic mercury with H2S.