The leaching ability of Leptospirillum ferrooxidans goes beyond the mere oxidation of Fe(2+) to Fe(3+). Addition of these bacteria to pyrite triggers interfacial phenomena that lead to bacterial attachment and local forms of corrosion (surface pitting). As the leaching process proceeds, bacterial cells undergo changes, characterized by the release of extracellular polymeric substances (EPS) and the uptake and storage of electro-dense nanoparticles. The latter are embedded in an exopolymeric capsule, which coats the bacterial surface leading to distinctive biomineralized assemblages. High-resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses, quantitative energy-dispersive X-ray measurements and electron diffraction established that the embedded electron-dense nanoparticles comprise pyrite with a well-defined stoichiometry. Addition of Fe(3+) alone did not induce any form of local corrosion on pyrite, which indicates that the reactions taking place between the attached bacteria and the underlying pyrite surface are responsible for the leaching patterns observed in this study. The observed corrosion process resembles that of 'electrochemical machining', because it uses a corrosion promoter, namely the locally concentrated Fe(3+) in the biofilm environment, formed by the attached cells.