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 d iseases on laboratory studies. This paper examines how Koch set up a plausi ble 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 te chniques of isolating and controlling microorganisms, Koch combined these d ifferent modes of representation in a way that made his claims convincing. Thus, the microorganism as a specific cause of disease emerged through a ch ain of repeated processes of selection and representation in the laboratory .