重金属胁迫对真菌生长及发酵液pH的影响

【背景】矿区废渣堆重金属污染严重,废渣堆分布着一些耐重金属的微生物。【目标】探究重金属胁迫对真菌生长及发酵液pH的影响。【方法】从金川矿区废渣堆采集土样,分离培养具有产酸能力的真菌,采用形态学与分子生物学技术鉴定这些菌株,并测定其产酸能力及其对Pb~(2+)、Cd~(2+)和Zn~(2+)的耐受性。【结果】形态学及18S rRNA基因序列分析获得黑曲霉ZJ-I (Aspergillus niger ZJ-I)和产黄青霉ZJ-V (Penicilium chrysogenum ZJ-V)两个产酸菌株。未加重金属培养时,与不接种真菌对照相比,上述2个菌株的发酵液pH分别下降0.58和0.69;添加重金属处理后,随着重金属浓度的增加,pH变化幅度变小,不同浓度Pb~(2+)使A.nigerZJ-I发酵液pH值分别下降0.53、0.39、0.34和0.39,使P. chrysogenum ZJ-V发酵液pH值分别下降0.21、0.23、0.14和0.09;不同浓度Cd~(2+)使A. niger ZJ-I发酵液pH值分别下降0.75、0.43、0.39和0.32,使P. chrysogenum ZJ-V发酵液pH值分别下降0.62、0.46、0.38和0.49;不同浓度Zn~(2+)可使A.nigerZJ-I发酵液pH分别下降0.87、0.61、0.57和0.43,使P. chrysogenum ZJ-V发酵液pH分别下降1.1、0.34、0.44和0.49;低浓度的Zn~(2+)对菌株A.niger ZJ-I和P. chrysogenum ZJ-V产酸都有促进作用,低浓度的Cd~(2+)对A. niger ZJ-I产酸有促进作用。当Cd~(2+)、Zn~(2+)与Pb~(2+)的浓度分别超过200、400、2 000 mg/L时,3种不同浓度的重金属对菌株A. niger ZJ-I的抑制率达到80%以上,抑制效果显著;当Cd~(2+)、Zn~(2+)与Pb~(2+)浓度分别超过200、1 000、2 000 mg/L时,3种不同浓度的重金属对菌株P.chrysogenumZJ-V抑制率达到80%以上,抑制效果显著。【结论】两株真菌均具有产酸能力和一定的重金属耐受性,菌株P. chrysogenum ZJ-V发酵液产酸性能与重金属耐受能力都要优于ZJ-I,菌株ZJ-V具备潜在的淋洗重金属污染土壤的能力。     

牛蒡根际可培养细菌多样性和镉耐性初步分析

从牛蒡根际土壤中分离可培养细菌,进行多样性分析,并对镉耐受性菌株进行筛选及其抗性和种群多样性进行了分析。限制性内切酶多态性分析显示,分离的菌株可分为9个操作分类单元(OUT),分别属于变形菌门、厚壁菌门和放线菌门,分属于6个科,9个属,其中隶属于肠杆菌属、芽胞杆菌属和假单胞菌属的是优势物种。分离到的耐镉菌株分别属于Bacillus subtilis、Enterobacter aerogenes、Enterobacter ludwigi、Klebsiellasp.、Pectobacterium carotovorum、Pseudomonassp.,而Pectobacterium carotovorumNP22、Enterobacter ludwigii NP23、Pseudomonassp.NP39三菌株可在Cd2+浓度为400 mg/L固体培养基上生长。     

Purification and properties of a manganese-stimulated deoxyribonuclease produced during sporulation of Bacillus subtilis.

A DNAase (deoxyribonuclease) was isolated from culture supernatants of sporulating Bacillus subtilis 168. The purified enzyme migrated as a single band during polyacrylamide-gel electrophoresis. The enzyme differs from other DNAases of B. subtilis in molecular weight, metal-ion requirement and mode of action. The enzyme was inactive in the absence of metal ions, and exhibited optimum activity with 10 mM-Mn2+, although Mg2+, Cd2+ and Co2+ could also permit some activity. The pH optimum for the enzyme was pH 7.5, and it degraded linear-duplex DNA or closed-circular-duplex DNA to acid-soluble material. There was little or no activity on single-stranded DNA or rRNA. Sucrose-gradient analysis of the products of DNAase action on bacteriophage T7 DNA showed that endonucleolytic cleavage had occurred by the introduction of single-strand breaks in both strands of the duplex. The molecular weight of the enzyme was determined, by gel filtration on Sephadex G-75, to be 12000.     

Metal-replacement studies in Bacillus stearothermophilus aldolase and a comparison of the mechanisms of class I and class II aldolases.

A comparison of the product-inhibition patterns during cleavage of D-fructose 1,6-diphosphate by aldolases from yeast, rabbit muscle and Bacillus stearothermophilus shows an ordered reaction sequence for all three enzymes, with dihydroxyacetone phosphate the last-leaving product. Addition of Zn2+, Co2+, Fe2+, Mn2+ or Cd2+ ions to the inactive apo-(Bacillus stearothermophilus aldolase) restores activity to different extents, whereas Ni2+, Mg2+ or Cu2+ ions have no effect. The cleavage activity of this aldolase is not enhanced by added K+ ion. The effects of metal replacement on thermal stability, Km and Vmax. are given and the possible role of the metal is discussed in the light of these results.     

Cadmium uptake by growing cells of gram-positive and gram-negative bacteria

The present study evaluates the effect of the cadmium (Cd2+) on the growth and protein synthesis of some Gram-positive (Staphylococcus aureus, Bacillus subtilis and Streptococcus faecium) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacteria and the cadmium uptake by the same micro-organisms. The Gram-negative bacteria tested were less sensitive to metal ions than the Gram-positive, and P. aeruginosa was the most resistant. The Gram-negative bacteria were also able to accumulate higher amounts of cadmium during growth than the Gram-positive bacteria. The maximum values of specific metal uptake (microgram of Cd2+ incorporated per mg of protein) were: 0.52 for S. aureus, 0.65 for S. faecium, 0.79 for B. subtilis, 2.79 for E. coli and 24.15 for P. aeruginosa, respectively. The differences in the ability to accumulate metal found between Gram-negative and Gram-positive bacteria seems to account for different mechanisms of metal resistance.     

Bacterial sorption of heavy metals.

Four bacteria, Bacillus cereus, B. subtilis, Escherichia coli, and Pseudomonas aeruginosa, were examined for the ability to remove Ag+, Cd2+, Cu2+, and La3+ from solution by batch equilibration methods. Cd and Cu sorption over the concentration range 0.001 to 1 mM was described by Freundlich isotherms. At 1 mM concentrations of both Cd2+ and Cu2+, P. aeruginosa and B. cereus were the most and least efficient at metal removal, respectively. Freundlich K constants indicated that E. coli was most efficient at Cd2+ removal and B. subtilis removed the most Cu2+. Removal of Ag+ from solution by bacteria was very efficient; an average of 89% of the total Ag+ was removed from the 1 mM solution, while only 12, 29, and 27% of the total Cd2+, Cu2+, and La3+, respectively, were sorbed from 1 mM solutions. Electron microscopy indicated that La3+ accumulated at the cell surface as needlelike, crystalline precipitates. Silver precipitated as discrete colloidal aggregates at the cell surface and occasionally in the cytoplasm. Neither Cd2+ nor Cu2+ provided enough electron scattering to identify the location of sorption. The affinity series for bacterial removal of these metals decreased in the order Ag greater than La greater than Cu greater than Cd. The results indicate that bacterial cells are capable of binding large quantities of different metals. Adsorption equations may be useful for describing bacterium-metal interactions with metals such as Cd and Cu; however, this approach may not be adequate when precipitation of metals occurs.     

Toxicity of zinc to fungi, bacteria, and coliphages: influence of chloride ions.

A 10 mM concentration of Zn2+ decreased the survival of Escherichia coli; enhanced the survival of Bacillus cereus; did not significantly affect the survival of Pseudomonas aeruginosa, Norcardia corallina, and T1, T7, P1, and phi80 coliphages; completely inhibited mycelial growth of Rhizoctonia solani; and reduced mycelial growth of Fusarium solani, Cunninghamella echinulata, Aspergillus niger, and Trichoderma viride. The toxicity of zinc to the fungi, bacteria, and coliphages was unaffected, lessened, or increased by the addition of high concentrations of NaCl. The increased toxicity of zinc in the presence of high concentrations of NaCl was not a result of a synergistic interaction between Zn2+ and elevated osmotic pressures but of the formation of complex anionic ZnCl species that exerted greater toxicities than did cationic Zn2+. Conversely, the decrease in zinc toxicity with increasing concentrations of NaCl probably reflected the decrease in the levels of Zn2+ due to the formation of Zn-Cl species, which was less inhibitory to these microbes than was Zn2+. A. niger tolerated higher concentrations of zinc in the presence of NaCl at 37 than at 25 degrees C.     

金属离子对枯草芽孢杆菌转酮酶缺失突变株合成D-核糖的影响

研究了金属离子Mn2 +、Fe2 +、Zn2 +对枯草芽孢杆菌 (Bacillussubtilis)转酮酶 (EC 2 .2 .1 .1 )缺失突变株FBL0 4 531D 核糖合成的影响。发现Mn2 +对该突变株合成D 核糖和形成芽孢具有非常显著的影响。     

The effect of transition metal ions on the resistance of bacterial spores to hydrogen peroxide and to heat.

The presence of 10 microM-Cu2+ increased the lethal effect of hydrogen peroxide on spores of Clostridium bifermentans but not on those of Clostridium sporogenes PA 3679, Clostridium perfringens, Bacillus cereus or Bacillus subtilis var. niger. Cu2+ at 100 muM also increased the lethal effect of heat on spores of C. bifermentans but not on those of B. sutilis var. niger. The rate and extent of Cu2+ uptake by spores of C. bifermentans and B. subtilis var. niger were similar, but examination of unstained sections of spores by electron microscopy suggested that Cu2+ is bound by the protoplasts of spores of C. bifermentans but not of B. subtilis var. niger.     

Mechanism of adsorption of hard and soft metal ions to Saccharomyces cerevisiae and influence of hard and soft anions.

The applicability of the hard-and-soft principle of acids and bases in predicting metal adsorption characteristics in a biological context was investigated for metabolism-independent uptake of the metal ions Sr2+, Mn2+, Zn2+, Cu2+, Cd2+, and Tl+ by Saccharomyces cerevisiae. Metal adsorption increased with external metal concentration (5 to 50 microM), although some saturation of uptake of the harder ions examined, Sr2+, Mn2+, and Zn2+, was evident at the higher metal concentrations. Cation displacement experiments indicated that, with the exception of Tl+, relative covalent bonding (H+ displacement) of the metals was greater at low metal concentrations, while weaker electrostatic interactions (Mg2+ plus Ca2+ displacement) became increasingly important at higher concentrations. These results were correlated with curved Scatchard and reciprocal Langmuir plots of metal uptake data. Saturation of covalent binding sites was most marked for the hard metals, and consequently, although no relationship between metal hardness and ionic/covalent bonding ratios was evident at 10 microM metal, at 50 microM the ratio was generally higher for harder metals. Increasing inhibition of metal uptake at increasing external anion concentrations was partially attributed to the formation of metal-anion complexes. Inhibitory effects of the hard anion SO42(-) were most marked for uptake of the hard metals Sr2+ and Mn2+, whereas greater relative effects on adsorption of the softer cations Cu2+ and Cd2+ were correlated with complexation by the soft anion S2O32(-). Inhibition of uptake of the borderline metal Zn2+ by SO42(-) and that by S2O32(-) were approximately equal.(ABSTRACT TRUNCATED AT 250 WORDS)     

Biosorption of Ni, Cr and Cd by metal tolerant Aspergillus niger and Penicillium sp. using single and multi-metal solution

Fungi including Aspergillus and Penicillium, resistant to Ni2+, Cd2+, and Cr6+ were isolated from soil receiving long-term application of municipal wastewater mix with untreated industrial effluents of Aligarh, India. Metal tolerance in term of minimum inhibitory concentration (MIC) was 125-550 microg/ml for Cd, 300-850 microg/ml for Ni and 300-600 microg/ml for Cr against test fungi. Two isolates, Aspergillus niger and Penicillium sp. were tested for their Cr, Ni and Cd biosorption potential using alkali treated, dried and powdered mycelium. Biosorption experiment was conducted in 100 ml of solution at three initial metal concentrations i.e., 2, 4 and 6 mM with contact time (18 hr) and pretreated fungal biomass (0.1g) at 25 degrees C. Biosorption of all metals was found higher at 4 mM initial metal concentration as compared to biosorption at 2 and 6 mM concentrations. At 4 mM initial metal concentration, chromium biosorption was 18.05 and 19.3 mg/g of Aspergillus and Penicillium biomasses, respectively. Similarly, biosorption of Cd and Ni ions was also maximum at 4 mM initial metal concentration by Aspergillus (19.4 mg/g for Cd and 25.05 mg/g of biomass for Ni) and Penicillium (18.6 mg/g for Cd and 17.9 mg/g of biomass for Ni). In general, biosorption of metal was influenced by initial metal concentration and type of the test fungi. The results indicated that fungi of metal contaminated soil have high level of metal tolerance and biosorption properties.     

Impact of Cell Wall Structure on the Behavior of Bacterial Cells as Sorbents of Cadmium and Lead

Batch metal sorption studies were conducted to compare the behavior of Gram-positive Bacillus subtilis and Gram-negative Escherichia coli as sorbents of Cd 2+ and Pb 2+ . A pH range from 3.0 to 6.5 was investigated at total metal concentrations of 1 2 10 -4.0 and 3.2 2 10 -5 M. Concentration apparent equilibrium sorption constants (K s n M ) and sorption capacity (S max n ) values were determined for the bacteria by fitting experimental data to one- ( n = 1) and two-site ( n = 2) Langmuir sorption isotherms. The sorption data for each of the bacteria were described well by a one-site model (r 2 > 0.9), Cd 2+ exhibited somewhat lower sorption affinities (log K s M =- 1.5 for B. subtilis , and -0.7 for E. coli ) than Pb 2+ (log K s M =-0.6 for B. subtilis and -0.8 for E. coli ). Corresponding S max values for Cd 2+ and Pb 2+ on B. subtilis were 0.36 mmole/g and 0.27 mmole/g, respectively. For E. coli Cd 2+ and Pb 2+ S max values were lower at 0.10 mmole/g and 0.21 mmole/g. A two-site sorption model yielded an improved fit for only the E. coli data with several orders of magnitude difference evident between high (Cd 2+ log K s1 M = 0.9; Pb 2+ log K s1 M = 1.5) and low (Cd 2+ log K s2 M =- 1.1; Pb 2+ log K s2 M = -1.6) affinity sorption sites. In addition, allowing for the presence of low affinity sorption (i.e., S max2 ) sites further increased the total E. coli metal sorption capacity closer to that of B. subtilis . As expected, the sorption of Cd 2+ and Pb 2+ by the bacteria exhibited a strong dependence on pH with sorption edges in the range of pH 4.2 to 5.6. The results of this study show that, despite differences in cell wall structure and composition, B. subtilis and E. coli exhibit remarkably similar sorption behavior toward Cd 2+ and Pb 2+ , respectively. These similarities can be attributed to the specific chemical reactivity of acidic functional groups (e.g., carboxyl, phosphoryl) that occur in the cell walls of both bacteria.     

Voltammetry on mercury and carbon electrodes as a tool for studies of metallothionein interactions with metal ions.

Rabbit liver Cd-metallothionein (CdMT) and Cd-complex of synthetically prepared pentapeptide (gamma-Glu-Cys)2-Gly were studied as examples of animal and plant metallothioneins. Using hanging mercury electrode, cathodic stripping voltammetry after adsorptive accumulation of the Cd(II)-SR complex at different potentials, is suitable for estimating changes occurring in metal coordination due to the presence of metal ions such as Zn2+, Cu2+, Hg2+ or excessive Cd2+. Conditions under which similar behaviour can be observed for both CdMT and Cd-pentapeptide complex are specified. On carbon electrodes, detailed study of reduction processes of Cd(II)-SR complexes is prevented by occurrence of a large catalytic current; oxidation processes are more suitable for study at these electrodes. Carbon composite paste electrode (10% SiO2) allows deposition of Cd(II)-SR complex during its reduction, as was demonstrated with Cd-cysteine, CdMT or Cd-pentapeptide complex. After deposition, oxidation peak of the uncomplexed Cd2+ ions and one or two oxidation peaks corresponding to a formation of the RS-Cd(II) complex are observed. Also, similarly as on Hg electrode, it was observed that excessive Cd2+ or Zn2+ ions influence oxidation peaks of the RS-Cd(II) complex formation. Combination of measurements on mercury electrode and composite paste electrode is recommended for studies of metallothionein interactions with metal ions or other metal complexes.     

Metal ion-induced conformational changes in Escherichia coli alkaline phosphatase.

Ultraviolet difference spectra are produced by the binding of divalent metal ions to metal-free alkaline phosphatase (EC 3.1.3.1). The interaction of the apoprotein with Zn2+, Mn2+, Co2+ and Cd2+, which induce the tight binding of one phosphate ion per dimer, give distinctly different ultraviolet spectra changes from Ni2+ and Hg2+ which do not induce phosphate binding. Spectrophotometric titrations at alkaline pH of various metallo-enzymes reveal a smaller number of ionizable tyrosines and a greater stability towards alkaline denaturation in the Zn2+- and Mn2+-enzymes than in the Ni2+-, Hg2+- and apoenzymes. The Zn2+- and Mn2+-enzymes have CD spectra in the region of the aromatic transitions that are different from the CD spectra of the Ni2+-, Hg2+- and apoenzymes. Modifications of arginines with 2,3-butanedione show that a smaller number of arginine residues are modified in the Zn2+-enzyme than in the Hg2+-enzyme. The presented data indicate that alkaline phosphatase from Escherichia coli must have a well-defined conformation in order to bind phosphate. Some metal ions (i.e. Zn2+, Co2+, Mn2+ and Cd2+), when interacting with the apoenzyme, alter the conformation of the protein molecule in such a way that it is able to interact with substrate molecules, while other metal ions (i.e. Ni2+ and Hg2+) are incapable of inducing the appropriate conformational change of the apoenzyme. These findings suggest an important structural function of the first two tightly bound metal ions in enzyme.     

Microbial and plant derived biomass for removal of heavy metals from wastewater

Discharge of heavy metals from metal processing industries is known to have adverse effects on the environment. Conventional treatment technologies for removal of heavy metals from aqueous solution are not economical and generate huge quantity of toxic chemical sludge. Biosorption of heavy metals by metabolically inactive non-living biomass of microbial or plant origin is an innovative and alternative technology for removal of these pollutants from aqueous solution. Due to unique chemical composition biomass sequesters metal ions by forming metal complexes from solution and obviates the necessity to maintain special growth-supporting conditions. Biomass of Aspergillus niger, Penicillium chrysogenum, Rhizopus nigricans, Ascophyllum nodosum, Sargassum natans, Chlorella fusca, Oscillatoria anguistissima, Bacillus firmus and Streptomyces sp. have highest metal adsorption capacities ranging from 5 to 641 mg g(-1) mainly for Pb, Zn, Cd, Cr, Cu and Ni. Biomass generated as a by-product of fermentative processes offers great potential for adopting an economical metal-recovery system. The purpose of this paper is to review the available information on various attributes of utilization of microbial and plant derived biomass and explores the possibility of exploiting them for heavy metal remediation.     

Raman spectroscopic study of the effects of Ca2+, Mg2+, Zn2+, and Cd2+ ions on calf thymus DNA: binding sites and conformational changes

The interaction of Mg2+, Ca2+, Zn2+, and Cd2+ with calf thymus DNA has been investigated by Raman spectroscopy. These spectra reveal that all of these ions, and particularly Zn2+, bind to phosphate groups of DNA, causing a slight structural change in the polynucleotide at very small metal: DNA (P) concentration ratio (ca. 1:30). This results in increased base-stacking interactions, with negligible change of the B conformation of DNA. Contrary to Zn2+ and Cd2+, which interact extensively with the nucleic bases (particularly at the N7 position of guanine), the alkaline-earth metal ions are bound almost exclusively to the phosphate groups. The affinity of both the Zn2+ and Cd2+ ions for G.C base pairs is comparable, but the Cd2+ ions interact more extensively with A.T pairs than Zn2+ ions. Interstrand cross-linking through the N3 atom of cytosine is suggested in the presence of Zn2+, but not Cd2+.     

Kinetic effects of ATP, divalent metal ions and pH on chicken liver mevalonate 5-diphosphate decarboxylase

The activity of chicken liver mevalonate 5-diphosphate decarboxylase was measured over a wide range of Mg2+ and ATP concentrations. It was found that free ATP activated the enzyme, whereas free Mg2+ had no effect on the enzyme activity. Computed analyses of free species concentrations and pH studies indicated that MgATP2- is the true substrate. The relative efficiencies of Mg2+, Mn2+, Cd2+, and Zn2+ as activating metal ions were evaluated in terms of V/Km for the corresponding (metal-ATP)2- complexes, and the relative ratios were: Mn2+ 100, Cd2+ 37, Mg2+ 14, Zn2+ 1.7. Inhibitory effects were demonstrated for all free divalent cations tested, except for Mg2+, and were in the order Zn2+ greater than Cd2+ greater than Mn2+.     

Metal ion interactions in the control of haem oxygenase induction in liver and kidney.

Mn2+ and Zn2+ exhibit a striking ability to block the induction by Sn2+ and Ni2+ of haem oxygenase (EC 1.14.99.3) in kidney. The blocking effects of Mn2+ and Zn2+ were found to be greatest on simultaneous administration, time-dependent when administered up to 8 h before the inducing metal ions, and ineffective when administered as little as 10 min after the inducing metal ions. The decreases in cytochrome P-450 and haem contents and the sequential changes in delta-aminolaevulinate synthase (EC 2.3.1.37) activity that occur concomitant with haem oxygenase induction were largely eliminated with simultaneous or prior treatment with Mn2+ or Zn2+, but not when Mn2+ or Zn2+ was administered after Sn2+ or Ni2+. Mn2+ and Zn2+ did not increase the catabolism of the enzyme in vivo. Zn2+ on simultaneous administration was also able substantially to block the induction of haem oxygenase by Co2+, Cd2+ and Ni2+ in liver. The Zn2+ blockade of Cd2+ induction was examined in detail, and prior or simultaneous administration of Zn2+ was found to be effective in blocking the induction of haem oxygenase and the concomitant decreases in cytochrome P-450 and haem contents, ethylmorphine demethylase activity and the sequential changes in delta-aminolaevulinate synthase activity. Zn2+ administration 10 min or more after Cd2+ was ineffective in preventing the occurrence of these perturbations in haem metabolism. These findings describe a new and striking biological property of Mn2+ and Zn2+, and indicate the existence of significant metal ion interactions in the control of haem metabolism.     

Comparative study of Cd(II) and Cr(VI) biosorption on Staphylococcus xylosus and Pseudomonas sp. in single and binary mixtures

Biosorption of Cd(II) and Cr(VI) ions in single solutions using Staphylococcus xylosus and Pseudomonas sp., and their selectivity in binary mixtures was investigated. Langmuir and Freundlich models were applied to describe metal biosorption and the influence of pH, biomass concentration and contact time was determined. Maximum uptake capacity of cadmium was estimated to 250 and 278 mg g(-1), whereas that of chromium to 143 and 95 mg g(-1) for S. xylosus and Pseudomonas sp., respectively. In binary mixtures with Cd(II) ions as the dominant species, there is a profound selectivity for cadmium biosorption, reaching 96% and 89% for Pseudomonas sp. and S. xylosus, respectively, at 10 mg l(-1) Cd(II) and 5 mg l(-1) Cr(VI). Interesting, when chromium (VI) ions are the dominant species, there is selectivity towards chromium around 92% with S. xylosus only.     

Bacterial sorption of heavy metals

Four bacteria, Bacillus cereus, B. subtilis, Escherichia coli, and Pseudomonas aeruginosa, were examined for the ability to remove Ag+, Cd2+, Cu2+, and La3+ from solution by batch equilibration methods. Cd and Cu sorption over the concentration range 0.001 to 1 mM was described by Freundlich isotherms. At 1 mM concentrations of both Cd2+ and Cu2+, P. aeruginosa and B. cereus were the most and least efficient at metal removal, respectively. Freundlich K constants indicated that E. coli was most efficient at Cd2+ removal and B. subtilis removed the most Cu2+. Removal of Ag+ from solution by bacteria was very efficient; an average of 89% of the total Ag+ was removed from the 1 mM solution, while only 12, 29, and 27% of the total Cd2+, Cu2+, and La3+, respectively, were sorbed from 1 mM solutions. Electron microscopy indicated that La3+ accumulated at the cell surface as needlelike, crystalline precipitates. Silver precipitated as discrete colloidal aggregates at the cell surface and occasionally in the cytoplasm. Neither Cd2+ nor Cu2+ provided enough electron scattering to identify the location of sorption. The affinity series for bacterial removal of these metals decreased in the order Ag greater than La greater than Cu greater than Cd. The results indicate that bacterial cells are capable of binding large quantities of different metals. Adsorption equations may be useful for describing bacterium-metal interactions with metals such as Cd and Cu; however, this approach may not be adequate when precipitation of metals occurs.