Journal of Industrial Microbiology & Biotechnology (1999) 22 , 88–92  1999 Society for Industrial Microbiology 1367-5435/99/$12.00 http:/ / www.stockton-press.co.uk/ jim The role of exopolymers in the bioleaching of a non-ferrous metal sulphide C Pogliani and E Donati Centro de Investigacio ´ n y Desarrollo de Fermentaciones Industriales CONICET, Facultad de Ciencias Exactas, 47 y 115 (1900) La Plata, Argentina Exocellular polysaccharides were extracted from Thiobacillus ferrooxidans cells grown in the presence of iron. Cells without these compounds could not adhere to covellite. The loss of the layer of exocellular polysaccharides also affected the direct mechanism of bioleaching of covellite in a negative way. This ability to attach to and leach covellite was restored within a few hours when exopolymeric material was produced again. The addition of exocellu- lar compounds to cells stripped of exocellular polymers also restored their ability to the same level as that of untreated cells. Thiobacillus thiooxidans was not able to attach to and leach covellite even when exocellular com- pounds from Thiobacillus ferrooxidans were added. Keywords: exopolymers; Thiobacillus ferrooxidans; covellite; bioleaching Introduction The ability of acidophilic bacteria to assist in the recovery of metals by the dissolution of sulphide minerals is well known but the mechanism is not fully understood. Although a variety of bacteria are involved in the leaching process, Thiobacillus ferrooxidans and Thiobacillus thiooxidans are the most effective microorganisms [2,8]. Two different mechanisms have been proposed to explain bacterial attack by T. ferrooxidans: a direct one and an indirect one. The direct mechanism is based on catalytic sulphide oxidation, while the indirect one implies sulphide oxidation by ferric ions. Both products, sulphur and ferrous ion are oxidised by the microorganisms allowing the iron redox cycle to be repeated. T. ferrooxidans is also able to use other terminal electron acceptors under anaerobic con- ditions [6]. T. thiooxidans oxidises elemental sulphur but not ferrous ions and is therefore unable to degrade sulphide minerals by the indirect mechanism described for T. ferrooxidans. Moreover, the ability of T. thiooxidans to oxidise sulphides has been demonstrated only for more soluble sulphides [12,17,18]. Because of the insolubility of metal sulphides, direct bac- terial attack must be initiated by adhesion of cells to the substrate surface. Physical evidence of the adsorption of T. ferrooxidans onto mineral surfaces has been described in several papers [3,14,16,23]. The surface properties of microorganisms influence the attachment of bacteria to ores [4]. Thus the partial loss of surface lipopolysaccharides affects the adhesion of T. ferrooxidans [1,9]. Recently, Gehrke et al [10] demonstrated that T. ferroox- idans possesses an extracellular layer of polysaccharides, proteins and lipids (exopolymeric substances, EPS). They Correspondence: Dr E Donati, Centro de Investigacio ´n y Desarrollo de Fermentaciones Industriales, CONICET, Facultad de Ciencias Exactas, 47 y 115 (1900) La Plata, Argentina Received 2 June 1998; accepted 8 January 1999 also showed that EPS was necessary for attachment and subsequent leaching of pyrite. Furthermore, the attachment ability was restored by addition of ferric ions to cells stripped of EPS (EPS-deficient cells). These studies were carried out using pyrite, which con- tains iron, allowing both mechanisms of bacterial action. Thus it is difficult to determine how the addition of ferric ion initiates the dissolution of pyrite. Gehrke et al [10] assumed that they stimulated the rapid re-establishment of an EPS layer by EPS-deficient cells. They also proposed that ferric ions were entrapped in the EPS layer allowing bacterial attachment to and leaching of pyrite. However, the addition of ferric ion could produce an initial chemical oxidation of pyrite generating a series of intermediate com- pounds (ferrous ion, polythionates) which can be used by T. ferrooxidans [20]. The production of ferric ion would continue the dissolution of pyrite. In the present work, we have studied the role of exocellu- lar polysaccharides on the attachment and the dissolution of non-ferrous sulphide (synthetic covellite, CuS) by T. fer- rooxidans. We also investigated whether T. thiooxidans was able to attach to and leach covellite when EPS from T. ferrooxidans was added to the culture. Materials and methods Bacteria A T. ferrooxidans strain from Santa Rosa de Arequipa (DSM 11477) was used. Bacteria were grown in 9 K medium [21] at pH 1.8 and harvested during the late logar- ithmic growth phase (85–90% of ferrous iron oxidised). After removal of jarosite by filtration through blue ribbon filter paper, the culture suspension was filtered through a 0.22-m pore size filter (Nuclepore, Osmonics Lab, Minne- sota, USA) and washed several times with acidified water (pH 1.5) to eliminate soluble ferric iron. Finally, the bac- terial pellet was suspended in iron-free 9 K medium and used as an inoculum (containing approximately 3 × 10 8 cells ml -1 ).