The working practices of biologists attest to a deep-seated belief in the unity of life. Biologists devote enormous attention to a handful of "model organisms," a group that includes, among others, the fruit fly Drosophila melanogaster, the weed Arabidopsis thaliana, and the brewer's yeast Saccharomyces cerevisiae. Although earth may contain up to 1 trillion species, biologists are convinced that, underlying this staggering diversity, there exists a shared set of molecular mechanisms. Intensive scrutiny of one species should yield insights that apply across all. How did this come to be? My historical research addresses this question by examining the field of microbial physiology in the 1920s and 30s. Using protozoa, flagellates and bacteria as models, researchers ascertained detailed chemical knowledge of the microbial metabolism. In the process, microbes attained an unprecedented status as indispensable tools in the science of life.