Even the novice student in an introductory microbiology course is struck by the degree to which this discipline is based in a set of specific laboratory practices. Far more than theory, these practices still dominate microbiology in comparison with many other modern sciences. This is not a mere historiographic matter: it is largely from microbiology (as much as or more than the more recent molecular biology) and its practices that the modern biotechnology industry got its start. This is the central argument and the main contribution of Christoph Gradmann's fine new study of Robert Koch: Gradmann realizes and helps us to see that bacteriology as founded by Koch was from its inception a practice-based discipline. It becomes clear that Koch saw bacteria as disease agents that he could stain, label and count; he never saw them as organisms, and certainly not (as Pasteur saw them) as ecological actors. The difference from Pasteur has been pointed out before, not least by René Dubos and by J. Andrew Mendelsohn. But no previous author has helped us understand how different this made the entire intellectual enterprise. Koch, we see, thinks via his techniques, and via the visualized images they create, rather than through any theory that would tie his enterprise more to natural history, botany or biology. Thus the medical bacteriology Koch created – even what one thought bacteria were – was deeply tied to concrete laboratory practices such as staining, growing pure cultures and photography. Medical microbiology done through Koch's methods – which spread virally to the world in the 1880s – was a discipline then with very practical goals, far less connected by any lineage to biology than we might imagine today looking back.

Gradmann portrays this unique feature of Koch's medical bacteriology very effectively, using case studies of Koch's work on traumatic wound infections, on tuberculosis and the treatment tuberculin Koch tried to develop, on experimental therapies, and on Koch's travelling tropical ‘microbe-hunter’ expeditions, especially in German colonial Africa, where Koch expanded his epidemiological interests. The author does not take Koch's accounts at face value, asking historically useful questions at each stage. How were government funding and Koch's personal career aspirations bound up with his premature marketing of tuberculin, which turned out in the end to have little effectiveness? In what way did the debacle over the public loss of confidence in tuberculin undermine Koch's negotiations to create his equivalent of an Institut Pasteur in Berlin, with himself as director? How did racism in European colonial Africa make clinical experimentation there simpler, lowering ethical barriers? Perhaps most of all, Gradmann does not accept Koch's claim that his bacteriology really had no meaningful antecedents. He gives us the first full-blooded account of rival research programmes on bacteria and infectious disease, both from the botany community (e.g. Ernst Hallier, Carl von Naegeli) and from medical researchers (e.g. Klebs, Billroth). Koch's narrative previously tended to make those research programmes disappear, as ‘losers’ at best – Koch ignored them entirely in his public accounts but Gradmann makes clear how Koch was responding to them in his thinking during the 1870s.

Gradmann shows how the supposed ‘forerunner’ of Koch's postulates, Jakob Henle, really did not have most of the crucial ideas that made Koch succeed, especially lacking any emphasis on animal experiments. Rather, Koch owed enormous amounts to histologist Carl Weigert, from whom he learned the power of staining techniques, and pathologist Julius Cohnheim, who taught Koch the anatomy needed to characterize the specific kinds of damage done by different bacteria. Yet Gradmann also recognizes, from a close study of Koch's laboratory notes as well as published papers, where Koch's own technical innovations acted as lenses through which he could see bacterial disease in new ways – for example Koch's initial use of transferring pathogens through a series of different species of host animals, using the selective defences of different hosts as the first means of creating what Koch saw as pure cultures, well before solid media such as agar plating in Petri dishes are usually said to be the crucial technical breakthrough that made pure cultures possible.

A few regrettable minor errors are present (on p. 23, for instance, it is Christian Gottfried Ehrenberg, not Johann). But these are few and in no way overshadow the significance of this major new reconceptualization of Koch and of how deeply different in kind his medical bacteriology was from other research programmes in the life sciences of that time and of our own. For those interested in visualization; in laboratory practices and their epistemological implications; and in the history of bacteriology, microbiology, medicine and biology in general, this is an important book.