Bacterial sorption of heavy metals

Staphylococcal surface display of metal-binding polyhistidyl peptides

Recombinant Staphylococcus xylosus and Staphylococcus carnosus strains were generated with surface-exposed chimeric proteins containing polyhistidyl peptides designed for binding to divalent metal ions. Surface accessibility of the chimeric surface proteins was demonstrated and the chimeric surface proteins were found to be functional in terms of metal binding, since the recombinant staphylococcal cells were shown to have gained Ni(2+)- and Cd(2+)-binding capacity, suggesting that such bacteria could find use in bioremediation of heavy metals. This is, to our knowledge, the first time that recombinant, surface-exposed metal-binding peptides have been expressed on gram-positive bacteria. Potential environmental or biosensor applications for such recombinant staphylococci as biosorbents are discussed.

Sub-lethal effects of heavy metals on activated sludge microorganisms

The effects of heavy metals, at sub-lethal concentrations, on activated sludge microbial ecosystem were investigated. Adsorption capacity and rate of copper, chromium, lead and zinc on microbial flocs were much faster than that of organic matters. Metals affected not only the adsorption rate of organic matters but also the COD adsorption capacity of the activated sludge. Effects of heavy metals, on wastewater treatment performance of a sequencing batch reactor were also studied. Metal-laden wastewater at sub-lethal levels affected the performance to different extents, depending on the hydraulic retention time. Metal ions acted as a strong competitor against the organic compounds for active sites on the bioflocs instead of acting as a toxic microbial inhibitor, thus hampering organic adsorption and affecting the COD removal efficiency under shorter HRTs.

Removal of silver from photographic wastewater effluent using Acinetobacter baumannii BL54

Acinetobacter baumannii BL54, a silver (Ag) resistant micro-organism was isolated from clinical samples collected at the Armed Forces Medical College hospital in Pune, India. The strain BL54 removed a high quantity of silver (2.85 mg/g biomass) from photographic wastewater effluent. Treatment of the cells with 10 mM EDTA or agitating the culture did not affect the removal process, while altering pH of the wastewater or pre-treating the cells with 0.5 mM 2,4-dinitrophenol (DNP), 20 μM N,N'-dicyclohexylcarbodiimide (DCC), 25 μg/mL cefotaxime, and polymyxin-B resulted in considerable decrease in removal of silver by the organism. Dead cells, or a Ags plasmid-cured derivative (BL54.1) removed little silver, which was mainly surface bound. The results, compared with accumulation of Ag by a sensitive culture of Escherichia coli K12 J53.2, suggest that A. baumannii BL54 has good potential for bioremediation of silver from photographic wastewater effluents. Key words: Acinetobacter, silver resistance, silver removal.

Metal binding by Pseudomonas aeruginosa PAO1 is influenced by growth of the cells as a biofilm

The metal-binding properties of Pseudomonas aeruginosa PAO1 biofilms were investigated using four metals (Cu, Fe, Au, and La). All but one of the metals (i.e., Cu) were bound by the biofilms in amounts that were significantly greater than those bound by planktonically grown cells of the same strain. Lanthanum precipitation appeared to be limited to the base of the biofilms and was not promoted by a shift in lipopolysaccharide production by the cells.Key words: metal binding, biofilms, Gram-negative bacteria, bioremediation.

Removal of cadmium from scallop hepatopancreas by microbial processes

A microbial process for removing cadmium from a homogenate of hepatopancreas, a waste of scallop processing, was devised to use this waste for value-added protein resources. Microorganisms were screened on the basis of the ability to remove cadmium from a medium with the initial concentration of 10 mg/l of cadmium. One soil isolate, identified as Xanthomonas sp. UR No. 2 by its taxonomical characteristics, removed 98% of the cadmium in the medium in 2 d. During cultivation of this strain in the homogenates of hepatopancreas digested by endopeptidases, 90% of cadmium was removed, while this strain had little effect on the simple non-digested homogenates. The mass balance of cadmium during homogenizations of the hepatopancreas tissues and cultivations in the protease-treated homogenate were examined. The content of crude proteins of culture supernatant treated by Xanthomonas sp. UR No. 2 was equivalent to those of various feedstuffs on the market.

Effect of O-side-chain-lipopolysaccharide chemistry on metal binding

Pseudomonas aeruginosa PAO1 produces two chemically distinct types of lipopolysaccharides (LPSs), termed A-band LPS and B-band LPS. The A-band O-side chain is electroneutral at physiological pH, while the B-band O-side chain contains numerous negatively charged sites due to the presence of uronic acid residues in the repeat unit structure. Strain PAO1 (A+ B+) and three isogenic LPS mutants (A+ B-, A- B+, and A- B-) were studied to determine the contribution of the O-side-chain portion of LPS to metal binding by the surfaces of gram-negative cells. Transmission electron microscopy with energy-dispersive X-ray spectroscopy was used to locate and analyze sites of metal deposition, while atomic absorption spectrophotometry and inductively coupled plasma-mass spectrometry were used to perform bulk quantitation of bound metal. The results indicated that cells of all of the strains caused the precipitation of gold as intracellular, elemental crystals with a d-spacing of 2.43 A. This type of precipitation has not been reported previously for gram-negative cells and suggests that in the organisms studied gold binding is not a surface-mediated event. All four strains bound similar amounts of copper (0.213 to 0.222 micromol/mg [dry weight] of cells) at the cell surface, suggesting that the major surface metal-binding sites reside in portions of the LPS which are common to all strains (perhaps the phosphoryl groups in the core-lipid A region). However, significant differences were observed in the abilities of strains dps89 (A- B+) and AK1401 (A+ B-) to bind iron and lanthanum, respectively. Strain dps89 caused the precipitation of iron (1.623 micromol/mg [dry weight] of cells) as an amorphous mineral phase (possibly iron hydroxide) on the cell surface, while strain AK1401 nucleated precipitation of lanthanum (0.229 micromol/mg [dry weight] of cells) as apiculate, surface-associated crystals. Neither iron nor lanthanum precipitates were observed on the cells of other strains, which suggests that the combination of A-band LPS and B-band LPS produced by a cell may result in a cell surface which promotes the formation of metal-rich precipitates. We therefore propose that the negatively charged sites located in the O-side chains are not directly responsible for the binding of metallic ions; however, the B-band LPS molecule as a whole may contribute to overall cell surface properties which favor the precipitation of distinct metal-rich mineral phases.

Asking for the indicator function of bioassays evaluating soil contamination: are bioassay results reasonable surrogates of effects on soil microflora?

In evaluating the biological effect of solid materials like soil a bacterial contact assay often shows higher sensitivity than elutriate testing. Results of the Bacillus cereus contact assay for some environmental important toxicants are presented in this article. A comparison with another heterotrophic soil bacterium, Arthrobacter globiformis, shows comparable sensitivity. In a bioassay approach organisms at the level of individuals or populations are exposed to soil material to determine the significance of contaminants. An investigation that incorporates community level processes in comparison with toxicity test results provides a better understanding of the indicator function of bioassays. Comparison of soil bioassays (aqueous and solid phase) with ecological parameters demonstrates the problems in predicting ecological effects of soil contamination.

Cadmium removal by a new strain of Pseudomonas aeruginosa in aerobic culture

A fluorescent pseudomonad (strain CW-96-1) isolated from a deep-sea vent sample grew at 30 degrees C under aerobic conditions in an artificial seawater medium and tolerated cadmium concentrations up to 5 mM. After 140 h, strain CW-96-1 removed > 99% of the cadmium from solution. Energy dispersive microanalysis revealed that the cadmium was removed by precipitation on the cell wall; sulfide production was confirmed by growth on Kligler's agar. Based on 16S ribosomal DNA sequencing and fatty acid analysis, the microorganism is closely related to Pseudomonas aeruginosa.

Interactions between biofilms and the environment

The surfaces of bacteria are highly interactive with their environment. Whether the bacterium is Gram-negative or Gram-positive, most surfaces are charged at neutral pH because of the ionization of the reactive chemical groups which stud them. Since prokaryotes have a high surface area-to-volume ratio, this can have surprising ramifications. For example, many bacteria can concentrate dilute environmental metals on their surfaces and initiate the development of fine-grained minerals. In natural environments, it is not unusual to find such bacteria closely associated with the minerals which they have helped develop. Bacteria can be free-living (planktonic), but in most natural ecosystems they prefer to grow on interfaces as biofilms; supposedly to take advantage of the nutrient concentrative effect of the interface, although there must also be gained some protective value against predators and toxic agents. Using a Pseudomonas aeruginosa model system, we have determined that lipopolysaccharide is important in the initial attachment of this Gram-negative bacterium to interfaces and that this surface moiety subtly changes during biofilm formation. Using this same model system, we have also discovered that there is a natural tendency for Gram-negative bacteria to concentrate and package periplasmic components into membrane vesicles which bleb-off the surface. Since some of these components (e.g., peptidoglycan hydrolases) can degrade other surrounding cells, the vesicles could be predatory; i.e., a natural system by which neighboring bacteria are targeted and lysed, thereby liberating additional nutrients to the microbial community. This obviously would be of benefit to vesicle-producing bacteria living in biofilms containing mixed microbial populations.

Bacteria as Mediators of Copper Sulfide Enrichment During Weathering

Supergene chalcocite enrichment during weathering is an economically vital natural process that may lead to severalfold increases in the copper content of sulfide deposits. A scanning electron microscope study of chalcocite (Cu2S) from major enriched copper deposits in northern Chile revealed myriad bacterioform bodies in original growth positions near replacement interfaces with remnant hypogene sulfide grains. These minute (0.03 to 0.2 micrometers) chalcocite bodies are interpreted as fossilized and metallized nannobacteria that promoted the fixation of mobilized copper ions. Bacterial activity may thus be a fundamental factor in supergene enrichment of copper deposits.

Identification of a high-molecular-weight cadmium-binding protein in copper-resistant Bacillus acidocaldarius cells

Bacillus acidocaldarius grown in the presence of Cu++ was capable of accumulating the metal in the form of a protease-sensitive high molecular weight (HMW) moiety whose formation was inhibited by actinomycin D. Only cells preadapted in Cu++ were able to grow in a Cd(++)-containing medium. A cell-free extract from cadmium-stressed cells was fractionated by gel-permeation chromatography. The majority of cadmium was found associated with a HMW protein fraction which was further purified by anion exchange chromatography and high-performance liquid chromatography. The molecular weight of the purified protein was estimated to be 23,000 by SDS-PAGE. Amino acid analysis showed a low cysteine content and an abundance of aspartate and glutamate. It is likely that the cadmium-binding protein is an essential component of the mechanism mediating recovery from heavy metal toxicity.

Microbial metal-binding mechanisms and their relation to nuclear waste disposal

The cell surface polymers of microorganisms readily bind a variety of metal ions, which enables the organisms to immobilize potentially toxic metal ions before they encounter the plasma membrane. Under appropriate chemical conditions, bound metal ions can form a variety of minerals that may be of major geological importance. Many studies have shown the occurrence of metal binding and biomineralization in nature, but detailed knowledge of the underlying mechanisms is lacking. The microbial influence of this binding may be indirect, such as physicochemical influences on the solution chemistry, Eh, and pH; or direct, when it is determined by the type of organisms present, their energy metabolism, and the structural and chemical characteristics of the cell surface and extracellular polymers. Metal binding by bacterial cell surfaces has several implications in nuclear waste disposal including adsorption of soluble radionuclides. A detailed knowledge of the chemical mechanisms of metal interactions with the microbial cell surface will enhance our understanding of the geochemical environment within a nuclear waste disposal vault.Key words: biomineralization, radionuclide immobilization, biofilm, bacterial cell surface.

Cadmium binding by bacteria: screening and characterization of new isolates and mutants

A fast and simple methodology was developed that enables screening of microbial strains for their ability to bind cadmium. It is based on the use of a radioisotope of cadmium (109Cd) for screening colonies and for evaluation of cadmium binding. The methods described here can be used to screen new environmental isolates or to obtain mutants with altered ability to bind cadmium. Examples for the two uses are described in the paper.

Mineral Precipitation by Epilithic Biofilms in the Speed River, Ontario, Canada

Epilithic microbial communities, ubiquitously found in biofilms on submerged granite, limestone, and sandstone, as well as on the concrete support pillars of bridges, were examined in the Speed River, Ontario, Canada. Transmission electron microscopy showed that attached bacteria (on all substrata) were highly mineralized, ranging from Fe-rich capsular material to fine-grained (<1 μm) authigenic (primary) mineral precipitates. The authigenic grains exhibited a wide range of morphologies, from amorphous gel-like phases to crystalline structures. Energy-dispersive X-ray spectroscopy indicated that the most abundant mineral associated with epilithic bacteria was a complex (Fe, Al) silicate of variable composition. The gel-like phases were similar in composition to a chamositic clay, whereas the crystalline structures were more siliceous and had compositions between those of glauconite and kaolinite. The consistent formation of (Fe, Al) silicates by all bacterial populations, regardless of substratum lithology, implies that biomineralization was a surface process associated with the anionic nature of the cell wall. The adsorption of dissolved constituents from the aqueous environment contributed significantly to the mineral formation process. In this regard, it appears that epilithic microbial biofilms dominate the reactivity of the rock-water interface and may determine the type of minerals formed, which will ultimately become part of the riverbed sediment. Because rivers typically contain high concentrations of dissolved iron, silicon, and aluminum, these findings provide a unique insight into biogeochemical activities that are potentially widespread in natural waters.

Characterization of the Binding of Gallium, Platinum, and Uranium to Pseudomonas fluorescens by Small-Angle X-Ray Scattering and Transmission Electron Microscopy

Small-angle X-ray scattering (SAXS) was used to determine the binding of Ga, U, and Pt to Pseudomonas fluorescens in aqueous buffer. Atomic absorption spectrophotometry was used to quantify the heavy metals during bulk analysis, whereas transmission electron microscopy of whole mounts and thin sections was used to determine the locations of the cell-bound metal precipitates, as well as their sizes and physical structures. Energy-dispersive X-ray spectroscopy confirmed the compositions and identities of the precipitates and helped show that they were associated primarily with the envelope layers of the bacteria. Unlike Ga and Pt, which were located only at the cell surface, U was also found intracellularly in ∼ 10% of the cells. This cytoplasmic location ultimately killed and lysed the cells. Surface-bound Ga and U were spread over the entire cell envelope (outer membrane-peptidoglycan-plasma membrane complex), whereas Pt was associated only with the lipopolysaccharide-rich, external face of the outer membrane. SAXS confirmed these data and showed that the bacteria were metal-enshrouded particles that were 1.0 to 1.5 μm in diameter. SAXS also provided a statistically significant representation of the bound metal precipitates, which ranged in size from 10 nm to 1 μm. The correlation between the microscopic data and the scattering data was extremely good. Since SAXS is performed in an aqueous milieu, it yields a more representative picture of the physical state of the metal bound to cell surfaces.

Remobilization of Heavy Metals Retained as Oxyhydroxides or Silicates by Bacillus subtilis Cells

The stability of heavy metals bound as either silicates or oxyhydroxides by the surfaces of Bacillus subtilis with changes in pH and presence of competing cations and EDTA was studied. Wall-bound silicate was very stable. Of all conditions, a pH of 3.0 (by HNO 3 ) promoted the greatest release of Cd (87% [wt/wt]), Pb (77%), or Cu (54%). Bacterial Pb resisted extraction by EDTA, and Cd that was included in bacterial-silicates proved difficult to remobilize.

Mechanism of silicate binding to the bacterial cell wall in Bacillus subtilis

To investigate the chemical mechanism of silicate binding to the surface of Bacillus subtilis, we chemically modified cell wall carboxylates to reverse their charge by the addition of an ethylenediamine ligand. For up to 9 weeks, mixtures of Si, Al-Fe-Si, and Al-Fe-Si plus toxic heavy metals were reacted with these cells for comparison with control cells and abiotic solutions. In general, more Si and less metal were bound to the chemically modified surfaces, thereby showing the importance of an electropositive charge in cell walls for fine-grain silicate mineral development. The predominant reaction for this development was the initial silicate-to-amine complexation in the peptidoglycan of ethylenediamine-modified and control cell walls, although metal ion bridging between electronegative sites and silicate had an additive effect. The binding of silicate to these bacterial surfaces can thus be described as outer sphere complex formation because it occurs through electrostatic interaction.

Quantitative and Qualitative Analysis of Bacteria in Er(III) Solution by Thin-Film Magnetopheresis

Magnetic deposition, quantitation, and identification of bacteria reacting with the paramagnetic trivalent lanthanide ion, Er 3+ , was evaluated. The magnetic deposition method was dubbed thin-film magnetopheresis. The optimization of the magnetic deposition protocol was accomplished with Escherichia coli as a model organism in 150 mM NaCl and 5 mM ErCl 3 solution. Three gram-positive bacteria, Staphylococcus epidermidis, Staphylococcus saprophyticus, and Enterococcus faecalis, and four gram-negative bacteria, E. coli, Pseudomonas aeruginosa, Proteus mirabilis, and Klebsiella pneumoniae, were subsequently investigated. Quantitative analysis consisted of the microscopic cell count and a scattered-light scanning of the magnetically deposited material aided by the computer data acquisition system. Qualitative analysis consisted of Gram stain differentiation and fluorescein isothiocyanate staining in combination with selected antisera against specific types of bacteria on the solid substrate. The magnetic deposition protocol allowed quantitative detection of E. coli down to the concentration of 10 5 CFU ml -1 , significant in clinical diagnosis applications such as urinary tract infections. Er 3+ did not interfere with the typical appearance of the Gram-stained bacteria nor with the antigen recognition by the antibody in the immunohistological evaluations. Indirect antiserum-fluorescein isothiocyanate labelling correctly revealed the presence of E. faecalis and P. aeruginosa in the magnetically deposited material obtained from the mixture of these two bacterial species. On average, the reaction of gram-positive organisms was significantly stronger to the magnetic field in the presence of Er 3+ than the reaction of gram-negative organisms. The thin-film magnetophoresis offers promise as a rapid method for quantitative and qualitative analysis of bacteria in solutions such as urine or environmental water.

Precipitation of cadmium by Clostridium thermoaceticum

Cadmium at an initial concentration of 1 mM was completely precipitated by cultures of Clostridium thermoaceticum in complex medium. The precipitation was energy dependent and required cysteine, although cysteine alone did not act as a growth substrate. Electron microscopic analysis revealed localized areas of precipitation at the surfaces of nonstarved cells as well as precipitate in the surrounding medium. The addition of cadmium had no apparent effect on growth or acetogenesis. However, nickel and cadmium were synergistically toxic at a concentration (1 mM) at which neither alone was toxic. The amount of protein extracted from cadmium-treated cultures was twofold higher than that in control extracts, and the amount of total sulfide was fourfold higher in cultures containing cadmium than in control cultures. Comparable levels of cysteine desulfhydrase activity were observed in extracts of both cadmium-treated and control cultures, but the enzyme activity was expressed maximally about 24 h earlier in the cadmium-treated cultures than in the untreated controls.

Deposition of bismuth by Yersinia enterocolitica

Yersinia enterocolitica 8081c cultures in exponential growth were incubated for 1 h in 0.1% microcrystalline bismuth subsalicylate (BSS) suspensions. Scanning electron microscopy (SEM) revealed microcrystals directly bound to BSS-treated bacteria. Energy dispersive spectroscopy (EDS) X-ray microanalysis of the attached microcrystals confirmed that the crystals were the microcrystalline BSS. X-ray spectra positive for bismuth were also obtained by SEM-EDS X-ray microanalysis of whole bacteria, suggesting metal incorporation into the bacteria in regions absent of bound microcrystals. Transmission electron microscopy of thin sections of embedded preparations of BSS-treated exponential-growth-phase bacteria showed electron-dense deposits in the periphery of the bacteria. Y. enterocolitica cultures that were in stationary phase at the time of incubation with microcrystalline BSS showed no evidence of the electron-dense deposits and EDS spectra were negative for bismuth. Bacteria incubated in the absence of microcrystalline BSS also lacked electron-dense deposits. Scanning transmission electron microscopy used in conjunction with EDS X-ray microanalysis to view and analyze semi-thick sections (250-300 nm) of embedded preparations of BSS-treated bacteria in exponential growth confirmed that the electron-dense deposits at the periphery of the bacteria are the sites of bismuth depositions.

The application of biotechnology to the treatment of wastes produced from the nuclear fuel cycle: biodegradation and bioaccumulation as a means of treating radionuclide-containing streams

Recent concerns on the radiotoxicity and longevity of nuclides have prompted the development of new technologies for their removal from wastes produced from nuclear power programs and nuclear fuel reprocessing activities. Alongside developments from traditional chemical treatment processes, interest has also centered on the application of biotechnology for efficient waste treatment. Many biological techniques have relied on empirical approaches in simple model systems, with scant regard to the nature and volume of actual target wastes; such considerations may limit the application of the new technologies in practice. This review aims to identify some of the likely problems, to discuss the various approaches under current consideration, and to evaluate ways in which either the target waste or the detoxifying biomass may be modified or presented for the most efficient treatment.

Remobilization of toxic heavy metals adsorbed to bacterial wall-clay composites

Significant quantities of Ag(I), Cu(II), and Cr(III) were bound to isolated Bacillus subtilis 168 walls, Escherichia coli K-12 envelopes, kaolinite and smectite clays, and the corresponding organic material-clay aggregates (1:1, wt/wt). These sorbed metals were leached with HNO3, Ca(NO3)2, EDTA, fulvic acid, and lysozyme at several concentrations over 48 h at room temperature. The remobilization of the sorbed metals depended on the physical properties of the organic and clay surfaces and on the character and concentration of the leaching agents. In general, the order of remobilization of metals was Cr much less than Ag less than Cu. Cr was very stable in the wall, clay, and composite systems; pH 3.0, 500 microM EDTA, 120-ppm [mg liter-1] fulvic acid, and 160-ppm Ca remobilized less than 32% (wt/wt) of sorbed Cr. Ag (45 to 87%) and Cu (up to 100%) were readily removed by these agents. Although each leaching agent was effective at mobilizing certain metals, elevated Ca or acidic pH produced the greatest overall mobility. The organic chelators were less effective. Lysozyme digestion of Bacillus walls remobilized Cu from walls and Cu-wall-kaolinite composites, but Ag, Cr, and smectite partially inhibited enzyme activity, and the metals remained insoluble. The extent of metal remobilization was not always dependent on increasing concentrations of leaching agents; for example, Ag mobility decreased with some clays and some composites treated with high fulvic acid, EDTA, and lysozyme concentrations. Sometimes the organic material-clay composites reacted in a manner distinctly different from that of their individual counterparts; e.g., 25% less Cu was remobilized from wall- and envelope-smectite composites than from walls, envelopes, or smectite individually in 500 microM EDTA. Alternatively, treatment with 160-ppm Ca removed 1.5 to 10 times more Ag from envelope-kaolinite composites than from the individual components. The particle size of the deposited metal may account for some of the stability changes; those metals that formed large, compact aggregates (Cr and Ag) as seen by transmission electron microscopy were less likely to be remobilized. In summary, it is apparent that remobilization of toxic heavy metals in sediments, soils, and the vadose zone is a complicated issue. Predictions based on single inorganic or organic component systems are too simplistic.

Bacterial interactions with silver

This review examines interactions between bacteria and the biologically non-essential metal, silver. Aspects of silver toxicity, tolerance and accumulation (possible binding and uptake as opposed to energy-dependent transport) in bacteria are discussed. In addition, plasmid biology is examined briefly since little information is available on the exact mechanism(s) of plasmid-endoced silver resistance in bacteria.