 Medicine changed fundamentally in the later part of the nineteenth century, and many historians of medicine locate the scrutiny of microorganisms via experimental laboratory methods at the heart of this transformation (e.g., Worboys 2000; Kreuder-Sonnen 2016). Pure culture, in which isolated and putatively causal microorganisms are grown in laboratories, was the first step. Normally, bacteria and other microbes live in complex multispecies communities. Such communities, however, are useless for identifying and intervening on the causes of diseases. Various innovations of nineteenth-century laboratory science (glassware, growth media, identification methods, micrography) enabled verifiable pure cultures of single species to be grown independently of their host organisms. Such cultures in turn allowed Koch’s postulates for causality to be implemented.