Abstract<br> The objective of this thesis was to investigate initial attachment and biofilm formation to pyrite surfaces of the mesophilic, acidophilic leaching bacteria Leptospirillum ferrooxidans DSM 2705, Acidithiobacillus ferrooxidans ATCC 23270 and Acidithiobacillus thiooxidans DSM 622 in pure and mixed cultures and their efficiency in pyrite dissolution.<br><br> The highest initial attachment of pure cultures to pyrite grains was detected for Leptospirillum ferrooxidans DSM 2705 and Acidithiobacillus thiooxidans DSM 622. However, Leptospirillum ferrooxidans DSM 2705 showed the highest leaching rates of pure cultures, whereas Acidithiobacillus thiooxidans DSM 622 is not able to leach pyrite at all. The highest attachment rate for mixed cultures to pyrite grains was detected for Acidithiobacillus ferrooxidans ATCC 23270 with Leptospirillum ferrooxidans DSM 2705. The highest leaching rates were detected for the pure culture of Leptospirillum ferrooxidans DSM 2705 and all mixed cultures including this strain. Contrary to the high attachment of the mixed culture composed of cells of Acidithiobacillus ferrooxidans ATCC 23270 and Acidithiobacillus thiooxidans DSM 622 to pyrite grains, leaching of pyrite grains was low. The investigation of bacterial attachment and leaching of pyrite grains indicated that high attachment rates do not necessarily correlate with high leaching rates.<br><br> In particular, initial attachment of Acidithiobacillus ferrooxidans ATCC 23270, Acidithiobacillus thiooxidans DSM 622 and Leptospirillum ferrooxidans DSM 2705 in pure and mixed cultures to pyrite coupons was investigated. A combination of atomic force microscopy and epifluorescence microscopy allowed the visualization of cells, biofilms and pyrite surfaces at the same side with high spatial accuracy. <br> Visualization of cells on pyrite coupons showed heterogeneous attachment and biofilm formation by both pure and mixed cultures. The bacteria attached as single cells or in small cell clusters to the mineral surface. Only cells of Leptospirillum ferrooxidans DSM 2705 showed rarely cluster formation with higher cell numbers on the pyrite surfaces. However, large areas of the pyrite surface remained cell-free, whereas, others where highly colonized. The highest attachment to pyrite coupons was determined for the pure culture of Leptospirillum ferrooxidans DSM 2705 and the mixed culture of Acidithiobacillus ferrooxidans ATCC 23070 with Acidithiobacillus thiooxidans DSM 622 and Leptospirillum ferrooxidans DSM 2705. The investigation of individual species in mixed cultures on pyrite coupons showed heterogeneously distributed cells with no physical interspecies contact. Depending on the composition of the mixed culture individual species showed increased (Acidithiobacillus ferrooxidans ATCC 23270) or decreased (L. ferrooxidans DSM 2705) attachment to pyrite coupons. Attachment of pure cultures of Leptospirillum ferrooxidans DSM 2705, Acidithiobacillus ferrooxidans ATCC 23270 and Acidithiobacillus thiooxidans DSM 622 considerable changed, if coupons were previously colonized by biofilms whose cells were heat-inactivated before secondary colonization. Increased attachment for Acidithiobacillus ferrooxidans ATCC 23270 and Acidithiobacillus thiooxidans DSM 622 and decreased attachment for Leptospirillum ferrooxidans DSM 2705 was detected. <br> However, no significant difference in leaching of virgin pyrite coupons compared to precolonized pyrite coupons by pure cultures of Leptospirillum ferrooxidans DSM 2705 and Acidithiobacillus ferrooxidans ATCC 23270 was detected.