Now showing items 1-20 of 3235

    • PD-1/PD-L1 pathway inhibition to restore effector functions in exhausted CD8+ T cells: chances, limitations and potential risks

      Veluswamy, Priya; Bruder, Dunja; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (AME Publishing Company, 2018-04)
      T cell exhaustion is a well-known mechanism involved in escape of degenerated cells or certain pathogens from CD8+ T cell-mediated immune surveillance, ultimately resulting in tumor development and chronic infections, respectively. Next to activated T cells, exhausted CD8+ T cells typically express high levels of the programmed cell death-1 (PD-1) receptor. While interaction of PD-1 with its ligand programmed death-ligand 1 (PD-L1) on hemotopoietic and non-hemotopoietic cells is important for the re-establishment of homeostasis following immune activation, PD-1/PD-L1 interaction represents a major drawback in certain other disease settings such as cancer or chronic viral infections. Here PD-1 signalling in T cells prevents efficient anti-tumor or anti-viral immune responses. Thus, therapeutic interference with the PD-1/PD-L1 pathway represents a promising approach for releasing exhausted CD8+ T cells from PD-1-dependent suppression and reactivation of effector functions. However, recent reports have highlighted unexpected outcomes of PD-1/PD-L1 pathway inhibition in the context of chronic infections. We provide here a comprehensive overview of the recent discoveries made in the context of PD-1/PD-L1 checkpoint inhibition that are considered relevant with respect to the targeted reactivation of effector functions in exhausted CD8+ T cells. We briefly discuss the impact of PD-1 signalling on the expression of certain transcription factors, on epigenetic modifications affecting chromatin accessibility, on cellular metabolism and the expression of certain cytokine receptors involved in immune homeostasis. These newly uncovered facts should be carefully considered before further development of therapies targeting the PD-1/PD-L1 pathway that are aiming at the restoration of pathogen-specific and anti-tumor CD8+ T cell effector functions in order to prevent adverse side effects. © 2018, Translational Cancer Research.
    • Structure of heme d-free cd nitrite reductase NirS.

      Klünemann, Thomas; Blankenfeldt, Wulf; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (International Union of Crystallography, 2020-05-29)
      A key step in anaerobic nitrate respiration is the reduction of nitrite to nitric oxide, which is catalysed by the cd1 nitrite reductase NirS in, for example, the Gram-negative opportunistic pathogen Pseudomonas aeruginosa. Each subunit of this homodimeric enzyme consists of a cytochrome c domain and an eight-bladed β-propeller that binds the uncommon isobacteriochlorin heme d1 as an essential part of its active site. Although NirS has been well studied mechanistically and structurally, the focus of previous studies has been on the active heme d1-bound form. The heme d1-free form of NirS reported here, which represents a premature state of the reductase, adopts an open conformation with the cytochrome c domains moved away from each other with respect to the active enzyme. Further, the movement of a loop around Trp498 seems to be related to a widening of the propeller, allowing easier access to the heme d1-binding side. Finally, a possible link between the open conformation of NirS and flagella formation in P. aeruginosa is discussed.
    • Polyketide-Derived Secondary Metabolites from a Dothideomycetes Fungus, . et . ., (Muyocopronales) with Antimicrobial and Cytotoxic Activities.

      Mapook, Ausana; Macabeo, Allan Patrick G; Thongbai, Benjarong; Hyde, Kevin D; Stadler, Marc; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (MDPI, 2020-04-08)
      Pseudopalawania siamensisgen. et sp. nov., from northern Thailand, is introduced based on multi-gene analyses and morphological comparison. An isolate was fermented in yeast malt culture broth and explored for its secondary metabolite production. Chromatographic purification of the crude ethyl acetate (broth) extract yielded four tetrahydroxanthones comprised of a new heterodimeric bistetrahydroxanthone, pseudopalawanone (1), two known dimeric derivatives, 4,4'-secalonic acid D (2) and penicillixanthone A (3), the corresponding monomeric tetrahydroxanthone paecilin B (4), and the known benzophenone, cephalanone F (5). Compounds 1-3 showed potent inhibitory activity against Gram-positive bacteria. Compounds 2 and 3 were inhibitory against Bacillus subtilis with minimum inhibitory concentrations (MIC) of 1.0 and 4.2 μg/mL, respectively. Only compound 2 showed activity against Mycobacterium smegmatis. In addition, the dimeric compounds 1-3 also showed moderate cytotoxic effects on HeLa and mouse fibroblast cell lines, which makes them less attractive as candidates for development of selectively acting antibiotics.
    • 2-Hydroxysorangiadenosine: Structure and Biosynthesis of a Myxobacterial Sesquiterpene-Nucleoside.

      Okoth, Dorothy A; Hug, Joachim J; Garcia, Ronald; Spröer, Cathrin; Overmann, Jörg; Müller, Rolf; HIPS, Helmholtz-Institut für Pharmazeutische Forschung Saarland, Universitätscampus E8.1 66123 Saarbrücken, Germany. (MDPI, 2020-06-09)
      Myxobacteria represent an under-investigated source for biologically active natural products featuring intriguing structural moieties with potential applications, e.g., in the pharmaceutical industry. Sorangiadenosine and the here-discovered 2-hydroxysorangiadenosine are myxobacterial sesquiterpene-nucleosides with an unusual structural moiety, a bicyclic eudesmane-type sesquiterpene. As the biosynthesis of these rare terpene-nucleoside hybrid natural products remains elusive, we investigated secondary metabolomes and genomes of several 2-hydroxysorangiadenosine-producing myxobacteria. We report the isolation and full structure elucidation of 2-hydroxysorangiadenosine and its cytotoxic and antibiotic activities and propose a biosynthetic pathway in the myxobacterium Vitiosangium cumulatum MCy10943T.
    • New Peptaibiotics and a Cyclodepsipeptide from : Isolation, Identification, Cytotoxic and Nematicidal Activities.

      Moussa, Ashaimaa Y; Lambert, Christopher; Stradal, Theresia E B; Ashrafi, Samad; Maier, Wolfgang; Stadler, Marc; Helaly, Soleiman E; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (MDPI, 2020-03-22)
      Fungal associations with nematodes have attracted scientific attention because of the need to develop new biocontrol agents. In this context, Ijuhya vitellina, an antagonistic fungus previously isolated from the plant parasitic cyst nematode Heterodera filipjevi, was selected to carry out an in-depth metabolomic study for its active metabolites. Herein, three new nonapeptide peptaibols with leucinostatin based sequences were isolated and identified by 1, 2D NMR, and HR-ESI-MS-MS. The absolute configuration was assigned based on Marfay's analysis and Mosher ester formation. The new leucinostatins manifested moderate nematicidal effect against the plant pathogenic nematode Pratylenchus penetrans with LD90 values ranging from 5 to 7 µg/mL. Furthermore, a cyclodepsipeptide, named arthrichitin D, with five amino acid residues attached to a 3-hydroxy-2,4-dimethylhexadeca-4,6-dienoic fatty acid chain was discovered and showed weak nematicidal effect against Caenorhabditis elegans. Chaetoglobosin B and its 19-O-acetyl derivative were also obtained as minor metabolites, and the activity of chaetoglobosin B on the actin cytoskeleton of mammalian cells was assessed.
    • The crystal structure of the heme d biosynthesis-associated small c-type cytochrome NirC reveals mixed oligomeric states in crystallo.

      Klünemann, Thomas; Henke, Steffi; Blankenfeldt, Wulf; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (: International Union of Crystallography, 2020-03-25)
      Monoheme c-type cytochromes are important electron transporters in all domains of life. They possess a common fold hallmarked by three α-helices that surround a covalently attached heme. An intriguing feature of many monoheme c-type cytochromes is their capacity to form oligomers by exchanging at least one of their α-helices, which is often referred to as 3D domain swapping. Here, the crystal structure of NirC, a c-type cytochrome co-encoded with other proteins involved in nitrite reduction by the opportunistic pathogen Pseudomonas aeruginosa, has been determined. The crystals diffracted anisotropically to a maximum resolution of 2.12 Å (spherical resolution of 2.83 Å) and initial phases were obtained by Fe-SAD phasing, revealing the presence of 11 NirC chains in the asymmetric unit. Surprisingly, these protomers arrange into one monomer and two different types of 3D domain-swapped dimers, one of which shows pronounced asymmetry. While the simultaneous observation of monomers and dimers probably reflects the interplay between the high protein concentration required for crystallization and the structural plasticity of monoheme c-type cytochromes, the identification of conserved structural motifs in the monomer together with a comparison with similar proteins may offer new leads to unravel the unknown function of NirC.
    • Foreword - Contents - List of authors

      Schwartz, Wilhelm; Kula, M.-R. (1977)
      FOREWORD It is a long way from theoretical considerations and laboratory experiments to technical applications, especially with the economic uncertainties lurking in the background. Since the publication in 1964 by Silverman and Ehrlich, containing a survey of what had until then been tried in microbiological formation and degradation of minerals by thiobacilli and other microorganisms, technical applications of thiobacilli have been developed on a broad scale only for sulphidic copper ores and for uranium ores along the lines of the primary program for low-grade ores, heap leaching and in-situ leaching of exhausted mines. During the last 10 years, new topics of leaching research have been discussed or are already developing: batch leaching with suspensions of ore concentrates; combined methods of chemical and microbial, and of electrolytic and microbial leaching; experiments to separate heavy-metal mixtures by microbial methods; leaching at high temperatures within the biological range or at high hydrostatic pressures; uranium leaching of phosphorites; leaching of industrial wastes and residues; application of acid-producing microorganisms other than thiobacilli; problems of freshwater shortage and its circumvention by the use of brackish water or sea water and adapted Thiobacillus strains; synergistic effects of thiobacilli and other microorganisms during the leaching process; treatment of mining waste waters with thiobacilli; relations between metal precipitation and solubilisation in the formation of sedimentary ore beds and in leaching processes. A quite different approach to leaching processes is the use of complexing or chelating metabolites produced by heterotrophic microorganisms. This had already been tried successfully by Perkins and Novielli (1962) with manganese oxides; it is again being discussed on a broader basis. Most of these topics were discussed or at least mentioned at our Conference, but some were not yet touched upon. As far as the technical and economic situation, the discussions on waste problems, environmental contamination, and shortages of raw materials are concerned, we may assume that biotechnical leaching has not yet reached its high point, but will continue to be extended and developed. | hope that the Conference will stimulate interest in this field of biotechnical research and also discussions between microbiologists and engineers of the mining and metallurgical industries on problems where interdisciplinary contacts may be profitable to both sides. | acknowledge with pleasure the support of this Conference by Dr. M.-R. Kula, Scientific Director of the GBF. | am especially grateful to the authors, and to Dr. Walsdorff of the GBF for their cooperation in preparing this volume. Braunschweig, July 1977 W. Schwartz Research and development at the GBF (Institute for Biotechnological Research Ltd.) is centered on the apparently unlimited biosynthetic capabilities of living cells: microorganisms as well as cell cultures. This potential is exploited for the production of organic chemicals, pharmaceuticals and materials such as single cell protein and specific enzymes. This also involves engineering, scale-up and development of new methods. Through the Leaching Conference, held here on 23-26 March 1977, the GBF has for the first time supported a scientific discipline that, although belonging to Biotechnology, is at present not being worked on at this Institute. However, the scientists at the GBF are aware that in recent years the leaching of low-grade ores has made fast progress in some areas. Thus we welcomed Prof. Schwartz’s suggestion to invite specialists in this field to a conference, in order to obtain a comprehensive and up-to-date survey of the field and to learn of problems and progress. We thank the participants of this Conference for their successful effort. It was decided to publish the papers of the Conference in the GBF Monograph Series, since, according to experts on modern leaching, there exists neither a comprehensive book nor a symposium volume that reflects the state of the art. This was clearly a correct decision, judging by the numerous requests already received. The present volume not only makes available a collection of the complete papers to the participants of the Conference, but is also intended to let other research groups in industry and academia gain insight into the area of microbial leaching, and to stimulate work in this field, especially in our country, where efforts are made to intensify work in this direction. Braunschweig-Stöckheim, July 1977 M.-R. Kula

      Kiel, Hildegard; Institut für Mikrobiologie der T.U. Braunschweig Arbeitsgruppe Prof.Dr.W,.Schwartz (1977)
      Leaching of carbonate and silicate ores with Thiobacilli under laboratory conditions has not been very successful. We have tested the leaching effects of organic acids and of acid producing heterotrophic microorganisms upon a low-grade copper ore, containing carbonates and silicates, from Timna, Israel. Good results were obtained with citric, lactic, glycollic, and tartaric acids. Citric acid, 0.05 M, for instance, solubilized 82% Cu. By lowering the pH with sulphuric acid to pH 2, lactic, eitric, and glycollic acids yielded almost 100% Cu. Citric acid producing strains of Aspergillus niger, growing in surface cultures on a Ssucrose fermentation medium, leached more than 80% Cu. Sulphite liquor may be used with fair results as a fermentation medium for A. niger. Anaerobic leaching experiments with whey, containing homofermentative lactobacilli, yielded 83% Cu.

      Berthelin, J.; Belgy, G.; Magne, R.; Centre de Pédologie biologique du C.N.R.S. B.P. 5 - 54500 Vandoeuvre-les-Nancy, France (*)now R. MAGNE is prospecting engineer at the C.E.A. (1977)
      In batch cultures, complex microflora from granitic mountain mass, promoted solubilization of U in presence of amino-acids as sole source of carbon and energy. Solubilized U amounted up to 100 mg U/1 in presence of microorganisms but was less than 35 mg U/1 in absence of microorganisms (sterile controls). Microflora involved contained different strains of bacteria, Pseudomonas fluorescens, P. putida, Achromobacter, Bacterium, Gaffkya or Peptococcus. In experimental design of factorial type with semi-continuous flow devices, the activity of different microflora from known forest soil was compared. Microflora specifically withstanding partial sterilization and comprising different Pseudomonas Bacillus licheniformis, B. cereus, B. lentus, B. polymyxa, B. megaterium, plus one or two unidentified yeasts promoted significantly microbial solubilization of U by synthesis of complexing agents with high complexing capacity for Al and Fe, but lower complexing capacity for U. But anaerobic microflora induced by waterlogging was much less active as compared by solubilization with control microflora. From the comparison of the different strains involved, Pseudomonas appeared to be the most active. In presence of an "organo-urany1" solution, obtained by adding glutamic or aspartic acids to uranium ore in sterile conditions, different bacteria originating from samples of granitic mountains mass could grow : Achromobacter, Brevibacterium, Acinetobacter, Gaffkya or Peptococcus, Pseudomonas, showing that such microorganisms could contribute to U deposition by metabolising organo-uranium compounds. Leaching and ecological implications of such processes are discussed.

      Ebner, H. G.; Universitat Dortmund, Technische Chemie B, Dortmund, FRG (1977)
      During the last two decades a lot of research has been carried out on microbiological leaching of low-grade ores. Since the UNCTAD conferences on raw materials have shown that besides the energy resources also the metallic resources of the world are limeted and what is even worse not renewable the idea of recycling has become more and more attractive. This means that waste raterials will have to be processed to regain valuable metals. Bacterial leaching of inorganic industrial waste or tailings can be regarded at least under two beneficial aspects: 1. toxic substances from the wastes will be removee thus the cost for their disposal can be lowered and the environment will be protected, 2. valuable metals can be gained from the wastes. This part is probably even more economic because of the rising prices for raw-materials in the near future.

      Szolnoki, J.; Hungarian Academy of Sciences, Laboratory for Geochemical Research, Budapest, Hungary (1977)
      Chemoautotrophic sulphur-bacteria were isolated from mine drainage waters /Th. ferrooxidans/, which are able to oxidize the ferrous-iron into ferric-iron in biocatalytic way in a heavy acidic medium. These bacteria were adapted to high metal especially to high iron, aluminium and sodium concentrations. The ferric-sulphate produced by the bacterial oxidation is a strong leaching agent. The optimal parameters were determined under which the acidic ferric-sulphate bearing solution is favourable from the point of view of practical utilization. During the biological oxidation process a part of dissolved iron precipitated in form of ferric-hydroxide which is also utilizable material.

      Zajic, J. E.; Jack, T. R.; Sullivan, E. A.; Faculty of Engineering Science The University of Western Ontario London, Ontario Canada (1977)
      The vast deposits of oil sands in the Province of Alberta Canada bear significant quantities of vanadium, nickel, titanium and iron in the bitumen component. During the production of oil from this bitumen, the metals are concentrated in the coke and coke ash refinery by-products. This study is concerned with the removal of metals, particularly vanadium, from the coke and coke ash obtained from different coking processes. The feasibility of marketable metal recovery by both chemical and microbially assisted leaching techniques has been assessed. Further the environmental impact of the solid waste disposal of untreated coke and coke ash has been evaluated by investigating the mechanism and extent of metal leaching under natural conditions and by investigating the toxicity of these "natural" leachates in a novel bioassay system. The study presents the data obtained in a critical evaluation of the leaching of metals from coke and coke ash in both economic and ecological contexts.

      Agate, A. D.; Deshpande, H. A.; Microbiology Department, Abasaheb Garware College Poona, INDIA (1977)
      When low grade manganiferous material (-65 mesh) obtained from Andhra Pradesh and Goa, India was subjected to leaching experiments , an efficiency of 70 to 85 % resulted,using an Arthrobacter species. The inoculum could be best cultivated in enriched soil extract - manganese medium and percolating columns with filtration were used for a period of 14 days. The optimum conditions for maximum leaching were worked out and the operation was found to be economically feasible on a large scale, when the adsorbed manganese was quantitatively precipitated with lime, Under the same conditions, other heterotrophic bacteria belonging to the genera Bacillus and Pseudomonas, isolated from fresh water pipeline deposits alongwith the predominant Arthrobacter species tested above, took upto 90 days to carry out the leaching of manganese,

      Tepper, K. P.; Näveke, R.; Lehrstuhl für Mikrobiologie, Technische Universitat Braunschweig Braunschweig, FRG (1977)
      The influence of direct current and of current with changing the poles two times per sec at intensities of 0,2...15 mA and at tensions of 12...170 Von leaching of pyrite dispersed in quartzite in percolators with Thiobacillus ferrooxidans was studied. The solution of iron was accelerated by bacteria without current and by current without bacteria, but in combination the acceleration did not exceed the sum of both single effects. The results suggest that there is no direct elec- trophysiological influence of current on bacteria under conditions of our experiments. At present we try to utilize indirect electrochemical effects of current on bacterial leaching of chalcopyrite.

      Illi, H.; Bertram, R.; Institut flir Physikalische Chemie, Technische Universitdt Braunschweig, Braunschweig, FRG (1977)
      Samples of naturally occurring chalcopyrite were dissolved anodically under potentiostatic conditions in various electrolytes. Limiting currents through the supply of holes were not observed. Because of the high current densities, even with a slight overpotential one observes a considerable resistance polarization depending on the conductivity of the electrolyte. A comparison with the results obtained through the leaching in the absence of current shows that the dissolution of the ore is strongly activated through the application of an external potential.

      Khalid, A. M.; Ralph, B. J.; School of Biological Technology, University of New South Wales, Kensington, New South Wales, Australia (1977)
      The availability of high-grade specimens of sphalerite, wurtzite and marmatite prompted a comparative study of the leaching rates of these minerals in the presence of Thiobacillus ferrooxidans, T. thiooxidans and T. thioparus. In this preliminary study, samples of the finely-ground minerals, of equivalent surface area, were subjected to attack by populations of equal magnitude of each of the three bacterial species in shake flasks at 30 C. The microorganisms were acclimatised to the particular substrates. The results indicate that (1) Wurtzite is much more slowly degraded than marmatite or sphalerite by all three organisms. (ii) Marmatite is leached more rapidly by T. ferrooxidans and T, thiooxidans than sphalerite, in both the presence and absence of soluble iron. (iii) Iron-free synthetic zinc sulphide is leached more rapidly by T. thiooxidans than by T. ferrooxidans or T. thioparus. The hexagonal crystalline structure of zinc sulphide in wurtzite appears to be more recalcitrant to microbial degradation than the cubical form of sphalerite, and the substitution of iron for some of the zinc in the marmatitic form of zinc sulphide appears to greatly facilitate biodegradation.

      Golding, R. M.; Harris, B.; Ralph, B. J.; Rickard, P. A. D.; Vanselow, D. G.; School of Chemistry, School of Metallurgy, School of Biological Technology, University of New South Wales, Kensington, N.S.W., Australia (1977)
      Evidence has been presented by Corrans et al. [1] that in the synthetic leaching of synthetic covellite, a direct mechanism operates by preventing the accumulation of a protective layer of sulphur and by the depolarisation of a cathodic reaction. The process of covellite passivation has been further studied, using sensitive methods for measurement of the consumption of oxygen and acid, and for the production of cupric and sulphate ions. The results indicate that passivation arises from the accumulation of approximately 30 micromole of elemental sulphur per square metre of covellite surface. The oxygen consumed during depassivation by various strains of Thiobacillus ferrooxidans supported this conclusion. Assuming an even distribution of sulphur over the surface, the passivation film has been calculated to be one or two atoms thick.

      Livesey-Goldblatt, E.; Tunley, T. H.; Nagy, I. F. (1977)
      Laboratory tests have established that a rapid oxidation of an acidified solution of ferrous sulphate to ferric can be achieved by passage of an air saturated solution over a film of Thiobacillus ferro-oxidans, Pilot-plant units of various types were constructed and operated to assess their oxidising efficiency using ferrous solutions ranging from 3,5 to 12 gram Fe II/litre and 3,1 to 10,5 gram H280,/litre. The best results were obtained with a plastic corrugated packing media to support the film of bacteria submerged in the iron sulphate solution thoroughly air saturated by forced aeration. The maximum specific rate of oxidation of Fe II to Fe III achieved was 7,5 gram per square metre of bacterial surface per hour.

      Torma, A. E.; Itzkovitch, I. J.; Department of Metallurgical and Materials Engineering, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA and Department of Metallurgy, Ontario Research Foundation, Mississauga, Ontario, Canada, L5K 1B3. (1977)
      Microbiological leaching as applied to uranium ores involves the metabolic oxidation by Thiobacillus ferrooxidans of associated pyrite to sulfuric acid and ferric sulfate. The sulfuric acid and ferric sulfate generated are effective leachants for common uranium minerals. Dissolved uranium can be recovered from the aqueous leach solutions by solvent extraction or ion exchange techniques. The present study explored the effects of potential solvent extraction reagents for uranium on pyrite oxidation ability of Thiobacillus ferrooxidans. For the solvents studied it was found that in all cases the dissolved organic matter decreased the pyrite oxidation activity of the bacteria, the surface tensionof the leach solutions and the oxygen saturation concentration. The following order of inhibition was established for the solvents and modifiers studied: aliquat 336 > nonyl phenol > kerosene 140 > alamine 310 > adogen 381 > di (2-ethylhexyl) phosphoric acid > adogen 365 > tri-n-butyl phosphate > ¥sodecanol > alamine 308 > alamine 336 > alamine 304. Suggestions to integrate solvent extraction and bacterial leaching for uranium by treating the recirculating raffinate are described.

      Ehrlich, Henry L.; Department of Biology, Rensselaer Polytechnic Institute Troy, NY 12181 (1977)
      A chalcopyrite-containing ore was shown to be able to retain measurable amounts of Cu2* ions from a 9 mM copper sulfate solution at pH values of 1.5 or above, which was percolated through it. The ore also exhibited significant buffering capacity when titrated with 0.01 N H,S0, or 0.01 N NaOH. Thiobacillus ferrooxidans strain Cu,S-2 accelerated leaching of the ore when using iron-free 1lixiviant at pH 1.5 or ferrous iron-containing lixiviant at pH 1.5. The total copper recovered in 20 weeks from inoculated percolation columns fed with ferrous lixiviant was the same as that recovered from inoculated columns fed with iron-free lixiviant, namely about 20% of the total copper in the ore, but the amount of copper recovered from uninoculated, sterile columns when using ferrous lixiviant was only about a third (3.2%) of that obtained with iron-free lixiviant fed to uninoculated, sterile columns in the same length of time (10.2%). When the ore was fed ferric lixiviant at pH 1.5, the bacteria exerted a strong retarding effect on the leaching process. In their absence, about 2.5 times as much copper (about 52% of the total copper in the ore) was recovered in 20 weeks than in their presence. The amount of copper recovered with bacteria in this case equalled approximately the amount recovered with bacteria using iron-free or ferrous ironcontaining lixiviants.