Linking cause and disease in the laboratory: Robert Koch's method of superimposing visual and 'functional' representations of bacteria.

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.