BACKGROUND: The metal respiring bacterium Shewanella oneidensis has previously been used for reduction of Pd(II) into Pd(0) nanoparticles. This study investigated whether Shewanella oneidensis could also perform the reduction of Au(III) to Au(0). The kinetics of both the biosorption and reduction of Au(III) were studied. RESULTS: Biosorption of Au(III) was a fast and efficient process, and depended on the presence of an electron donor, the pH and the medium used. The reduction process, however, appeared to be a slow process, requiring the presence of an electron donor. As reduction also occurred in heat-killed cells, it is suggested that the reduction is non-enzymatic. At a concentration of 100 mg L-1 Au(III), the nanoparticles were mainly smaller than 10 nm and precipitated intracellularly. With H-2 as the electron donor, it was shown that the location of the particles and the size could be steered by changing the concentration of Au(III). CONCLUSIONS: After a fast biosorption and slow reduction process, Au(0)-nanoparticles were formed inside the cells or on the cell wall of Shewanella oneidensis. Inmost cases, these particles had interesting properties, such as small size and a narrow size distribution, which can make them suitable for applications in, for example, catalysis. (C) 2010 Society of Chemical Industry