 Although it can be argued that microbiology began with microscopy in the early seventeenth century (e.g., Gest 2004; Lane 2015), the disciplinary starting point for bacteriology and eventually microbiology is usually bestowed on its experimental turn a couple of centuries later (Bulloch 1938). Early microscopy allowed observational microbiology for sure, but it gave rise to limited classification, and even more limited insights into what microbes could do consistently. The second half of the nineteenth century is when experimentation on microbes was codified, with the aim of understanding the specific and stable effects of particular microorganisms on disease in animals and plants. A central part of this emerging tradition involved pure culture techniques, Koch’s postulates, and germ theory. Each of these has ongoing philosophical ramifications.