Robert Koch based his claim that specific microorganisms cause particular diseases on laboratory studies. This paper examines how Koch set up a plausible line of argument by using special methods of representing bacteria. One kind of representation consisted in making the bacteria visible; the other mode of representation was based on disease phenomena. Using a range of techniques of isolating and controlling microorganisms, Koch combined these different modes of representation in a way that made his claims convincing. Thus, the microorganism (...) as a specific cause of disease emerged through a chain of repeated processes of selection and representation in the laboratory. (shrink)